Department of Electrical and Computer Systems Engineering

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1 Department of Electrcal and Computer Systems Engneerng Techncal Report MECSE A Survey of IEEE MAC Mechansms for Qualty of Servce (QoS) n Wreless Local Area Networks (WLANs) D. Pham, A. Sekercoglu and G. Egan

2 MECSE : "A Survey of IEEE MAC Mechansms for...", D. Pham, A. Sekercoglu and G. Egan A Survey of IEEE MAC Mechansms for Qualty of Servce (QoS) n Wreless Local Area Networks (WLANs) Mnh-Duc Pham Y. Ahmet Şekercoğlu Gregory K. Egan March 29, 2004 Contents 1 Introducton 4 2 Legacy IEEE MAC Dstrbuted Coordnaton Functon Pont Coordnaton Functon IEEE e MAC Enhancements Enhanced Dstrbuted Coordnaton Functon Hybrd Coordnaton Functon Dstrbuted Approaches Based on DCF Approaches based on prorty Approaches based on IFS Approaches based on CW Approaches based on combnaton of IFS and CW Dscusson Approaches based on far schedulng Matchng prorty to IFS Matchng prorty to CW Matchng prorty to BI Dscusson Centralzed Approaches Based on PCF Prorty-based approaches TDMA-lke approaches Adaptve pollng approach Contenton-based multpollng approach Dscusson Summary and Concluson 25 Lst of Fgures 1 Two superframes DCF access method A typcal superframe e EDCF access method

3 MECSE : "A Survey of IEEE MAC Mechansms for...", D. Pham, A. Sekercoglu and G. Egan e EDCF queues vs DCF queue per staton A typcal e superframe Lst of Tables 1 Mappng from Prorty to Access Category n EDCF Abstract Wreless local area network (WLAN) s becomng the edge network of choce n today s network nfrastructure. The IEEE , whch nvolves the Medum Access Control (MAC) and physcal (PHY) layers, s so far the most wdely used WLAN standard. However, t does not support Qualty of Servce (QoS) requrements of an ncreasng number of multmeda servces beng used on the networks. Therefore, a number of technques for QoS support at MAC layer have been proposed. Ths artcle revews some of the presented methods and explans the prncples, the advantages and drawbacks of each method. From ths survey, t seems that selectng the proper QoS mechansm and the best set of MAC parameters stll remans a topc requrng more research. Keywords IEEE , IEEE e, MAC (Medum Access Control), QoS (Qualty of Servce), WLAN (Wreless Local Area Network), DCF (Dstrbuted Coordnaton Functon), PCF (Pont Coordnaton Functons), EDCF (Enhanced Dstrbuted Coordnaton Functon), HCF (Hybrd Coordnaton Functon). Lst of Abbrevatons AC ACK ADB AEDCF AI AID AIFS AIFSD AP AR ARME BC BCUT BEB BI BSS CA CAP CF CFACK CFB CFP CFPRI CoS CP Access Category Acknowledgment Age Dependent Backoff Adaptve Enhanced DCF Assocaton Informaton Assocaton ID Arbtraton Inter-Frame Space AIFS Duraton Access Pont Assocaton Request Assured Rate MAC Extenson Backoff Counter Backoff Counter Update Tme Bnary Exponental Backoff Backoff Interval Basc Servce Set Collson Avodance Controlled Access Perod Contenton Free Contenton Free Acknowledgment Contenton Free Burst Contenton Free Perod Contenton Free Perod Repetton Interval Class of Servce Contenton Perod 2

4 MECSE : "A Survey of IEEE MAC Mechansms for...", D. Pham, A. Sekercoglu and G. Egan CSMA Carrer Sense Multple Access CTS Clear To Send CW Contenton Wndow CW mn Contenton Wndow Mnmum CW max Contenton Wndow Maxmum DBRP Dstrbuted Bandwdth Reservaton Protocol DC Defct Counter DCF Dstrbuted Coordnaton Functon DCF/SC DCF/Shortened Contenton Wndow DDRR Dstrbuted Defct Round Robn DFS Dstrbuted Far Schedulng DIFS DCF Inter-Frame Space DLC Data Lnk Control DRR Defct Round Robn DS Dstrbuton System DU Data Unt DWFQ Dstrbuted Weghted Far Queung EDCF Enhanced DCF EIED Exponental Increase Exponental Decrease FIFO Frst In Frst Out FTP Fle Transfer Protocol HAD Hybrd Actvty Detecton HCF Hybrd Coordnaton Functon HIPERLAN Hgh Performance LAN IEEE Insttute of Electrcal and Electroncs Engneers IETF Internet Engneerng Task Force IFS Inter-Frame Space IP Internet Protocol ISM Industral, Scentfc and Medcal ISO Internatonal Organzaton for Standardzaton JW Jammng Wndow LAN Local Area Network LT Lfe Tme MAC Medum Access Control M-DCF Modfed DCF MF Multplcator Factor M-PCF Modfed PCF MPDU MAC Protocol Data Unt MPEG2 Moton Pcture Experts Group 2 MPX Moton Pcture Extreme Compressed Moves MS Moble Staton MSDU MAC Servce Data Unt NAV Network Allocaton Vector NRTDU Non-Real-Tme Data Unt NTS Nothng To Send OFDM Orthogonal Frequency Dvson Multplexng PC Pont Coordnator PCF Pont Coordnaton Functon PF Persstent/Persstence/Prorty Factor PHY Physcal layer PI Pollng Informaton PIFS PCF Inter-Frame Space P-MAC Prorty-based MAC PVT Packet Vrtual Tme 3

