Reducing Channel Zapping Time in IPTV Based on User s Channel Selection Behaviors

Transcription

1 > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < Reducng Channel Tme n IPTV Based on User s Channel Selecton Behavors Chae Young Lee, Member, IEEE, Chang K Hong and Kang Yong Lee* Department of Industral Engneerng, KAIST, 373- Kusung Dong, Taeon, Korea *ETRI, 38 Gaeongno, Taeon, Korea Abstract Channel zappng tme s a crucal ssue n Internet Protocol Televson (IPTV) Qualty of Experence (QoE) performance. One way to reduce channel zappng tme s a predctve tunng method, whch reduces channel zappng tme by preonng channels that are lely to be selected next, n addton to the currently watched channel. Ths paper presents an mproved predctve tunng method that satsfes requred channel zappng tme wth mnmzed bandwdth usage. Unle exstng methods, the proposed method determnes the most effcent number of preonng channels n a surfng perod and n a vewng perod dfferently by estmatng expected channel zappng tme and expected bandwdth usage wth a Sem-Marov Process. We also propose a preonng channel selecton method accordng to the combned button and channel preference. Mathematcal model and smulaton show that the proposed method acheves the requred channel zappng tme wth less bandwdth consumpton and a more stable networ than prevous wors. Index Terms Channel selecton behavor, channel zappng tme, predctve tunng, IPTV. I. INTRODUCTION he rapd dffuson of hgh speed nternet and the fast Tadvance of broadband networng technology have been breang down the walls between telecommuncaton and broadcastng. IPTV (Internet Protocol Televson) s one of the ey applcatons n the telecommuncaton maret whch gves an opportunty for telephone companes to beneft from vdeo delvery over IP networs. Currently, there are actve trals and commercal deployments across the world, ncludng n North Amerca, Europe, and Asa/Pacfc []. However, the nature of the IPTV mechansm lmts ts qualty of servce (QoS), especally n the aspect of channel zappng tme. Channel zappng tme s defned as the tme dfference between the user asng for a channel change by pressng some buttons on the remote control and the dsplay of the frst frame of the requested channel on the TV screen [2]. Compared wth the conventonal analog TV that broadcasts all Manuscrpt receved June 9, 29. Ths wor was supported n part by the ETRI under Grant BS F. A. Author s wth the Natonal Insttute of Standards and Technology, Boulder, CO 835 USA (correspondng author to provde phone: ; fax: ; e-mal: author@ boulder.nst.gov). S. B. Author, Jr., was wth Rce Unversty, Houston, TX 775 USA. He s now wth the Department of Physcs, Colorado State Unversty, Fort Collns, CO 8523 USA (e-mal: author@lamar.colostate.edu). channels, t multcasts only the currently watched channel stream to each Set Top Box (STB) at any moment through the IP networ. Accordng to the QoE requrements of DSL Forum, the zappng tme should be lmted to a maxmum of 2 seconds [3] and ITU-T FG IPTV s also consderng t as one of the QoE metrcs. Channel zappng tme usually taes up to second for MPEG-2 and as long as 2 seconds for H.264/MPEG-4 AVC, forcng the vewer to wat for the pcture [4]. However, a model where Mean Opnon Score (MOS) depends on the channel zappng tme on a logarthmc scale shows that the channel zappng tme should be less than.43 seconds for an MOS score of 3.5 [5]. Channel zappng tme conssts of a) Internet Group Management Protocol (IGMP) nternal processng delay, b) decodng delay, and c) bufferng delay. If the user swtches the channel, the STB has to perform the process of sendng IGMP multcastng tree, leavng and onng messages to obtan a new channel. After watng for the obect vdeo stream to come, STB wats more for a decodable frame, whch s called an Intra-coded frame (I-frame). Then STB buffers some frames to avod the unsmooth dsplay caused by the delay tter over the Internet. Many research efforts have been undertaen to reduce channel zappng tme. Methods addng tune-n servers at the end of the core networ have been proposed [6], [7]. However, hgh-cost dedcated servers whch provde hgh rate uncast streamng must be added nto core networs, and STBs should also shft precsely between those temporary uncast streams and fnal multcastng streams to acheve ths purpose [4][8]. Some methods that mprove the vdeo encodng or RTP protocols have been proposed [], [9]-[5]. These may stll be unsatsfactory due to the mandatory decodng delay, whch s lmted by the vdeo codng standard, and bufferng delay [4]. One of the approaches to reduce the channel zappng tme s for the STB to on the adacent channels of the current channel n advance [6]. If the user swtches to an adacent channel, the user can watch the selected adacent channel wthout channel zappng tme because the stream of the adacent channel has already been sent to the STB. Ths method can be expanded to on more channels hghly lely to be selected next [][4][7]-[2]. Ths nd of method s called preonng method or predctve tunng. However, predctve tunng consumes addtonal bandwdth

