Information Conveyed Along An R2-signalled Trunk
A signalling protocol is best visualized by two contexts: what information it conveys and where its participants are in the network. This section presents those two contexts.
Each national variant in the family of R2 signalling protocols conveys at least the following, where forward is the direction from the dialling telephone's switch to the called telephone's switch and where backward is the direction from the called telephone's switch to the calling telephone's switch:
| R2 line signalling | acquisition (termed seizure) of an idle DS0 channel |
| R2 forward register signalling | the digits 1 through 10 of the destination telephone number (termed called-party address) |
| R2 forward register signalling | the digits 1 through 10 of the origin telephone number (termed calling-party address) |
| R2 forward register signalling | the digits 11 through 14 for special-service requests (e.g., route to operator, add echo-suppression) |
| R2 forward register signalling | the calling party's category (e.g., normal subscriber, high-priority subscriber, operator, coin-operated telephone) |
| R2 forward register signalling | the disposition of the routing of the telephone call attempt (e.g., called-party's telephone is currently busy, called-party's telephone is now ringing, called-party's telephone is out of service) |
| R2 line signalling | the called party has now lifted the handset from its hook to answer this call attempt in order to transition from R2 signalling toward the establishment of speech-capable audio to fully establish the call (termed going off hook) |
| R2 line signalling | the release of the call (e.g., the handset of the called-party's telephone has now been returned to its on-hook position, ending this call; the telephone company or trouble in the network is explicitly or implicitly forcing the ending of this call) |
| R2 backward register signalling | the set of explicit requests corresponding to each of the forward register-signalling data. The client–server requesting of each of these data differentiates R2 from the Bell System R1 MF-tone signalling, where in R1 the called-party's switch unilaterally sends some of these data as a timed sequence without explicit intervening requests from the calling-party's switch. |
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