Research Article A Novel Adaptation Method for HTTP Streaming of VBR Videos over Mobile Networks

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1 Moble Informaton Systems Volume 216, Artcle ID 29285, 11 pages Research Artcle A Novel Adaptaton Method for HTTP Streamng of VBR Vdeos over Moble Networks Hung T. Le, 1 Ha N. Nguyen, 2 Nam Pham Ngoc, 2 Anh T. Pham, 1 and Truong Cong Thang 1 1 Unversty of Azu, Tsuruga, Ikk-mach, Azu-Wakamatsu , Japan 2 Hano Unversty of Scence and Technology, 1 Da Co Vet, Hano 1, Vetnam Correspondence should be addressed to Truong Cong Thang; thang@u-azu.ac.jp Receved 16 November 215; Accepted 12 June 216 Academc Edtor: Q Wang Copyrght 216 Hung T. Le et al. Ths s an open access artcle dstrbuted under the Creatve Commons Attrbuton Lcense, whch permts unrestrcted use, dstrbuton, and reproducton n any medum, provded the orgnal work s properly cted. Recently, HTTP streamng has become very popular for delverng vdeos over the Internet. For adaptvty, a provder should generate multple versons of a vdeo as well as the related metadata. Varous adaptaton methods have been proposed to support a streamng clent n copng wth strong bandwdth varatons. However, most of exstng methods target at constant btrate (CBR) vdeos only. In ths paper, we present a new method for qualty adaptaton n on-demand streamng of varable btrate (VBR) vdeos. To cope wth strong varatons of VBR btrate, we use a local average btrate as the representatve btrate of a verson. A buffer-based algorthm s then proposed to conservatvely adapt vdeo qualty. Through experments n the moble streamng context, we show that our method can provde qualty stablty as well as buffer stablty even under very strong varatons of bandwdth and vdeo btrates. 1. Introducton Recently, vdeo streamng has rapdly ganed popularty over themobleinternet.itspredctedthatglobalvdeotraffcwll reach 8 percent of total consumer Internet traffc n 219 [1]. Besdes, HTTP protocol has become a cost-effectve soluton thanks to the abundance of Web platform and broadband connectons [2, 3]. Furthermore, for nteroperablty of HTTP streamng n the ndustry, ISO/IEC MPEG has developed Dynamc Adaptve Streamng over HTTP (DASH) [4] as the frst standard for vdeo streamng over HTTP. Due to the heterogenety of communcaton networks nowadays, adaptvty s a prncpal requrement for any streamng clents. In DASH, multple versons of an orgnal vdeo as well as related metadata (e.g., descrbng btrates and resolutons) are generated and stored at servers [4, 5]. Based on the nformaton of metadata as well as the termnal and networks, a clent can adaptvely decde whch/when data parts should be downloaded. Currently, the queston of vdeo adaptaton for HTTP streamng s stll an open ssue [6]. Varous adaptaton methods for HTTP streamng have been proposed over the past few years [7 17]. These methods can be roughly classfed nto two groups, throughput-based and buffer-based; each has ts own strengths and weaknesses [18]. Throughput-based methods decde the verson based on the estmated throughput only, whle buffer-based methods manly use buffer characterstcs as references for makng decsons. Throughput-based methods are usually able to react quckly to throughput varatons; however, the streamng qualty may be unstable [7]. Meanwhle, buffer-based methods try to mantan a smooth vdeo stream, but may cause sudden changes n vdeo qualty when the buffer level drastcallydrops[8 11].Itshouldbenotedthatasthedatasze of each segment vares accordng to the requested verson, the buffer sze and buffer level should be measured n seconds of meda. So far, exstng adaptaton methods have mostly focused on CBR (constant btrate) vdeos. The research on HTTP streamng for VBR (varable btrate) vdeos s stll lmted. The problem wth VBR vdeos s that even though the throughput s stable, strong fluctuatons of vdeo btrate may result n buffer underflows [12]. Our prevous work n [12] s the frst study on HTTP streamng that supports VBR vdeos by estmatng both the nstant btrate and the nstant throughput. In the context of managed IPTV networks, where

