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Constellation diagram significance; E1 vs T1

Short answer

A constellation diagram plots digital symbols in I–Q space, showing symbol count, minimum distance (noise immunity), power, and impairments. E1 has 32 slots, 30 voice, 2.048 Mbps, A-law; T1 has 24 voice, 1.544 Mbps, µ-law, with robbed-bit signalling. Both have hierarchical multiplexing: E1/2/3/4 and T1/2/3/4.

What is the significance of constellation diagram? Compare E1 and T1 TDM PCM hierarchy.

Significance of constellation diagram

A constellation diagram plots every possible symbol of a digital modulation scheme as a point in the I–Q (in-phase vs quadrature) plane. The distance of a point from the origin shows the symbol's amplitude, and its angle shows the symbol's phase.

      Q                      Q
      |                  o   |   o
  o---+---o   I          ----+----  I
      |                  o   |   o
     BPSK                   QPSK

Why it matters:

  • It shows the number of symbols M, and so the bits per symbol (log⁡2M\log_2 M), at a glance.
  • The minimum distance between points sets the noise margin. Points that are farther apart give a lower bit error rate for the same noise.
  • It shows the average and peak power of the scheme: the mean and maximum squared distance from the origin.
  • On a received-signal display, spread-out clouds show noise. A rotated pattern shows phase error, and a squeezed pattern shows amplitude non-linearity. This makes it a practical test tool.
  • It lets us compare schemes such as BPSK, QPSK and 16-QAM by bandwidth efficiency and noise immunity.

Comparison of E1 and T1 hierarchy

PointT1 (North America, Japan)E1 (Europe, ITU-T, Nepal)
Voice channels2430 (+2 overhead slots)
Time slots per frame24 + 1 framing bit32
Bits per frame24×8+1=19324 \times 8 + 1 = 19332×8=25632 \times 8 = 256
Frame duration125 µs125 µs
Bit rate1.544 Mb/s2.048 Mb/s
Framing1 F-bit per frame, pattern over 12/24 framesTS0 carries frame alignment word
SignallingRobbed-bit (LSB of frames 6 and 12)Separate slot TS16 (CAS)
MultiframeSuperframe 12 frames / ESF 24 frames16 frames (2 ms)
Companding lawµ-law, μ=255\mu = 255A-law, A=87.6A = 87.6
Line codeAMI / B8ZSHDB3
Higher levels6.312, 44.736, 274.176 Mb/s8.448, 34.368, 139.264, 564.992 Mb/s

Frame structures:

|<--------- 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)

|<-------- 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)

✓ Answer checked by Claude Opus 5.5 (high effort)

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