Abstract -- The IEEE 802.11 MAC protocol supports twomodes of operation, a random access mode for non-real-time dataapplications, and a polling mode for real-time applications. Wedesign and analyze a system that uses the polling mode for inter-active voice traffic. With larger inter-poll periods, more voicecalls can be accommodated, but at the expense of increased delay.For example, our analysis shows that with an inter-poll period of90 ms, a maximum of 26 voice calls can be handled with a worst-case delay of 303 ms, whereas with an inter-poll period of 60 ms,a maximum of 17 voice calls can be handled with a worst-casedelay of 213 ms. We also carry out an error analysis that demon-strates the need for error correction of voice packets.I. INTRODUCTIONThe IEEE 802.11 wireless LAN [1] is gaining popularity fordata applications in campus networks, such as in universitycampuses and airports. Data rates of these indoor wirelessLANs are in the order of 11 Mbps, which is considerablyhigher than outdoor wireless data services offered through cel-lular base stations. However, while the use of 802.11 LANs forInternet applications such as web browsing and electronic mailis increasing, there appears to be no immediate commercialinterest in using these LANs to carry interactive voice. This isevident in that most commercially-available offerings onlyimplement the 802.11 mode of operation that supports dataservices, called the Distributed Coordination Function (DCF)mode, and not the second 802.11 mode of operation, designedfor real-time services, called the Point Coordination Function(PCF) mode.The problem statement of this work is to determine whetherthe 802.11 PCF mode is suitable for supporting interactivevoice services. This mode of operation uses a polling schemeto provide resource guarantees for real-time sessions. There-fore, more generally, the results of our work are applicable toany polling-based scheme.The motivation for this work comes from an observationthat the PCF mode offers a “packet-switched connection-ori-ented” service, which is well suited for telephony traffic. Tele-phony traffic has been shown to have alternating periods oftalk spurts and silences [2][3]. Packet-switched solutions thattake advantage of silences in a given voice call by multiplex-ing voice data from other calls are more bandwidth-efficientthan circuit-switched solutions. This has been one of the pri-mary reasons for the ongoing movement in the telecommuni-cations industry toward moving telephony traffic from DS0based circuit-switched networks on to packet-switched net-works. In wireless networks, where bandwidth is more con-strained, the use of packet-switched techniques for carryingvoice are indeed needed. While currently most wireless usersuse cellular or cordless telephones within buildings, the avail-ability of an 802.11 wireless LAN will enable a more efficientusage of overall wireless bandwidth. By having in-buildingusers use wireless LAN access for their voice calls, more ofthe cellular resources are available for outdoors users andbuildings without 802.11 LANs. Thus, our motivation is totake advantage of the packet-switched aspect of 802.11 to sup-port bursty telephony traffic, and thus achieve better overallwireless bandwidth utilization.The “connection-oriented” aspect of the PCF mode wouldallow the network to provide delay guarantees necessary forinteractive voice. The end-to-end delay requirement for inter-active voice is 25ms without echo cancellers, 150ms with echocancellers for excellent quality voice, and 400ms with echocancellers for acceptable quality voice [4]. The PCF modewould allow for delay guarantees to be made for voice calls. Insummary, given the packet-switched and connection-orientedaspects of the PCF mode, the problem addressed in this paperis to determine how exactly to use the PCF mode to carry tele-phony traffic. Problems associated with the coexistence ofDCF and PCF are also addressed.Section II briefly summarizes the operation of an 802.11LAN and surveys related work on this topic. Section IIIdescribes our proposed solution. The solution is more thansimply stating that we use PCF to carry voice. The IEEE802.11 specification has many options and management-setta-ble parameters. Specific choices of how to operate such aLAN to support voice calls are required. Section IV providesnumerical results for the maximum number of voice calls thatcan be supported and end-to-end delays. While in Sections IIIand IV we assume error-free transfers on the wireless medium,in Section V, we consider the effect of errors. Section VI pre-sents our conclusions.II. BACKGROUND AND RELATED WORKThis section provides some background information on802.11 LANs and reviews prior work on supporting real-time(interactive) traffic using different MAC schemes.A. BackgroundAn 802.11 LAN can be operated in an ad hoc configuration,i.e., without an Access Point (AP), or in an infrastructure con-figuration, i.e., with an AP. The AP serves as a MAC layerbridge between wireless stations as well as between wirelessand wired stations. The 802.11 standard specifies a MAC pro-tocol (with the DCF and PCF modes), and three physical layeroptions: Frequency Hopping Spread Spectrum (FHSS), DirectSupport of voice services in IEEE 802.11 wireless LANsMalathi Veeraraghavan, Nabeel Cocker, Tim MoorsPolytechnic University{[email protected], [email protected], [email protected]}0-7803-7018-8/01/$10.00 (C) 2001 IEEE IEEE INFOCOM 2001Sequence Spread Spectrum (DSSS) and InfraRed (IR). Themain focus of our work is on the MAC sublayer and particu-larly the PCF. We therefore provide a review of the MAC layerwith an emphasis on the PCF mode.The DCF mode is the fundamental access method of the802.11 MAC sublayer and is based on Carrier Sense MultipleAccess with Collision Avoidance (CSMA/CA). The timeperiod during which the LAN operates in the DCF mode isknown as the Contention Period (CP). Access priority to themedium is controlled through the use of InterFrame Spaces(IFS), i.e., the time interval between frames. There are threetypes of interframe spaces: the Short IFS (SIFS), the Pointcoordination function IFS (PIFS) and the Distributed coordi-nation function IFS (DIFS). The SIFS is the shortest intervaland is used for transmission of acknowledgements, stationsresponding to polls from the point coordinator (usually theAP) during the PCF mode, and between fragments if an MACService Data Unit (MSDU) is fragmented.
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