2 2 Moble Informaton Systems Tme Server Alternatve 1 Alternatve 2 HTTP requestresponse transactons Alternatve 3 Throughput Clent Play Fgure 1: Meda delvery herarchy n HTTP streamng. the bandwdth s allocated n advance, the delay-qualty tradeoff of a VBR vdeo s optmally acheved by replacng some hgh-btrate segments wth low-btrate segments [13]. In [9], a buffer-based adaptaton method s proposed for VBR vdeo streamng by usng a partal-lnear buffer predcton model along wth a strategy to select versons n dfferent buffer ranges. However, ths method does not consder btrate values, and so t cannot avod sudden changes of qualty when vdeo btrate and throughput are drastcally varyng. In ths paper, we present a novel adaptaton method whch can effectvely support VBR vdeos. By extendng our prelmnary work n [14], the proposed method can cope wth varatons of throughput as well as vdeo btrate. Our method especally takes nto account the movng average of bandwdth and vdeo btrate to provde stable streamng qualty wthout sudden changes. The expermental results n the moble streamng context show that our approach can provde consstent VBR vdeo streamng wth smooth vdeo qualty and stable buffer level. To the best of our knowledge, ths s the frst method that can provde smooth verson transtons for VBR vdeo streamng over HTTP. It should be noted that our method does not requre the exact values of vdeo segment btrates to make decsons. The organzaton of ths paper s as follows. Secton 2 presents an overvew of HTTP streamng and the related work. The prncples of our method as well as the algorthm descrpton are presented n detal n Secton 3. Secton 4 provdes our expermental results and dscussons. Fnally, concluson and drecton for future work are gven n Secton Related Work The general archtecture of an HTTP streamng system conssts of servers, delvery networks, and clents [3, 4]. In MPEG DASH termnology, to support adaptvty, a vdeo s encoded n multple versons (also called alternatves or representatons), each of whch s further dvded nto short segments. Vdeo segments together wth metadata are hosted at a server and wll be requested by the clent. In most cases, for each request from the clent, the server wll send one segment. Therefore, a vdeo wll be delvered by a sequence of HTTP request-response transactons. The verson (low or hgh) of a requested segment s decded based on the metadata and status of termnals/networks. Fgure 1 depcts an llustraton of meda delvery n DASH. More nformaton about the HTTP streamng structure as well as DASH concepts could be found n [2, 4]. In general, an adaptaton method needs to answer two key questons: (1) should the current verson be mantaned? and (2) f not, whch verson should be swtched to?asalreadymentoned,exstngmethodscanbedvded nto a throughput-based group and a buffer-based group. Throughput-based methods are dfferent n the ways they estmate or use the throughput. In terms of throughput estmaton, the smplest way s to use the measured throughput rght after havng fully receved a segment (called nstant throughput) as the throughput estmate of the next segment. Another approach s to use a smoothed throughput measure [3,1]toavodshort-termfluctuatons,whchareadrawback of usng nstant throughput. However, ths may cause late reacton of the clent to large throughput drops. In [3] we propose a throughput estmaton method that has the advantages of both nstant throughput and smoothed throughput. Furthermore, other studes also present ways to obtan the estmatedthroughputbasedonsampledthroughputvalues and RTT [12], probng or stored data (lookup table) [15]. Once the clent obtans the estmated throughput, the verson can be decded n many ways. A smple soluton s usng a safety margn to compute an approprate verson for the next segment [3]. In [7], the verson s controlled by a TCPlke mechansm where a measure proportonal to the nstant throughput s used as the key nput. Buffer-based methods, whch manly use buffer characterstcs to decde the vdeo versons, may take nto account a throughput estmate as well. A popular strategy of these methodssdvdngthebufferntomultplerangeswth buffer thresholds β 1, β 2, β 3, β max (<β 1 <β 2 <β 3 β max ) [8, 1, 11]. When the buffer level stays n dfferent ranges,

