Monday, January 07, 2008

ISDN's Primary Rate Interface

ISDN, the Integrated Services Digital Network, is a digital telephone standard that made voice and data services widely available long before the more recent deployment of Enterprise VoIP and MPLS networks. ISDN's Primary Rate Interface is more in demand today than ever. That's because it is tightly integrated with the installed base of telephone trunk lines that are almost universally available.

ISDN was the telcos' response to a change in technology that saw modems being installed on analog telephone lines to transfer data over long distances. The toll network was only designed for a limited voice bandwidth and tops out at about 56 Kbps when used as data bandwidth. In addition, analog modems use the same frequencies as telephone audio. You can use the line for data or voice, but only not both at the same time.

ISDN drew on existing digital carrier technology based on 64 Kbps channels. One channel can carry one telephone conversation, once the call is digitized using a CODEC or Coder / Decoder. These channels are the building blocks of T1, DS3, SONET and other digital carriers. The smallest unit is the DS0 which is a single 64 Kbps channel.

What ISDN did was to create two services for digitized voice and data. Note that once voice is digitized it actually is a data stream and can be carried by a digital trunk line side by side with computer data. Unlike VoIP, which consists of voice packets that all flow down the same line with data packets, ISDN keeps the voice and data channels separate.

The BRI or Basic Rate Interface consists of two 64 Kbps B or bearer channels plus one 16 Kbps D or delta channel. Each of the bearer channels can carry one telephone conversation or a 64 Kbps data link. The delta channel is used for signaling and other services such as Caller ID.

BRI was originally intended for residential and small business use. With a ISDN BRI connection you could have a telephone line and a 64 Kbps Internet connection, 2 telephone lines, or combined data of 128 Kbps. BRI was never widely adopted because other services such as DSL and Cable Internet became available with far greater bandwidths.

While BRI came too late to be a dominant digital line service, PRI or Primary Rate Interface has flourished. PRI consists of 23 bearer channels and 1 delta channel. Shorthand for this is 23 B + D. In this case all channels are 64 Kbps. The larger D channel can handle signaling for all 23 of the other channels. The total of 24 channels is exactly the right size for a T1 line, which is widely available. Thus, T1 lines are used to carry ISDN PRI. In this case, they are often called T1 PRI lines or T1 PRI service.

What has made T1 PRI such a popular service is that it is the perfect digital trunk to support PBX or Private Branch eXchange telephone systems used by businesses and call centers. Each T1 PRI line can replace up to 23 analog telephone lines. It's usually much more cost effective to order T1 PRI service than to bring in a dozen or more separate analog lines. Using the D channel to send signaling to the telephone company and receive Caller ID information results in faster call set-ups and disconnects, which can be important in busy call center operations.

Another popular use for T1 PRI has been to support modem banks. A dozen or more dial-up modems can be deployed by an Internet Service Provider at a single location and will communicate with the ISP via the T1 PRI connection. Each modem is supported by a single bearer channel.

Is ISDN PRI service priced right for your company? You can find out with a quick online price and availability search right now. Our friendly consultants will also be happy to review your needs and recommend the most cost effective solutions from a variety of competing carriers.

Click to check pricing and features or get support from a Telarus product specialist.

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