There are two main services associated with ISDN – Basic Rate Interface (BRI) and Primary Rate Interface (PRI). Both services consist of multiple channels over which data can be sent (known as B channels) and also include a signaling channel (the D channel). The D channel is used for control and signaling purposes, such as setting and tearing down ISDN call. Referred to as “out-of-band” signaling, this method ensures that other ISDN calls do not interfere with existing connections, that bandwidth on the B channels is reserved for data only, and ultimately results in quicker call setup and teardown.
Basic Rate Interface (BRI) ISDN
Basic Rate Interface ISDN is made up of two 64 Kbps B channels that are used for sending and receiving data in full duplex, and one 16K D channel for signaling. In total, an ISDN BRI interface provides 144K of bandwidth (64+64+16). ISDN BRI is often referred to as 2B+D. Many Cisco router models include built-in BRI interfaces, but they can also be added using modular WAN interface cards.
Primary Rate Interface (PRI) ISDN
For companies with higher bandwidth requirements, ISDN Primary Rate Interface (PRI) service exists. In North America, PRI service consists of 23 64 Kbps B channels, and one 64Kbps D channel, for a total possible bandwidth of 1.544Mbps (T1 equivalent). In Europe, PRI service consists of 30 B channels and 1 D channel, for a total bandwidth of 2.048Mbps (E1 equivalent). ISDN PRI interfaces are typically implemented as modular WAN interfaces on Cisco routers, although some models do include built-in PRI ports.