Computer Networks
Computer Networks covers layered protocols, error and flow control, addressing and routing, and transport behaviour. GATE questions are usually numerical - utilisation, subnetting, delay - or conceptual about which layer or protocol does what.
Data link layer
- Framing, error detection by CRC and checksum, and sliding-window flow control.
- Stop-and-wait efficiency = 1/(1 + 2a) where a = propagation delay / transmission time; pipelining with window N raises it toward 1.
- CSMA/CD requires a minimum frame size so a collision is detected before transmission ends.
Network layer
- IPv4 header: 20-byte minimum, TTL prevents loops, fragmentation uses identification, flags and fragment offset.
- CIDR: a /26 leaves 6 host bits, so 64 addresses and 62 usable hosts.
- RIP is distance vector with hop count and a maximum of 15; OSPF is link state; BGP is path vector between autonomous systems.
Transport layer
- TCP three-way handshake: SYN, SYN-ACK, ACK. Flow control uses the receiver window; congestion control uses cwnd with slow start and AIMD.
- UDP is connectionless and has no reliability or ordering guarantees.
Worked example
A 10 Mbps link with a 40 ms round-trip time and a 5000-byte window. Bandwidth-delay product = 10⁷ × 0.04 = 400,000 bits = 50,000 bytes. Utilisation = 5000 / 50,000 = 10%. Thus a window of 50,000 bytes would be needed to fill the pipe.
Traps that cost marks
- Using one-way delay where RTT is required (or the reverse).
- Forgetting to subtract network and broadcast addresses in subnet host counts.
- Assuming CRC corrects errors - it only detects them.