3 Moble Informaton Systems Btrate (kbps) Tme (seconds) Btrate (kbps) Tme (seconds) QP =22 QP =28 QP =34 QP =38 QP =42 QP =48 QP =22 QP =28 QP =34 QP =38 QP =42 QP =48 (a) Sony Demo (b) Termnator 2 Fgure 2: Btrates of the versons of two test vdeos [19]. dfferent actons are appled. For nstance, methods of [8, 1] try to mantan the current verson n a specfc buffer range (e.g., β 2 β 3 for the method of [8] and β 2 β max for themethodof[1])anddynamcallyswtchup/downthe qualty n other buffer ranges. Meanwhle, the method of [11] chooses vdeo versons followng the varatons of nstant throughput (also wth the use of upscalng and downscalng factors) based on dfferent buffer ranges. In [16] we ntroduce a trells-based method that represents all possble changes of the versons and correspondng buffer levels n the near future. Thus, ths approach can make good decsons on the btrates of some future segments. In [17], the buffer level devaton and nstant throughput are employed as nputs of a proportonal-ntegral controller for adaptaton. Yet, most of the exstng methods have been developed only for the context of CBR vdeos, where the btrate of a verson s constant. Compared to CBR vdeos, vdeos encoded nvbrmodehavemportantadvantagesntermsofqualty and network resource usage [2]. However, the varatons of vdeo btrate over tme, together wth throughput fluctuatons, result n a bg challenge for HTTP adaptve streamng [12]. Two examples of how btrates of dfferent versons vary, especally n some scene changes, are llustrated n Fgure 2. Detaled nformaton of these vdeos wll be descrbed n Secton 4. Our prevous work n [12] s the frst study on VBR vdeo streamng over HTTP. Besdes throughput estmaton, ths method also consders the estmaton of the nstant vdeo btrate, whch can be dvded nto (1) ntrastream estmaton and (2) nterstream estmaton. The former estmates thebtratesofsegmentswthnaverson,whlethelatter estmates the btrates of segments across dfferent versons. Thsmethodcanprovdeaverystablebufferand,moreover, can support a CBR-lke streamng servce from VBR vdeos. In [9], a buffer-based adaptaton method for VBR vdeos s proposed, where the buffer s dvded nto multple ranges. In order not to take nto account varyng vdeo btrates, ths method uses a partal-lnear trend predcton of the buffer level for choosng versons n dfferent buffer ranges. If no sgnfcant change of buffer level s estmated, the clent wll mantan the current verson for the next segment. However, ths method stll causes sudden verson changes f the actual buffer level declnes drastcally. In ths paper, we propose a novel adaptaton method that can effectvely support VBR vdeos n on-demand streamng. The dstngushng features of our method nclude the followng: () To cope wth strong varatons of vdeo btrates, we proposeusngalocalaveragebtrateasthe(movng) representatve btrate of a verson. Snce ths representatve btrate s stable n short term, t helps the clent clearly dfferentate between the avalable versons and make a good verson selecton at each tme nstance. () Because the clent does not have enough nformaton aboutthesegmentbtratesofallversons,weprovde an algorthm to obtan an estmated representatve btrate of each verson. () Regardng the frst key queston, qualty stablty s effectvely mantaned by (1) usng a smoothed throughput estmate when the buffer s not n danger, (2) usng the representatve btrate, and (3) beng conservatve n qualty swtchng. (v) Regardng the second key queston, smooth transtons of qualty are supported by avodng jumpng smply to the lowest verson n panc case and by early swtchng down when there s a throughput-btrate msmatch. 3. The Proposed Adaptaton Method In ths secton, we present a new buffer-based qualty adaptaton method for VBR vdeo streamng. Some notatons along wth ther defntons used n the paper are provded n Notatons Handlng Throughput and Btrate Fluctuatons. Generally, basedonthemeasuredthroughputandthevdeobtrate, theclentshouldchooseanapproprateversonforthenext segment. Suppose that after recevng the current (or last)

4 4 Moble Informaton Systems segment of verson I, the clent measures the btrate B,I and throughput T of ths segment. Now the clent wll decde the verson I +1 for the next segment +1.Ourproposed method wll also leverage the clent buffer to cope wth the fluctuatons of both the throughput and the vdeo btrate. Dependng on the current buffer level β cur,theclentwll decde whether the verson should be ncreased, decreased, or mantaned. For the verson selecton of the next segment, t s necessary to estmate the throughput based on the throughput hstory of receved segments. To avod the effects of shortterm fluctuatons of nstant throughput, we use a smoothed throughput measure T s [3, 1] as the throughput estmate Test +1 for the next segment +1: +1 =Ts = { (1 δ) T s 1 +δ T { T { T est f >; f =, where