Experimental Tuning of the AIFSN Parameter to Prioritize Voice Over Data Transmission in E WLAN Networks

Transcription

1 Dubln Insttute of Technology Conference papers Communcatons Network esearch Insttute Expermental Tunng of the AIFSN Parameter to Prortze Voce Over Data Transmsson n 82.11E WLAN Networks Mroslaw Narbutt Dubln Insttute of Technology, narbutt@cnr.dt.e Mark Davs Dubln Insttute of Technology, mark.davs@dt.e Follow ths and addtonal works at: Part of the Systems and Communcatons Commons ecommended Ctaton Narbutt, M. & Davs, M. (27) Expermental tunng of the AIFSN parameter to prortze voce over data transmsson n 82.11E WLAN networks. ACM Internatonal Conference on Wreless Communcatons and Moble Computng (IWCMC 27), Hawa, August, 27. Ths Conference Paper s brought to you for free and open access by the Communcatons Network esearch Insttute at AOW@DIT. It has been accepted for ncluson n Conference papers by an authorzed admnstrator of AOW@DIT. For more nformaton, please contact yvonne.desmond@dt.e, arrow.admn@dt.e, bran.wdds@dt.e.

2 27 IEEE Internatonal Conference on Sgnal Processng and Communcatons (ICSPC 27), November 27, Duba, Unted Arab Emrates EXPEIMENTAL TUNING OF THE AIFSN PAAMETE TO PIOITIZE VOICE OVE DATA TANSMISSION IN 82.11E WLAN NETWOKS Mroslaw Narbutt and Mark Davs Communcatons Network esearch Insttute, School of Electronc and Communcatons Engneerng, Dubln Insttute of Technology, Ireland ABSTACT In ths paper we expermentally study the mpact of one EDCA parameter, namely AIFSN on a mxed voce/data wreless transmsson. In partcular we nvestgate how the tunng of ths parameter affects both the voce transmsson qualty and background data throughput. We predct end-to-end voce transmsson qualty from tme varyng transmsson mparments usng the latest Appendx to the ITU-T E-model. Our expermental results show that the tunng of the AIFSN parameter can successfully prortze voce transmsson over data n a real 82.11e network. To the best of our knowledge, ths s the frst expermental nvestgaton on tunng of MAC layer parameters n a real 82.11e WLAN network and ts effect on end-to-end voce transmsson qualty. Index Terms speech communcaton, data transmsson, wreless LAN, voce transmsson qualty, IEEE82.11e WLAN. 1. INTODUCTION eal-tme voce transmsson over wreless LAN (VoWLAN) mposes strngent requrements on transmsson mparments such as end-to-end delays, jtter, and packet loss. The responsblty of meetng these requrements s shared between the varous communcaton layers. Actons at the applcaton layer nclude effcent encodng and packetzaton schemes, packet loss concealment (PLC) technques, adaptve dejtter bufferng, echo cancellaton, etc. On the network sde, the new IEEE 82.11e protocol supports voce traffc by dfferentatng channel access probablty among dfferent traffc categores. In partcular, the new, extended channel access mechansm (EDCA) allows for adjustng a number of channel access parameters at the L2/MAC layer to prortze VoIP packets over other traffc types. Applcaton-layer adaptaton mechansms and MAC-layer parameters tunng can greatly mtgate the effect of transmsson mparments and thus mprove speech transmsson qualty. However, these mechansms are often complex and dffcult to tune properly. We clam that f a part of the VoIP transmsson path s beng tuned, the mpact of local tunng actons on the whole end-toend (mouth-to-ear) transmsson has to be taken nto account. For ths reason we developed a method for evaluatng user satsfacton regardng end-to-end VoIP transmsson qualty from tme varyng transmsson mparments. Ths method has shown to be partcularly effectve n evaluatng varous playout buffer algorthms [1, 2], assessng VoIP performance n Voce over WLAN systems [3, 4, 5], and was recently standardzed by the ITU-T [6] as an Appendx to [7]. In ths paper we use ths method to expermentally evaluate the capablty of the EDCA mechansm to support voce traffc n a mxed voce/data transmsson over 82.11e WLAN. We nvestgate how real-tme voce can be supported by tunng one EDCA parameter, namely AIFSN and how ths mpacts background data transmsson. Ths paper s structured as follows. In Secton 2, the 82.11e WLAN expermental setup s descrbed, EDCA mechansm s outlned and proper dejtter bufferng at applcaton layer s addressed. Expermental results are presented and dscussed n Secton 3. Fnally, the paper s concluded n Secton E WLAN EXPEIMENT 2.1. Expermental