2 > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 2 Fg.. IPTV Servce System for preonng channels, whch can cause congeston n the access networ. To reduce bandwdth consumpton, a method of preonng channels only whle a user s surfng channels s proposed [4] and [9]. Whle watchng a channel (called vewng perod ), the STB only receves the currently watched channel. When the user starts to swtch the channel, the STB ons more channels n addton to the currently watched channel. Ths perod s called the surfng perod. When the user decdes to watch the channel, the STB leaves the extra channels. Ths can reduce average bandwdth usage. However, the frst swtch of each surfng always has to suffer from a very large channel zappng tme, whch maes the requred channel zappng tme unattanable. Another aspect of predctve tunng s the accuracy of predcton for the next channel. Many researches consder channel popularty, personal channel preference, and behavors n operatng the remote control [][4][7]-[2]. However, the user s button preference, whch refers to whch buttons on the remote controller a user uses most frequently, hasn t been consdered. In predctve tunng, the button preference s more relevant than the channel preference. In ths paper, we present an mproved predctve tunng method that satsfes average channel zappng tme wth mnmzed bandwdth consumpton. The proposed method has two dfferent ams: ) determnng the most effcent number of preonng channels based on the user s channel selecton behavors and 2) selectng preonng channels wth a consderaton of the button preference. To satsfy requred channel zappng tme wth less bandwdth consumpton, the proposed method preons a small number of channels durng a vewng perod and preons more channels durng a surfng perod. A Sem-Marov Process (SMP) s used to analyze the user s channel selecton behavor. The optmal number of preonng channels s obtaned from expected channel zappng tme and bandwdth usage calculated from the proposed model. The proposed method also taes advantage of H.264 Scalable Vdeo Codng (SVC,) whch was proposed by Lee, et al. [9]. The rest of ths paper s organzed as follows. We ntroduce the detals of IPTV servce system, predctve tunng, and H.264/SVC n Secton II. Then, we develop a mathematcal model to descrbe the vewer channel selecton behavors n secton III. In Secton IV, we descrbe the mproved predctve tunng method, and Secton V combnes and derves a detaled analyss of the channel zappng tme and the bandwdth usage. Numercal results, ncludng the smulaton to verfy t, are n Secton VI. We also present a smple algorthm to apply the proposed method to the practcal IPTV servce system n secton VII. Fnally, concludng remars are gven n Secton VIII. II. BACKGROUND A. IPTV Servce System A typcal IPTV system conssts of four elements, as shown n Fgure. The vdeo headend s responsble for encodng content receved through satellte, terrestral, or fber networs nto MPEG-2 or MPEG-4 formats. The content encoded wth MPEG-2 taes a bt rate of 3~8 Mbps for standard defnton (SD) qualty, and 5~8 Mbps for hgh defnton (HD) qualty, whle the content encoded wth H.264/MPEG-4 AVC taes a bt rate of.5~8 Mbps for SD qualty, and 8~2 Mbps for HD qualty [3]. The content, whch s encapsulated nto IP pacets, s sent to the core networ usng IP multcast or IP uncast. The core networ groups the encoded vdeo streams nto ther respectve channels. The access networ contans the aggregaton node and the access node. The aggregaton node, whch s generally called broadband access server (BRAS), s responsble for mantanng user polcy management, such as authentcaton and subscrpton detals. At the end of the access networ, the access node, such as a dgtal subscrber lne access multplexer (DSLAM,) provdes varous nternet access technologes to end users. It can be the 2 Mbps promse of asymmetrc dgtal subscrber lne 2+ (ADSL2+), the 5 Mbps capablty of very hgh dgtal subscrber lne (VDSL2), or the Mbps potental of fber to the x (FTTx) [22]. The home networ dstrbutes the data, voce, and IPTV traffc n subscrbng homes wth the home gateway. Each home can have two or more TV sets and other equpment usng an IP networ, such as VoIP phones or laptop computers. If each set shows one channel, then the home networ should support at least two HDTV channels, whch taes a bt rate of 36 Mbps n the case of MPEG-2 smultaneously. Therefore, the IPTV servce system should be desgned to manage the bandwdth of the channels effectvely because each channel necesstates hgh bandwdth [][8][23]. B. Predctve Tunng Cho, et al. [6] presented a method of reducng channel zappng tme by sendng the adacent channels of the current

