S2M interface
The S2M interface is part of the primary multiplex connection(PMxA). It provides 30 basic channels, which are the B channels, each with 64 kbit/s and additionally one signaling channel, which is the D channel, with a capacity of 64 kbit/s, which is used to transmit all the information required for signaling between the subscriber area and the local exchange.
The net transmission capacity is 1,984 kbit/s. This is also the origin of the abbreviation S2M, which is based on S2Mbit/s. In the USA, the PRI interface(Primary Rate Interface) supports 23 B channels and one D channel. The data rate thus corresponds to that of DS-1 with 1.544 Mbit/s.
The individual functions of the S2M interface include:
The B channels: The S2M interface provides up to 30 independent B-channels, each with 64 kbit/s, which can be used full- duplex. The user data is transmitted in the B-channels.
The D channel: The signaling channel also has a transmission rate of 64 kbit/s. It is used for line-bound out-band signaling.
The bit clock: The bit clock is the clock rate at which the analogsignal is digitized. It is 64 kbit/s. From the bit clock, the network termination can derive information from the entire bit stream.
The octet clock: This 8- kHz clock is used to correctly assign channels to the corresponding channel time segments of a frame.
Frame synchronization: This signal is used to synchronize the transmitter and receiver. For this purpose, a special bit pattern is transmitted in time channel 0.
The CRC procedure: Cyclic block checking is used to prevent faulty synchronizations
The supply: The NT2M network termination is supplied from the telecommunications system via separate lines. No emergency power supply from the ISDN DIVO is possible. The S2M interface is constantly active.