- 2081 Chaitra · 5 marks
Short note: T1 and E1 hierarchy
Short answer
T1 carries 24 voice channels, one framing bit, 193 bits per 125 µs frame, rate 1.544 Mbps; E1 uses 32 slots (30 voice, TS0 sync, TS16 signalling), 2.048 Mbps. Multiplexing: T1 DS1→DS2→DS3→DS4, E1→E2→E3→E4. T1 uses µ-law, E1 A-law.
Write a short note on T1 and E1 hierarchy used in telephony.
T1 and E1 are the two standard TDM-PCM digital carrier systems used in telephony. Each one interleaves several 64 kb/s PCM voice channels into one serial stream. T1 is the North American (Bell) standard. E1 is the European (ITU-T) standard, and Nepal Telecom uses it too. Both sample speech at 8 kHz with 8 bits per sample, so one frame lasts µs.
T1 system
- 24 voice channels × 8 bits + 1 framing bit = 193 bits per frame.
- Mb/s.
- Signalling is "robbed bit": the LSB of each channel in frames 6 and 12 of a superframe.
E1 system
- 32 time slots × 8 bits = 256 bits per frame. TS0 carries frame sync, TS16 carries signalling, and the other 30 slots carry voice.
- Mb/s.
Hierarchies
Higher levels are made by multiplexing lower-level streams. Extra bits are added for framing and bit stuffing, so each higher rate is a little more than the sum of its inputs.
| T1 level | Rate (Mb/s) | Channels | E1 level | Rate (Mb/s) | Channels |
|---|---|---|---|---|---|
| DS1 (T1) | 1.544 | 24 | E1 | 2.048 | 30 |
| DS2 = 4 DS1 | 6.312 | 96 | E2 = 4 E1 | 8.448 | 120 |
| DS3 = 7 DS2 | 44.736 | 672 | E3 = 4 E2 | 34.368 | 480 |
| DS4 = 6 DS3 | 274.176 | 4032 | E4 = 4 E3 | 139.264 | 1920 |
T1 uses µ-law companding () and the AMI/B8ZS line code. E1 uses A-law () and the HDB3 line code.
✓ Answer checked by Claude Opus 5.5 (high effort)
Related questions
- T1 and E1 TDM-PCM hierarchy with frame diagrams · Communication Systems, chapter 5
- Companding; T1 hierarchy of TDM-PCM telephony · Communication Systems, chapter 5
- FDMA vs TDMA; T1 and E1 hierarchy · Communication Systems, chapter 5
- Differences between T1 and E1 hierarchy · Communication Systems, chapter 5
- FDM and FDM hierarchy in telephony · Communication Systems, chapter 5 · asked 4×
- E1 digital hierarchy; SQNR in uniformly quantized PCM · Communication Systems, chapter 5 · asked 2×
- North American T1 vs European E1 hierarchy · Telecommunications, chapter 3 · asked 2×
- T1 and E1 frame structures and bit rates · Telecommunications, chapter 3
- TDM and T1/E1 carrier standards · Telecommunications, chapter 3
- E1 telephone hierarchy · Computer Networks and Security, chapter 2