WANs usually cover a geographical area of tens to thousands of kilometers,and are able to connect multiple cities or countries, or span several continents andmeanwhile provide long-distance communication, thus forming an internationalremote network. For example, an enterprise has two LANs in Beijing andShanghai, and when these two LANs are connected, a WAN is created. Gener-ally, WAN requires to lease the line of Internet Service Provider (ISP) and pay acertain fee to ISP for bandwidth every year. The bandwidth is related to the feepaid. In the early days, households used Asymmetric Digital Subscriber Line(ADSL) to dial up to access the Internet, so there were different bandwidthstandards such as 2 Mbit/s bandwidth, 4 Mbit/s bandwidth and 8 Mbit/s band-width, and corresponding charges. The technologies used in WAN include PPP,PPPoE and HDLC. The larger ISPs in China are China Telecom, China Mobileand China Unicom.