testbed. The 82.11e wreless/wred test bed conssts of 15 desktop PCs actng as wreless VoIP termnals, one desktop PC actng as a background traffc generator, and one desktop PC actng as an access pont (AP). All machnes n the test bed use PCMCIA wreless cards based on Atheros chpsets controlled by MadWF wreless drvers and Lnux OS (kernel 2.6.9). The MadWF drvers (elease.9.1 and above) provde workng mplementaton of IEEE 82.11e EDCA mechansm [8]. All of the nodes are also equpped wth a 1Mbps Ethernet cards. The PC that acts as access pont routes traffc between the wred network and the wreless clents, and vce versa (each PC has two nterfaces: one on the wreless and one on wred network). Durng the experments each VoIP termnal runs one VoIP sesson and all sessons are b-drectonal. In ths way each termnal acts as the source of an uplnk flow (.e. sender) and the snk (.e. recever) of a downlnk for a VoIP sesson. The wred nterface off one PC s used to generate background traffc whch s routed va the AP to /7/$ IEEE 784

3 the wreless nterface of the same PC. All generated traffc nvolved a wred and a wreless nterface so that no traffc was generated between wreless nterfaces. The wreless statons were located wthn 5 meters range from the AP to ensure that the wreless lnk qualty s good. Ths test bed s llustrated n Fg. 1. Voce traffc was generated usng TPtools [9] whch generated G.711 encoded voce packets (8bytes audo frames created every 1ms) wth fxed IP packet overhead of 12bytes for TP, 8bytes for UDP, and 2bytes for IP layer. Fgure 1. Expermental 82.11e test bed Durng the experments b-drectonal transmsson of packets was realzed n the form of alternatng actve and passve perods modeled as a four state Markov chan (talker A actve, talker B actve, both actve, both slent). The duraton of states and the transtons between them followed the ITU-T recommendaton P.59. [1]. Ths resulted n an ON-OFF modulated CB traffc stream beng generated. Background traffc n the form of Posson dstrbuted UDP packet flow was generated usng MGEN traffc generator [11]. For the experments we used 1, 2, and 4Mbps background traffc. To measure effectve throughput (goodput) of the background traffc we used TP package [12]. The sze and sendng rate of the IP packets comprsng the load s specfed n Table I. Table 1. The sze and sendng rate of the packets comprsng the background load IP packet sze [Bytes] 1Mbps load 2Mbps load 4Mbps load The reasonng behnd choosng UDP and not TCP as a transport protocol for carryng background traffc s threefold: 1) UDP background traffc gves more accurate estmate of the actual load n the network (no retransmssons at transport layer) 2) results obtaned wth UDP consttute an upper bound for the throughput possble wth TCP; 3) retransmssons of lost or corrupted packets s done by the MAC-layer so TCP do net get affected by the packet loss [13]. Durng experments all the measured VoIP data (packet arrval tmes, tmestamps, sequence numbers, and marker bts) was collected at all recevng termnals to be processed later (off-lne) by a program that smulated the behavor of the de-jtterng buffer. Fnally, the qualty assessment algorthm was used to predct the qualty of voce transmsson MAC-layer parameters tunng The newer IEEE standard 82.11e offers two modes of MAC operaton: contenton-based channel access called Enhanced Dstrbuton Coordnate Access (EDCA) and contenton-free channel access called Hybrd Controlled Channel Access (HCCA). In our experments we have focused on the performance of the EDCA mode that dfferentates the channel access probablty among dfferent traffc categores (TCs). When ths operatonal mode s used, the packets are categorzed n dfferent TCs, and later mapped to four prortzed output queues called access categores (ACs). Each AC uses a set of parameters that controls the access probablty to the wreless medum: AIFSN parameter controls the dle tme (.e. the the arbtraton nterframe space, AIFS) after whch a transmsson may occur; CWmn and CWmax parameters defne the range of the contenton wndow (CW) values from whch the back-off tme s randomly selected; TXOP parameter controls the tme nterval for whch a staton holds the channel (transmsson opportunty) allowng for multple packet transmsson on a sngle channel access opportunty. Confgurng these parameters for each AC separately enables access probablty dfferentaton between TCs. Snce a staton wth packet to