First segment partition for video-on-demand broadcasting protocols

Computer Communications 26 (2003) 1698–1708 www.elsevier.com/locate/comcom

First segment partition for video-on-demand broadcasting protocols W.-F. Poona, K.-T. Loa,*, J. Fengb a

Centre for Multimedia Signal Processing, Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, People’s Republic of China b Department of Computer Engineering and Information Technology, City University of Hong Kong, Tat Chee Avenue, Hong Kong, People’s Republic of China Received 20 March 2002; revised 20 September 2002; accepted 31 October 2002

Abstract Broadcasting is an efficient transmission scheme to provide on-demand service for very popular movies in a multicast environment. In this paper, a comparative study is first performed to evaluate various broadcasting protocols in terms of maximum waiting time, client buffer requirement and receiver bandwidth. It is found that some existing broadcasting schemes are not practical to provide a small delay video-ondemand (VoD) service if the client buffer size is not large enough. It is shown that the staggered [1] and skyscraper [3] protocols are the only feasible solutions to support an insensitive delay VoD system with limited client buffer and receiver bandwidth. Two first segment partition schemes are then proposed to further reduce the maximum waiting time of these two broadcasting schemes. The results show that if 15 min of video data can be stored in the buffer, Skyscraper with our proposed schemes can serve the customers within 10 s using 16 video channels. For Staggered broadcasting with our schemes, the maximum waiting time can also be reduced from 7.5 min to less than 1 min. q 2002 Elsevier Science B.V. All rights reserved. Keywords: VoD service; Client buffer; Protocols

1. Introduction With the advances in digital video technology, the videoon-demand (VoD) service has come into practice in recent years. The VoD service allows geographically distributed users to interactively access video files from a number of remote video servers. Subscribers can request the videos at any time and can have the video playback within a very short time. In the last few years, many researchers [23 – 25] have paid enormous efforts on the design of a VoD system. The main reason for the lack of success of the current VoD system is its high operating cost as each customer has to open a high bandwidth dedicated channel to receive the high quality video data. In view of this, numerous proposals aimed at reducing the cost of VoD by sharing the system resources. Previous works [1 –8] suggested that the broadcasting protocols could significantly improve the efficiency of a VoD system in multicast environments. Rather than transmitting the dedicated stream to each customer, these protocols * Corresponding author. Tel.: þ85-2276-662-56; fax: þ 85-2236-284-39. E-mail address: [email protected] (K.-T. Lo).

repeatedly broadcast the video over several data channels in such a way that customers only wait for a few minutes before watching the video. Many different broadcasting protocols have been developed in the last few years. Typical examples include staggered [1], skyscraper [3], fast data broadcasting [4] and harmonic [5]. Some of these protocols such as fast data broadcasting are very efficient in terms of waiting time but they require a huge buffer to be installed at the receiver. If the buffer size is limited, some protocols will even not be able to implement. In addition, some techniques like harmonic are actually impractical to support less-thanminutes VoD service because too many simultaneous connections are required. A review and a comparative study of different data broadcasting techniques for VoD services can be found in Ref. [9,10], respectively. To reduce the system resources, some researchers [14,15] also considered a hybrid VoD system in which some kind of broadcast protocols are used to transmit the popular videos and some other schemes are used for the videos with less arrival rate. Besides that, since the demands on videos may change by time, some dynamic broadcast schemes were developed. In Ref. [11], a seamless channel transition scheme on top of the fast data broadcasting was proposed

0140-3664/03/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 4 0 - 3 6 6 4 ( 0 2 ) 0 0 2 6 4 - 5

