Data Communications and Computer Networks

Data Communications and Computer Networks
by Curt M. White

Table of Contents 1. Introduction to Computer Networks and Data Communications Fundamentals of Data and Signals Conducted and Wireless Media Making Connections Making Connections Efficient: Multiplexing and Compression Errors, Error Detection, and Error Control Local Area Networks: Part

8. Local Area Networks: Part 2. -----246 9
8. Local Area Networks: Part Introduction to Metropolitan Area Networks and Wide Area Networks The Internet Voice and Data Delivery Networks Network Security Network Design and Management

Chapter 1 Introduction to Computer Networks and Data Communications

Introduction Who today has not used a computer network?
Mass transit, interstate highways, 24-hour bankers, grocery stores, cable television, cell phones, businesses and schools, and retail outlets support some form of computer network

The Language of Computer Networks
Computer network—an interconnection of computers and computing equipment using either wires or radio waves over small or large geographic areas Local area network—networks that are small in geographic size spanning a room, floor, building, or campus Metropolitan area network—networks that serve an area of 1 to 30 miles, approximately the size of a typical city

Wide area network—a large network that encompasses parts of states, multiple states, countries, and the world Personal area network—a network of a few meters, between wireless devices such as PDAs, laptops, and similar devices Voice network—a network that transmits only telephone signals (almost extinct) Data network—a network that transmits voice and computer data (replacing voice networks)

Data communications—the transfer of digital or analog data using digital or analog signals Telecommunications—the study of telephones and the systems that transmit telephone signals (becoming simply data communications) Network management—the design, installation, and support of a network, including its hardware and software

The Big Picture of Networks
Networks are composed of many devices, including: Workstations (computers, tablets, wireless phones, etc) Servers Network hubs and switches Routers (LAN to WAN and WAN to WAN) Telephone switching gear

The Big Picture of Networks

Communications Networks—Basic Connections
Microcomputer-to-local area network Microcomputer-to-Internet Local area network-to-local area network Personal area network-to-workstation Local area network-to-metropolitan area network

Communications Networks—Basic Connections
Local area network-to-wide area network Sensor-to-local area network Satellite and microwave Cell phones Computer terminal / microcomputer-to-mainframe

Microcomputer-to-Local Area Network Connections
Highly common throughout business and academic environments, and now homes Typically a medium- to high-speed connection Computer (device) requires a NIC (network interface card) NIC connects to a hub-like device (switch)

Microcomputer-to-Local Area Network Connections

Microcomputer-to-Internet Connections
Popular with home users and small businesses For some, a dial-up modem is used to connect user’s microcomputer to an Internet service provider Technologies such as DSL and cable modems are replacing modems

Microcomputer-to-Internet Connections

Local Area Network-to-Local Area Network Connections
Found in systems that have two or more LANs and a need for them to intercommunicate A bridge-like device (such as a switch) is typically used to interconnect LANs Switch can filter frames

Local Area Network-to-Local Area Network Connections

Personal Area Network-to-Workstation Connections
Interconnects wireless devices such as PDAs, laptops and notebooks, and music playback devices Used over short distances such as a few meters

Personal Area Network-to-Workstation Connections

Local Area Network-to-Metropolitan Area Network Connections
Used to interconnect companies (usually their local area networks) to networks that encompass a city High-speed networks with redundant circuits Metro Ethernet is latest form of metropolitan LAN

Local Area Network-to-Metropolitan Area Network Connections

Local Area Network-to-Wide Area Network Connections
One of the most common ways to interconnect a user on a LAN workstation to the Internet (a wide area network) A router is the typical device that performs LAN to WAN connections Routers are more complex devices than switches

Local Area Network-to-Wide Area Network Connections

Wide Area Network-to-Wide Area Network Connections
High-speed routers and switches are used to connect one wide area network to another Thousands of wide area networks across North America, many interconnected via these routers and switches

Sensor-to-Local Area Network Connections
Not all local area networks deal with microcomputer workstations Often found in industrial and laboratory environments Assembly lines and robotic controls depend heavily on sensor-based local area networks

Sensor-to-Local Area Network Connections

Satellite and Microwave Connections
Typically long distance wireless connections Many types of applications including long distance telephone, television, radio, long-haul data transfers, and wireless data services Typically expensive services but many companies offer competitive services and rates Newer shorter-distance services such as Wi-Max

Satellite and Microwave Connections

Cell Phone Connections
Constantly expanding market across the U.S. and world Third generation services available in many areas and under many types of plans with fourth generation services starting to appear Latest generation includes higher speed data transfers (100s to 1000s of kilobits per second)

Cell Phone Connections

Terminal/Microcomputer-to-Mainframe Computer Connections
Predominant form in the 1960s and 1970s Still used in many types of businesses for data entry and data retrieval Few dumb terminals left today—most are microcomputers with terminal emulation card, a web browser and web interface, Telnet software, or a thin client

Terminal/Microcomputer-to-Mainframe Computer Connections

Convergence An Additional Basic Connection—telephone-to-network
Telephone systems are ubiquitous and now carry more data than voice Common configuration—telephone connected to POTS Newer configuration (VoIP)—telephone-to-LAN via gateway or telephone to gateway via DSL/cable

Network Architectures
A reference model that describes the layers of hardware and software necessary to transmit data between two points or for multiple devices / applications to interoperate Reference models are necessary to increase likelihood that different components from different manufacturers will converse Two models to learn: TCP/IP protocol suite and OSI model

The TCP/IP Protocol Suite

Application layer Where the application using the network resides Common network applications include web browsing, , file transfers, and remote logins Transport layer Performs a series of miscellaneous functions (at the end-points of the connection) necessary for presenting the data package properly to the sender or receiver

Network (Internet or internetwork) layer Responsible for creating, maintaining and ending network connections Transfers data packet from node to node (e.g. router to router) within network Network access (data link/physical) layer Responsible for taking the data and transforming it into a frame with header, control and address information, and error detection code, then transmitting it between the workstation and the network Handles the transmission of bits over a communications channel Includes voltage levels, connectors, media choice, modulation techniques

The OSI Model

The OSI Model Application layer
Equivalent to TCP/IP’s application layer Presentation layer Responsible for “final presentation” of data (code conversions, compression, encryption) Session layer Responsible for establishing “sessions” between users

The OSI Model Transport layer Equivalent to TCP/IP’s transport layer
Network layer Equivalent to TCP/IP’s network layer Data link layer Responsible for taking the data and transforming it into a frame with header, control and address information, and error detection code

The OSI Model Physical layer
Handles the transmission of bits over a communications channel Includes voltage levels, connectors, media choice, modulation techniques

Logical and Physical Connections
A logical connection is one that exists only in the software, while a physical connection is one that exists in the hardware Note that in a network architecture, only the lowest layer contains the physical connection, while all higher layers contain logical connections

Logical and Physical Connections

Network Connections In Action

The TCP/IP Protocol Suite In Action
Note the flow of data from user to Web browser and back At each layer, information is either added or removed, depending on whether the data is leaving or arriving at a workstation The adding of information over pre-existing information is termed encapsulation

The TCP/IP Protocol Suite In Action

Chapter 2 Fundamentals of Data and Signals

Introduction - Data and Signals

Analog versus Digital Analog Digital

All Signals Have Three Components
Amplitude Frequency Phase

Amplitude The amplitude of a signal is the height of the wave above or below a given reference point.

Frequency The frequency
the number of times a signal makes a complete cycle within a given time frame.

Spectrum and Bandwidth
e.g. The average voice has a frequency range of roughly 300 Hz to 3100 Hz. The spectrum is The bandwidth is

Phase The phase of a signal is the position of the waveform relative to a given moment of time or relative to time zero. A change in phase can be any number of angles between 0 and 360 degrees. Phase changes often occur on common angles, such as 45, 90, 135, etc.

Signal Strength All signals experience loss (attenuation) due to friction in transmission. Attenuation is denoted as a decibel (dB) loss. dB is a relative measure. Decibel losses (and gains) are additive. Total: -5dB loss

Digital Data with Digital Signals – Encoding
NRZ-L NRZ-I Manchester Differential Manchester 4B/5B Digital Encoding

Baud Rate and BPS Baud rate BPS 1s Baud rate = BPS =

4B/5B Digital Encoding Yet another encoding technique that converts four bits of data into five-bit quantities. The five-bit quantities are unique in that no five-bit code has more than 2 consecutive zeroes. The five-bit code is then transmitted using an NRZ-I encoded signal. overhead

Digital Data with Analog Signals - Modulation
Amplitude Modulation

Digital Data with Analog Signals - Modulation
Frequency Modulation Phase Modulation Quadrature phase modulation Quadrature amplitude modulation

Analog Data into Digital Signals
Pulse Code Modulation The analog waveform is sampled at specific intervals and the “snapshots” are converted to binary values Higher sampling rate, or more quantization levels, improve the resolution, but will also increase the cost

Analog Data into Digital Signals
Delta Modulation An analog waveform is tracked using delta steps Output 1 to represent a rise in voltage, and a 0 to represent a drop.

Analog Data with Analog Signals
Analog signals serve as carriers Modulated into different amplitude (AM) or frequencies (FM)

Spread Spectrum Technology
A secure encoding technique that uses multiple frequencies or codes to transmit data.

Data Codes The set of all textual characters or symbols and their corresponding binary patterns is called a data code. There are two basic data code sets ASCII EBCDIC

Chapter 3 Conducted and Wireless Media

Introduction The world of computer networks would not exist if there were no medium by which to transfer data The two major categories of media include: Conducted media Wireless media

Twisted Pair Wire One or more pairs of single conductor wires that have been twisted around each other Twisted pair wire is classified by category. Twisted pair is currently Category 1 through Category 7, although Categories 1, 2 and 4 are nearly obsolete Twisting the wires helps to eliminate electromagnetic interference between the two wires Shielding can further help to eliminate interference

Twisted Pair Wire

Coaxial Cable A single wire wrapped in a foam insulation surrounded by a braided metal shield, then covered in a plastic jacket. Cable comes in various thicknesses Baseband coaxial technology uses digital signaling in which the cable carries only one channel of digital data Broadband coaxial technology transmits analog signals and is capable of supporting multiple channels

Coaxial Cable

Fiber-Optic Cable A thin glass cable approximately a little thicker than a human hair surrounded by a plastic coating and packaged into an insulated cable A photo diode or laser generates pulses of light which travel down the fiber optic cable and are received by a photo receptor

Fiber-Optic Cable

Fiber-Optic Cable Fiber-optic cable is capable of supporting millions of bits per second for 1000s of meters Thick cable (62.5/125 microns) causes more ray collisions, so you have to transmit slower This is step index multimode fiber Typically use LED for light source, shorter distance transmissions Thin cable (8.3/125 microns)—very little reflection, fast transmission, typically uses a laser, longer transmission distances; known as single mode fiber

Fiber-Optic Cable

Fiber-Optic Cable Fiber-optic cable is susceptible to reflection (where the light source bounces around inside the cable) and refraction (where the light source passes out of the core and into the surrounding cladding) Thus, fiber-optic cable is not perfect either. Noise is still a potential problem

Fiber-Optic Cable

Conducted Media

Wireless Media Radio, satellite transmissions, and infrared light are all different forms of electromagnetic waves that are used to transmit data Technically speaking—in wireless transmissions, space is the medium Note in the following figure how each source occupies a different set of frequencies

Wireless Media

Terrestrial Microwave Transmission
Land-based, line-of-sight transmission Approximately miles between towers Transmits data at hundreds of millions of bits per second Signals will not pass through solid objects Popular with telephone companies and business to business transmissions

Terrestrial Microwave Transmission

Satellite Microwave Transmission
Similar to terrestrial microwave except the signal travels from a ground station on earth to a satellite and back to another ground station Can also transmit signals from one satellite to another Satellites can be classified by how far out into orbit each one is (LEO, MEO, GEO, and HEO)

