hich also applies to 802.11b, is considerable radio frequency interference from other 2.4 GHz devices, such as the newer cordless phones. 802.11a is a standard that operates in the 5 GHz frequency band with 12 separate non- overlapping channels. As a result, you can have up to 12 access points set to different channels in the same area without them interfering with each other. In addition, radio frequency interference is much less likely because of the less-crowded 5 GHz band. Similar to 802.11g, 802.11a delivers up to 54 Mbit/s, with extensions to even higher data rates possible by combining channels. Due to higher frequency, however, the range (around 25 m) is somewhat less than lower frequency systems (i.e., 802.11b and 802.11g). This increases the cost of the overall system because it requires a greater number of access points, but the shorter range enables a much greater capacity in smaller areas via a higher degree of channel reuse. 802.11a is not directly compatible with 802.11b or 802.11g networks. In other words, a user equipped with an 802.11b or 802.11g radio card will not be able to interface directly to an 802.11a access point. The real operational bit rates are much lower than the ideal ones. The gross digit speeds are summarized in Figure 13.38. 802.11a 54 Mbit/s TCP/UDP less 5 GHz band 802.11b 11 Mbit/s 2.4 GHz 802.11g 54 Mbit/s 2.4 Ghz Server Internet From line-of-sight to a number of walls Figure 13.38 Gross digit speeds - the net speed with TCP/IP traffic is considerably less than the gross digit speed Access Network Mobility management Subscriber management Signalling access network Billing/charging Security management 2G/3G core network Functionality within WLAN access network and access to 2G/3G core network functionality provided by solutions based on IETF protocols Figure 13.39 Cooperation between WLAN and 2 G/3G mobile systems WLAN as an Extension to Mobile Systems (ex GPRS and UMTS) For a mobile operator, WLAN is a complementary access technology, especially for city area packet data ser