A SYNCHRONIZATION CONTROL SCHEME FOR REAL-TIME STREAMING MULTIMEDIA APPLICATIONS

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1 A SYNCHRONIZATION CONTROL SCHEME FOR REAL-TIME STREAMING MULTIMEDIA APPLICATIONS Hnn Lu, Md El Zrk School of Informton nd Computer Scence Unersty of Clforn, Irne Irne, CA 9697 {hnn, ABSTRACT In ths pper, we propose n dpte synchronzton control scheme to chee both ntr-strem nd nterstrem synchronzton for rel-tme stremn multmed pplctons on the Internet. Bsed upon synchronzton errors clculted n rel-tme, our scheme pecewsely dusts the end-to-end dely to compenste for the dely rton. It does so by controlln the rtul clock mplemented t the destnton end. Smulton results show tht our control scheme s ble to mntn ood synchronzton performnce under dfferent network condtons. 1. INTRODUCTION The dncements n communctons nd med codn technoloes he mde rel-tme stremn of multmed pplctons, such s VoIP, nd deo conferencn, relty on the current Internet. These multmed pplctons contn ether snle or multple udo/deo obects, nd med dt unts (MDU s) wthn ech obect re pcketzed nd trnsported from the source to the destnton n seprte strem. To fthfully restore the ornl form of the multmed presentton, both the temporl ordern mon the MDU s n strem nd the relte temporl reltonshp mon dfferent strems he to be mntned. In other words, end-to-end rel-tme stremn multmed pplctons demnd both ntr-strem synchronzton nd nter-strem synchronzton. A typcl exmple of nter-strem synchronzton s the lp-sync n n udo/deo presentton. Howeer, due to the sttstcl multplexn feture of pcket swtch networks, the dely requrements cn rrely be stsfed wthout pproprte control mechnsms. For nstnce, IP networks, whch only prode best-effort serces, mke no urntees for on-tme delery of rel-tme dt. Vrble pcket delys nd unpredctble pcket losses my seerely dstort both ntr nd nter strem temporl reltonshps, nd mpr the fnl presentton qulty. Due to the lck of QoS support on the Internet, recent reserch emphss hs shfted to pplctonleel solutons for synchronzton control. A rety of pplcton-leel lorthms he been proposed to chee the ntr-strem nd nter-strem synchronzton [1]-[5]. The lorthms dffer n terms of ols nd pplcton scenros. In enerl, one cn dentfy three types of synchronzton control technques tht re employed by most lorthms, nmely, bsc control, preente control nd recte control 1. Bsc control, whch conssts of ppendn synchronzton nformton (tmestmp, sequence number, etc.) to MDU s nd buffern the dt t the receer sde, s essentl for ll lorthms. Preente control conssts of technques used to od synchrony, e.. the destnton chnes the buffern tme of the MDU s bsed upon n estmton of network dely. Recte control s used to recoer synchronzton fter synchrony occurs. Approches such s recte skppn nd pusn, shortenn/extendn plybck durton, nd rtul locl tme contrcton or expnson (referred s rtul locl tme control herefter) ll belon to recte control. Anyone of these technques, ether lone or n combnton wth others, cn be employed to chee the desred synchronzton for the treted pplcton [1]. Amon ll the technques proposed, the de of rtul locl tme control s more ttrcte becuse t 1) does not rely on synchronzed network clock, whch s not lwys fesble to procure on the Internet, ) cn be mplemented wth low oerhed [5], nd 3) cn produce resonbly ood qulty een when the network dely tter s ery hh [3]. Three lorthms tht use rtul locl tme control technque he been proposed n the lterture 1 In [1], the technques tht cn be used s both preente nd recte control ones re defned s nother type of technques, common control technques.

