A T1 line is a circuit for transporting voice or data traffic at a rate of 1.544 Mbps, or 1,544,000 bits per second. The reliability of a T1 line is much greater than standard telephone lines or DSL. Telephone companies tend to give T1 lines higher service level agreements (SLA) than common DSL or telephone lines. Most T1’s come with an SLA that will guarantee such things as speed, uptime, and latency (the time it takes for the signal to travel from one point to the other).



Digital “pipelines" or carriers are available in many different sizes that can supply virtually any data delivery requirement. The standard North American carriers are:

DS0 – 64 kilobits per second (equivalent to 1 voice grade line)

T1 – 1.544 megabits per second (24 DS0 lines)

T3 – 43.232 megabits per second (28 T1s)

OC3 – 155 megabits per second (100 T1s)

OC12 – 622 megabits per second (4 OC3s)

OC48 – 2.5 gigabits per seconds (4 OC12s)

OC192 – 9.6 gigabits per second (4 OC48s)

VOICE T1: A Voice T1 can handle up to 24 calls simultaneously, even more if the equipment has voice compression, which is “compressing" the standard 64 kbps bandwidth per call to as low as 8 kbps. However, there is no limit to how many telephone numbers that can point to a T1, which allows some companies to have hundreds of numbers pointing to a single T1. These telephone numbers are called Direct Inward Dialing (DID) numbers.

A DID can give a business great flexibility with their communications. Phone numbers can be programmed to ring directly to a department or to an individual’s desk, easing the workload for an overwhelmed receptionist, and give customers greater accessibility to their representatives. DID’s operate on the assumption that everyone in the company will not be on the phone at the same time.

CHANNELIZED T1: A Channelized T1 is a T1 that is split up into 24 equal channels. Each voice grade channel, called a DS0 (Digital Signal, level zero) can transport 64 kbps—64,000 bits per second. A Channelized T1 can plug directly into a PBX for phone service. These T1’s have no features. All the special features are provided by the PBX itself. Internet Service Providers also use this type of service to connect customers using dial up modems.

ISDN PRI: Another type of T1 is an Integrated Services Digital Network - Primary Rate Interface (ISDN-PRI), often referred to simply as a PRI. The difference between a normal T1 and a PRI is that a PRI uses one of the 24 channels available in a T1 as the D channel, or data channel. This channel carries the customer call information and control signals rather than an actual call. Consequently, a PRI can provide the information for Caller ID and screen pops, customer information and history that “pops up" on a compute screen prior to an incoming call being answered.

DATA T1: A Data T1 is exactly what the name implies, a T1 configured to transport data signals rather than voice traffic. Also called an Internet T1, a data T1 uses the full 1.544 Mbps for internet traffic. Data T1’s have becoming increasingly more popular with the increased demand for VoIP (Voice over Internet Protocol) services, as good VoIP service requires a reliable and stable internet connection to work properly.

PRIVATE LINE: T1’s can also be used as Private Lines. A private line is a dedicated circuit between two or more locations. The terms point-to-point and point-to-multipoint are used to describe this service. Private lines are used for direct and secure access between locations, providing a way to connect PBX’s or multiple servers in a private network that probably does not touch a public network or the Internet.

FRACTIONAL T1: A Fractional T1 is a T1 with limited access. The T1 is limited to a predetermined number of channels or bandwidth. A fractional T1 can be configured in the same format a full T1 can (channelized, PRI, data, etc.). Fractional T1’s are typically used for small businesses and small branch offices. A Fractional T1 will normally carry the same reliability and guarantees as a full T1.

INTEGRATED T1: An Integrated T1 combines both voice and data services on the same T1. Integrated T1 service can be configured by allocating a certain number of channels to voice service and leaving the rest of the bandwidth to data or internet service. When configured in this fashion, the bandwidth for the Internet never changes. The voice channels simply lay dormant when now phone calls are being made or received.

DYNAMIC T1: Another type of Integrated T1 is a Dynamic T1. A dynamic T1 is an integrated T1 that will instantly allocate bandwidth to a phone call on an as needed basis. When a call comes in or when an outside line is accessed, a device referred to as a CSU/DSU pulls the required bandwidth needed for the call and dedicates it to that conversation. As soon as the call is completed the bandwidth is released and again made available for data use.

BONDED T1: A Bonded T1 can give speeds over 1.544 Mbps. Bonded T1’s combine multiple T1’s to make them work as a single circuit.

FRACTIONAL T3: A T3 is a carrier which is made up of 28 T1's. Occasionally, when a user requires more bandwidth than a single T1 can provide, upgrading to a fractional or dynamic T3 is a better soltuion, dpending on pricing and bandwidth requirement. From the user's standpoint, there is no bandwidth difference between a bonded T1 circuit and a fractional T3 circuit, as long as both have the same number of T1s in the circuit.