Chapter 6Modem Fundamentals

Part II: Understanding Internet Access Technologies

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Topics Addressed in Chapter 6

Dial-up access via ISPsData codesTransmitting encoded dataInterfaces and interface standardsSignal representation and modulationModem capabilitiesError detection and correctionModem/computer communicationsSpecial-purpose modems

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Dial-Up Access Via ISPsConsumers and businesses typically gain Internet access via ISPs. Many ISPs provide a variety of connection interfaces including:

Dial-in modem connectionsISDNxDSLCable modemsT-n and fractional T-n

Wireless service providers (WSPs) provide wireless Internet access for users with wireless modems, smart phones, and Web-enabled PDAs, or handheld computersDespite increasing use of DSL and cable modems, dial-in access over voice-grade analog circuits is the most common form of Internet access for consumersPoint-to-point (PPP) protocol is the most widely used protocol over dial-up connections

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Character EncodingEncoding is one of the first requirements of a data communication network (see Figure 6-1)Character encoding involves the conversion of human-readable characters to corresponding fixed-length series of bitsBits can be represented as discrete signals and therefore can be easily transmitted or received over communication media

When bits are represented as discrete signals, such as different voltage levels, they are in a digital format

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Data CodesSeveral character encoding schemes are widely used in data communication systems including:

ASCII (American Standard Code for Information Interchange) – See Table 6-2EBCDIC (Extended Binary-Coded Decimal Interchange Code) – See Table 6-3Unicode (aka ISO 10646)Touch-tone telephone code

As illustrated in Table 6-1, these vary in the number of bits used to represent each character as well as the total number of characters that can be represented

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Transmitting Encoded Data

The bits that represent encoded characters can be transmitted simultaneously (parallel transmission) or one at time (serial transmission) – see Figure 6-2

Serial transmission is more widely used than parallel transmission for data communicationParallel transmission is used for communication between components within a computer

In serial transmission, encoded characters can either be transmitted one at a time (asynchronous transmission) or in blocks (synchronous transmission) – see Figure 6-5

Figure 6-4 illustrates asynchronous transmission of a single character.UART provides the interface between parallel transmission within the computer and serial transmission ports. It also plays a key role in formatting encoded characters for asynchronous transmission

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Figure 6-2

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Figure 6-4

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Figure 6-5

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Data FlowData communication networks, including modem-to-modem communications, must have some mechanism for control over the flow of data between senders and receiversThree elementary kinds of data flow are:

SimplexHalf-duplex Full-duplex

These are illustrated in Figures 6-6 and 6-7Most modems in use today support both full- and half-duplex communication

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Figure 6-7

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Interfaces and Interface Standards

There are two major classes of data communication equipment:

Data communication equipment (DCE): this includes modems, media, switches, routers, satellite transponders, etc.)Data terminating equipment (DTE): this includes terminals, servers, workstations, printers, etc.)

The physical interface is the manner in these two classes are joined together (see Figure 6-8)A wide range of interface standards exist including

RS-232-CRS-422, RS-423, RS-449A variety of ISO and ITU interfacesUSB and FireWire

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Figure 6-8

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RS-232-CEIA’s RS-232-C standard is arguably the most important physical layer standardIt is the most widely accepted standard for transferring encoded characters across copper wires between a computer or terminal and a modemRS-232-C uses voltage levels between –15 and +15 volts (see Figure 6-9); negative voltages are used to represent 1 bits and positive voltages are use to represent 0 bitsThis standard does not specify size or kind of connectors to be used in the interface. It does define 25 signal leads (see Table 6-4). 25-pin connectors and 9-pin connectors are most common, but other kinds of connectors are sometimes used

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Figure 6-9

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Digital Data TransmissionAll communication media are capable of transmitting data in either digital or analog form.Voice-grade dial-up circuits are typically analog, however, relative to analog transmission, digital transmission has several advantages:

Lower error ratesHigher transmission speedsNo digital-analog conversionSecurity

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Analog TransmissionData is represented in analog form when transmitted over analog voice-grade dial-up circuits (see Figure 6-14)This is done by varying the amplitude, frequency, or phase of the carrier signal (carrier wave) raised during the handshaking process at the start of a communication session between two modems

During handshaking, the two modems raise a carrier signal and agree on how it will be manipulated to represent 0 and 1 bitsIn some modulation schemes, more than one of the carrier signal’s characteristics are simultaneously manipulated

Modems (modulator/demodulators) are the devices used to translate the digital signals transmitted by computers into corresponding analog signals used to represent bits over analog dial-up circuits (see Figure 6-13)

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Figure 6-13

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Figure 6-17

Figure 6-19

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Figure 6-20

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Phase ModulationFigure 6-24

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Bit Rates and BandwidthThe bandwidth of an analog channel is the difference between the minimum and maximum frequencies it can carry

A voice-grade dial-up circuit can transmit frequencies between 300 and 3400 Hz and thus has a bandwidth of 3100 Hz

For digital circuits, bandwidth is a measure of the amount of data that can be transmitted per unit. Bits per second (bps) is the most widely used measure for digital circuitsOver time, bit rates (bps) have also become on of the key measures of modem performance (e.g. a 56 Kbps modem)

However, modem bit rates are not necessarily an accurate reflection of their data throughput rates

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Baud RateBaud rate is a measure of the number of discrete signals that can be transmitted (or received) per unit of timeA modem’s baud rate measures the number of signals that it is capable of transmitting (or receiving) per second

Baud rate represents the number of times per second that a modem can modulate (or demodulate) the carrier signal to represent bits