W.-F. Poon et al. / Computer Communications 26 (2003) 1698–1708

such that the number of channels for a video can be adjusted on-the-fly. For the dynamic broadcast protocols, interested readers can refer to Refs. [12,13] for more details. In this paper, we first evaluate different existing broadcasting protocols and identify the efficient and feasible broadcasting solutions under different VoD environments in terms of maximum waiting time, client buffer requirement and receiver bandwidth. We then propose two segmentation schemes, namely First Segment Partition Scheme (FSPS) and Fast Data for First Segment Partition Scheme (FD_FSPS), to further reduce the start-up latency of the staggered and skyscraper broadcasting, which are considered as the only efficient and feasible schemes under our comparative study. Basically, both schemes try to minimize the size of the first segment of the video to achieve a smaller waiting time. The rest of the paper is organized as follows. A comparative study on different broadcasting solutions will first be discussed in Section 2. To reduce the start-up delay, FSPS and FD_FSPS are then proposed in Sections 3 and 4, respectively. The results will be presented in Section 5 and some concluding remarks are finally given in Section 6.

2.1. Previous proposed schemes Staggered broadcasting (STB) [1] is the simplest broadcasting protocol proposed in the early days. The approach of STB is to open the video channels at a fixed regular interval. Suppose that the video length is L seconds. The protocol allocates K channels each with bandwidth C to transmit the whole video. The video is then broadcast at its transmission rate over the channels at a phase delay. The maximum access time is equal to L=K: For example, if 12 channels are allocated to a video with 2 h long, the access latency will be as long as 10 min. To reduce the start-up delay, Pyramid broadcasting (PB) was introduced in [2]. The principle behind PB is to divide a video into N segments of geometrically increasing size so that the video channel Ci will periodically broadcast the video segment Si for the M videos in turns. In order to provide on time delivery of the videos, each channel has to transmit the segments in a very high rate. Both client I/O and storage requirement are very high. The access time of the video TPB and the storage requirement BufPB are then given by Eqs. (1) and (2), respectively.

2. Overview of the broadcasting protocols

TPB ¼

Broadcast schemes can generally work with various network infrastructures [19], including cable TV, direct broadcast satellite and local area network. In this paper, it is assumed that a generic network infrastructure that supports broadcasting operations is used to implement the broadcasting schemes. Because most of the existing work on periodic broadcasting [1 – 3] is based on Constant Bit Rate (CBR) videos such as MPEG-2, it is further assumed that the videos are encoded as CBR. With CBR encoding, the bitrate of resulting encoded video only fluctuates around the target CBR rate. The video can thus be transmitted at the CBR with a small smoothing buffer1 at the client [16]. To facilitate our discussion, we define the following notations:

Buf PB

B M L C N

Server bandwidth in Mbits/s Number of videos being broadcast Length of each video in seconds Bandwidth of each video in Mbits/s Number of data segment in each video file

CLMNða 2 1Þ BðaN 2 1Þ
 CNDN;PB þ DN21;PB ¼ DN;PB 2 B

where Di;PB ¼

a¼

(

ð1Þ ð2Þ

) ai21 ða 2 1Þ L aN 2 1

B CMN

Skyscraper broadcasting (SB) was then reported in Ref. [3] to further enhance the system performance in terms of the waiting time and the buffer requirement. In SB, a new video fragmentation function was developed. The system then broadcasts the video segments over K channels. At the client side, reception of segments is done in terms of transmission group, which is defined as consecutive segments having the same sizes. Users need to download from at most two streams at any time and the receiver buffer requirement is constrained by the size of the last segment. The size of the ith video segment, in the units of the first segment size D1;SB, is given by

8 > 1 > > > > < 2 f ðiÞ ¼ >
  
  
  > > i i i 2 4bi=4c > > 2 i f ði 2 1Þ þ 1 þ 2 2i 1þ : 2þ2 2 2 2

1 The smoothing buffer is not included in the calculation of the buffer requirement for the following schemes.

1699

i ¼ 1; i ¼ 2; 3

:

ð3Þ

otherwise

DSBMAX is defined to restrict the segment from becoming too large. Thus, the start-up latency TSB, that is equal to the size of

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the first segment, is TSB ¼D1;SB ¼

L N X

ð4Þ

minðf ðiÞ;DSBMAX Þ

j¼i

The storage requirement BufSB can be computed by Eq. (5). Buf SB ¼D1;SB ðDSBMAX 21Þ

