Basic Principles of Digital Communication-3

Error Detection:
All practical data communications channels are subject to noise, particularly copper cables in industrial environments with high electrical noise. Noise can result in incorrect reception of the data. The basic principle of error detection is for the transmitter to compute a check character based on the original message content. This is sent to the receiver on the end of the message and the receiver repeats the same calculation on the bits it receives. If the computed check character does not match the one sent, we assume an error has occurred.

The simplest form of error checking in asynchronous systems is to incorporate a parity bit, which may be even or odd.

Even parity requires the total number of data bits at logic 1 plus the parity bit to equal an even number. The communications hardware at the transmission end calculates the parity required and sets the parity bit to give an even number of logic 1 bits. Odd parity works in the same way as even parity, except that the parity bit is adjusted so that the total number of logic 1 bits, including the parity bit, equals an odd number.

The hardware at the receiving end determines the total number of logic 1 bits and reports an error if it is not an appropriate even or odd number. The receiver hardware also detects receiver overruns and frame errors. Statistically, use of a parity bit has only about a 50% chance of detecting an error on a high speed system. This method can detect an odd number of bits in error and will not detect an even number of bits in error. The parity bit is normally omitted if there are more sophisticated error checking schemes in place.

Signalling Rate:
The signaling rate of a communications link is a measure of how many times the physical signal changes per second and is expressed as the baud rate. An oscilloscope trace of the data transfer would show pulses at the baud rate. For a 1000 baud rate, pulses would be seen at multiples of 1 ms. With asynchronous systems, we set the baud rate at both ends of the link so that each physical pulse has the same duration.

Bit Rate:
The data rate or bit rate is expressed in bits per second (bps), or multiples such as kbps, Mbps and Gbps (kilo, mega and gigabits per second). This represents the actual number of data bits transferred per second. An example is a 1000 baud RS-232 link transferring a frame of 10 bits, being 7 data bits plus a start, stop and parity bit. Here the baud rate is 1000 baud, but the data rate is 700 bps.
Although there is a tendency to confuse baud rate and bit rate, they are not the same.
Whereas baud rate indicates the number of signal changes per second, the bit rate indicates the number of bits represented by each signal change. In simple baseband systems such as RS-232, the baud rate equals the bit rate. For synchronous systems, the bit rate invariably exceeds the baud rate. For ALL systems, the data rate is less than the bit rate due to overheads such as stop, stand, and parity bits (synchronous systems) or fields such as address and error detection fields in synchronous system frames.

There are sophisticated modulation techniques, used particularly in modems that allow more than one bit to be encoded within a signal change. The ITU V.22bis full duplex standard, for example, defines a technique called quadrature amplitude modulation, which effectively increases a baud rate of 600 to a data rate of 2400 bps. Irrespective of the methods used, the maximum data rate is always limited by the bandwidth of the link.

Bandwidth:
The single most important factor that limits communication speeds is the bandwidth of the link. Bandwidth is generally expressed in hertz (Hz), meaning cycles per second. This represents the maximum frequency at which signal changes can be handled before attenuation degrades the message. Bandwidth is closely related to the transmission medium, ranging from around 5000 Hz for the public telephone system to the GHz range for optical fiber cable.
As a signal tends to attenuate over distance, communications links may require repeaters placed at intervals along the link, to boost the signal level. Calculation of the theoretical maximum data transfer rate uses the Nyquist formula and involves the bandwidth and the number of levels encoded in each signaling element. 

SNR:
The signal to noise (S/N) ratio of a communications link is another important limiting factor. Sources of noise may be external or internal.

The maximum practical data transfer rate for a link is mathematically related to the bandwidth, S/N ratio and the number of levels encoded in each signaling element. As the S/N decreases, so does the bit rate.

Data throughput:
As data is always carried within a protocol envelope, ranging from a character frame to sophisticated message schemes, the data transfer rate will be less than the bit rate.The amount of redundant data around a message packet increases as it passes down the protocol stack in a network. This means that the ratio of non-message data to ‘real’ information may be a significant factor in determining the effective transmission rate, sometimes referred to as the throughput.

Error Rate:
Error rate is related to factors such as S/N ratio, noise, and interference. There is generally a compromise between transmission speed and the allowable error rate, depending on the type of application. Ordinarily, an industrial control system cannot allow errors and is designed for maximum reliability of data transmission. This means that an industrial system will be comparatively slow in data transmission terms. As data transmission rates increase, there is a point at which the number of errors becomes excessive. Protocols handle this by requesting a retransmission of packets. Obviously, the number of retransmissions will eventually reach the point at which a high apparent data rate actually gives a lower real message rate, because much of the time is being used for retransmission.