2.3 How WANs relate to the OSI Model
2.3.3 Name, and briefly describe, six WAN data link encapsulations

The WAN data link layer defines how data is encapsulated for transmission to remote sites. WAN data-link protocols describe how frames are carried between systems on a single data path. Figure shows the common data-link encapsulations associated with WAN lines, which are:

  • Frame Relay -- By using simplified encapsulation with no error correction mechanisms over high-quality digital facilities, Frame Relay can transmit data very rapidly compared to the other WAN protocols.
  • Point-to-Point Protocol (PPP) -- Described by RFC 1661, PPP was developed by the IETF. PPP contains a protocol field to identify the network-layer protocol.
  • ISDN -- A set of digital services that transmits voice and data over existing phone lines.
  • Link Access Procedure, Balanced (LAPB) -- For packet-switched networks, LAPB is used to encapsulate packets at Layer 2 of the X.25 stack. It can also be used over a point-to-point link if the link is unreliable or there is an inherent delay associated with the link, such as in a satellite link. LAPB provides reliability and flow control on a point-to-point basis.
  • Cisco/IETF -- Used to encapsulate Frame Relay traffic. The Cisco option is proprietary and can be used only between Cisco routers.
  • High-Level Data Link Control (HDLC) -- An ISO standard, HDLC might not be compatible between different vendors because of the way each vendor has chosen to implement it. HDLC supports both point-to-point and multipoint configurations.