ð5Þ

Later on, a more efficient broadcast scheme called fast-data broadcasting (FB) [4] was proposed to divide a video into a geometrical series of [1,2,4,…,2K21,2K]. Compared with other broadcasting methods, this protocol is the most efficient in terms of server bandwidth requirement but the receiver is required to download the video segments from all K channels simultaneously. The maximum delay TFB and the buffer requirement BufFB are given by Eqs. (6) and (7), respectively. TFB ¼

L

ð6Þ

2K 21

! 2K21 L Buf FB ¼ 12 K 2 21

ð7Þ

In summary, all the above protocols are to divide a video into N increasing size of segments and transmit them into logical channels of same bandwidth. In Ref. [5], a new broadcasting scheme called harmonic broadcasting (HB) was initiated to divide a video into equal-size segments and transmit them into logical channels of decreasing bandwidth. In HB, Si is put on channel Ci with a bandwidth of C=i: Client starts receiving data from each segment stream after it begins to download the first segment of the video. Thus, the total bandwidth allocated for the video is given by BHB ¼

N X C i¼1

i

:

ð8Þ

When the client is ready to consume Si, it will have received i 2 1 slots of data from that segment and the last slot of that segment can be received during the segment playout time. The start-up time THB is equal to L THB ¼ : N

However, it was found that HB could not always deliver all the data on time. Thus, cautious harmonic broadcasting (CHB) and quasi-harmonic broadcasting (QHB) [6] were proposed to solve this problem. In addition, some hybrid protocols called pagoda broadcasting [7] and new pagoda broadcasting [8] were then developed. These protocols tried to partition each video into fixed size segments and map them into video streams of equal bandwidth but use time-division multiplexing to minimize the access latency and bandwidth requirement. 2.2. Performance evaluation We now compare the performance of the above mentioned protocols in terms of maximum waiting time, client buffer requirement and receiver bandwidth. Figs. 1 and 2 show the waiting time and the buffer requirement against the bandwidth requirement of different broadcasting protocols, respectively. In our study, the video length is fixed as 2 h. To calculate the results of the PB scheme, we set   B N¼ CMe and then compute a ¼ BCMN where e is the Euler’s constant (e < 2.72) [3]. For the SB scheme, DSBMAX is equal to 52 [3]. In Fig. 1, we can see that the pyramid scheme performs the worst and both HB and FB can achieve less waiting time than the SB scheme. When four channels are allocated for a video, the waiting time of HB is only 4 min but the SB scheme requires users to wait as long as 12 min. However, we would like to point out that when the number of channels is increased to further reduce the delay, HB needs many concurrent data streams for each video. For example, 615 segments are used to reduce the waiting time to about 10 s with 7 video channels. Actually, managing such a large number of independent streams is a tedious task for the video server, which hinders the implementation of HB in real applications. On the other hand, for the FB protocol, if

ð9Þ

The buffer requirement BufHB can be calculated by Eq. (10) Buf HB ¼max{Zi li¼1;…;N 21}

ð10Þ

where Z1 ¼I1 Zi ¼Zi21 þIi 2Oi N L X 1 N j¼iþ1 j
 L i21 Oi ¼ N i

Ii ¼

½1#i,N ½1#i#N

Fig. 1. The waiting time against the number of channels for various broadcasting schemes.

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interactive functions in a VoD system is beyond the scope of this paper. Reader can refer to the related topics for more details.

3. First segment partition scheme (FSPS)

Fig. 2. The client buffer size against the number of channels for various broadcasting Schemes.

