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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