δ s a weghtng value, whch s set to.1 n ths paper. As vdeo btrate s hghly fluctuatng, we propose usng a representatve btrate for each verson, whch can dfferentatetheavalableversonsandcanalsobeappropratefor the representatve btrate for verson k at segment ndex. Inourmethod,B rep,k s calculated as the average btrate of N recent segments of verson k. The problem s that the clent only knows the btrates of the receved segments, whch may belong to dfferent versons. So, after recevng each segment, wewllestmate the segment btrates of other versons wth the same ndex. Ths s enabled by the btrate estmaton method proposed n our prevous study [12], where the segment btrates of other versons are estmated from the btrate of the receved segment usng the nterstream btrate predcton. Specfcally, mantanng qualty stablty. Denote B rep,k (1) the estmated btrate B e,k of verson k canbecalculatedfrom the (actual) btrate B o,n of the receved segment wth the selected versonn as follows: B e,k =θ Bo,n 2(QP n QP k )/6, (2) where QP k and QP n are the quantzaton parameter (QP) values of the versons and θ = 1.5 s an emprcal factor used as the compensaton for the approxmaton error of the model [12]. In our notaton (see Notatons), btrate B,k can be ether the actual btrate B o,k or the estmated one Be,k.Oncethe btrates of all segments are obtaned, the representatve btrate of each verson at segment ndex s calculated as the average of the btrates of segment and N 1 prevous segments n that verson. The algorthm to calculate the representatve btrates s provded n Algorthm 1. In ths algorthm, the current representatve btrate s actually computed usng the prevous representatve btrate. Also, t should be noted that, when <N, N wll be set to. In Secton 4, we wll nvestgate how dfferent values of N affect the performance of our method Adaptaton Algorthm. Our algorthm wll address the twokeyquestonsabove,soastoavodrebufferngandto Input: N, B rep 1,k 1 k V, B j,k j<n,1 k V Output: B rep,k 1 k V for k 1,2,...,V do // Check f btrate s the orgnal btrate f k=i then B,k B o,k ; // Otherwse the btrate s the estmated btrate else end Estmate B e,k by (2); B,k B e,k ; end // Compute B rep B rep,k Brep,k 1,k +(B,k B N,k )/N; Algorthm 1: Representatve btrate computaton (after recevng the last segment and for all vdeo versons). reduce the number and the degree of (verson) swtches. Based on the current buffer level of the clent, we defne four possble cases n a streamng sesson, whch are uptrend, stable, downtrend, and panc cases. In these four cases, the clent wll be lkely to swtch up, mantan, swtch down, or aggressvely decrease the verson. For ths purpose, our method dvdes the buffer nto three ranges wth thresholds β mn and β th (β mn <β th <β max ). Here β max s the buffer sze andalsothetargetbufferleveloftheadaptatonmethod. When the current buffer level exceeds β max,theuptrend case s actvated (for the reason why the buffer level could be hgher than β max,pleaserefertoourprevouswork[18]). However, t s not good f the clent frequently goes back and forth between the uptrend case and downtrend case. To avod ths fluctuaton, our method swtches up to the next hgher verson (I +1) only f the representatve btrate B rep,i +1 of that verson s smaller than the throughput estmate of the next segment (.e., B rep,i +1 < Test +1 ); otherwse, the clent wll mantan the current verson. The stable case s determned by the condton β th β cur < β max. In ths case, the buffer level s judged as n a very safe condton, so no change n qualty s needed. The clent just mantans the current verson to avod unnecessary swtches. The downtrend case s actvated when β mn β cur < β th. In ths case, the clent needs to carefully decde the requested verson n order to avod buffer underflows as well as sudden qualty changes when the throughput and/or the vdeo btrate change drastcally. In general, the verson should be decreased; however, t s unnecessary to swtch down always when the buffer level s n ths range. In ths process, we defne the target btrate for the next segment +1 as the hghest representatve btrate, whch s lower than the throughput estmate: B tar +1 = max{brep,k Brep,k <Test +1 }.Ifthe nstant btrate and the representatve btrate do not exceed the target btrate (B,I B tar +1 and Brep,I B tar +1 ), the current verson wll be mantaned. Otherwse, the clent wll swtch down to the next lower verson.

