The transmission media alters the electromagnetic signals by resisting their flow. The resistance of the media effectively decreases the strength of the electrical signal until the strength of the signal becomes too low for the receiving device to interpret. This process is called attenuation.

In addition to attenuation, the connecting media changes the electromagnetic signals by adding additional components to the signal.

The sub atomic particles and molecules in the media vibrate. This vibration generates a meaningless electromagnetic signal, which has no legitimate meaning. - This is electronic noise. Thus the receiving device must be able to decipher a weak signal mixed in with noise.

There are a number of options available to militate against poor signal quality being received by the destination receiver as follows.

• Filters - these devices electronically filter and eliminate the noise without changing the signal
• Amplifiers and Repeaters - these devices increase the signal level or regenerate the signal

Channel Types

There are two fundamentally different types of channel that are used in voice and data communications - analogue and digital.

Analogue circuits have a waveform that is a continuously changing in a continuous manner. These are the sorts of signals that occur during a conversation between people and are found in telephone communications.

Digital circuits have a waveform with sharp changes in the strength of the signal. Thus digital circuits are ideally suited for the on-off type of activity that is the characteristic of a computer environment.

Analogue Channel Capacity

All physical channels impose a limitation to the amount of traffic that they can accommodate. The total amount of traffic that a given channel can pass over a given period is known as the bandwidth of the channel.

An analogue channel can accommodate multiple analogue signals that span a broad band of individual frequencies. This is known as a Broadband channel. The effective capacity of a analogue channel is determined by the frequency of the analogue circuit signal.

This is derived by the formula.

FREQUENCY = VELOCITY/WAVELENGTH

Where velocity is the speed (in metres per second) that the signal is moving through the channel and the wavelength is the distance (meters) it takes for the signal to go through a full cycle.

Digital Channel Capacity

The capacity of a digital channel is limited by the rate that a signal can change state over a given period.

The digital signal switches between two energy states. This process makes step changes in the voltage that are distinctly different from the continuous voltage changes associated with analogue signals. Devices transmit digital signals without the use of an analogue based carrier signal.

The digital channel transmits a signal at a discrete energy level at one frequency or it transmits no signal at all. This is known as baseband transmission.

The capacity of the digital channel is the number of digital values that the channel can convey in one second. This is measured in bits per second.

Telephone Line Limitations for Analogue Signals

The frequency range for the human voice is around zero to 12,000 hertz. The power of the lower end of the frequency is much greater than at the higher frequency levels.

However, the public switched telephone service can only support a tiny proportion of this bandwidth, approximately between 100 and 3500 hertz. This is a severe limitation to the use of telephone lines for transmitting data between computer systems.

Asynchronous Data Transmission

Data characters in a computer system are represented by a data code, each element of which consists of a group of binary digits (bits), which can only take the value of 1 or 0. A group of 8 bits is known as a byte.

The object of serial data transmission is to send these bytes from point to point over a single channel.

The method used is to define successive short intervals of time as representing successive bits in a byte.

Two possible conditions termed "mark" and "space" are imposed on the line by the transmitting terminal to represent binary 1 and 0 respectively.

Assuming the receiving terminal samples the line at the same intervals starting at the same time, it will be able to recreate the byte at the receiving end.

To allow the receiving terminal to start its sampling at the correct instant, the transmitter always reverts to a mark condition (1) for at least one interval at the end of each byte and always goes to a space (0) condition for one interval before starting to send the next byte.

Some slower speed transmission systems need two stop bits to function correctly. Apart from the start and stop bits, there is also an optional parity bit that can be set to give a modicum of error checking. There are two types of parity - odd and even.

Synchronous Data Transmission

Asynchronous transmission is very simple to implement, but is relatively inefficient because ten or eleven bits have to be transmitted to deliver each character of data.

Better use of the channel could result if only the data bits were transmitted.

This is achieved by Synchronous transmission. This system synchronises a high quality clock at the receiving terminal with a similar clock on the sending terminal.

Bytes are transmitted continuously without gaps for as long as the transmission lasts and any gaps in the data stream are filled by "idle" bytes as "padding".
For correct operation, it is essential that the receiving terminal starts to sample the line at the correct instant i.e. the first data bit of the first byte. The necessary "byte synchronisation" is achieved by preceding the data stream by two or more SYN bytes using a predetermined bit pattern which can be recognised by the receiving terminal.