Primary Rate Interface vs. Basic Rate Interface

Last time, we began a retrospective look at ISDN. In particular, we examined the Primary Rate Interface in some detail, so this time we’ll move on to the Basic Rate Interface.

The BRI was designed to provide two bearer channels for digital voice (or data), for a total of 128K bit/sec. Additionally, the “D” channel consisted of 16K bit/sec of control data, for a total capacity of 144K bit/sec. For both the 23B+D format of the PRI and for the BRI, there was a little additional overhead from framing, so that the total bit rate is/was 1.544M bit/sec and 160K bit/sec, respectively.

At the time that the BRI was designed, it was envisioned that this would be primarily a residential service and small office (SOHO) type of service – and the implicit assumption was that 128K bit/sec would be sufficient bandwidth for this market. However, almost as soon as the service started to roll out, issues started cropping up. For instance, there were initial distance limitations so that one had to be relatively close to a central office to be a candidate for the service. And while these issues were resolved, they contributed to a slow start.

But in spite of this slow start, there are a few particularly salient points that ISDN brought to the telecomm marketplace:

1) ISDN signaled a move from analog to digital services at sub-T-1 speeds via the BRI.  Previously, one had to use an incredibly expensive, dedicated “DDS” 56K bit/sec (or slower) service – or make the jump to a T-1/E-1 service. 

2) ISDN services are switched – as opposed to dedicated point-to-point – services.

3) The B channels are formatted and switched as dedicated bandwidth.

4) The “D” channel is in a packet format.

5) Because the signaling is in the D channel, all 64K bit/sec of each DS-0 is available for data transport, which was previously limited to 56K bit/sec.

6) The fundamental format for the “D” channel became the basis for frame relay.

7) Even early in its lifecycle, ISDN started to meet resistance from the “data” side of the market. In fact, an enhanced service called Broadband ISDN (B-ISDN) provided the basis for ATM services; ATM was chosen as the service name for marketing purposes due to a perceived market liability in associating the ISDN name with the service.