5 Moble Informaton Systems 5 Input: T, β cur, I, B,I, B rep,i, T est +1 Output: I +1 // Uptrend case (1) f β cur >β max then f B rep,i <T est +1 then I +1 I +1; else I +1 I ; end // Stable case (2) else f β [β th,β max ) then I +1 I ; // Downtrend case (3) else f β [β mn,β th ) then B tar +1 max{brep f B,I B tar +1,k Brep,k and Brep,I I +1 I ; else I +1 I 1; end // Panc case (4) else Select I +1 based on (4); end <Test +1 }; B tar +1 then Algorthm 2: Adaptaton algorthm for VBR vdeo streamng. We can see that sometmes the nstant throughput mght be much smaller than the nstant btrate. Even though the buffer s not n danger yet, the downtrend case should be actvated earler to avod havng to quckly reduce the verson nthefuture.thssenabledbyncreasngthevalueofβ th. In our method, β th s controlled usng a logstc functon as follows: β th =β max 1 1+e σ (β max β mn ), where σ=1 T B,I. In ths way, the larger the msmatch between T and B,I becomes, the hgher the value of β th wll be, whle stll satsfyng the condton β mn β th <β max. The fnal case, called the panc case, s when the buffer level s n danger; that s, β cur <β mn.toensurethatthebuffer wll not be empty, the clent wll aggressvely swtch down the verson. Yet, nstead of jumpng drectly to the lowest verson, the clent employs the method of our prevous work [12] n ths case. Specfcally, the clent wll choose a verson of whch the nstant btrate s the hghest but stll lower than the nstant throughput: I +1 = arg max {B,k B,k <T }. (4) 1 k V The algorthm of our method s summarzed by pseudocode as n Algorthm 2. Generally, t can be seen that our method tres to provde a smooth vdeo stream by two technques. Frst, t (3) Web server IP networks Fgure 3: Test-bed organzaton for experments. DummyNet Clent s somewhat conservatve n ncreasng the qualty because that s allowed only when the buffer s full. In addton, the selected verson s constraned by the long-term values of throughput and btrate. Second, the downtrend case s also conservatve by avodng contnuously swtchng down the qualty. Meanwhle, n the panc case, we take nto account the nstant btrate and nstant throughput. Thus, the clent can swtch down gradually when possble; and there s no need to swtch to the lowest verson f the nstant btrate of a hgher verson stll meets the throughput constrant. 4. Expermental Results and Dscussons In ths secton, we wll evaluate our proposed method and two reference methods n the context of on-demand VBR streamng over moble networks, focusng on the behavors of verson swtchng and buffer level after the ntal bufferng stage. Two bandwdth traces, a smple one and a complex one, are employed n our experments Experment Setup. Our test-bed organzaton used for the experments s smlar to that of [18], whch conssts of an HTTP Web server, a streamng clent, and an IP network as n Fgure 3. The IP network ncludes a router and wred connectons connectng the server and the clent. The server s an Apache HTTP server of verson runnng on Ubuntu Our test-bed uses DummyNet tool [21] nstalled at the clent sde to emulate network characterstcs. The packet loss rate s set to %, assumng that the bandwdth trace used n the experments already takes nto account the fluctuatons caused by packet loss. RTT value of DummyNet s set to 4 