tailieunhanh - CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 56

Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 56 is the Cisco approved textbook to use alongside version of the Cisco Networking Academy Program CCNA 1 and CCNA 2 web-based courses. The topics covered provide you with the necessary knowledge to begin your preparation for the CCNA certification exam (640-801, or 640-821 and 640-811) and to enter the field of network administration. | Page 519 Tuesday May 27 2003 8 57 AM WAN Characteristics 519 Figure 10-4 illustrates some of the common data link encapsulations associated with synchronous serial lines. Figure 10-4 Data Link Encapsulations HDLC PPP Frame Relay isdN Figure 10-5 CSU DSU Figure 10-5 illustrates the synchronous serial communications model where services offered to the router are made available through a modem or a CSU DSU. WAN Connection Options You can use different WAN connection options to interconnect LANs. The following subsections give a brief description of the most common WAN connection services Circuit-switched services Packet-switched services Cell-switched services Dedicated digital services Dialup cable and wireless services Figure 10-6 depicts the different WAN connection services. Page 520 Tuesday May 27 2003 8 57 AM 520 Chapter 10 WANs and Routers Figure 10-6 WAN Connection Services Circuit-Switched Services Circuit switching is a WAN switching method in which a dedicated physical circuit through a carrier network is established maintained and terminated for each communication session. ISDN is an example of a circuit-switched WAN technology. Figure 10-7 illustrates a network topology that supports circuit switching. Figure 10-7 Circuit Switching Table 10-3 describes the characteristics of two most common circuit-switched services plain old telephone service POTS and narrowband Integrated Services Digital Network ISDN . Page 521 Tuesday May 27 2003 8 57 AM WAN Characteristics 521 Table 10-3 Circuit-Switched Services Circuit-Switched Service Characteristics POTS Not a computer data service but included because many of its technologies are part of the growing data infrastructure and it is an incredibly reliable easy-to-use wide-area communications network. Typical medium is twisted-pair copper wire. Narrowband ISDN A versatile widespread historically important technology. Was the first all-digital dialup service. Cost is moderate. Maximum