Telecom Network Fundamentals: Switching, Transmission and Optical Fibre
What this domain covers
This domain links three ideas: how calls are switched, how signals are carried, and why optical fibre dominates backbone transmission. Expect questions on switching types, multiplexing hierarchies, fibre construction and optical testing.
Core rules
- Circuit switching reserves an end-to-end path for the call duration (PSTN); packet switching sends labelled chunks that share links (internet); message switching stores and forwards whole messages.
- Exchanges form a hierarchy: local exchange to tandem/transit to trunk exchange, with subscriber lines on one side and trunks on the other.
- Multiplexing: FDM shares frequency, TDM shares time, WDM shares wavelength. E1 = 32 time slots × 8 bits × 8000 frames/s = 2.048 Mbps, carrying 30 speech channels plus slot 0 (sync) and slot 16 (signalling). T1 = 1.544 Mbps with 24 channels.
- Fibre structure: core, cladding, buffer/coating, strength members, outer jacket. Light is guided by total internal reflection, which needs the core refractive index to be greater than the cladding.
- Single-mode fibre: about 9 micrometre core, 1310/1550 nm, long haul. Multimode: 50 or 62.5 micrometre core, 850/1300 nm, short reach.
- Typical attenuation: 0.35 dB/km at 1310 nm and 0.2 dB/km at 1550 nm.
Worked example
An E1 link is required. One frame = 32 slots × 8 bits = 256 bits; frames repeat 8000 times a second; 256 × 8000 = 2,048,000 bit/s = 2.048 Mbps. Because two slots are overhead, the payload is 30 voice channels. Reasoning: the frame rate is fixed at 8 kHz to match voice sampling.
Traps
- Saying E1 carries 32 voice channels - it carries 30.
- Swapping single-mode and multimode core sizes.
- Thinking WDM works on copper pairs.
- Using an OTDR to measure absolute optical power, which is the power meter's job.