The Transmission of Binary Data in Communication Systems The proliferation of applications that send digital data over communication channels has resulted in the need for efficient methods of transmission, conversion, and reception of digital data.
Topics Covered in Chapter 11 Digital Codes Principles of Digital Transmission Transmission Efficiency Modems Error Detection and Correction Protocols
Digital Codes Data processed and stored by computers can be numerical or text. The signals used to represent computerized data are digital. Even before the advent of computers, digital codes were used to represent data.
Early Digital Codes The Morse code was originally designed for wired telegraph, but was later adapted for radio communication. The Morse code consists of a series of dots and dashes that represent letters of the alphabet, numbers, and punctuation marks. The Baudot code was used in the early teletype machine, a device for sending and receiving coded signals over a communication link.
Modern Binary Codes For modern data communication, information is transmitted using a system in which the numbers and letters to be represented are coded, usually by way of a keyboard, and the binary word representing each character is stored in a computer memory.
American Standard Code for Information Interchange The most widely used data communication code is the 7-bit binary code known as the American Standard Code for Information Interchange (ASCII). ASCII code can represent 128 numbers, letters, punctuation marks, and other symbols. ASCII code combinations are available to represent both uppercase and lowercase letters of the alphabet. Several ASCII codes have two- and three-letter designations which initiate operations or provide responses for inquiries.
Hexadecimal Values Binary codes are often expressed using their hexadecimal, rather than decimal values. To convert a binary code to its hexadecimal equivalent, first divide the code into 4-bit groups. Start at the least significant bit on the right and work to the left. (Assume a leading zero on each of the codes.)
Extended Binary Coded Decimal Interchange Code The Extended Binary Coded Decimal Interchange Code (EBCDIC) was developed by IBM. The EBDIC is an 8-bit code allowing a maximum of 256 characters to be represented. The EBCDIC is used primarily in IBM and IBM- compatible computing systems and is not widely used as ASCII.
Principles of Digital Transmission Data can be transmitted in two ways: Parallel Serial
Serial Transmission Data transfers in long-distance communication systems are made serially. In a serial transmission, each bit of a word is transmitted one after another. Parallel data transmission is not practical for long- distance communication.
Serial Transmission of the ASCII Letter M
Serial Data Rate The speed of data transfer is usually indicated as number of bits per second (bps or b/s). Another term used to express the data speed in digital communication systems is baud rate. Baud rate is the number of signaling elements or symbols that occur in a given unit of time. A signaling element is simply some change in the binary signal transmitted.
Asynchronous Transmission In asynchronous transmission each data word is accompanied by start and stop bits that indicate the beginning and ending of the word. When no information is being transmitted, the communication line is usually high, or binary 1. In data communication terminology, this high level is referred to as a mark. Most low-speed digital transmission (the to 56,000-bps range) is asynchronous. Asynchronous transmissions are extremely reliable.
Synchronous Transmission The technique of transmitting each data word one after another without start and stop bits, usually in multiword blocks, is referred to as synchronous data transmission. To maintain synchronization between transmitter and receiver, a group of synchronization bits is placed at the beginning, and at the end of the block. Each block of data can represent hundreds or even thousands of 1-byte characters.
Synchronous Data Transmission
Encoding Whether digital signals are being transmitted by baseband methods or broadband methods, before the data is put on the medium, it is usually encoded in some way to make it compatible with the medium. Primary encoding methods are: Nonreturn to zero (NRZ) Return to Zero Manchester
Encoding Methods In the nonreturn to zero (NRZ) method of encoding the signal remains at the binary level assigned to it for the entire bit time. In return to zero (RZ) encoding the voltage level assigned to a binary 1 level returns to zero during the bit period. Manchester encoding, also referred to as biphase encoding, is widely used in LANs. In this system a binary 1 us transmitted first as a positive pulse, for one half of the bit interval, and then as a negative pulse for the remaining part of the bit interval.
Transmission Efficiency Transmission efficiency is the accuracy and speed with which information, whether it is voice or video, analog or digital, is sent and received over communication media.
Hartleys Law The amount of information that can be sent in a given transmission is dependent on the bandwidth of the communication channel and the duration of transmission. The greater the number of bits transmitted in a given time, the greater the amount of information that is conveyed. The higher the bit rate, the wider the bandwidth needed to pass the signal with minimum distortion.
Transmission Media and Bandwidth The two most common types of media used in data communication are wire cable and radio. The two types of wire cable used are coaxial and twisted pair. Coaxial cable has a center conductor surrounded by an insulator over which is a braided shield. The entire cable is covered with a plastic insulation. A twisted-pair cable is two insulated wires twisted together.
Transmission Media and Bandwidth (Continued) Twisted-pair is available as unshielded (UTP) or shielded. Coaxial cable and shielded twisted-pair cables are usually preferred, as they provide some protection from noise and cross talk. The bandwidth of any cable is determined by its physical characteristics. All wire cables act as low-pass filters because they are made up of wire that has inductance, capacitance, and resistance.
