E1 is an ITU-T standard that defines the basic protocol and structure for transmission of TDM data over a physical circuit (most often the physical connections use twisted pairs and conform to G.703). It is widely used in Europe.
An E1 primary rate trunk has a total of 32 time slots allocated within each frame of data. In TDM telephony applications, time slot 16 is used as a data channel (the D channel) and carries call setup, clear down and other non-voice information. Time slot zero is the framing synchronisation time slot. The remaining 30 time slots are used for the 30 voice channels (B channels) – each time slot holds an 8-bit sample, normally encoded using A-Law PCM. The total bandwidth required for E1 is 2.048Mb/s which allows for 8kHz sampling of the voice in each B channel.
As an aside, North American standards are different and the most widely used standard for TDM digital telephony trunks is T1 which has 24 time slots and requires a bandwidth of 1.544Mb/s. The most common standard used for voice encoding on T1 circuits is u-Law PCM. E1 was developed from T1.
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