T1 and E1 TDM-PCM hierarchy with frame diagrams

TDM-PCM telephony carries many 64 kb/s PCM voice channels on one line by interleaving their 8-bit samples in time. The two standards are T1 (North America, Japan) and E1 (Europe, ITU-T, used in Nepal). Both sample speech at 8 kHz, so each frame lasts $125$ µs.

T1 frame

|<--------- one frame = 125 us = 193 bits --------->|
+---+--------+--------+--------+-------+--------+
| F |  Ch 1  |  Ch 2  |  Ch 3  |  ...  | Ch 24  |
+---+--------+--------+--------+-------+--------+
 1 bit  8 bits  8 bits  8 bits           8 bits
 F = framing bit (pattern spread over 12 frames)

$$\begin{aligned} \text{Bits per frame} &= 24 \times 8 + 1 = 193\ \text{bits} \\ \text{Frames per second} &= f_s = 8000\ \text{frames/s} \\ R_b &= 193 \times 8000 = 1\,544\,000\ \text{b/s} = 1.544\ \text{Mb/s} \end{aligned}$$

• 12 frames form a superframe (24 in the extended superframe, ESF). The F-bits of these frames carry the frame alignment pattern (100011011100 for the superframe).
• Signalling uses the robbed-bit method: in frames 6 and 12, the LSB of every channel carries signalling (A and B bits).
• µ-law companding ($\mu = 255$) and AMI/B8ZS line coding are used.

E1 frame

|<-------- 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)

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

• TS0 carries the frame alignment word (0011011) in alternate frames, plus alarm bits.
• TS16 carries channel-associated signalling. 16 frames form a multiframe of 2 ms. TS16 of frame 0 carries the multiframe alignment pattern, and TS16 of each of frames 1–15 carries the signalling bits of 2 channels (4 bits each).
• A-law companding ($A = 87.6$) and HDB3 line coding are used.

T1 hierarchy

 24 voice ch x 64 kb/s
        |
   [ DS1 / T1 ]  1.544 Mb/s    24 ch
        | x4
   [ DS2 / T2 ]  6.312 Mb/s    96 ch
        | x7
   [ DS3 / T3 ]  44.736 Mb/s   672 ch
        | x6
   [ DS4 / T4 ]  274.176 Mb/s  4032 ch

E1 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

At each higher level, the higher rate is a little more than (number of inputs × lower rate). The extra bits are used for framing and bit stuffing (justification), which align input streams whose clocks differ slightly. For example, $4 \times 1.544 = 6.176$ Mb/s, but DS2 runs at 6.312 Mb/s.