we want to reduce the waiting time to say less than 15 s, 9 channels are required. However, because the user has to receive all the streams simultaneously, the receiver I/O bandwidth has to be very large. For a 4 Mbits/s MPEG-2 video, it requires a bandwidth 36 Mbits/s in the client side. Up to now, the fastest residential access rate cannot achieve such a high data rate. Therefore, the FB scheme is also impractical for implementation if a small waiting time is required. In addition to the above problems, the large buffer requirement is another constraint for the implementation of a VoD system. In Fig. 2, it can be observed that the client buffer of FB requires 50% of the entire storage size of the video even if the number of channels is increased. Although the HB scheme requires less buffer than FB, it still requires 37% of the whole video that is about 1.3 Gbytes for a 4 Mbits/s MPEG-2 video. It is known from the figure that if the receiver buffer size is limited, only the SB protocol is feasible for providing the small-delay VoD service, however, more broadcasting channels are required. From the above observation, it is found that if the receiver buffer is very large, FB and HB are the most efficient protocols in terms of waiting time for the broadcasting VoD system. However, as mentioned before, it would be difficult to reduce the waiting time to less than 10 s for the HB and FB schemes due to the requirement of too many parallel streams and very large receiver I/O bandwidth, respectively. Thus, if the storage size is limited and nearly true VoD service (few seconds start-up latency) is provided, the SB protocol is the most appropriate choice. If the system also supports the VCR functions, to the best of our knowledge, it is still an open question in these broadcasting protocols. Some previous research works [17 – 21] studied that the STB scheme can provide all the VCR functions by exploiting the receiver buffer and the jumping group property of the network. They suggested that some emergency channels could be used to serve the interactive customers. After the VCR functions, they can then join back to one of the broadcast channels. Providing

To further reduce the start-up latency, a method called FSPS is proposed in this section. Actually, FSPS can be applied to different broadcasting protocols. In this section, we would only focus on a limited buffer environment. As we have said, if a small buffer is installed in the receiver, the STB and SB protocols can only be used by a broadcast VoD system. The latter performs better but the former can support VCR operations. Thus, we simply consider the performance of STB and SB with FSPS in this section. FSPS is basically inspired by Patching [22]. Before we go into the details of FSPS, Patching is briefly described. With Patching, the system allows the customers to join an existing multicast group. The idea is that a client first downloads data on two channels simultaneously. One is called a regular channel that is used to serve a batch of customers. The other is called a patching channel that is used to provide the leading portion of the video such that the customer can be served without waiting for the next regular channel. When the customer start playing the video, the patching data can be consumed as soon as they arrive and the data from the regular channel will be buffered. Once the customer consumes the data from the regular channel, the patching channel can be released. 3.1. Staggered broadcasting with FSPS In STB, each video is broadcast at its transmission rate over a fixed number of logical channels, which we call broadcasting channels, at a phase delay of W seconds. Thus, each video is broadcast over ðL=WÞ number of logical channels. The maximum access time is then equal to W seconds. In our proposed FSPS scheme, the first W seconds of the video (we call first segment of the video in this paper) is further divided into k subsegments. At the beginning of each sub-segment, the user can start receiving the data from the broadcasting channel. However, depending on the location of the arrival, there are still some data missing at the beginning of the video. The missing data can then be obtained from the so-called patching channels. To prevent the buffer underflow, the bandwidth of the patching channels should be equal to that of the broadcast channels. Fig. 3 illustrates the idea of FSPS by dividing the first W seconds of the video into 5 sub-segments. In the figure, two patching channels are available for the users to retrieve the missing data. Assume that customer A arrives between t1 and t2, he can start receiving data D3 from the broadcasting channel C1 at t2. However, D3 is not the beginning of the video. He/she should receive the missing data D1 and D2 from the patching channel P1 at t2 and t3,

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Since at most 2Np data will be early stored in the receiver, the buffer requirement BufSTB_FS is thus given by Eq. (13). Buf STB_FS ¼ 2Np TSTB_FS

ð13Þ

3.2. Skyscraper broadcasting with FSPS

Fig. 3. The staggered broadcasting with FSPS.

respectively. Similarly, customer B will start receiving data D5 from C2 at t9 and then the missing data D1, D2, D3 and D4 from the patching channel accordingly as illustrated by the shaded area in the figure. We can see that the client downloads the data from at most two channels simultaneously but the start-up latency can be significantly reduced. Assume that the first segment is divided into k segments. If the customer arrives within (t j,t jþ1] where 0 # j , k 2 2,j þ 1 sub-segments are downloaded from the patching channels. Thus, if we have Np patching channels, then kX 22

