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T-1 is a digital transmission system designed for transport of all traffic types (e.g., voice and data) over four wire copper facilities. It operates at the standard DS-1 rate of 1.544 Mbps, and is commonly configured in the telephony world as the channelized combination of 24 standard DS-0 channels running at 64 kbps each (e.g., 24 voice conversations).

T-1 was designed to replace noisy analog carrier facilities and to consolidate interoffice trunks. T-1 has become the industry standard for both interoffice and intraoffice connectivity. T-1 provides services for access to data, such as frame relay, to customer locations. In private networks, T-1 is commonly used for applications like interconnection of PBXs and LANs. Although higher-speed services are available (e.g., DS-3 and SONET), T-1 still remains a popular choice.

T-1’s benefits to the user are listed below.

  • Integration: All traffic types can be consolidated on a T-1 circuit.
  • Simplification of the communications network: Use of T-1 for network consolidation reduces the number of separate facilities, simplifying *the network and facilitating network management.
  • Economy: For organizations with large traffic volumes, the T-1 reduces the expense of multiple DS-0 circuits. Due to easy installation of the product and plummeting prices, smaller customers are also realizing economic benefits with T-1.

The T-carrier system is sometimes referred to as the North American digital carrier system, since its most widespread deployment is in the U.S. and Canada. T-carrier is also used in a number of other countries, in particular, Japan and South Korea. Defined by ETSI, the E-carrier system is used in the rest of the world. Most discussions concerning T-carrier apply equally to E-carrier since both systems use the DS-0 signal level as the basic building block.

T-1 Requirement

The requirement for T-1 connectivity in the telephone network came about primarily as a result of noise problems and a scarcity of copper loops between the subscriber and the central office, sometimes called the loop plant. In addition, the FDM analog carriers of the time were notoriously noisy when bundled into shared cables. A more efficient, less noisy multiplexing approach had to be developed in order to avoid the massive cost associated with increasing the capacity of the physical plant.

In the late 1950s, at Bell Telephone Laboratories, such a multiplexing technology was under development. Using TDM, the new T-carrier system would offer greater channel capacity and would be less noisy than the analog systems it was destined to replace. The first deployment of T-1 carrier was by Illinois Bell in 1962, when they connected their central office in downtown Chicago with the northern suburb of Skokie. The basic characteristics of T-1 have not changed to this day. T-1 has the capacity to carry 24 64 kbps (DS-0) channels at 1.536 Mbps (24 x 64 kbps = 1.536 Mbps). T-1 framing bits, required by the network equipment, add an additional 8 kbps of overhead to the T-1 stream. Thus, the aggregate T-1 line rate is 1.544 Mbps.

Today T-1 is still an extremely popular service with customers. Carriers often deliver the service to customers using a variety of technologies. The term T-1 implies very specific circuit requirements. However, carriers frequently use technologies with less stringent constraints around the loop characteristics and repeater spacing to deliver T-1 service to customers. High-speed digital subscriber line 2 (HDSL2) is commonly used.

T-1 Equipment

Terminating Equipment

The T-1 carrier network may be built with several types of equipment, depending on the application for the specific network. Typically, a simple point-to-point T-1 circuit contains three basic categories of equipment: terminating, user interface, and transmission equipment.

Terminating equipment accepts low-speed data streams, (e.g., voice), and multiplexes them into the 1.544 Mbps (DS-1) rate of a T-1 carrier. Channel banks are considered terminating equipment.

User interface equipment connects the terminating equipment to the T-1 circuit. It performs basic Physical Layer functions like line coding and framing to ensure compatibility with the network and often some error detection. An example of user interface equipment is the CSU.

Finally, the transmission equipment is the actual physical media: twisted-pair copper. This media, together with line repeaters used to correct for attenuation, exemplify the transmission equipment functions.

T-1 Terminating Equipment

T-1 terminating equipment gets its name because it terminates the T-1 formats and distributes the channels of the DS-1 signal level to various users (e.g., individual telephones, computer applications, and LANs).

The most common variety of terminating equipment in carrier environments is the channel bank, or T-1 multiplexer. This device uses synchronous time division multiplexing to combine many low-speed voice and data signals (subrates) onto the high-speed (aggregate) T-1 link. When presented with analog voice or modem inputs, the channel bank is responsible for analog-to-digital conversion (A/D), most often using the mu-255 encoding scheme. ISLUs that support the PRI are examples of carrier T-1 terminating equipment, as are DLC remote terminals.

Terminating equipment on a simple point-to-point customer application might be multiplexers at each end for the aggregation of low-speed services, or we might find direct router connections to LANs that need a high-speed dedicated WAN linkage. T-1 links between PBXs for the support of private voice networking are common elements of enterprise networking. T-1 links are also a common interface for frame relay service.

T-1 User Interface Equipment

When the LEC or T-1 access provider delivers service to the customer, it is normally at the network demarcation point (“demarc”), called an NIU. Handoff can be via an individual RJ-48 jack or a multiple-circuit RJ-21 telco interface. The NIU is commonly called a “smart jack” because of its ability to react to an incoming loop command from the CO. Customers are responsible for the purchase and installation of their own interface equipment. The CSU is the user interface to the T-1 facility. The NIU is a network provider-owned interface that supplies remote network loopback and test capability through the provider’s OAM&P system, and the RJ-48 provides the user connection.

User Interface Equipment

The CSU performs many functions necessary to maintain and use T-1 network connections. First, it provides a termination point for the four wire copper facilities. Second, it ensures that T-1 terminating equipment complies with T-1 line code specifications, such as ones density, bipolar AMI, proper voltage formats (+3 and -3 volts), and pulse width requirements. Other CSU functions include diagnostic and customer loopback capability, which are maintenance and test functions. Another important function provided by a CSU is generation of a “keep alive” signal that allows the network connection to be maintained when the terminating equipment is disconnected or not functioning. The CSU is customer-provided equipment and is powered locally by the customer power supply, not the telco.

Modern T-1 terminating equipment (e.g., PBX, router, frame relay access device, or multiplexer) often has the CSU function built into the box. However, upgrades to OAM&P functions provided by the networks might be more difficult and costly to deploy than for individual stand-alone or rack mounted plug-in CSUs.


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