5 MECSE : "A Survey of IEEE MAC Mechansms for...", D. Pham, A. Sekercoglu and G. Egan QoS QSTA RCG RF RR RTDU RTS SAD SCFQ SD SID S-PCF SS STA SVT TBTT TC TCMA TCP TDMA TOS TXOP UAT UDP UNII UP VDCF WLAN Qualty of Servce QoS Staton Reservaton Cycle Generator Reservaton Frame Round Robn Real-Tme Data Unt Request To Send Statstcal Actvty Detecton Self-Clocked Far Queung Slow Decrease Sequence ID Slotted PCF Slave Scheduler Staton Staton Vrtual Tme Target Beacon Transmsson Tme Traffc Category Tered Contenton Multple Access Transmsson Control Protocol Tme Dvson Multple Access Type of Servce Transmsson Opportunty Urgency Arbtraton Tme User Datagram Protocol Unlcensed Natonal Informaton Infrastructure User Prorty Vrtual DCF Wreless Local Area Network 1 Introducton Over the past few years, many multmeda applcatons such as voce telephony, streamng audo and vdeo on demand have become ncreasngly popular under the Internet Protocol (IP) networks. However, IP was not desgned to support multmeda servces wth strngent requrements on mnmum data rate, delay and jtter. The desre to use these multmeda applcatons over IP networks has led to the need for enhancng the exstng networks wth end-to-end QoS support. QoS s the ablty to offer some persstent data transmsson over the network wth dfferent treatment for dfferent traffc classes [36]. The Internet Engneerng Task Force (IETF) s currently workng on servce dfferentaton at the IP layer to support varous traffc classes. However, for optmal result, there s a need for QoS support from lower layers [52], especally Data Lnk Control (DLC) layer. The IEEE WLAN standard, whch covers the MAC sublayer of the data lnk layer and the physcal layer, s ganng growng popularty, acceptance and s beng deployed everywhere, such as hot spots n coffee shops, hotels and arports. However, the standard does not currently provde QoS support for multmeda applcatons. As a result, numerous proposals for QoS support at the MAC layer have been proposed by an actve research communty. In ths artcle, we present a survey of research efforts focusng on IEEE MAC QoS mechansms for WLANs. The artcle s organzed as follows: The legacy IEEE MAC s descrbed n Secton 2, whle ts e enhancements are presented n Secton 3. Secton 4 revews the dstrbuted mechansms for QoS support based on Dstrbuted Coordnaton Functon (DCF). The centralzed mechansms for QoS support based on Pont Coordnaton Functon (PCF) are evaluated n Secton 5. The artcle concludes wth a summary and concluson n Secton 6. 4

6 MECSE : "A Survey of IEEE MAC Mechansms for...", D. Pham, A. Sekercoglu and G. Egan 2 Legacy IEEE MAC An IEEE WLAN can operate n two modes: ad-hoc or nfrastructure. In ad-hoc mode, moble statons (MSs) can drectly communcate wth each other. In the nfrastructure mode, an access pont (AP) allows the MSs to communcate wth each other, and also connects them to a dstrbuton system (DS). In IEEE parlance, the cluster of communcatng MSs (and the AP) s called Basc Servce Set (BSS). In the nfrastructure mode APs, through the DS, lnk the BSSs together. The tradtonal IEEE MAC [1] contans algorthms to arbtrate meda access (called coordnaton functons), whch control the operaton of MSs wthn a BSS to decde when a MS s allowed to send or receve frames va the wreless medum. Two coordnaton functons are defned, whch nclude the compulsory DCF based on Carrer Sense Multple Access/Collson Avodance (CSMA/CA) and the optonal PCF based on pollng. The DCF may be used n both ad-hoc or nfrastructure networks, whle the PCF can only be used n the nfrastructure networks. The DCF and PCF modes of operaton are multplexed n a superframe, whch s repeated over tme. In a superframe, a Contenton Free Perod (CFP), n whch PCF s used for medum access, s followed by a Contenton Perod (CP), n whch DCF s used. A dagram of two superframes s shown n Fgure 1. All the parameters such as Short Inter-Frame Space (SIFS), PCF Inter-Frame Space (PIFS), DCF Inter-Frame Space (DIFS), SlotTme, Contenton Wndow Mnmum (CW mn ), Contenton Wndow Maxmum (CW max ) depend on the selecton of the underlayng physcal layer. The IEEE standard conssts of a famly of standards. The orgnal standard [1] provdes data rates up to 2 Mb/s at 2.4 GHz ndustral, scentfc, and medcal (ISM) band. Its enhanced verson IEEE b [3] acheves the data rates up to 11 Mb/s n the ISM band. Another verson, IEEE a [2] extends the data rates up to 54 Mb/s, usng orthogonal frequency dvson multplexng (OFDM) at 5 GHz unlcensed natonal nformaton nfrastructure (UNII) band. However, regardless of the selected physcal layer, followng equaltes are always vald: PIFS SIFS SlotTme and DIFS PIFS SlotTme. The operaton of DCF and PCF are descrbed n detals n Secton 2.1 and Secton 2.2, respectvely. Delay (due to busy medum CFP repetton perod Contenton Free Perod Contenton Perod Due to busy medum n the prevous CFP repetton perod Foreshortened CFP Contenton Perod B PCF DCF Busy B PCF Medum DCF NAV Access Pont broadcasts the Beacon Frame Access Pont polls the statons NAV Varable Length (per SuperFrame) B = Beacon Frame In ths perod, statons defer accessng the medum Fgure 1: Two superframes 5

