mobile network, PLMN. Figure 5.8 A mobile satellite system. Most satellite projects have been financial catastrophes. Their business plans were done at a time when international mobile communication services were not available and when the expansion of digital cellular systems started. By the time satellite services became available, most business travelers already carried their own digital mobile telephones and the market for satellite serv- ice was much reduced. 5.4.8 WLANs Many working environments would benefit from having available short-haul high-data-rate wireless data transmission. Examples include hospitals, factory floors, stores, and conference and exhibition centers. An approach similar to that of a wired private LAN is needed and that approach is a WLAN. A major step in the development of WLAN technology was Standard IEEE 802.11b, which was approved in 1999. Earlier standards had many imple- mentation options and compatibility between different products was not good enough to make them popular. Standard IEEE 802.11b uses a 2.4-GHz license free frequency band and its maximum data rate over the air interface is 11 Mbps. To be compati- ble with earlier 1- and 2-Mbps IEEE 802.11 standards, Standard IEEE 802.11b sends all frame header information at 1 Mbps, which reduces the Mobile Communications user data rate. Acknowledgments and the channel reservation mechanism handle a share of the air-interface capacity, and the actual higher protocol data rate is of the order of 6 Mbps, which is shared by all users and between the two transmission directions. Standard IEEE 802.11b uses four different modulation schemes, one for each of four data rates: 1, 2, 5.5, and 11 Mbps. If the quality of the radio channel becomes worse, a more noise-tolerant modulation scheme is accessed and the data rate is reduced. The base stations of WLAN systems are called access points (APs) and they are connected to wire-line Ethernet. WLANs are actually designed to operate as wireless extensions to wire-line backbone Ethernet. The ban