Quality of Service Provision for IP Traffic over Wireless Local Area Networks

Transcription

1 Qualty of Servce Provson for IP Traffc over Wreless Local Area Networks Dmtrs Skyranoglou 1 Natonal and Kapodstran Unversty of Athens Department of Informatcs and Telecommuncatons dmsk@d.uoa.gr Abstract. Ths PhD dssertaton deals wth the provson of guaranteed qualty of servce (QoS) to the users of a Wreless LAN (WLAN) and the nterworkng between WLANs and the 3rd generaton and IP networks. The work s dvded nto three man parts: ) Study and development of Wreless Adaptaton Layer (WAL), a shm, transparent both for the IP layer and the underlyng WLAN- layer that supports the nterworkng of WLANs wth IP networks and provdes guaranteed QoS over WLANs by utlzng the QoS mechansms of WAL.. ) Study of traffc schedulng algorthms for WLANs that are based on IEEE 80211e protocol, the extenson of legacy IEEE protocol for supportng guaranteed QoS over WLANs. More specfcally a new traffc schedulng algorthm for e called ARROW (Adaptve Resource Reservaton Over Wreless) and an extenson of ARROW called ARROW (P- ARROW) was developed and evaluated. ) Study of nterworkng between WLANs and UMTS for the provson of guaranteed QoS for the moble users that perform a handover from one network to the other. The focus was on how the QoS mechansms of UMTS and WLANs can nterwork and combne so as to offer guaranteed QoS servce to the users that perform a handover. Keywords: Qualty of Servce (QoS), DffServ, Wreless Adaptaton Layer (WAL), IEEE e, Traffc Schedulng Algorthms, ARROW Scheduler, UMTS/WLAN Interworkng, Seamless Handover 1 Introducton The rapd development and the hgh transmsson rates attaned by the Wreless Local Area Networks (Wreless LANs - WLANs) have establshed them as one of the most attractve choces for supportng alternatve access to large 3rd generaton networks (3G) lke UMTS or metropoltan IP networks. The nstallaton of WLANs n places wth a dense moble user populaton (.e. hot-spots lke malls, arports, hosptals etc.) releves the traffc load towards the metropoltan networks whle, at the same tme, acheves an mproved level of qualty of servce for the moble users. 1 Dssertaton Advsor: Lazaros Merakos, Professor

2 The work n hand deals wth the provson of guaranteed qualty of servce (QoS) to the users of a WLAN and the nterworkng between WLANs and the 3rd generaton and IP networks. The provson of qualty of servce to WLAN users at a level at least equal to that offered by the metropoltan network s deemed as especally mportant snce the objectve s to offer the moble users a unform level of qualty of servce regardless of ther current locaton. In ths respect ths work proposes the ntroducton of a new shm-layer called Wreless Adaptaton Layer (WAL) that les between the IP and the underlyng wreless LAN DLC layer and ams at provdng or complementng the QoS support for the underlyng WLAN platform [1]-[3]. Further to ths, the work delved nto the QoS support mechansm of IEEE e WLAN protocol and proposed a novel traffc schedulng algorthm named ARROW (Adaptve Resource reservaton Over Wreless) together wth an extenson of ARROW called P-ARROW (Prortzed-ARROW) [4]- [8]. Fnally the work examned the nterworkng of WLANs wth 3G networks lke UMTS focusng agan on the provson of QoS and proposng an archtecture for supportng seamless handover for voce and vdeo streams from one platform to the other [9]-[15]. 1.1 Wreless Adaptaton layer (WAL) Several solutons are avalable n the lterature, copng wth lmtatons of the wreless lnks. Most of these solutons propose enhancements at the Transport or Applcaton layers, whle others focus on the Lnk Layer tryng to transparently mprove hgher layers performance and thus avod modfcatons. A number of these solutons fall nto the category of Performance Enhancng Proxes (PEPs) that are defned as elements used to mprove the performance of Internet Protocols on network paths where natve performance suffers due to characterstcs of a lnk or subnetwork path. The approach proposed n ths work s n lne wth the dea of PEPs but also tres to expand and generalze t. More specfcally, t s based on the ntroducton of an ntermedate layer called Wreless Adaptaton Layer (WAL) between the IP and the Lnk Layer. WAL ncorporates a set of functonal modules, vewed as generalzed PEPs, that can be dynamcally combned and adapted to the specal characterstcs of the wreless lnk and the transport protocol. WAL archtecture s shown n Fg 1. A novel and key feature of the WAL s that t s an abstracton used for servce provsonng at the lnk layer [1]-[3]. Each IP packet s classfed by WAL nto classes and assocatons. A WAL class defnes the servce offered to a partcular set of IP packets and corresponds to a partcular sequence of WAL modules that provde such a servce. A WAL assocaton dentfes a stream of IP packets classfed for the same WAL class and destned to or orgnated from a specfc moble termnal (MT). In other words, a WAL assocaton corresponds to a partcular type of servce offered to a partcular MT. In ths way, we can dfferentate the operaton of WAL on a per-user bass. In addton, servces for partcular users can be customzed to meet ther specfc QoS requrements and to mplement a dfferentated-chargng polcy.

