E1 frame, TDM hierarchy and signaling rate

E1 frame

E1 is the ITU-T TDM-PCM carrier that carries 30 voice channels. Each voice channel is sampled at 8 kHz and coded with 8 bits (A-law), giving 64 kb/s. One frame takes one byte from each of 32 time slots:

|<-------- 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)

• TS0: frame alignment word (0011011) in alternate frames, plus alarm/CRC bits.
• TS16: channel-associated signalling. 16 frames form a 2 ms multiframe.
• 30 slots carry speech.

Signalling (bit) rate:

$$\begin{aligned} \text{Bits per frame} &= 32 \times 8 = 256 \\ R_b &= 256 \times 8000 = 2\,048\,000\ \text{b/s} = 2.048\ \text{Mb/s} \end{aligned}$$

Bit duration $T_b = 1/2.048\ \text{MHz} \approx 0.488$ µs.

E1 TDM hierarchy

 30 voice ch x 64 kb/s (+ 2 overhead slots)
        |
   [ E1 ]  2.048 Mb/s     30 ch
        | x4
   [ E2 ]  8.448 Mb/s     120 ch
        | x4
   [ E3 ]  34.368 Mb/s    480 ch
        | x4
   [ E4 ]  139.264 Mb/s   1920 ch
        | x4
   [ E5 ]  564.992 Mb/s   7680 ch

Each level combines 4 streams of the level below. A few extra bits are added for framing and justification, so for example $4 \times 2.048 = 8.192$ Mb/s, but E2 = 8.448 Mb/s.