31 Days Before Your CCNA Exam

310 31 Days Before Your CCNA ExamFigure 4-1WAN Physical Layer TerminologyWAN ServiceProvider NetworkCompany (Subscriber)Data TerminalEquipment (DTE)CO SwitchLocal LoopSData CommunicationEquipment (DCE)DemarcationPointLocal CentralOffice (CO)WAN Service ProviderCustomer PremisesEquipment (CPE)Service ProviderEquipmentWANs use numerous types of devices that are specific to WAN environments:■■■■■■Modem: Modulates and demodulates between analog and digital signals.CSU/DSU: A channel service unit (CSU) and a data service unit (DSU) often combined intoa single piece of equipment to provide the termination for the digital signal and ensure connectionintegrity through error correction and line monitoring. It interprets frames from thecarrier into frames that LAN devices can interpret and vice versa.Access server: Concentrates dial-in and dial-out user communications. An access servermight have a mixture of analog and digital interfaces and support hundreds of simultaneoususers.WAN switch: A multiport internetworking device used in carrier networks. These devicestypically switch traffic such as Frame Relay, ATM, or X.25 and operate at the data link layerof the OSI reference model. Public switched telephone network (PSTN) switches might alsobe used within the cloud for circuit-switched connections such as (ISDN) or analog dialup.Router: Provides internetworking and WAN access interface ports that are used to connect tothe service provider network.Core router: A service provider router that resides within the middle or backbone of theWAN rather than at its periphery. To fulfill this role, a router must be able to support multipletelecommunications interfaces of the highest speed in use in the WAN core, and it must beable to forward IP packets at full speed on all those interfaces. The router must also supportthe routing protocols being used in the core.