3 Wreless Adaptaton Layer 6 1 IPv4 / IPv6 2 WAL Coordnator Module X 4 Module Y QoS Module 3 Module Z 2 6 Sgnalng: LLCT for Hperlan2 LLCT for Bluetooth LLCT for IEEE Packet Interface Connecton Measurement Data Exchange: (UpStream) 1 7 Wreless Drver Interface (Generc Ethernet Interface) (DownStream) Fg. 1. WAL Archtecture The WAL Coordnator shown n Fg. 1 can be vewed as the central ntellgence of the WAL. Both downstream (from IP layer) and upstream (to IP layer) traffc passes through the WAL Coordnator before beng processed by other modules. The QoS module (shown n Fg. 1) provdes flow solaton and farness guarantees through traffc shapng and schedulng. On the other hand, modules X/Y/Z comprse a pool of functonal modules, amng to mprove performance n a number of ways. The modules that have been dentfed so far are: ARQ module, FEC module, Fragmentaton module, IP Header Compresson module, and SNOOP module. Fnally, n order to nterface wth a number of wreless drvers of dfferent wreless technologes (such as IEEE , Bluetooth, HperLAN/2, etc.), one Logcal Lnk Control Translator (LLCT) module for each dfferent wreless technology has been ntroduced. The man functons of ths module manage the connecton status wth the wreless drver, and ensure the stream conversons toward the wreless drver. For the classfcaton of the IP packets to WAL classes a servce dfferentaton s needed. Servce dfferentaton n WAL s based on the DffServ archtecture. In ths respect, the wreless access system can be vewed as a DffServ doman wth the Access Pont actng as the DffServ boundary node, nterconnectng the wreless access system wth the core network or other DffServ domans.

4 1.2 Traffc Schedulng n IEEE e The IEEE standard s consdered today the domnant technology for wreless local area networks (WLANs). Besdes great research nterest, has enjoyed wdespread market adopton n the last few years, manly due to low-prce equpment combned wth hgh bandwdth avalablty. Recent mprovements n the physcal (PHY) layer provde transmsson speeds up to hundreds of Mbps per cell, facltatng the use of broadband applcatons. However, one of the man weaknesses of the orgnal , towards effcent support of multmeda traffc, s the lack of enhanced Qualty of Servce (QoS) provson n the Medum Access Control (MAC) layer. In order to elmnate these weaknesses and respond to busness requrements for multmeda over WLANs, IEEE s currently workng on a set of QoS-orented specfcaton amendments, referred to as IEEE e, that enhance the exstng MAC protocol and facltate the multmeda QoS provson. In IEEE e, the QoS mechansm s controlled by the Hybrd Coordnator (HC), an entty that mplements the so-called Hybrd Coordnaton Functon (HCF). The HC s typcally located n an Access Pont (AP) and utlzes a combnaton of a contenton-based scheme, referred to as Enhanced Dstrbuted Coordnaton Access (EDCA), and a pollng-based scheme, referred to as HCF Controlled Channel Access (HCCA), to provde QoSenhanced access to the wreless medum. EDCA provdes dfferentated QoS servces by ntroducng classfcaton and prortzaton among the dfferent knds of traffc, whle HCCA provdes parameterzed QoS servces to Statons (QSTAs) based on ther traffc specfcatons and QoS requrements. To perform ths operaton, the HC has to ncorporate a schedulng algorthm that decdes on how the avalable rado resources are allocated to the polled QSTAs. Ths algorthm, usually referred to as the Traffc Scheduler, s one of the man research areas n e, as ts operaton can sgnfcantly affect the overall system performance [4]. Traffc Schedulers allocates resources to the QSTAs n the form of Transmsson Opportuntes (TXOPs). A TXOP s an nterval of tme when a QSTA obtans permsson to transmt onto the shared wreless channel. In the open techncal lterature, only a lmted number of e traffc schedulers have been proposed so far and ths work partally ams at fllng ths gap. The draft amendment of IEEE e ncludes an example schedulng algorthm, referred to as the Smple Scheduler, to provde a reference for future, more sophstcated solutons. The dea of ths algorthm s to schedule fxed batches of TXOPs at constant tme ntervals. Each batch contans one fxed length TXOP per QSTA, based on the mean data rates as declared n the respectve Traffc Specfcatons (TSPECs). Wth ths dscplne the Smple Scheduler respects the mean data rates of all TSs and performs well when the ncomng traffc load does not devate from ts mean declared value (e.g., constant bt rate traffc). On the other hand, ts performance deterorates sgnfcantly when t comes to bursty traffc, as t has no means to adjust TXOP assgnments to traffc changes. Identfyng the weaknesses of the Smple Scheduler, SETT-EDD (Schedulng based on Estmated Transmsson Tmes - Earlest Due Date) scheduler provdes mproved flexblty by allowng the HC to poll each QSTA at varable ntervals, assgnng varable length TXOPs. Wth SETT-EDD TXOP assgnments are based on earlest deadlnes, to reduce transmsson delay and packet

