Communication Methods

There are two main methods of transmitting data - serial and parallel. Both have advantages and disadvantages. Serial, at least in principle, only requires a single cable which means that each bit follows another and is generally slow. Parallel transmission requires several cables and so is bulkier but faster. Parallel transmission would normally only be used over short distances (eg in the CPU) whereas serial is used over long distances (over network cable or over the telephone lines).

A recent article about increasing the speed of a CPU implies that designers are finding it increasingly difficult to sort out the timings of parallel circuits inside the CPU and so serial may end up being more efficient and faster! - don't mention this in the exam it might confuse the examiner.

Definitions: serial is "bits transmitted one after the other (along a single channel/ wire/ line)"; parallel is "bits transmitted (along several wires/ channels/ lines) at the same time."

With serial transmission there must be some way of "waking-up" the receiving device. This is normally achieved by using START and STOP bits. The data itself would probably be 7 data bits and a parity bit.

The rules that govern the transmission (number of start bits, stop bits, etc) are called PROTOCOLS.

It is important to realise at this stage that the sending device and the receiving device must operate at the same speed. If, in the above diagram, each signal was on or off for 0.001 of a second as far as the sending device was concerned but for 0.0005 for the receiving device the data pattern would be 00111100.

The fact that you also need start and stop bits means that the proportion of good, useful data to the number of bits transmitted is reduced.

This type of data transmission is termed ASYNCHRONOUS which means that the data can be transmitted when it is needed. There is no need to send data at regular times. The data line can stay asleep until the start bits wake it up.

With most parallel systems there needs to be a different way of signalling if data is to be transmitted. Sometimes this is given the name “handshaking” because it initializes communication. If we wish to send signals from a computer to a printer one of the pins in the printer port is set to a value, say 1. If the printer is ready it, in turn, will set the value on another pin to 1. If the printer buffer is full then the printer sets this value to 0 which is decoded as the message “stop sending data”. When some of the data has been processed and a portion of the buffer cleared the “clear to send” signal will be sent. Obviously this method could not be used since there is only one signal line – there are no free lines for handshaking.

When we are transmitting any form of data there is a possibility of interruption or breakdown of the signal. One way of detecting this is to use the concept of parity. This comes in two forms: odd and even. In this example we will use even parity but it should be obvious how odd parity works. With even parity we count the number of 1’s in the pattern and, if it is an odd number, we add an extra one to make it up to an even number of 1’s. Otherwise we add a zero.

If we are working with ASCII codes only 7 bits are used. The eighth bit is used as the parity bit. Eg 0101001 would have parity bit set to 1.

An even better example occurs when error-correction routines automatically detect errors and correct them. Imagine that we wish to transmit a two byte pattern. This is split into a 4 x 4 grid together with a parity bit for the vertical and horizontal pattern (shown in grey). Altogether we now have 24 bits and this is what is transmitted. At the receiving end the process repeat. Any error in a row and column would have an incorrect parity bit and so the fault can be corrected.

So, although we wanted to transmit 16 bits, we have actually transmitted 24 bits – all to make sure that the data has been received correctly. Often you will be asked about the "Baud Rate". This is "rate that signals/voltage changes" whereas the "bit-rate" is the number of bits transmitted per second. Another concept is the “bandwidth” which can be defined as the amount of data that can be sent through a given communications circuit per second but the exam board prefer "the range of frequencies a medium is capable of transmitting". So the greater the bandwidth the greater the bit rate. Some circuits can “multiplex” more than data signal at the same time.

When data signals are sent from a computer to an analogue telephone line then it needs to be converted. Traditionally an acoustic coupler or MODEM (Modulator/Demodulator) is used. Within a home environment Modems are the normal way of connecting a home user to the Internet. Modem’s use a DAC as output from the computer and an ADC when the signals are entering the computer.

ISDN lines are digital so the digital signals from the computer can travel directly to the digital phone lines without requiring a modem.

Modern home broadband connections probably use ADSL which is an alternative method of transmitting these signals over copper telephone lines.