2 [4]-[6]. In [4], the de of chen nter-strem synchronzton by prodn ntr-strem synchronzton for ech strem s proposed. To ttn ntr-strem synchronzton, the plyout tme of ech MDU n strem s determned by comprn ts rrl tme wth seerl threshold lues, fter tht, the decson of ether plyn out or skppn s mde. Once ntr-strem synchronzton s estblshed, nter-strem synchronzton cn be mntned wth certn probblty []. The mpct of MDU losses or skppn on synchronzton dstorton s not consdered n ths lorthm. In [5], scheme whch dptely dusts the rtul plyout clock bsed on the frequency of occurrence of three synchronzton eents (MDU s 1) plyed out on tme, ) delyed, nd 3) lost) s presented. The lues of the three counters re obtned upon n ssumed worst-cse dely dstrbuton, nd the nter-strem synchronzton s exerted when clcultn the clock dustment mount. In [6], n dpte synchronzton frmework, whch s ble to mntn both ntr-strem synchronzton nd nter-strem synchronzton effectely, s proposed for wreless PCS networks. The pproch loses ts enerlty n tht t ssumes the mster strem does not experence ny dely rton n the network. In ths pper, we propose synchronzton control scheme tht 1) drectly ncorportes the qulty requrements of the pplcton nto the prmeters of the lorthm, ) clcultes synchronzton errors n rel-tme, 3) pecewsely dusts end-to-end dely by controlln the rtul locl clock to dpt to the network dely rton, nd 4) rcefully recoers the synchronzton f synchronzton error occurs. Our work hs been lrely nspred by the fndns n [3] nd the lorthms proposed n [4], [5] nd [6]. Smulton results show tht our scheme cn mntn ood synchronzton performnce under seere network condtons, e.. fluctutn delys..1 Vrtul Locl Tme Control In rel-tme multmed pplcton oer IP networks, the udo/deo MDU s re enerted t the source end nd sent cross the network. At the destnton end, the ornl temporl ordern s enerlly destroyed due to the dely rton cused by the network. A strhtforwrd soluton to ths problem s to buffer the receed MDU s t the destnton to compenste for the dely tter, nd then ply bck bsed on pre-determned sttc tmelne. Ths pproch ntroduces n equlzed end-to-end dely between the source nd the destnton. Howeer, becuse the network behor s unpredctble, t s not lwys fesble to fnd n optml sttc equlzton dely lue tht stsfes the requrements of the pplcton. A smll end-toend equlzton dely most lkely wll ncur synchronzton errors resultn from skpped MDU s (MDU s dd not rre pror to the scheduled plyout tme) s llustrted by sttc plyout tmelne (1) n Fure 1. On the other hnd, lre equlzton dely cn reduce the synchronzton error by extendn the plyout dedlne. But, s llustrted by the sttc plyout tmelne () n Fure 1, t leds to lrer end-to-end dely, whch wll deterorte the perceed qulty of the rel-tme pplcton. An dpte control mechnsm, whch s cpble of dustn the end-to-end dely bsed upon the obsered network dely nd synchronzton errors, s desrble under such crcumstnce. Ths s llustrted by the dpte plyout tmelne shown n Fure 1, the control mechnsm enbles the system to ncrese the end-toend dely f the synchronzton error exceeds certn threshold lue, nd decrese t f ll MDU s he rred hed of the scheduled tmelne durn certn perod. Sequence Number enerton tmelne dpte plyout tmelne sttc plyout tmelne (1) The rest of ths pper s structured s follows. In secton, we frst dscuss the bscs of the rtul tme control model nd the synchronzton model. We then descrbe new closed-loop synchronzton control scheme. Secton 3 detls synchronzton control lorthm tht uses rel-tme clculted synchronzton errors. Secton 4 presents the smulton results of the performnce eluton of the lorthm, nd we conclude n secton 5. end-to-end dely t1 end-to-end dely t sttc plyout tmelne () tme. SYSTEM MODEL Fure 1. Dfferent plyout strtees