Twisted-Pair Unshielded Cable (UTP)
Multiple Coding Levels Channel capacity can be modified by using multiple- level encoding schemes that permit more bits per symbol to be transmitted. It is possible to transmit data using more than just two binary voltage levels or symbols. Multiple voltage levels can be used to increase channel capacity. Other methods, such as using different phase shifts for each symbol, are used.
Impact of Noise in the Channel An important aspect of information theory is the impact of noise on a signal. Increasing bandwidth increases the rate of transmission but also allows more noise to pass. Typical communication systems limit the channel capacity to one-third to one-half the maximum to ensure more reliable transmission in the presence of noise.
Modems Broadband communication techniques involving modulation, which are implemented by a modem, a device containing both a modulator and demodulator. Modems convert binary signals into analog signals capable of being transmitted over telephone and cable TV lines.
Modems (Continued) Telephone networks, originally designed to carry voice signals, are now widely used to carry digital information, as well as, linking computers and computer networks across the globe. Cable TV networks designed to carry analog TV signals now also routinely carry digital data for Internet access. There are three widely used modem types, conventional analog dial-up, digital subscriber line (DSL), and cable TV modems.
Modem and Digital Data Transmission
Modulation for Data Communication The four main types of modulation used in modern modems are: Frequency-shift keying (FSK) Phase-shift keying (PSK) Quadrature amplitude modulation (QAM) Orthogonal frequency division multiplexing (OFDM)
Frequency-Shift Keying (FSK) Frequency-shift keying (FSK) is the oldest and simplest form of modulation used in modems. In FSK, two sine-wave frequencies are used to represent binary )s and 1s. A binary 0, usually called a space, has a frequency of 1070 Hz. A binary 1, referred to as a mark, is 1270 Hz. These two frequencies are alternately transmitted to create the serial binary data.
Frequency-Shift Keying – Binary and FSK Signals
Phase-Shift Keying In phase-shift keying (PSK), the binary signal to be transmitted changes the phase shift of a sine-wave character depending upon whether a binary 0 or binary 1 is to be transmitted. A phase shift of 180 degrees, the maximum phase difference that can occur, is known as a phase reversal, or phase inversion. During the time that a binary 0 occurs, the carrier is transmitted with one phase; when a binary 1 occurs, the carrier is transmitted with a 180 degree phase shift.
Binary Phase-Shift Keying (BPSK)
Quadrature Phase Keying One way to increase the binary data rate while not increasing the bandwidth required for the signal transmission is to encode more than 1 bit per phase change. With quadrature phase keying (QPSK) more bits per baud are encoded, the bit rate of data transfer is higher than the baud rate, yet the signal does not take up additional bandwidth. In QPSK, each pair of successive digital bits in the transmitted word is assigned a particular phase. Each pair of serial bits, called a dibit, is represented by a specific phase.
QPSK Modulator Operation The QPSK modulator consists of a 2-bit shift register implemented with flip-flops, commonly known as a bit splitter. The serial binary data train is shifted through the register. The bits from the flip-flops are applied to balanced modulators. The carrier oscillator is applied to one balanced modulator and through a 90 degree phase shifter to another balanced modulator. The outputs of the balanced modulators are linearly mixed to produce the QPSK signal.
Quadrature Amplitude Modulation One of the most popular modulation techniques used in modems for increasing the number of bits per baud is quadrature amplitude modulation (QAM). QAM uses both amplitude and phase modulation of a carrier. In 8-QAM, there are four possible phase shifts and two different carrier amplitudes. Eight different states can be transmitted. With eight states, 3 bits can be encoded for each baud or symbol transmitted. Each 3-bit binary word transmitted uses a different phase- amplitude combination.
Orthogonal Frequency Division Multiplexing (OFDM) A data modulation method called OFDM is growing in popularity. OFDM is also known as multicarrier modulation (MCM). Although OFDM is known as a modulation method, the term frequency division multiplexing is appropriate because the method transmits data by simultaneously modulating segments of the high- speed serial bit stream onto multiple carriers spaced throughout the channel bandwidth.
Orthogonal Frequency Division Multiplexing (OFDM) (Continued) The carriers are frequency-multiplexed in the channel. The data rate on each channel is very low, making the symbol time much longer than predicted transmission delays. This technique spreads the signals over a wide bandwidth, making them less sensitive to the noise, fading, reflections, and multipath transmission effects common in microwave communication.
Analog Telephone Modem The most commonly used modem is one that connects personal computers to the telephone line. A typical dial-up modem consists of both transmitter and receiver sections. Most modern modems are implemented using digital signal processing (DSP) techniques. Modems are packaged on a single small printed circuit board and are designed to plug into the PC bus. The modem takes its power from the PC power supply. An RJ-11 modular connector attaches the modem to the telephone line.
Modem Classification and Standards The International Telecommunications Union (ITU) sponsors, negotiates, and maintains modem and many other communication standards. Modem standards are designated by a special V.xx symbol. Modems are usually capable of operating in several different V.xx modes. Most modems is use today are the V.90 or V.92 type and are capable of speeds up to 56 kbps.
Cable Modems Many cable TV systems are set up to handle high-speed digital data transmission. The digital data is used to modulate a high-frequency carrier that is frequency-multiplexed onto the cable that also carries the TV signal. The TV signals are frequency-division-multiplexed onto the cable. Cable modems provide significantly higher data rates than can be achieved over the standard telephone system.