7 MECSE : "A Survey of IEEE MAC Mechansms for...", D. Pham, A. Sekercoglu and G. Egan Immedate access w hen medum s dle >= DIFS DIFS PIFS Contenton Wndow DIFS Busy Medum SIFS Backoff Wndow Next Frame Defer Access Slot Tme Select Slot and decrement backoff as long as medum stays dle Fgure 2: DCF access method 2.1 Dstrbuted Coordnaton Functon The access method of DCF s shown n Fgure 2. Each MS has a Frst In Frst Out (FIFO) transmsson queue. A staton can sense whether the medum s busy (a staton s transmttng) or dle (there s no transmsson). When a frame (or n ISO terms, a MAC Servce Data Unt (MSDU), whch s the unt of data enterng the MAC layer from hgher layer) reaches the front of a staton s transmsson queue, the staton checks the state of the medum. If the medum s busy, the staton wats untl the medum s dle. After that, t wats for a duraton of DIFS. If the medum s stll dle durng the DIFS nterval, the staton ntates the backoff procedure. Ths s the Collson Avodance (CA) mechansm to reduce the probablty of frame collsons f two or more statons perceve the medum dle at the same tme. Backoff Counter (BC) s chosen as a random nteger n the unformly dstrbuted nterval [0, CW], where CW s the current contenton wndow of ths staton. The ntal value of CW s CW mn. The Backoff Interval (BI) s the backoff tme of the staton, whch s equal the BC multpled by the SlotTme. If the medum s dle for SlotTme, the BC s decremented by 1. When BC s 0, the staton transmts the frame. If durng the backoff procedure, the medum becomes busy (another staton fnshes ts backoff procedure and transmts a frame), the BC s suspended. Ths staton wll have to wat for the medum to become dle agan, wat for extra DIFS duraton before contnung the backoff procedure wth the suspended BC value. If the transmtted frame s receved successfully, the recevng staton wats for a duraton of SIFS and sends back an Acknowledgment (ACK) frame. The sendng staton, upon recevng ths ACK frame, resets ts CW to CW mn, defers for DIFS and ntates a post backoff procedure, even f there s no frame watng n the transmsson queue. The post backoff procedure guarantees that there s at least one BI between transmsson of two successve MSDUs. If the sendng staton does not receve the ACK frame after some tme (ether due to two or more statons fnsh backoff process at the same tme and the transmtted frames collde, the transmtted frame s lost or the ACK frame s lost), t assumes that the frame transmsson s unsuccessful. It ncreases the CW to the new value of 2 CW 1 1, wth upper lmt of CW max, wats for the medum to become dle agan, wats for DIFS and performs backoff process wth ths new value of CW. In case when an MSDU reaches the front of a staton s transmsson queue and the staton s performng DIFS deferral or post backoff process, the frame s transmtted when the backoff procedure completes successfully. If the MSDU reaches the front of the queue and the backoff procedure has fnshed and the medum has been dle for at least DIFS, the frame s transmtted nstantly. The MSDU can be of any sze up to 2304 bytes. A large MSDU can be fragmented nto smaller frames. To solve the hdden staton problem, an optonal Request To Send/Clear To Send (RTS/CTS) mechansm s ntroduced. Instead of transmttng a DATA frame after ganng access to the medum, a staton sends a short RTS frame. The recevng staton wats for SIFS and reples wth a short CTS frame. Upon recevng the CTS frame, the sendng staton wats for SIFS and transmts the DATA frame. The RTS and CTS frames stores nformaton about transmsson duraton of the DATA frame and are receved by adjacent statons, whch could be hdden from sendng and/or recevng statons. These adjacent statons set ther Network Allocaton Vector 6

8 MECSE : "A Survey of IEEE MAC Mechansms for...", D. Pham, A. Sekercoglu and G. Egan (NAV) and restran from transmttng for ths duraton, thus avod collson. 2.2 Pont Coordnaton Functon The PCF s only applcable for nfrastructure WLAN, whch conssts of an AP and a number of statons assocated wth t. The operaton of statons s controlled by a staton called Pont Coordnator (PC). At the start of a superframe, the PC, whch s usually located at the AP, generates a beacon frame perodcally, whether PCF s actve or not. Other statons know the tme when the next beacon frame wll arrve, ths tme s desgnated as Target Beacon Transmsson Tme (TBTT). The beacon frame transports the management nformaton to other statons to synchronze local tmers n statons and provde protocol related parameters. Statons can use the nformaton n the beacon frame to assocate wth the PC durng CP to have ther transmssons scheduled n the next CFP. After the beacon frame, the PC wats for SIFS and polls the statons regstered n ts pollng lst by sendng a CF-Poll frame to a staton. If the PC has data or acknowledgment destned for ths staton, t pggybacks the CF-Poll frame on the DATA or CF-Ack frames, respectvely. When a staton receves the CF-Poll frame, t wats for SIFS and reples wth a DATA, CF-Ack, DATA + CF-Ack or NULL frame (the NULL frame s transmtted when there s no data to send and no pendng acknowledgment). If the polled staton does not reply after PIFS, the PC polls the next staton. Ths s repeated untl the CFP expres. The PC sends a CF-End frame to ndcate the end of the CFP. A typcal superframe s shown n Fgure 3. Contenton Free Repeton Perod SIFS Contenton Free Perod SIFS PIFS SIFS SIFS Contenton Perod NAV Beacon D1+poll D 2+a c k +poll D 3+a ck +poll D 4 +poll U 2 U1+ack +a ck PIFS SIFS SIFS SIFS No response to CF- Poll U4+ack CF- E nd Reset NAV Dx = Frames sent by Pont Coordnator Ux = Frames sent by polled statons Contenton Free Maxmum duraton Fgure 3: A typcal superframe 3 IEEE e MAC Enhancements To support QoS, the IEEE Task Group E propose enhancements to the above standard n the IEEE e draft [4]. Wthn e, the Enhanced Dstrbuted Coordnaton Functon (EDCF) s an enhancement of the DCF n the legacy whle a new medum access mechansm called Hybrd Coordnaton Functon (HCF) s ntroduced. The EDCF s part of the HCF. The HCF combnes aspects of both DCF and PCF wth the multplex of CFP and CP n a e superframe, whch s repeated over tme. EDCF s used only n CP, whle HCF may be used n both CP and CFP. The EDCF and HCF are descrbed n Secton 3.1 and Secton 3.2, respectvely. 7

9 MECSE : "A Survey of IEEE MAC Mechansms for...", D. Pham, A. Sekercoglu and G. Egan 3.1 Enhanced Dstrbuted Coordnaton Functon Support for QoS s provded by the ntroducton of Traffc Categores (TCs). Each staton has up to four Access Categores (ACs) to support up to eght User Prortes (UPs). One or more UPs are assgned to one AC. The mappng of UPs to ACs s shown n Table 1. Prorty Access Category Desgnaton 1 0 Best-effort 2 0 Best-effort 0 0 Best-effort 3 1 Vdeo probe 4 2 Vdeo 5 2 Vdeo 6 3 Voce 7 3 Voce Table 1: Mappng from Prorty to Access Category n EDCF In Table 1, the prortes are sorted n ascendng order wth the excepton of relatve prorty 0 beng placed between prortes 2 and 3. Ths s orgnated from IEEE d brdge specfcaton [5]. Each AC s a varant of DCF wth ts own parameters AIFSD[AC], CW[AC], CW mn AC and CW max AC nstead of DIFS, CW, CW mn and CW max. The AIFSD[AC], whch s usually referred to as Arbtraton Inter-Frame Space (AIFS), s calculated as AIFSD AC SIFS AIFS AC SlotTme, where AIFS[AC] s an nteger greater than zero. Furthermore, the BC s chosen from [1, 1+CW[AC] ] rather than [0, CW] as n the DCF. An AC wth hgher prorty s assgned smaller CW mn and CW max values and/or shorter AIFS. The IEEE e EDCF access method s shown n Fgure 4. Immedate access when medum s dle >= AIFSD[A C] + SlotTme AIFSD[A C] PIFS Contenton Wndow from [1, CWmn[AC] +1] AIFSD[A C] + SlotTme Busy Medum SIFS Backoff Wndow Next Frame Defer Access Slot Tme Select Slot and decrement backoff as long as medum stays dle Fgure 4: e EDCF access method Each AC wthn a staton behaves as a vrtual staton and contends for Transmsson Opportuntes (TXOPs), a tme nterval when t can ntate transmssons, usng ts own parameters and ts own BC. If the BCs of two or more ACs wthn a staton reach zero at the same tme, the frame from the AC wth hghest prorty receves the TXOP. ACs wth lower prorty behave as f there were an external collson n the medum and perform backoff process wth ncreased CW values. The four ACs of a e staton and one prorty of a legacy staton and s shown n Fgure 5. The AP determnes and broadcasts the parameters for each AC plus the lmt of a TXOP nterval for each AC TXOPLmt[AC] n beacon frames perodcally. Durng a TXOP, multple MAC Protocol Data Unts (MPDUs) from the same AC n a staton can be transmtted wth an SIFS tme nterval between an ACK and the next frame transmsson. Ths multple transmsson of MPDUs s called Contenton Free Burst (CFB). 8

