primary rate interface (ISDN) (PRI)
The primary multiplex connection (PMxA) of ISDN is used to connect medium to large private branch exchanges and offers significantly higher transmission speeds compared with the basic connection.
The primary multiplex connection enables subscriber equipment to be connected to the local exchange, with a maximum information capacity of 30 basic channels, each with 64 kbit/s, and an additional D channel with a capacity of 64 kbit/s being available to the end user via the S2M interface. These channels are realized on PCM-30 based multiplex systems.
At the S2M interface, the useful signals and the control signals are combined in time division multiplex in a 2 Mbit/s frame. In each direction, 8,000 frames are transmitted per second, corresponding to a frame duration of 125 microseconds. The frame consists of 32 time slots of 8 bits each. 31 time slots are used for the 30 base channels and the D channel, the 32nd for internal controls. This is transmitted first in time slot 0. The total bit rate thus comes to 2.048 Mbit/s.
Another variant for the primary multiplex connection described under ITU Recommendation I.412, which is used in the USA, has 23 B channels and one D channel, which corresponds to a data rate of 1.544 Mbps. Most European countries use the 2 Mbit/s connection, which corresponds to the E transmission interface.
The PMxA can be up to approx. 250 m long. This length is determined by the maximum permissible attenuation. To be able to set up the PMxA over a further distance, intermediate regenerators must be used.
On the physical layer, the primary multiplex connection uses HDB3 coding behind the network termination on the user side, but different line coding on the network side: 1T/2B coding is used for optical fibers as the UG2 interface.