Introduction to Telecom and Communication Systems
What this domain covers
Communication is the transfer of information from a sender to a receiver through a medium. The Madhyamik Telecom paper tests the basic block diagram - source, transmitter/encoder, channel, receiver/decoder, destination - and the impairments that sit between them: attenuation, distortion and noise.
Core rules
- Simplex: one direction only (radio broadcast, TV). Half-duplex: both directions but not at the same time (walkie-talkie, push-to-talk). Full-duplex: both directions simultaneously (telephone).
- Analogue signals vary continuously; digital signals take discrete levels (0/1). Digital transmission resists noise and can be regenerated.
- The telephone voice band is 300–3400 Hz (nominally 4 kHz), not the 20 Hz–20 kHz audio range of music.
- Bandwidth is the range of frequencies a channel carries, measured in Hz; bit rate is bits per second. They are related but not identical.
- Modulation places a baseband signal on a carrier so it radiates efficiently: it reduces antenna size, allows multiplexing and separates channels.
- Decibel: loss/gain in dB = 10 log10 (Pout/Pin). 3 dB is about double or half the power; 10 dB is ten times.
Worked example
A 4 kHz voice channel is sampled at the Nyquist rate of 8000 samples/s and each sample is coded into 8 bits. Bit rate = 8000 × 8 = 64,000 bit/s = 64 kbps. Reasoning: sampling must be at least twice the highest frequency, and bit rate is samples per second multiplied by bits per sample. This 64 kbps is the standard voice channel used in digital hierarchies.
Traps
- Calling the telephone channel 20 kHz - that is hi-fi audio.
- Mixing up half-duplex and full-duplex.
- Writing bandwidth in bit/s when the question asks for Hz.
- Forgetting that a 3 dB loss halves power, not amplitude.