ms. The clent s mplemented n Java and runs on a Wndows 7 notebook wth Core GHz CPU and 4 GB RAM. The test vdeos are Sony Demo and Termnator 2 [19] wth a frame rate of 3fps and a resoluton of The duraton of each vdeo s 6 seconds. We encode 6 VBR versons by the hgh profle of H.264/AVC [22], correspondng to 6 dfferent values of QP, namely, 22, 28, 34, 38, 42, and 48. Each verson s dvded nto small vdeo segments of 2 seconds. The verson ndex, QP, and the average btrate of each verson are lsted n Table 1. The btrate traces of the vdeo versons are shown n Fgure 2. As we focus on on-demand streamng, the buffer sze of the clent s set to 5 s (.e., 25 segment duratons), whch s smlar to prevous work (e.g., [9, 14]). For comparson, two reference methods whch are the nstant throughput-nstant btrate based method [12] (called ) and the buffer-based method wth trend predcton [9] (called ) are employed. The method s mplemented wth buffer thresholds (B mn,b low,b hgh,b max ) = (1 s,2s,4s,5s),whcharethe

6 6 Moble Informaton Systems Bandwdth (kbps) Tme (s) Versson Tme (s) Bandwdth (a) Segment btrate Bandwdth (kbps) AVG-1 AVG-3 AVG-5 AVG Tme (s) Bandwdth AVG-1 AVG-3 AVG-5 (c) Resultng buffer level AVG-3 AVG-5 (b) Requested verson Fgure 4: Adaptaton results of the three methods n smple bandwdth scenaro Buffer level (seconds) Table 1: Verson nformaton of the two test vdeos. Index QP Average btrate (kbps) Sony Demo Termnator same as the settngs n [9]. In our method, β mn and β max are 1 s and 5 s, respectvely. Also, we nvestgate whether the number of segments N used for representatve btrates affects our method s performance. The values of N consdered are 1, 3, and 5, and these optons of our method are referred to as AVG-1, AVG-3, and AVG-5, respectvely Smple Bandwdth Scenaro. Frst, we nvestgate the performance of the methods n a smple bandwdth scenaro, when the avalable bandwdth drops suddenly. The bandwdth has a rectangular shape wth two bandwdth levels, 25 kbps and 5 kbps, as shown n Fgure 4(a). Ths case s mportant n evaluatng adaptaton methods because we need to know how they perform when the bandwdth drops drastcally. In ths case, the Sony Demo vdeo s used. Fgure 4 shows the comparsons of requested versons, btrates, and buffer levels of the three methods. It s clear that the method tres to mantan the hgh qualty (verson 5) for too long even when the avalable bandwdth drops and stays at the low level for a long nterval, resultng n the worst buffer level curve (Fgure 4(c)) as well as a drastc drop of qualty (around t = 125 s, from verson 5 to verson 1 n Fgure 4(b)). As for the method, because t requests versons followng the varatons of nstant throughput and nstant vdeo btrate, the qualty s aggressvely changed over tme whle the buffer level varatons are very small. Asforourmethod,allthethreeoptonshavesmlar behavorsntermsofbtrate,versonswtch,andbufferlevel. Our method reduces the vdeo qualty gradually wth no swtches larger than 1 whle the buffer level s hgher than 25 seconds. Also, the mnmum verson provded by our method s 2, whle the other two methods sometmes jump to verson Complex Bandwdth Scenaro. In ths part, we evaluate the adaptaton methods wth a complex bandwdth trace (Fgure 5), whch was obtaned from a moble network [11].

