Communication and Internet Technologies

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Cambridge International AS & A Level Computer Science (9618)

Communication and Internet Technologies

The big idea: Computers can talk only if they follow the same rules. The TCP/IP stack splits those rules into four layers, and packet switching lets millions of conversations share the same links at once.

Summary — What This Chapter Covers

A protocol is a set of rules for communication (format, timing, error checking). Without an agreed protocol, devices from different makers could not exchange data.
Protocols are arranged in a stack: each layer has its own job and only talks to the layers directly above and below it.
TCP/IP has four layers: Application, Transport, Internet and Link.
Application-layer protocols: HTTP (web), FTP (files), SMTP (send email), POP3 and IMAP (receive email), BitTorrent (peer-to-peer file sharing).
Circuit switching sets up a dedicated path for the whole communication; packet switching sends independent packets that may take different routes.
Routers read the destination IP address of each packet and forward it towards its destination using a routing table.

1. Protocols

14.1

Why protocols, and the TCP/IP stack

A protocol is an agreed set of rules for transmitting data: how data is formatted, addressed, sent, checked for errors and acknowledged. It is essential because the sender and receiver may use different hardware and software; only shared rules let them understand each other.

Protocols are implemented as a stack of layers. Each layer does one job, uses the services of the layer below and serves the layer above. Layers can be changed independently (Wi-Fi can replace Ethernet without changing the web browser), and the problem is easier to design and debug.

Layer	Purpose and function	Examples
Application	provides services to applications; formats the data for the user's program and starts the communication	HTTP, HTTPS, FTP, SMTP, POP3, IMAP, BitTorrent, DNS
Transport	splits data into segments (packets) and numbers them; adds source and destination port numbers so data reaches the right application; with TCP, makes sure all packets arrive (acknowledgements, retransmission) and reassembles them in order; controls flow	TCP, UDP
Internet	adds source and destination IP addresses; routes each packet across networks, choosing the next hop	IP, routers
Link (network access)	moves the data across one physical link as frames; adds MAC addresses; converts to signals on the medium	Ethernet, Wi-Fi

Worked Example

Describe what happens at each TCP/IP layer when an email is sent from a laptop to a mail server.

Protocol	Purpose
HTTP / HTTPS	transfers web pages and other resources between a web server and a browser (HTTPS adds TLS encryption)
FTP	transfers files between a client and a server; supports login, upload, download, renaming and deleting files
SMTP	a push protocol: sends email from a client to a mail server and between mail servers
POP3	downloads email from the server to one device; messages are usually deleted from the server afterwards
IMAP	email stays on the server and is synchronised, so it can be read and organised from several devices
BitTorrent	peer-to-peer file sharing: the file is split into pieces downloaded from many peers at once

How BitTorrent works

torrent file tracker swarm seed leecher

Practice Question

2. Circuit Switching, Packet Switching

14.2

Dedicated path vs independent packets

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Communication and Internet Technologies