Short note: E1 digital hierarchy

E1 is the ITU-T (European) standard TDM-PCM digital carrier. It multiplexes 30 voice channels of 64 kb/s plus 2 overhead channels into a 2.048 Mb/s stream. It is used in Europe and most other countries, including Nepal.

E1 frame

• Speech is sampled at 8 kHz, so a frame lasts 125 µs.
• Each frame has 32 time slots × 8 bits = 256 bits.
• TS0: frame alignment word and alarm bits.
• TS16: signalling (channel-associated). 16 frames form a 2 ms multiframe.
• TS1–TS15 and TS17–TS31: 30 voice channels.

|<-------- one frame = 125 us = 256 bits -------->|
+-----+-----+-----+-----+------+------+-----+------+
| TS0 | TS1 | ... |TS15 | TS16 | TS17 | ... | TS31 |
+-----+-----+-----+-----+------+------+-----+------+
 sync  voice       voice signal voice        voice
 each time slot = 8 bits; 32 slots per frame
 voice channels: TS1-TS15 and TS17-TS31 (30)

$$R_b = 32 \times 8 \times 8000 = 2.048\ \text{Mb/s}$$

E1 digital hierarchy

Four lower-level streams are combined at each step. Extra bits for framing and justification (bit stuffing) are added.

E1 uses A-law companding ($A = 87.6$) and the HDB3 line code. Compared with T1 (24 channels, 1.544 Mb/s), E1 carries more channels and has a separate slot for signalling, so all 8 bits of each voice slot stay available for speech.