10 MECSE : "A Survey of IEEE MAC Mechansms for...", D. Pham, A. Sekercoglu and G. Egan e staton: four Access Categores/multple prortes Legacy staton: sngle prorty Access Category Backoff DIFS BC Backoff AIFS[3] BC[3] Backoff AIFS[2] BC[2] Backoff AIFS[1] BC[1] Backoff AIFS[0] BC[0] Vrtual Collson Handler Transmsson Attempt Transmsson Attempt Fgure 5: e EDCF queues vs DCF queue per staton 3.2 Hybrd Coordnaton Functon The HC, whch s usually located at the AP, may assgn TXOPs to tself or other statons after the medum s dle for PIFS wthout any backoff procedure, therefore t has hgher prorty than ACs snce ACs, usng (E)DCF cannot contend for medum access at least after DIFS dle duraton. The HC traffc delvery and TXOP assgnment may be scheduled durng both CP and CFP. The beacon frame s broadcast by the HC at the begnnng of each superframe. Durng CFP, only HC can allow access to the medum by gvng TXOPs to statons usng QoS CF-Poll frames, whch specfy the startng tme and maxmum duraton of each TXOP. The CFP ends f the duraton reaches the tme specfed n the beacon frame or the HC sends a CF-End frame. Durng CP, each TXOP begns when a staton gans access to the medum usng EDCF or when ths staton receves a QoS CF-Poll frame from the HC. A Controlled Access Perod (CAP) s composed of several ntervals wthn one CP when short bursts of frames are transmtted usng pollng. A typcal e superframe s shown n Fgure 6. 4 Dstrbuted Approaches Based on DCF In the dstrbuted approach, each staton n a WLAN determnes to access the medum wthout control of a partcular staton. Servce dfferentaton between traffc classes s based on dfferentaton of the tme the traffc has to wat before transmsson. Two man parameters that decde the watng tme of traffc are Inter-Frame Space (IFS) and CW. The dstrbuted approach may be dvded further nto prorty-based approach and far schedulng approach. The prorty-based approach and far schedulng approach are descrbed n Secton 4.1 and Secton 4.2, respectvely. 9

11 MECSE : "A Survey of IEEE MAC Mechansms for...", D. Pham, A. Sekercoglu and G. Egan e perodc superframe CFP (pollng through HCF) QoS CF- Poll CF- End CP (Both EDCF and pollng through HCF) QoS CF- Poll Transmtted by HC Beacon Transmtted by (Q) STAs Polled TXOP (CFP) TXOP TXOP Polled TXOP (CAP) TBTT RTS/CTS/fragmented DATA/ ACK (polled by HC) RTS/CTS/DATA/ACK (after DIFS +backoff) RTS/CTS/fragmented DATA/ ACK (polled by HC) TBTT Fgure 6: A typcal e superframe 4.1 Approaches based on prorty In the prorty-based approach, the prorty s mapped nto medum access. Traffc wth hgher prorty s assgned smaller values for IFS and/or CW (leadng to smaller BI value), therefore low-prorty has to wat longer than hgh-prorty before transmsson Approaches based on IFS Aad et al. [7] recommend a scheme n whch hgher prorty j 1 and lower prorty j have IFS values of DIFS j 1 and DIFS j, respectvely, such that DIFS j 1 DIFS j. The maxmum random range RR j 1 of prorty j 1 s defned as the maxmum BI of that prorty. If the strct condton RR j 1 DIFS j DIFS j 1 s satsfed, then all packets of prorty j 1 have been transmtted before any packet of prorty j s transmtted. In less strngent condton, RR j 1 DIFS j DIFS j 1, a packet whch could not access the medum the frst tme may have ts prorty decreased n the subsequent attempts. Smulatons were carred out and the results show that the method does not change the system effcency, wth data rate sums reman the same. The method works well for both Transmsson Control Protocol (TCP) and User Datagram Protocol (UDP) flows wth more sgnfcant effect on UDP flows than on TCP flows. It also works n nosy envronment and keeps the same stablty of the system. Benvenste [15] suggests a technque to dfferentate servces based on Urgency Arbtraton Tme (UAT), whch s the tme a staton has to wat before a transmsson attempt followng a perod when the medum s busy. AIFS and Backoff Counter Update Tme (BCUT) are generalzaton of DIFS and SlotTme, respectvely. Traffc wth hgher prorty s assgned shorter AIFS and/or shorter BCUT values. The hghest prorty has AIFS hgh pro PIFS and a mnmum backoff tme of 1 n order to prevent conflct wth medum access by centralzed protocol PCF. Smulaton of two traffc classes wth AIFS hgh pro PIFS, AIFS low pro DIFS, CW hgh pro 1, 32 and CW low pro 0, 31 was conducted. The outcomes reveal that the delay and jtter of hgh-prorty traffc are decreased and under moderate load condton, the performance of low-prorty traffc s also mproved as compared wth legacy DCF. Deng et al. [19] propose a method to support two prortes. Hgh and low prortes have to wat for the medum to be dle for PIFS and DIFS, respectvely, before ntatng backoff procedure. Smulaton of vdeo, voce and data traffc wth prortes of 3, 2 and 0, and traffc ratos of 1 : 1 : 2, respectvely, was performed. The results demonstrate that the approach, when combned wth separaton of random backoff tme, can be used to support vdeo, voce and data traffc n heavy load condton (say 90%). In heavy load condton, the hghest prorty vdeo traffc uses most of the bandwdth (55%) and lower prortes use the remanng bandwdth, whle n lght 10

