Special Purpose Communication Devices
1. Data link layer functions:
- controls access to the circuit; formats messages to mark their beginning and end; and controls errors
2. Protocol converter functions:
- translates between different communications formats used by different devices
3. Message switching functions:
- rerouting a message back to the network without going through the host
4. Message processing functions:
- receiving a message from a remote terminal, processing the message and retrieving the data required, send data back to the remote terminal