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minal mobility at velocities of up to 10 m/s.In addition, it provides a means of handlingdifferent interference and propagation environments,with the aim of• maintaining the communications link atlow signal-to-interference ratios;• maintaining quality of service; and• finding a suitable trade-off between communicationsrange and data rate.The air interface of the HIPERLAN/2 standardis based on time-division duplex(TDD) and dynamic time-division multipleaccess (TDMA). HIPERLAN/2 is a flexibleplatform on which a variety of business andhome multimedia applications can be basedto provide bit rates of up to 54 Mbit/s. In atypical business application scenario, a mobileterminal receives services over a fixedcorporate or public network infrastructure.In addition to quality of service, the networkprovides mobile terminals with security andmobility management services when theymove between networks—for example,when terminals move between local area andwide area networks or between corporateand public networks. In a home applicationscenario, low-cost and flexible networkingis supported to interconnect wireless digitalconsumer devices.HIPERLAN/2 relies on cellular networkingtopology combined with ad hoc networkingcapability. It supports two basicmodes of operation: centralized mode (CM)and direct mode (DM).The centralized mode of operation appliesto the cellular networking topology whereeach radio cell is controlled by an accesspoint (AP) that covers a certain geographicalarea. In this mode, mobile terminalscommunicate with one another or with thecore network through the access point. Thecentralized mode of operation is mainly usedin indoor and outdoor business applicationswhere the area to be covered is larger than aradio cell.The direct mode of operation applies tothe ad hoc networking topology of privatehome environments and where the entireserving area is covered by one radio cell. Inthis mode, mobile terminals in a single-cellhome network can exchange data directlywith one another. The access point controlsthe assignment of radio resources to the mobileterminals.Convergence layerThe convergence layer (CL) has two mainfunctions: it adapts service requests fromhigher layers to the service offered by theFigure 5General structure of the convergence layer.DLC, and it converts higher-layer packets offixed or variable length into a fixed-lengthservice data unit (SDU) that is used withinthe DLC.The convergence layer thus maps incomingdata onto different bearers of the DLC.For example, if we assume that Ethernetquality of service is supported via IEEE802.lp, then the priority indicated in theadditional tag field stipulates the type oftraffic to be carried in the packet. 10 The convergencelayer maps different traffic typesinto different classes and consequently ontodifferent radio bearers.There are two types of convergence layer:• a cell-based convergence layer, whichhandles higher layers with fixed-lengthpackets—for instance, ATM-based corenetworks; and• a packet-based convergence layer, whichhandles higher layers with variablelengthpackets—for instance, Ethernet.Separate service-specific convergence sublayers(SSCS) have been defined to make theappropriate service adaptation for Ethernet,IEEE 1394, PPP, and the universal mobiletelecommunications system (UMTS). Figure5 depicts the basic structure of each typeof convergence layer.The padding, segmentation and reassemblefunction of the fixed-length DLC servicedata units is a key feature that makes it pos-Ericsson Review No. 2, 2000 111