T1 Basics

T1 is able to take advantage of these formats and technologies:

A Four-Wire Circuit – This technology evolved from the old twisted-pair environment.  Four wires are used. Two wires transmit and two receive.

Full Duplex – Transmission and reception are simultaneous.   Applications include remote file printing, data transfer, and other services including voice communications.

Digital – Data, analog fax, and analog voice is all converted to digital pulses for transmission.

Time-Division Multiplexing – The digital stream carries a standard 64Kbps channel; 24 channels are multiplexed to create 1.536 Mbps. Time division enables a channel to use 1/24 of the time. 

Pulse Code Modulation – Analog voice or other signals are sampled 8000 times a second; an 8-bit word represents a sample, yielding 64 Kbps per channel.

Framed Format – 24 channels are time-division multiplexed into a frame to for transmission. Each frame represents one sample (8 bits) from each of the 24 channels, added to this is a frame resulting in a frame that is 193 bits. Framing accounts for 8 Kbps overhead, add this to the 1.536 Mbps discussed in time-division multiplexing and you get 1.544 Mbps.

Bipolar Format – T1 uses voltage to represent the pulses (1’s), with alternate use of positive and negative voltage. This bipolar format serves two purposes:  First, it reduces required bandwidth from 1.5 MHz to 772 kHz, increasing repeater spacing. Second, since signal voltage averages out to zero (nulls), direct current (dc) power can be simplexed onto the line to drive intermediate repeaters.  The bipolar format is also called alternate mark inversion (AMI).

Byte-Synchronous Transmission – Timing for the channels derives from pulses that appear within samples. This timing provides a coordinating reference for all devices in the circuit (without synchronization, the circuit’s processing architecture is unable to distinguish between one message and another). This enables an individual T1 to coordinate all signals carried on it, but not to other T1 carriers.  For multiplexing two or more T1s lines, additional bits may be added to achieve synchronization among discreet lines.

Channelized Services – T1 lines originally consisted of 24 channels at 64 Kbps each, plus 8 Kbps of overhead. This describes channelized service. New multiplexing equipment allows differing channel widths to be multiplexed onto one carrier.  For example, T1 lines can be configured at 384 Kbps for data and 768 Kbps for video.

Digital Capacity

The T-carrier system has become the standard building block in the multiplexing hierarchy. In high-speed transmissions, a carrier combines many lower speed signals for transmission. To simplify this, standard rates have been defined:

DS0

The DS0 is a 64 Kbps signal that is the basis for  DS1 transmission (see below). Twenty-four DS0s combine to form one DS1.

DS1

The digital signal level 1 is a time-division multiplexed (TDM) pulse-code modulation aggregate of 1.544 Mbps.

DS1C

This equals two DS1s combining into one 3.152 Mbps channel. Few of these circuits exist today.

DS2

This is a composite of four DS1s multiplexed together yield 6.312 Mbps. 96 DS0s can travel across DS2 to deliver the local loop to customers.  This extends the useful life of wire pairs already in place.  Download rates of 6 Mbps or more are sufficient for leaving wire pairs in place.

DS3

The T3 service is for large customers and equals 28 DS1s at 44.736 Mbps. The costs of a T3 are out of range for smaller companies.

DS4

This 274.176 Mbps signal equals 6 DS3s or 4,032 DS0s.