Secure Communications Interoperability Protocol - Signalling Plan (210)

Signalling Plan (210)

The SCIP signalling plan in common to all national and multinational modes of SCIP. SCIP has two mandatory types of transmission. The mandatory data service uses an ARQ protocol with forward error correction (FEC) to ensure reliable transmission. The receiving station acknowledges accurate receipt of data blocks and can ask for a block to be re-transmitted, if necessary. For voice, SCIP simply sends a stream of voice data frames (typically MELPe frames, but possibly G.729D or another codec if that has been negotiated between the terminals). To save power on voice calls, SCIP stops sending if there is no speech input. A synchronization block is sent roughly twice a second in place of a data frame. The low order 14 bits of the encryption counter are sent with every sync block. The 14 bits are enough to cover a fade out of more than six minutes. Part of the rest of the state vector are sent as well so that with receipt of three sync blocks, the entire state vector is recovered. This handles longer fades and allows a station with the proper TEK to join a multi station net and be synchronized within 1.5 seconds.