3 Centered round the boe obserton, the rtul locl tme control model ntroduces rtul clock t the destnton end n ddton to the ctul tme. The MDU s re plyed out lon ths rtul clock xs t the destnton end [4][5][6]. The dpte plyout tmelne s cheed by modfyn the plyout tme of the dt unt bsed on the rrl tme smpled loclly, the enerton tme stmped t the source, nd other synchronzton error nformton. The clock cn be slowed down rtully by ncresn the scheduled plyout tme lue relte to the rrl tme. Ths s lso equlent to hn the end-to-end equlzton dely ncresed. Smlrly, t cn be rtully speeded up by don the reerse. We defne the bsc notton used throuhout ths pper s follows wth rerd to ths rtul tme control model: T : The enerton tme of MDU n n strem bsed on the source clock,.e. the lue of the tme stmp ppended to the unt. Note tht f the rtul plyout clock t the destnton strts upon the recept of the frst MDU nd there s no dely rton n the network, T lso represents the scheduled plyout tme of unt n. T : The rrl tme t whch MDU n completely rres t the destnton bsed on the rtul clock t the destnton. T p : The plyout tme t whch MDU n s decoded nd rendered, wth reference to the rtul clock t the destnton. The plyout tme s determned by the lorthm descrbed n secton 3. σ : The threshold Root Men Squre Error (RMSE) for strem whch represents the mxmum ntrstrem synchronzton error tht strem cn tolerte. l : The threshold loss rto for strem whch represents the mxmum loss percente tht strem cn tolerte. σˆ : Clculted RMSE for strem oer smpln wndow. lˆ : Clculted loss rto for strem oer smpln wndow. E : The threshold of the nter-strem nt synchronzton error.. Synchronzton Performnce Eluton The qulty of the ntr-strem synchronzton for strem cn be eluted by the RMSE of the ntersmple tme of the MDU s n the strem [5]. For N MDU s plyed out n strem, the RMSE s defned s: N [( Tp Tp ( n 1)) ( T T ( n 1))] n= σ = (1) N 1 Smlrly, for the nter-strem synchronzton, the qulty cn lso be eluted by the RMSE of the closest correspondn dt unts mon strems [6]. For exmple, n the cse of udo/deo pplctons, the RMSE of nter-strem synchronzton s defned s N [( Tp ( m) Tp ) ( T ( m) T )] n= σ = () N 1 where MDU m n the udo strem corresponds to MDU n n the deo strem, nd N s the totl number of MDU s n udo strem. Whle equtons (1) nd () elute the synchronzton phse dstorton between the MDU s tht re plyed bck, t does not cpture n mportnt source of the synchronzton error dscontnuty resultn from lost/skpped MDU s. For exmple, f consecute MDU s n strem re lost or skpped, een thouh the plyed MDU s re synchronzed precsely, the perceptul synchronzton qulty wll stll be ffected. We therefore propose to tke nto ccount both fctors nd elute the synchronzton error for W consecutely enerted MDU s wthn strem, by two prmeters: 1) Clculted RMSE M [( Tp Tp ( n 1)) ( T T ( n 1))] n= σ ˆ =. (3) M ) Loss rto l W M = 4) W where M ( M < W ) represents the number of MDU s plyed bck.

4 We propose synchronzton control scheme whch controls the rtul clock t the destnton end bsed on rel-tme clculton of the synchronzton errors s defned by equton (3) nd (4). We next expln how the synchronzton control s mplemented wth the d of these two lues..3 Synchronzton Control Scheme We dopt the mster-sle concept [1] [6], nd pply the nter-strem constrnt to the sle strems. The essence of our scheme s tht t s dren by the ntrstrem synchronzton errors from mster nd sle strems nd pecewsely dust the end-to-end dely to stsfy the error threshold lues requred by the pplctons. Fure shows how ths scheme works wth mster strem nd snle sle strem. For the sle strem, the plyout tme of the rred MDU s frst determned under the ntr-strem synchronzton constrnt. It s then dusted to stsfy the nter-strem synchronzton requrement. The synchronzton errors re clculted fter ths step. For