LEO (Low-Earth-Orbit)—100 to 1000 miles out Used for wireless , special mobile telephones, pagers, spying, videoconferencing MEO (Middle-Earth-Orbit)—1000 to 22,300 miles Used for GPS (global positioning systems) and government GEO (Geosynchronous-Earth-Orbit)—22,300 miles Always over the same position on earth (and always over the equator) Used for weather, television, government operations

HEO (Highly Elliptical Earth orbit)—satellite follows an elliptical orbit Used by the military for spying and by scientific organizations for photographing celestial bodies

Satellite microwave can also be classified by its configuration: Bulk carrier configuration Multiplexed configuration Single-user earth station configuration (e.g. VSAT)

Cellular Telephones Wireless telephone service, also called mobile telephone, cell phone, and PCS To support multiple users in a metropolitan area (market), the market is broken into cells Each cell has its own transmission tower and set of assignable channels

Cellular Telephones

Cellular Telephones 1st Generation
AMPS (Advanced Mobile Phone Service)—first popular cell phone service; used analog signals and dynamically assigned channels D-AMPS (Digital AMPS)—applied digital multiplexing techniques on top of AMPS analog channels

Cellular Telephones 2nd Generation
PCS (Personal Communication Systems)—essentially all-digital cell phone service PCS phones came in three technologies: TDMA—Time Division Multiple Access CDMA—Code Division Multiple Access GSM—Global System for Mobile Communications

Cellular Telephones 2.5 Generation
AT&T Wireless, Cingular Wireless, and T-Mobile now using GPRS (General Packet Radio Service) in their GSM networks (can transmit data at 30 kbps to 40 kbps) Verizon Wireless, Alltel, U.S.Cellular, and Sprint PCS are using CDMA2000 1xRTT (one carrier radio- transmission technology) (50 kbps to 75 kbps) Nextel uses IDEN technology

Cellular Telephones 3rd Generation
UMTS (Universal Mobile Telecommunications System)—also called Wideband CDMA The 3G version of GPRS UMTS not backward compatible with GSM (thus requires phones with multiple decoders) 1XEV (1 x Enhanced Version) –3G replacement for 1xRTT Will come in two forms: 1xEV-DO for data only 1xEV-DV for data and voice

Cellular Telephones 4th Generation
LTE (Long Term Evolution)—3 to 5 Mbps? WiMax—introduced in a couple slides UMB (Ultra Mobile Wideband) - ??? Too new to even discuss; may not even make it to market with LTE and WiMax available

Infrared Transmissions
Transmissions that use a focused ray of light in the infrared frequency range Very common with remote control devices, but can also be used for device-to-device transfers, such as PDA to computer

Broadband Wireless Systems
Delivers Internet services into homes and businesses Designed to bypass the local loop telephone line Transmits voice, data, and video over high frequency radio signals

Multichannel multipoint distribution service (MMDS) and local multipoint distribution service (LMDS) looked promising a few years ago but died off Now companies are eyeing WiMAX, an IEEE standard Initially 300 kbps to 2 Mbps over a range of as much as 30 miles Forthcoming standard (802.16e) will allow for moving devices

Bluetooth Bluetooth is a specification for short-range, point-to-point or point-to-multipoint voice and data transfer Bluetooth can transmit through solid, non-metal objects Its typical link range is from 10 cm to 10 m, but can be extended to 100 m by increasing the power

Bluetooth Bluetooth will enable users to connect to a wide range of computing and telecommunication devices without the need of connecting cables Typical uses include phones, pagers, modems, LAN access devices, headsets, notebooks, desktop computers, and PDAs

Wireless Local Area Networks
This technology transmits data between workstations and local area networks using high-speed radio frequencies Current technologies allow up to 54 Mbps (theoretical) data transfer at distances up to hundreds of feet Three popular standards: IEEE b, a, g More on this in Chapter Seven (LANs)

Free Space Optics and Ultra-Wideband
Uses lasers, or more economically, infrared transmitting devices Line of sight between buildings Typically short distances, such as across the street Newer auto-tracking systems keep lasers aligned when buildings shake from wind and traffic

Free space optics (continued) Current speeds go from T-3 (45 Mbps) to OC-48 (2.5 Gbps) with faster systems in development Major weakness is transmission thru fog A typical FSO has a link margin of about 20 dB Under perfect conditions, air reduces a system’s power by approximately 1 dB/km Scintillation is also a problem (especially in hot weather)

UWB not limited to a fixed bandwidth but broadcasts over a wide range of frequencies simultaneously Many of these frequencies are used by other sources, but UWB uses such low power that it “should not” interfere with these other sources Can achieve speeds up to 100 Mbps but for small distances such as wireless LANs

Ultra-wideband (continued) Proponents for UWB say it gets something for nothing, since it shares frequencies with other sources. Opponents disagree Cell phone industry against UWB because CDMA most susceptible to interference of UWB GPS may also be affected One solution may be to have two types of systems—one for indoors (stronger) and one for outdoors (1/10 the power)

ZigBee Based upon IEEE 802.15.4 standard
Used for low data transfer rates ( Kbps) Also uses low power consumption Ideal for heating, cooling, security, lighting, and smoke and CO detector systems ZigBee can use a mesh design ZigBee-enabled device can both accept and then pass on ZigBee signals

Wireless Media

Media Selection Criteria
Cost Speed Distance and expandability Environment Security

Cost Different types of costs
Initial cost—what does a particular type of medium cost to purchase? To install? Maintenance / support cost ROI (return on investment)—if one medium is cheaper to purchase and install but is not cost effective, where are the savings?

Speed Two different forms of speed:
Propagation speed—the time to send the first bit across the medium This speed depends upon the medium Airwaves and fiber are speed of light Copper wire is two thirds the speed of light Data transfer speed—the time to transmit the rest of the bits in the message This speed is measured in bits per second

Expandability and Distance
Certain media lend themselves more easily to expansion Don’t forget right-of-way issue

Environment Many types of environments are hazardous to certain media

Security If data must be secure during transmission, it is important that the medium not be easy to tap

Conducted Media In Action: Two Examples
First example: simple local area network Hub typically used To select proper medium, consider: Cable distance Data rate

Second example: company wishes to transmit data between buildings that are one mile apart Is property between buildings owned by company? If not consider using wireless When making decision, need to consider: Cost Speed Expandability and distance Environment Security

Wireless Media In Action: Three Examples
First example: you wish to connect two computers in your home to Internet, and want both computers to share a printer Can purchase wireless network interface cards May consider using Bluetooth devices Second example: company wants to transmit data between two locations, Chicago and Los Angeles Company considering two-way data communications service offered through VSAT satellite system

Third example—second company wishes to transmit data between offices two miles apart Considering terrestrial microwave system

Chapter 4 Making Connections

Text-to-Text Text-to-Self Text-to-World Text-to-Media
Making Connections Text-to-Text Text-to-Self Text-to-World Text-to-Media

What are Text Connections?
Making connections will allow us to better relate and understand ways in which reading connects to: your own life and experiences other forms of literature society and the world film, theatre, television, radio and music Making connections elicit (BRINGS OUT or ENHANCES) thinking skills such as: comparing and contrasting, cause and effect, recalling, inferring, synthesizing, and evaluating.

How Will Making Connections Help?
Recognizing connections stimulates our cognitive (knowledge and thoughts) and affective (feelings and emotions) perception. This increased awareness makes reading more relevant and meaningful. You will become more involved, engaged, and invested in reading and viewing an abundance of different forms of literature. You will “dive into” what you read, making it your own, and, in doing so, become proficient in the process of reading and comprehending.

Activating Prior Knowledge
Also known as activating schema, this skill is simply using what we have already absorbed from reading other texts, our lively experiences, watching movies, and observing the world on a day to day basis. Prior knowledge is what we already know. When making text connections between what we already know and what we are reading, we will build a stronger base of knowledge and be better equipped to understand and comprehend.

Cognitive Connections
Cognitive connections stimulate our thought processes and works with our prior knowledge as we recall concrete facts and our tactile experiences. When making these types of links we are using our reasoning skills.

Affective Connections
Affective connections stimulate our emotions and feelings and can cause our moods to change. We are activating past experiences that were happy or sad, exciting or boring, painful or soothing.

Connections Improve Comprehension!

4 Types of Text Connections
Text-to-Self (TS) - in this instance, we make personal connections with events or characters in the text; they remind us of people we know, things we did, places we have been, experiences we have had, etc. Text-to-Text (TT) - this is when we connect events or characters we are reading about with other texts we have read. Text-to-World (TW) - This is when we connect events, characters, or concepts in a book with real life events, people, or issues. This includes: social, political, economic, environmental, and cultural issues, as well as race relations and class systems, conflicts, and wars.

This is a connection to a historical time period, a historical figure, or an event in history. This can also be a current event that is happening NOW in our WORLD. Text-to-Media (TM) - this is when we connect events or characters we are reading about with a television show, a movie, a play, a radio show, or music we have viewed or listened to.

Making Connections Text-to-Text (T-T) Text-to-Self (T-S)
Text-to-World (T-W) Text-to-Media (T-M)

Text-to-Self The easiest connection to understand is text-to-self. This category represents the personal connections we develop between a specific written text and our own experiences. When we say, “This story reminds me of my grandfather who took me to the beach. Similar to my main character, grandpa taught me…….” Here we are expressing a text-to-self connection.

Making a connection between a story helps us better understand the tale. You should make a connection to a previous experience, memory, lesson you learned, or emotion you felt just as your main character does. It can also connect to anything the author indicates in his/her writing. Text-to-self connections help us in visualizing a scene, sympathizing with a character, or predicting possible meanings of unfamiliar vocabulary.

Text-to-Text Text-to-Text connections can be made across: themes
literary elements, features, devices, and techniques fiction and nonfiction genres paired reading selections vocabulary Effective readers think within the text. Effective readers think beyond the text. Effective readers think about the text.

Genres (a type of literature)
- Fiction: fable, tall tale, legend, myth, realistic fiction, historical fiction, play, adventure, science fiction, fantasy, mystery - Nonfiction: informational article, biography, autobiography, diary, journal, magazine article, news story, book review, persuasive essay, editorial, interview

Text-to-World We all have ideas about how the world works that go far beyond our own intimate personal experiences. We encounter the world vicariously, through newspapers and the nightly news. We observe others as they relate their personal experiences. We form ideas from our observations of and interactions with these people. This is when we connect our answer/response to a person in history, a time in history, or a specific event in history. This could also include a current event that is happening in our world.

Text-to-World connections are greater connections we make to events, issues, and concerns in society. What message or universal themes can we learn from the text? Does any of the situations relate to what’s happening in the world today? Is history repeating? What’s going on in the news?

Text-to-Media Film, television, theatre and music provide visual and auditory enhancement of many forms of literature. As we read we will find that we can make connections to a plethora (several or many) of movies or plays we have seen. A character from a novel is more likely to come to life if we can link their personalities to that of our favorite actor or entertainer. When we make these connections we are strengthening our understanding of the text.

4 Types of Text Connections

Introductory Phrases for Connections I
I have experienced a similar situation as…………………………. I have shared an experience with…… I can relate to………………………….because….…… I have also experienced………………………………………………………………. I have undergone the same trials and tribulations as……… There is a connection between myself and………….…because

Introductory Phrases for Connections II
A similar connection can be made to… …brings to mind… …relates to… …provides a direct connection between… …is a contrast/correlation/comparison to… …is the epitome (best example) of… …is the antithesis (opposite) of…

More “Lead-In Lines” The author probably feels this way because……..
The character probably acts this way because……. This also reminds me of………… Many people feel this way because…… We see this in society when…….