3 > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 3 one n advance. Predctve tunng has two benefcal characterstcs: Frst, t s a pure clent technology and can be mplemented wthout changes to the IPTV system and n combnaton wth other solutons to fast channel change. Second, overall channel zappng tme can be reduced to nearly zero by decodng data streams of preonng channels or bufferng ey frames, whch are called I-frames, of preonng channels. The second advantage of the predctve tunng s remarable, because when a user swtches to a preonng channel, the user can watch the scene mmedately - at least the stll cut of the channel. Varous predctve tunng methods are also proposed, focusng on how to select preonng channels. Instead of adacent channels, a predctve tunng based on channel popularty, whch s aggregated from the rate server, has been proposed [7]. Other researchers [8] have proposed a method that selects the canddate channels by consderng the user s remote control behavor and personal channel preference. They consder that vewers tend to eep pushng the same button to change channels, and the STB can match a channel number, called an expected channel, wth any pushed button. There s also a method of preonng channels that have been recently watched and are numercally adacent to the current channel [9]. One of the wea ponts n predctve tunng s that t consumes addtonal bandwdth. Bascally, predctve tunng consumes sgnfcant bandwdth because each channel necesstates hgh bandwdth. If an STB preons two channels n parallel to the current channel, t taes a bt rate of 36 Mbps for HD qualty n the case of H.264/MPEG-4 AVC. If there are two or more STBs n a home, t wll cause lac of bandwdth of the access networ. To reduce bandwdth consumpton, the authors proposed the method of preonng channels only whle a user s surfng channels [4], [9]. Whle watchng a channel, called a vewng perod, the STB receves the currently watched channel only. When the user starts to swtch the channel, the STB ons more channels n addton to the currently watched channel. Ths perod s called the surfng perod. When the user decdes to watch a channel, the STB leaves the extra channels. Ths can reduce average bandwdth usage, but the frst swtch of each surfng perod always has to suffer from a very large channel zappng tme. Ths affects the average channel zappng tme sgnfcantly. C. H.264 Scalable Vdeo Codng Another approach to reduce the bandwdth usage of predctve tunng s usng H.264/Scalable Vdeo Codng (SVC). H.264/SVC s an extenson of H.264/AVC (Advanced Vdeo Codng) whch acheves a hgh degree of spato-temporal and qualty scalablty. An SVC bt-stream conssts of a base layer and one or more enhancement layers. The base layer s an H.264/AVC bt stream, ensurng bacward compatblty for legacy decoders and provdng low qualty of vdeo wth low resoluton, low frame rate (temporal resoluton), or low pcture fdelty (PSNR). The enhancement layers nclude the nformaton of the frames wth hgher resoluton, frame rate, or Fg. 2. CDFs of the number of swtches pror to vewng and Posson Dstrbuton PSNR, but t cannot be decoded wthout a base layer. Low-end devces, whch have low resoluton, low networ capacty or low computng power, can tae only the base layer and hgh-end devces can tae both the base layer and enhancement layers. Whle the hgh qualty of vdeo s served wth the base layer and the enhancement layers of the currently watched channel, the STB can preon the base layers of the canddate channels, because base layers consume less bandwdth. Lee et al. show that acceptable vdeo qualty for surfng s acheved wth the base layer, whch consumes tmes less bandwdth than an overall SVC stream [9]. Therefore, the STB can preon channels wth the bandwdth usage of a hgh qualty channel. III. BEHAVIOR MODEL There are two types of vewer behavors n channel selecton: (a) how many channels users swtch and how long they spend swtchng before they enter a vewng perod, and (b) whch channels they choose whle swtchng. The frst one can be called channel swtchng behavor, and second one channel surfng behavor. A. Channel Swtchng Behavor Fortunately, there s a measurement for ths behavor based on real IPTV servce. In [24], the traces from a large-scale commercal IPTV are used to study how users