the mster strem, the plyout tme of the rred MDU s only determned under ts own ntrstrem synchronzton constrnt.the synchronzton errors of both strems re montored durn the whole process, once the synchronzton error (ether the RMSE or the loss rto) of ether strem exceeds the threshold lue, the rtul clock wll be slowed down correspondnly. The rtul clock cn be speeded up f ether strem s synchronzton errors (both the RMSE nd the loss rto) re equl to durn certn perod of tme. Note tht n our scheme, unlke the schemes n [4][5][6], only one rtul clock s mntned for roup of strems t the destnton. The rtul clock s pecewsely dusted n closedloop mnner. 3. ALGORITHM In ths secton, we frst expln how to determne the plyout tme, nd then descrbe the detls of the rtul clock dustment lorthm. 3.1 Plyout Tme Determnton It s well known tht certn mount of rendern tter s tolerble for some multmed pplctons. Ths mens tht for the MDU s tht do not mss the schedule by too much, f controlled properly, they cn be rendered wthout cusn notceble phse dstorton. Bsed on ths feture, we set lte boundry δ for ech MDU nd prtton the rrl tme of the MDU s wthn strem nto two reons s shown n Fure 3. For MDU n wthn strem, f T T + δ, where δ represents the mxmum tolerble lte tme lue ccordn to the current rtul clock, the dt unt cn be rendered. If T > T + δ, the dt unt s smply skpped to od error. Intutely, the lue of δ s the trdeoff between the RMSE nd the loss rto. If we rse the lte boundry, then the RMSE wll ncrese nd the loss rto wll drop. If the lte boundry s too close to the scheduled plyout tme, then more MDU s wll be skpped. δ s determned by the fctor to whch the pplcton s more senste. T plyout T T + δ dscrd T ( n +1) rtul clock Determne Plyout Tme under Intr-Sync Constrnt Adust Plyout Tme under Inter-Sync Constrnt Clculte Sync Error Ply Bck Fure 3. Prtton of dt unt rrl tme Sle Strem Vrtul Clock The plyout tme determnton lorthm works recursely s follows: Mster Strem Determne Plyout Tme under Intr-Sync Constrnt Clculte Sync Error Ply Bck 1) If the rrl tme of MDU n flls n the dscrd reon, tht s, T - T >δ, then n s dscrded. Fure. Closed-loop synchronzton control scheme bsed on ESD ) If the rrl tme of MDU n flls n the plyout reon, whch mens T - T <δ, then there re two possble scenros:

5 ) If T p ( n 1) s equl to T ( n 1), whch mples tht unt ( n 1) ws plyed out on tme ccordn to the current rtul clock, then Else e nt < E, nt T = T ( m) + T T ( m) E (9) p p nt Tp = mx( T, T ) (5) Equton (5) ensures tht MDU n s plyed out t ts scheduled tme f t rres erly. Otherwse, t s plyed bck mmedtely t the tme t rres. b) If T p ( n 1) s lrer thn T ( n 1), ths mples tht unt ( n 1) s plyed bck fter the scheduled tme, nd synchronzton error hs occurred. To reduce the mpct of ths slp, t needs to be recoered from smoothly. Accordnly, the plyout tme of n s set to be Tp = mx[ T, T, Tp ( n 1) + T T ( n 1) α] (6) where α s the smoothn prmeter to ensure tht unt ( n 1) s rendered wth enouh tme [6]. Equton (6) urntees tht MDU n s plyed bck fter t rres, nd menwhle, mnmzes the synchronzton error s much s possble. Accordn to our scheme shown n Fure, the plyout tme of MDU n hs to be re-checked to stsfy the nter-strem synchronzton requrement f strem s sle strem. If we cn locte the closest correspondn MDU (ccordn to ts enerton tme) n the mster strem, we cn esly dust T p s n [6]. Howeer, the correspondn dt unt my be lost or rred too lte to be referenced. To oercome ths, the determnton lorthm pcks the MDU m n the mster strem, whch s the most closely enerted one mon the MDU s tht he rred hed of n, nd forces MDU n to synchronze to t. Frst, the nter-strem synchronzton error e nt s clculted s e nt = [ T T ( m)] [ T T ( m)] (7) p p If e nt < E, where nt E nt s the threshold lue of nter-strem