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Cambridge IAL Computer Science study materials
Cambridge IAL Computer Science study resources

Cambridge IAL Computer Science: Communication

Cambridge IAL Computer Science · Work through your exam, one topic at a time.

All guide sections

Communication

Communication

This domain covers how data moves between devices: network models, the layered protocol stack, addressing, and the protocols that sit on top. Questions usually ask you to match a protocol to a layer or purpose, or to calculate addressing values.

Layers and encapsulation

The TCP/IP model has four layers - application, transport, internet, link - mapping onto the seven OSI layers. Each layer adds its own header (encapsulation) and the receiving host strips them in reverse order. Know the mapping: HTTP, DNS and SMTP are application; TCP and UDP are transport; IP is internet; Ethernet and Wi-Fi are link.

Protocols and ports

  • TCP: connection-oriented, three-way handshake, sequencing, acknowledgements and retransmission - used where integrity matters (HTTP, SMTP, FTP).
  • UDP: connectionless, no acknowledgement, lower overhead - used for live streaming, VoIP, DNS queries and gaming.
  • Ports identify the service: 80 HTTP, 443 HTTPS, 25 SMTP, 110 POP3, 143 IMAP, 53 DNS, 67/68 DHCP.

Packet switching

Data is split into packets, each carrying source and destination addresses and a sequence number. Routers forward packets independently, so they may arrive out of order and are reassembled at the destination. This is more resilient than circuit switching, which reserves a dedicated path for the whole session.

Worked example

Host 192.168.10.130 uses mask /26. Which subnet is it in, and how many usable hosts does that subnet have?

A /26 borrows 2 host bits: mask = 255.255.255.192, block size = 256 − 192 = 64. Valid subnets start at .0, .64, .128, .192. Since 130 lies between 128 and 191, the network address is 192.168.10.128 and the broadcast is 192.168.10.191. Usable hosts = 2⁶ − 2 = 62, subtracting the network and broadcast addresses.

Traps that cost marks

  • Confusing MAC (link-layer hardware address) with IP (logical, routable address).
  • Saying a switch routes packets between networks; routers do that, switches forward within a LAN.
  • Forgetting to subtract 2 for network and broadcast addresses.
  • Claiming UDP is never used because it is unreliable, instead of trading reliability for latency.
  • Assigning a protocol to the wrong layer, e.g. placing TCP at the internet layer.