E1 is one of the standard Transmission Techniques.
It is the Infrastructure needed to transmit Traffic (Voice native, Voice over IP, Data…) or to transmit signalling (MAP, ISUP, C7, …)
it consists of 32 time slots, each can be used for one application or one call channel.
if the maximum frequency in human voice is 4K Hz this voce should be sampled minimum at 8K sample per Second so that it could be re-constructed without voice quality degradation.
This 8000 sample is coded with 8 Bits PCM coding the resulting is 64000 bit per second.
This is one voice sample for one Voice channel at a time.
for the complete 32 Channel it becomes 32*64000 which is the 2048 K bps or the 2 Mbps.
For some applications that requires bandwidth more than 64 K, group of channels (5 time Slots for example) can be considered as a one set capable of providing (5*64K =320K).
some other applications requires the complete 32 Time Slots to be considered as a one channel with complete 2 Mbps Bandwidth)
E1 is working in Layer 1 and Layer 2 of the OSI Reference Model provided by ISO.
For Layer 2, it allocates one time slot from the 32 Time Slots for E1 availability and reliability Management; this reduces the number of Time Slots that could be used for transmission to 31 Time Slots.