Protocol control information

In telecommunication, the term protocol-control information or PCI has the following meanings:

1. The queries and replies among communications equipment to determine the respective capabilities of each end of the communications link.
2. For layered systems, information exchanged between entities of a given layer, via the service provided by the next lower layer, to coordinate their joint operation.

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".