Wide Area Network ( WAN ) refers to a large scale network. If you have more than one Local Area Network and are connecting them, you are creating a Wide Area Network ( WAN ). A WAN may occur more than one offices, homes, schools or etc.
There should be large distances between that places. Mostly an ISP’s ( Internet Service Provider ) backbone network is used for WAN connection like MPLS , ATM or SDH. Also radio-link devices can be used for providing connectivity between different locations too. In the past, ISPs were using TDM based Leased Line technologies over copper too but nowadays all backbone is MPLS almost.
WAN protocols mostly work on the Layer-2 ( Data Link Layer ) of the OSI reference model. MPLS is known as a Layer 2,5 technology because that kind of devices can do both routing ( Level -3 ) and switching ( Layer – 2 ). These MPLS switches are in many locations belong to ISP and provide long distance connection. It is not recommended provide a wireless connection ( like radio link ) between MPLS switches because of the weather conditions. So that, mostly MPLS devices are connected via network fiber optic cable.
SDH devices were being used before MPLS very often but nowadays most of the SDH devices are becoming empty with no circuit on them. Mostly SDHs are being used to carry ISDN calls. ATM backbone is about to “die” to. Most of the ATM devices used in ISP backbones are being shut because of the access devices are IP based anymore.