- Point to point communication not usually practical
- Devices are too far apart
- Large set of devices would need impractical number of connections
- Solution is a communications network
Simplified Network Model
Wide Area Networks
- Large geographical area
- Crossing public rights of way
- Rely in part on common carrier circuits
- Alternative technologies
- Circuit switching
- Packet switching
- Frame relay
- Asynchronous Transfer Mode (ATM)
Circuit Switching
- Dedicated communications path established for the duration of the conversation
- e.g. telephone network
Packet Switching
- Data sent out of sequence
- Small chunks (packets) of data at a time
- Packets passed from node to node between source and destination
- Used for terminal to computer and computer to computer communications
Frame Relay
- Packet switching systems have large overheads to compensate for errors
- Modern systems are more reliable
- Errors can be caught in end system
- Most overhead for error control is stripped out
Asynchronous Transfer Mode
- ATM
- Evolution of frame relay
- Little overhead for error control
- Fixed packet (called cell) length
- Anything from 10Mbps to Gbps
- Constant data rate using packet switching technique
