# R15 signaling

R15 TCPs implement a number of different protocols used in the Russian telephone network. The protocols run on E1 trunks. Two separate TCPs are used for inbound and outbound calls, since the protocols' structure excludes the possibility of two-way trunks. They are named r150.tcp and r151.tcp. Separate call setup schemes are used for local and long distance calls as well. Both TCPs support both schemes.

R1.5 protocols use two signaling bits (A and B) per direction to control the state of calls. Line signaling is a combination of signaling bit variations and in-band tones.

Register signaling is usually performed by acknowledged MF tones. This means that the outbound equipment plays a forward tone (representing one of the address digits), and the inbound equipment requests the next tone by playing a backward MF tone. Both tones are timed, not compelled.

However, it is possible for the outbound to be requested to switch to decadic pulse dialing during setup of a call, since not all switches in the Russian networks are equipped with MF tone detectors.

MF-pulse shuttle protocol is the default register signaling for both inbound and outbound protocols and can be combined or replaced with other types of register signaling.