So, what is a WAN? A WAN is a data communications network that serves users across a broad geographic area and often uses transmission facilities provided by common carriers such as telephone companies. These providers are companies like MCI, AT&T, UuNet, and Sprint. There are also many small service providers that provide connectivity to one of the larger carriers’ networks and may even have email servers to store clients mail until it is retrieved.
- – Telephone service is commonly referred to as plain old telephone service (POTS).
- – WAN technologies function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer.
Common WAN network components include WAN switches, access servers, modems, CSU/DSUs, and ISDN Terminals.
WAN Devices
A WAN switch is a multiport internetworking device used in carrier networks. These devices typically switch traffic such as Frame Relay, X.25, and SMDS and operate at the data link layer of the OSI reference model. These WAN switches can share bandwidth among allocated service priorities, recover from outages, and provide network design and management systems.
A modem is a device that interprets digital and analog signals, enabling data to be transmitted over voice-grade telephone lines. At the source, digital signals are converted to analog. At the destination, these analog signals are returned to their digital form.
An access server is a concentration point for dial-in and dial-out connections.
A channel service unit/digital service unit (CSU/DSU) is a digital interface device that adapts the physical interface on a data terminal equipment device (such as a terminal) to the interface of a data circuit terminating (DCE) device (such as a switch) in a switched-carrier network. The CSU/DSU also provides signal timing for communication between these devices.
An ISDN terminal is a device used to connect ISDN Basic Rate Interface (BRI) connections to other interfaces, such as EIA/TIA-232. A terminal adapter is essentially an ISDN modem.
WAN Terminating Equipment
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 (the customer’s device). In this model, the services offered to the DTE are made available through a modem or channel service unit/data service unit (CSU/DSU).
CSU/DSU (Channel Service Unit / Data Service Unit) Device that connects the end-user equipment to the local digital telephone loop or to the service providers data transmission loop. The DSU adapts the physical interface on a DTE device to a transmission facility such as T1 or E1. Also responsible for such functions as signal timing for synchronous serial transmissions.
Unless a company owns (literally) the lines over which they transport data, they must utilize the services of a Service Provider to access the wide area network.