12 MECSE : "A Survey of IEEE MAC Mechansms for...", D. Pham, A. Sekercoglu and G. Egan load condton, the lower prorty traffc has the requred bandwdth. It s also llustrated that vdeo and voce traffc have lower access delay and lower packet loss probablty than n DCF, but data traffc has hgher access delay and hgher packet loss probablty than n DCF Approaches based on CW Separaton of CW Aad et al. [8] ntroduce a dfferentaton mechansm based on CW mn separaton, n whch hgher prorty traffc has lower CW mn value. Smulatons of a WLAN consstng of an AP and three statons wth CW mn values of 31, 35, 50 and 65, respectvely, were conducted wth both TCP and UDP flows. The results reveal that for the same set of CW mn values, the dfferentaton effect s more sgnfcant on UDP flows than on TCP flows. The per-flow dfferentaton s ntroduced, n whch the AP sends back ACK packets wth prortes proportonal to prortes of the destnatons. In other words, the AP wats for a perod of tme whch s proportonal to the delay from a destnaton before transmttng an ACK packet to that destnaton. Barry et al. [12, 49] recommend usng dfferent values of CW mn and CW max for dfferent prortes, n whch hgher prorty has lower CW mn and CW max values than those of lower prorty. Smulatons of hgh-prorty traffc wth CW mn between [8,32] and CW max 64, and low-prorty traffc wth CW mn between [32,128] and CW max 1024 were performed. The outcomes show that the hgh-prorty and low-prorty traffc undergo dfferent delay. Deng et al. [19] propose a scheme based on separaton of CW. Orgnally, the random BI s unformly dstrbuted between 0, 2 2 1, n whch s the number of tmes the staton attempted transmsson of the packet. In Deng s scheme, the hgh and low prortes have random BI values unformly dstrbuted n ntervals 0, and 2 2 2, 2 2 1, respectvely. Ths approach can be combned wth the approach based on IFS mentoned n the last paragraph of Secton The smulaton results reveal some mprovement n delay and jtter for hgh prortes (voce and vdeo). Xaohu et al. [53] present the Modfed DCF (M-DCF) scheme, whch uses dfferent values of CW mn and CW max for servce dfferentaton. Smulatons of ad-hoc WLAN wth 10 data statons and between 10 and 35 voce statons were performed. Voce servce had CW mn 7 and CW max 127, whle data servce had CW mn 15 and CW max 255. The outcomes llustrate that M-DCF decreases the total packet droppng probablty and the droppng probablty of voce packets as well as reduces the contenton delay of both voce and data packets as compared wth DCF. Dfferent Prorty/Persstent/Persstence Factor (PF) Aad et al. [6, 7] propose a method based on the backoff ncrease functon. In the orgnal DCF, the CW s multpled by a Prorty Factor (PF) of 2 after each collson. In Aad s method, a hgher prorty traffc has a lower PF P j. Smulatons of three prortes wth PF values of 2, 6 and 8 were conducted. The results demonstrate that ths method works well wth UDP flows, but does not work well wth TCP flows or n nosy envronment. The effcency s not lost but the stablty of the system s decreased. Benvenste [15] recommends a technque based on Persstent Factor (PF). After each collson, the CW s multpled by a PF. Hgher prorty traffc has lower value for PF. For tme senstve applcatons and wth capablty for congeston estmaton, PF value should be less than 1. Otherwse, a value between 1 and 2 should be chosen. Smulaton of traffc wth two prortes and AIFS hgh pro PIFS, AIFS low pro DIFS, PF hgh pro 0.5 and PF low pro 2 was carred out. The outcomes show that the hgh-prorty traffc performance s mproved wthout any sgnfcant effect on low-prorty traffc. Furthermore, the delay and jtter s less than 10 ms, whch could not be obtaned wth dfferentaton based on IFS alone. Song et al. [46] ntroduce a scheme called Exponental Increase Exponental Decrease (EIED), n whch after a collson or a successful transmsson, the CW s multpled or dvded by factor 11

13 MECSE : "A Survey of IEEE MAC Mechansms for...", D. Pham, A. Sekercoglu and G. Egan r I and r D, respectvely. Smulatons of 5, 10, 40 and 60 statons wth packet arrval rate between 10 to 160 packets/s and packet length of 1024 bytes were performed usng the Bnary Exponental Backoff (BEB) and EIED wth [r I, r D ] = [2, ], [2, ], [2 2, 2 2], [2, 2]. The results reveal that the delay of EIED s smaller than that of BEB for all packet arrval rates and all number of statons. The throughput of EIED s the same as throughput of BEB when the packet arrval rate s small (less than 80 packets/s) and hgher than throughput of BEB when the packet arrval rate s hgh Approaches based on combnaton of IFS and CW Wong et al. [52] present a scheme named Age Dependent Backoff (ADB), whch s an mprovement of EDCF. For each TC, after each collson, the CW[TC] s multpled by a Persstence Factor (PF) gven by the formula 2 PF TC Age 2 LT TC LT TC and Age are the packet lfetme (LT) and age of the packet n transmsson queue, respectvely. Packets wth Age LT are dscarded snce they would be obsolete by the tme they get to the recpent. PF TC s between 1 and 2 n the frst half of packet lfetme and s between 0 and 1 n the second half. CW TC s always less than CW max TC but could be less than CW mn TC to dfferentate between a retransmtted packet and a newly arrved one. Smulatons of an ad-hoc network wth 10 voce statons, 4 vdeo statons and a number of Fle Transfer Protocol (FTP) statons (between 5 and 25), n whch half of the FTP statons usng DCF and the other half usng EDCF, were conducted. LTs of voce and vdeo packets were set to 25 ms and 75 ms, respectvely. DCF had DIFS 50µs, CW mn 31, CW max For voce servce, AIFS 1 Voce 50µs, CW mn 1 7, CW max The vdeo servce had AIFS 2 Vdeo 70µs, CW mn 2 15, CW max 2 63, whle data servce had AIFS 3 Data 110µs, CW mn 3 15, CW max The results show that ADB mproves the packet delay, jtter and drop rate of both voce and vdeo traffc as compared wth EDCF wth fxed PF values of 2 and 1.5. At the same tme, the best-effort FTP traffc does not suffer from starvaton. Romdhan et al. [40] ntroduce a method called Adaptve EDCF (AEDCF), n whch CW s dynamcally calculated after each successful transmsson or collson accordng to the network condtons. After each successful transmsson of a packet of class, the CW s updated as CW new max CW mn, CW old MF where hgher prorty has lower multplcator factor MF. If MF s fxed at 0.5, then the scheme s called Slow Decrease (SD) scheme. Each traffc class has a dfferent multplcator factor (MF), whch should not exceed 0.8 as suggested by the authors. MF mn 1 2 f j avg, 0.8 The average collson rate at step j s f j avg 1 α fcurr j α f j 1 avg where α s the weght or smoothng factor. The average collson rate s calculated every T update expressed n SlotTme. The estmated current collson rate s f j curr E collson j p E data sent j p where E collson j p and E data sent j p are the number of collson and the number of packets sent at staton p n the perod j, respectvely. After each collson, CW of class s updated as CW new mn CW max, CW old PF 12

