, the term primary channel
has the following meanings:
1. The channel that is designated as a prime transmission channel and is used as the first choice in restoring priority circuits.
2. In a communications network, the channel that has the highest data rate of all the channels sharing a common interface.
Note: A primary channel may support the transfer of information in one direction only, either direction alternately, or both directions simultaneously.
Source: from Federal Standard 1037C and from MIL-STD-188