Important Hardware Modems (amplitude and frequency Modulation) FAX machines Multiplexers (time and frequency division multiplexing) Inverse Multiplexers Front-end communications processors Remote intelligent controllers/line splitter (concept of polling) Line interface module/Terminal Controller Port Sharing Device. Cellular Phones (frequency reuse) [Note: Main source of pictures in this document is Fitzgerald and Dennis, Wiley, 1998]

Modems Modem is an acronym for Modulator/ Demodulator, and takes digital electrical pulses from a computer, terminal, or microcomputer and converts them into a continuous analog signal, for transmission over an analog voice grade circuit. It then re-converts the analog signal to its original digital format. Most modems accept commands from a computer keyboard.

Modem Operation

Modulation Modulation is the technique that modifies the form of a digital electrical signal so the signal can carry information on a communications media. There are three fundamental methods of analog modulation of an analog signal: Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation(PM)

Amplitude Modulation

Frequency Modulation and FSK (Frequency Shift Keying)

Phase Modulation and PSK

Sending Multiple Bits Simultaneously In practice, the maximum number of bits that can be sent with any one of these techniques is about five bits. The solution is to combine modulation techniques. One popular technique is quadrature amplitude modulation (QAM) involves splitting the signal into eight different phases, and two different amplitudes for a total of 16 different possible values.

Sending Multiple Bits Simultaneously Trellis coded modulation (TCM) is an enhancement of QAM that combines phase modulation and amplitude modulation. The problem with high speed modulation techniques such as TCM is that they are more sensitive to imperfections in the communications circuit.

Bit Rate Versus Baud Rate Versus Symbol Rate The bit rate and the symbol rate (or baud rate) are the same only when one bit is sent on each symbol. If we use QAM or TCM, the bit rate would be four to eight times the baud rate.

Modem Standards There are many different types of modems available today. Most modems support several standards so that they can communicate with a variety of different modems. Better modems can change data rates during transmission to reduce the rate in case of noisy transmission (fast retrain). 28800 bps V.34 Modem                         Time Elapsed: 30 seconds (using a Half-fast modem) V.32 Modem 14400 bps Time Elapsed: 30 seconds (good time to go for coffee) V.90 Modem 50000 bps Time Elapsed: 30 seconds OK! I get the picture!                                                  Source: http://www.v90.com/graphic.htm

Modem Standards Modem Standard Maximum Symbol Rate Bits per Data Rate ( bps) V.22 1200 1 2400 V.32 4 9600 V.32bis 6 14,400 V.34 3429 8.4 28,800 V.34+ 9.8 33,600 V.42, V.42bis N/a 56,000 V.90 Digital N/a 56,000 downstream More information: http://www.v90.com/

Digital Transmission of Analog Data Analog voice data can be sent over digital networks using a pair of special devices called CODECs (Coder/Decoder).

Pulse Amplitude Modulation (PAM) Analog voice data must be translated into a series of binary digits before they can be transmitted. With Pulse Amplitude Modulation, the amplitude of the sound wave is sampled at regular intervals and translated into a binary number. The difference between the original analog signal and the translated digital signal is called quantizing error.

Pulse Amplitude Modulation/Code Modulation Source: Goldman, Wiley.

Pulse Amplitude Modulation For standard voice grade circuits, the sampling of 3000 Hz at an average of 2 samples/Hz would result in a sample rate of 6000 times per second.

Reducing Quantizing Error There are two ways to reduce quantizing error and improve the quality of the PAM signal. Increase the number of amplitude levels Sample more frequently (oversampling).

Pulse Code Modulation Pulse Code Modulation is the most commonly used technique in the PAM family and uses a sampling rate of 8000 samples per second. Each sample is an 8 bit sample resulting in a digital rate of 64,000 bps (8 x 8000). A T-1 channel (capacity = 1.544 Mbps) can carry 24 simultaneous voice conversations. Kai Larsen: For more information on PCM, see Stallings (2003) “Data & Computer Communications” Sixth Edition (available in my office).

Multiplexing There are three major types of multiplexers Time division multiplexers (TDM) Frequency division multiplexers (FDM) Statistical time division multiplexers (STDM)

Multiplexed Circuit

Opticom - 16E1 Multiple E1 and Ethernet Multiplexer (Source: Opticom Communications)

Time Division Multiplexing

Time Division Multiplexing Time division multiplexing shares a circuit among two or more terminals by having them take turns, dividing the circuit “vertically.” Time on the circuit is allocated even when data are not transmitted, so that some capacity is wasted when a terminal is idle. Time division multiplexing is generally more efficient and less expensive to maintain than frequency division multiplexing, because it does not need guardbands.

TIME DIVISION MULTIPLEXING 1 Signal 1 Signal 2 Signal 3 Signal 4 Multiplexed Signal

Frequency Division Multiplexing (FDM)

Frequency Division Multiplexing (FDM) Frequency division multiplexers are somewhat inflexible because once you determine how many channels are required, it may be difficult to add more channels without purchasing an entirely new multiplexer.

