CHAPTER 12
Data Communications
1. Data communications is the transmission and reception of binary data between computers and other digital equipment.
2. The earliest form of electronic communications, the telegraph, was a type of data communications.
3. Turning a carrier off and on in a code of dots and dashes is a kind of data communications known as continuous wave (CW).
4. Teletype is a form of telegraph that uses the 5-bit Baudot code to transmit between typewriters like units.
5. The most widely used binary data communications code is the 7-bit American Standard Code for Information Interchange (ASCII).
6. Another popular code is the 8-bit Extended Binary Coded Decimal Inter- change Code (EBCDIC) used mainly in IBM systems.
7. The two main methods of data transmission are serial and parallel. In serial transmission, each bit is transmitted sequentially. In parallel transmission, all bits are transmitted simultaneously.
8. Serial transfers are slower than parallel transfers but require only a single line or channel. Parallel transfers require multiple channels of lines called a bus.
9. The speed of data transmission is designated in terms of bits per second (bits/s) or baud.
10. Baud rate is the number of symbol changes per second. A symbol is an amplitude, frequency, or phase change.
11. The channel-or bit-rate capacity of a channel is directly proportional to the channel bandwidth and the time of transmission.
12. The channel capacity or binary signal transmission speed C in bits per second is equal to twice the channel bandwidth B when no noise is present (C = 2B).
13. When multiple levels or symbols are used to encode the data, the channel capacity C is greater for a given bandwidth B, or C = 2BLog2N where N is the number of symbols used.
14. The channel capacity C in bits per second is proportional to the channel bandwidth B and the power SIN ratio, or C = Blog2 t1 + SIN).
15. The, bit rate is higher than the baud (symbol) rate if multiple-level (symbol) encoding is used.
16. The two methods of data transmission are asynchronous and synchronous. In asynchronous transmission, data is sent one character at a time with start and stop bits. In synchronous transmission, data is sent as a continuous block of multiple characters framed with synchronization characters.
17. Synchronous transmission is faster than asynchronous transmission.
18. In data communications, a binary 1 is referred to as a mark and a binary 0 as a space.
19. Signals, whether voice, video, or binary, transmitted directly over a cable are known as baseband signals.
20. Voice and video signals are analog but may be converted to digital for data communications transmission.
21. Signals that involve a modulated carrier a called broadband signals.
22. Communications of binary data signal over the telephone network which is designed for analog signals is made possible by using a modem.
23. A modem is a modulator-demodulator unit that converts digital signals to analog and vice versa.
24. The most commonly used modulation techniques in modems are frequency-shift keying (FSK), phase-shift keying (PSK), and quadrature amplitude modulation (QAM).
25. Frequency-shift keying uses two frequencies for binary 0 and 1 (1070 and 1270 Hz or 2025 and 2225 kHz). It operates at speeds of 300 baud or less.
26. Modems capable of transmitting at standard higher rates of 1200, 2400, 4800, and 9600 bits/s use PSK and/or QAM.
27. Binary PSK (BPSK) uses a carrier of 1600 or 1700 Hz where a phase of 0° represents a binary 0 and a 1800 phase shift represents a binary 1, or vice versa.
28. Binary PSK is generated by a balanced modulator.
29. Binary PSK is demodulated by a balanced modulator.
30. To properly demodulate BPSK, the carrier at the demodulator must have exactly the same phase as the transmitting carrier.
31. A special carrier recovery circuit in the receiver produces the correct phase carrier from the BPSK signal.
32. Differential PSK eliminates the need for a special reference phase carrier by using a coding technique where the phase of each bit is referenced to the previous bit.
33. Quadrature PSK uses four equally spaced phase shifts of the carrier to represent two bits (dibit). For example, 00 = 45°, 01 = 135°,11 = 225°, 10 = 315°.
34. In 8-PSK, 3 bits are coded per phase change. In 16-PSK, 4 bits are coded per phase change. Thus the bit rate is 3 or 4 times the symbol rate change or baud rate.
35. Quadrature amplitude modulation uses a combination of QPSK and two-level AM to code 3 bits per baud. Each of the eight possible 3-bit combinations is represented by a unique phase and amplitude signal.
36. A protocol is a rule or procedure that defines how data is sent and received.
37. Protocols include "handshaking" signals between the transmitter and receiver that indicate the status of each.
38. The Xmodem protocol is widely used in personal computers.
39. In synchronous communications, a variety of special characters are sent before and after the block of data to ensure that the data is correctly received.
