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Data and Computer Communications

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1 Data and Computer Communications
Chapter 1 – Data Communications, Data Networks, and the Internet Ninth Edition by William Stallings

2 Data Communications, Data Networks, and the Internet
“The fundamental problem of communication is that of reproducing at one point either exactly or approximately a message selected at another point” - The Mathematical Theory of Communication, Claude Shannon Message

3 1-1 DATA COMMUNICATIONS The term telecommunication means communication at a distance. The word data refers to information presented in whatever form is agreed upon by the parties creating and using the data. Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable. 1.#

4 Figure 1.1 Five components of data communication

5 Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)

6 1-2 NETWORKS A network is a set of devices (often referred to as nodes) connected by communication links. A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network. 1.#

7 Figure 1.3 Types of connections: point-to-point and multipoint

8 Technological Advancement Driving Forces
Development of new services Advances in technology Traffic growth at a high & steady rate

9 Changes in Networking Technology
* Emergence of high-speed LANs * Corporate WAN needs * Digital electronics

10 Communications Model

11 Communications Tasks Transmission system utilization Addressing
Interfacing Routing Signal generation Recovery Synchronization Message formatting Exchange management Security Error detection and correction Network management Flow control

12 Transmission Lines Capacity
The basic building block of any communications facility is the transmission line. Reliability The business manager is concerned with a facility providing the required capacity, with acceptable reliability, at minimum cost. Cost Transmission Line

13 Transmission Mediums Wireless transmissions
Two mediums currently driving the evolution of data communications transmission are: Fiber optic transmissions and Wireless transmissions

14 Networking Advances in technology have led to greatly increased capacity and the concept of integration, allowing equipment and networks to work simultaneously. Voice Data Image Video The number of computers in use worldwide is in the hundreds of millions, with pressure from users of these systems for ways to communicate among all these machines being irresistible. Advances in technology have led to greatly increased capacity and the concept of integration, allowing equipment and networks to deal simultaneously with voice, data, image, and even video.

15 Network Hardware Local Area Networks Metropolitan Area Networks
Classifying networks based on their scale: Local Area Networks Metropolitan Area Networks Wide Area Networks Wireless Networks Home Networks Internetworks

16 Network Hardware

17 LANs and WANs There are two broad categories of networks:
Local Area Networks (LAN) Wide Area Networks (WAN) Networks are currently classified into two broad categories: Local Area Networks (LANs) and Wide Area Networks (WANs).

18 Wide Area Networks (WANs)
Span a large geographical area Require the crossing of public right-of-ways Rely in part on common carrier circuits Typically consist of a number of interconnected switching nodes Wide area networks generally cover a large geographical area, require the crossing of public right-of-ways, and rely at least in part on circuits provided by a common carrier. Typically, a WAN consists of a number of interconnected switching nodes. A transmission from any one device is routed through these internal nodes to the specified destination device. These nodes are not concerned with the content of the data; rather, their purpose is to provide a switching facility that will move the data from node to node until they reach their destination.

19 Wide Area Networks Alternative technologies used include:
Circuit switching Packet switching Frame relay Asynchronous Transfer Mode (ATM) Traditionally, WANs have been implemented using one of two technologies: circuit switching and packet switching. More recently, frame relay and ATM networks have assumed major roles.

20 Circuit Switching Uses a dedicated communications path
Connected sequence of physical links between nodes Logical channel dedicated on each link Rapid transmission The most common example of circuit switching is the telephone network

21 Network Hardware Wide Area Networks

22 Packet Switching Data are sent out in a sequence of small chunks called packets Packets are passed from node to node along a path leading from source to destination Packet-switching networks are commonly used for terminal-to-terminal computer and computer-to-computer communications

23 Asynchronous Transfer Mode (ATM)
Referred to as cell relay Culmination of circuit switching and packet switching Uses fixed-length packets called cells Works in range of 10’s and 100’s of Mbps and in the Gbps range Data rate on each channel dynamically set on demand

24 Local Area Networks (LAN)
Smaller scope, typically a single building LANs are usually owned by the same organization that owns attached devices Internal data rates greater than WANs Most common configurations are switched LANs and wireless LANs

25 Metropolitan Area Networks (MAN)
Covers a geographic area such as a town, city or suburb Middle ground between LAN and WAN Supports both data and voice Private or public network

26 Network Hardware Wireless Networks
Categories of wireless networks: System interconnection Wireless LANs Wireless WANs

27 Network Hardware Wireless Networks
System interconnection Bluetooth a short-range wireless network. Allows system components together, digital cameras, headsets, scanners, and other devices to connect to a computer by merely being brought within range. Wireless LANs Every computer has a radio modem and antenna with which it can communicate with other systems. Standard for wireless LANs: IEEE , which most systems implement and which is becoming very widespread.

28 Network Hardware Wireless Networks
Wireless WANs (cont.) Wireless LANs an operate at rates up to about 50 Mbps over distances of tens of meters. While cellular systems (Wireless WANs) operate below 1 Mbps, but the distance between the base station and the computer or telephone is measured in kilometers rather than in meters. High-bandwidth wide area wireless networks are also being developed (IEEE ).

29 The Channel Allocation Problem
To allocate a single broadcast channel among competing users, we can use: Static Channel Allocation in LANs and MANs Dynamic Channel Allocation in LANs and MANs

30 Static Channel Allocation in LANs and MANs
Frequency Division Multiplexing (FDM) is an example of static channel allocation where the bandwidth is divided among a number of N users. When there is only a small and constant number of users, each of which has a heavy (buffered) load of traffic (e.g., carriers' switching offices), FDM is a simple and efficient allocation mechanism. However, when the number of senders is large and continuously varying or the traffic is bursty, FDM presents some problems. 1) when fewer than N users are currently interested in communicating, a large piece of valuable spectrum will be wasted. 2) when more users wants to communicate, those who have not been assigned a frequency will be denied permission. 3) even assuming that the number of users could somehow be held constant at N, each user traffic usually changes dynamically over time.

