Telecom and Network Engineering
Telecom and network engineering in the BSNL JTO technical section tests how voice and data move from access to core: transmission, multiplexing, switching, IP routing and optical/cellular fundamentals. Questions are usually numerical or definitional and reward knowing standard telecom rates and layered network behaviour.
Core rules and concepts
- PCM: band-limit voice to about 3.4 kHz, sample at 8 kHz (Nyquist), quantize to 8 bits, giving 64 kbps per channel.
- E1: 32 timeslots × 8 bits × 8000 frames/s = 2.048 Mbps; 30 voice channels, one sync and one signalling. T1 is 1.544 Mbps with 24 channels.
- Multiplexing: FDM divides frequency, TDM divides time; WDM is optical wavelength division.
- Switching: circuit switching reserves a path; packet switching shares links and uses store-and-forward.
- OSI layers: physical, data link, network, transport, session, presentation, application. IP is connectionless layer 3; TCP is reliable/connection-oriented layer 4; UDP is unreliable.
- IPv4 subnetting: /n leaves 32−n host bits; usable hosts = 2^(32−n) − 2.
- Optical: single-mode has a small core for long haul; multimode suits short LAN links; 1310 nm and 1550 nm are low-loss windows.
- Cellular: FDMA, TDMA, CDMA and OFDMA separate users by frequency, time, code and subcarriers.
Worked example
E1 payload: 32 × 8 × 8000 = 2,048,000 bit/s = 2.048 Mbps. Of the 32 timeslots, 30 carry voice, so usable voice = 30 × 64 kbps = 1.920 Mbps; the remaining 128 kbps is sync and signalling overhead. The arithmetic is simple, but the conclusion must distinguish line rate from voice payload.
Traps that cost marks
Confuse E1 with T1; forget to subtract 2 hosts for network and broadcast; use 4 kHz instead of 8 kHz sampling; assume TCP for DNS; mix dB (a ratio) with dBm (power relative to 1 mW); label IP as layer 2.