Page 209 - Asterisk™: The Future of Telephony

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 comprised 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 American and Japan.

The various other T-carriers (T2, T3, and T4) are multiples of the T1, each based on
the humble DS-0. Table 7-2 illustrates the relationships between the different T-carrier
circuits.
Table 7-2. 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.

SONET and OC circuits
The Synchronous Optical Network (SONET) was developed out of a desire to take the
T-carrier system to the next technological level: fiber optics. SONET is based on the
bandwidth of a T3 (44.736 Mbps), with a slight overhead making it 51.84 Mbps. This
is referred to as an OC-1 or STS-1. As Table 7-3 shows, all higher-speed OC circuits
are multiples of this base rate.
Table 7-3. OC circuits

Carrier Equivalent data bitrate Number of DS-0s Data bitrate
OC-1 1 DS-3 (plus overhead) 672 51.840 Mbps
OC-3 3 DS-3s 2,016 155.520 Mbps
OC-12 12 DS-3s 8,064 622.080 Mbps
OC-48 48 DS-3s 32,256 2488.320 Mbps
OC-192 192 DS-3s 129,024 9953.280 Mbps

* Conférence Européenne des Administrations des Postes et des Télécommunications.

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