Chapter 5 Making Connections Efficient: Multiplexing and Compression

Introduction Under the simplest conditions, a medium can carry only one signal at any moment in time For multiple signals to share one medium, the medium must somehow be divided, giving each signal a portion of the total bandwidth. The current techniques that can accomplish this include: Frequency division multiplexing Time division multiplexing Wavelength division multiplexing Discrete Multitone Code division multiplexing

Frequency Division Multiplexing
Assignment of non-overlapping frequency ranges to signal on a medium. All signals are transmitted at the same time, each using different frequencies. A multiplexor accepts inputs and assigns frequencies to each device. The multiplexor is attached to a high-speed communications line. A corresponding multiplexor, or demultiplexor, is on the end of the high-speed line and separates the multiplexed signals. Analog signaling is used to transmit signals. Used in broadcast radio and television, cable television, and the AMPS cellular phone systems. More susceptible to noise.

Time Division Multiplexing
Sharing of the signal is accomplished by dividing available transmission time on a medium among users. Digital signaling. Two basic forms: Synchronous time division multiplexing Statistical time division multiplexing

Synchronous Time Division Multiplexing
The multiplexor accepts input from attached devices in a round-robin fashion and transmit the data in a never ending pattern. For devices that generate data at a faster rate than other devices, the multiplexor must either: sample the incoming data stream from that device more often than it samples the other devices, or buffer the faster incoming stream. For devices that has nothing to transmit, the multiplexor insert a piece of data from that device into the multiplexed stream. T1, ISDN, and SONET/SDH are common examples of synchronous time division multiplexing.

Synchronization The transmitting multiplexor insert alternating 1s and 0s into the data stream for the receiver to synchronize with incoming data stream.

Statistical Time Division Multiplexing
Transmits only the data from active workstations. No space is wasted on the multiplexed stream. Accepts the incoming data streams and creates a frame containing only the data to be transmitted. An address is included to identify each piece of data. A length is also included if the data is of variable size. The transmitted frame contains a collection of data groups.

Wavelength Division Multiplexing
Wavelength division multiplexing multiplexes multiple data streams onto a single fiber optic line. Different wavelength lasers (called lambdas) transmit the multiple signals. Each signal carried on the fiber can be transmitted at a different rate from the other signals. Dense wavelength division multiplexing combines many (30, 40, 50, 60, more?) onto one fiber Coarse wavelength division multiplexing combines only a few lambdas

Discrete Multitone (DMT)
A multiplexing technique commonly found in digital subscriber line (DSL) systems DMT combines hundreds of different signals, or subchannels, into one stream Each subchannel is quadrature amplitude modulated recall - eight phase angles, four with double amplitudes Theoretically, 256 subchannels, each transmitting 60 kbps, yields Mbps Unfortunately, there is noise

Code Division Multiplexing
Also known as code division multiple access Advanced technique that allows multiple devices to transmit on the same frequencies at the same time. Each mobile device is assigned a unique 64-bit code (Chip spreading code). To send a binary 1, mobile device transmits the unique code To send a binary 0, mobile device transmits the inverse of code Receiver gets summed signal, multiplies it by receiver code, adds up the resulting values Interprets as a binary 1 if sum is near +64 Interprets as a binary 0 if sum is near –64

Code Division Multiplexing
Three different mobile devices use the following codes: Mobile A: Mobile B: Mobile C: Three signals transmitted: Mobile A sends a 1, or , or Mobile B sends a 0, or , or Mobile C sends a 1, or , or Summed signal received by base station: +3, -1, -1, +1, +1, -1, -3, +3 Base station decode for Mobile A: Signal received: +3, -1, -1, +1, +1, -1, -3, +3 Mobile A’s code: +1, -1, +1, +1, +1, -1, -1, +1 Product result: +3, +1, -1, +1, +1, +1, +3, +3 Sum of products: +12 Decode rule: For result near +8, data is binary 1

Compression Compression is another technique used to squeeze more data over a communications line If you can compress a data file down to one half of its original size, file will obviously transfer in less time Two basic groups of compression: Lossless – when data is uncompressed, original data returns (Compress a financial file) Examples of lossless compression include: Huffman codes, run-length compression, and Lempel-Ziv compression Lossy – when data is uncompressed, you do not have the original data (Compress a video image, movie, or audio file) Examples of lossy compression include: MPEG, JPEG, MP3

Lossless Compression Run-length encoding
Replaces runs of 0s with a count of how many 0s. … ^ (30 0s) Replace each decimal value with a 4-bit binary value (nibble) Note: If you need to code a value larger than 15, you need to use two consecutive 4-bit nibbles The first is decimal 15, or binary 1111, and the second nibble is the remainder For example, if the decimal value is 20, you would code which is equivalent to If you want to code the value 15, you still need two nibbles: The rule is that if you ever have a nibble of 1111, you must follow it with another nibble

Lossy Compression Relative or differential encoding
Video does not compress well using run-length encoding In one color video frame, not much is alike But what about from frame to frame? Send a frame, store it in a buffer Next frame is just difference from previous frame Then store that frame in buffer, etc. First Frame Second Frame Difference

Images One image (JPEG) or continuous images (MPEG)
A color picture can be defined by red/green/blue, or luminance/chrominance/chrominance which are based on RGB values Either way, you have 3 values, each 8 bits, or 24 bits total (224 colors!) A VGA screen is 640 x 480 pixels 24 bits x 640 x 480 = 7,372,800 bits And video comes at you 30 images per second

JPEG Compresses still images Lossy
JPEG compression consists of 3 phases: Discrete cosine transformations (DCT) Quantization Run-length encoding

JPEG - DCT Divide image into a series of 8x8 pixel blocks
If the original image was 640x480 pixels, the new picture would be 80 blocks x 60 blocks If B&W, each pixel in 8x8 block is an 8-bit value (0-255) If color, each pixel is a 24-bit value (8 bits for red, 8 bits for blue, and 8 bits for green) Takes an 8x8 array (P) and produces a new 8x8 array (T) using cosines T matrix contains a collection of values called spatial frequencies These spatial frequencies relate directly to how much the pixel values change as a function of their positions in the block

An image with uniform color changes (little fine detail) has a P array with closely similar values and a corresponding T array with many zero values An image with large color changes over a small area (lots of fine detail) has a P array with widely changing values, and thus a T array with many non-zero values

JPEG - Quantization The human eye can’t see small differences in color
So take T matrix and divide all values by 10 Will give us more zero entries More 0s means more compression! But this is too lossy And dividing all values by 10 doesn’t take into account that upper left of matrix has more action (the less subtle features of the image, or low spatial frequencies)

Divided into 8 x 8 Pixel Blocks
640 x 480 VGA Screen Image Divided into 8 x 8 Pixel Blocks 184

U matrix Q[i][j] = Round(T[i][j] / U[i][j]), for i = 0, 1, 2, …7 and j = 0, 1, 2, …7

JPEG - Run-length encoding
Now take the quantized matrix Q and perform run-length encoding on it But don’t just go across the rows Longer runs of zeros if you perform the run-length encoding in a diagonal fashion

JPEG Uncompress Undo run-length encoding
Multiply matrix Q by matrix U yielding matrix T Apply similar cosine calculations to get original P matrix back

Chapter 6 Errors, Error Detection, and Error Control

Introduction All transmitted signals will contain some rate of error (>0%) Popular error control methods include: Parity bits (add a 1 or 0 to the end of each seven bits) Longitudinal redundancy checking (LRC) Polynomial checking

What’s an “error”? Human errors:
Incorrect IP address assignment, or subnet mask, etc., etc. Network errors: Lost data Corrupted data (received, but garbled)

Error Prevention To prevent errors from happening, several techniques may be applied: Proper shielding of cables to reduce interference Telephone line conditioning or equalization Replacing older media and equipment with new, possibly digital components Proper use of digital repeaters and analog amplifiers Observe the stated capacities of the media

Error Detection Methods
The only way to do error detection and correction is to send extra data with each message Two common error detection methods: Parity checking Simple parity Longitudinal parity Cyclic redundancy checksum (CRC)

Simple Parity 1 Even parity 1 1 1 Odd parity
Add an additional bit to each byte in the message: Even parity causes the sum of all bits (including the parity bit) to be even Odd parity causes the sum of all bits to be odd 1 Even parity 1 1 1 Odd parity

BCC Longitudinal Parity 1 1 1 1 1 1 1 1 1 1 1 1 1
Add block check character (BCC) to the end of the message: Perform odd parity checking on the block of bits for each character in the message 1 1 1 1 1 1 1 1 BCC 1 1 1 1 1

Parity Checks Both simple parity and longitudinal parity do not catch all errors Simple parity only catches odd numbers of bit errors Longitudinal parity is better at catching errors But requires too many check bits added to a block of data We need a better error detection method What about cyclic redundancy checksum?

Arithmetic Checksum Used in TCP and IP on the Internet
Characters to be transmitted are converted to numeric form and summed Sum is placed in some form at the end of the transmission Receiver performs same conversion and summing and compares new sum with sent sum TCP and IP processes a little more complex but idea is the same

Polynomial Checking D A T 68 65 84 282 255 = 1 remainder 27 1 Checksum
Adds a character (or series of characters) to the end of the message based on a mathematical algorithm: Checksum Sum the message values and divide by 255. The remainder is the checksum D A T 68 65 84 282 255 = 1 remainder 27 1 Checksum

Cyclical redundancy check
CRC error detection method treats packet of data to be transmitted as a large polynomial Transmitter Using polynomial arithmetic, divides polynomial by a given generating polynomial Quotient is discarded Remainder is “attached” to the end of message Message (with the remainder) is transmitted to the receiver Receiver divides the message polynomial plus the remainder (checksum) by same generating polynomial If a remainder of zero results Ú no error during transmission If a remainder not equal to zero results Ú error during transmission

Error Control Once an error is detected, the receiver can:
Toss the frame/packet Some newer systems such as frame relay perform this type of error control Return an error message to the transmitter Stop-and-wait error control Sliding window error control Fix the error with no further help from the transmitter

Toss frame/packet Seems like a strange way to control errors but some lower- layer protocols such as frame relay perform this type of error control For example, if frame relay detects an error, it simply tosses the frame No message is returned Frame relay assumes a higher protocol (such as TCP/IP) will detect the tossed frame and ask for retransmission

Return A Message Once an error is detected, an error message is returned to the transmitter Two basic forms: Stop-and-wait error control Sliding window error control

Stop-and-wait Error Control
A transmitter sends a frame then stops and waits for an acknowledgment If a positive acknowledgment (ACK) is received, the next frame is sent If a negative acknowledgment (NAK) is received, the same frame is transmitted again

Sliding Window Error Control
These techniques assume that multiple frames are in transmission at one time A sliding window protocol allows the transmitter to send a number of data packets at one time before receiving any acknowledgments Depends on window size When a receiver does acknowledge receipt, the returned ACK contains the number of the frame expected next Older sliding window protocols numbered each frame or packet that was transmitted More modern sliding window protocols number each byte within a frame

Notice that an ACK is not always sent after each frame is received
It is more efficient to wait for a few received frames before returning an ACK How long should you wait until you return an ACK? 206

TCP/IP Rule 1: If a receiver just received data and wants to send its own data, piggyback an ACK along with that data Rule 2: If a receiver has no data to return and has just ACKed the last packet, receiver waits 500 ms for another packet If while waiting, another packet arrives, send the ACK immediately Rule 3: If a receiver has no data to return and has just ACKed the last packet, receiver waits 500 ms No packet, send ACK

Packet Lost If a frame is lost, the following frame will be “out of sequence” The receiver will hold the out of sequence bytes in a buffer and request the sender to retransmit the missing frame

ACK Lost If an ACK is lost, the sender will wait for the ACK to arrive and eventually time out When the time-out occurs, the sender will resend the last frame

Correct the Error For a receiver to correct the error with no further help from the transmitter requires a large amount of redundant information to accompany the original data This redundant information allows the receiver to determine the error and make corrections This type of error control is often called forward error correction and involves codes called Hamming codes Hamming codes add additional check bits to a character These check bits perform parity checks on various bits

For example, what if bit b9 flips?
The c8 check bit checks bits b12, b11, b10, b9 and c8 (01000) This would cause a parity error The c4 check bit checks bits b12, b7, b6, b5 and c4 (00101) This would not cause a parity error (even number of 1s) The c2 check bit checks bits b11, b10, b7, b6, b3 and c2 (100111) This would not cause a parity error The c1 check bit checks b11, b9, b7, b5, b3 and c1 (100011) Writing the parity errors in sequence gives us 1001, which is binary for the value 9 Thus, the bit error occurred in the 9th position

Chapter 7 Local Area Networks: Part 1

Introduction A local area network is a communication network that interconnects a variety of data communicating devices within a small geographic area and broadcasts data at high data transfer rates with very low error rates. Since the local area network first appeared in the 1970s, its use has become widespread in commercial and academic environments.

