Complete study guide for Electrical Engineering students covering PCM multiplexing, E1/T1 framing, bit stuffing, and digital transmission hierarchies.
Plesiochronous comes from Greek "plēsios" (near) + "chronos" (time). It describes a system where network elements operate at nearly the same clock rate but are not synchronized to a common master clock.
PDH was the first generation of digital telecommunications (1960s-1980s), enabling multiple voice channels to share high-bandwidth links through Time Division Multiplexing (TDM).
Pulse Code Modulation (PCM) is the foundation of PDH. Voice signals are digitized through sampling, quantization, and encoding.
Nyquist rate: 8 kHz (8000 samples/sec) for 4 kHz voice bandwidth
8 bits per sample (256 levels) using A-law (E1) or μ-law (T1)
Resulting bit rate: 64 kbps per voice channel (DS0/E0)
Two regional standards: European (E-system) and North American (T-system)
| Level | Bit Rate | Channels | Multiplexing | Overhead |
|---|---|---|---|---|
| E0 | 64 kbps | 1 | Basic PCM channel | - |
| E1 | 2.048 Mbps | 30 voice + 2 signaling | 32 × E0 | TS0 (sync), TS16 (signaling) |
| E2 | 8.448 Mbps | 120 | 4 × E1 | 256 kbps overhead |
| E3 | 34.368 Mbps | 480 | 4 × E2 | ~1.5 Mbps overhead |
| E4 | 139.264 Mbps | 1,920 | 4 × E3 | ~4.8 Mbps overhead |
| E5 | 564.992 Mbps | 7,680 | 4 × E4 | ~21 Mbps overhead |
32 timeslots × 8 bits = 256 bits per frame. Frame duration: 125 μs (8000 frames/sec)
Frame Alignment Signal (FAS): 0011011 in bits 2-8 of even frames. Odd frames carry alarms.
Channel Associated Signaling (CAS). In multiframe, carries signaling for voice channels.
30 voice/data channels at 64 kbps each. Can carry PCM voice or digital data.
In PDH, each multiplexer has its own independent clock. When multiplexing 4 E1 streams into E2, each E1 may have slightly different bit rates (±50 ppm tolerance).
Positive Justification (Stuffing): When tributary is slower than multiplexer clock, extra "stuffing" bits are inserted. Control bits tell the receiver which bits to discard.
| Feature | PDH | SDH/SONET |
|---|---|---|
| Synchronization | Plesiochronous (nearly synchronous) Independent clocks, ±50ppm tolerance |
Synchronous Common master clock across network |
| Multiplexing | Bit-interleaving with bit stuffing Complex, requires full demux to access channel |
Byte-interleaving (synchronous) Direct add/drop of channels possible |
| Max Capacity | 566 Mbps (E5/J5) Limited scalability |
40+ Gbps (STM-256/OC-768) Highly scalable |
| Network Topology | Point-to-point only No protection switching |
Ring, mesh, star, linear Automatic protection switching (<50ms) |
| Overhead | Minimal OAM overhead Limited monitoring capabilities |
Rich overhead bytes Comprehensive monitoring, management |
| Standardization | Regional standards (E/T/J) Interconnection requires conversion |
Global standard (STM/OC) Universal interoperability |
| Cost | Lower initial cost | Higher initial cost |
Complete study guide for Electrical Engineering students covering PCM multiplexing, E1/T1 framing, bit stuffing, and digital transmission hierarchies.
Plesiochronous comes from Greek "plēsios" (near) + "chronos" (time). It describes a system where network elements operate at nearly the same clock rate but are not synchronized to a common master clock.
PDH was the first generation of digital telecommunications (1960s-1980s), enabling multiple voice channels to share high-bandwidth links through Time Division Multiplexing (TDM).
Pulse Code Modulation (PCM) is the foundation of PDH. Voice signals are digitized through sampling, quantization, and encoding.
Nyquist rate: 8 kHz (8000 samples/sec) for 4 kHz voice bandwidth
8 bits per sample (256 levels) using A-law (E1) or μ-law (T1)
Resulting bit rate: 64 kbps per voice channel (DS0/E0)
Two regional standards: European (E-system) and North American (T-system)
| Level | Bit Rate | Channels | Multiplexing | Overhead |
|---|---|---|---|---|
| E0 | 64 kbps | 1 | Basic PCM channel | - |
| E1 | 2.048 Mbps | 30 voice + 2 signaling | 32 × E0 | TS0 (sync), TS16 (signaling) |
| E2 | 8.448 Mbps | 120 | 4 × E1 | 256 kbps overhead |
| E3 | 34.368 Mbps | 480 | 4 × E2 | ~1.5 Mbps overhead |
| E4 | 139.264 Mbps | 1,920 | 4 × E3 | ~4.8 Mbps overhead |
| E5 | 564.992 Mbps | 7,680 | 4 × E4 | ~21 Mbps overhead |
32 timeslots × 8 bits = 256 bits per frame. Frame duration: 125 μs (8000 frames/sec)
Frame Alignment Signal (FAS): 0011011 in bits 2-8 of even frames. Odd frames carry alarms.
Channel Associated Signaling (CAS). In multiframe, carries signaling for voice channels.
30 voice/data channels at 64 kbps each. Can carry PCM voice or digital data.
In PDH, each multiplexer has its own independent clock. When multiplexing 4 E1 streams into E2, each E1 may have slightly different bit rates (±50 ppm tolerance).
Positive Justification (Stuffing): When tributary is slower than multiplexer clock, extra "stuffing" bits are inserted. Control bits tell the receiver which bits to discard.
| Feature | PDH | SDH/SONET |
|---|---|---|
| Synchronization | Plesiochronous (nearly synchronous) Independent clocks, ±50ppm tolerance |
Synchronous Common master clock across network |
| Multiplexing | Bit-interleaving with bit stuffing Complex, requires full demux to access channel |
Byte-interleaving (synchronous) Direct add/drop of channels possible |
| Max Capacity | 566 Mbps (E5/J5) Limited scalability |
40+ Gbps (STM-256/OC-768) Highly scalable |
| Network Topology | Point-to-point only No protection switching |
Ring, mesh, star, linear Automatic protection switching (<50ms) |
| Overhead | Minimal OAM overhead Limited monitoring capabilities |
Rich overhead bytes Comprehensive monitoring, management |
| Standardization | Regional standards (E/T/J) Interconnection requires conversion |
Global standard (STM/OC) Universal interoperability |
| Cost | Lower initial cost | Higher initial cost |