Error Detection and Correction When high-speed binary data is transmitted over a communication link, whether it is a cable or radio, errors will occur. These errors are changes in the bit pattern caused by interference, noise, or equipment malfunctions. Such errors will cause incorrect data to be received. The number of bit errors that occur for a given number of bits transmitted is referred to as the bit error rate (BER).
Error Detection Many different methods have been used to ensure reliable error detection and include the following. Redundancy is a method that ensures error-free transmission by sending each character or message multiple times until it is properly received. Encoding schemes like the RZ-AMI are used whereby successive binary 1 bits in the bit stream are transmitted with alternating polarity. One of the most widely used systems is known as parity, in which each character transmitted contains one additional bit, known as a parity bit.
Error Detection (Continued) The horizontal or longitudinal redundancy check (LRC) is the process of logically adding, by exclusive ORing, all characters in a specific block of transmitted data. The final bit value for each horizontal row becomes one bit in a character known as the block-check character (BCC), or the block-check sequence (BCS). The cyclical redundancy check (CRC) is a mathematical technique used in synchronous data transmission.
Error Correction A number of efficient error-correction schemes have been devised to complement the parity and BCC error detection methods. The process of detecting and correcting errors at the receiver so that retransmission is not necessary is called forward error correction (FEC). The most popular FEC codes are the Hamming and Reed Solomon codes. These codes add extra bits to a transmitted word, process them, and bit errors are identified and corrected.
Protocols Protocols are rules and procedures used to ensure compatibility between the sender and receiver of digital data regardless of the hardware and software being used. Protocols are used to identify the start and end of a message, identify the sender and receiver, state the number of bytes to be transmitted, state the method of error detection, and for other functions. Various protocols, and various levels of protocols, are used in data communication.
Asynchronous Protocols Three popular protocols for asynchronous ASCII-coded data transmission between personal computers, via modem are: Xmodem Kermit MPN.
Xmodem In Xmodem, the data transmission procedure begins with the receiving computer transmitting a negative acknowledge (NAK) character to the transmitter. NAK is a 7-bit ASCII character that is transmitted serially back to the transmitter every 10 seconds until the transmitter recognizes it. Once the transmitter recognizes the NAK character, it begins sending a 128-byte block of data, known as a frame (packet) of information.
Xmodem Protocol Frame
Kermit The Kermit protocol transmission begins with an SOH character followed by a length (LEN) character, which tells how long the block of data is. A block can be up to 94 bytes long. Next is a packet sequence number (SEQ). There can be up to 63 blocks, and these are given a sequence number so that both transmitter and receiver can keep track of long messages. Kermit is reliable because it requires every packet sent be acknowledged by the receiver as being read correctly.
MNP Microcom Networking Protocols (MNPs) are a series of protocols developed by the manufacturer Microcom to be used with asynchronous modems. They specify ways to handle error detection and correction and how to specify whether or not data compression is used. There are 10 classes of protocols. MNPs are easy to implement because they can be programmed into the control microcomputer used in most modems.
Synchronous Protocols Protocols used for synchronous data communication are more complex than asynchronous protocols. Like asynchronous systems, they use various control characters for signaling purposes at the beginning and ending of the block of data to be transmitted.
Bisync IBMs Bisync protocol, which is widely used in computer communication, usually begins with the transmission of two or more ASCII sync (SYN) characters. These characters signal the beginning of the transmission and are also used to initialize the clock timing circuits in the receiving modem. This ensures proper synchronization of the data transmitted a bit at a time.
SDLC One of the most flexible and widely used synchronous protocols is the synchronous data link control (SDLC) protocol. SDLC is used in networks that are interconnections of multiple computers.
Open Systems Interconnection Model The International Organization for Standardization (ISO) has attempted to standardize data communication procedures. The ISO has come up with a framework, or hierarchy, that defines how data can be communicated. This hierarchy, known as the open systems interconnection (OSI) model, is designed to establish general interoperability guidelines for developers of communication systems and protocols.
OSI Model (Continued) The OSI hierarchy is made up of seven levels, or layers. Each layer is defined by software (or, in one case, hardware) and is clearly distinct from the other layers. These layers are not protocols, but they provide a way to define and partition protocols to make data transfers in a standardized way.
The Seven OSI Layers
Spread Spectrum Spread spectrum (SS) is a modulation and multiplexing technique that distributes a signal and its sidebands over a very wide bandwidth. There are two basic types of spread spectrum, frequency-hopping (FH) and direct-sequence (DS).
Spread Spectrum (Continued) In frequency-hopping SS, the frequency of the carrier of the transmitter is changed according to a predetermined sequence, called pseudorandom, at a rate higher than the serial binary data modulating the carrier. In direct-sequence SS, the serial binary data is mixed with a higher-frequency pseudorandom binary code at a faster rate and the result is used to phase-modulate a carrier.
Benefits of Spread Spectrum Spread spectrum is being used in more and more applications in data communications. Benefits are: Security Resistance to jamming and interference Band sharing Resistance to fading Precise timing