While only relatively recently standardized, IEEE 802.16 orWiMAX networks are receiving a great deal of attention in both industry and research. This is so because with the increased emphasis on multimedia data, apart from the general advantage of wireless, 802.16 promises wider bandwidth and QoS as part of the standard. As a back haul network for other networks, in particular the 802.11a/b/g/e or WiFi networks, it is well suited. As for any new technology, there are many open questions of which Transmission Scheduling and Connection Admission Control (CAC) are the most prominent. The standard intentionally makes no statement about either function. Different from other performance models we have seen, we consider an analytical framework which takes into account the close relationship between the CAC algorithms and the Scheduler algorithms and is applicable to each mode of operation and admission control paradigm specified by the standard. The long term objective of this work is to present a hybrid analytic and simulation model, based on the proposed framework, for modelling QoS metrics in 802.16 networks

Iazeolla, Prof G

Kritzinger, Prof PS

Pileggi, Mr PP

UCT Computer Science Research Document Archive

1Modelling Quality of Service in IEEE 802.16NetworksGiuseppe Iazeolla1, Pieter Kritzinger2 and Paolo Pileggi21Software Engineering and System Performance Modelling GroupUniversity of Roma ‘Tor Vergata’, Italy2Data Network Architecture GroupUniversity of Cape Town, South AfricaEmail:{iazeolla@info.uniroma2.it}{psk,ppileggi}@cs.uct.ac.zaAbstract—While only relatively recently standardized, IEEE802.16 or WiMAX networks are receiving a great deal of attentionin both industry and research. This is so because with theincreased emphasis on multimedia data, apart from the generaladvantage of wireless, 802.16 promises wider bandwidth andQoS as part of the standard. As a back haul network for othernetworks, in particular the 802.11a/b/g/e or WiFi networks, it iswell suited. As for any new technology, there are many openquestions of which Transmission Scheduling and ConnectionAdmission Control (CAC) are the most prominent. The standardintentionally makes no statement about either function. Differentfrom other performance models we have seen, we consideran analytical framework which takes into account the closerelationship between the CAC algorithms and the Scheduleralgorithms and is applicable to each mode of operation andadmission control paradigm specified by the standard. The longterm objective of this work is to present a hybrid analyticand simulation model, based on the proposed framework, formodelling QoS metrics in 802.16 networks.Index Terms—WiMAX, scheduling, call admissionI. INTRODUCTIONWhile only relatively recently standardized, IEEE 802.16or WiMAX networks are receiving a great deal of attention inboth industry and research. A network consists of individualwireless cards, such as that shown in Figure 1 (taken from[1]) which act either as a Base Station (BS) or as a SubscriberStation (SS). Resource allocation is performed in the MACunder the control of the Driver.The host interface communicates with the MAC controllerand most of the time directly to memory using Direct MemoryAccess (DMA). That is, the host writes packets to a specificmemory where the controller, using a particular schedulingalgorithm, reads them and sends them. QoS in the MAC-basedbandwidth reservation scheme or Scheduler of IEEE 802.16,cannot be decoupled from the QoS routing protocols, and playa significant role in the determining routing performance. Withthe Scheduler we understand not only the scheduling algorithmbut also the buffer management required to ensure the QoSassociated with a certain Traffic Class (TC) described below.The Scheduler is not involved in routing and call admissionis done in the host. A cross-layer approach [2], [3] is neededFig. 1: Architecture of a Wireless Cardto make effective use of the MAC layer mechanisms whenprovisioning end-to-end QoS. Further work in this area isrequired to analyze the dependencies across layers and to findan optimal solution jointly across all protocol layers. Thisis the focus of this paper, in addition to analyzing soundalgorithms to service the packet transmission queues to ensurethat QoS requirements of applications are met as discussed,inter alia, by Wongthavarawat [4].Though central to performance, IEEE 802.16-2004 stan-dard [5] writers have purposely left the scheduling and Con-nection Admission Control (CAC) details out of the standard.Support for QoS, however, is a fundamental part of 802.16Medium Access Control (MAC) layer design [6] where QoSis principally provided by the Scheduler and CAC. The factthat the 802.16 MAC is connection-oriented gives an 802.16network the advantage of having greater control over networkresources sharing amongst individual connections, making itpossible to provide better QoS. The standard provides a basicQoS platform to promote inter-operability between 802.16-based networks. Scheduling services are defined and shouldbe supported by each 802.16 station for connection admissionand transmission. These services are:• Unsolicited grant service (UGS) that caters for real-time,fixed-size data packets with constant bit rate (CBR). Thisservice is granted at regular time intervals without arequest or polls.