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Chapter 1 Introduction
T1/FT1 OVERVIEW
T1 digital communication links have been used by telcos for the transmission of voice since the early
sixties. The D4 channel bank is an example of a T1 digital carrier system that was introduced in the mid-
seventies and is still widely used by the telcos. Communication demands of business continued to grow to
the point that the telcos began offering T1 service directly to the public. D4 channel banks were 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 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 installed approximately 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 (SF), which identifies how the T1
is multiplexed. An enhancement of that framing format, called extended superframe (ESF), is available.
ESF provides a non-disruptive means of full-time monitoring on the digital facility. It was originally used
by service providers to monitor the performance of their service offering. Since the introduction of ESF,
equipment installed in private networks can also provide this performance information to the user.
T1 ESF CSU ACE OVERVIEW
Most carriers (regional or local telcos), when supplying a T1 line to customer premises equipment (CPE),
require an interface to monitor the T1 line. The T1 ESF CSU ACE (extended superframe channel service
unit advanced communication equipment) provides the required interface between the CPE (such as DSUs,
channel banks, T1 multiplexers, and PBXs) and telco or private T1 facilities. Figure 1-1 depicts a typical
application.