7 Moble Informaton Systems 7 Bandwdth (kbps) Tme (s) Fgure 5: The bandwdth trace used n the complex bandwdth scenaro. Both test vdeos, Sony Demo and Termnator 2, are employed n ths scenaro. Fgure 6 shows the expermental results for the Sony Demo vdeo. As seen n Fgure 6(a), t s very dffcult to use the curves of segment btrates to compare the adaptaton methods. From Fgure 6(b), t s obvous that the method s behavor s smlar to that n the smple bandwdth case. Usually, ths method tres to mantan a hgh verson as long as the buffer level permts. Ths behavor results n sudden drops of qualty (e.g., from verson 6 to verson 3 at 1sandfromverson5toverson1at24s).Also,thebuffer level curve of ths method s the worst (Fgure 6(c)), as n the prevous scenaro. As for the method, ts btrate curve closely follows the throughput curve, resultng n a hghly fluctuatng versoncurvewthmanyswtches,ncludngswtchesoflarge degrees. Anyway, ths method has the most stable buffer level curve as seen n Fgure 6(c). Meanwhle, our method provdes both qualty stablty and buffer stablty. The verson curves of our method (Fgure 6(b)) have no sudden swtches. Moreover, when the throughput decreases, the selected verson of our method s always hgher than or equal to those of the other methods. The buffer level curve of our method s not as stable as that of the method; however, t s always hgher than that of the method. Compared to the method, our method s more conservatve n swtchng-up and less conservatve n swtchng-down. Somestatstcsoftheadaptatonresultsareprovdedn Table 2. The statstcs are related to btrate, requested verson, verson swtch, and buffer level. In terms of btrate, the method has the lowest average btrate. Its qualty s also very fluctuatng wth many swtches and large swtch degree/devaton. The method has the hghest average btrate snce ths method tends to select and mantan the best possble qualty. However, ts average verson s nterestngly lower than that of our AVG-1 opton (4.19 vs. 4.3). Even, the method has very low average btrate, but ts average verson s hgher than that of the method. In fact, the average verson values of all methods are very smlar (about 4.2 or 4.3). Among the three optons, the AVG-1 opton s the most aggressve as t always tres to request hgher versons; however, ths causes a lttle more swtches than other optons. These ponts wll be explaned and dscussed further n the next part. Table 2: Statstcs of the reference methods and the proposed method wth three dfferent optons for Sony Demo. Here, STD s the standard devaton. Statstcs AVG-1 AVG-3 AVG-5 Average btrate (kbps) Average verson Maxmum verson Mnmum verson Number of swtches Maxmum swtch degree STD of swtch degrees Mnmum buffer level (s) STD of buffer levels (s) Table 3: Statstcs of the reference methods and the proposed method wth three dfferent optons for Termnator 2. Here, STD s the standard devaton. Statstcs AVG-1 AVG-3 AVG-5 Average btrate (kbps) Average verson Maxmum verson Mnmum verson Number of swtches Maxmum swtch degree STD of swtch degrees Mnmum buffer level (s) STD of buffer levels (s) Moreover, our method provdes the smallest values n terms of the number of swtches and the degree of swtches. In addton, the mnmum verson of our method s hgher than the other methods. The mnmum value and standard devaton (STD) of buffer level of our method are also much better than those of the method. For more nformaton about the buffer, the cumulatve dstrbuton functons (CDFs) of the buffer levels are provded n Fgure 7(a). Agan, tsevdentthatthebufferofourmethodsmuchmorestable than that of the method. Fgure 8 shows the expermental results for Termnator 2 vdeo, where smlar behavors of the methods can be found. Our method agan provdes a smoother verson curve and a more stable buffer level curve than the method. The aggressveness of AVG-1 can also be seen n Fgure 8(b). For example, durng 33 s 37 s, the AVG-1 opton shortly ncreases the qualty to verson 6 and then reduces the qualty to verson 5 and verson 4. Meanwhle, the other two optons stll request verson 5 durng the same nterval. The statstcs of the adaptaton results for Termnator 2 vdeo s provded n Table 3. Wth ths vdeo, the average verson values of our method are all hgher than that of the method. The average btrate of the AVG-1 opton especally s hgher than that of the method. Ths s because verson btrates of Termnator 2 are smaller than those of Sony Demo, so the AVG-1 opton can easly reach the hghest possble qualty. In addton, the other parameters

8 8 Moble Informaton Systems 8 9 Btrate (kbps) Verson Tme (s) Tme (s) Throughput (a) Segment btrate 8 AVG-1 AVG-3 AVG-5 AVG-1 AVG-3 AVG-5 (b) Requested verson 8 Throughput (kbps) Tme (s) Buffer level (seconds) Throughput AVG-1 AVG-3 AVG-5 (c) Resultng buffer level Fgure 6: Adaptaton results of the three methods n the complex bandwdth scenaro wth Sony Demo vdeo. CDF Buffer level (s) AVG-1 (a) Sony Demo AVG-3 AVG-5 CDF Buffer level (s) AVG-1 (b) Termnator 2 AVG-3 AVG-5 Fgure 7: Cumulatve dstrbuton functons (CDFs) of buffer level n the complex bandwdth scenaro. also show that our method provdes a smoother qualty and a more stable buffer than the method. Besdes, the CDFs of buffer levels (Fgure 7(b)) confrm the stablty of buffer level of our method. Fgure 7(b) also showsthatthebufferlevelsoftheavg-3andavg-5 optons are a lttle better than that of the AVG-1 opton. ThssbecausetheAVG-1optonsmoreaggressvethanthe other optons. Especally compared to the case of Sony Demo vdeo (Fgure 7(a)), the buffer of the method s worse whle the buffer of our method s better. Ths s actually due to the characterstcs of the Termnator 2 vdeo as dscussed n the next part Dscussons. From the above expermental results, we can see some nterestng ponts. Frst, n some cases the average btrate of the method s hgher than that of our proposed method, whle ts average verson s lower than that of our method. Ths can be explaned by the fact that sometmes the nstant btrate s very hgh. So, havng some more segments of hgh btrates, especally those belongng to the top verson, wll sgnfcantly ncrease the average btrate. In case of Termnator 2 vdeo, whch has lower verson btrates than Sony Demo vdeo, the average btrate of method s not better than that of our method. Meanwhle, n low-throughput perods, the selected versons of the method are lower than those of our method, resultng n a lower value of average verson. So, as the btrate of VBR vdeo s hghly fluctuatng, the parameter of average btrate should not be the man ndcator to evaluate the performance of VBR vdeo streamng (as n [9]); rather, the parameter of average verson should be consdered. Ths s dfferent from CBR vdeo streamng, where the average btrate s always of hgh nterest.