• Real-time polling service (rtPS) that caters for real-time data packets that vary in size generated at periodicintervals, such as MPEG video and VoIP with silencesuppression, where packet sizes are variable nature of thepacket sizes.• Non-real-time Polling Service (nrtPS), designed for con-nections that do not have delay requirements. nrtPS issimilar to BE. It only differs from BE in that it guaranteesa minimum BW. An example of nrtPS traffic is FTP.• Best-effort (BE) that service makes no guarantee ofservice. To guarantee a minimum BW the connectionmust subscribe to the nrtPS service. An example of BEtraffic is web browsing data.• Extended real-time variable rate (ertPS), which wasadded in 802.16e-2005 (or Mobile-WiMAX), that sup-ports real-time applications where the applications re-quire guaranteed data rate and delay. This service isfor applications that would typically, in 802.16-2004,subscribe to the rtPS service even though they maybehave similarly to UGS traffic at times, such as VoIPwith silence suppression.The 802.16 standard describes either a Point-to-MultiPoint(PMP) or Mesh (MSH) mode. In Section II we discuss thesemodes and the different admission control paradigms. InSection III we explain the roles of, and the functional rela-tionship between, the 802.16 Scheduler and CAC. The CACand Scheduler play very different roles in that the former isclosely related to routing and has to be located in the networklayer. The Scheduler is a MAC function since it controls theup-link (UL) and down-link MAP (DL). Section IV presentsthe proposed Scheduler and CAC relationship framework andshow that it can represent each of the possible modes ofoperation identified in Section II.II. OPERATION AND ADMISSION CONTROLThe 802.16 standard supports two basic topological modesof operation: A Point-to-MultiPoint (PMP) and Mesh (MSH)modes [7] of operation, shown in Figure 2. In PMP modesubscribers or Subscriber Stations (SS), communicate withthe Base Station (BS) which acts as a central hub. The BSregulates up-link and down-link transmission between itselfand its set of SSs, in a star topology.The Mesh (MSH) mode of operation is moreover charac-terised by a multi-hop environment where every SS can com-municate with every other SS as well as the BS. In this mode,802.16 defines two mechanisms to schedule transmissions;either centralized or distributed scheduling [8]. In centralizedscheduling the BS acts as a cluster-head, determining howSSs share the available time slots in an up-link radio frame.Fig. 2: Network OverviewThis centrally coordinated approach to network managementis much simpler than distributed scheduling where a numberof BSs compete for channel access, called uncoordinateddistributed MSH mode.Even though CAC is not a MAC function, it is central to theQoS operation of 802.16 in either modes of operation. Apartfrom the modes, there are two admission control paradigms ortraffic granting schemes defined in 802.16: Either Grant-Per-Connection (GPC) or Grant-Per-Subscriber-Station (GPSS).Under GPC, the CAC algorithm considers each individualconnection arriving from an SS, while for GPSS each SSmanages admission of its own individual connections beforesending a single bandwidth (BW) request to the BS.From what we have written so far it is clear that there arefour combinations of operation and call admission, which weshall refer to as admission control (AC) modes in this paper.The combinations and our designation are:• AC mode 1: PMP mode and GPC admission control• AC mode 2: PMP mode and GPSS admission control• AC mode 3: MSH mode and GPC admission control• AC mode 4: MSH mode and GPSS admission controlIII. SCHEDULING AND CAC IN 802.16Performance models found in the literature [6], [9], [10]either consider the 802.16 Scheduler or CAC, but nowhere thatwe have seen do researchers distinguish adequately betweenthe roles of either or identify the relationship between the two.While the Scheduler independently builds the DL-MAP andUL-MAP, the CAC needs to closely consult these in order todetermine the available resources and consequently, whetherto admit or deny a connection of a particular traffic type. Suchinformation or control flow is often confused with the flow ofdata, perhaps because it is not clearly recognized that the twofunctions are located at different layers of the protocol stack.Fig. 3: Roles and Relationship between the 802.16 Schedulerand CAC from the CAC’s perspectiveFig. 