Each T1 line has a data transmission speed of 1.54Mb per second – for both downloads and uploads – and utilizes separate pairs of fiber optic or copper wires for transmitting and receiving.

Integrated Services Digital Network (ISDN) offers two speeds of service:

• Basic Rate Interface (BRI) – intended for small business and residential application, BRI is a low volume service
• Primary Rate Interface (PRI) – a high capacity service that divides a T1 signal into 24 channels, and uses the 24^th channel for special features, including caller ID and automatic number identification, and signaling information.

Channelized T1 vs. Un-channelized T1

With channelized T1s, each of the 24 channels can be allocated to telephone calls; however, the signaling information identifying when a phone is off or on line is carried within each channel by “robbing” or using the least significant bit because there is no separate signaling channel.

Unfortunately, channelized T1s do not provide the capability necessary for caller ID or ANI – a PRI service is required.

Un-channelized T1s do not offer assigned signaling or dedicated phone channels for switched telephone service. Intended to be handled – not by PBX systems – but by digital routers, an un-channelized T1 considers all 24 channels to be one large, combined channel for VoIP broadband phone, point-to-point data transmission, and carrying Internet service.

There are two basic services that one can usually expect from an ISDN business line. The first is an ISDN connection with a basic rate interface (BRI), while the second is a connection that uses primary rate interface (PRI). In general, the two services are highly similar: they still use multiple channels to send data out of your network. They use these B channels to transmit data, while a secondary D channel controls and detects / acts upon signals sent by your transmitters. This is called “out of band” signaling, a style that helps you ensure that other ISDN calls won’t end up crossing over into your line. This way, you can assure that the bandwidth you spend on your B channel is dedicated only to the transmission of data. This setup is geared towards ensuring that the quality of your calls is never compromised.

Different types of ISDN

But what’s the difference between BRI and PRI? If they both send out information through multiple channels, what makes one service recognizable from the other?

BRI is a form of ISDN that uses the full capacity of 64 kbps lines to send and receive data in an orderly and efficient fashion. These two lines are both B channels, and the data sent is in full duplex, which means that the full capacity of the lines are used to ensure that the data you send out is crisp, clear, and uncompromised. It also uses a 16 K D channel as a signaling component.

Overall, BRI is capable of taking in a full 144 K load of full bandwidth in increments of 64+64+16. Because of the unique setup of its channels, some service providers refer to BRI as 2B+D, and can be transformed into a WAN using modular interface cards.

On the other hand, if you’re looking for a service that can fit your extremely high bandwidth needs, you need to get yourself a service that can work with you in order to give you the best ISDN quality possible. This is why PRI solutions are available throughout the industry – occasionally, getting a private line is a better option than spending a lot for a full-blown infrastructure installation.

A regular PRI channel is a high-bandwidth line that uses a variety of channels in order to transmit their data. Capable of reaching 64 Kbps of transmission speed, PRI uses twenty-three of these B channels and one 64 Kbps D channel to transmit data – which means that upon installing a PRI system in your office, you’re getting the equivalent of a T1 connection (that’s a total of 1.544 Mbps in channels). If you happen to live in Europe, you’re even luckier, because European PRI structures use thirty B channels, which add up to an entire megabyte of additional circuits to choose from.