select channels n the real-world. The study shows that users generally swtch 4 channels on average before they decde on a channel, and % of users watch the frst channel they swtch to, whle % of users swtch more than 6 tmes pror to vewng. Fgure 2 shows that the cumulatve dstrbuton functon (CDF) of the number of changes pror to vewng a channel, whch approxmates to CDF of Posson dstrbuton. Therefore, the number of swtches pror to vewng can be assumed to follow Posson dstrbuton. A Sem-Marov Process (SMP) was used to analyze ths behavor. It wors as follows: Each state ndcates the number of swtches, the transton to the next state ndcates swtchng and the transton to the state ndcates enterng vewng

4 > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 4 Fg. 3. State transton dagram of channel swtchng perod. Whle a user s watchng a channel, the user s n the state and when the user swtches the channel, the user transts to state. If the user decdes to watch the channel, the user goes bac to the state. Instead, f the user swtches agan, the user transts to state 2. Ths model s llustrated n Fg 3. Based on the assumpton, the transton Probablty ( P ), can be calculated from the Posson dstrbuton. If a user swtches once pror to vewng, t means that the user transts from state to state, and then the user transts from state to state. Therefore, the probablty of swtchng once pror to vewng becomes e λ λ Pr{swtchng once pror to vewng} = = P P! We can expend ths equaton to Pr{swtchng tmes pror to vewng} e λ λ = P, P, =,( 2)! =,, The sum of the outgong probabltes from each state should be.. Hence P, =, P, = P, + Whch gves e λ λ,,! = P = / P,( 2) () We assume that the number of the state s lmted, whch s reasonable n the aspect of human behavor. In [24], the number of swtchng s lmted to. B. Channel Surfng Behavor Users generally use a remote control that has varous buttons: up/down, toggle, preset, and numerc buttons. There are two types of preferences: (a) button preference and (b) channel preference. Button preference refers to whch buttons on the remote control a user uses more frequently and channel preference refers to how often a user watches a channel. Sometmes a user swtches to the obectve channel, whch follows the user s channel preference; sometmes the user ust surfs channels to fnd an nterestng channel, whch follows the user s button preference. However, even when the user has the obectve channel, the user should select a button to push wth the button preference. Therefore, t can be assumed that the user selects the next channel as follows: frst, users decde whch button to push. If they decde to push the up, down, or toggle button, the next channel wll be determned by the currently or prevously watched channel. If they decde to push numerc buttons, they should choose the next channel by themselves. IV. PROPOSED METHOD A. Determnng the Number of Preonng Channels Preonng durng a vewng perod has two aspects. Frst, t affects the average channel zappng tme more than preonng durng a surfng perod because the frst swtchng of every surfng s nfluenced by t. Second, t consumes bandwdth sgnfcantly because the duraton of a vewng perod s much longer than that of a surfng perod. Therefore, we need to fnd the optmal number of preonng channels n the vewng perod and the surfng perod. From the proposed Sem-Marov Model (SMP), the expected channel zappng tme and expected bandwdth usage can be calculated. The number of preonng channels n a vewng perod and the number of a surfng perod are assgned as varables. By adustng those varables, the varaton of the expected bandwdth usage and expected channel zappng tme can be observed. The detaled analyss for the expected channel zappng tme and expected bandwdth usage s descrbed n Secton 5, and a smple algorthm to apply the result n the practcal STB s descrbed n Secton 7. B. Selectng Preonng Channels A remote control has varous buttons: up/down, toggle, preset, and numerc buttons. Therefore, a user can use varous ways to swtch to the obectve channel. For example, f a user s watchng channel 9, and the user decdes to swtch to channel 2, whch s the most preferred channel of the user, the user can push the up button three tmes or use numerc buttons by pushng 2 drectly as shown n Fgure 4. In the predctve tunng, the most mportant channel s not the obectve channel but the next channel, because STB reselects preonng channels whenever a user swtches the channel. That mples that button preference affects more than channel preference. In the example shown n fgure 4-(b), f the STB preons channel 2, t s of no use because he wll swtch to channel frst by pushng the up button.