14 MECSE : "A Survey of IEEE MAC Mechansms for...", D. Pham, A. Sekercoglu and G. Egan where hgher prorty traffc has lower persstent factor PF. Smulatons of an ad-hoc WLAN wth the number of statons between 2 and 44 (correspondng to load rate from 6.7% to 149%) were carred out. Each staton had three servce classes, wth hgh, medum and low classes havng PF values of 2, 4 and 5, respectvely. T update 5000 SlotTme and α 0.8. Hgh, medum and low classes had AIFS values of 34µs, 43µs and 52µs, respectvely. CW mn, CW max of hgh, medum and low classes were [5,200], [15,500] and [31,1023], respectvely. The outcomes demonstrate that AEDCF has lower mean access delay than EDCF and SD schemes, and ts delay s less than 10 ms. AEDCF and SD has hgher goodput than EDCF. Furthermore, AEDCF has hgher gan on goodput than SD, especally under hgh load condton. The medum utlzaton decreases when traffc load ncreases, but AEDCF has hgher utlzaton than SD, whch has hgher utlzaton than EDCF. Of all the three schemes, AEDCF has lowest collson rate whle EDCF has hghest collson rate. Km et al. [29, 30, 31] propose a technque called DCF wth Shorten Contenton Wndow (DCF/SC), whch provdes support for three servce classes, premum (low latency and jtter), assured (guaranteed bandwdth) and best-effort (no guarantee). The super perod, whch orgnally conssts of PCF and DCF, s dvded nto two perods. In the frst perod, only premum servce s allowed to access the medum wth DCF/SC after sensng the medum dle for PIFS. In the second perod, premum and assured servces access the medum wth DCF/SC and besteffort servce access medum wth DCF after medum dle tme of DIFS. DCF/SC has shorter CW than orgnal IEEE DCF. Smulatons of nfrastructure WLAN wth premum servce such as voce, Moton Pcture Extreme Compressed Moves (MPX) and Moton Pcture Experts Group 2 (MPEG2), assured servce (assured bulk data) and best-effort servce (bulk data) were conducted. DCF/SC had CW values from [11, 17, 23, 29, 35, 41] whle DCF had CW values from [16, 32, 64, 128, 256, 512, 1024]. The results show that the method ncreases utlzaton (defned as the rato of total successful transmsson tme of pure data (excludng backoff tme, preambles and header) to the total smulaton tme) and ncreases stream throughput (defned as the rato of the number of successful packets to the generated packets) as compared wth the DCF. It also decreases the latency and jtter of the real-tme servces. Banchs et al. [11] suggest a scheme for dfferentaton of real-tme and best-effort servces. Real-tme statons wats for medum to be dle for PIFS and generates elmnaton burst EB 1, whose duraton s multple of SlotTme. The length of EB 1 has the followng dstrbuton P E1 p n n E1 1 p m E1 1 E1, n m E1 1 p E1, 1 n m E1 where n s the length of EB 1 n SlotTme, p E1 s a probablty between 0 and 1 and m E1 s the maxmum length of EB 1 n SlotTme. If the staton senses the medum busy after transmsson of EB 1 or EB 2, t defers access to the medum untl the medum s dle for PIFS. The staton wth the longest EB 1 or EB 2 gans access to the medum. If two statons have the same EB 1 and EB 2, collson occurs and would be detected. Data statons wat for the medum to be dle for DIFS and use backoff algorthm. Each staton has a share value correspondng to QoS level of ths staton. The basc servce has share value of 1 and hgher servce has share value greater than 1. For all r statons, the rato w s should be equal, where r and s are throughput and the share assgned to staton, respectvely. r s updated after a packet transmsson as follows r new 1 e t k l t e t k where l and t are the length and nter-arrval tme of the transmtted packet, and k s a constant. Each staton calculates ts own w and ncludes ths n the transmtted packet s header. A staton observes a packet and notces the value of w n ths packet header. If the staton s w s greater than ths packet s w, the staton ncreases ts CW by a small amount, and vce versa. The algorthm for calculaton of a staton CW s f w own w rcv then r old 13

