System information

Even the ubiquitous analog circuit is sampled into a DS-0 as soon as possible. Some-
times this happens where your circuit terminates at the central office, and sometimes
well before.
§
T-carrier circuits
The venerable T1 is one of the more recognized digital telephony terms. A T1 is a digital
circuit consisting of 24 DS-0s multiplexed together into a 1.544-Mbps bit stream.
‖
This
bit stream is properly defined as a DS-1. Voice is encoded on a T1 using the μlaw
companding algorithm.
The European version of the T1 was developed by the European Con-
ference of Postal and Telecommunications Administrations
#
(CEPT),
and was first referred to as a CEPT-1. It is now called an E1.
The E1 is composed of 32 DS-0s, but the method of PCM encoding is
different: E1s use alaw companding. This means that connecting be-
tween an E1-based network and a T1-based network will always require
a transcoding step. Note that an E1, although it has 32 channels, is also
considered a DS-1. It is likely that E1 is far more widely deployed, as it
is used everywhere in the world except North America and Japan.
The various other T-carriers (T2, T3, and T4) are multiples of the T1, each based on
the humble DS-0. Table A-2 illustrates the relationships between the different T-carrier
circuits.
Table A-2. T-carrier circuits
Carrier Equivalent data bitrate Number of DS-0s Data bitrate
T1 24 DS-0s 24 1.544 Mbps
T2 4 T1s 96 6.312 Mbps
T3 7 T2s 672 44.736 Mbps
T4 6 T3s 4,032 274.176 Mbps
At densities above T3, it is very uncommon to see a T-carrier circuit. For these speeds,
optical carrier (OC) circuits may be used.
§ Digital telephone sets (including IP sets) do the analog-to-digital conversion right at the point where the
handset plugs into the phone, so the DS-0 is created right at the phone set.
‖ The 24 DS-0s use 1.536 Mbps, and the remaining .008 Mbps is used by framing bits.
#Conférence Européenne des Administrations des Postes et des Télécommunications.
The Digital Circuit-Switched Telephone Network | 611