synchronzton error, T p s kept s t s. Else f e > E nt nt T = mx[( T ( m) + T T ( m) Ent ), T ] (8) p p + 3. Vrtul Clock Adustment In our control scheme, the ntl lue of the rtul clock s smply set to be the lue of the tmestmp crred by the frst receed MDU n the strem. If there re multple strems n the pplcton, the clock s set to be the lue of the tmestmp crred by the frst receed MDU n the mster strem. Recll tht the tmestmp represents the enerton tme of the MDU t the source, nd the scheduled plyout tme t the destnton. After MDU n s ctul plyout tme hs been determned, the control scheme then updtes the lue of the synchronzton errors for the strem to whch the dt unt belons. Recll tht the synchronzton error of ech strem should be eluted by two prmeters: RMSE σˆ nd lost rto lˆ. We use smpln wndow nd pcket lost counter to montor the synchronzton errors. If MDU n s chosen to be plyed bck, t s put nto the smpln wndow to clculte the RMSE lue. If MDU n s dscrded or lost, the counter lue ncreses by one. The sze of the smpln wndow lues rnes nd W, where W s the upper lmt of the smpln wndow sze for strem ; the counter lue rnes from to W l. The wndow sze ncreses by one fter n MDU hs entered the wndow untl the sze reches W. At the bennn or fter clock dustment, both the wndow sze nd the counter lues re reset to be. In the cse of multple strems, both prmeters for ll the strems re reset to be. The rtul plyout clock s dusted under the follown two condtons: 1) Once the sze of the smpln wndow exceeds two, we strt clcultn the synchronzton errors for ll the strems. For strem, L [( Tp Tp ( n 1)) ( T T ( n 1))] n= σ ˆ = (1) W 1 where L s the current wndow sze. At the sme tme, ψ lˆ = (11) W

6 where ψ s the current loss count lue. If ether σˆ s reter thn the threshold lue σ, or lˆ s reter thn the threshold lue l, the rtul clock wll be slowed down. If σˆ exceeds the threshold lue, we et the mxmum dely tht the dt unt n the smpln wndow experenced, whch s equl to = mx( T T ), n smpln wndow for strem. (1) Then the rtul plyout clock s dusted by -, ths s equlent to hn the end-to-end dely ncresed by. If ψ exceeds the threshold lue, we dust the rtul plyout clock by - δ. If two consecute clock dustments re trered by the loss counter s result of hhly rble network dely, we ncrese the upper lmt of smpln wndow for ll strems by β. By don ths, we he thtened the RMSE constrnt nd relxed the MDU loss constrnt for the next smpln perod. W s then chned to W = mn( W + β, W ) (13) where W s the mxmum possble sze of the mx smpln wndow. Equton (13) ensures tht W wll not exceed W. mx ) When the wndow sze reches W, the rrl tmes of the MDU s n the wndow re checked. If the rrl tme of eery MDU wthn the smpln wndow s hed of ts scheduled plyout tme ccordn to the current rtul plyout clock, the clock needs to be speeded up. The dusted mount s -, where = mx( T T ) mx, n smpln wndow for strem. (14) Note s less thn n ths cse, nd the end-to-end dely s decresed by -. Menwhle, we thten the MDU loss constrnt nd relx the RMSE constrnt for the next smpln perod by decresn the upper lmt of the smpln wndow for ll strems by β,.e. W = mx( W β, W ) (15) where W s the mnmum pplcble sze of the mn smpln wndow. Equton (15) ensures tht W wll not be lower thn W. mn mn 4. PERFORMANCE EVALUATION In ths secton, we nestte the performnce of our synchronzton control lorthm by mens of smultons. We show how the proposed scheme performs under dfferent network condtons. Both snle-strem nd mult-strem pplcton scenros re consdered, nd the controlled end-to-end dely, synchronzton errors nd ere buffern lenth re mesured wth respect to dfferent trnsport chnnel behors. For smplcty, we ssume tht ech MDU fts nto one pcket for trnsmsson. 