1.4.1 Network aspects of the digital interface
The output of the source encoder is usually segmented into packets (and in many cases, such
as email and data files, is already segmented in this way). Each of the network layers then
adds some overhead to these packets, adding a header in the case of TCP (transmission control
protocol) and IP (internet protocol) and adding both a header and trailer in the case of DLC
(data link control). Thus what enters the channel encoder is a sequence of frames, where each
frame has the structure illustrated in Figure 1.7.