The Data link Layer was covered in week 4 of Data communications, which was another high detail lecture [All Trains Now Depart Promptly -> Application, Transport, Network, Data Link, Physical] .

Using both Logical Link Control [LLC] and Media Access Control [MAC] the Data Link layer controls when computers transmit data, detects and correct transmission errors and identifies the beginning and end of messages.

Reminder, half duplex is one message at a time, full duplex (enabled by co-axial cables) message can pass each other.


Sending messages over a multipoint network requires message control. This can be accomplished somewhat inefficiently on networks less than about 20 computers through polling techniques. Contention, sending whenever the network is open is more efficient for low traffic networks (less than about 20 nodes).

Network errors involve corrupted or lost data, error rates and burst error describe the amount of network error occurring. We then discuss a number of causes of errors (ie: line outage, white noise, impulse noise, cross talk,  echo and attenuation) and some solutions to them.

A more interesting point was that of error detection:

  • Parity checks – very basic odd even check detecting about 50% of errors
  • Checksum – 95% effective, usually 16bit addition to the message
  • Cyclic Redundancy Check – 99.998 -100% effective

Correcting the errors was the next logical topic, which brought us to ‘Automatic Repeat Request’ [ARQ]. Continuous vs Stop and Wait ARQ where covered which tied into another subject, Grid Computing and some of the advances in protocols such as SCTP(see: