Telecommunication Fundamentals (signals, transmission media, telephony concepts)
This section covers Telecommunication Fundamentals: signals, transmission media, and telephony concepts. The exam tests whether you can describe how information is carried, measured, and impaired from microphone to receiver.
Core rules and concepts
- A signal has amplitude, frequency, and phase. Wavelength λ = c/f; at 100 MHz, λ = 3 m in free space.
- Analog signals vary continuously; digital signals use discrete levels. Bandwidth is the range of frequencies a channel carries; bit rate is how many bits per second it carries. They are related but not identical.
- Impairments: attenuation (loss), noise, distortion, echo, and crosstalk. SNR compares signal power with noise power. For power, dB = 10 log10(P2/P1); for voltage, dB = 20 log10(V2/V1).
- Modulation places information on a carrier: AM, FM, ASK, FSK, PSK, QAM. Multiplexing shares a medium: FDM by frequency, TDM by time.
- Telephony voice band is about 300-3400 Hz. PCM samples at 8 kHz, quantizes to 8 bits, giving 64 kbps per channel. E1 carries 32 timeslots at 2.048 Mbps.
- Media: twisted pair for local loops, coaxial cable, optical fiber for high capacity, and radio for mobile and microwave.
Worked example
A PCM channel samples at 8,000 samples/s with 8 bits per sample: 8,000 × 8 = 64,000 bit/s = 64 kbps. Thirty-two channels give 32 × 64 kbps = 2.048 Mbps, the E1 rate. Nyquist requires sampling at least twice the highest frequency, so a 3.4 kHz voice signal needs at least 6.8 kHz; 8 kHz is chosen to leave filtering room.
Traps that cost marks
Confusing Hz bandwidth with bit/s; using 20 log for power ratios; treating dB as an absolute unit; forgetting that PCM bit rate = sampling rate × bits per sample; and assuming a higher carrier frequency automatically means a higher data rate.