Artech House - Voice.over.802.11.pdf - VirtuaLitera

Vo802.11: Range Is a Matter of Engineering 91TDMA) technique. DAMA is a capacity assignment technique that adapts asneeded to respond to demand changes among multiple stations. TDMA is thetechnique of dividing time on a channel into a sequence of frames, each consistingof a number of slots, and allocating one or more slots per frame to form alogical channel.With DAMA-TDMA, the assignment of slots to channels varies dynamically.For transmission from a base station to subscribers, the standard specifiestwo modes of operation, one targeted to support a continuous transmissionstream (mode A), such as audio or video, and one targeted to support a bursttransmission stream (mode B), such as IP-based traffic. Both are TDM schemes.Above the physical layer are the functions associated with providing serviceto subscribers. These functions include transmitting data in frames and controllingaccess to the shared wireless medium, and are grouped into the MAC layer.The MAC protocol defines how and when a base station or subscriber stationmay initiate transmission on the channel. Because some of the layers above theMAC layer, such as ATM, require quality of service, the MAC protocol must beable to allocate radio channel capacity to satisfy service demands.In the downstream direction (base station to subscriber stations), there isonly one transmitter, and the MAC protocol is relatively simple. In theupstream direction, multiple subscriber stations compete for access, resulting ina more complex MAC protocol. In both directions, a TDMA technique is used,in which the data stream is divided into a number of time slots.The sequence of time slots across multiple TDMA frames that is dedicatedto one subscriber forms a logical channel, and MAC frames are transmitted overthat logical channel. IEEE 802.16.1 is intended to support individual channeldata rates of from 2 to 155 Mbps.Above the MAC layer is a convergence layer that provides functions specificto the service being provided. For IEEE 802.16.1, bearer services includedigital audio/video multicast, digital telephony, ATM, Internet access, wirelesstrunks in telephone networks, and frame relay [8].Consecutive Point NetworkIn a WMAN, reliability of the network can be ensured by implementing consecutivepoint network (CPN) technology (Figure 6.4). Like a SONET fiber ring,the data flow of the network around the wireless ring would reverse flow in theevent of a disruption in the network. This ensures that only a limited part of thenetwork is down due to a disruption.Extending Range Via an Ad Hoc Peer-to-Peer NetworkAd hoc peer-to-peer technologies extend the maximum range of Wi-Fi networksfrom distances typically measured in hundreds of feet to several miles