4.1 Network Dely Model In the smulton, we frst enerte seres of pseudonetwork dely lues. We do not use the wdely ppled snle norml dstrbuton model [1], s t does not ccurtely reflect the network coneston stuton. Insted, two-stte Mrko model tht hs shown better pproxmton [8] s chosen to smulte the trnsport chnnel. 1-q Good p q Fure 4. Trnsport chnnel model Wth the two-stte Mrko model shown n Fure 4, the trnsport chnnel s ssumed to he two sttes Good or Bd. p s the probblty wth whch the network trnsts nto the coneston stte, nd q s the probblty the network resumes the unconested stte. Wthn ech stte, the network dely s ssumed to conform to norml dstrbuton wth dfferent men U nd stndrd deton D. Then by ssnn dfferent trnst probbltes nd dstrbuton prmeters, we cn et dfferent chnnel behors. In Fure 5, the dely dstrbuton of two dfferent chnnel behors, whch re lbeled s Moderte nd Bd, re presented. The dely lues we obtned cpture the hey tl feture of network dely experenced by rel-tme stremn pplctons on the current Internet [9]. We then combne the two chnnel behors nd crete new seres of pseudo-network dely lues to In order to et the lower bound of the network dely, we et rd of the lues tht re less thn U D wthn ech stte Bd 1-p

7 smulte the stuton of brupt chnes n network condton. We then lbel ths behor s Seere. We demonstrte how the proposed control lorthm works wth dfferent pplctons under these three dfferent network condtons. Frequency p=.1, q=.1, U1= 5ms, D1=1ms, U=75ms, D=1ms Frequency Dely Dstrbuton Dely lue (ms) w th bn=5ms () Moderte Dely Dstrbuton Dely lue (ms) w th bn=5ms p=.1, q=.4, U1= 1ms, D1=5ms, U=18ms, D=7ms (b) Bd Fure 5. Dely dstrbuton of Moderte nd Bd chnnel behor 4. Snle-strem Applcton We frst elute how the control lorthm performs wth snle-strem pplcton. In ths cse, we ssume the MDU nterl s 3 ms, whch s usully the frme durton n VoIP, nd set the σ, l nd δ to be ms,. 3 nd 5ms 4, respectely. The smoothn prmeter α s set to be 5ms, whch n turn urntees tht ech udo MDU s rendered for 3 Threshold lue of RMSE for snle udo strem cn rne from to 5ms [11]; mxmum loss rto of most commonly used oce codecs rnes from 1% to % [7] 4 In order to reduce the buffer lenth nd the end-to-end dely, δ ws chosen to be less thn the nterl lue between two consecute MDU s n the udo strem,.e. T + δ T ( n + 1). A lrer lue, lthouh potentlly cceptble by the pplctons, wll result n lrer end-to-end dely nd lre buffer sze. t lest 3-5=5 ms. In order to keep ech clock dustment fxed lue for t lest 15s (frequent clock dustment lso ntroduces extr synchronzton error [6]), the W should be t lest mn 15/.3= 5. In our smultons, we set W mn to be 6. W s set to be 9. The smulton lenth s 1 mx mnutes ( MDU s). We rn the lorthm for 1 tertons wth dfferent dely seres for ech network condton. Fure 6 shows exmples of the operton of the synchronzton control lorthm under dfferent network condtons. The performnce results reflectn the 95% confdence nterls re summrzed n Tble I. Note tht we use the source clock s the common reference pont for the mesurement. As shown n Fure 6, our control lorthm dusts the end-to-end dely dptely to ccommodte dfferent network condtons. It keeps the end-to-end dely t n pproprte leel whle presern ood synchronzton performnce. The RMSE lue nd the loss rto re kept frly low under Moderte nd Bd network condtons. Een thouh the trnsport chnnel behor bruptly chned twce under the Seere network condton, the resultn RMSE s slhtly hher thn the threshold lue oer the whole plyout perod. It s lso to be noted tht, wth our synchronzton control lorthm, the loss rto stll remns resonbly low when seere chnes n network condton occurs. 4.3 Mult-strem Applcton We next nestte how the proposed control lorthm opertes wth mult-strem pplcton. Ths tme we ssume n pplcton tht hs one udo strem nd one deo strem. We set the prmeters bsed on the chrcterstcs of deo conferencn. For the udo strem, we stll ssume tht the MDU nterl n the udo strem s 3 ms nd set the other prmeters the sme s for the boe snle-strem. For the deo strem, we ssume tht the MDU nterl s ms (15 frmes/sec). σ nd l re set to be 5ms nd.3 5 respectely; the smoothn prmeter α s set to be ms [6]. E s set to nt be 8ms whch s the mxmum llowed nter-strem skew between udo nd deo [1]. The smulton 5 Threshold lue of RMSE for snle deo strem cn rne up to 1ms. We choose 5ms s n [6]. Mxmum tolerble loss rto of deo codecs rnes from % to 3% [7]

