WAN Structure Facts

WAN Structure Facts

A typical WAN structure includes the following components.

Component	Description
Consumer Premises Equipment (CPE)	Devices physically located on the subscriber's premises. CPE includes the telephone wire, telephone, modem, and other equipment, both the devices the subscriber owns and the ones leased from the WAN provider. The wiring typically includes UTP cable with RJ-11 or RJ-45 connectors. CPE is sometimes used synonymously with DTE.
Data Terminal Equipment (DTE)	A device on the network side of a WAN link that sends and receives data. The DTE resides on the subscriber's premises, and marks the point of entry between the LAN and the WAN. DTEs are usually routers, but computers and multiplexers can also act as DTEs. Broadly, DTEs are any equipment at the customer's site, and can include all computers. In a narrow sense, the DTE is the device that communicates with the DCE at the other end.
Channel Service Unit/Data Service Unit (CSU/DSU)	The CSU/DSU is a device that connects a physical circuit installed by the telco to some CPE device, adapting between the voltages, current, framing, and connectors used in the circuit to the physical interface supported by the DTE.
Demarcation point (demarc)	The point where the telephone company's telephone wiring connects to the subscriber's wiring. The demarc can also be called the network interface or point of presence. Typically, the customer is responsible for all equipment on one side of the demarc. The phone company is responsible for all equipment on the other side of the demarc.
Local loop	Cable that extends from the demarc to the central telephone office. The demarc media is owned and maintained by the telephone company. Typically, it is UTP, but it can also be one or a combination of UTP, fiber optic, or other media. Fiber optic cable to the demarc is rare.
Central Office (CO)	The switching facility closest to the subscriber, and the nearest point of presence for the WAN provider. It provides WAN-cloud entry and exit points for incoming and outgoing calls, and acts as a switching point to forward data to other central offices. A CO provides services such as switching incoming telephone signals to outgoing trunk lines. It also provides reliable DC power to the local loop to establish an electric circuit. COs use long-distance, or toll, carriers to provide connections to almost anywhere in the world. Long-distance carriers are usually owned and operated by companies such as AT&T or MCI.
Data Communication Equipment (DCE)	A device that communicates with both DTEs and the WAN cloud. DCEs are typically routers at the service provider that relay messages between the customer and the WAN cloud. In a strict sense, a DCE is any device that supplies clocking signals to DTEs. Thus, a modem or CSU/DSU at the customer site is often classified as a DCE. DCEs may be devices similar to DTEs (such as routers), except that each device plays a different role.
WAN cloud	The hierarchy of trunks, switches, and central offices that make up the network of telephone lines. It is represented as a cloud because the physical structure varies, and different networks with common connection points may overlap. Few people thoroughly understand where data goes as it is switched through the "cloud." What is important is that data goes in, travels through the line, and arrives at its destination.
Packet-Switching Exchange (PSE)	A switch on a carrier's packet-switched network. PSEs are the intermediary points in the WAN cloud.