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ISDN Connectivity

by Jeff Newman and David Willis

What ISDN Is: BRI and PRI

Because ISDN is a digital transmission service, as opposed to the analog method that POTS connections use, it is highly reliable and eliminates the need to modulate and demodulate (modem) data before transmitting between digital devices such as computers, routers, and so on. Basic Rate Interface (BRI) is the most commonly implemented of ISDN's flavors, and it allows digital transmission of data at up to 128 Kbps reliably (and up to 512 Kbps with compression). Because its physical medium consists of the same single pair of copper wire that POTS uses, ISDN can be implemented over most of the already-existing telephone wiring found in residential households and business facilities. Most telephone installations contain two pair of copper wire, with the second pair commonly go ing unused, so the need to rewire a business or residential location is eliminated in most situations, which greatly reduces the time and cost of ISDN installation and allows the POTS connection to coexist with the ISDN connection.

BRI ISDN is made up of three logical channels--D, B1 and B2. Each channel represents a slot in a time-division multiplexing (TDM) scheme with the local telephone switch, and it creates a logical interface for the end point equipment connected to the circuit. The D channel operates at only 16 Kbps, but serves an important role. The D channel carries a wide variety of information about the ISDN call, which we will discuss in more detail later, and it can carry packet switched data. One of the D channel's most important tasks is the signaling for setting up calls and dynamically allocating the remaining bearer, or B, channels.

Each B channel can be assigned to operate either in voice mode or data mode and it can carry data at up to 64 Kbps. Each channel has the capability to operate independently of the other channel. As we will discuss in detail later, the two B channels can also be combined to aggregate the bandwidth of the connection. BRI can be used to carry voice and/or data concurrently to any other ISDN connection, allowing for dynamic connections and flexible networking.

Primary Rate ISDN (PRI) is like BRI on steroids. It functions essentially the same as BRI, only it has 23 B and one D channel. There are different implementations of PRI in different locations around the globe. Some have different numbers of B channels and variations of services. North American PRI typically comes in the 23 B + D channel configuration. Unlike BRI, PRI's D channel operates at 64 Kbps along with the 64 Kbps of each of its 23 B channels.

You now say to yourself, "Wait a second. That's 1.5 Mbps--the same as a T1 link?" Well, that's exactly what it is, in fact, PRI operates as a service for an existing T1 link. Think of PR I as a T1 with brains. PRI is not meant to provide 1.544-Mbps aggregated throughput, but instead is mostly meant to provide a flexible and manageable pipe to a large amount of wide-area networking services.

PRI gives a T-1 customer the ability to allocate dynamically each of the channels for voice, data or any other service that can be provided by a digital link on the fly, without having to involve the telephone company. When the right equipment is used to delegate the many services over the connection, PRI can be a powerful, if expensive, technology.

May 16, 1996