Cisco CCNA4 - chap 1, выдержки

The three major characteristics of WANs:

WANs generally connect devices that are separated by a broader geographical area than can be served by a LAN. WANs use the services of carriers, such as telephone companies, cable companies, satellite systems, and network providers. WANs use serial connections of various types to provide access to bandwidth over large geographic areas.

WAN Physical Layer Terminology

One primary difference between a WAN and a LAN is that a company or organization must subscribe to an outside WAN service provider to use WAN carrier network services. A WAN uses data links provided by carrier services to access the Internet and connect the locations of an organization to each other, to locations of other organizations, to external services, and to remote users. The WAN access Physical layer describes the physical connection between the company network and the service provider network.

- Customer Premises Equipment (CPE) - The devices and inside wiring located at the premises of the subscriber and connected with a telecommunication channel of a carrier. The subscriber either owns the CPE or leases the CPE from the service provider. A subscriber, in this context, is a company that arranges for WAN services from a service provider or carrier.

- Data Communications Equipment (DCE) - Also called data circuit-terminating equipment, the DCE consists of devices that put data on the local loop. The DCE primarily provides an interface to connect subscribers to a communication link on the WAN cloud.

- Data Terminal Equipment (DTE) - The customer devices that pass the data from a customer network or host computer for transmission over the WAN. The DTE connects to the local loop through the DCE.

- Demarcation Point - A point established in a building or complex to separate customer equipment from service provider equipment. Physically, the demarcation point is the cabling junction box, located on the customer premises, that connects the CPE wiring to the local loop. It is usually placed for easy access by a technician. The demarcation point is the place where the responsibility for the connection changes from the user to the service provider. This is very important because when problems arise, it is necessary to determine whether the user or the service provider is responsible for troubleshooting or repair.

- Local Loop - The copper or fiber telephone cable that connects the CPE at the subscriber site to the CO of the service provider. The local loop is also sometimes called the "last-mile."

- Central Office (CO) - A local service provider facility or building where local telephone cables link to long-haul, all-digital, fiber-optic communications lines through a system of switches and other equipment.

WAN Devices

WANs use numerous types of devices that are specific to WAN environments, including:

- Modem-Modulates an analog carrier signal to encode digital information, and also demodulates the carrier signal to decode the transmitted information. A voiceband modem converts the digital signals produced by a computer into voice frequencies that can be transmitted over the analog lines of the public telephone network. On the other side of the connection, another modem converts the sounds back into a digital signal for input to a computer or network connection. Faster modems, such as cable modems and DSL modems, transmit using higher broadband frequencies.

- CSU/DSU-Digital lines, such as T1 or T3 carrier lines, require a channel service unit (CSU) and a data service unit (DSU). The two are often combined into a single piece of equipment, called the CSU/DSU. The CSU provides termination for the digital signal and ensures connection integrity through error correction and line monitoring. The DSU converts the T-carrier line frames into frames that the LAN can interpret and vice versa.

- Access server-Concentrates dial-in and dial-out user communications. An access server may have a mixture of analog and digital interfaces and support hundreds of simultaneous users.

- WAN switch-A multiport internetworking device used in carrier networks. These devices typically switch traffic such as Frame Relay, ATM, or X.25, and operate at the Data Link layer of the OSI reference model. Public switched telephone network (PSTN) switches may also be used within the cloud for circuit-switched connections like Integrated Services Digital Network (ISDN) or analog dialup.

- Router-Provides internetworking and WAN access interface ports that are used to connect to the service provider network. These interfaces may be serial connections or other WAN interfaces. With some types of WAN interfaces, an external device such as a DSU/CSU or modem (analog, cable, or DSL) is required to connect the router to the local point of presence (POP) of the service provider.

- Core router-A router that resides within the middle or backbone of the WAN rather than at its periphery. To fulfill this role, a router must be able to support multiple telecommunications interfaces of the highest speed in use in the WAN core, and it must be able to forward IP packets at full speed on all of those interfaces. The router must also support the routing protocols being used in the core.

Data Link Protocols

The most common WAN data-link protocols are:

- Frame Relay

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