4: Roles and Relationship between the 802.16 Schedulerand CAC from the Scheduler’s perspectiveIn 802.16 the CAC decides whether a connection requestis granted or whether a bandwidth (BW)-change is allowed.As seen in Figure 3, the CAC has as input either a DynamicService Addition (DSA; essentially a new connection), Dy-namic Service Change (DSC) or a Dynamic Service Deletion(DSD). These need to be considered in terms of a set ofpredefined QoS parameters. It also needs to know the currentresource state of the network which it can only determine byconsulting the Scheduler. With that information, it applies theparticular CAC algorithm and, as output in the figure, informsthe scheduler of whether a request has been admitted or not.The Scheduler shares the network resources amongst theconnections that have previously been admitted. As seen inFigure 4, the Scheduler is responsible for both the UL-MAP,which decides the order of transmission from the SS to the BS,and the DL-MAP, which transmits data downward to the SS.Note that we do not explicitly consider data traffic destinedfor an Internet fixed-line network, since we assume that thesewill be sent from the BS on a high bandwidth fixed line whichdoes not involve the Scheduler. Data arriving at the BS fromthe Internet environment, or new connection requests arrivingfrom there, are treated as wireless traffic originating at an SS.The Scheduler receives connection updates and schedulesthese requests in the UL-MAP by executing the UL algo-rithm that uses the virtual queueing information and theQoS parameters. The DL-MAP is updated by consulting theMAC Memory Buffers and executing the DL algorithm todecide the allocation of the down-link resource for theserequests. Together the UL-MAP and DL-MAP are built bythe Scheduler and they decide the MAP information used tocoordinate transmission amongst the stations in the network.The Scheduler therefore has as output the updated MAP.In the left-hand part of Figure 5, the protocol stack showsthe data flow for up-link and down-link traffic arriving at anddeparting, respectively, from the 802.16 BS. On the right-hand side, the relationship between the Scheduler and CACare shown as discussed above.On the up-link information from the wireless mediumenters the 802.16 Physical Layer (PHY), and the MAP mapsFig. 5: Information flow between the 802.16 Scheduler andCAC and data flow.incoming data or change requests with a particular connectionas scheduled in the previous DL-MAP. This information entersthe MAC layer, either being buffered there or not, dependingon the implementation and then passes through the SecuritySub-Layer, Common Part Sub-Layer (CPS) and the Conver-gence Sub-Layer (CS), in that order. The data or requests thenenter the Network layer where the CAC would consider changerequests as explained. Data, not destined for the BS, will berouted appropriately.On the down-link, information that does not have as desti-nation the BS, i.e. information that must be routed to anotherstation, together with information generated by admitted con-nections of the BS itself, passes down into the MAC layer,through the CS, CPS and Security Sub-Layer. Informationentering the CPS is queued in the MAC memory buffers. In-formation about these queues is, as mentioned before, used bythe Scheduler to allocate or re-allocate resources. Informationenters the PHY from the MAC memory buffers according tothe DL-MAP and is transmitted over the wireless medium.IV. PROPOSED MODELLING FRAMEWORKOur proposed modelling framework is based upon individ-ual models of the BS, in Figure 6, and the SS illustrated inFigure 7.A. Base Station ModelFrames arriving at the BS were previously scheduled onthe UL-MAP to be either BW requests or data PDUs to beFig. 6: Base Station Modelforwarded on the DL or, less frequently, data PDUs destinedfor the BS itself (not indicated in the figure).A BW request is taken up by the CAC that decides whetherto admit the request and, if so, will pass this information tothe Scheduler. Some authors, e.g., Cicconetti [6]et al. refer to“virtual queues”, a designation we prefer not to use.Information about these newly arrived connections or band-width change requests as well as buffer lengths, traffic typesand so on of current connections are used by some schedulingalgorithm to update the UL-MAP and DL-MAP for the nexttransmission. The UL-MAP essentially informs the schedulersat the SSs in the network of their turn to transmit on the uplink,assuming TDD, while the DL-MAP