|  | 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.