15 MECSE : "A Survey of IEEE MAC Mechansms for...", D. Pham, A. Sekercoglu and G. Egan CW 1 1 CW else f (queue empty) then CW 1 1 CW else CW 1 1 CW end f where CW Mn CW CW Max and w own s calculated by the staton, w rcv s the value of w n the header of observed packet, 1 s calculated as 1 k w own w own w rcv w rcv where k s another constant, whch s dfferent from k n the formula for r new. It s clamed that the resdual collson rate (two or more statons collde after transmsson of EB 1 and EB 2) s very small and only depends on p E1, m E1 and m E2, but not the number of contendng statons. Sheu et al. [41, 42] present a scheme called Dstrbuted Bandwdth Reservaton Protocol (DBRP) to support both real-tme and non real-tme traffc. Data statons use DCF: they wat for the medum to be dle for DIFS and start backoff algorthm wth CW mn and CW max values of 31 and 1023, respectvely. Voce statons stay n one of three states: ntal state where each staton starts wth, reservaton state where voce statons contend to reserve access to the medum and transmsson state where voce statons transmt perodcally wthout contenton. A voce staton has an Sequence ID (SID) for access sequence and an Actve Counter (AC) for the number of actve voce statons. A voce staton that wants to access the medum at tme t senses the medum for the reservaton frame (RF) n perod t, t D max, where D max s the maxmum acceptable delay of voce packets. The RF stores the number of actve voce statons (AN). If there s no RF frame, and f the medum s dle n the perod t D max, t D max PIFS, the voce staton performs backoff algorthm wth backoff tme unformly dstrbuted between 0 and 3 SlotTme. When the backoff tme goes to zero, the staton enters Send RTS procedure and transmts RTS frame to destnaton. If there s no collson, the staton would receve CTS frame, enters Transmsson State and becomes the Reservaton Cycle Generator (RCG), whch s the frst voce staton n WLAN and has to generate RF frame perodcally. It sets ts SID and Actve Counter to 1 and transmts RF frame and ts voce packet. If there s RTS collson, the colldng voce staton would use p-persstent scheme for retransmsson of RTS, wth probablty p p n the next tme slot. If there s RF frame n perod t, t D max, the voce statons enters RF receved procedure, sets ts Actve Counter as the value of AN n the RF frame (denoted as RF.AN). The voce staton enters Wat to content procedure and wats for a duraton of WT RF.AN T voce before tryng to access the medum. If the medum s dle for SlotTme durng ths perod, a voce staton has left and WT should be decreased by one T voce. After WT, the voce staton goes to backoff procedure to send a RTS frame. The condton D max RP max T maxmpdu s requred, where RP max s the sum of the maxmum voce packets reservaton perod and the voce staton contenton perod, T maxmpdu s the transmsson tme of a maxmum MPDU. If there s no collson of RTS/CTS, the staton ncreases ts Actve Counter by 1 and sets ts SID to the content of Actve Counter. The staton sends the frst packet and enters Transmsson State. All other statons ncrease ther Actve Counter by 1 when they hear a CTS. If there s collson of RTS, the colldng statons use p-persstent scheme for contenton resoluton. After the last access that s over the boundary of RP max, a voce staton senses the medum dle for PIFS. If the voce staton SID s 1, t s the RCG and sends the RF frame and the voce packet mmedately. Other statons sense the medum, and f t s dle for SlotTme, the RCG has left and all statons decrease ts SID and Actve Counter by 1. The new RCG has to transmt the RF frame before ts voce packet. When an RF frame s receved, a voce staton sets ts cd tmer as cd tmer SID 1 T voce. When the cd tmer reaches zero, the staton transmts ts packet. Chen et al. [16] recommend a method to dfferentate between real-tme and non-real-tme traffc based on IFS, CW mn and retransmsson procedure of real-tme packets. After the medum s dle for DIFS, statons wth real-tme packets for retransmsson generate jammng nose. The 14

16 MECSE : "A Survey of IEEE MAC Mechansms for...", D. Pham, A. Sekercoglu and G. Egan staton wth longest jammng nose then transmts ts packet. The length of jammng nose has a truncated geometrc dstrbuton. The probablty of a jammng nose wth length of f n unt of SlotTme s: p f 1 j 1 p j, P j 1 f JW f p JW 1 j, f JW p j s a probablty value between 0 and 1, Jammng Wndow JW s the maxmum jammng tme defned as JW mn JW p JW 1 1 j N, where N s the number of statons that smultaneously generate jammng nose. Smulatons of seventy non-real-tme flows and a number of actve realtme flows were performed usng Chen s scheme, Tered Contenton Multple Access (TCMA) [13, 14] and Vrtual DCF (VDCF) [17, 18]. (CW mn, IFS) for real-tme and non-real-tme were (15, 40 µs) and (31, 50 µs), respectvely. p j was 0.35 and JW was 9. The results show that Chen s scheme has lower mean MAC delay, lower packet droppng rate, smlar average jtter and hgher channel utlzaton than TCMA. Ths scheme also has lower mean MAC delay, smlar packet droppng rate, lower average jtter and lower channel utlzaton than VDCF. Sobrnho et al. [43, 44, 45] propose a scheme called Blackburst. Defne τ to be the maxmum propagaton delay between any two statons. There are three nter-frame spacngs that are requred for the access procedures of data and real-tme statons, whch must satsfy the condtons t short 2 τ t med and t med 2 τ t long. A data staton that wants to transmt data senses the medum for a perod t long. If the medum s dle, ths staton transmts data. Otherwse, t wats for the medum to be dle for t long and enters the backoff procedure, wth the BI f data c n unt of whte slot t wslot as f data c rand f data 0 2 c where rand a returns a random number between 0 and a 1, c s the number of collsons that the data packet has experenced and f data 0 s the ntal CW. A real-tme staton that wants to transmt packets senses the medum for a perod t med. If the medum s dle, ths staton transmts a packet. Otherwse, t wats untl the medum s dle for t med agan and enters the blackburst contenton perod. The staton jams the medum wth a black burst n unt of black slot t bslot, whose duraton s proportonal to the tme that ths staton has been watng. After the transmsson of the black burst, the staton wats for t obs to see f any staton s transmttng a longer black burst. The wnnng staton, whch s the staton that has been watng the longest tme, would get access to the medum and other statons would contend for the medum wth blackburst n the next cycle. In every black burst contenton perod, there s only one unque wnner. By ensurng that 2 τ t obs mn t bslot, t med, a real-tme staton can always see f ts blackburst s shorter than that of another and real-tme statons do not attempt to access the medum durng the observaton nterval. When a real-tme staton has access to the medum, t transmts a packet that must be at least t pkt and schedules the next access nstant to be t sch n the future. The real-tme statons have hgher prorty than the data statons, snce no data staton would perceve the medum dle for t long untl all real-tme statons have access to the medum. The real-tme statons get access to the medum n the round robn order and share the medum n a Tme Dvson Multple Access (TDMA) manner. After all the real-tme statons have synchronzed ther transmssons, the black burst contenton s only ntated agan f some transmsson of data statons dsturbs the order Dscusson Aad s scheme [7] can support a large number of prortes wth the proper selecton of DIFS j for each prorty. Benvenste s approach [15] of usng BCUT for dfferentaton s not possble because the hgher prorty could not use a shorter BCUT as the SlotTme used n the orgnal DCF s the mnmum possble. Deng s method [19] uses only two IFS values, whch are PIFS and DIFS, therefore t only allows for dfferentaton of two prortes. In order to support more than two prortes, more IFS values must be used. However, except PIFS, all other values of IFS are at least DIFS and therefore there can be only one hgher prorty than the legacy DCF traffc f dfferentaton s merely based on IFS values. Furthermore, the total tme a staton has to wat 15