Primary Function of a LAN
To provide access to hardware and software resources that will allow users to perform one or more of the following activities: File serving - A large storage disk drive acts as a central storage repository. Print serving - Providing the authorization to access a particular printer, accept and queue print jobs, and providing a user access to the print queue to perform administrative duties.

Primary Function of a LAN
Video transfers - High speed LANs are capable of supporting video image and live video transfers. Manufacturing support - LANs can support manufacturing and industrial environments. Academic support – In classrooms, labs, and wireless. support. Interconnection between multiple systems.

Advantages of Local Area Networks
Ability to share hardware and software resources. Individual workstation might survive network failure. Component and system evolution are possible. Support for heterogeneous forms of hardware and software. Access to other LANs and WANs Private ownership. Secure transfers at high speeds with low error rates.

Disadvantages of Local Area Networks
Equipment and support can be costly. Level of maintenance continues to grow. Private ownership possible. Some types of hardware may not interoperate. Just because a LAN can support two different kinds of packages does not mean their data can interchange easily. A LAN is only as strong as it weakest link, and there are many links.

Basic Local Area Network Topologies
Local area networks are interconnected using one of four basic configurations: 1. Bus/tree 2. Star-wired bus 3. Star-wired ring 4. Wireless

Bus/Tree Topology The original topology.
Workstation has a network interface card (NIC) that attaches to the bus (a coaxial cable) via a tap. Data can be transferred using either baseband digital signals or broadband analog signals.

Bus/Tree Topology Baseband signals are bidirectional and more outward in both directions from the workstation transmitting. Broadband signals are usually uni-directional and transmit in only one direction. Because of this, special wiring considerations are necessary. Buses can be split and joined, creating trees.

Star-wired Bus Topology
Logically operates as a bus, but physically looks like a star. Star design is based on hub. All workstations attach to hub. Unshielded twisted pair usually used to connect workstation to hub. Hub takes incoming signal and immediately broadcasts it out all connected links. Hubs can be interconnected to extend size of network.

Star-wired Bus Topology
Modular connectors and twisted pair make installation and maintenance of star-wired bus better than standard bus. Hubs can be interconnected with twisted pair, coaxial cable, or fiber optic cable. Biggest disadvantage: when one station talks, everyone hears it. This is called a shared network. All devices are sharing the network medium.

Star-wired Ring Topology
Logically operates as a ring but physically appears as a star. Star-wired ring topology is based on MAU (multi-station access unit) which functions similarly to a hub. Where a hub immediately broadcasts all incoming signals onto all connected links, the MAU passes the signal around in a ring fashion. Like hubs, MAUs can be interconnected to increase network size.

Wireless LANs Not really a specific topology since a workstation in a wireless LAN can be anywhere as long as it is within transmitting distance to an access point. Newer IEEE and b standard defines various forms of wireless LAN connections. Speeds up to 11 Mbps with b standard. Workstations reside within a basic service set, while multiple basic service sets create an extended service set.

Wireless LANs Two basic components necessary: the client radio, usually a PC card with an integrated antenna, and the access point (AP), which is an Ethernet port plus a transceiver. The AP acts as a bridge between the wired and wireless networks and can perform basic routing functions. Workstations with client radio cards reside within a basic service set, while multiple basic service sets create an extended service set.

Wireless LANs With directional antennae designed for point-to-point transmission (rare), b can work for more than 10 miles. With an omni-directional antenna on a typical AP, range may drop to as little as 100 feet. Distance is inversely proportional to transmission speed - as speed goes up, distance goes down.

Wireless LANs In actual tests, 11 Mbps b devices managed 5.5 Mbps (from a July 2000 test by Network Computing). To provide security, most systems use Wired Equivalent Privacy (WEP), which provides either 40- or 128-bit key protection. What will Bluetooth’s impact be on b?

Other Wireless Standards
IEEE (older 2 Mbps) IEEE b (11 Mbps, 2.4 GHz) IEEE a (54 Mbps, 5 GHz, in 2002) IEEE g (54 Mbps, 2.4 GHz, in 2002) HiperLAN/2 (European standard, 54 Mbps in 5 GHz band)

Peer-to-Peer LANs Not as common as server-based LANs
Less, if any reliance on servers Most peer-to-peer LANs still use one or more servers Interesting collaborative-type applications (world-wide law firm)

Medium Access Control Protocols
How does a workstation get its data onto the LAN medium? A medium access control protocol is the software that allows workstations to “take turns” at transmitting data. Three basic categories: 1. Contention-based protocols 2. Round robin protocols 3. Reservation protocols

Contention-Based Protocols
Essentially first come, first served. Most common example is carrier sense multiple access with collision detection (CSMA/CD). If no one is transmitting, a workstation can transmit. If someone else is transmitting, the workstation “backs off” and waits.

Contention-Based Protocols
If two workstations transmit at the same time, a collision occurs. When the two workstations hear the collision, they stop transmitting immediately. Each workstation backs off a random amount of time and tries again. Hopefully, both workstations do not try again at the exact same time. CSMA/CD is an example of a non-deterministic protocol.

Round Robin Protocols Each workstation takes a turn transmitting and the turn is passed around the network from workstation to workstation. Most common example is token ring LAN in which a software token is passed from workstation to workstation. Token ring is an example of a deterministic protocol. Token ring more complex than CSMA/CD. What happens if token is lost? Duplicated? Hogged? Token ring LANs are losing the battle with CSMA/CD LANs.

Reservation Protocols
Workstation places a reservation with central server. Workstation cannot transmit until reservation comes up. Under light loads, this acts similar to CSMA/CD. Under heavy loads, this acts similar to token ring. Powerful access method but again losing out to CSMA/CD. Most common example of reservation protocol is demand priority protocol.

Medium Access Control Sublayer
To better support local area networks, the data link layer of the OSI model was broken into two sublayers: 1. Logical link control sublayer 2. Medium access control sublayer Medium access control sublayer defines the frame layout and is more closely tied to a specific medium at the physical layer. Thus, when people refer to LANs they often refer to its MAC sublayer name, such as 10BaseT.

IEEE 802 Frame Formats The IEEE 802 suite of protocols defines the frame formats for CSMA/CD (IEEE 802.3) and token ring (IEEE 802.5). Each frame format describes how the data package is formed. Note how the two frames are different. If a CSMA/CD network connects to a token ring network, the frames have to be converted from one to another.

Local Area Network Systems
Ethernet or CSMA/CD Most common form of LAN today. Star-wired bus is most common topology but bus topology also available. Ethernet comes in many forms depending upon medium used and transmission speed and technology.

Ethernet Originally, CSMA/CD was 10 Mbps.
Then 100 Mbps was introduced. Most NICs sold today are 10/100 Mbps. Then 1000 Mbps (1 Gbps) was introduced. 10 Gbps is now beginning to appear.

Ethernet 1000 Mbps introduces a few interesting wrinkles:
Transmission is full duplex (separate transmit and receive), thus no collisions. Prioritization is possible using 802.1p protocol. Topology can be star or mesh (for trunks).

Ethernet A few more interesting wrinkles:
Cabling can be either UTP or optical (but 10 Gbps Ethernet may not work over UTP due to radio frequency interference). Where 10 Mbps Ethernet has less than 30% utilization due to collisions, 1000 Mbps is limited only by traffic queueing. Distance with 10 Mbps is limited by CSMA/CD propagation time, whereas 1000 Mbps is limited only by media.

IBM Token Ring Deterministic LAN offered at speeds of 4, 16 and 100 Mbps. Very good throughput under heavy loads. More expensive components than CSMA/CD. Losing ground quickly to CSMA/CD. May be extinct soon.

FDDI (Fiber Distributed Data Interface) Based on the token ring design using 100 Mbps fiber connections. Allows for two concentric rings - inner ring can support data travel in opposite direction or work as backup. Token is attached to the outgoing packet, rather than waiting for the outgoing packet to circle the entire ring.

100VG-AnyLAN Deterministic LAN based on demand priority access method. Similar to hub topology (star design). Two levels of priority - normal and high. Supports a wide-variety of media types. Losing ground quickly to CSMA/CD.

Chapter 8 Local Area Networks: Part 2

LAN Applications(1) Personal computer LANs Low cost Limited data rate
Back end networks Interconnecting large systems (mainframes and large storage devices) High data rate High speed interface Distributed access Limited distance Limited number of devices

LAN Applications (2) Storage Area Networks
Separate network handling storage needs Detaches storage tasks from specific servers Shared storage facility across high-speed network Hard disks, tape libraries, CD arrays Improved client-server storage access Direct storage to storage communication for backup High speed office networks Desktop image processing High capacity local storage Backbone LANs Interconnect low speed local LANs Reliability Capacity Cost

Storage Area Networks

LAN Architecture Topologies Transmission medium Layout
Medium access control

Topologies Tree Bus Special case of tree One trunk, no branches Ring
Star

LAN Topologies

Bus and Tree Multipoint medium
Transmission propagates throughout medium Heard by all stations Need to identify target station Each station has unique address Full duplex connection between station and tap Allows for transmission and reception Need to regulate transmission To avoid collisions To avoid hogging Data in small blocks - frames Terminator absorbs frames at end of medium

Frame Transmission on Bus LAN

Ring Topology Repeaters joined by point to point links in closed loop
Receive data on one link and retransmit on another Links unidirectional Stations attach to repeaters Data in frames Circulate past all stations Destination recognizes address and copies frame Frame circulates back to source where it is removed Media access control determines when station can insert frame

Frame Transmission Ring LAN

Star Topology Each station connected directly to central node
Usually via two point to point links Central node can broadcast Physical star, logical bus Only one station can transmit at a time Central node can act as frame switch

Choice of Topology Reliability Expandability Performance
Needs considering in context of: Medium Wiring layout Access control

Bus LAN Transmission Media (1)
Twisted pair Early LANs used voice grade cable Didn’t scale for fast LANs Not used in bus LANs now Baseband coaxial cable Uses digital signalling Original Ethernet

Bus LAN Transmission Media (2)
Broadband coaxial cable As in cable TV systems Analog signals at radio frequencies Expensive, hard to install and maintain No longer used in LANs Optical fiber Expensive taps Better alternatives available Not used in bus LANs All hard to work with compared with star topology twisted pair Coaxial baseband still used but not often in new installations

Ring and Star Usage Ring Very high speed links over long distances
Single link or repeater failure disables network Star Uses natural layout of wiring in building Best for short distances High data rates for small number of devices

Choice of Medium Constrained by LAN topology Capacity Reliability
Types of data supported Environmental scope

