T Carrier Technical Summary

What is a T Carrier?

T Carrier is a generic name for any of several digitally multiplexed carrier systems. T Carrier systems were originally designed to transmit digitized voice signals. Current applications also include digital data transmission. T1 and T3 are the most popular T Carrier links used in United States and Canada. E1 and E3 are similar links used in Europe, and J1 and J3 are similar links used in Japan. The North American, European, and Japanese versions differ somewhat in the transmission rates and signaling protocols used. The "T" in T Carrier stands for "Trunk". If an "F" precedes the "T", it refers to an optical fiber cable system, but at the same speeds.

T Carrier Rates

The following tables list the designators and rates for current T Carrier systems:

North American Hierarchy
Designator	Transmission Rate	Number of Voice Channels
DS-0	64 Kbps	1
T-1 (DS-1)	1.544 Mbps	24
T-1C (DS-1C)	3.152 Mbps	48
T-2 (DS-2)	6.312 Mbps	96
T-3 (DS-3)	44.736 Mbps	672
T-4 (DS-4)	274.176 Mbps	4032

European Hierarchy
Designator	Transmission Rate	Number of Voice Channels
E-0	64 Kbps	1
E-1	2.048 Mbps	30
E-2	8.448 Mbps	120
E-3	34.368 Mbps	480
E-4	139.268 Mbps	1920
E-5	565.148 Mbps	7680

Japanese Hierarchy
Designator	Transmission Rate	Number of Voice Channels
J-0	64 Kbps	1
J-1	1.544 Mbps	24
J-1C	3.152 Mbps	48
J-2	6.312 Mbps	96
J-3	32.064 Mbps	480
J-3C	97.728 Mbps	1440
J-4	397.200 Mbps	5760

T-1

T-1, also spelled T1, stands for Trunk Level 1. It is a digital transmission link with a total signaling speed of 1.544 Mbps (1,544,000 bits per second). T-1 is a standard for digital transmission in North America - the United States and Canada. It is part of a hierarchy of digital transmission pipes known generically as the DS (Digital Signal Level) hierarchy. T-1 equates to DS-1 in the DS hierarchy.

A T-1 link has the following characteristics:

• Four wire circuit. T-1 originated from phone circuits using two pairs of unshielded twisted copper wires - one pair for transmit and one pair for receive. Today, T-1 is often delivered on fiber optic media. It can also run over other media types such as coaxial cable, microwave, or satellite systems.
• Full duplex. Transmission and reception of data can take place simultaneously.
• Digital. T-1 is an all digital service. Data, analog voice, and analog fax are all converted into digital pulses (1s and 0s) for transmission on the line.
• Time-division multiplexing. T-1 consists of digital streams carrying 64-kbps in each channel. 24 channels are multiplexed together to create an aggregate of 1.536 Mbps. Time-division allows a channel to use a slot one-twenty-fourth of the time. These can be fixed time slots made available to each channel.
• Pulse code modulation. Analog voice or any other service is sampled 8,000 times a second. An 8-bit word represents each sample, thus yielding a 64-kbps channel capacity.
• Framed format. As the pulse code modulation scheme is used, the 24 channels are time-division multiplexed into a frame to be carried along the line. Each frame represents an 8-bit sample for each of the 24 channels. Added to this is the framing bit. The net result is a 193-bit frame. There are 8,000 frames per second, therefore a frame is 125 microseconds long. Framing accounts for 8-kbps overhead (1-bit x 8,000 frames). Adding this 8-kpbs to the 1.536 Mbps described above yields an aggregate of 1.544 Mbps.
• Bipolar format. T-1 uses an electrical voltage across the line to represent the pulses (1s). The bipolar format serves two purposes: it reduces the required bandwidth from 1.5 MHz to 770 kHz (which increases repeater spacing), and it averages out the signal voltage to zero to allow dc power to be simplexed on the line to power intermediate regenerators. Every other pulse will be represented by the negative equivalent of the pulse. For example, the first pulse will be represented by a positive 3 volts (+3V), the next pulse will be represented by negative 3 volts (-3V) and so on. This effectively yields a 0 voltage on the line, since the positives and negatives equalize the current. This bipolar format is also called alternate mark inversion (AMI). The mark is a digital one (1). Alternate ones are inverted in polarity (+, -).
• Byte synchronous transmission. Each sample is made up of 8 bits from each channel. Timing for the channels is derived for the pulses that appear within the samples. If a long series of zeros are transmitted, the device on the receiving end of the line may lose synchronization with the transmitter due to the lack of pulses on the line. This can cause bits that are part of one channel to be mistakenly interpreted as part of another channel. To avoid this problem, techniques such as bit stuffing and bipolar 8 zero substitution (B8ZS) are used to ensure that 1s appear on the line frequently enough to maintain synchronization.
• Channelized or unchannelized. Generically, T-1 is 24 channels of 64 kbps each plus 8 kbps of overhead. This is considered channelized service. However, the multiplexing equipment can be configured in a number of ways. For example, the T-1 can be used as a single channel of 1.536 Mbps. Or it can be configured as two high-speed data channels at 384 kbps each, and a video channel at 768 kbps. Or as a high-speed data channel of 512 kbps, plus 16 channels of lower speed data and voice at 64 kbps each. These examples can be mixed into a variety of offerings. The point is that the service does not have to be "channelized" into 24 separate streams, but can be "unchannelized" into any usable data stream needed (equipment allowing of course).

Fractional T-1

Leasing a T-1 line means paying for the entire 1.544 Mbps bandwidth 24 hours a day, whether it is used or not. Fractional T-1 (FT-1) lets you lease any 64 kbps submultiple of a T-1 line. You might, for example, lease only six of the 64 kbps channels for an aggregate bandwidth of 384 kbps. Fractional T-1 is useful whenever the cost of a dedicated T-1 would be prohibitive.

T-3

T-3 is the North American standard for DS-3 (Digital Signal Level 3). T-3 operates at a signaling rate of 44.736 Mbps, equivalent to 28 T-1s. It is commonly referred to as 45 Megabits per second, rounded up. Capable of handling 672 voice conversations, T-3 runs on fiber optic or microwave transmission media, as twisted pair is not capable of supporting such a high signaling rate over long distances.

Copyright © 1999-2002 TechFest.com. All Rights Reserved.