17 MECSE : "A Survey of IEEE MAC Mechansms for...", D. Pham, A. Sekercoglu and G. Egan before ganng access to the medum s the sum of IFS and the random backoff tme. Therefore, even f hgh-prorty traffc has lower IFS value, t can stll have hgher total wat tme than lowprorty traffc and loses the medum contenton to low-prorty traffc. Although Barry s, Deng s and Xaohu s methods [12, 19, 53] only provde servce dfferentaton for two prortes, the same prncple can be appled to support more prortes. Song s scheme [46] shows mprovement n delay and throughput compared wth DCF, however t does not ntroduce dfferentaton of servces. Aad s methods [6, 7] based on IFS or backoff ncrease functon does not work well for TCP flows, snce the AP always uses ts own prorty to send back TCP-ACKs to dfferent statons. For example, f two statons wats for t 1 and t 2 before transmttng a packet on average, then the data rate rato s t 2 t 1. For TCP flow, the ACK produces addtonal delay t 0 and the data rate becomes t 2 t 0 t 1 t 0. If the AP s slow, t 0 s hgh and rato t 2 t 0 t 1 t 0 s very much dfferent from the desred rato t 2 t 1 [8]. In usng dfferent CW mn and/or CW max values for dfferent prortes, low-prorty traffc has to produce longer backoff tme even f there s no hgh-prorty traffc, leadng to longer delay. Choosng a random BI n the CW range produces unpredctable varatons n delay and throughput, whch s undesrable for tme-senstve traffc. Wth regard to the PF, f PF value s greater than 1, after each collson, the CW s ncreased and the collded packet has to wat longer before beng retransmtted, resultng n longer delay and hgher jtter, whch s not desrable for real-tme packets. However, f PF value s less than 1, CW s reduced, and under the heavy load condton, congeston may ncrease leadng to more collsons. In Sheu s method [41, 42], DIFS PIFS 4 SlotTme, whch s nconsstent wth the current standard, n whch DIFS PIFS SlotTme. Sobrnho s method [43] can provde guarantee on delay. However, t s only optmzed for sochronous traffc and therefore can be a drawback for varable rate traffc [49]. Bandwdth s also wasted by sendng bursts for each packet to access the medum [42]. The approach usng combnaton of IFS and CW can support more prortes than each of the IFS or CW approach alone. However, the values of IFS, CW and BI are determnstc for each prorty once chosen. Ths may lead to unfar medum access snce hgh prortes tend to seze the medum and low prortes may suffer starvaton. Romdhan s and Wong s methods [40, 52] calculate the CW dynamcally based on network condtons, and therefore can adapt better to traffc load and can avod starvaton of low-prorty traffc. 4.2 Approaches based on far schedulng Far schedulng approach guarantees that traffc n each class has a far chance for transmsson. The approach ams to ensure that the bandwdth gven to traffc flows s proportonal to ther weghts Matchng prorty to IFS Pattara-Aukom et al. [37] propose a method called Dstrbuted Defct Round Robn (DDRR) based on Defct Round Robn (DRR) schedulng. Traffc s categorzed nto classes. Traffc class has a servce quantum Q bts every T seconds, such that Q T s equal the desred throughput of class. The Defct Counter (DC) of traffc class, at MS j at any gven tme s DC j and s proportonal to the bandwdth avalable to ths traffc class at that tme. DC j s ncreased Q bts every T seconds and s decreased by the sze of the frame whenever a frame s transmtted. At tme t, IFS for traffc class at MS j, IFS j t s calculated as IFS j t DIFS DC j t DC j α DC j t t DC j DC j t Q Q T t t t Frame Sze t 16 random 1.0, β

18 MECSE : "A Survey of IEEE MAC Mechansms for...", D. Pham, A. Sekercoglu and G. Egan α s a scalng factor, β between 1 and β. 1 and random 1.0, β returns a random number unformly dstrbuted 0 DC j DIFS PIFS α If DC j 0, then traffc class has to wat untl DC j s greater than 0 agan before next transmsson. The above equatons leads to PIFS IFS j DIFS From ths last equaton, DDRR s backward compatble wth IEEE When a MS senses the medum s dle, t wats for IFS before transmttng a traffc class frame. If the medum s busy, the MS wats untl t becomes dle agan, wats for addtonal tme IFS (as calculated at ths tme) and then transmts the frame. There s no backoff procedure n ths scheme. The rght to access the medum depends on DC j but does not depend on the requred throughput, resultng n far share of the medum between dfferent traffc classes Matchng prorty to CW Banchs et al. [9] present an approach called Dstrbuted Weghted Far Queung (DWFQ), n r whch for all statons, the rato L W should be equal, where r and W are throughput and the weght assgned to staton, respectvely. The assumpton s all packets at a staton are from one flow. IEEE statons have default weght of 1, and other weght values must be greater than or equal to 1, whch means better than or best-effort servce. r s updated after a packet transmsson as follows r new 1 e t K l e t K r old t where l and t are the length and nter-arrval tme of the transmtted packet, and K s a constant. Each staton calculates ts own L and ncludes ths n the transmtted packet s header. A staton observes a packet and notces the value of L n ths packet header. If the staton s L s greater than ths packet s L, the staton ncreases ts CW by a small amount, and vce versa. The algorthm for calculaton of a staton CW s f (overload) then p 1 2 p else f L own L rcv then p 1 1 p else f (queue empty) then p 1 1 p else p 1 1 p end f p mn p, 1 CW p CW where s a constant, L own s calculated by the staton, L rcv s the value of L n the header of observed packet, p s a scalng factor and 1 s calculated as 1 k L own L rcv L own where k s another constant, whch s dfferent from k n the formula for r new. The condton overload occurs when there are a large number of statons wth hgh weghts, resultng n statons choosng small values for CW and there are a large number of collsons. Ths condton could be tested lke ths f (av nr coll c) then L rcv Q 17