Np ¼

ðj þ 1Þ

j¼0

k

:

ð11Þ

It is assumed that the available Nall channels are divided into Nb broadcasting channels and Np patching channels. From Eq. (11), we have k ¼ 2Np þ 1: The maximum waiting time of the system is thus equal to TSTB_FS ¼

L : Nb ð2Np þ 1Þ

ð12Þ

8 1 > > > < 2 gðiÞ ¼
  
  > > > : 2 þ 2 i 2 1 2 ði 2 1Þ f ði 2 1Þ þ 1 þ 2 i 2 1 2 2

In SB, the broadcast series is [1,2,2,5,5,25,25,52,]. The cycle time of segment 1 (S1) is twice that of S2. Thus, FSPS cannot be simply applied by dividing S1 into k subsegments. The problem is illustrated in Fig. 4, in which S1 is divided by 5 subsegments. When a customer arrives between t3 and t4, he/she can start receiving D5 and D1 at C1 and P1, respectively. However, because S2 starts at t10, the customer will experience data loss between t9 and t10. In order to guarantee the on-time data delivery, the broadcast series of SB is modified to [1,1,2,2,5,5,25,25,52,…]. In this case, we see that the series is similar to the original SB scheme but the first two segments have the same size. Since we have to ensure the continuous playback with our FSPS scheme, S1 and S2 should have the same cycling time. Thus, the customers can receive all the missing data of S1 from the patching channels before the SB protocol starts. Fig. 5 shows the example of the SB scheme with FSPS. S2 is downloaded to the customer at t5. The transmission pattern is then the same as SB. All the data will therefore be delivered to the customers on time. Since SB requires clients to receive 2 channels simultaneously. If FSPS is applied, 3 channels will be listened by the receiver to reduce the start-up time. For example, a customer arrives between t8 and t9. By the SB protocol, S2 and S3 should be downloaded at the same time but D2 to D4 is also receiving by the customer after t10. The fragmentation function of the SB scheme with FSPS is given by i ¼ 1; 2 
  ði 2 1Þ 2 4bði 2 1Þ=4c 2 ði 2 1Þ 1 þ 2

i ¼ 3; 4

:

otherwise ð14Þ

Fig. 4. The problem in the skyscraper broadcasting with FSPS.

W.-F. Poon et al. / Computer Communications 26 (2003) 1698–1708

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Fig. 5. The skyscraper broadcasting with FSPS.

In SB, the maximum delay is S1. Because S1 is further divided into k segments with FSPS, the maximum latency becomes TSKY_FS ¼ XN

L

b

i¼1

 minðgðiÞ; DSBMAX Þ ð2NP þ 1Þ

ð15Þ

For the receiver buffer requirement, as illustrated in Fig. 5, since some data have been stored in the receiver when S2 is being downloaded (the first segments of the original SB scheme), apart from the buffer for the SB protocol, an extra buffer is required. The buffer size can be calculated by Eq. (16). LðD 2 1Þ Buf SKY_FS ¼ 2Np TSKY_FS þ XNb SBMAX minðgðiÞ; DSMAX Þ i¼1

ð16Þ

The first and second terms are the buffer requirements for FSPS and SB, respectively. Now, we consider how to place the sub-segments into the patching channels so that the data can be delivered to the customers on time. From Eq. (11), Np channels can support 2Np þ 1 sub-segments. Intuitively, the requested data should be available as latest as possible but before the display time. In addition, it is desirable that all the missing data can be obtained from one patching channel such that the receiver is not required to tune the channel frequently. An algorithm shown in Fig. 6 is developed to allocate the subsegments into the patching channels to

Fig. 6. The algorithm to allocate the sub-segments into patching channels.

