Chapter 2: Connecting to the WAN Part 2

WAN Standards and OSI Model

WANs function on the Physical Layer, Layer 1, and the Data Link Layer, Layer 2 of the OSI Model.

WAN standards describe both Physical Layer delivery methods and Data Link Layer requirements, physical addressing, flow control, and encapsulation.

WAN access standards are defined by the usual authorities:

• Telecommunication Industry Association and Electronic Industries Alliance (TIA/EIA)
• International Organization for Standardization (ISO)
• Institute of Electrical and Electronics Engineers (IEEE)

Layer 1 protocols describe how to provide electrical, mechanical, operational, and functional connections to the services of a service provider.

Layer 2 protocols define how data is encapsulated for transmission and the mechanisms for transferring the resulting frames. A variety of technologies are used such as Point-to-Point protocol (PPP), Asynchronous Transfer Mode (ATM), and High-level Data Line Control (HDLC) mechanism. Some of the former protocols use the same basic formatting or subset of HDLC.

Most WAN links are Point-to-Point so the address field in the Layer 2 frame is usually not used.

 

Common WAN Terminology and Devices

Customer Premises Equipment (CPE) are devices and inside wiring located on the enterprise edge that connect the organization to the carrier link. Can be owned or leased by the subscriber.

Data Communications Equipment (DCE), also called Data Circuit-Terminating equipment, consists of devices that put data on the Local Loop. Provides an interface to connect subscribers to the external link. *Examples include modems and edge routers. DCE Devices also provide clocking*

Data Terminal Equipment (DTE) are customer devices that connect to the DCE.

Demarcation Point is an established point that separates customer equipment and responsibility from service provider equipment and responsibility. Usually the cabling junction box on the customer premises.

Local Loop is the actual copper/fiber media that connects the CPE to the Central Office of the Service Provider. (Also known as the Last Mile)

Central Office (CO) is the service providers facility that connects the CPE to the provider network.

Toll Network consists of the long-haul/high speed links and equipment inside the service providers WAN.

Dialup Modems are considered legacy technology. The modem modulates the digital signals of the subscriber network into voice frequencies for the analog phone network. The signals received from the phone network are demodulated from the analog voice signals to digital signals. *Modulator + Demodulator = Modem*

Access Server is a device used to concentrate the dial-in dial-out user communications of dialup modems. Also considered legacy technology. The access server may have many analog an digital interfaces to support hundreds of simultaneous users.

Broadband Modem is like a dialup modem but used with high-speed DSL or cable internet services. Broadband modems use the higher broadband frequencies and transmission rates.

Channel Service Unit/Data Service Unit (CSU/DSU) is required for digital leased lines. It can be a separate device or a router interface. CSU provides digital signal termination and ensures connection stability with error correction and link monitoring. DSU converts frames so either the LAN or WAN can understand them.

WAN Switch is a layer 2 multiport device used on service provider networks. *its a switch as we know it used in a WAN*

Router is a CPE device that provides internetworking and WAN ports to the service provider WAN. The interfaces may be serial, Ethernet, or others. Other External devices may be necessary to connect to the WAN (CSU/DSU or modem).

Core Router/Multilayer Switch reside in the service providers backbone. The devices must support routing protocols and multiple high-speed ports. Key Core Routers connect other provider networks.

©2014 Cisco Press, Connecting Networks Companion Guide

This chapter will be continued in Part 3.