5 > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 5 Fg. 4. Two cases of channel surfng from channel 9 to channel 2 Therefore, the channel preference and the button preference can be combned to the probablty of a channel that a user wll swtch to the channel next at a partcular moment. Preonng channels are selected n descendng order of the probablty. If there are N channels and K types of buttons, Let η denote button preference of button (, K), ρ denote channel preference of channel (, N) and η ndcate the button preference of numerc buttons. Then, we can defne the combned probablty ω of each channel at a partcular moment as follows K = 2 ω = η ρ + η β, N, (2), f channel corresponds to button β, otherwse β s a correspondng functon between buttons and channels. Except numerc buttons, correspondng channel to a button depends on the current channel, the prevous channel, or user confguraton. Thus, the STB can obtan the correct value of β n each swtch. However, ω can t be used to calculate the expected channel zappng tme n the proposed SMP model, because ω s not for a steady state, but a partcular moment. Instead of ω, We use an approxmate value n the steady state for the proposed SMP model. We assume that there are K- more channels, whch corresponds to buttons except numerc buttons. The approxmate probablty Π of channel s obtaned as follows ηρ, N Π = (3) η N +, N+ N + K tme Marov chan (DTMC) and transton probablty. In the nature of predctve tunng, f a user swtches to the channel whch s not preoned, he has to tae on compulsory channel zappng tme to fnsh the process of exchangng IGMP messages, stream watng and decodng. We call ths Full Delay and denote t wth F. If the user swtches to the preoned channel, there would be no channel zappng tme. If we defne C as the set of preonng channels n state, the expected delay n state can be presented as follows ED [ ] = Π + ( Π ) F (4) C C To calculate expected channel zappng tme, we should consder when the channel zappng tme occurs. Swtch means the transton from to +, whch maes channel zappng tme. Thus, the proporton of E[ D ] n the expected channel zappng tme can be obtaned from the steady-state probablty of the transton from to +, whch can be presented as follows: π P, + If t s assumed that the number of state s lmted to m +, The steady-state probablty of embedded DTMC can be obtaned from those equatons whch gve π = πp, + π2p2, πmpm, π = π P, π = π P,..., π = π P, 2,2 m m m, m m π = = π =, π = π P, ( ) (5) m, P, + P, + = = The probablty that the transton from to + occurs n the steady-state s πp, + = π. Therefore, the expected channel + zappng tme s obtaned as follows m π + = ED [ ] = ED [ ] (6) V. ANALYSIS A. Expected Channel Tme The expected channel zappng tme s only related to the number of transtons between the states, not to the tme spent n the states. Therefore, expected channel zappng tme can be obtaned from the steady-state probablty of embedded dscrete B. Expected Bandwdth Usage Usng SVC n IPTV, each channel s splt nto a base layer and enhancement layers, whch are allocated to dfferent multcast groups [9]. In a vewng perod, the STB receves both layers of watchng channel to produce full-qualty vdeo and the base layers of preonng channels. But n a surfng perod, the STB receves only the base layers of watchng

6 > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 6 channel and preonng channels as shown n Fgure. If we defne n as the number of preonng channels n the state, the bandwdth usage n each state can be obtaned as follows Fg. 5. Bandwdth usage n the proposed method ( n + ) base + enh, = ( n + ) base, If we defne μ as the amount of the tme spent n the state, the steady-state probablty of SMP s obtaned from the steady-state probablty of embedded DTMC and the tme spent n the states. P = π μ m = π μ (7) The expected bandwdth usage can be calculated through t m. = E[ ] = P VI. NUMERICAL RESULT In ths secton, the analyss on the channel zappng tme and bandwdth usage of varous predctve tunng methods s presented. In Secton VI.A, the performance evaluaton about the polcy for the number of preonng channel s provded. It shows that our analytcal results closely match the smulaton results. In Secton VI.B, the performance of the proposed preonng channel selecton method s compared wth prevous methods n terms of the average channel zappng tme. To mae more precse comparson, we compare the polces of the number of preonng channels n the same preonng channel selecton method, and the preonng channel selecton methods n the same polcy for the number of preonng channels. The default values of the parameters for analyss and smulaton are: Full Delay (F) s 2 seconds [4], total number of channels (N) s 5, bandwdth of base layer ( base ) s Mbps, bandwdth of enhanced layers ( enh ) are 8 Mbps, whch s requred for the HD qualty of vdeo [3]. As descrbed above, channel preference s assumed to follow a Zpf-le dstrbuton wth s =, and the number of swtchng pror to watchng s assumed to be Posson-dstrbuted wth λ = 3.7. The amount of the tme spent n a vewng perod s assumed to be 2 mnutes, whle the amount of the tme spent n a surfng perod ( μ, ) s assumed to be 9 seconds.