|  | 2.1 | WAN Service Providers
| 2.1.2 | CPE, demarc, "last-mile", CO switch, and toll network
WAN Service Providers Advances in technology over the past decade have made a number of additional WAN solutions available to network designers. When you're selecting an appropriate WAN solution, you should discuss the costs and benefits of each with your service providers. When your organization subscribes to an outside WAN service provider for network resources, the provider gives connection requirements to the subscriber, such as the type of equipment to be used to receive services. As shown in Figure , the following are the most commonly used terms associated with the main parts of WAN services: Customer premises equipment (CPE) -- Devices physically located on the subscriber's premises. Includes both devices owned by the subscriber and devices leased to the subscriber by the service provider. Demarcation (or demarc) -- The point at which the CPE ends and the local loop portion of the service begins. Often occurs at the POP of a building. Local loop (or "last-mile") -- Cabling (usually copper wiring) that extends from the demarc into the WAN service provider's central office. CO switch -- A switching facility that provides the nearest point of presence for the provider's WAN service. Toll network -- The collective switches and facilities (called trunks) inside the WAN provider's cloud. The caller's traffic may cross a trunk to a primary center, then to a sectional center, and then to a regional- or international-carrier center as the call travels the long distance to its destination. A key interface in the customer site occurs between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE). Typically, the DTE is the router, and the DCE is the device used to convert the user data from the DTE into a form acceptable to the WAN service's facility. As shown in Figure , the DCE is the attached modem, channel service unit/data service unit (CSU/DSU), or terminal adapter/network termination 1 (TA/NT1). The WAN path between the DTEs is called the link, circuit, channel, or line. The DCE primarily provides an interface for the DTE into the communication link in the WAN cloud. The DTE/DCE interface acts as a boundary where responsibility for the traffic passes between the WAN subscriber and the WAN provider. The DTE/DCE interface uses various protocols (such as HSSI and V.35) that establish the codes that the devices use to communicate with each other. This communication determines how call setup operates and how user traffic crosses the WAN.
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