40. Bit errors that occur during transmission are caused primarily by noise.
41. The ratio of the number of bit errors that occur for a given number of bits transmitted is known as the bit error rate (BER).
42. Error-detection and -correction schemes have been devised to reduce bit errors and increase data accuracy.
43. One of the most widely used error detection schemes add a parity bit to each character transmitted, making the total number of binary Is transmitted odd or even. If a bit error occurs, the parity bit derived at the receiver will differ from the one transmitted.
44. Parity generator circuits are made up of multiple levels of exclusive OR (XOR) gates.
45. Another name for parity is vertical redundancy check (VRC).
46. The longitudinal redundancy check (LRC) is another way to test for errors. Corresponding bits in adjacent data words are exclusive-ORed to generate a block check character (BCC) that is appended to the transmitted block.
47. The VRC and LRC provide a coordinate system that will identify the exact location of a bit error so that it may be corrected.
48. A widely used error-detection scheme is the cyclical redundancy check (CRC) where the data block is divided, by a constant to produce a quotient and remainder. The remainder is the CRC character which is attached to the transmitted data block and compared to the CRC computed at the receiver.
49. A network is any interconnection of two or more stations that can communicate with one another.
50. An example of a wide area network (WAN)is the telephone system. An example of a metropolitan area network (MAN) is a cable TV system.
51. A local area network (LAN)is an interconnection of stations in a small area over short distances such as in an office building, on a military base, or on a college campus.
52. Local area networks were conceived to allow PC users to share expensive peripherals such as hard disks and printers but are now used for general communication and the sharing of software and data.
53. The most common physical configurations or topologies of LANs are the star, ring, and bus.
54. The bus is the fastest. The ring is the least expensive but is disabled if one station fails.
55. The star configuration is commonly used to connect many terminals and PCs to a larger mainframe or minicomputer.
56. The three most commonly used transmission media in networks are twisted pair, coax, and fiber-optic cable.
57. Twisted pair is inexpensive and easy to work with and is widely used for short distance, low-speed LANs. It is not shielded, so it is susceptible to noise.
58. Coax is the most widely used medium because of its high-speed capability and its shielding against noise.
59. Fiber-optic cable is growing in usage as its price declines. It has the highest speed capability of any medium.
60. Local area networks use both baseband and broadband techniques. Baseband refers to transmitting the information signal directly on the medium.
61. Broadband refers to using modulation techniques to transmit the data on carriers that can be assigned specific channels over a wide frequency range on a common medium.
62. Broadband systems require the use of modems at each node.
63. Spread spectrum (SS) is a modulation and multiplexing technique used primarily in data communications that deliberately spreads the signal out over a wide bandwidth rather than trying to restrict it to a narrow band.
64. The two most widely used types of spread spectrum are frequency hopping (FH) and direct sequence (DS).
65. Spread spectrum offers the benefits of privacy or security of communications, immunity to jamming, and lower sensitivity to frequency-selective fading.
66. In frequency-hopping SS, the serial binary data usually modulates a carrier by FSK. The FSK signal is mixed with a sine wave from a frequency synthesizer to form the final RF signal. The frequency synthesizer is switched at a rate of speed higher than the rate of the data signal, dwelling only briefly on each of many channel frequencies. Thus, the signal is broken up into small pieces and spread over a wide frequency range.
67. The frequency-hopping scheme is controlled by a pseudorandom binary code that switches at random from binary 0 to 1 and vice versa. The random nature of the signal causes the frequency to jump all over the band, distributing pieces of the signal hither and yon. The pseudorandom code acts like digital noise and thus is called a pseudorandom noise (PSN) code.
68. In direct-sequence SS, the serial data is mixed with a higher-frequency PSN code in an X-OR circuit. The resulting higher frequency binary signal produces more higher-frequency sidebands, thereby spreading the signal out over a wider bandwidth. The X-OR output usually phase-modulates the final carrier.
69. Because an SS signal is spread out randomly over a wide bandwidth, many signals can share a band without interference. A narrowband receiver will no respond to an SS signal except to interpret the random signals as a form of low-level noise.
70. The main problem in receiving SS signals is in acquiring the signal and synchronizing the transmitted PSN code to the same internally generated PSN code in the receiver. In direct-sequence SS, an electronic correlator circuit is responsible for achieving synchronism.
71. Different stations sharing a given band are defined and identified by their unique PSN code.
72. Spread spectrum was originally developed for and used in military equipment. In 1985, the FCC authorized SS for commercial or civilian use in the 902- to 928-, 2400- to 2483-, and 5725- to 51150-MHz bands.
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