5.1 ISDN Architecture
5.1.5 PRI - Reference points

PRI technology is a bit simpler than BRI technology. The wiring is not multipoint, and there is only the straight connection between the channel service unit/data service unit (CSU/DSU) and the PRI interface (see the Figure).

A regular ISDN "S" interface can have several endpoint devices with different capabilities occupying the same bus (the "S" bus). When the switch can communicate with multiple devices, it is typically referred to as "multipoint." Unfortunately, this calls for complexity in terms of service profile identifiers (SPIDs) and endpoint identifiers (EIDs), both in ISDN device setup and call processing.

As a result, there is arbitration at Layers 1 and 2, so multiple devices can access the network without collisions or interruptions between devices that need to share the ISDN network. Because there are not multiple devices, PRI does not require this arbitration.