[24] A remote control has four types of buttons: numerc, up, down, toggle, and the button preference follows the user type, whch s descrbed n the Secton 6.B. To compare the methods n the same condtons, all methods tae advantage of H.264/SVC, whch means a preonng channel uses only the base layer of the channel. The comparson of the pea bandwdth usage of each method s also performed. The pea bandwdth usage also affects the congeston of the networ sgnfcantly because the swtches caused by commercal brea may account for up to 95% of a vewer s total swtches [4], meanng that channel swtches can be crowded n a certan moment and usng too much bandwdth n the surfng perod can be crtcal to the congeston of the networ. A. The Polcy for the Number of Preonng Channels Three polces for the number of preonng channels are evaluated, comparng channel zappng tme and bandwdth usage as shown n Table I. As mentoned before, every polcy selects preonng channels based only on the channel preference for the comparson under the same condtons. Fgure 6 shows the expected channel zappng tme of each polcy wth the dfferent number of preonng channels. The expected channel zappng tme should be below.43 second to satsfy QoE [5]. The always method provdes the soluton wth 2 preonng channels and proposed (v=5) method wth 6 preonng channels n the surfng perod and 5 preonng channels n the vewng perod. However, the surfng only method can t provde the soluton. It also shows that ncreasng the number of preonng channels becomes neffectve gradually. TABLE I POLICIES FOR THE NUMBER OF PREJOINING CHANNELS Polcy Descrpton Always Preons a varous number of channels, always [4,5,6,8] Surfng only Preons a varous number of channels durng surfng perod only [7,] Proposed (v=5) Preons a varous number of channels n surfng perod and 5 channels n vewng perod

7 > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 7 Fg. 6. Expected channel zappng tme per number of preonng channels Fg. 8. Pea bandwdth usage and expected channel zappng tme per number of preonng channels Fg. 7. Expected bandwdth usage per number of preonng channels Fgure 7 showss the expected bandwdth usage of each polcy wth the dfferent number of preonng channels. The always method wth 2 preonng channels consumes 2.6 Mbps, whle the proposed (v=5) method wth 6 preonng channels n the surfng perod and 5 n the vewng perod consumes 4.2 Mbps, whch s 3.% bandwdth mprovement. It also shows that preonng more channels n the surfng perod scarcely affects the average bandwdth usage. Fgure 8 presents the pea bandwdth usage and the expected channel zappng tme of each polcy. The always method wth 2 preonng channels consumes 2. Mbps, whle the proposed (v=5) method requres 7. Mbps, whch s 9.% mprovement. We develop a Java-based smulaton program to verfy our mathematcal analyss. We smulate a set-top box whch taes, channel swtches n each case of the channel surfng and swtchng model descrbed n Secton 3. By tang every channel zappng tme of each swtch and bandwdth usage n each state under the varous predctve tunng methods average channel zappng tme and bandwdth usage of each swtch s obtaned. Fgure 9 shows that our smulaton results closely match the analytcal results n the aspect of channel zappng tme. Fg. 9. Comparson of smulaton and analyss To mae the surfng only method provde a soluton, we relaxed the requred average channel zappng tme to 7 seconds and compared the performance of each polcy as shown n Table II. It shows that the surfng only method provdes a soluton usng mnmum average bandwdth (.2 Mbps). But ts pea bandwdth s too hghh (28. Mbps) ), because t preons tooo many channels to meet the channel zappng tme requrement. On the other hand, the proposed (v=2) method provded a soluton usng sutable average bandwdth (.5 Mbps) and pea bandwdth usage (2. Mbps). In the case of the always method, both average bandwdth usage (4.57 Mbps) and pea bandwdth usage (28. Mbps) are hgher than the proposed method.

