3Com 3C888 User Guide - Page 138

138 APPENDIX A: NETWORKING PRIMER 10 Mbps Ethernet LAN More Connected 3 Com Figure 69 Example of a LAN What is a WAN? A WAN (Wide Area Network), often referred to as "the WAN," is that portion of a network which involves use of the pre-existing public telephone network and network infrastructures, such as the public Internet, or long-distance carrier services such as DSL or cable, for transporting data over long distances and a wide geographical area. The WAN segment of a network is that segment for which access is typically subscriber-based-meaning its use is essentially "borrowed," through, for instance, dial-up or leased-line accounts. It is the WAN, with its unlimited reach in terms of distance, which enables two or more LANs located at far geographical distances to be linked as part of a single network. Whereas a LAN is limited by the physical limits of its direct cabling, the WAN faces no such limits. Ultimately, it is this absence of any distance limit to the WAN which gives networks the potential to have global reach and expanse. Networks which extend into the WAN require the use of WAN devices, such as routers, which can link the LAN with the WAN by providing a means of access to these borrowed services of the WAN, typically, through the device's ability to make dial-up calls and connections. How is a LAN connected You can connect a LAN to the WAN using a device such as a router, which has the to the WAN? ability to establish a connection out to the WAN and, in some cases, receive connections in the from the WAN. Since different WAN services use different physical media and different technologies for transporting data (for example, cable is different from analog, which is different from ISDN), a WAN device has to have the proper "interface" to match the particular type of WAN service that is being utilized. Since a WAN device must also serve as a link to the LAN, it must have another interface to match the media and technology used for data transport on the LAN (typically, Ethernet). Thus, for example, the Dual 56K LAN Modem has Ethernet ports which serves as its link to the LAN on one side, and analog modem ports which serve as its link to the analog telephone network on the other. What is a LAN Modem? A LAN Modem is a hybrid device which combines the dial-up capabilities of a standard modem with the LAN expansion and sharing capabilities of an Ethernet hub. Because a computer's access to the modems in the LAN Modem is achieved through a 10 Mbps Ethernet connection rather than through a serial (COM) port, the traditional COM port speed bottleneck associated with serial port-based modems is eliminated. At the same time, the LAN Modem's Ethernet hub provides local networking capability among the attached computers.