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second period, the BER is 10

−7

, an average of 1 error in 10

6

bits. The

acceptable level of transmission is determined by the type of traffic.

For PCM voice traffic, bit errors manifest themselves as clicks and

a threshold of 10

−6

(1 click approximately every 15 seconds) is usually

considered acceptable. At this threshold the speech is intelligible, but

beyond it the clicks become annoying and intelligibility falls rapidly.

For data with error correction a higher BER of 10

−8

is normally

acceptable (Communications International, 1989).

11.6 Terminology

A number of different units are used worldwide to define the

performance and transmission levels of a radio relay system (for

decibel definitions see Chapter 2). Important international definitions

and units