send must wat untl the medum s dle and then wat for an addtonal perod of tme AIFS, the AIFSN parameter for the voce AC_VO (AIFSN [AC_VO] ) should be smaller than the AIFSN parameter for the background AC_BK (AIFSN [AC_BK] ). In ths way tme-senstve voce traffc wll contend sooner for accessng the wreless medum and thus wll on average more transmsson opportuntes over the lesssenstve background traffc. After the AIFS perod, the statons wth a packet to send select random numbers between the CWmn and CWmax for each contendng AC. Snce the smallest number ndcates the wnner, the values of CWmn and CWmax should be lower for the voce queue than for the background queue. In general the combnaton of AIFSN, CWmn and CWmax should be confgured so that hgh-prorty voce packets wn transmsson opportuntes over background traffc. However, to avod stuatons n whch the low-prorty traffc s completely blocked, the sum of AIFSN plus CWmax for hgh-prorty voce should be greater than AIFSN for low-prorty traffc. In our experments the voce packets were mapped nto the voce AC (AC_VO) queue whle the data traffc was mapped nto the 785

4 background (AC_BK) queue based on ther TOS values n IP packets headers. 1 qualty at wreless sde (BK traffc 1Mbps) Durng the experment we prortzed voce over background traffc by ncreasng the number of tme slots comprsng the background AIFS perod (AIFSN [AC_BK] ) from 2 to 15 slots. All the other AC_BK parameters were: CWmn=7, CWmax=123, TXOP= and they were kept fxed for the duraton of the frst experment. The parameters under consderaton for both AC_BK and AC_BK are lsted n Table II Table 2. EDCA parameters settngs durng the experments Access Parameter AC_VO (STAs and AP) AC_BK (STAs and AP) CWmn 7 7 CWmax AIFSN 2 2,3, 14,15 TXOP 2.3. Applcaton-layer parameters tunng In our experments we used amjee s algorthm [14] whch s often used as a reference playout buffer controller. The algorthm uses the same playout delay throughout a gven talkspurt but permts dfferent playout delays for dfferent talkspurts. We modfed the orgnal amjee s algorthm by addng one parameter, namely playout_offset that represents addtonal pre-bufferng delay. In our soluton the playout tme p at whch the the -th packet, assumed to be the frst packet n a talkspurt (played at the destnaton) s calculated as follow: (1) p = t + d + β v + playout _ offset where d and v are the estmates of delay of the -th packet delay n and ts varance respectvely and are calculated as follows: (2) d = α d 1 + (1 α ) n qualty at wred sde (BK traffc 1Mbps) 15Bpp@83pps 124Bpp@122pps 512Bpp@244pps 256Bpp@488pps goodput (BK traffc 1Mbps) Fgure 2. Qualty of voce transmsson and effectve throughput of 1Mbps background traffc vs AIFSN [AC_BK] qualty at wreless sde (BK traffc 2Mbps) qualty at wred sde (BK traffc 2Mbps) 15Bpp@167pps 124Bpp@244pps 512Bpp@488pps 256Bpp@977pps goodput (BK traffc 2Mbps) Fgure 3. Qualty of voce transmsson and effectve throughput of 2Mbps background traffc vs AIFSN [AC_BK] v = α v + (1 α ) d n 1 (3) 1 qualty at wreless sde (BK traffc 4Mbps) Parameter β controls the delay/packet loss rato whle parameter α controls the ablty of the algorthm to follow the changes n the delay. By expermentng wth dfferent values of α, β, and playout_offset n a real wreless envronment we were able to chose the values (.e. α =.9982, β = 2, playout_offset = 4ms) that maxmzed ratng factor for all possble AIFSN and CWmn settngs. 3. EXPEIMENTAL ESULTS Experments covered 3 background traffc loads (1, 2, 3Mbps) 4 packetzaton schemes for background (256Bytes, 512Bytes, 124Bytes and 15Bytes packets) qualty at wred sde (BK traffc 4Mbps) 15Bpp@336pps 124Bpp@488pps 512Bpp@977pps 256Bpp@1954pps goodput (BK traffc 4Mbps) Fgure 4. Qualty of voce transmsson and effectve throughput of 4Mbps background traffc vs AIFSN [AC_BK] 786