guarantee the continuous playback. Denote Di , for 1 # i # 2Np þ 1 as the ith sub-segment of S1. Pm.j is defined as the jth slot time of the mth patching channel where 0 # j # 2Np and 1 # m # Np and each slot time is used to transmit one sub-segment. Since the patching channels will periodically transmit the subsegments and the cycling time is 2Np þ 1; Pm;j is equal to Pm;j%ð2Np þ1Þ where % is the modulus operator. In the proposed algorithm, we examine each slot time of S1. When customers arrive between ( j,j þ 1), they request D1 to Djþ1 from slot time j þ 1 to 2j þ 1: At each of their request time, we find the available slot for the sub-segment. Based on the algorithm, the placement of sub-segments with different Np is shown in Fig. 7. It is found that most of customers can download the sub-segments from one patching channel. They are not required to frequently tune to the other patching channels while receiving the missing data.

4. Fast data for first segment partition scheme (FD_FSPS) As mentioned in Section 2, FB does not work if the receiver buffer size is not large enough. In addition, since it requires the receiver to download the data from many channels simultaneously (9 channels) to achieve a small waiting time (14 s), it is not feasible for implementation. Actually, under the current technology environment, the bandwidth at the client side is about 3– 10 Mbits/s. It is the bandwidth of about 2– 6 video streams depending on the coding formats. In the STB protocol with FSPS, the receiver would download 2 channels at the same time. If the receiver can download more than 2 channels, it is believed that the waiting time can be further reduced. Thus, in this section, we propose another protocol called FD_FSPS, which is to apply the FB scheme in the first segment to reduce the startup latency. Because only the first segment is transmitted by the FB scheme, the buffer size is not as large as 50% of the whole video. Fig. 8 shows how to apply FD_FSPS in the STB scheme. Assume 2 channels are allocated to do the patching in the system. In STB, the video is broadcast over a number of

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Fig. 7. The placement of sub-segments with different Np :

logical channels. Because the broadcasting channels have a phase difference, it can be considered that one broadcast channel is repeatedly transmitting S1. The system thus has 3 channels to broadcast S1 with the FB protocol [4] and S1 can be divided into 7(23 2 1) sub-segments. The figure shows how the subsegments are transmitted with the STB scheme. When A arrives, D1 is downloaded at t2. D2 and D6 are also downloaded at the same time. Therefore, the receiver listens 3 channels simultaneously. In general, the receiver at most download data from Np þ 1 channels simultaneously. The download sequence is illustrated by the shaded area in the figure. Since D1 is transmitted in the regular broadcast channels, Np þ 1 channels are used for FB. The waiting time is TFD_STB ¼

L Nb ð2

Np þ1

2 1Þ

:

ð17Þ

The buffer size is given by Eq. (18). It is similar to Eq. (11) but L is replaced by the length of S1. Thus, at most 50% of the first segment is stored in the receiver. ! 2 Np Buf FD_STB ¼ 1 2 Np þ1 ð18Þ ð2Np þ1 2 1ÞTFD_STB 2 21

To apply FD_FSPS in the SB protocol, for the same reason mentioned in Section 3, the broadcast series is [1,1,2,2,5,5,25,25,52,…]. The first two segments should be the same size in order to guarantee the continuous playback. In Fig. 9, since one regular channel is used to transmit S1, if 2 patching channels are available, we can have 7 sub-segments. We see that C1 is used to transmit D1. The two patching channels are providing the missing data with the FB protocol. Because SB requires customers to download data from 2 channels, the customers would have to receive data from 4 channels in this case. If Np patching channels are allocated for FD_FSPS, the receiver listens at most Np þ 2 channels at the same time. The example shows that when a customer arrives between t3 and t4, he/she will start the video at t4 and the data will be downloaded accordingly as shown in the figure. Because S1 is divided into 2Np þ1 2 1 segments, the waiting time can then be calculated by Eq. (19). TFD_SKY ¼ XN b

L

  minðgðiÞ; D Þ 2Np þ1 2 1 SBMAX i¼1

Fig. 8. FD_FSPS in the staggered broadcasting.

ð19Þ

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Fig. 9. FD_FSPS in the skyscraper broadcasting.

