Previous Table of Contents Next


L-carrier (L = Larger) was quite an improvement, employing coaxial cable and an analog transmission scheme. L5E, the last L-carrier system, used 22 coaxial pairs (tubes) to carry a total of 132,000 simultaneous voice grade conversations. Although this was an impressive improvement over N-carrier, the inherent problems of analog transmission were still present. Additionally, the coax cables were expensive and bulky, and the analog Radio Frequency (RF) amplifiers were expensive and prone to failure [8-2] and [8-3].

Digital carrier was first introduced by the U.S. Bell System in 1957 in Newark, New Jersey in 1957. Designated T-carrier (T = Trunk), it referred to a specific set of cable pairs and digital repeaters spaced every 6,000 ft. [8-5]. T-carrier was rapidly and extensively deployed throughout the carrier networks, initially for short-haul interexchange trunking [8-2]. T1, also known as digroup (digital group), was first offered commercially by AT&T in 1977 on the basis of a special assembly tariff, and was added to the interstate DDS Tariff #267 in December 1981 [8-6]. In 1983 it was tariffed by AT&T under the name Accunet 1.5. T1 provides 24 channels based on a convenient multiple of 2 x 12 channels that formed the basis for the original analog N-carrier [8-2]. Currently, there are about 200,000 private and public T1 links in service, and about 1,200 private networks are based on T3 [8-7].

T-carrier Concept

T-carrier is a dedicated, digital, leased-line service offering that employs time division multiplexing (TDM) in order to derive multiple channels from a single four-wire circuit operating in full duplex (FDX) transmission mode. In capsule, T-carrier offers the advantages of digital error performance, increased bandwidth, and improved bandwidth utilization. As is the case with digital services it also delivers increased management and control capabilities to the carriers and end users. Additionally, T-carrier is medium-independent; it can be provisioned over twisted-pair, coax, microwave, satellite, infrared, or fiber optic cable. As is the case with any dedicated service offering, T-carrier cost is sensitive to distance and bandwidth. While T-carrier initially was deployed in support of voice transmission, it supports data, image and video, as well. Further, T-carrier will support any and all such information streams on an unbiased basis—it offers the advantage of supporting integrated communications. As shown in Figure 8.3, T-carrier can obviate the need for multiple voice, facsimile, data, video and image networks [8-3] and [8-4].


Figure 8.3  T-carrier as a replacement for multiple dedicated, leased-line networks.

The significance of T-carrier extends well beyond its practical advantages. Specifically, as the first digital carrier system, it set the standards for digital transmission and switching, including the use of Pulse Code Modulation (PCM) for digitizing analog voice signals. T-carrier not only set the basis for the North American digital hierarchy (Table 8.1), but it led to the development of similar standards, such as E-carrier in Europe (Table 8.2) and J-carrier in Japan. Ultimately, the CCITT (now ITU-T) developed international standards in order to ensure interconnectivity. Although T-carrier, E-carrier, and J-carrier are very different in terms of certain specifics of the protocols employed (e.g., transmission rates, encoding techniques. and signaling and control methods), their basic characteristics are much the same.

Table 8.1 North American Digital Carrier Hierarchy (T-carrier)

Digital Signal (DS) Number Data Rate (Mbps) Number of 64 Kbps Channels (DS-0’s) Equivalent Number of Tx’s

DS-1 (T1) 1.544 24 1 T1
DS-1C (T1C) 3.152 48 2 T1
DS-2 (T2) 6.312 96 4 T1, 2 T1C
DS-3 (T3) 44.736 672 28 T1, 14 T1C, 7 T2
DS-4 (T4) 274.176 4032 168 T1, 84 T1C, 42 T2, 6 T3

Table 8.2 International (ITU-T) Digital Carrier Hierarchy (E-carrier)

Level Number Data Rate (Mbps) Number of 64 Kbps Channels (DS-0s)

1 2.048 30
2 8.448 120
3 34.368 480
4 139.264 1920
5 565.148 7680

Channelized T-carrier

The basic component of T-carrier is a 64 Kbps channel, referred to as DS-0 (Digital Signal #0). Digital carrier is a channelized service, at least in a standard implementation. In other words, a single, high-capacity digital circuit supports multiple logical channels, with each channel supporting a separate conversation. A T1 circuit, for instance, operates at 1.544 Mbps, supporting a standard 24 Time Division Multiplexed (TDM) channels, each of 64 Kbps. E1 supports 30 TDM channels of 64 Kbps; J1 supports 24 channels as does T1.

T-carrier hierarchy standards (Table 8.1) were set by the American National Standards Institute (ANSI) in its T1.107 specifications. Beginning at the T1 level, the hierarchy progresses up to T3, which provides bandwidth of approximately 45 Mbps in support of 672 channels. Most end users subscribe to T1 services, as one or more T1’s satisfy their bandwidth requirements. Generally, the next step is T3, with the intermediate levels being unusual in end-user implementation, although they are employed extensively in the carrier networks. As one might expect, as the data rate increases, the carrier reference frequency increases, and issues of signal attenuation and crosstalk increase; this fact creates special engineering problems [8-6] that can be resolved by various means including spacing repeaters more closely together [8-5].

The process of transmitting data (voice, data, video, or image) in given and consistently repeated channels, or time slots, is know as byte interleaving. Each byte of a given conversation to be transmitted is accepted by the MUX, assuming capacity is available, and is assigned one or more time slots. In a baseline example, those 64 Kbps time slots are reserved for that conversation, with the MUX providing the transmitting device with regular and repeated access to them for the duration of the communication.


Previous Table of Contents Next