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T1 and E1 Overview
Page 33-2
T1 and E1 Overview
Carrier digital services were designed primarily to support digitized voice over long distances.
Digital services are the primary method for carrying voice between two endpoints using two
pairs of copper wire. Digital wide-area data networking uses the same digital services that
were originally designed for digitized voice.
Analog to Digital Conversion
To improve quality and reliability, long-distance phone networks upgraded their backbones
from analog Frequency Division Multiplexing (
FDM
) to digital Time Division Multiplexing
(
TDM
). In
TDM
, analog data is converted to digital data using a
CODEC
device that employs a
method called Pulse Code Modulation (
PCM
).
In Pulse Code Modulation, the
CODEC
samples the analog signal 8,000 times a second and
converts each sample to an 8-bit digital value. These 8,000 8-bit samples yield a total digital
data rate of
64
,
000 BPS
for one voice service. This service is also known as Digital Service
Zero (
DS0
), which is the basis for T1 and E1 connections.
These 8,000 8-bits in time are also known as a time slot . A channel is a time slot that can
carry voice or data. Using Time Division Multiplexing,
24
channels (for
T1)
or 32 channels (for
E1)
are multiplexed to create a service called Digital Service 1 (
DS1
). The more common name
for
DS1
is T1 or E1 .
T1 Framing
A T1 frame consists of 24, 8-bit time slots and a 1-bit synchronization and control bit. Twelve
(12) T1 frames can be grouped into a
SuperFrame (SF/D4) , or 24 T1 frames can be grouped
into an
Extended SuperFrame
. In each SuperFrame, the 6th and 12th frame may contain
“robbed bit” (A, B) signalling, which means the least significant bit is robbed from each time
slot in the 6th and 12th frame and used for signalling. In Extended SuperFrames, this robbed-
bit signalling (A, B, C, D) occurs in the 6th, 12th, 18th, and 24th frames.
E1 Framing
The E1 frame consists of 32, 8-bit time slots (two of these time slots are used for synchroniza-
tion and multiframe signalling) for 256 bits per frame at 2.048 megabits per second. Sixteen
(16) E1 frames are grouped into a multiframe. An E1 multiframe can use Channel Associated
Signalling (
CAS
) contained in time slot 16. Timeslot 16 in multiframe 0 is used for multiframe
synchronization and control. Timeslot 16 of multiframes 1 through 15 are used to carry A, B,
C, and D signaling bits.