ISDN PRI - Introduction

The ISDN S2M interface is a four-wired, whole duplex interface multiplexed on the principle of time-sharing (TDM) with the transmission rate of 2048 kbit/s.

Its main application field is realization of the access between subscribers and the network in ISDN networks. This interface is a kind of implementation of digital trunks. S2M interface has a higher rate, as the S0 interface providing access with a basic rate, so we can access subexchanges, data terminals with high speed and videoterminals to the network through this interface.

Division of 2048 kbit/s transmission rate:

Marking of the two sides of the interface:

                             S0
                  +--+       |       +--+
                  |TEĆ---------------“NT|
                  +--+       |       +--+

        NT - network termination (network (NET) side)
        TE - terminal equipment  (subscriber (user) side)
    Interface is asymmetric, its two sides are not built at the same way. 
Other names of the interface: Elements of the standard, describing the interface: Interface is built on the basis of the OSI model recommendations describing the interface specify the first three layers of the model (physical, network and data connection).

ITU-T recommendations describing the interface:

Considering recommendations we can state, that the ISDN S2M interface is an ITU-T G.703 2048 kbit/s interface with a PCM31_CRC typed framing, taking its physical layer into consideration. (There is no CAS multiframe in the 16. timeslot, messages of channel D are forwarded from here in an HDLC-framed format.) Operation of the second and third layer of the interface compounds with the operation of data connection and network layer of the ISDN S0 interface. (There are small differences, e.g. the S0 can be a several points operation, while the S2M can provide only a point-to-point connection.)

Reference: AT&T Definity Vol. Implementation: Trunk Groups-> ISDN PRI 5-195...