A hierarchy of digital transmission formats published by the ITU that are used everywhere except for North America, Hong Kong and Japan. The name comes from the fact it was developed in Europe.
The basic unit of the E-carrier system is the 64 Kbps DS-0, which is multiplexed to form transmission formats with higher speeds.
The first E* format is composed of 32 DS-0s: 30 of which are used for data, and two which are basically used for overhead (signalling of the rest). Each of following E* formats is the preceding one multiplexed four times. In other words:
- E1, 2.048 Mbps,
- E2, 4 * E1, 8.448 Mbps,
- E3, 4 * E2, 16 * E1, 34.368 Mbps,
- E4, 4 * E3, 64 * E1, 139.264 Mbps, and
- E5, 4 * E4, 256 * E1, 565.148 Mbps.
E1 and E3 are the most popular ones and are often offered by the telecommunication companies.
The E-carrier system is quite similar to, and compatible with, the T-carrier system used in North America, but it has higher capacity since it uses out-of-band signalling.