T-carrier is a series of digital communication services provided by telcos for high-speed permanent voice and data connections that was first developed by Bell in the 1960s. Read the full article.

What is T-carrier?

A series of digital communication services provided by telcos for high-speed permanent voice and data connections. T-carrier services were first developed by Bell in the 1960s for digital transmission of analog voice communication. Telcos typically use T1 lines to connect telephone exchange switching equipment within the telco’s central office (CO).

Common uses for T1 lines from a networking perspective include the following:

  • Building dedicated, high-speed wide area networks (WANs)
  • Providing corporate networks with high-speed access to the Internet
  • Connecting corporate intranet/extranet web servers to the Internet
  • Providing high-speed remote access solutions for companies with mobile users
  • Providing integrated voice/fax/data services to businesses

Graphic T-3. Uses for T1 routers.

How T-carrier Works

The T-carrier system is based on the DS1 signaling standard defined by AT&T. A DS1 channel is formed from a combination of 24 DS0 (Digital Signal Zero) channels with 64 Kbps of bandwidth each, for a total bandwidth of 1.544 Mbps. This configuration is called a T1 circuit and is the base circuit from which other T-carrier circuits are derived. The 24 DS0 channels can either be used separately for voice and data or be combined by using a technique called time-division multiplexing (TDM), in which voice or data information from each channel is interleaved into a single bit stream. A DS1 frame is thus composed of 1 byte (8 bits) from each DS0 channel plus 1 bit of framing control. The transmission rate of frames is set at 8000 frames per second, which means that the total bandwidth of a T1 circuit or DS1 communication channel can be calculated using this formula:

T1 = 8000 frames/sec x ((24 x 8) + 1) bits/frame
= 1544000 bits/sec
= 1.544 Mbps

Because of the framing bit, the actual bandwidth of a T1 line for data transmission is slightly less than this, at 1.536 Mbps. Synchronization is maintained between T1 equipment at the customer premises and the telco CO by varying the framing bit using a predetermined algorithm. The most flexible T1 solution for customer premises is to use a T1 channel bank to interface local area networks (LANs), Private Branch Exchanges (PBXs), and telephone and fax equipment with a T1 circuit leased from a local telco.

Other special-purpose T1 equipment that you can buy or lease includes

  • T1 CSU/DSU, a Channel Service Unit/Data Service Unit for connecting bridges or routers to T1 circuits
  • T1 MUX, a multiplexer for aggregating T1 circuits for high-speed communication
  • T1 bridges and routers, with or without integrated T1 CSU/DSUs, for dedicated point-to-point or multipoint WAN connections for enterprise-level internetworks
  • T1 access routers for providing remote access services and allowing multiple, simultaneous remote access connections to be channeled through a single T1 line at the customer premises

Other T-carrier services provided by telecommunications carriers include the following:

  • Fractional T1 circuits, consisting of 4, 8, 12, or more DS0 channels multiplexed together
  • Multiples of T1, such as T2, T3, and T4, as shown in the following table
T-Carrier Services

T-Carrier Service Number of DS0 Channels Bandwidth
1.544 Mbps
4 x 24 = 96
6.312 Mbps
18 x 24 = 432
44.736 Mbps
168 x 24 = 4032
274.176 Mbps

T-carrier services such as T1 lines are often used to provide networks with high-bandwidth, permanent WAN connections between sites. T1 is the preferred technology for combining voice, fax, and data transmissions over an enterprise-level internetwork. T1 lines can be expensive because, whether or not they are being used, they are always “on.” A cheaper solution is to lease a fractional T1 service such as 4 x DS0 = 256 Kbps and then upgrade it to higher speeds as necessary. Fractional T1 is usually cheaper than using individual DS0 circuits at the customer premises and multiplexing them together.

T1 cannot be run over Plain Old Telephone Service (POTS) lines. They must use specially conditioned two-pair copper lines. Two wires are used for transmission (TX interface) and two for receiving (RX interface) for full-duplex communication. A repeater is typically required every 915 meters or 3000 feet to regenerate the signal. T1 lines typically terminate at the customer premises using an RJ-48 jack, which looks like an RJ-45 jack but is pinned differently. T1 lines can use unshielded twisted-pair (UTP) cabling, coaxial cabling, or fiber-optic cabling.

T1 circuits use either AMI (Alternate Mark Inversion) or B8ZS line coding. AMI encodes zeros as 01 and ones alternately as 00 and 11. B8ZS substitutes a special byte if eight consecutive zero bits are detected in order to maintain a specific ones density to help maintain synchronization. (Ones density is a scheme that allows a CSU/DSU to recover the data clock reliably.

The CSU/DSU derives the data clock from the data that passes through it. To recover the clock, the CSU/DSU hardware must receive at least one 1-bit value for every 8 bits of data that pass through it. Ones density is also called pulse density.)


To test T1 customer premises equipment (CPE) such as channel banks and CSU/DSUs, you can use a cable simulator, a passive device that simulates a standard 22-gauge twisted-pair T1 line that is 400 meters (1310 feet) long. Connect two cable simulators between your CPE and your T1 test equipment using the TX and RX interfaces to analyze the performance of your device. (Or use 400 meters of 22-gauge twisted-pair cabling instead!)

A “wet” T1 line carries a small DC current of about 140 mA (milliamperes) at several hundred volts for powering the CSU/DSU at the customer premises. “Dry” lines carry no current, so CSU/DSUs must be powered from the customer premises. Don’t touch a T1 line - a wet line can give you a serious shock!

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