European TDM system

The European TDM system is the CEPT (ITU-T G.732/G.751) plesiochronous digital hierarchy, also used in Nepal, Asia and most of the world outside North America and Japan. Its basic building block is the E1 (2.048 Mbps) 30-channel PCM system.

E1 (30-channel PCM) frame

• Each voice channel: 8000 samples/s × 8 bits = 64 kbps, coded with A-law companding ($A = 87.6$).
• One frame = 32 time slots × 8 bits = 256 bits in 125 µs.

+-----+-----+-----+------+------+-----+------+
| TS0 | TS1 | ... | TS15 | TS16 | ... | TS31 |
+-----+-----+-----+------+------+-----+------+
 sync  <-voice 1-15->  sig   <-voice 16-30->
<------ 32 x 8 = 256 bits in 125 us ------->

$$\begin{aligned} \text{Bits per frame} &= 32 \times 8 = 256\ \text{bits} \\ R_{E1} &= 256 \times 8000 = 2.048\ \text{Mbps} \end{aligned}$$

• TS0: frame-alignment word (×0011011) in even frames; alarm and national bits in odd frames.
• TS16: signalling. For channel-associated signalling, 16 frames form a multiframe (2 ms); TS16 of frame 0 carries the multiframe alignment, and frames 1–15 each carry 4 signalling bits for two channels, so all 30 channels are served. TS16 can instead be a 64 kbps common-channel (SS7) link.
• TS1–TS15 and TS17–TS31: 30 voice channels.
• Line code: HDB3, at 2.048 Mbps with ±50 ppm tolerance.

Higher levels

Each higher order combines four lower-order tributaries by bit interleaving with pulse stuffing (justification), plus frame-alignment and service bits.

30 x 64k --> [MUX] --> E1  2.048 Mbps (30 ch)
 4 x E1  --> [MUX] --> E2  8.448 Mbps (120 ch)
 4 x E2  --> [MUX] --> E3  34.368 Mbps (480 ch)
 4 x E3  --> [MUX] --> E4  139.264 Mbps (1920 ch)
 4 x E4  --> [MUX] --> E5  564.992 Mbps (7680 ch)

$$\begin{aligned} E2 &: 4 \times 2.048 = 8.192;\ \ 8.448 - 8.192 = 0.256\ \text{Mbps overhead} \\ E3 &: 4 \times 8.448 = 33.792;\ \ 34.368 - 33.792 = 0.576\ \text{Mbps overhead} \\ E4 &: 4 \times 34.368 = 137.472;\ \ 139.264 - 137.472 = 1.792\ \text{Mbps overhead} \\ E5 &: 4 \times 139.264 = 557.056;\ \ 564.992 - 557.056 = 7.936\ \text{Mbps overhead} \end{aligned}$$

Features: regular ×4 structure; signalling does not steal voice bits; overhead 2/32 at E1. Higher levels are now carried in SDH (STM-1 at 155.52 Mbps can hold 63 E1s).