8 > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 8 TABLE II COMPARISON OF POLICIES FOR THE NUMBER OF PREJOINING CHANNELS Preonng 6 channels 'always' Preonng 27 channels durng 'surfng only' Preonng 2 channels n vewng perod and channels n surfng perod Average Channel Tme (seconds) Average Bandwdth Usage (Mbps) Pea Bandwdth Usage (Mbps) TABLE III PREJOINING CHANNEL SELECTION METHODS Method Descrpton PREF Preons most preferred channels [5] ADJ-PREF Preons 2 adacent channels and most preferred channels [4,7] EXP-PREF Preons expected channel and most preferred channels [6] PROPOSED Preons channels based-on combned button-channel preference TABLE IV USER TYPES FOR BUTTON PREFERENCE Method Descrpton Numerc only Uses only numerc buttons wth channel preference Numerc preferred Uses numerc buttons 6% of tmes wth channel preference Up/Down preferred Uses up/down buttons 6% of tmes Surfng Model [6] Has the tendency to eep pushng the same button B. Preonng Channel Selecton Four preonng channel selecton methods, ncludng our method, are evaluated comparng channel zappng as shown n Table III. As mentoned before, every method preons the same number of channels for the comparson under the same condtons. Every method preons 2 channels n the vewng perod and channels n the surfng perod. The performance of the preonng channel selecton depends on the user s preference n channels and buttons. As mentoned before, channel preference s usually assumed to follow Zpf-le dstrbuton, but there s no gude for button preference. Therefore, we made four types of users whch have unque characterstcs to represent varous users, as shown n Table IV. Channel tme (sec.) TABLE V COMPARISON OF USER TYPES FOR BUTTON PREFERENCE Numerc only Numerc preferred Up/Down preferred Surfng Model [6] Table V shows the average channel zappng tme of each method for the dfferent types of users. In the case of Numerc only, PREF, EXP-PREF and PROPOSED methods provde mnmum channel zappng tme. If a user only pushes numerc buttons, those three methods perform n the same manner because EXP-PREF and PROPOSED methods consder the button that the user pushed recently or frequently. In the cases of Numerc preferred and Up/Down preferred, preonng adacent channels s more effectve. The reason s f the user decdes to push numerc buttons, t s hard to predct the next channel. But f he decdes to push the up or down button, the STB can predct the next channel. That maes the ADJ-PREF method provde a better soluton than other prevous methods because the ADJ-PREF method gves more weght to adacent channels than preferred channels. The proposed method also consders these characterstc. In addton, t uses combned button-channel preference, whch provdes more accurate weght between adacent channels and the preferred channel. As a result, the proposed method provdes mnmum channel zappng tme. In the case of the Surfng Model, ntroduced n [8], the EXP-PREF method gves mnmum channel zappng tme among the prevous methods because t consders the buttons that the user pushed recently. But the proposed method provdes better performance because we can preon many channels by usng H.264/SVC and we can consder more preferred buttons nstead of one recently used button. As a result, the proposed method provdes mnmum channel zappng tme. VII. ALGORITHM FOR THE PRACTICAL SYSTEM Avg. PREF ADJ-PREF EXP-PREF PROPOSED Numercal results show the outstandng performance of the proposed predctve tunng method. However, t seems hard to apply the proposed method to the practcal system because t s analytcal rather than practcal. We wll show the smple algorthm to apply our method n the practcal IPTV system, focused on a set-top box. The proposed algorthm has several characterstcs compared wth the analytcal process. Frst, t uses collected data of the user s real behavor nstead of data from models or statstcs, whch provde more effectve solutons. The STB can aggregate the nformaton on the channel swtchng easly: how many channels the user swtches pror to vewng, how many tmes a channel s selected and how many tmes a button s pushed.