8 lenth s 4 seconds (1 MDU s for udo strem, nd 54 MDU s for deo strem). Wth dfferent choces of δ for the deo strem, we compre the resultnt synchronzton dstorton lues under Seere network condtons. Tble II shows the results. An, we run the lorthm for 1 tertons, nd ech lue n the tble s expressed n the form of 95% confdence nterl. We obsere tht the resultnt RMSE, especlly the nter-strem RMSE, ncreses drmtclly wth the lrer δ for the deo strem, whle the loss rto of the deo strem decreses t the sme tme. We conclude tht, n the cse of mult-strem pplctons n whch strems he nter-strem synchronzton reltonshps between ech other, dfferent δ lues cn result n extr nter-strem synchronzton errors. Consequently, n order to reduce the nter-strem dstorton to mproe the perceptul qulty 6, we set δ of the deo strem to be equl to tht of the udo strem nd rn the smulton n. Fure 7 shows the operton of the end-to-end dely control for udo nd deo strems under seere network condtons (wth δ = δ ). The results re presented n Tble III nd Tble IV. To summrze, wth our lorthm, both ntrstrem synchronzton nd nter-strem synchronzton for n A/V pplcton re presered under dfferent network condtons. When the trnsport chnnel behes well (low ere network dely nd moderte dely tter), our control scheme s cpble of mntnn low end-to-end dely wth smll buffer lenths. When the network becomes conested, our control scheme pecewsely dusts the end-to-end dely to n pproprte lue nd ncreses the buffer lenths dptely to ensure the synchronzton errors re mntned below the threshold lues. When the network resumes moderte stte, the control scheme rdully dusts the end-to-end dely lue to reflect ths chne. Note tht oer the trnston perod durn whch the network condton bruptly chnes from moderte to bd, our control scheme s stll ble to mntn ood synchronzton wth resonbly low loss rto. 6 In [3], t hs been shown tht nter-strem synchronzton s closely relted to the fnl presentton qulty. The less the nter-strem synchronzton error, the better the qulty. Dely (ms) Netw ork Dely Controlled End-to-End Dely MDU Sequence Number Dely (ms) Dely (ms) () Under Moderte Network Condton Netw ork Dely 5 45 Controlled End-to-End Dely MDU Sequence Number (b) Under Bd Network Condton 45 4 Netw ork Dely Controlled End-to-End Dely MDU Sequence Number (c) Under Seere Network Condton Fure 6. Controlled end-to-end dely determned by the synchronzton control lorthm for snle-strem pplcton (MDU nterl= 3ms, σ =ms, l =., δ =5ms, α =5ms, W =6, mn W =9, β =1) mx 5. CONCLUSIONS In ths pper, we presented synchronzton control scheme for rel-tme multmed pplctons oer the Internet n whch the synchronzton errors re used to dust the end-to-end equlzton dely. We nestted the performnce of the scheme n snlestrem nd mult-strem pplcton scenros. Smulton results show tht our scheme cn presere ood synchronzton performnce for these pplctons under dfferent network condtons.

