 to 54 Mbit/s within the 2.4 GHz band using OFDM (orthogonal frequency division multiplexing) technology. Because of backward compatibility, an 802.11b radio card will interface directly with an 802.11g access point (and vice versa) at 11 Mbit/s or lower depending on the range. Upgrading newer 802.11b access points to be 802.11g compliant is possible via relatively easy firmware upgrades. A complete upgrade in an area would create a need to move the access points closer together and include additional ones to accommodate higher data rates. Application scenarios GPRS/UMTS Bluetooth Internet On the move Office Ethernet Figure 13.36 WLAN Applications Understanding Changing Telecommunications - Building a Successful Telecom Business 2.4 GHz 802.11 802.11b 802.11e 802.11g Some QoS 802.11a 802.11e/h 5 GHz Coexistence Harmonised HiperLAN2 interworking 60 GHz 1-2 Mbps 11 Mbps 20-54 Mbps >100 Mbps Figure 13.37 WLAN development A big issue with 802.11g, which 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 ca