Media Available (1) Voice grade unshielded twisted pair (UTP) Cat 3
Cheap Well understood Use existing telephone wiring in office building Low data rates Shielded twisted pair and baseband coaxial More expensive than UTP but higher data rates Broadband cable Still more expensive and higher data rate

Media Available (2) High performance UTP Cat 5 and above
High data rate for small number of devices Switched star topology for large installations Optical fiber Electromagnetic isolation High capacity Small size High cost of components High skill needed to install and maintain Prices are coming down as demand and product range increases

Protocol Architecture
Lower layers of OSI model IEEE 802 reference model Physical Logical link control (LLC) Media access control (MAC)

IEEE 802 v OSI

802 Layers - Physical Encoding/decoding Preamble generation/removal
Bit transmission/reception Transmission medium and topology

802 Layers -Logical Link Control
Interface to higher levels Flow and error control

Logical Link Control Transmission of link level PDUs between two stations Must support multiaccess, shared medium Relieved of some link access details by MAC layer Addressing involves specifying source and destination LLC users Referred to as service access points (SAP) Typically higher level protocol

LLC Services Based on HDLC Unacknowledged connectionless service
Connection mode service Acknowledged connectionless service

LLC Protocol Modeled after HDLC
Asynchronous balanced mode to support connection mode LLC service (type 2 operation) Unnumbered information PDUs to support Acknowledged connectionless service (type 1) Multiplexing using LSAPs

Media Access Control Assembly of data into frame with address and error detection fields Disassembly of frame Address recognition Error detection Govern access to transmission medium Not found in traditional layer 2 data link control For the same LLC, several MAC options may be available

LAN Protocols in Context

Media Access Control Where Central Greater control
Simple access logic at station Avoids problems of co-ordination Single point of failure Potential bottleneck Distributed How Synchronous Specific capacity dedicated to connection Asynchronous In response to demand

Asynchronous Systems Round robin
Good if many stations have data to transmit over extended period Reservation Good for stream traffic Contention Good for bursty traffic All stations contend for time Distributed Simple to implement Efficient under moderate load Tend to collapse under heavy load

MAC Frame Format MAC layer receives data from LLC layer MAC control
Destination MAC address Source MAC address LLS CRC MAC layer detects errors and discards frames LLC optionally retransmits unsuccessful frames

Generic MAC Frame Format

Bridges Ability to expand beyond single LAN
Provide interconnection to other LANs/WANs Use Bridge or router Bridge is simpler Connects similar LANs Identical protocols for physical and link layers Minimal processing Router more general purpose Interconnect various LANs and WANs see later

Why Bridge? Reliability Performance Security Geography

Functions of a Bridge Read all frames transmitted on one LAN and accept those address to any station on the other LAN Using MAC protocol for second LAN, retransmit each frame Do the same the other way round

Bridge Operation

Bridge Design Aspects No modification to content or format of frame
No encapsulation Exact bitwise copy of frame Minimal buffering to meet peak demand Contains routing and address intelligence Must be able to tell which frames to pass May be more than one bridge to cross May connect more than two LANs Bridging is transparent to stations Appears to all stations on multiple LANs as if they are on one single LAN

Bridge Protocol Architecture
IEEE 802.1D MAC level Station address is at this level Bridge does not need LLC layer It is relaying MAC frames Can pass frame over external comms system e.g. WAN link Capture frame Encapsulate it Forward it across link Remove encapsulation and forward over LAN link

Connection of Two LANs

Fixed Routing Complex large LANs need alternative routes
Load balancing Fault tolerance Bridge must decide whether to forward frame Bridge must decide which LAN to forward frame on Routing selected for each source-destination pair of LANs Done in configuration Usually least hop route Only changed when topology changes

Bridges and LANs with Alternative Routes

Spanning Tree Bridge automatically develops routing table
Automatically update in response to changes Frame forwarding Address learning Loop resolution

Frame forwarding Maintain forwarding database for each port
List station addresses reached through each port For a frame arriving on port X: Search forwarding database to see if MAC address is listed for any port except X If address not found, forward to all ports except X If address listed for port Y, check port Y for blocking or forwarding state Blocking prevents port from receiving or transmitting If not blocked, transmit frame through port Y

Address Learning Can preload forwarding database Can be learned
When frame arrives at port X, it has come form the LAN attached to port X Use the source address to update forwarding database for port X to include that address Timer on each entry in database Each time frame arrives, source address checked against forwarding database

Spanning Tree Algorithm
Address learning works for tree layout i.e. no closed loops For any connected graph there is a spanning tree that maintains connectivity but contains no closed loops Each bridge assigned unique identifier Exchange between bridges to establish spanning tree

Loop of Bridges

Layer 2 and Layer 3 Switches
Now many types of devices for interconnecting LANs Beyond bridges and routers Layer 2 switches Layer 3 switches

Hubs Active central element of star layout
Each station connected to hub by two lines Transmit and receive Hub acts as a repeater When single station transmits, hub repeats signal on outgoing line to each station Line consists of two unshielded twisted pairs Limited to about 100 m High data rate and poor transmission qualities of UTP Optical fiber may be used Max about 500 m Physically star, logically bus Transmission from any station received by all other stations If two stations transmit at the same time, collision

Hub Layouts Multiple levels of hubs cascaded
Each hub may have a mixture of stations and other hubs attached to from below Fits well with building wiring practices Wiring closet on each floor Hub can be placed in each one Each hub services stations on its floor

Two Level Star Topology

Buses and Hubs Bus configuration
All stations share capacity of bus (e.g. 10Mbps) Only one station transmitting at a time Hub uses star wiring to attach stations to hub Transmission from any station received by hub and retransmitted on all outgoing lines Only one station can transmit at a time Total capacity of LAN is 10 Mbps Improve performance with layer 2 switch

Shared Medium Bus and Hub

Shared Medium Hub and Layer 2 Switch

Layer 2 Switches Central hub acts as switch
Incoming frame from particular station switched to appropriate output line Unused lines can switch other traffic More than one station transmitting at a time Multiplying capacity of LAN

Layer 2 Switch Benefits No change to attached devices to convert bus LAN or hub LAN to switched LAN For Ethernet LAN, each device uses Ethernet MAC protocol Device has dedicated capacity equal to original LAN Assuming switch has sufficient capacity to keep up with all devices For example if switch can sustain throughput of 20 Mbps, each device appears to have dedicated capacity for either input or output of 10 Mbps Layer 2 switch scales easily Additional devices attached to switch by increasing capacity of layer 2

Types of Layer 2 Switch Store-and-forward switch
Accepts frame on input line Buffers it briefly, Then routes it to appropriate output line Delay between sender and receiver Boosts integrity of network Cut-through switch Takes advantage of destination address appearing at beginning of frame Switch begins repeating frame onto output line as soon as it recognizes destination address Highest possible throughput Risk of propagating bad frames Switch unable to check CRC prior to retransmission

Layer 2 Switch v Bridge Layer 2 switch can be viewed as full-duplex hub Can incorporate logic to function as multiport bridge Bridge frame handling done in software Switch performs address recognition and frame forwarding in hardware Bridge only analyzes and forwards one frame at a time Switch has multiple parallel data paths Can handle multiple frames at a time Bridge uses store-and-forward operation Switch can have cut-through operation Bridge suffered commercially New installations typically include layer 2 switches with bridge functionality rather than bridges

Problems with Layer 2 Switches (1)
As number of devices in building grows, layer 2 switches reveal some inadequacies Broadcast overload Lack of multiple links Set of devices and LANs connected by layer 2 switches have flat address space Allusers share common MAC broadcast address If any device issues broadcast frame, that frame is delivered to all devices attached to network connected by layer 2 switches and/or bridges In large network, broadcast frames can create big overhead Malfunctioning device can create broadcast storm Numerous broadcast frames clog network

Problems with Layer 2 Switches (2)
Current standards for bridge protocols dictate no closed loops Only one path between any two devices Impossible in standards-based implementation to provide multiple paths through multiple switches between devices Limits both performance and reliability. Solution: break up network into subnetworks connected by routers MAC broadcast frame limited to devices and switches contained in single subnetwork IP-based routers employ sophisticated routing algorithms Allow use of multiple paths between subnetworks going through different routers

Problems with Routers Routers do all IP-level processing in software
High-speed LANs and high-performance layer 2 switches pump millions of packets per second Software-based router only able to handle well under a million packets per second Solution: layer 3 switches Implementpacket-forwarding logic of router in hardware Two categories Packet by packet Flow based

Packet by Packet or Flow Based
Operates insame way as traditional router Order of magnitude increase in performance compared to software-based router Flow-based switch tries to enhance performance by identifying flows of IP packets Same source and destination Done by observing ongoing traffic or using a special flow label in packet header (IPv6) Once flow is identified, predefined route can be established

Typical Large LAN Organization
Thousands to tens of thousands of devices Desktop systems links 10 Mbps to 100 Mbps Into layer 2 switch Wireless LAN connectivity available for mobile users Layer 3 switches at local network's core Form local backbone Interconnected at 1 Gbps Connect to layer 2 switches at 100 Mbps to 1 Gbps Servers connect directly to layer 2 or layer 3 switches at 1 Gbps Lower-cost software-based router provides WAN connection Circles in diagram identify separate LAN subnetworks MAC broadcast frame limited to own subnetwork

Typical Large LAN Organization Diagram

Chapter 9 Introduction to Metropolitan Area Networks and Wide Area Networks

Introduction As we have seen, a local area network covers a room, a building or a campus. A metropolitan area network (MAN) covers a city or a region of a city. A wide area network (WAN) covers multiple cities, states, countries, and even the solar system.

Metropolitan Area Network Basics
MANs borrow technologies from LANs and WANs. MANs support high-speed disaster recovery systems, real-time transaction backup systems, interconnections between corporate data centers and Internet service providers, and government, business, medicine, and education high-speed interconnections. Almost exclusively fiber optic systems

MANs have very high transfer speeds MANs can recover from network faults very quickly (failover time) MANs are very often a ring topology (not a star-wired ring) Some MANs can be provisioned dynamically

SONET versus Ethernet MANs
Most MANs are SONET network built of multiple rings (for failover purposes) SONET is well-proven but complex, fairly expensive, and cannot be provisioned dynamically. SONET is based upon T-1 rates and does not fit nicely into 1 Mbps, 10 Mbps, 100 Mbps, 1000 Mbps chunks, like Ethernet systems do. Ethernet MANs generally have high failover times

SONET versus Ethernet MANs

Metro Ethernet One of the latest forms of the metropolitan area network is metro Ethernet Metro Ethernet is a service in which the provider creates a door-to-door Ethernet connection between two locations For example, you may connect your business with a second business using a point-to-point Ethernet connection (Figure 9-4a)

Metro Ethernet

Metro Ethernet You may also connect your business with multiple businesses using a connection similar to a large local area network (Figure 9-4b) Thus, by simply sending out one packet, multiple companies may receive the data Neat thing about metro Ethernet is the way it seamlessly connects with a company’s internal Ethernet network(s)

Metro Ethernet

Wide Area Network Basics
WANs used to be characterized with slow, noisy lines. Today WANs are very high speed with very low error rates. WANs usually follow a mesh topology.

A station is a device that interfaces a user to a network. A node is a device that allows one or more stations to access the physical network and is a transfer point for passing information through a network. A node is often a computer, a router, or a telephone switch. The sub-network or physical network is the underlying connection of nodes and telecommunication links.

Types of Network Structures
Circuit switched network - a sub-network in which a dedicated circuit is established between sender and receiver and all data passes over this circuit. The telephone system is a common example. The connection is dedicated until one party or another terminates the connection. AT&T announced end of 2009 that they will begin phasing out their switched networks

Packet switched network - a network in which all data messages are transmitted using fixed-sized packages, called packets. More efficient use of a telecommunications line since packets from multiple sources can share the medium. One form of packet switched network is the datagram. With a datagram, each packet is on its own and may follow its own path. Virtual circuit packet switched network create a logical path through the subnet and all packets from one connection follow this path.

