Digital Trunking and the T1 PRI Protocol

The T1 or Primary Rate Interface (PRI), in simple terms, describes a digital telephone line that carries a large volume of voice calls through a single circuit. It is the typical method in which larger offices with considerable call volumes receive telecommunication services.

A T1 line is a single digital circuit that provides a bandwidth of up to 1.5Mbps. By using a method called time division multiplexing (TDM), this circuit can be divided into 24 different channels each with a bandwidth of 64kbps. This allows a business to have up to 23 voice calls over a single circuit (the 24th channel is used for control signaling).

When business communication needs grow, a T1/PRI circuit is often considered. As you approach 23 analog telephone lines at your business, you will find that the cost becomes more than the cost of a single T1/PRI circuit. There are a variety of reasons for this, but the main one is the cost to maintain the 23 individual pairs of copper that provide the analog lines. With a T1/PRI, only a single circuit needs to be maintained for up to 23 voice calls.

Along with regular inbound and outbound calling, T1/PRI circuits also feature Direct Inward Dial (DID) services. DIDs provide outside callers with the ability to dial an employee in the business without going through a receptionist or auto-attendant. This allows businesses to give each employee a phone number that may be called directly without adding additional lines.

T1 circuits can also be used to network remote phone systems within the same business. For instance an office in Michigan can be networked using a T1 tie trunk to another office in California. A user in Michigan can then connect to a user in California simply by dialing their extension number. This bypasses tolls and allows employees in the business to communicate seamlessly. Routing tables can also be set up to find the most cost effective ways to distribute calls through the Michigan and California offices.

Typical PBXs that service the medium sized business market, allow up to 3 T1/ PRI circuits to be added to the system. Larger enterprise class PBXs like those used in call centers can add many more.

With T1 PRI, businesses can enjoy a large number of communication channels at a lower cost and also get great features standard, including Caller ID and call forwarding. It is an extremely cost effective option for large businesses that need a multitude of telecommunication services.

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