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Chapter 1. Introduction
61200.076L2-1 TSU 600 User Manual 1
Chapter 1
Introduction
T1/FT1 OVERVIEW
The T1 digital communications link has been used by
telephone companies (telcos) for transmitting voice
since the early sixties. The D4 channel bank is a T1
digital carrier system that was introduced in the mid
seventies and is still widely used by telcos. Communi-
cation demands of businesses continued to grow to the
point that the telcos began offering T1 service directly
to the public. D4 channel banks began to be used for
T1 in corporate network topographies for voice. The
technological advances in computer development also
created a demand for T1 data communication which
now is a large part of the T1 traffic.
T1 Service Offerings
T1 is a digital service that is delivered to the user over
two pairs of wires from the service provider. The
signal operates at 1.544 Mbps and is usually extended
by repeaters that are installed about every mile after
the first 6000 feet. The T1 signal is divided into 24
time slots which operate at 64 kbps (DS0s). Each time
slot is occupied by digitized voice or by data.
The T1 signal originally used a type of framing known
as D4 Superframe which identifies how the T1 is
multiplexed. An enhancement of that framing format,
called Extended Superframe Format (ESF), is avail-
able. It provides a non-disruptive means of full time
monitoring on the digital facility. The ESF was
originally used by the service provider to monitor the