Broadcast network - a network typically found in local area networks but occasionally found in wide area networks. A workstation transmits its data and all other workstations “connected” to the network hear the data. Only the workstation(s) with the proper address will accept the data.

Summary of Network Structures

Connection-oriented versus Connectionless
The network structure is the underlying physical component of a network. What about the software or application that uses the network? A network application can be either connection-oriented or connectionless.

A connection-oriented application requires both sender and receiver to create a connection before any data is transferred. Applications such as large file transfers and sensitive transactions such as banking and business are typically connection-oriented. A connectionless application does not create a connection first but simply sends the data. Electronic mail is a common example.

A connection-oriented application can operate over both a circuit switched network or a packet switched network. A connectionless application can also operate over both a circuit switched network or a packet switched network but a packet switched network may be more efficient.

Routing Each node in a WAN is a router that accepts an input packet, examines the destination address, and forwards the packet on to a particular telecommunications line. How does a router decide which line to transmit on? A router must select the one transmission line that will best provide a path to the destination and in an optimal manner. Often many possible routes exist between sender and receiver.

Routing

Routing The communications network with its nodes and telecommunication links is essentially a weighted network graph. The edges, or telecommunication links, between nodes, have a cost associated with them. The cost could be a delay cost, a queue size cost, a limiting speed, or simply a dollar amount for using that link.

Routing

Routing The routing method, or algorithm, chosen to move packets through a network should be: Optimal, so the least cost can be found Fair, so all packets are treated equally Robust, in case link or node failures occur and the network has to reroute traffic. Not too robust so that the chosen paths do not oscillate too quickly between troubled spots.

Least Cost Routing Algorithm
Dijkstra’s least cost algorithm finds all possible paths between two locations. By identifying all possible paths, it also identifies the least cost path. The algorithm can be applied to determine the least cost path between any pair of nodes.

Least Cost Routing Algorithm

Flooding Routing When a packet arrives at a node, the node sends a copy of the packet out every link except the link the packet arrived on. Traffic grows very quickly when every node floods the packet. To limit uncontrolled growth, each packet has a hop count. Every time a packet hops, its hop count is incremented. When a packet’s hop count equals a global hop limit, the packet is discarded.

Flooding Routing

Centralized Routing One routing table is kept at a “central” node.
Whenever a node needs a routing decision, the central node is consulted. To survive central node failure, the routing table should be kept at a backup location. The central node should be designed to support a high amount of traffic consisting of routing requests.

Centralized Routing

Distributed Routing Each node maintains its own routing table.
No central site holds a global table. Somehow each node has to share information with other nodes so that the individual routing tables can be created. Possible problem with individual routing tables holding inaccurate information.

Distributed Routing

Adaptive Routing versus Static Routing
With adaptive routing, routing tables can change to reflect changes in the network Static routing does not allow the routing tables to change. Static routing is simpler but does not adapt to network congestion or failures.

Routing Examples - RIP Routing Information Protocol (RIP) - First routing protocol used on the Internet. A form of distance vector routing. It was adaptive and distributed Each node kept its own table and exchanged routing information with its neighbors.

Routing Examples - RIP Suppose that Router A has connections to four networks (123, 234, 345, and 789) and has the following current routing table: Network Hop Cost Next Router B C C D

Routing Examples - RIP Now suppose Router D sends out the following routing information (note that Router D did not send Next Router information, since each router will determine that information for itself): Network Hop Cost 123 4 345 5 567 7

Routing Examples - RIP Router A will look at each entry in Router D’s table and make the following decisions: 1. Router D says Network 123 is 4 hops away (from Router D). Since Router D is 1 hop away from Router A, Network 123 is actually 5 hops away from Router A. That is better than the current entry of 8 hops in Router A’s table, so Router A will update the entry for Network 123. 2. Router D says Network 345 is 5 hops away. Add one hop to get to Router D and Network 345 is 6 hops away. That is currently the same hop count as shown in Router A’s table for Network 345, so Router A will not update its table.

Routing Examples - RIP Router A will look at each entry in Router D’s table and make the following decisions: 3. Router D says Network 567 is 7 hops away. Add 1 hop to get to Router D, giving 8 hops. Since Router A has no information about Network 567, Router A will add this entry to its table. And since the information is coming from Router D, Router A’s Next Router entry for network 567 is set to D. 4. Router D says Network 789 is 10 hops away. Add 1 hop to get to Router D. The value of 11 hops is worse than the value currently in Router A’s table. Since Router A currently has information from Router D, and Router D is now saying it takes more hops to get to Network 789, then Router A has to use this information. (Note – the book has this point wrong)

Routing Examples - RIP Router A’s updated routing table will thus look like the following: Network Hop Cost Next Router D C C D D

Routing Examples - OSPF
Open Shortest Path First (OSPF) - Second routing protocol used on the Internet A form of link state routing It too was adaptive and distributed but more complicated than RIP and performed much better

Network Congestion When a network or a part of a network becomes so saturated with data packets that packet transfer is noticeably impeded, network congestion occurs. What can cause network congestion? Node and link failures; high amounts of traffic; improper network planning. When serious congestion occurs buffers overflow and packets are lost.

Network Congestion What can we do to reduce or eliminate network congestion? An application can observe its own traffic and notice if packets are disappearing. If so, there may be congestion. This is called implicit congestion control. The network can inform its applications that congestion has occurred and the applications can take action. This is called explicit congestion control.

Congestion Avoidance Before making a connection, user requests how much bandwidth is needed, or if connection needs to be real-time Network checks to see if it can satisfy user request If user request can be satisfied, connection is established If a user does not need a high bandwidth or real-time, a simpler, cheaper connection is created This is often called connection admission control Asynchronous transfer mode is a very good example of this (Chapter Eleven)

WANs In Action: Making Internet Connections
Home to Internet connection - modem and dial-up telephone provide a circuit switched network, while connection through the Internet is packet switched. The application can be either a connection-oriented application or a connectionless application.

A work to Internet connection would most likely require a broadcast network (LAN) with a connection to the Internet (packet switched network).

Chapter 10 The Internet

Some Common Terms The Internet is a network of computers spanning the globe. It is also called the World Wide Web. An Internet Browser is a software program that enables you to view Web pages on your computer. Browsers connect computers to the Internet, and allow people to “surf the Web.” Internet Explorer is one of the browsers most commonly used. There are other browsers available as well, including Netscape.

A site or area on the World Wide Web that is accessed by its own Internet address is called a Web site. A Web Page is like a page in a book. Websites often have several pages that you can access by clicking on links. A Web site can be a collection of related Web pages. Each Web site contains a home page (this is the original starting page) and may also contain additional pages. Different computers will have different home pages. You can set your own webpage.

Layout of a Web Page Title bar – tells you the name of the web page
Menu bar – has commands for moving around the webpage, printing, etc Tool bar – short cuts to commands. Each picture represents a command Address bar – webpage address. If you want to go directly to a web page, you will need to know the address.

Parts of a Web Address A web address is typically composed of four parts: For example, the address is made up of the following areas: This Web server uses Hypertext Transfer Protocol (HTTP). This is the most common protocol on the Internet. www This site is on the World Wide Web. google The Web server and site maintainer. ca This tells us it is a site in Canada.

Endings of web pages tells us a bit about the page
Endings of web pages tells us a bit about the page. Some common endings to web addresses are: com (commercial) edu (educational institution) gov (government) net (network) org (organization) You might also see addresses that add a country code as the last part of the address such as: ca (Canada) uk (United Kingdom) fr (France) us (United States of America) au (Australia)

How to Search the Internet
Two basic ways if you know the address of the web page (example: 2. Using a search engine like Google to find the address. This is called a keyword search

Typing in the Web Site Address
Go to the address bar. Click once to highlight the address. (It should turn blue). Hit the Delete key on your keyboard. Enter the following address: Then press Enter on the keyboard or click on the word Go on the right side of the Address Bar.

Search the Internet If you don’t know the address of the webpage, but want to learn more about a topic or find a particular website, you will need to do a search. There are several handy search engines out there that will locate information for you. Two of the mostly commonly used are:

Practice Exercise Type into the address bar. ( is a Canadian version of the search engine. will search US sites first) Hit the Enter key or Go on the toolbar. You should see the Google web page.

3. Now click on the Google box. You should see a flashing cursor
3. Now click on the Google box. You should see a flashing cursor. Type in the topic. Hit enter.

Chapter 11 Voice and Data Delivery Networks

Telephone Lines, Trunks, and Numbers
The local loop is the telephone line that runs from the telephone company’s central office to your home or business Central office – building that houses the telephone company’s switching equipment and provides a local dial tone on your telephone If you place a long-distance call, the central office passes your telephone call off to a long-distance provider

The country is divided into a few hundred local access transport areas (LATAs) If your call goes from one LATA to another, it is a long-distance call and is handled by a long-distance telephone company If your call stays within a LATA, it is a local call and is handled by a local telephone company

Trunk – special telephone line that runs between central offices and other telephone switching centers Usually digital, high-speed, and carries multiple telephone circuits Typically a 4-wire circuit, while a telephone line is a 2-wire circuit

A trunk is not associated with a single telephone number like a line is A telephone number consists of an area code, an exchange, and a subscriber extension The area code and exchange must start with the digits 2-9 to separate them from long distance and operator services Author has added additional slides on phone systems

When telephone company installs a line, it must not proceed any further than 12 inches into the building This point is the demarcation point, or demarc Modular connectors, such as the RJ-11, are commonly used to interconnect telephone lines and the telephone handset to the base When handset is lifted off base (off-hook), an off-hook signal is sent to the central office

When off-hook signal arrives at central office, a dial tone is generated and returned to telephone When user hears the dial tone, they dial (or press) number The central office equipment collects dialed digits, and proceeds to place appropriate call

PBX Private Branch Exchange (PBX) – common internal phone switching system for medium- to large-sized businesses Provides advanced intelligent features to users, such as: 4-digit internal dialing Special prefixes for WATS, FX, etc (private dialing plans) PBX intelligently decides how to route a call for lowest cost

PBX More PBX features: Voice mail
Routes incoming calls to the best station set (automatic call distribution) Provides recorded messages and responds to touch-tone requests (automated attendant) Access to database storage and retrieval (interactive voice response) VoIP

PBX PBX components: CPU, memory, telephone lines, trunks
Switching network Supporting logic cards Main distribution frame Console or switchboard Battery back-up system

Automated Attendant Plays a recorded greeting and offers a set of options Lets the caller enter an extension directly (touch tone or voice) and bypass an “operator” Forwards the caller to a human operator if the caller does not have a touch tone phone Available as an option on a PBX

Automatic Call Distributor
When you call a business and are told all operators / technicians / support staff / etc. are busy and that your call will be answered in the order it was received Used in systems where incoming call volume is large, such as customer service, help desk, order entry, credit authorization, reservations, and catalog sales Early systems used hunt groups Original systems routed call to first operator in line (kept person very busy!)