informs the SSs whichdata in the frame is destined for them.B. Subscriber Station ModelThe SS model is illustrated in Figure 7. Information arrivingat the SS, as transmitted by the BS on the down-link accordingto the BS DL-MAP, are either updates for the SS UL-MAP,data PDUs to be routed through the SS, or data destined forthe SS (not indicated in the figure).The SS Scheduler has not been described in detail becauseit adheres to the MAP specified by the BS. However, in ACmodes 2 and 4, the SS still adheres to the MAP scheduleas dictated by the BS, but must also manage connectiontransmission schedules internally since, in those modes, theSS has been allocated resource for a group of connections.The SS therefore has an SS UL Schedule to manage theseconnections including the data to be routed. The SS MACmemory buffers are managed according to the UL-MAP andadditionally by the SS scheduler in AC modes 2 and 4.V. MODEL FRAMEWORK EXTENSIONThe framework we have described are for the BS in PMPmode and with GPC admission control (AC mode 1). We shallperform analytic and simulation studies by use of a hybridmodel where the Scheduler and the CAC are sub-models.These sub-models will be changeable, depending on variousFig. 7: Subscriber Station Modelstrategies involving the Scheduler and CAC algorithms andhow the latter interoperates with the Scheduler and variousscheduling services (UGS, rtPS, nrtPS, BE, ertPS).Using QoS metrics such as BS utilization, bandwidth use,frame throughput, provider return throughput, call drop andcall refusal probabilities, etc. we shall be able to predict theeffect of each strategy.A secondary objective would be to extend the model fromGPC to GPSS admission control paradigm (i.e., AC mode2), meaning that the SS Scheduler algorithm will be morecomplex but the SS model would not change. The BS modelwould remain the same and the GPSS call admission mayimply less BW change requests.Extending from AC mode 1 to 3, i.e. from PMP to MSH,means that the Scheduler and CAC at the BS will be far morecomplex and lead to worse performance at the BS. However,no structural changes need to be made to the BS model.Extending from AC mode 2 to 4 can be done in thesame way as was done for extending from AC mode 1 to 3.Therefore we believe the framework is suitable for developingperformance models of 802.16 networks operating in any oneof the four AC modes.VI. CONCLUSIONIn this paper we have focused on the dependencies acrosslayers and the relationships between the Scheduler and CACin 802.16 networks, and outlined BS and SS models to beused for analytic and simulation studies to investigate theeffect of various strategies involving the Scheduler and CACalgorithms, and their effect on network productivity in termsof frame throughput, provider return throughput, bandwidthuse and other performance indices.REFERENCES[1] J. Tourrilhes, “Anatomy of a radio LAN,”http://www.hpl.hp.com/personal/Jean Tourrilhes/Linux/Wireless.html,August 2000.[2] T. S. R. S. Shakkottai and P. C. Karlsson, “Cross-layer design forwireless networks,” IEEE Communications Magazine, vol. 41, no. 10,p. 7480, October 2003.[3] V. Srivastava and M. Motani, “Cross-layer design: a survey and the roadahead,” IEEE Communications Magazine, no. 43, p. 112119, 12 2005.[4] K. Wongthavarawat and A. Ganz, “Packet scheduling for QoS support inIEEE 802.16,” International Journal of Communication Systems, vol. 16,pp. 81 – 96, 2003.[5] IEEE, IEEE Standard for Local and Metropolitan Area Networks, IEEE802.16 Standard, 2004.[6] C. Cicconetti, L. Lenzini, E. Mingozzi, and C. Eklund, “Quality ofservice support in ieee 802.16 networks,” Network, IEEE, vol. 20, no. 2,pp. 50–55, March-April 2006.[7] B. Li, Y. Qin, C. P. Low, and C. L. Gwee, “A survey on mobile wimax[wireless broadband access],” Communications Magazine, IEEE, vol. 45,no. 12, pp. 70–75, December 2007.[8] M. Cao, W. Ma, Q. Zhang, X. Wang, and W. Zhu, “Modelling andperformance analysis of the distributed scheduler in ieee 802.16 meshmode,” in MobiHoc ’05: Proceedings of the 6th ACM internationalsymposium on Mobile ad hoc networking and computing. New York,NY, USA: ACM, 2005, pp. 78–89.[9] D. Niyato and E. Hossain, “QoS-aware bandwidth allocation and admis-sion control next term in previous term IEEE 802.16 broadband wirelessaccess next term networks: A non-cooperative game theoretic approach,”Computer Networks, vol. 51, no. 11, pp. 3305–3321, August 2007.[10] A. Sayenko and O. Alanen, “Scheduling solution for the IEEE 802.16base station,” Computer Networks, vol. 52, pp. 96–115, 2008.

Modelling Quality of Service in IEEE 802.16 Networks

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Modelling Quality of Service in IEEE 802.16 Networks

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