A WAN is said to operate at the physical layer and at the data link layer. This does not mean that the other five layers of the OSI model are not found in a WAN. It simply means that the characteristics that separate a WAN from a LAN are typically found at the physical layer and the data link layer. In other words, the standards and protocols used in WANs at Layer 1 and Layer 2 are different from those used in LANs at the same layers.
The WAN physical layer describes the interface between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE). Generally, the DCE is the service provider and the DTE is the attached device. In this model, the services offered to the DTE are made available through a modem or a CSU/DSU. ![]()
The principal function of a router is routing. Routing occurs at the network layer, Layer 3, but if a WAN operates at Layers 1 and 2, is a router a LAN device or a WAN device? The answer is both, as is so often the case in the field of networking. A router may be exclusively a LAN device, it may be exclusively a WAN device, or it may sit at the boundary between a LAN and a WAN and be a LAN and WAN device at the same time.
One of the roles of a router in a WAN is to route packets at Layer 3, but this is also a role of a router in a LAN. Therefore routing is not strictly a WAN role of a router. When a router uses the physical and data link layer standards and protocols that are associated with WANs, it is operating as a WAN device. The primary WAN roles of a router are therefore not routing, but providing connections to and between the various WAN physical and data-link standards. For example, a router may have an ISDN interface using PPP encapsulation and a serial interface terminating a T1 line using Frame Relay encapsulation. The router must be able to move a stream of bits from one type of service, such as ISDN, to another, such as a T1, and change the data link encapsulation from PPP to Frame Relay.
Many of the details of WAN Layer 1 and Layer 2 protocols will be covered later in the course, but some of the key WAN protocols and standards are listed here for reference.
WAN physical layer standards and protocols:
- EIA/TIA-232
- EIA/TIA-449
- V.24
- V.35
- X.21
- G.703
- EIA-530
- ISDN
- T1, T3, E1, and E3
- xDSL
- SONET (OC-3, OC-12, OC-48, OC-192)
WAN data link layer standards and protocols: ![]()
- High-level data link control (HDLC)
- Frame Relay
- Point-to-Point Protocol (PPP)
- Synchronous Data Link Control (SDLC)
- Serial Line Internet Protocol (SLIP)
- X.25
- ATM
- LAPB
- LAPD
- LAPF
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