A WAN is a data communications network that operates beyond a LAN's geographic scope. One way that a WAN is different from a LAN is that you must subscribe to an outside WAN service provider, such as a regional Bell operating company (RBOC) to use WAN carrier network services. A WAN uses data links, such as Integrated Services Digital Network (ISDN) and Frame Relay, that are provided by carrier services to access bandwidth over wide-area geographies. A WAN connects the locations of an organization to each other, to locations of other organizations, to external services (such as databases), and to remote users. WANs generally carry a variety of traffic types, such as voice, data, and video. WAN technologies function at the three lowest layers of the OSI reference model: the physical layer, the data link layer, and the network layer. Telephone and data services are the most commonly used WAN services. Telephone and data services are connected from the building point of presence (POP) to the WAN provider's central office (CO). The CO is the local telephone company office to which all local loops in a given area connect and in which circuit switching of subscriber lines occurs. An overview of the WAN cloud organizes WAN provider services into three main types: Call setup- sets up and clears calls between telephone users. Also called signaling, call setup uses a separate telephone channel not used for other traffic. The most commonly used call setup is Signaling System 7 (SS7), which uses telephone control messages and signals between the transfer points along the way to the called destination. Time-division multiplexing (TDM)-Information from many sources has bandwidth allocation on a single medium. Circuit switching uses signaling to determine the call route, which is a dedicated path between the sender and the receiver. By multiplexing traffic into fixed time slots, TDM avoids congested facilities and variable delays. Basic telephone service and ISDN use TDM circuits. Frame Relay-Information contained in frames shares bandwidth with other WAN Frame Relay subscribers. Frame Relay is statistical multiplexed service, unlike TDM, which uses Layer 2 identifiers and permanent virtual circuits. In addition, Frame Relay packet switching uses Layer 3 routing with sender and receiver addressing contained in the packet. WANs use numerous types of devices, including the following: Routers, which offer many services, including LAN and WAN interface ports. WAN switches, which connect to WAN bandwidth for voice, data, and video communication. Modems, which interface voice-grade services. Modems include CSUs/ DSUs and TA/NT1 devices that interface ISDN services. Communication servers, which concentrate dial-in and dial-out user communication. The Figure shows the icons used for these WAN devices. WAN links can be ordered from the WAN provider at various speeds that are stated in bits per second (bps) capacity. This bps capacity determines how fast data can be moved across the WAN link. 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. 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.