5 losses due to expraton. SETT-EDD s a flexble and dynamc scheduler, but t lacks an effcent mechansm for calculatng the exact requred TXOP duraton for each QSTA transmsson. Each TXOP duraton s estmated based on the mean data rate of each TS and the tme nterval between two successve transmssons. In order to overcome the dsadvantages of Smple and SETT-EDD schedulers ths work proposes a new schedulng algorthm, referred to as Adaptve Resource Reservaton Over WLANs (ARROW) [4], [6], [7], that adapts TXOP duratons based on the backlogged traffc reports ssued by QSTAs. The novel characterstc of ARROW s that t explots the Queue Sze (QS) feld, ntroduced by e as part of the new QoS Data frames, not supported by legacy systems. The QS feld can be used by the QSTAs to ndcate the amount of buffered traffc for ther TSs,.e., ther transmsson requrements. Furthermore, n order to take advantage of the perodc nature of CBR streams, a CBR-enhancement of ARROW was also developed. Smulaton results show that ARROW acheves much more effcent use of the avalable bandwdth, compared to Smple and SETT-EDD, leadng to better channel utlzaton and hgher throughput. The ncreased transmsson overhead percentage of the proposed scheduler turned to be not a sgnfcant performance ssue. Fnally, t s mportant to note that ARROW does not mandate any standards changes. It could be readly deployed and mplemented n practce, provded that STAs populate the QS feld as defned n the e standard. An mportant extenson of ARROW s P-ARROW (Prortzed ARROW) [5]. The man enhancement of P-ARROW compared to ARROW s ts ablty to effcently handle dfferent traffc classes. The novel characterstc of P-ARROW s the ntroducton of Prorty Factor a, and the use of traffc prortes based on delay constrants. Performance results extracted from smulaton models, show that P-ARROW s very effcent n supportng the desred level of servce dfferentaton and prortzaton among dfferent traffc classes. 1.3 UMTS/WLAN Interworkng As the Internet technologes evolve, more sophstcated and Qualty of Servce (QoS) demandng multmeda servces are beng requested by the users. The Internet Protocol (IP), together wth ts QoS enhancement frameworks (namely the Integrated Servces - IntServ - and the Dfferentated Servces - DffServ), s currently the man transport technology for supportng all these new servces and n ths respect the motto Everythng over IP and IP over everythng has become the trend of the day. On the other hand, both UMTS and Wreless LANs (WLANs) are already commercally avalable and become ncreasngly popular. The number of moble users s growng rapdly and so does the demand for wreless access to the Internet servces, mposng the need for a unfed QoS support framework n both UMTS and WLANs. Despte the ntal mpresson, expressed by several network technology vendors, that UMTS and WLANs wll be competng technologes t appears that they can be combned and complement each other n an effectve way. The approach followed n ths work s that both UMTS and WLANs can act as access systems to one common