Automatic Call Distributor
Modern systems perform more advanced functions, such as: Prioritize the calls Route calls to appropriate agent based on the skill set of the agent If all agents busy, deliver call to waiting queue and play appropriate message (like how long they may have to wait) Forward calls to another call center, or perform automatic return call

Interactive Voice Response
IVR is similar to automated attendant except: IVR incorporates a connection to a database (on a mainframe or server) IVR allows caller to access and/or modify database information IVR can also perform fax on demand

Interactive Voice Response
Common examples of IVR include: Call your bank to inquire about an account balance University online registration system Brokerage firm taking routine orders from investors Investment fund taking routine requests for new account applications A company providing employees with info about their benefit plans

Key Telephone System Used within a small office or a branch office, a key telephone system (KTS) is an on-premise resource sharing device similar to a PBX Example – key system might distribute 48 internal telephone sets over 16 external phone lines The business would pay for the 16 individual lines but have 48 telephone sets operating User selects outside line by pressing corresponding line button on key set (phone)

Basic Telephone Systems Services
Foreign exchange service (FX) - customer calls a local number which is then connected to a leased line to a remote site Wide area telecommunications services (WATS) – discount volume calling to local- and long-distance sites Off-premises extensions (OPX) – dial tone at location B comes from the PBX at location A

Other Players in the Market
Alternate operator services Pay phones, hotel phones Aggregator – pulls a bunch of small companies together and goes after phone discounts Reseller – rents or leases variety of lines from phone companies, then resells to customers Specialized mobile radio carriers – mobile communication services to businesses and individuals, including dispatch, paging, and data services ARDIS and RAM Mobile Data two good examples

The Telephone Network Before and After 1984
In 1984, U.S. government broke up AT&T Before then, AT&T owned large majority of all local telephone circuits and all the long-distance service With Modified Final Judgment of 1984, AT&T had to split off local telephone companies from long-distance company The local telephone companies formed seven Regional Bell Operating Companies Today, there are only 4 left: BellSouth, SBC, Qwest (US West), and Verizon (Bell Atlantic)

Another result of the Modified Judgment was creation of LATA (local access and transport area) Local telephone companies became known as local exchange carriers (LECs), and long distance telephone companies became known as interexchange carriers (IEC, or IXC) Calls that remain within LATA are intra-LATA, or local calls Calls that pass from one LATA to another are inter-LATA, or long distance

Before 1984, telephone networks in the U.S. resembled a large hierarchical tree, with Class 5 offices at the bottom and Class 1 offices at the top Users were connected to Class 5 offices The longer the distance of a telephone call, the further up the tree the call progressed Today’s telephone structure is a collection of LECs, POPs, and IECs

Telephone Networks After 1996
Another landmark ruling affecting the telephone industry was the Telecommunications Act of 1996 Opened up local telephone market to competitors Now cable TV companies (cable telephony), long-distance telephone companies, or anyone that wants to start a local telephone company can offer local telephone service Local phone companies that existed before the Act are known as incumbent local exchange carriers (ILEC) while the new companies are competitive local exchange carriers (CLEC)

Telephone Networks After 1996
LECs are supposed to allow CLECs access to all local loops and switching centers / central offices If a local loop is damaged, the LEC is responsible for repair The LEC is also supposed to provide the CLEC with a discount to the dial tone (17-20%) LECs can also provide long-distance service if they can show there is sufficient competition at the local service level

Limitations of Telephone Signals
POTS lines were designed to transmit the human voice, which has a bandwidth less than 4000 Hz A telephone conversation requires two channels, each occupying 4000 Hz

A 4000 Hz analog signal can only carry about 33,600 bits per second of information while a 4000 Hz digital signal can carry about 56,000 bits per second If you want to send information faster, you need a signal with a higher frequency or you need to incorporate more advanced modulation techniques POTS cannot deliver faster signals What will?

The 56k Dial-Up Modem A 56k modem (56,000 bps) achieves this speed due to digital signaling as opposed to analog signaling used on all other modems Would actually achieve 64k except: Local loop is still analog, thus analog signaling Analog to digital conversion at the local modem introduces noise/error Combined, these shortcomings drop the speed to at best 56k

The 56k Dial-Up Modem Does not achieve 56k either
FCC will not let modem transmit at power level necessary to support 56k, so the best modem can do is approximately 53k Will not even achieve 53k if connection between your modem and remote computer contains an additional analog to digital conversion, or if there is significant noise on line

The 56k Dial-Up Modem

The 56k Dial-Up Modem There is always one analog to digital conversion on the downstream link – at the users modem. This drops the 64Kbps stream down to 56 Kbps.

The 56k Dial-Up Modem Based upon one of two standards: V.90
Upstream speed is maximum 33,600 bps V.92 Newer standard Allows maximum upstream speed of 48 kbps (under ideal conditions) Can place a data connection on hold if the telephone service accepts call waiting and a voice telephone call arrives

Digital Subscriber Line
Digital subscriber line (DSL) is a relative newcomer to the field of leased line services DSL can provide very high data transfer rates over standard telephone lines Unfortunately, less than half the telephone lines in the U.S. are incapable of supporting DSL And there has to be a DSL provider in your region

DSL Basics DSL, depending on the type of service, is capable of transmission speeds from 100s of kilobits into single-digit megabits Because DSL is highly dependent upon noise levels, a subscriber cannot be any more than 5.5 kilometers (2-3 miles) from the DSL central office DSL service can be: Symmetric – downstream and upstream speeds are identical Asymmetric – downstream speed is faster than the upstream speed

DSL Basics DSL service Often connects a user to the Internet
Can also provide a regular telephone service (POTS) The DSL provider uses a DSL access multiplexer (DSLAM) to split off the individual DSL lines into homes and businesses A user then needs a splitter to separate the POTS line from the DSL line, and then a DSL modem to convert the DSL signals into a form recognized by the computer

DSL Basics

Cable Modems Allow high-speed access to wide area networks such as the Internet Most are external devices that connect to the personal computer through a common Ethernet card Can provide data transfer speeds between 500 kbps and 25 Mbps

Cable Modems

T-1 Leased Line Service T-1 – digital service offered by the telephone companies that can transfer data as fast as Mbps (both voice and computer data) To support a T-1 service, a channel service unit / data service unit (CSU/DSU) is required at the end of the connection

T-1 Leased Line Service T-1 Mux T-1 Line DSU CSU ? Customer Supplied
Data Loopback control DSX-1 Interface T-1 Mux T-1 Line DSU CSU ? Customer Supplied From Telco

T-1 Leased Line Service A T-1 service
Is a digital, synchronous TDM stream used by businesses and telephone companies Is always on and always transmitting Can support up to 24 simultaneous channels These channels can be either voice or data (PBX support) Can also be provisioned as a single channel delivering Mbps of data (LAN to ISP connection)

T-1 Leased Line Service A T-1 service (continued)
Requires 4 wires, as opposed to a 2-wire telephone line Can be either intra-LATA (local) which costs roughly $350-$400 per month, or inter-LATA (long distance) which can cost thousands of dollars per month (usually based on distance) A customer may also be able to order a 1/4 T-1 or a 1/2 T-1

T-1 Leased Line Service Constantly transmits frames (8000 frames per second) Each frame consists of one byte from each of the 24 channels, plus 1 sync bit (8 * = 193 bits) 8000 frames per second * 193 bits per frame = Mbps If a channel is used for voice, each byte is one byte of PCM-encoded voice If a channel is used for data, each byte contains 7 bits of data and 1 bit of control information (7 * 8000 = 56 kbps)

CSU (Channel Service Unit)
First (last) piece of equipment on a T-1 line Can perform various loop-back tests CSU can also generate “keep alive” signal when the attached DTE fails to deliver a valid stream of data or DTE is disconnected CSU can also collect error statistics for the phone company

DSU (Digital Service Unit)
Shapes the T-1 signal being sent Prepares the customer data to meet the requirements of the DSX-1 interface Suppresses long strings of zeros with special coding Provides the terminal (user) with remote and local loopback tests DSU if often built into the terminal equipment or multiplexor and should eventually disappear

ISDN Allows digital transmission of voice and data over traditional copper lines We have already seen the ISDN frame layout in the chapter on Multiplexing Three basic types of ISDN Basic rate Primary rate Broadband

Basic Rate ISDN Entry level service 144 kbps service
2 – 64 kbps bearer (data) channels (DS-0) and 1 – 16 kbps delta channel (for signaling or data) (also known as 2B+D) User can bond both data channels together for a 128 kbps channel Rarely used in the US; some degree of use in Japan / England / Europe; fairly popular in Germany (29% of all subscriber lines as of 2003; 20% of all ISDN lines worldwide) (source:Wikipedia)

Primary Rate ISDN In Europe, this consists of 30 B-channels of 64 kbps each plus 1 D-channel of 64 kbps and is carried over an E-1 In North America, this service is 23 B-channels and 1 D-channel and carried over a T-1 (J-1 in Japan) PRI-ISDN is popular through-out the world and is used to connect PSTN to company PBXs In US, PRI-ISDN is used on connection of non-VoIP PBXs to PSTN

Chapter 12 Network Security

What is “Security” Dictionary.com says:
1. Freedom from risk or danger; safety. 2. Freedom from doubt, anxiety, or fear; confidence. 3. Something that gives or assures safety, as: 1. A group or department of private guards: Call building security if a visitor acts suspicious. 2. Measures adopted by a government to prevent espionage, sabotage, or attack. 3. Measures adopted, as by a business or homeowner, to prevent a crime such as burglary or assault: Security was lax at the firm's smaller plant. …etc.

1. Freedom from risk or danger; safety. 2. Freedom from doubt, anxiety, or fear; confidence. 3. Something that gives or assures safety, as: 1. A group or department of private guards: Call building security if a visitor acts suspicious. 2. Measures adopted by a government to prevent espionage, sabotage, or attack. 3. Measures adopted, as by a business or homeowner, to prevent a crime such as burglary or assault: Security was lax at the firm's smaller plant. …etc. In other words, having systems in place beforehand which prevent attacks before they begin.

1. Freedom from risk or danger; safety. 2. Freedom from doubt, anxiety, or fear; confidence. 3. Something that gives or assures safety, as: 1. A group or department of private guards: Call building security if a visitor acts suspicious. 2. Measures adopted by a government to prevent espionage, sabotage, or attack. 3. Measures adopted, as by a business or homeowner, to prevent a crime such as burglary or assault: Security was lax at the firm's smaller plant. …etc. Related to the first definition, having peace of mind knowing that your systems are safe and protected.

1. Freedom from risk or danger; safety. 2. Freedom from doubt, anxiety, or fear; confidence. 3. Something that gives or assures safety, as: 1. A group or department of private guards: Call building security if a visitor acts suspicious. 2. Measures adopted by a government to prevent espionage, sabotage, or attack. 3. Measures adopted, as by a business or homeowner, to prevent a crime such as burglary or assault: Security was lax at the firm's smaller plant. …etc. This includes contingency plans for what to do when attackers strike, keeping up with the latest CERT advisories, hiring network security consultants to find insecurities in your network, etc.

Why do we need security? Protect vital information while still allowing access to those who need it Trade secrets, medical records, etc. Provide authentication and access control for resources Ex: AFS Guarantee availability of resources Ex: 5 9’s (99.999% reliability)

Who is vulnerable? Financial institutions and banks
Internet service providers Pharmaceutical companies Government and defense agencies Contractors to various government agencies Multinational corporations ANYONE ON THE NETWORK

Common security attacks and their countermeasures
Finding a way into the network Firewalls Exploiting software bugs, buffer overflows Intrusion Detection Systems Denial of Service Ingress filtering, IDS TCP hijacking IPSec Packet sniffing Encryption (SSH, SSL, HTTPS) Social problems Education

Firewalls Basic problem – many network applications and protocols have security problems that are fixed over time Difficult for users to keep up with changes and keep host secure Solution Administrators limit access to end hosts by using a firewall Firewall is kept up-to-date by administrators

Firewalls A firewall is like a castle with a drawbridge
Only one point of access into the network This can be good or bad Can be hardware or software Ex. Some routers come with firewall functionality ipfw, ipchains, pf on Unix systems, Windows XP and Mac OS X have built in firewalls Why good? Because it lets you filter what comes in and what goes out. Why bad? If that point goes down, you are cut off from everyone else. Also, may have lots of congestion at that one point.

