On each WAN connection, data is encapsulated into frames before crossing the WAN link. To ensure that the correct protocol is used, the appropriate Layer 2 encapsulation type must be configured. The choice of protocol depends on the WAN technology and the communicating equipment. The figure displays the more common WAN protocols and where they are used. The following are short descriptions of each type of WAN protocol:
- HDLC - The default encapsulation type on point-to-point connections, dedicated links, and circuit-switched connections when the link uses two Cisco devices. HDLC is now the basis for synchronous PPP used by many servers to connect to a WAN, most commonly the Internet.
- PPP - Provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. PPP works with several network layer protocols, such as IPv4 and IPv6. PPP uses the HDLC encapsulation protocol, but also has built-in security mechanisms such as PAP and CHAP.
- Serial Line Internet Protocol (SLIP) - A standard protocol for point-to-point serial connections using TCP/IP. SLIP has been largely displaced by PPP.
- X.25/Link Access Procedure, Balanced (LAPB) - An ITU-T standard that defines how connections between a DTE and DCE are maintained for remote terminal access and computer communications in public data networks. X.25 specifies LAPB, a data link layer protocol. X.25 is a predecessor to Frame Relay.
- Frame Relay - An industry standard, switched, data link layer protocol that handles multiple virtual circuits. Frame Relay is a next generation protocol after X.25. Frame Relay eliminates some of the time-consuming processes (such as error correction and flow control) employed in X.25.
- ATM - The international standard for cell relay in which devices send multiple service types, such as voice, video, or data, in fixed-length (53-byte) cells. Fixed-length cells allow processing to occur in hardware; thereby, reducing transit delays. ATM takes advantage of high-speed transmission media such as E3, SONET, and T3.