|  | 2.1 | WANs
| 2.1.2 | WAN standards

WAN physical layer protocols describe how to provide electrical, mechanical, operational, and functional connections for WAN services. These services are most often obtained from WAN service providers such as RBOCs, alternate carriers, and post-telephone and telegraph (PTT) agencies. WAN data link protocols describe how frames are carried between systems on a single data link. They include protocols designed to operate over dedicated point-to-point, multipoint, and multi-access switched services such as Frame Relay. WAN standards are defined and managed by a number of recognized authorities, including the following agencies: International Telecommunication Union-Telecommunication Standardization Sector (ITU-T), formerly the Consultative Committee for International Telegraph and Telephone (CCITT) International Organization for Standardization (ISO) Internet Engineering Task Force (IETF) Electronic Industries Association (EIA) WAN standards typically describe both physical layer and data link layer requirements. The WAN physical layer describes the interface between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE). Typically, 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. Several physical layer standards specify this interface: EIA/TIA-232 EIA/TIA-449 V.24 V.35 X.21 G.703 EIA-530 The common data link encapsulations associated with synchronous serial lines are listed in Figure : High-Level Data Link Control (HDLC) -- an IEEE standard; may not be compatible with different vendors because of the way each vendor has chosen to implement it. HDLC supports both point-to-point and multipoint configurations with minimal overhead Frame Relay -- uses high-quality digital facilities; uses simplified framing with no error correction mechanisms which means it can send Layer 2 information much more rapidly than other WAN protocols Point-to-Point Protocol (PPP) -- described by RFC 1661; two standards developed by the IETF; contains a protocol field to identify the network layer protocol Simple Data Link Control Protocol (SDLC) -- an IBM-designed WAN data link protocol for System Network Architecture (SNA) environments; largely being replaced by the more versatile HDLC Serial Line Interface Protocol (SLIP) -- an extremely popular WAN data link protocol for carrying IP packets; being replaced in many applications by the more versatile PPP Link Access Procedure Balanced (LAPB) -- a data link protocol used by X.25; has extensive error checking capabilities Link Access Procedure D-channel (LAPD) -- the WAN data link protocol used for signaling and call setup on an ISDN D-channel. Data transmissions take place on the ISDN B channels Link Access Procedure Frame (LAPF) -- for Frame-Mode Bearer Services; a WAN data link protocol, similar to LAPD, used with frame relay technologies