9 The next step s to pply our lorthm to rel trces from the Internet to test the performnce of the scheme. We wll lso do n ctul mplementton of the control mechnsm n W-F deo conferencn proect n our lb. Dely (ms) Netw ork Dely 45 Controlled End-to-End Dely MDU Sequence Number n Audo Strem () Operton of end-to-end dely control for udo strem Buffer Lenth ( Number of MDUs) Dely (ms) Audo MDU Sequence Number (b) Buffern lenth of udo strem Netw ork Dely 45 Controlled End-to-End Dely MDU Sequence Number n Vudo Strem (c) Operton of end-to-end dely control for deo strem Buffer Lenth (Number of MDUs) Vdeo MDU Sequence Number (d) Buffern lenth of deo strem Fure 7. Operton of the end-to-end dely control for udo nd deo strems under seere network condton. (Audo MDU nterl= 3ms, σ =ms, l =., δ =5ms, α =5ms, Vdeo MDU nterl=66.667ms, σ =5ms, l =.3, δ =5ms, α =16.667ms, W =6, mn W =9, β =1) mx 6. REFERENCES 1. Y. Ishbsh nd S. Tsk, A Comprte Surey of Synchronzton Alorthms for Contnuous Med n Network Enronments, LCN, pp E. Bersck nd W. Geyer, Synchronzed Delery nd Plyout of Dstrbuted Stored Multmed Strems, Multmed Systems 7, pp. 7-9, Y. Ishbsh, S. Tsk nd H. Ow, A Comprson of Med Synchronzton Qulty mon Recte Control Schemes, INFOCOM 1, pp Y. Ishbsh nd S. Tsk, A Synchronzton Mechnsm for Contnuous Med n Multmed Communctons, INFOCOM 1995, pp Y. Xe, C. Lu, M. J. Lee, T. nd N. Sdw, Adpte Multmed Synchronzton n Teleconference System, Multmed Systems 7, pp , H. Lu nd M. El Zrk, Dely nd Synchronzton Control Mddlewre to support Rel-Tme Multmed Serces oer Wreless PCS Networks, IEEE Journl on Selected Ares n Communctons, Vol. 17, No. 9, pp , D. Mrs, A surey on Network QoS Needs of Adnced Internet Applctons, Workn Document of Internet QoS Workn Group, 8. U. Horn, K. Stuhlmuller, M. Lnk, nd B. Grod, Robust Internet Vdeo Trnsmsson Bsed on Sclble Codn nd Unequl Error Protecton, Ime Comuncton: Specl Issue on Rel-Tme Vdeo oer the Internet, pp , Sept D. Louno nd H. Rdh, End-to-End Internet Vdeo Trffc Dynmcs: Sttstcl Study nd Anlyss, INFOCOM. 1. R. Stenmetz, Humn Percepton of Jtter nd Med Synchronzton, IEEE Journl of Selected Ares n Communctons, ol. 14, pp , Sept R. Stenmetz nd C. Enler, Humn Percepton of Jtter nd Med Synchronzton, Internl Report #43.931, IBM Europen Networkn Center, Hedelber, Germny 1993.

10 Tble I Performnce of the synchronzton control lorthm for snle-strem pplcton under dfferent network condtons (MDU nterl= 3ms, σ =5ms, l =., δ =5ms, α =5ms, W =6, mn W =9, β =1) mx RMSE (ms) Loss rto Aere buffer lenth (number of dt unts) Aere controlled end-to-end dely (ms) Loss rto of frst 3s fter network condton chne Moderte 1.1 ±.. ±.3.6 ± ±.8 N/A Bd 1.4 ±.1.8 ±. 4.9 ± ± 1.6 N/A Seere.1 ±.1.6 ±.3 N/A N/A.6 ±.4 Tble II Performnce of synchronzton control lorthm wth dfferent δ for deo strem n A/V pplcton (udo MDU nterl= 3ms, σ =ms, l =., δ =5ms, α =5ms, deo MDU nterl=66.667ms, σ =5ms, l =.3, α =16.667ms, W =6, mn W =9, β =1) mx Inter-strem RMSE (ms) Intr-strem RMSE of udo strem (ms) Loss rto of udo strem Intr-strem RMSE of deo strem (ms) Loss rto of deo strem δ =6ms 4.1 ± ±..9 ± ± 1.. ±.1 δ =5ms.6 ±.8.5 ±..8 ±. 3.1 ±.9.8 ±.4 Tble III Synchronzton dstorton of A/V pplcton under dfferent network condtons (Audo MDU nterl= 3ms, σ =3ms, l =., δ =5ms, α =5ms, Vdeo MDU nterl=66.667ms, σ =5ms, α =16.667ms, W =6, mn W =9, β =1) mx Inter-strem RMSE (ms) Intr-strem RMSE of udo strem (ms) Loss rto of udo strem Intr-strem RMSE of deo strem (ms) Loss rto of deo strem Loss rto of frst 3s fter network condton chne (udo) l =.3, δ =5ms, Loss rto of frst 3s fter network condton chne (deo) Moderte 1.4 ± ±.5.3 ± ±.6.4 ±.3 N/A N/A Bd. ± ±.3.96 ± ±.5.96 ±.4 N/A N/A Seere.6 ±.9.5 ±..8 ±. 3.1 ±.9.8 ±.4.58 ± ±.19 Tble IV Controlled end-to-end dely nd ere buffer lenth for A/V strems (wth the sme set of prmeter s n Tble III) Aere controlled end-to-end dely of udo strem (ms) Aere buffer lenth of udo strem (number of MDU s) Aere controlled end-toend dely of deo strem (ms) Aere buffer lenth of deo strem (number of MDU s) Moderte 85.8 ± ± ± 6.4 ± Bd 89. ± ± ± ±. Seere N/A 3.5 ±.3 N/A 1.3 ±.1