A generic wide area network uses the following components: Component | Description | WAN cloud | The WAN cloud is the collection of equipment that makes up the WAN network. The WAN cloud is owned and maintained by large telecommunications companies. 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. | Central Office (CO) | The central office is a switching facility connected to the WAN, and is the nearest point of presence for the WAN provider. It provides WAN-cloud entry and exit points. | Local loop | The local loop is the cable that extends from the central office to the customer location. The local loop is owned and maintained by the WAN service provider. Typically, it is UTP, but it can also be one or a combination of UTP, fiber optic, or other media. The local loop is often referred to as the "last mile" because it represents the last portion of the WAN up to the customer premises. | Demarcation point (demarc) | When you contract with a local exchange carrier (LEC) for data or telephone services, they install a physical cable and a termination jack onto your premises. The demarcation point is the line that marks the boundary between the telco equipment and the private network or telephone system. * Typically, the LEC is responsible for all equipment on one side of the demarc, and the customer is responsible for all equipment on the other side of the demarc. * The demarc is also called the minimum point of entry (MPOE) or the end user point of termination (EU-POT). * The demarc is typically located in the bottom floor of a building, just inside the building. For residential service, the demarc is often a small box on the outside of the house. | Consumer Premises Equipment (CPE) | Devices physically located on the subscriber's premises are referred to as the consumer (or customer) premises equipment. CPE includes the telephone wire, telephone, modem, and other equipment, both the devices the subscriber owns and the ones leased from the WAN provider. | Channel Service Unit/Data Service Unit (CSU/DSU) | A CSU/DSU is a device that converts the signal received from the WAN provider into a signal that can be used by equipment at the customer site. A CSU/DSU is composed of two separate devices: * The CSU terminates the digital signal and provides error correction and line monitoring. * The DSU converts the digital data into synchronous serial data for connection to a router. The CSU/DSU might be two separate devices, one combined device, or it might be integrated into a router. |
WANs employ one of the two following methods to transfer data: Method | Description | Circuit Switching | A circuit switched network uses a dedicated connection between sites. Circuit switching is ideal for transmitting data that must arrive quickly in the order it is sent, as is the case with real-time audio and video. | Packet Switching | A packet switched network allows data to be broken up into packets. Packets are transmitted along the most efficient route to the destination. Packet switching is ideal for transmitting data that can handle transmission delays, as is often the case with Web pages and e-mail. |