31 Multiple Access Protocols
ALOHA Carrier Sense Multiple Access Protocols Collision-Free Protocols Limited-Contention Protocols Wavelength Division Multiple Access Protocols Wireless LAN Protocols

32 Evolution of random-access methods

33 ALOHA network

34 Procedure for ALOHA protocol

35 Pure ALOHA The basic idea of an ALOHA system is simple: let users transmit whenever they have data to be sent. There will be collisions, of course, and the colliding frames will be damaged. If the frame was destroyed, the sender just waits a random amount of time and sends it again. How the channel know that there is a collision: Due to the feedback property of broadcasting, a sender can always find out whether its frame was destroyed by listening to the channel, the same way other users do. With a LAN, the feedback is immediate; with a satellite, there is a delay of 270 msec before the sender knows if the transmission was successful. If listening while transmitting is not possible for some reason, acknowledgements are needed.

36 In pure ALOHA, frames are transmitted at completely arbitrary times.
The throughput of ALOHA systems is maximized by having a uniform frame size rather than by allowing variable length frames.

37 Other protocols Slotted ALOHA: It assumed the time is divided into discrete intervals. The station can send at the beginning of the next time interval whenever it have data ready after the start of the current time interval. 1- Persistent CSMA: When a station has data to send, it first listens to the channel to see if anyone else is transmitting at that moment. if the channel is idle, it start transmission. If the channel is busy, the station waits until it becomes idle. When the station detects an idle channel, it transmits a frame. If a collision occurs, the station waits a random amount of time and starts all over again. Nonpersistent CSMA: same as 1-persistent except that the station does not continually sense the channel when it finds it busy, rather it waits a random period of time and then sense the channel again. When the channel becomes idle it transmit. p-Persistent CSMA: same as Nonpersistent CSMA but the station transmit with probability p when the channel is idle.

38 Persistence strategies

39 CSMA/CD procedure

40 CSMA/CA procedure

41 Wireless LAN Protocols
A system of notebook computers that communicate by radio can be regarded as a wireless LAN A common configuration for a wireless LAN is an office building with base stations (also called access points) strategically placed around the building. All the base stations are wired together using copper or fiber. A simplifying assumption that all radio transmitters have some fixed range will be used follow. When a receiver is within range of two active transmitters, the resulting signal will generally be garbled and useless.

42 Wireless LAN Protocols (2)
A naive approach to using a wireless LAN might be to try CSMA: just listen for other transmissions and only transmit if no one else is doing so. The trouble is, this protocol is not really appropriate because what matters is interference at the receiver, not at the sender. A wireless LAN. (a) A transmitting. (b) B transmitting.

43 Wireless LAN Protocols (3)
When A is transmitting to B (previous figure part a) If C senses the medium, it will not hear A because A is out of range, and thus falsely conclude that it can transmit to B. If C does start transmitting, it will interfere at B, wiping out the frame from A. The problem of a station not being able to detect a potential competitor for the medium because the competitor is too far away is called the hidden station problem.

44 Wireless LAN Protocols (4)
When B transmitting to A (previous figure part b) If C senses the medium, it will hear an ongoing transmission and falsely conclude that it may not send to D, when in fact such a transmission would cause bad reception only in the zone between B and C, where neither of the intended receivers is located. This is called the exposed station problem.

45 Hidden station problem

46 Note The CTS frame in CSMA/CA handshake can prevent collision from a hidden station.

47 Use of handshaking to prevent hidden station problem

48 Exposed station problem

49 Use of handshaking in exposed station problem

50 Wireless LAN Protocols (5)
The problem is that before starting a transmission, a station really wants to know whether there is activity around the receiver. An early protocol designed for wireless LANs is MACA (Multiple Access with Collision Avoidance) (Karn, 1990). The basic idea behind it is for the sender to stimulate the receiver into outputting a short frame, so stations nearby can detect this transmission and avoid transmitting for the duration of the upcoming (large) data frame.

51 Wireless LAN Protocols (6)
Let us now consider how A sends a frame to B. - A starts by sending an RTS (Request To Send) frame to B. This short frame (30 bytes) contains the length of the data frame that will eventually follow. - Then B replies with a CTS (Clear to Send) frame. The CTS frame contains the data length (copied from the RTS frame). Upon receipt of the CTS frame, A begins transmission.

52 Wireless LAN Protocols (7)
The MACA protocol. (a) A sending an RTS to B. (b) B responding with a CTS to A.

53 Topics discussed in this section:
UNICAST ROUTING PROTOCOLS A routing table can be either static or dynamic. A static table is one with manual entries. A dynamic table is one that is updated automatically when there is a change somewhere in the Internet. A routing protocol is a combination of rules and procedures that lets routers in the Internet inform each other of changes. Topics discussed in this section: Optimization Intra- and Interdomain Routing Distance Vector Routing and RIP Link State Routing and OSPF Path Vector Routing and BGP

54 Figure Autonomous systems

55 Figure 22.13 Popular routing protocols

56 Figure 22.14 Distance vector routing tables

57 Figure 22.15 Initialization of tables in distance vector routing

58 immediate neighbors periodically and when there is a change.
Note In distance vector routing, each node shares its routing table with its immediate neighbors periodically and when there is a change.

59 Figure Updating in distance vector routing

60 Figure Two-node instability

61 Summary Trends challenging data communications: Transmission mediums
traffic growth development of new services advances in technology Transmission mediums fiber optic wireless Network categories: WAN LAN Internet evolved from the ARPANET TCP/IP foundation

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