5 and 14 settngs of the AIFSN [AC_BK] parameter: 2, 3 14, and 15 tme slots. Fgures 2, 3 and 4 shows the average voce transmsson qualty (at wreless and wred nterface) calculated for all 15 VoIP termnals and effectve throughput (goodput) as a functon of AIFSN [AC_BK] for three background traffc loads (1Mbps, 2Mbps, and 4 Mbps respectvely). It can be seen that voce transmsson at thte wreless network can be effectvely prortzed over data by tunng the AIFSN [AC_BK]. Increasng AIFSN [AC_BK] essentally promotes the AC_VO queue at the expense of the AC_BK queue n terms of probablty access. The bgger the dfference n AIFSN values, the easer t s for the AC_VO queue to wn transmsson opportuntes from the AC_BK. As a result, transmsson mparments (delay, jtter and packet loss) are reduced and the overall transmsson qualty s mproved. For example, when the AIFSN dfference between AC_VO and AC_BK was 6 (AIFSN [AC_BK] =8 and AIFSN [AC_VO] =2), all VoIP statons could experence at least tool voce transmsson qualty (ndcated by 7) for all examned background traffc loads and packetzaton schemes. Conversely a substantal reducton n the background traffc goodput was observed. In some cases (.e. the 256 Bytes background packets load) the goodput of the background traffc was almost halved. Increasng the AIFSN dfference between AC_BK and AC_VO further penalzes background traffc by makng t more dffcult to wn transmsson opportuntes. 4. CONCLUSIONS In ths paper we have expermentally evaluated the capablty of the new 82.11e MAC protocol to support voce calls n a mxed voce/data transmsson over WLANs. In our experments we have focused on the contenton-based mode of MAC operaton called Enhanced Dstrbuted Channel Access (EDCA) and more specfcally on one qualty enhancement parameter that controls the arbtraton nterframe space (AIFS),.e. AIFSN parameter. Our results show that the tunng of the AIFSN parameter can mprove the qualty of voce transmsson at the wreless network whle reducng the goodput of the background data traffc. For example, when the AIFSN dfference between AC_VO and AC_VO was 6 AIFSN [AC_BK] =8 slots and AIFSN [AC_VO] =2), all ffteen VoIP termnals could experence at least toll voce transmsson qualty (ndcated by 7) n the presence of the heavy background traffc njected to the network. The AIFSN dfferentaton s an effectve mechansm for voce prortzaton over data because of the very exstence of dscrete nstants of tmes (protected slots represented by the AIFSN dfference) where a lower number of statons may compete and access the channel. Ths ncreases the effectveness of the overall random access mechansm for the hgh-prorty statons [15]. To our knowledge, ths s the frst expermental demonstraton of voce prortzaton n the real 82.11e WLAN network from the perspectve of end-to-end speech transmsson qualty and end user satsfacton. 5. ACKNOWLEDGMENTS Ths work was supported by Scence Foundaton Ireland grant 3/IN3/I EFEENCES [1] Mroslaw Narbutt, Andrew Kelly, Lam Murphy, Phlp Perry, "Adaptve VoIP Playout Schedulng: Assessng User Satsfacton," IEEE Internet Computng Magazne, vol. 9, no. 4, July/August 5. [2] Mroslaw Narbutt, Mark Davs, "Assessng the Qualty of VoIP Transmsson Affected by Playout Buffer Scheme," Proc. of the ETSI/IEE Measurement of Speach and Audo Qualty n Networks Conference 25 (MESAQIN 25), Prague, June 5. [3] Mroslaw Narbutt, Mark Davs, "An Assessment of the Audo Codec Performance n Voce over WLAN (VoWLAN) Systems," Proc. of the Internatonal Conference on Moble and Ubqutous Systems: Networkng and Servces, (MOBIQUITOUS 25), San Dego, July 5. [4] Mroslaw Narbutt, Mark Davs "Gaugng VoIP Call Qualty from 82.11b esource Usage", Proc of the IEEE Internatonal Symposum on a World of Wreless, Moble and Multmeda Networks (WoWMoM6), Buffalo-NY, June 6 [5] Mroslaw Narbutt, Mark Davs, "Expermental nvestgaton on VoIP performance and the resource utlzaton n 82.11b WLANs", Proc of the 31st IEEE Conference on Local Computer Networks (LCN 6), Tampa, November 6 [6] ITU-T ecommendaton G.19 Appendx I (1/27) The E-model based qualty contours for predctng speech transmsson qualty and user satsfacton from tme-varyng transmsson mparments [7] ITU-T ec. G.19 Defnton of categores of speech transmsson qualty, September 99 [8] H. Yoon, Test of MADWIFI-ng WMM/WME nwlans, T nr 1, February 6 [9] TPtools: [1] ITU-T ecommendaton P.59, Artfcal conversatonal speech, March 93 [11] MGEN, The Mult-Generator Toolset: [12] Tace Plot eal-tme package (TP) [13] S. Garg, M. Kappes An Expermental Study o f Throughput for UDP and VoIP Traffc n IEEE 82.11b Networks, Proc of the IEEE Wreless Communcatons and Networkng Conference, WCNC 23, New Orelan, 3 [14]. amjee, J. Kurose, D. Towsley, and H. Schulzrnne, Adaptve playout mechansms for packetzed audo applcatons n wde-area networks, Proc. of the IEEE INFOCOM, Toronto, 99 [15] G. Banch, I. Tnnrello, L. Scala, Understandng 82.11e contenton-based prortzaton mechansms and ther coexstence wth legacy statons IEEE Network 19(4): (25) 787