6 IP core network, effcently coverng both wde areas and hot-spots. One of the man requrements of ths system s a unfed QoS support for IP traffc. As RSVP s consdered the domnant sgnalng protocol of IP traffc, the dscusson focuses on the adopton of RSVP messages and parameters by UMTS or WLAN QoS mechansms [12]-[15]. Further to ths, as the next-generaton networks (NGN) are expected to support a wde varety of servce types, especally broadband multmeda servces, ncludng vdeo conference, streamng, and advanced telephony servces, a major objectve s how these servces should operate seamlessly across all dverse access systems (e.g. WLAN, UMTS, fxed broadband, WMAX, cable, etc). Ths seamless operaton presents several challenges especally when the dfferent access systems are loosely coupled and therefore lack the tght ntegraton we experence n GSM/UMTS rado envronments for nstance. To address ths ssue for the case of UMTS/WLAN nterowrkng ths work proposes a specfc archtecture for the support of seamless voce and vdeo handover between the two platforms [9]-[11]. The basc dea of the proposed archtecture s that a new nternal entty of UMTS calles Seamless Handover Control Functon, located at the IMS (Internet-Multmeda System) wll act as an anchor-pont hdng user moblty from the external IP network. Both UMTS and WLAN are also equpped wth approprate enttes that take care of nterworkng procedures such as re-routng of traffc and authentcaton of the roamng users. The proposed archtecture for the case of seamless voce handover s depcted n Fg. 2. IP Core Network Multmeda Subsystem (IMS) UE UMTS CS Doman RNS MSC MGW-n IMS-based Applcaton Server I-CSCF SHCF MGW-out S-CSCF PSTN RNS MGCF-n MGCF-out HSS P-CSCF BGCF AP WLAN DHCP WLAN AAA PSTN user 3GPP AAA Control plane movement AP DNS Interworkng WLAN Intra-WLAN Anchor WAG 3GPP DHCP 3GPP Core PDG Data plane Fg. 2. IMS-based archtecture for enablng seamless voce handover across UMTS and WLAN. Smulaton results ndcate that WLAN can accommodate a lmted number of UMTS roamers (.e. users that perform a handover from UMTS to WLAN). Ths number depends on the bandwdth allocated for these users, ther QoS requrements and also on the QoS support mechansm of WLAN [9]-[11].

7 2 The ARROW Scheduler In IEEE e the traffc scheduler has to decde on the next TXOP assgnment takng nto account traffc characterstcs and QoS requrements expressed through TSPEC parameters. As already mentoned, TXOP assgnments are performed per QSTA, whle TSPECs are defned per TS. Therefore, for each QSTA havng n actve TSs (where s the ndex of the QSTA), the traffc scheduler has to utlze some aggregate parameters, derved from the ndvdual TSPECs, whch are calculated as follows: Mnmum TXOP duraton (mtd): Ths s the mnmum TXOP duraton that can be assgned to a QSTA and equals the maxmum tme requred to transmt a packet of maxmum sze for any of the QSTA s TSs. Thus, mtd of QSTA s calculated as: M j mtd = max, j [1,n ] (1) Rj where M j and R j are the maxmum MSDU sze and the mnmum physcal rate for TS j, respectvely. Maxmum TXOP duraton (MTD): Ths s the maxmum TXOP duraton that can be assgned to a QSTA. It should be less than or equal to the transmsson tme of the Aggregate Maxmum Burst Sze (AMBS) of a QSTA. The AMBS s the sum of the maxmum burst szes (MBSs) of all TSs of a QSTA. Thus for QSTA t holds: and, n = AMBS MBS j j= 1 AMBS (2) MTD (3) R where R s the mnmum physcal bt rate assumed for QSTA (R =mn(r j ), j [1,n ]). Mnmum Servce Interval (msi): It s the mnmum tme gap requred between the start of two successve TXOPs assgned to a specfc QSTA. It s calculated as the mnmum of the msis of all the QSTA s TSs: msi = mn, j [1,n ] (4) ( ) msi j If not specfed n the TSPEC, msi j of TS j s set equal to the average nterval between the generaton of two successve MSDUs,.e., msi j = L j /ρ j. Maxmum Servce Interval (MSI): It s the maxmum tme nterval allowed between the start of two successve TXOPs assgned to a QSTA. Although no specfc gudelnes for calculatng MSI are provded, an upper lmt exsts to allow an MSDU generated rght after a TXOP assgnment to be transmtted at the next TXOP. Accordngly: MSI D MTD (5) where D s defned as the mnmum delay bound of all TSs of QSTA (D =mn(d j ), j [1,n ]). Ths s an upper lmt that ensures that successve TXOPs wll be assgned close enough to preserve delay constrants.

