TRANSMISSION ERRORS

Errors are introduced in the data bits during their transmission across a sub network. These errors can be categorised into :

·          Content errors

·          Flow integrity errors

Content errors are the errors in the content or a message, e.g. a "1" may be received as "0". This type of errors gets introduced due to impairment of the electrical signal in the transmission media.

Flow integrity errors refer to missing blocks of data. For example, a data block may be lost in the sub-network due to its having been delivered to a wrong destination.

In voice communication, the listener can tolerate a good deal of signal corruption during transmission. But data is very sensitive to errors. Measures are, therefore, built into a data communication system to counteract the effect of errors. These measures include:

·                     Introduction of additional check bits in the data bits to detect and correct content errors.

·                     Establishing procedures of data exchange which enable recovery of corrupted/lost messages.

BIT ERROR RATE (BER)

In analog transmission, signal quality is specified in terms of Signal to Noise ratio (S/N) which is usually expressed in decibels. In digital transmission, the quality of received digital signal is expressed in terms of Bit Error Rate (BER) which is number of errors in a fixed number of transmitted bits. A typical error rate on a high quality leased telephone line is as low as 1 error in 10⁶ bits or simply 1 x 10^-6.