he subscriber. The fibre plant is generally a star configuration with all optical node fibres terminating at a head- end. The coaxial cable part of the system is generally a trunk-and-branch configuration. The solution is called HFC (cable network). The upstream frequency used to transmit data from the cable modem to the CMTS is normally in the 5-42 MHz range for US systems and 5-65 MHz for European systems. The downstream frequency used for transmitting data from the CMTS to the cable modem is normally in the 42/65-850 MHz range depending on the actual cable plant capabilities. Data over cable service interface specification (DOCSIS) is the dominating cable modem standard. It defines technical specifications for both cable modem and CMTS. Access Network 13.17 WLAN The increasingly popular WLAN or wireless LAN is used as a common LAN in office environment but also as a home LAN and as a public access network to the Internet at hot spots, such as airports and conference areas. The WLAN network consists of base stations, called access points and wireless clients. The clients can be 'anything', such as PDAs and portable or stationary PCs. Soon most new portable computers are expected to be equipped with WLAN chips. IP telephony and WLAN is a new combination. The access points are bridging wireless and wired communication. They can be con- nected as an extension to an existing wired network, but they can also be connected to other base stations by means of radio in totally wireless networks. Some possible accesses are illustrated in Figure 13.36. The WLAN development is shown in Figure 13.37. The first 802.11 standard came out in 1997. It specifies wireless communication with the speeds of 1 and 2 Mbit/s in the 2.4 GHz band. The next standard was called 802.11b. It specifies radio communication with speeds of 5.5 and 11 Mbit/s in the 2.4 GHz band. 802.11b is backward compatible with 802.11. 802.11g is an extension to 802.11b, the basis of the majority of WLANs in existence today. 802.11g broadens 802.11b's data rates