Overview of Switched WAN Networks

Apart from point-to-point WAN Networks (where PPP is used), there is another type of WAN network known as switched WAN Network. As the name indicates, a switched WAN network is used to connect multiple end nodes through a common WAN network. The end nodes connect to a switched WAN network to either reach other nodes connected to the switched network or to connect to the public Internet.
X.25, Frame Relay, ATM, MPLS are examples of popular  switched WAN protocols.
The diagram given below illustrates a typical switched WAN network
A typical Switched WAN Network

A typical Switched WAN Network

As shown in the above diagram, end nodes typically consist of DTE (Data Terminal Equipment) and DCE (Data Communicating Equipment) combinations. While the end computer is the DTE, the DCE is a modem that implements one of the switched WAN protocols (like X.25, ATM etc.) and is responsible for interfacing with the Switched WAN network. The Switched WAN network consists of multiple protocol specific WAN Switches (like ATM switches or  MPLS switches) that basically implement Layer 2 Switching of data frames. The Switched Network could act as a multi-point to multi-point network connecting the different end nodes or it could also act as a VPN (Virtual Private Network) connecting specific end nodes or end networks. The Switched WAN network is also typically connected to an external gateway router for enabling the end nodes to connect to the public Internet.

Basic Theory of Operation of Switched WAN Networks

All switched WAN networks work on the principle of  VC (Virtual Circuit) based packet switching. ( http://computernetworkingsimplified.in/physical-layer/overview-circuit-switching-packet-switching/ )
This implies that there would be a signalling phase and then a data transfer phase.
During the signalling phase, the WAN switches communicate among themselves using a signalling protocol to establish a virtual connection between the end points that need to communicate. As part of this signalling phase, a suitable path is determined and then VC table entries are created in each of the switches that are along the path of the communicating nodes. The VC table entries typically consists of protocol specific VC identifiers like ATM VPI/VCI, MPLS labels, Frame Relay DLCIs etc.
After the signalling phase, a virtual logical connection is established between the communicating end nodes through the switched WAN network. The actual data is then carried in Layer 2 frames (data transfer phase) with suitable VC Identifiers (VPI/VCI, labels etc.). The intermediate switches look at the VC Identifiers in the incoming frames, lookup the VC table for identifying the appropriate outgoing interface and also the outgoing VC Identifiers. It then replaces the incoming VC IDs with the outgoing VC IDs and sends out data frames via. appropriate outgoing interfaces.
As data is switched by looking at short VC IDs, the switching is faster than Layer 3 Routing (that used IP address lookup). Additionally, as there is an explicit signalling phase for VC establishment, QOS, and Traffic Engineering could be implemented easily by giving different priorities to different end flows, by marking VC entries with priorities. Similarly, point to point VPNs could also created between different end points by creating dedicated VC paths and giving unique VC IDs to the VPN connections.
Also both PVC (Permanent Virtual Ciruits) and SVC (Switched Virtual Circuits) are supported by these protocols. While leased lines use PVCs, general connections use SVCs.
While X.25 and Frame Relay use Frames as the basic unit of transmission, ATMs use fixed size cells as their basic data unit.

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