Although baud rate and bit rate are sometimes used interchangeably to refer to modem data transfer speeds, these are only identical when each signal transmitted (or received) represents a signal bit

A modem’s bit rate is typically higher than its baud rate because each signal transmitted or received may represent a combination of two or more bits

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Dibits, Tribits, Quadbits, and QAM

Dibits are a transmission mode in which each signal conveys two bits of data With tribits, each carrier signal modulation represents a 3-bit combinationQuadbits is a transmission mode in which each signal represents a 4-bit combination. Sixteen distinct carrier signal modulations are required for quadbitsPhase modulation is common on today’s modems because it lends itself well to the implementation of dibits, tribits, and quadbits (see Figure 6-27)Quadrature amplitude modulation (QAM) is widely used in today’s modems. Many versions of QAM represent far more than 4-bits per baud

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Figure 6-27

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Modem CapabilitiesModems differ in several dimensions including:

The type of medium they can be connected to (copper-based, fiber-optic, wireless)SpeedConnection options (such as support for call waiting)Support for voice-over-data Data compression algorithmsSecurity features (such as password controls or callback)Error detection and recovery mechanisms

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Modem SpeedOver time, the evolution of modem standards has corresponded with increases in modem speeds (see Table 6-6)In 2002, V.92 is the newest modem standard

V.92 is backward compatible with V.90 but is capable of upstream data rates of 48,000Like V.90, V.92 modems leverage PCM for downstream links

A variety of factors contribute to modem speed and data throughput including:

Adaptive line probingDynamic speed shiftsFallback capabilitiesFallforword capabilitiesData compression

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Table 6-6

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Data CompressionModem data compression capabilities enable modems to have data throughput rates greater than their maximum bit ratesThis is accomplished by substituting large strings of repeating characters or bits with shorter codesThe data compression process is illustrated in Figure 6-29Widely supported standards for data compression include (see Table 6-7):

V.42bis --- up to 4:1 compression using the Lempel Ziv algorithmMNP Class 5 --- supports 1.3:1 and 2:1 ratios (via Huffman encoding and run-length encoding)MNP Class 7 – up to 3:1 compression

V.44 --- capable of 20% to 100% improvements over V.42bis

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Figure 6-29

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Table 6-7

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Error Detection and Recovery

In order to ensure that data is not changed or lost during transmission, error-detection and recovery processes are standard aspects of modem operationsThe general process is as follows (see Figure 6-30)

During handshaking, the modem pair determines the error checking approach that will be usedThe sender sends the error-check along with the dataThe receiver calculates its own error-check on received data and compares it to that transmitted by the senderIf the receiver’s error-check matches the sender’s, no error is detected; a mismatch indicates a transmission errorDetected errors trigger error recovery mechanisms

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Figure 6-30

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Error SourcesThere are many sources of data communication transmission errors including:

Signal attenuationImpulse noiseCrosstalkEchoPhase jitterEnvelope delay distortionWhite noiseElectromagnetic interference (EMI)

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Error ImpactsErrors cause bits to be changed (corrupted) during transmission; without error-detection mechanisms, erroneous data could be received and used in application processingFigure 6-32 illustrates a transmission error caused by noiseTable 6-8 indicates that longer impulse noises can corrupt multiple bits, especially as transmission speed increases

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Figure 6-32

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Table 6-8

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Error PreventionError prevention approaches used in data communications include:

Line conditioningAdaptive protocols (such as adaptive line probing, fallback, adaptive size packet assembly)ShieldingRepeaters and amplifiersBetter equipmentFlow control

RTS/CTSXON/OFF

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Error Detection Approaches

Error detection processes vary in complexity and robustness. They include:

Parity checking (see Table 6-9)Longitudinal redundancy checks (LRC) – see Table 6-10ChecksumsCyclical redundancy checks (most widely used and robust)

CRC-12CRC-16CRC-32

Sequence checksOther approaches include check digits, hash totals, byte counts, and character echoing

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Table 6-9

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Table 6-10

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Error RecoveryAutomatic repeat request (ARQ) is the most widely used error-recovery approach in data communications. In this approach, the receiver requests retransmission if an error occurs. There are three major kinds of ARQ:

Discrete ARQ (aka stop-and-wait ARQ). Sender waits for an ACK or NAK before transmitting another packetContinuous ARQ (aka go-back-N ARQ). Sender keeps transmitting until a NAK is returned; sender retransmits that packet and all others after itSelective ARQ. Sender only retransmits packets with errors

Forward error correction codes involve sending additional redundant information with the data to enable receivers to correct some of the errors they detect. Hamming code and Trellis Coded Modulation are examplesError control/recovery standards include MNP Class 4, V.42, and LAP-M (see Table 6-12)

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Modem/Computer Communications

One of the roles of communication software is to enable users to view and modify modem settings (see Figure 6-33) such as:

error control (see Figure 6-33a and Figure 6-33c)transmission speed (see Figure 6-33b)flow control (see Figure 6-33c)data compression (see Figure 6-33c)UART settings (see Figure 6-33d)

Most communication software issues Hayes AT command set instructions to modemsWhen a user wants to establish a communication session over a dial-up connection, communication software sends a setup string to the modem.

The setup string specifies what settings are to be used for communicating with other modems and how the modem and computer will interact.

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Figure 6-33c

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Special Purpose ModemsA variety of special purpose modems are found in data communication networks including:

multiport modemsshort-haul modems (see Table 6-13)modem eliminators (see Figure 6-34)fiber optic modemscable modemsISDN modemsDSL modemsCSU/DSUs

Chapter 6Modem Fundamentals

Part II: Understanding Internet Access Technologies