A WAN provides long-distance transmission of data, voice, image and video information over large geographical areas that may comprise a country, a continent or even the world. In contrast to LANs (which depend on their own hardware for transmission), WANs can utilise public, leased or private communication devices, usually in combination.
- Point-to-Point Protocol (PPP)
The Point-to-Point Protocol (PPP) is designed to handle the transfer of data using either asynchronous modem links or high-speed synchronous leased lines. The PPP frame uses the following format:
- Flag field: Each frame starts with a one-byte flag whose value is 7E(0111 1110). The flag is used for synchronisation at the bit level between the sender and receiver.
- Address field: This field has the value of FF(1111 1111).
- Control field: This field has the value of 03(0000 0011).
- Protocol field: This is a two-byte field whose value is 0021(0000 0000 0010 0001) for TCP/IP.
- Data field: The data field ranges up to 1500 bytes.
- CRC: This is a two-byte cyclic redundancy check. Cyclic redundancy check (CRC) is implemented in the physical layer for use in the data link layer. A sequence of redundant bits (CRC) is appended to the end of a data unit so that the resulting data unit becomes exactly divisible by a predetermined binary number. At its destination, the incoming data unit is divided by the same number. If there is no remainder, the data unit is accepted. If a remainder exists, the data unit has been damaged in transit and therefore must be rejected.
X.25 is widely used, as the packet switching protocol provided for use in a WAN. It was developed by the ITU-T in 1976. X.25 is an interface between data terminal equipment and data circuit terminating equipment for terminal operations at the packet mode on a public data network.
X.25 defines how a packet mode terminal can be connected to a packet network for the exchange of data. It describes the procedures necessary for establishing connection, data exchange, acknowledgement, flow control and data control.
- Frame Relay
Frame relay is a WAN protocol designed in response to X.25 deficiencies. X.25 provides extensive error-checking and flow control. Packets are checked for accuracy at each station to which they are routed.
Each station keeps a copy of the original frame until it receives confirmation from the next station that the frame has arrived intact. Such station-to-station checking is implemented at the data link layer of the OSI model, but X.25 only checks for errors from source to receiver at the network layer.
The source keeps a copy of the original packet until it receives confirmation from the final destination. Much of the traffic on an X.25 network is devoted to error-checking to ensure reliability of service. Frame relay does not provide error-checking or require acknowledgement in the data link layer.
Instead, all error-checking is left to the protocols at the network and transport layers, which use the frame relay service. Frame relay only operates at the physical and data link layer.
- Asynchronous Transfer Mode (ATM)
ATM is a revolutionary idea for restructuring the infrastructure of data communication. It is designed to support the transmission of data, voice and video through a high data-rate transmission medium such as fibre-optic cable. ATM is a protocol for transferring cells.
A cell is a small data unit of 53 bytes long, made of a 5-byte header and a 48-byte payload. The header contains a virtual path identifier (VPI) and a virtual channel identifier (VCI). These two identifiers are used to route the cell through the network to the final destination.
An ATM network is a connection-oriented cell switching network. This means that the unit of data is not a packet as in a packet switching network, or a frame as in a frame relay, but a cell.
However, ATM, like X.25 and frame relay, is a connection-oriented network, which means that before two systems can communicate, they must make a connection. To start up a connection, a system uses a 20-byte address. After the connection is established, the combination of VPI/VCI leads a cell from its source to its final destination.