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Chapter 6 Modem Fundamentals

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  1. Chapter 6Modem Fundamentals Part II: Understanding Internet Access Technologies

  2. Topics Addressed in Chapter 6 • Dial-up access via ISPs • Data codes • Transmitting encoded data • Interfaces and interface standards • Signal representation and modulation • Modem capabilities • Error detection and correction • Modem/computer communications • Special-purpose modems

  3. Dial-Up Access Via ISPs • Consumers and businesses typically gain Internet access via ISPs. Many ISPs provide a variety of connection interfaces including: • Dial-in modem connections • ISDN • xDSL • Cable modems • T-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 computers • Despite increasing use of DSL and cable modems, dial-in access over voice-grade analog circuits is the most common form of Internet access for consumers • Point-to-point (PPP) protocol is the most widely used protocol over dial-up connections

  4. Character Encoding • Encoding 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 bits • Bits 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

  5. Data Codes • Several character encoding schemes are widely used in data communication systems including: • ASCII (American Standard Code for Information Interchange) – See Table 6-2 • EBCDIC (Extended Binary-Coded Decimal Interchange Code) – See Table 6-3 • Unicode (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

  6. 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 communication • Parallel 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

  7. Figure 6-2

  8. Figure 6-4

  9. Figure 6-5

  10. Data Flow • Data communication networks, including modem-to-modem communications, must have some mechanism for control over the flow of data between senders and receivers • Three elementary kinds of data flow are: • Simplex • Half-duplex • Full-duplex • These are illustrated in Figures 6-6 and 6-7 • Most modems in use today support both full- and half-duplex communication

  11. Figure 6-7

  12. 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-C • RS-422, RS-423, RS-449 • A variety of ISO and ITU interfaces • USB and FireWire

  13. Figure 6-8

  14. RS-232-C • EIA’s RS-232-C standard is arguably the most important physical layer standard • It is the most widely accepted standard for transferring encoded characters across copper wires between a computer or terminal and a modem • RS-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 bits • This 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

  15. Figure 6-9

  16. Digital Data Transmission • All 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 rates • Higher transmission speeds • No digital-analog conversion • Security

  17. Analog Transmission • Data 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 bits • In 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)

  18. Figure 6-13

  19. Figure 6-17 Figure 6-19

  20. Figure 6-20

  21. Phase ModulationFigure 6-24

  22. Bit Rates and Bandwidth • The 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 circuits • Over 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

  23. Baud Rate • Baud rate is a measure of the number of discrete signals that can be transmitted (or received) per unit of time • A 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

  24. 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 combination • Quadbits is a transmission mode in which each signal represents a 4-bit combination. Sixteen distinct carrier signal modulations are required for quadbits • Phase 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

  25. Figure 6-27

  26. Modem Capabilities • Modems differ in several dimensions including: • The type of medium they can be connected to (copper-based, fiber-optic, wireless) • Speed • Connection options (such as support for call waiting) • Support for voice-over-data • Data compression algorithms • Security features (such as password controls or callback) • Error detection and recovery mechanisms

  27. Modem Speed • Over 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 48,000 • Like V.90, V.92 modems leverage PCM for downstream links • A variety of factors contribute to modem speed and data throughput including: • Adaptive line probing • Dynamic speed shifts • Fallback capabilities • Fallforword capabilities • Data compression

  28. Table 6-6

  29. Data Compression • Modem data compression capabilities enable modems to have data throughput rates greater than their maximum bit rates • This is accomplished by substituting large strings of repeating characters or bits with shorter codes • The data compression process is illustrated in Figure 6-29 • Widely supported standards for data compression include (see Table 6-7): • V.42bis --- up to 4:1 compression using the Lempel Ziv algorithm • MNP 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

  30. Figure 6-29

  31. Table 6-7

  32. 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 operations • The general process is as follows (see Figure 6-30) • During handshaking, the modem pair determines the error checking approach that will be used • The sender sends the error-check along with the data • The receiver calculates its own error-check on received data and compares it to that transmitted by the sender • If the receiver’s error-check matches the sender’s, no error is detected; a mismatch indicates a transmission error • Detected errors trigger error recovery mechanisms

  33. Figure 6-30

  34. Error Sources • There are many sources of data communication transmission errors including: • Signal attenuation • Impulse noise • Crosstalk • Echo • Phase jitter • Envelope delay distortion • White noise • Electromagnetic interference (EMI)

  35. Error Impacts • Errors cause bits to be changed (corrupted) during transmission; without error-detection mechanisms, erroneous data could be received and used in application processing • Figure 6-32 illustrates a transmission error caused by noise • Table 6-8 indicates that longer impulse noises can corrupt multiple bits, especially as transmission speed increases

  36. Figure 6-32

  37. Table 6-8

  38. Error Prevention • Error prevention approaches used in data communications include: • Line conditioning • Adaptive protocols (such as adaptive line probing, fallback, adaptive size packet assembly) • Shielding • Repeaters and amplifiers • Better equipment • Flow control • RTS/CTS • XON/OFF

  39. 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 6-10 • Checksums • Cyclical redundancy checks (most widely used and robust) • CRC-12 • CRC-16 • CRC-32 • Sequence checks • Other approaches include check digits, hash totals, byte counts, and character echoing

  40. Table 6-9

  41. Table 6-10

  42. Error Recovery • Automatic 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 packet • Continuous ARQ (aka go-back-N ARQ). Sender keeps transmitting until a NAK is returned; sender retransmits that packet and all others after it • Selective 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 examples • Error control/recovery standards include MNP Class 4, V.42, and LAP-M (see Table 6-12)

  43. 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 modems • When 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.

  44. Figure 6-33c

  45. Special Purpose Modems • A variety of special purpose modems are found in data communication networks including: • multiport modems • short-haul modems (see Table 6-13) • modem eliminators (see Figure 6-34) • fiber optic modems • cable modems • ISDN modems • DSL modems • CSU/DSUs

  46. Chapter 6Modem Fundamentals Part II: Understanding Internet Access Technologies