8 2.2 Operaton of ARROW Scheduler Both Smple and SETT-EDD, as brefly descrbed above, decde on TXOP duratons usng some knd of estmaton of the amount of data watng to be transmtted by every QSTA. ARROW tres to overcome ths drawback by adaptng TXOP duratons based on traffc feedback reports ssued by QSTAs The novel characterstc of ARROW s that t explots the Queue Sze (QS) feld, ntroduced by e as part of the new QoS Data frames, not supported by legacy systems [4], [6], [7]. The QS feld can be used by the QSTAs to ndcate the amount of buffered traffc for ther TSs,.e., ther transmsson requrements. An example of the use of the QS feld n ARROW s depcted n Fg. 3. The allocaton procedure wll be descrbed n detal later n ths secton. For smplcty reasons, one TS per QSTA s assumed. At tme t (x), QSTA s assgned TXOP (x), accordng to requrements expressed through the QS feld of the prevous TXOP as well as traffc characterstcs and QoS requrements declared n the respectve TSPEC. Usng a QoS Data frame, QSTA transmts ts data together wth the current sze of ts queue n the QS feld (QS (x)). At tme t (x+1) the scheduler assgns TXOP (x+1) to QSTA, n order to accommodate the requrements of QS (x). Durng the nterval [t (x), t (x+1)] new data s generated n QSTA, therefore QSTA uses the QoS Data frame transmtted at TXOP (x+1) to ndcate the new queue sze (QS (x+1)). In the same manner, at t (x+2) the scheduler assgns TXOP (x+2) to QSTA, accommodatng the requrements of QS (x+1) and gets the new queue sze from QSTA (QS (x+2)). As clearly shown, by utlzng the QS feld, ARROW has very accurate nformaton about the tme varyng propertes of each TS, and s able to adapt the TXOP duraton accordngly. Ths s consdered essental, especally n the case of bursty and VBR traffc, where transmsson requrements feature large tme varatons. QS (x)=5 QS (x+1)=3 QS (x+2)=6 TXOP (x) TXOP (x+1) TXOP (x+2) TD (x)=4 TD (x+1)=5 TD (x+2)=3 <= MSI <= MSI t (x) t (x+1) t (x+2) t <= D : data arrvals Fg. 3 TXOP assgnment wth ARROW As can be observed n Fg. 3, for every QSTA, data arrvng wthn the nterval [t (x), t (x+1)] can be transmtted no earler than TXOP (x+2) startng at t (x+2). Therefore, n order not to exceed the delay deadlne of MSDUs, assumng the worst

9 case that servce ntervals are equal to MSI and TXOP (x+2)=mtd, t should hold that: D 2 MSI + MTD D MTD MSI (6) If the scheduler should also take nto account possble retransmssons, relaton (6) becomes: MSI 2 D MTD 2 + m where m s the number of maxmum retransmsson attempts. ARROW ncorporates a traffc polcng mechansm to ensure that the transmsson requrements expressed through the QSs do not volate traffc characterstcs expressed through the TSPECs. For that purpose, a TXOP tmer s used, that mplements the operaton of a leaky bucket of tme unts. The TXOP tmer value T for a QSTA havng n actve TSs, ncreases wth rate r(t ): n L L j j r(t ) = ( + O) / (8) j= 1 Rj ρ j ι where O s the overhead due to PHY and MAC headers measured n seconds. Equaton (8) means that durng the tme nterval needed for the generaton of an MSDU of Nomnal Sze at mean data rate, the TXOP Tmer should be ncreased by the tme requred for the transmsson of ths MSDU. Τhe maxmum TXOP Tmer value max(t ) equals the tme requred for the transmsson of all maxmum bursts: n MBS j max(t ) = + O (9) j= 1 Rj Accordng to the operaton of ARROW descrbed below, no TXOP longer than the current value of T can be assgned to QSTA at any tme. After each TXOP assgnment, the value of the respectve TXOP tmer s reduced accordngly. The operaton of ARROW can be dvded n the followng steps: 1. The scheduler wats for the channel to become dle. 2. When the channel becomes dle at a gven moment t, the scheduler checks for QSTAs that: a. can be polled wthout volatng msi,.e., for a QSTA that was last polled at tme t, t should hold that: t t + msi (10) and, b. ther TXOP tmer value T s greater than the value of ther mtd, to ensure enough tme for the mnmum TXOP duraton. 3. If no QSTAs are found, the scheduler wats untl (10) becomes true at least for one QSTA and returns to step In dfferent case, the scheduler polls the QSTA wth the earlest deadlne. The deadlne for a QSTA s the latest tme that ths QSTA should be polled,.e., t +MSI, where t s the tme of the last poll for QSTA. (7)

