Switched WAN network is a network that has several end nodes. There are two kinds of WAN networks point-to-point WAN network which connects two end nodes using a dedicated communication line and the other is switched WAN network that is a mesh of a point-to-point network that provides access to several end nodes.
In our previous content i.e., point-to-point WAN network we have seen several technologies to implement point-to-point WAN such as leased line, DSL, cable modem, SONET, T-lines, and PPP. In this section, we will be studying technologies that can be used to implement switched WAN.
Switched WAN Technologies
In the switched WAN network, the data sent from a source node is routed to the destination node by being switched from one node to another in the network. Switched WAN networks are suitable for long-distance transmission.
Switched WAN network uses a connection-oriented technology where the switches are used to establish the path between source and destination node.
If a source node wants to send data to the destination node at first a path is established between them and then the data is transmitted over the established path. Once the communication is over the path between the sender and receiver is terminated.
The switched WAN can be a circuit-switched network or packet-switched network.
1. Circuit Switched Network
In the circuit-switched network, when the source node wants to communicate with the destination node, the physical connection is established from the source node to the destination node. The source then transmits the entire message over this physical connection.
Once the message is transferred the source node informs the network to terminate the connection. The circuit switching is always applied at the physical layer.
2. Packet Switching
In packet switching the entire message to be transmitted is divided into small chunks which refer to as a packet. Now each packet travels the optimum route from source to destination.
At the destination, all the packets are assembled and then forwarded to the upper layer. The packet switching is performed at the network layer.
X.25 is the first switched WAN which was introduced in the year 1970 and is a packet-switched WAN network. In circuit switching, there is a committed path between the two communicating end nodes.
While in the packet switching the data has to be organized into the packets for transmission over the network. So, a certain level of cooperation is required between the end nodes and the network through which the packets have to be transmitted.
The X.25 specifies the set of protocols that defines communication in the packet-switched wide area network. X.25 carries the IP packets between the two end nodes of a wired area network.
But the IP is a network layer protocol and IP packets have to be carried inside the frame at the data link layer. But as X.25 was introduced before the evolution of the OSI model, it is a three-layer protocol i.e., it is comprised of its own network layer.
So, now the IP packet has to be first enclosed in the X.25 network layer packet and this is further encapsulated into frames at the data link layer.
The X.25 protocol suite was introduced when there were no reliable transmission media. That’s why it has a strong error control mechanism. And due to this the X.25 provides slow transmission which gives the reason for its replacement. X.25 was later replaced frame relay protocol.
Frame relay is used to implement switched WANs which uses packet-switched technology. The frame relay standard was introduced to replace the slow X.25 standard. The frame relay provides services at the physical layer and data link layer.
In frame relay as such, there are no specifications for the physical layer. The implementer can make use of whatever is available as it supports all the protocols identified by ANSI. At the data link layer, the error detection mechanism is implemented and the error control mechanism is left for the upper layers.
In frame relay, the error correction is not performed at every hop of a WAN network instead the error correction is left for the communicating end nodes.
The frame relay makes use of the virtual circuits that can either be a permanent virtual circuit (PVC) or switched virtual circuit (SVC). The PVC network establishes a permanent virtual connection between the two communicating nodes of WAN which exist even when the nodes are communicating. The SVC network establishes a short-term virtual connection between the two communicating nodes of WAN which exist only when the nodes are communicating.
Let us overview the advantages of frame relay over the X.25 standard.
- The switched WAN implemented using frame relay provides high data transmission rates as compared to X.25. Earlier the data rate provided by frame relay is 1.54 Mbps but now it has 44.73 Mbps.
- Frame relay supports bursty data that means it provides the required bandwidth spontaneously to the communicating node.
- As frame relay does not support error checking and left this task for communicating end stations.
ATM (Asynchronous Transmission Mode)
The ATM is referred to as a cell relay protocol. ATM is a standard used to implement a switched WAN which uses packet switching technology. Similar to X.25 and frame relay ATM also transmits data in small chunks which is referred to as cells.
The ATM uses fixed-sized and fixed-format cells which improves the transmission speed of the cells over the high-speed network. Entire data is packed into fixed-sized cells. The cells from different end nodes are multiplexed using asynchronous time-division multiplexing and are routed over the transmission path.
ATM uses the virtual circuit to transmit cells from one node to another node. The transmission path has several virtual paths and inside each virtual path, there are multiple virtual circuits that are used to carry the cells from source to destination.
Consider transmission path as a collection of all the highways, where all the highways connect two cities. Among this set of highways, a single highway is a virtual path and a lane in a highway is a virtual circuit.
All cells from the same message travel through the same virtual circuit and maintain the same order till they reach the destination node.
ATM standard defies three layers application adaption layer, ATM layer, and physical layer. The application adaption layer can interface with the existing system and provide interconnectivity between the new ATM system and the existing system.
So, this is all about the switched WAN that connects several end nodes. The switch WAN network makes use of switches to create a wide area network.