Wavelength Division Multiplexing (WDM) …is a version of FDM used in fiber optic cables. WDM should reach 25 Terabits per second within three years.                                                       Kai Larsen: SWDM= Selective Wave Division Multiplexing (DWDM) DSLAM=Digital Subscriber Line Access Multiplexer Source: http://www.lightreading.com

Voice Circuit

FDM Example

Statistical Time Division STDM provides more efficient use of the circuit and saves money. However, STDM introduces two complexities: 1. STDM can cause time delays, if all terminals decide to transmit simultaneously. 2. All data must be identified by an address that specifies the device to which terminal it belongs. Most STDM multipexers do not send one bit at a time from each terminal, but send groups of characters at one time (say, a byte).

STDM 4 8 4 8 4 8 4 8 xxxxxxxxxxxxxxx

NASA’s Ground Network

Inverse Multiplexing Inverse multiplexing (IMUX) combines several low speed circuits to appear as one high speed circuit. One of the most common uses is to provide T-1 (1.544 Mbps) circuits for wide area networks, by combining 24 slower (64 Kbps) circuits. T1 actually uses a multiplexer/inverse multiplexer pair.

Inverse Multiplexing

Front End Processors (FEP)

Front End Communications Processor (FEP) Objective: Specialized computer to take away communications related functions from the host computer. Functions Communications Line Control: (Polling/Selecting) Message Formatting (Assembly/Disassembly) Protocol Conversion Message Editing: Compression/Decompression etc. Message Buffering Error Control Message Recording/Trail Keeping Statistics

Remote Intelligent Controller or Cluster Controller Small computer which reduces load on host computer. A group of terminals are connected to the host via the cluster controller instead of being directly connected. Functions Control operation of cluster. (e.g., data collection, screen formatting, etc.) Some processing is done within cluster controller Reduce need for interaction with host. Reduce traffic Step towards distributed processing.

Polling Polling is the process of sending a signal to a client that gives it permission to transmit or to ask it to receive. There are two common types of polling. Roll call polling, the FEP works consecutively through a list of clients, first 1, then 2 etc. until all are polled. Hub go-ahead polling (a.k.a. token passing) One computer starts the poll and passes it to the next system on the multipoint circuit, which sends its message or passes it to the next system, and so on.

Network Configuration Network configuration is the basic physical layout of the network. 3 common types of circuits are used to configure networks: Point-to-point circuits (or two-point) - sometimes called dedicated circuits. Multipoint circuits (or multidrop). Multiplexed circuits. Most complex computer networks have many circuits, some of each type.

Multipoint Configuration

Example of a Bank Network

The Cellular Phone Network                                                 

How does the Cellular Phone Network Work? Source: Carr and Snyder, Irwin.

Frequency Reuse                                        4 6 1 2 7 5 3 This is a repeating pattern with 7 frequencies. The same set of frequencies is repeated two cells away. Source for monkey: gettyimages.com

Cellular Phone Standards AMPS: Advanced Mobile Phone Services. (Analog Standard in North America. Uses 30 KHz per channel. Frequency band: 824-894 MHz.) TDMA: Time Division Multiple Access. (From 3 to 15 times the capacity of analog; caller ID; text messaging; better battery life; seamless roaming between AMPS and PCS.) CDMA: Code Division Multiple Access. (All the advantages of TDMA with twice the capacity, better voice quality, and lower costs.) GSM: Global System for Mobile Communications. (Similar to TDMA. Pan-European Standard.)

Team exercise Form teams Go to computer labs (10 minutes, no more), use WWW to do task Either: Map different cell-phone providers back to their underlying technologies Find maps of coverage areas and/or cooperative deals between providers Come back to classroom, 5 mins to write up findings Enlighten the class with your findings…

TDMA Utilizes time-sharing methods of centralized computers with large numbers of users. Stems from time-division multiplexing used by phone companies to put multiple calls on each digital line. Used in GSM Conservative digital system that multiplies the number of users in an analog cellular channel by three. Developers founded Qualcomm and developed CDMA.

CDMA Differentiates calls by multiplying the digital signal with a code resembling white noise. Spreads the signal across a 1.25-megahertz swath of cellular spectrum. Allows many users to share the spectrum at the same time. Each phone is programmed with a specific pseudonoise code to spread the signal over a wide frequency band.

CDMA continued… Base station uses same code in inverted form (1's are 0's and 0's are 1's) to "despread" the original signal. All other codes remain spread out and indistinguishable from background noise. Keeps transmit power at lowest feasible level.

Cocktail Party Example CDMA Everyone can talk at once, but in different languages. Everyone listens for messages in their own languages and ignores all other sounds as background noise. If someone begins to yell they drown out surrounding messages reducing the number of sustainable conversations . TDMA Each person restricts talk to a specific time slot while everyone else is silent. Reasonable as long as party was run by a dictator who controlled conversation with complex rules and a rigid clock.

Advantages of CDMA over TDMA Longer battery life in cell phone. Combines three strongest signals into one. TDMA perceives the combination as interference. CDMA does not divide time slots --> multipath signals come in time to strengthen message. CDMA allows simple and soft hand-offs Utilizes spread spectrum facilitating easy movement from one cell to another.

Summary We studied some important concepts, and devices used in computer networks. These are the building blocks of networks and are used to design networks. General trends: - Several devices are becoming smaller, faster and more capable with developments in chip technology. - Many devices are getting integrated into other devices. - Prices are falling rapidly.