9 > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 9 From ths nformaton, the STB can estmate more accurate channel and button preference, channel zappng tme and bandwdth usage. Second, the proposed algorthm uses a fnte duraton nstead of the steady-state, whch s assumed n the analyss, whch means that the STB re-estmates all values perodcally. The proposed algorthm assumes that the number of preonng channels s lmted because of the lac of the capacty of the access networ, and that the channel zappng tme of non-preoned channels s constant. The process of the proposed algorthm s as follows: ) In a certan perod, the STB aggregates the nformaton. n (): The number of vstng states n our model, whch can be obtaned by countng the number of swtches pror to vewng. c( ): The number of tmes selectng channel to watch. b ( ): The number of tmes pushng button. 2) At the end of the perod, the STB calculates those values, whch are needed to estmate the channel zappng tme. A. Transton probablty ( P, + ) and steady-state probablty ( π ) P n ( + ) n ( ) =, π = n () n( ), + m = B. Button preference ( η ) and channel preference ( ρ ) b ( ) c( ) η =, ρ K = N b () c () = = 3) From those values and the analyss descrbed n Secton 5, we can estmate the channel zappng tme of the next perod. So we can determne the number of preonng channels n vewng perod and surfng perod to meet the requred channel zappng tme as follows: Step : Calculate the average channel zappng tme n the prevous perod. Step 2: Adust the obectve of the channel zappng tme of next perod ( Ob ). If the average channel zappng next tme of prevous perod ( D ) s longer than the prevous overall obectve of the channel zappng tme ( Ob ), overall decrease the obectve of the channel zappng tme of next perod. Otherwse, ncrease t. It can be descrbe as follows: Ob = Ob + ( Ob D ) next overall overall prevous Channel tme (sec.) PREF /always[5] ADJ-PREF /always[4,6] EXP-PREF /always[6] PROPOSED /always PROPOSED /adaptve TABLE VI SIMULATION RESULT OF PROPOSED ALGORITHM (sec.) (Mbps) (sec.) (Mbps) (sec.) (Mbps) (sec.) (Mbps) (sec.) (Mbps) Numerc only Up/Down preferred Surfng Model [6] Avg Step 3: Determne the number of preonng channels n surfng perod and vewng perod. Frst, Increase the number of preonng channels n the surfng perod from to the maxmum lmt, examnng whether the expected zappng tme, calculated from the data descrbed n 2)-A, meets the obectve of channel zappng tme n next perod ( Ob ). next Step 4: If ths fals, ncrease the number of preonng channels n vewng perod and go bac to Step 3. Step 5: Whenever a user swtches the channel, calculate combned probablty ( ω ) of each channel from the data descrbed n 2)-B, and preon the number of channels, determned n the prevous steps, n descendng order of ω. To evaluate the proposed and prevous methods [6, 7, 8], the smulaton explaned n Secton 6 s performed for channel zappng tme and average bandwdth usage. The number of preonng channels s lmted to 2 wth 2 Mbps capacty., channel swtches are expermented. The obectve of the channel zappng tme s.43 sec [5]. In each preonng channel selecton method always polcy s adopted based on Table I for the number of preonng channels. Each method chooses the number of preonng channels to meet the channel zappng tme requrement of.45 sec. We also added the proposed algorthm, called PROPOSED/adaptve, whch chooses the mnmum number of preonng channels for the channel zappng tme requrement. Table VI shows that the proposed algorthm requres the lowest bandwdth n most user types. On the average, the proposed algorthm requres 3% less bandwdth than other methods.

10 > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < VIII. CONCLUSION In ths paper, an mproved predctve tunng method wth two aspects s descrbed. It features a) determnng the most effcent number of preonng channels n a surfng perod and a vewng perod and b) selectng preonng channels wth combned button-channel preference. Also t taes advantage of H.264/SVC, whch allevates the bandwdth consumpton. The user s channel selecton behavors s analyzed wth Sem-Marov Processes, wth whch we can estmate average channel zappng tme and average bandwdth usage and obtan the optmal number of preonng channels n a surfng perod and n a vewng perod. To verfy ts accuracy, a smulaton s also conducted. Studes show that the proposed method acheved requred channel zappng tme wth less bandwdth consumpton and a more stable networ than prevous wor. Ths also shows that the proposed preonng channel selecton method wth combned button-channel preference provdes shorter channel zappng tme than any prevous methods n the envronment of the varous types of users. A smple algorthm to apply the proposed method to the practcal IPTV system s also presented. It s demonstrated that the proposed algorthm satsfes the QoE requrement of the channel zappng tme. The research on the channel zappng tme n ths paper s more orented to the set-top box. For more comprehensve study a comparatve analyss on the aggregaton and access nodes n the networ s necessary. REFERENCES [] H. Joo, H. Song, D.B. Lee, I. 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