10 5. Assumng QSTA havng n actve TSs s selected for pollng, the scheduler calculates TD, as follows: a. For every TS j of QSTA (j [1,n ]), the scheduler calculates TD j, as the maxmum of () the tme requred to accommodate the pendng traffc, as ndcated by the queue sze of that TS (QS j ), plus any overheads (O), and, () mtd j, to ensure that the assgned TXOP wll have at least the mnmum duraton: QSj TD j = max + O, mtd j (11) Rj In the specal case where QS j s equal to zero, TD j s set equal to the tme for the transmsson of a Null-Data MSDU. In ths way, QSTA s allowed to transmt a Null- Data MSDU, n order to update the queue sze nformaton for TS j. TD for QSTA s calculated as the sum of all TD j : n TD = TD j (12) j= 1 b. Fnally TD obtaned from (12) s compared wth the current TXOP Tmer value T, to ensure conformance wth the negotated traffc profle: TD = mn( TD, T ) (13) 6. After the scheduler assgns the TXOP, t reduces the respectve TXOP tmer value accordngly and returns to step 1: T = T TD (14) 2.2 Smulaton results To measure the performance ARROW aganst Smple and SETT-EDD, a specalzed e smulaton tool developed by ATMEL Hellas was used [8]. The smulaton scenaros consdered an ncreasng number of QSTAs attached to a QAP. All QSTAs and the QAP were supportng the extended MAC layer specfed n IEEE e and the PHY layer specfed n IEEE g, wth a transmsson rate of 12Mbps. Each QSTA had two actve sessons: a. a b-drectonal G.711 voce sesson (CBR traffc), mapped nto two TSs (one per drecton), and, b. an uplnk (from QSTA to QAP) H.261 vdeo sesson at 256 Kbps (VBR traffc), mapped nto one uplnk TS. Fg. 4 depcts throughput of non-delayed MSDUs for voce and vdeo traffc. For voce traffc (Fg. 4a), basc ARROW accommodates up to 18 QSTAs, whle SETT- EDD can manage up to 14 QSTAs and Smple up to only 7 QSTAs. Usng the enhancement for CBR traffc, the number of QSTAs can be ncreased to 19 wth CBR-enhanced ARROW, as a result of less requred overhead. For vdeo traffc (Fg. 4b), basc and CBR-enhanced ARROW outperform both SETT-EDD and Smple, accommodatng up to 19 QSTAs, as opposed to 13 wth SETT-EDD and 6 wth Smple. The man reason for the consderably mproved performance of basc ARROW s the accurate TXOP assgnment t performs, by utlzng the queue sze

11 nformaton. Ths s also shown n detal usng more metrcs later n ths secton. From Fg 4a and 4b t s clear that CBR-enhanced ARROW can extend the admsson capablty of the system, as t can accommodate up to 19 QSTAs wth voce and vdeo TSs. G.711 Non-Delayed MSDU Throughput Throughput (Kbps) Smple SETT-EDD Basc ARROW CBR-Enhanced ARROW Offered Load QSTAs (a) G.711 Voce H.261 Non-Delayed MSDU Throughput Throughput (Kbps) Smple SETT-EDD Basc ARROW CBR-Enhanced ARROW Offered Load QSTAs (b) H.261 vdeo Fgure 2. Throughput of Non-Delayed MSDUs It s nterestng to observe that throughput of SETT-EDD and ARROW (both basc and CBR-enhanced) reduces rapdly mmedately after reachng ts maxmum value. The reason s that, due to the dynamc TXOP assgnment performed by these algorthms, new TSs enterng the system can partcpate equally to the channel assgnment. Ths means that, after the overall nput traffc exceeds a value that