Firewalls DMZ Internet Intranet
Web server, server, web proxy, etc

Firewalls Used to filter packets based on a combination of features
These are called packet filtering firewalls There are other types too, but they will not be discussed Ex. Drop packets with destination port of 23 (Telnet) Can use any combination of IP/UDP/TCP header information man ipfw on unix47 for much more detail But why don’t we just turn Telnet off?

Firewalls Here is what a computer with a default Windows XP install looks like: 135/tcp open loc-srv 139/tcp open netbios-ssn 445/tcp open microsoft-ds 1025/tcp open NFS-or-IIS 3389/tcp open ms-term-serv 5000/tcp open UPnP Might need some of these services, or might not be able to control all the machines on the network

Firewalls What does a firewall rule look like?
Depends on the firewall used Example: ipfw /sbin/ipfw add deny tcp from cracker.evil.org to wolf.tambov.su telnet Other examples: WinXP & Mac OS X have built in and third party firewalls Different graphical user interfaces Varying amounts of complexity and power

Intrusion Detection Used to monitor for “suspicious activity” on a network Can protect against known software exploits, like buffer overflows Open Source IDS: Snort,

Intrusion Detection Uses “intrusion signatures”
Well known patterns of behavior Ping sweeps, port scanning, web server indexing, OS fingerprinting, DoS attempts, etc. Example IRIX vulnerability in webdist.cgi Can make a rule to drop packets containing the line “/cgi-bin/webdist.cgi?distloc=?;cat%20/etc/passwd” However, IDS is only useful if contingency plans are in place to curb attacks as they are occurring

Minor Detour… Say we got the /etc/passwd file from the IRIX server
What can we do with it?

Dictionary Attack We can run a dictionary attack on the passwords
The passwords in /etc/passwd are encrypted with the crypt(3) function (one-way hash) Can take a dictionary of words, crypt() them all, and compare with the hashed passwords This is why your passwords should be meaningless random junk! For example, “sdfo839f” is a good password That is not my andrew password Please don’t try it either

Denial of Service Purpose: Make a network service unusable, usually by overloading the server or network Many different kinds of DoS attacks SYN flooding SMURF Distributed attacks Mini Case Study: Code-Red

Denial of Service SYN flooding attack
Send SYN packets with bogus source address Why? Server responds with SYN ACK and keeps state about TCP half-open connection Eventually, server memory is exhausted with this state Solution: use “SYN cookies” In response to a SYN, create a special “cookie” for the connection, and forget everything else Then, can recreate the forgotten information when the ACK comes in from a legitimate connection Forge source IP so that the victim can’t figure out who you are.

Denial of Service

Denial of Service SMURF
Source IP address of a broadcast ping is forged Large number of machines respond back to victim, overloading it

Denial of Service

Denial of Service Distributed Denial of Service
Same techniques as regular DoS, but on a much larger scale Example: Sub7Server Trojan and IRC bots Infect a large number of machines with a “zombie” program Zombie program logs into an IRC channel and awaits commands Example: Bot command: !p Result: runs ping.exe l n 10000 Sends 10,000 64k packets to the host (655MB!) Read more at:

Denial of Service Mini Case Study – CodeRed
July 19, 2001: over 359,000 computers infected with Code-Red in less than 14 hours Used a recently known buffer exploit in Microsoft IIS Damages estimated in excess of $2.6 billion

Denial of Service Why is this under the Denial of Service category?
CodeRed launched a DDOS attack against www1.whitehouse.gov from the 20th to the 28th of every month! Spent the rest of its time infecting other hosts

Denial of Service How can we protect ourselves? Ingress filtering
If the source IP of a packet comes in on an interface which does not have a route to that packet, then drop it RFC 2267 has more information about this Stay on top of CERT advisories and the latest security patches A fix for the IIS buffer overflow was released sixteen days before CodeRed had been deployed!

TCP Attacks Recall how IP works…
End hosts create IP packets and routers process them purely based on destination address alone Problem: End hosts may lie about other fields which do not affect delivery Source address – host may trick destination into believing that the packet is from a trusted source Especially applications which use IP addresses as a simple authentication method Solution – use better authentication methods

TCP Attacks TCP connections have associated state
Starting sequence numbers, port numbers Problem – what if an attacker learns these values? Port numbers are sometimes well known to begin with (ex. HTTP uses port 80) Sequence numbers are sometimes chosen in very predictable ways

TCP Attacks If an attacker learns the associated TCP state for the connection, then the connection can be hijacked! Attacker can insert malicious data into the TCP stream, and the recipient will believe it came from the original source Ex. Instead of downloading and running new program, you download a virus and execute it

TCP Attacks Say hello to Alice, Bob and Mr. Big Ears

TCP Attacks Alice and Bob have an established TCP connection

TCP Attacks Mr. Big Ears lies on the path between Alice and Bob on the network He can intercept all of their packets

TCP Attacks First, Mr. Big Ears must drop all of Alice’s packets since they must not be delivered to Bob (why?) Packets Alice can send a RESET The Void

TCP Attacks Then, Mr. Big Ears sends his malicious packet with the next ISN (sniffed from the network) ISN, SRC=Alice

TCP Attacks What if Mr. Big Ears is unable to sniff the packets between Alice and Bob? Can just DoS Alice instead of dropping her packets Can just send guesses of what the ISN is until it is accepted How do you know when the ISN is accepted? Mitnick: payload is “add self to .rhosts” Or, “xterm -display MrBigEars:0”

TCP Attacks Why are these types of TCP attacks so dangerous?
Malicious user can send a virus to the trusting web client, instead of the program they thought they were downloading. Web server Trusting web client Malicious user

TCP Attacks How do we prevent this? IPSec
Provides source authentication, so Mr. Big Ears cannot pretend to be Alice Encrypts data before transport, so Mr. Big Ears cannot talk to Bob without knowing what the session key is

Five Minute Break For your enjoyment, here is something completely unrelated to this lecture:

Packet Sniffing Recall how Ethernet works …
When someone wants to send a packet to some else … They put the bits on the wire with the destination MAC address … And remember that other hosts are listening on the wire to detect for collisions … It couldn’t get any easier to figure out what data is being transmitted over the network!

Packet Sniffing This works for wireless too!
In fact, it works for any broadcast-based medium

Packet Sniffing What kinds of data can we get?
Asked another way, what kind of information would be most useful to a malicious user? Answer: Anything in plain text Passwords are the most popular

Packet Sniffing How can we protect ourselves? SSH, not Telnet
Many people at CMU still use Telnet and send their password in the clear (use PuTTY instead!) Now that I have told you this, please do not exploit this information Packet sniffing is, by the way, prohibited by Computing Services HTTP over SSL Especially when making purchases with credit cards! SFTP, not FTP Unless you really don’t care about the password or data Can also use KerbFTP (download from MyAndrew) IPSec Provides network-layer confidentiality

Social Problems People can be just as dangerous as unprotected computer systems People can be lied to, manipulated, bribed, threatened, harmed, tortured, etc. to give up valuable information Most humans will breakdown once they are at the “harmed” stage, unless they have been specially trained Think government here…

Social Problems Fun Example 1:
“Hi, I’m your AT&T rep, I’m stuck on a pole. I need you to punch a bunch of buttons for me”

Social Problems Fun Example 2:
Someone calls you in the middle of the night “Have you been calling Egypt for the last six hours?” “No” “Well, we have a call that’s actually active right now, it’s on your calling card and it’s to Egypt and as a matter of fact, you’ve got about $2000 worth of charges on your card and … read off your AT&T card number and PIN and then I’ll get rid of the charge for you”

Social Problems Fun Example 3: Who saw Office Space?
In the movie, the three disgruntled employees installed a money-stealing worm onto the companies systems They did this from inside the company, where they had full access to the companies systems What security techniques can we use to prevent this type of access? Security techniques: IDS can be configured to look for internal inconsistencies in traffic patterns Firewalls can be configured to block off one part of a corporate network from another part to further restrict access Can also use hardware based identification tokens with strong encryption to identify who is doing what

Social Problems There aren’t always solutions to all of these problems
Humans will continue to be tricked into giving out information they shouldn’t Educating them may help a little here, but, depending on how bad you want the information, there are a lot of bad things you can do to get it So, the best that can be done is to implement a wide variety of solutions and more closely monitor who has access to what network resources and information But, this solution is still not perfect

Chapter 13 Network Design and Management

Analysis and Design Phases
Request Feasibility Study Analysis (requirement) Alternatives (cost and benefit) Design Selection Cost Documentation Management approval

Implementation Phases
Purchasing and vendor agreement Installation Training, and testing Conversion Follow up audit

Request Source User Senior managers Communication department
External environment (customer, government, etc.) Form: formal or informal Prioritization (costs & benefits analysis) Outcome: approval, deny, or on hold by management

Feasibility Study Team User, specialist, and management Problems
Technical or non-technical Analysis (technical, operational, economical, legal, schedule) Report and presentation Go or stop

Analysis Analysis items Geographic requirement (scope)
Traffic load analysis Peak load (no. of message and length) National & international busy hour Traffic flow pattern by individual location (map) Response time, reliability, & availability (cost) Terminal operators (end users & their education background and needs) Capacity growth projection (6 months to 5 years) Constraint: time, cost, and compatibility Documentation and reports

Alternatives Sources (vendors)
Availability (enchancements or new products) Incompatibility issues Costs/services trade off

Design Items Configuration diagram Routing time (peak and average)
Response time (peak and average) Delay consideration (queue) Simulation What if for routing, response, and delay

Selection Procedures Request for proposal (RFP) or request for quotation (RFQ) Selection criteria and its weight

Request for Proposal Title Table of contents
Description of organization Problem definition Operation requirements Format of response Evaluation criteria Decision schedule References

Description of Organization
Organization chart Organization locations Overview of system

Problem Definition Error rate Misrouting Time consuming Security

Operation Requirements
Reliability Down time Misrouting Lost message Transmission error Performance Throughput Response time Error rate

Format of Response - I Title Table of contents Overview
Software (function & feature) Hardware (function, feature, & capacity) Performance (future growth) Site requirement Conversion (installation, testing, & schedule)

Format of Response - II Maintenance (support) Costs Warranty Coverage
User Training References Other related information

Selection criteria Proposed solution Compatibility Security and backup
Price Technical support Product maintenance Repair service Financial viability

Vendor Response to RFP - I
Introduction Executive summary Table of contents System design System feature Growth capacity Installation and testing methods Maintenance arrangements Ongoing support Installation schedule Pricing and timing of payments Warranty coverage Training and education Other Recommendation

Documentation Diagrams and maps Configuration and wiring
Components list Hardware: model and specification Software: version, release, no. of copies Implementation plan (Gantt chart) Milestones Activities Schedule

Management Approval Review and verification Users Operators
Management: budget

Purchasing Vendor agreement Equipment configuration specification
Acceptance test Schedule: delivery, installation, testing, operation Location: delivery, installation, testing, operation Other terms and conditions: payment, warranty, maintenance Get a experienced lawyer

Training Types of instruction Classroom Hand on In house Outside
Educating Users Operators Maintenance crew Security and backup

Testing Testing time Testing Tools Software & hardware
IBM’s Telecommunication Network Simulator Testing areas Performance Stress testing (load) Error handling Error recovery procedures

Conversion Types Parallel Cut over Pilot Piece by piece Help desk
Vendor technicians or consultants Implementation problems cleanup

Audit After six months Check performance or success criteria
Formal report to management for proper actions

LAN Design Number of nodes Usage Traffic: day and time Disk storage
Speed Security and backup Compatibility Management

Voice Network Design Peak load and average load Grade of service
Capacity & usage of circuits Overdesigning or planned delay

Points to Remember Analysis and Design Implementation LAN
Voice Network Design

Discussion Develop a simple request for proposal to add e-business to a traditional department store.

Data Communications and Computer Networks

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Data Communications and Computer Networks

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