Fractional T1, Voice T1, and PRI
A Fractional T1 line usually describes an Internet circuit that uses a piece or a fraction of a Full T1. Fractional T1s are used for telephone lines and integrated T1’s, but are used less often then for Internet access. Fractional T1s are sold in nearly any fraction of a twenty-four channel, 1.54Mbps circuit. For example, out of the twenty-four channels in a Full T1 line, 12 channels might be used in a Fractional T1 to provide a 768k connection, which is half of 1.54Mbps. Other Fractional sizes include 1.1Mbps, 512k, 384k, 256, 128k, and even 64k, although the smaller sizes are very uncommon. As T1 Providers have become more competitive, prices on Full T1s have come down so much that Fractional T1s have become less cost effective. For example, a Fractional T1 of 768k carries only half of the bandwidth of a Full T1, but usually saves only 5% to 15% of the price of a Full T1. This does not make much sense in applications where only one or two T1 lines are needed, unless the budget is extremely tight. However, this can add up to significant savings when using hundreds of T1's across a large company's multi-location VPN or frame relay network.
A PRI is also called a Primary Rate Interface line. It is somewhat different and more popular than a standard Voice T1 because it is true digital trunking, which allows for even more advanced calling features. A PRI line provides twenty-three telephone lines per T1, because the twenty-fourth channel is used to carry useful data for signaling and other features like special caller ID information called ANI is also known as a Automatic Number Identification. Since the transmission is digital, the quality of a telephone connection over a PRI line is unsurpassed, providing clear reception that is noticeable to the user.
The use of DID, or Digital Inward Dial numbers, and calling features like hunting and rolling, allow you to use twenty-three digital telephone lines to provide service to potentially thirty to forty employees, depending on how often they will need to use their phone. Instead of guessing on the number of DID's to put on a PRI line, it is important to figure out the maximum number of simultaneous calls that could be placed at any given time. Having too few telephone lines causes’ costs more in a loss of productivity than it would cost to add more phone lines to keep your employees busy. This is why it is important to figure out the proper balance of telephone lines vs. DID numbers. The cost of a PRI is also typically lower than other forms of telephone service.
Voice and PRI T1 lines are common among businesses and call centers with multiple telephone lines and large call volumes. They are far less expensive and more efficient than installing dozens of POTS lines. Each of the 24 channels on a T1 line is versatile, allowing it to be used for either bandwidth or for a single telephone line. Therefore, a pure Voice T1 line will have 24 telephone lines with reliable service, good reception, good long distance rates, depending on the carrier, and a wide selection of calling features.
