What is a T1 line and what's it used for ?

T1 Definition: T1 is a member of the T Carrier family. Originally called a T1.5, the "T" was used by AT&T to distinguish between a terrestrial network as opposed to satellite. T1 is a standard 1.544Mbps (Megabits or million bits per second) carrier system used to transport 24 "time division multiplexed" (TDM) 64 Kbps (Kilobits or thousand bits per second) channels (DS0's) of voice or data services, using pulse code modulation, from one point to another.

Often interchanged carelessly with the term "DS1" or "Digital Signal One", there is an important technical difference. The "T Carrier" (T1) is the type of transmission system. The T Carrier can be any terrestrial network capable of carrying the 1.544 Mbps signal, including wire, which was the original medium, fiber and microwave radio. The "DS1" is the electrical signal that carries the information, and it conforms to DS1 "physical layer" electrical standards for voltage, pulse shape, etc..

This DS1 signal carries the information in various DS1 formatted frame patterns including D1, D2, D3, D4, DCT, ESF, etc., and is digitally encoded within the Pulse Code Modulation using either B8ZS or AMI coding techniques. The wire T Carrier (T1) carries the DS1 frames over a -135 Volt four-wire circuit, two wires for transmit and two wires for receive. T1 is the standard carrier for the United States, Canada, Japan, and Singapore. All other countries use the E1 standard, (30 channels on four wires). A T1's use of voice or data determines the variables of framing format and digital line encoding. The 24 channels within the T1 can be split into any combination of voice and N x 64 Kbps data, up to a total of 1.544 Mbps. If all channels of the T1 are not used, this is called a Fractional T1 (FT1). If you're a real 'techie', click here for almost everything you ever wanted to know about T1, including an in depth technical explanation and history of T1.

What's a T1 used for ?

Dedicated T1 Internet access - A dedicated T1 connection to the Internet is a high speed connection that is always on. It's comprised of a T1 local loop that takes you to the ISP's Point of Presence (POP) and a full T1 speed connection to the Internet at the POP. Unlike dial-up, DSL, cable or satellite, dedicated Internet access carries a Service Level Agreement (SLA) that guarantees at least 99.9% of the T1 bandwidth will be available to you at least 99.9% of the time or you will be refunded for a portion or all of the days on which full access was not acheived. Unlike all the other mentioned types of Internet access, dedicated access, properly delivered, is never shared, thus always providing you with maximum speed that should rarely be subject to the slow downs shared services experience at peak hours.
Dedicated T1 Long Distance Service - Bypass the local telephone company by leasing a T1 local loop directly to your long distance carrier's switch. You can save a bundle this way. Be sure to read how in our article All About Dedicated T1 Long Distance.
T1/PRI Local Service - If you have lots of phone lines, T1 local service is an alternative to using many individual pairs of wires to bring in all those phone lines. One T1 carries 24 voice lines over one set of 4 wires. There are other advantages as well, such as digital quality, fast call setup and, sometimes, access to better local and/or long distance rates.
Private Line - a private dedicated "inter-office" circuit that is always on and is used to carry voice or data between 2 locations. Businesses, ISP's, local and long distance telephone companies use private lines T1's to carry voice, data or a combination thereof from one location to another.
Local Loop - A local loop is the "last mile" or last leg of the route required to deliver service to and from the telephone company, ISP, cable company or other service provider's central office or head end equipment. In the case of a long distance or local service T1, the local loop is a private line that you lease for a flat monthly fee which delivers service to and from your local or long distance carrier's switch.
Frame Relay - A packetized data service/protocol used for private networking and local loop transmission protocol for access to the public Internet.

Related articles and resources:
All About Dedicated T1 Long Distance