|  | 2.1 | WAN Service Providers
| 2.1.1 | WAN services

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. Figure illustrates the relationship between the common WAN technologies and the OSI reference model. 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 (see Figure ) 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.
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