1.1 Remote Access
1.1.5 WAN encapsulation protocols
In order to exchange traffic over a WAN link, the packets must be encapsulated into a Layer 2 frame while it is crossing the WAN link. You need to configure the correct Layer 2 encapsulation type to ensure that the correct protocol is used. Some encapsulation types are listed on the Figure. 

Encapsulation must be configured on the router when configuring the interface. In an ISDN environment, the Point-to-Point Protocol (PPP) is the Layer 2 encapsulation of the B channel. LAPD is the encapsulation for the D channel.

Either the proprietary Cisco or Internet Engineering Task Force (IETF) (defined in RFC 1490) encapsulations are the Layer 2 encapsulations for Frame Relay. Link Access Procedure, Balanced (LAPB) is the X.25 encapsulation.

The choice of encapsulation protocol depends on the WAN technology and the communicating equipment. Typical WAN protocols include the following:

  • PPP -- PPP is a standards-based protocol for router-to-router and host-to-network connections over synchronous and asynchronous circuits.
  • Serial Line Internet Protocol (SLIP) -- SLIP is an older standard protocol for point-to-point serial connections using a variation of TCP/IP. SLIP is the predecessor of PPP.
  • High-Level Data Link Control (HDLC) -- On a Cisco router, HDLC is the default encapsulation type on serial interfaces. As a bit-oriented synchronous data link layer protocol, HDLC specifies a data-encapsulation method on synchronous serial links using frame characters and checksums. HDLC implementations are proprietary, so Cisco's HDLC is typically used only when connecting two Cisco devices. When connecting routers from different vendors, PPP (which is standards-based) is used instead.
  • X.25/LAPB -- X.25 is an ITU-T standard that defines the way connections between DTE and DCE devices are maintained for remote terminal access and computer communications in public data networks. It uses LAPB to provide high reliability.
  • Frame Relay -- Frame Relay is a high-performance, packet-switched, WAN protocol that can be used over a variety of network interfaces. It is a very streamlined protocol that does not have the windowing and reliability of X.25.
  • Asynchronous Transfer Mode (ATM) -- ATM is an international standard for cell relay, in which multiple service types (such as voice, video, or data) are conveyed in fixed-length (53-byte) cells. Fixed-length cells allow processing to occur in hardware, thereby reducing transit delays. ATM is designed to take advantage of high-speed transmission media such as E3, Synchronous Optical Network (SONET), and T3.