The E interfaces are based on the basic bit rate of the PDH multiplex structure (Plesiochronous Digital Hierarchy), which is 64 kbit/s in Europe. This basic bit rate is designated E0. The E interfaces are documented by the International Telecommunication Union( ITU) as G.7xx.
The individual interfaces( E1, E2, E3, E4, and E5) correspond to a multiplex hierarchy in which the higher- order interface forms the 4-way multiplex of the lower-order interface: Four E1 lines are multiplexed to form an E2 channel, four E2 channels to form an E3, four E3 lines to form an E4 channel, and four E4 channels to form an E5 channel.
Channels 0 and 16 are reserved for transmission management, all other channels can be occupied by user data. This reduces the bandwidth for the user data to 1.920 Mbit/s. This corresponds to a bandwidth utilization of 93.8%. Channel 0 provides a unique bit pattern for synchronization and also indicates transmission errors. Channel 16 is used for signaling and is responsible for establishing and terminating connections for the individual user channels. The resulting bit stream is encoded in HDB3 coding.
The data rate of the E2 interface is 8.448 Mbit/s, that of E3 is 34.368 Mbit/s, that of the E4 interface is 139.264 Mbit/s, and that of E5 is 564.992 Mbit/s. The E3 data transmission frame is 1,536 bits long, consisting of 4 subframes of 384 bits each. The E4 data transmission frame consists of 6 subframes with a length of 488 bits each.