1-5 Encoding Techniques in Local Area Networks
WILLIAM STALLINGS
The nonreturn-to-zero (NRZ) code common to computing equipment interfaces is not appropriate for local area networks (LANs). As higher speeds and lower-quality transmission media are increasingly employed, the design of the signal encoding scheme becomes more critical.
DIGITAL SIGNALING
The increasing proliferation of different types of LANs and the ongoing evolution of public and private wide area networks (WANs) toward digital technology and services have led to an increased interest in designing efficient digital signaling techniques.
A bit stream is generated by a source, such as a personal computer or a voice digitizer. In either case, the data is typically represented as discrete voltage pulses, using one voltage level for binary 0 and another for binary 1. This traditional encoding technique is known as nonreturn-to-zero (NRZ) and is common in physical interfaces such as Electronic Industries Association-232 (EIA-232).
A common means of transmitting digital data is to pass it through a modem, then transmit it as analog signals. There are a number of cases where this is not done, such as:
- Baseband LANs (i.e., Ethernet and Token Ring).
- Digital PBX connections for digital telephones and data processing devices.
- Digital access to public telecommunications networks over a digital local loop.
In all of these cases, the digital data is transmitted as a series of voltage pulses, which is referred to as digital signaling. Although it is possible to use NRZ directly for signaling, NRZs form is not compatible with the data rates and/or distances of LANs and digital WANs. Instead, the NRZ stream is encoded to enhance performance. The various encoding approaches used in LANs are described and compared in this chapter. Two of the major issues are signaling-rate requirements and performance.
There are two important tasks in interpreting digital signals at the receiver. First, the receiver must know the timing of each bit. That is, the receiver must know with some accuracy when a bit begins and ends, so that the receiver may sample the incoming signal each bit time to recognize the value of each bit. Second, the receiver must determine whether the signal level for each bit position is high or low.
A number of factors determine how successful the receiver will be in interpreting the incoming signal, including the signal-to-noise ratio (S/N), the data rate, and the bandwidth of the signal. With the type and length of transmission medium held constant, these factors affect the interpretation of the incoming signal in the following ways:
- An increase in data rate increases bit error rate (the probability that a bit is received in error).
- An increase in S/N decreases bit error rate.
- An increase in bandwidth of the transmission medium allows increased data rate.
Another factor that can be used to improve performance, the encoding scheme, is simply the mapping from data bits to signal elements. A variety of approaches have been tried, all of which involve mapping a stream of bits into a signal encoding format.
Some of the approaches also involve a bit transformation on the bit stream prior to signal encoding to improve signal characteristics. The following sections evaluate the various techniques.
Signal Spectrum
A lack of high-frequency (high relative to the data rate) components means that less bandwidth is required for transmission. Lack of a direct current (DC) component is also desirable. With a DC component to the signal, there must be direct physical attachment of transmission components. With no DC component, AC coupling via transformer is possible. This provides excellent electrical isolation and reduces interference. Also, the magnitude of the effects of signal distortion and interference depend on the spectral properties of the transmitted signal. In practice, it usually happens that the transmission fidelity of a channel is worse near the band edges. Therefore, a good signal design should concentrate the transmitted power in the middle of the transmission bandwidth. This results in less distortion in the received signal. To meet this objective, codes can be designed with the aim of shaping the spectrum of the transmitted signal.
Clocking
Determining the beginning and end of each bit position is no easy task. One rather expensive approach is to provide a separate clock lead to synchronize the transmitter and receiver. The alternative is to provide some synchronization mechanism that is based on the transmitted signal. This can be achieved with suitable encoding.
Error Detection
Error detection is primarily the responsibility of a data link or transport layer above the physical signaling level. However, it is useful to have some error detection capability built into the physical signaling encoding scheme. This permits errors to be detected more quickly.
Signal Interference and Noise Immunity
Certain codes exhibit superior performance in the presence of noise. This is usually expressed in terms of a bit error rate.
Cost and Complexity
Although digital logic continues to drop in price, this factor should not be ignored. In particular, the higher the signaling rate to achieve a given data rate, the greater the cost. Some codes require a signaling rate that is greater than the actual data rate.
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