12 corresponds to the maxmum capablty of the scheduler, none of the TSs (new or old) s servced as requred. The Smple Scheduler on the other hand, manages to provde a stable throughput regardless of the offered load, because statc allocatons for exstng TSs are not affected by the traffc load ncrease. Ths effect hghlghts the need for an effectve admsson control scheme for SETT-EDD and ARROW that would prevent the offered load from exceedng the maxmum schedulng capablty. References 1. D.Skyranoglou, N.Passas, A. K. Salkntzs, E. Zervas A Generc Adaptaton Layer for Dfferentated Servces and Improved Performance n Wreless Networks, n Proc. PIMRC 2002, Lsbon, Portugal, September D.Skyranoglou, N.Passas, L.Merakos, Improvng QoS n a Mult-Hop Wreless Envronment, n Proc. Med-Hoc-Net 2002, Sardegna, Italy, September D.Skyranoglou, N.Passas and S.Kampourdou, A DffServ-based Classfcaton Scheme for Internet Traffc Over Wreless Lnks, n Proc. ICWLHN 2001, Sngapore, December D.Skyranoglou, N.Passas and A.Salkntzs, ARROW: An Effcent Traffc Schedulng Algorthm for IEEE e HCCA, IEEE Transactons on Wreless Communcatons, vol. 5, no.12, December N. Passas, D. Skyranoglou, and P. Mouzouras, Prortzed Support of Dfferent Traffc Classes n IEEE e Wreless LANs, Elsever Computer Communcatons Journal, vol. 29, no. 15, September D.Skyranoglou, N.Passas and A.Salkntzs, Traffc Schedulng for Multmeda QoS Over Wreless LANs, n Proc. ICC 2005, Seoul, Korea, May D.Skyranoglou, N.Passas and A.Salkntzs, "Traffc Schedulng n IEEE e Networks Based on Actual Requrements", n Proc. Moble Venue 2004, Athens, Greece, May Α.Floros, D. Skyranoglou, N. Passas, T. Karoubals, A Smulaton Platform for QoS Performance Evaluaton of IEEE e, The Medterranean Journal of Computers and Networks, vol. 2, no. 2, Aprl A. Salkntzs, N. Passas, and D. Skyranoglou, On the Support of Voce Call Contnuty across UMTS and Wreless LANs, Wley Wreless Communcatons and Moble Computng (WCMC) Journal, vol. 8, ssue 7, Sep Α. Salkntzs, N. Passas, and D. Skyranoglou, Seamless Voce Call Contnuty n 3G and WLANs, n Proc. 9th Internatonal Symposum on Wreless Personal Multmeda Communcatons (WPMC), San Dego, CA, September Α. Salkntzs, G. Dmtrads, D. Skyranoglou, N. Passas, F.-N. Pavldou, Seamless Contnuty of Real-Tme Vdeo Across UMTS and WLAN Networks: Challenges and Performance Evaluaton, Specal ssue on Towards Seamless Interworkng of WLAN and Cellular Networks, IEEE Wreless Communcatons Magazne, vol. 12, no. 3, Jun A Salkntzs, D. Skyranoglou and N. Passas, "Seamless Multmeda QoS Across UMTS and WLAN Networks", n Proc. IEEE Vehcular Technology Conference (VTC) Sprng 2005, Stockholm, Sweden, May D. Skyranoglou, N. Passas and A. Salkntzs, Support of IP QoS over Wreless LANs, n Proc. VTC Sprng 04, Mlan, Italy, May D. Skyranoglou and N. Passas, A Framework for Unfed IP QoS Support Over UMTS and Wreless LANs, n Proc. European Wreless 2004, Barcelona, Span, February N. Passas, D. Skyranoglou and A. Salkntzs, "Supportng UMTS QoS n WLANs" n Proc. Personal Wreless Communcatons (PWC) 2003, Vence, Italy, September 2003.