10.4 PERFORMANCE CRITERIA

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The performance of a digital communication system is measured in terms of the Bit Error Rate (BER). BER is measured in terms of the ratio of the number of bits in error as a percentage of the total number of bits received.

Depending on the application, the BER requirements vary. For applications such as banking, a very high BER is required, of the order of 10⁻¹² i.e., out of 10¹² bits only one bit can go wrong (even that has to be corrected or a retransmission requested). For applications such as voice, a BER of 10⁻⁴ is acceptable.

As we discussed in the previous section, BER is dependent on the modulation technique used. The performance in terms of the BER also is dependent on the transmission medium. A satellite channel, for example, is characterized by a high bit error rate (generally, around 10⁻⁶; in such a case, a higher-layer protocol (data link layer) has to implement error detection techniques and automatic repeat request (ARQ) protocols such as stop-and-wait and sliding window.

Because it is not possible to achieve a completely error-free transmission, errors should be detected or corrected using error-detection or error-correcting codes. After detection of errors, using automatic repeat request (ARQ) protocols, the receiver has to request retransmission of data.

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