9 Moble Informaton Systems Btrate (kbps) Verson Tme (s) Tme (s) Throughput (a) Segment btrate 1 AVG-1 AVG-3 AVG-5 AVG-1 AVG-3 AVG-5 (b) Requested verson 8 Throughput (kbps) Tme (s) Throughput AVG-1 AVG-3 AVG-5 (c) Resultng buffer level Buffer level (seconds) Fgure 8: Adaptaton results of the three methods n the complex bandwdth scenaro wth Termnator 2 vdeo. The results of our adaptaton method show that a normal buffer sze (.e., smlar to that n CBR vdeo streamng) s already enough to cope wth strong btrate varatons of VBR vdeos. Also, a representatve btrate for each verson s mportant for VBR vdeos. Even the method benefts a lot from learnng the nstant btrate. Though havng a hghly fluctuatng verson curve, ths method can be used for lve streamng thanks to ts very stable buffer. On the other hand, the method does not consder btrate values and thus poorly handles the strong varatons of VBR vdeo. For example, although Termnator 2 vdeo has lower verson btrates than Sony Demo vdeo, the buffer stablty of the method n the case of Termnator 2 vdeo s worse than n the case of Sony Demo vdeo. Ths s just because the verson btrates of Termnator 2 vdeo are extremely varyng (wth repeated and quck changes fromahghvaluetoalowvalue),whereastheperformance of our method s not degraded n the case of Termnator 2vdeo. The three optons of our method are smlar n terms of average verson values. Yet, among the three optons, the AVG-1 opton seems to be more aggressve n swtchng up and mantanng a hgh qualty. Ths s because the representatve btrate of ths opton s more local and can quckly detect low-btrate segment ntervals, where the clent can swtch to a hgher verson when the throughput permts. Such property can ncrease the average verson; however, that also causes more swtches as seen n the statstcs of the AVG- 1 opton. The above statstcs show that AVG-3 and AVG-5 optons have nearly the same performance wth good qualty stablty and good buffer stablty. In general, t s shown that our method s more effectve than the reference methods n provdng stable streamng qualty, wth smooth verson transtons even when the avalable bandwdth dramatcally drops. If a user prefers streamng wth the hghest possble qualty, more local representatve btrates should be used. On the other hand, f the user prefers a stable streamng sesson wth fewer verson swtches, more global representatve btrates should be used. It should be noted that, actually, the qualty of a CBR vdeo s varable and the qualty of a VBR vdeo s constant. Meanwhle, as shown n [12], t s totally feasble to provde a quas-cbr streamng servce from VBR vdeo data sets. The above results also show that, wth the smple method of representatve btrate estmaton, we do not need to know exactly the btrates of all VBR vdeo segments even though the nstant btrate s extremely varyng. Ths means that VBR vdeos and the clent-based adaptaton method proposed n ths paper can be easly deployed n DASH-complant

10 1 Moble Informaton Systems streamng systems wthout the need to modfy the current standard specfcaton. 5. Concluson In ths paper, we have presented an adaptaton method for VBRvdeosandevaluatedthemethodnthemoblestreamngcontext.Tocopewthstrongvaratonsofvdeobtrate,we employed a local average btrate as the representatve btrate of a verson. A buffer-based algorthm was then proposed to conservatvely adapt vdeo qualty by takng nto account a smoothed throughput estmate, the representatve btrate, and even the nstant btrate. The expermental results showed thatourmethodcanprovdesmoothvdeoqualtyandstable buffer level, even under very strong varatons of bandwdth andvdeobtrates.forfuturework,wewllfocusonlve streamng scenaros such as survellances wth VBR vdeo sources. Notatons T : Thethroughputofsegment T est +1 : Thethroughputestmateofsegment+1 β cur : Thecurrentbufferlevel β mn : The mnmum buffer threshold β th : The flexble buffer threshold β max : The buffer sze and also the target buffer level B,k : Thebtrateofthesegment n verson k. It could be the actual value B o,k or the estmated value B e,k B rep,k : The representatve btrate of verson k at segment V: The number of avalable vdeo versons I : The ndex of the verson whch s chosen for segment (verson of hgher qualty has a hgher ndex value). Competng Interests The authors declare that they have no competng nterests. References [1] Csco, Csco vsual networkng ndex: forecast and methodology, , Whte Paper, 215. [2] A.C.Begen,T.Akgul,andM.Baugher, Watchngvdeoover the web: part 1: streamng protocols, IEEE Internet Computng, vol. 15, no. 2, pp , 211. 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Lu, Towards agle and smooth vdeo adaptaton n dynamc HTTP streamng, n Proceedngs of the 8th ACM Internatonal Conference on Emergng Networkng Experments and Technologes (CoNEXT 12), pp , Nce, France, December 212. [18] T.C.Thang,H.T.Le,A.T.Pham,andY.M.Ro, Anevaluaton of btrate adaptaton methods for HTTP lve streamng, IEEE Journal on Selected Areas n Communcatons,vol.32,no.4,pp , 214. [19] H264/AVC vdeo trace lbrary, [2]T.V.Lakshman,A.Ortega,andA.R.Rebman, VBRvdeo: tradeoffs and potentals, Proceedngs of the IEEE,vol.86,no.5, pp , 1998.

11 Moble Informaton Systems 11 [21] L. Rzzo, Dummynet: a smple approach to the evaluaton of network protocols, ACM SIGCOMM Computer Communcaton Revew,vol.27,pp.31 41,1997. [22] T. Wegand, G. J. Sullvan, G. Bjøntegaard, and A. Luthra, Overvew of the H.264/AVC vdeo codng standard, IEEE Transactons on Crcuts and Systems for Vdeo Technology, vol. 13,no.7,pp ,23.

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