Communication concepts

The essentials

Data communication is the exchange of data between two devices through a transmission medium. Its components are the sender, the receiver, the message, the communication medium (channel) and the protocol (set of rules both sides follow).

Key points

• Simplex: data flows in one direction only. Examples: keyboard to computer, TV broadcast.
• Half-duplex: both directions, but only one direction at a time. Example: walkie-talkie.
• Full-duplex: both directions at the same time. Example: a telephone call.
• Circuit switching: a dedicated path is set up between sender and receiver before communication and is kept for the whole session, as in a traditional telephone call. Resources stay reserved even when no data is sent.
• Packet switching: the message is broken into small packets that travel independently, possibly by different routes, and are reassembled at the destination. No dedicated path is needed, so the links are shared efficiently. The Internet uses packet switching.
• Channel: the path through which data is carried from sender to receiver, wired or wireless.
• Bandwidth: the range of frequencies a channel can carry, measured in hertz (Hz). Higher bandwidth means more data can be carried.
• Data transfer rate: the number of bits sent per second, measured in bps, Kbps, Mbps or Gbps.
• Time to send data = amount of data ÷ data transfer rate. For example, 40 megabits at 8 Mbps take 5 seconds.

Exam tips

• Give one example for each mode of communication; examiners like walkie-talkie for half-duplex.
• Keep units straight: bandwidth is in Hz, data rate is in bits per second.

Common mistakes

• Calling a walkie-talkie full-duplex.
• Assuming all packets of a message follow the same route in packet switching.