In addition to the buffer for the SB protocol, some extra buffer is also required to implement FB. Thus, the receiver buffer requirement is given by Eq. (20). !   2 Np 2Np þ1 2 1 TFD_SKY Buf FD_SKY ¼ 1 2 N þ1 2 p 21 LðD 2 1Þ þ XNb SBMAX minðgðiÞ; DSBMAX i¼1

ð20Þ

5. Analytical results In this section, we study the performance of our proposed segmentation schemes. Assume that the video is fixed as 2hour long. Because we focus on a limited buffer environment, we further assume that the receiver buffer can store up to 15 min of video data unless otherwise specified. The performance metric we concern is the maximum waiting time of customers. To calculate the waiting time, for FSPS, we look for the optimal number of patching channels on condition that the receiver buffer is not overflowed. However, in SB, DSBMAX limits the buffer requirement. It is a tedious task to find the optimal number of patching channels if we consider all the value of DSBMAX. For simplicity, we only set DSBMAX as the segment size of the broadcast series that is [1,1,2,2,5,5,25,25,52,…]. For FD_FSPS, in addition to the above criteria, we also consider receiver I/O bandwidth as a constraint when the waiting time is calculated. Fig. 10 shows the waiting time of customers of STB and SB with our proposed schemes. ‘FSPS_STA’ represents the STB with FSPS scheme and ‘FSPS_SKY’ represents the SB with FSPS protocol. Because the receiver I/O bandwidth is also a parameter to determine the waiting time in FD_FSPS, ‘FD_STA(xxx)/FD_SKY(xxx)’ represents the corresponding broadcasting protocol with FD_FSPS, where ‘xxx’ is the number of channels the receiver can download simultaneously. In Fig. 10, ‘inf’ means the receiver can concurrently listen to infinity number of channels. The result shows that, in STB, FSPS can greatly reduce the startup delay. When 14 channels are available in the system, the waiting time can be reduced from 514 to 69 s. For the SB protocol with FSPS, although SB is an efficient protocol,

more than 10 s reduction can still be obtained when the number of channels is smaller than 14. If the receiver can download more channels simultaneously, we can see that the waiting time can be further reduced. In addition, it can be found that the latency is insensitive to the customers in FD_STA(inf) because many channels are used for FB if the receiver is able to listen many channels at the same time. The performance of FD_STA(inf) is only bound by the buffer requirement that is 15 min of video data in this case. On the other hand, the gain from FD_SKY(inf) is not obvious because the system is not prone to the FB protocol due to the small buffer size. Smaller than 3 patching channels are assigned to reduce the start-up delay even if the receiver I/O bandwidth is unlimited. Actually, in real situation, the receiver can listen to 2– 6 channels simultaneously. In view of this, the waiting time against the number of channels with different receiver I/O requirement is plotted in Fig. 11. As we expect, if the receiver can download more channels, the waiting time can be further reduced. In Fig. 11(a), it is shown that FSPS_STA performs better than FD_STA(3). The reason is that, in FSPS_STA, the receiver at most listens two channels at the same time regardless the number of patching channels used. However, FD_STA(3) can only use up to 2 patching channels to minimize the latency even if the buffer size is very large. In SB shown in Fig. 11(b), it can be observed that the performance of FSPS and FD_FSPS is similar when the number of streams is more than 12. We also find that

Fig. 10. The waiting time of broadcasting protocols with FSPS and FD_FPSP.

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Fig. 11. The waiting time against the number of channels with different I/O.

the waiting time of FD_SKY(5) is very close to FD_SKY(inf). It means that 5 channels are enough for the receiver to minimize the start-up latency. The customers can wait less than 5 s when there are 16 channels for the video. The relationship of the buffer requirement with our proposed schemes is shown in Fig. 12. We can see that, in Fig. 12(a), the buffer requirement of FSPS_STA is decreasing when the number of channels is increased. It is because more channels can reduce the first segment size. For the same reason, the buffer requirements of FD_STA are decreasing except FD_STA(inf). As we said, in FD_STA(inf), the performance is bound by buffer size. Thus, the buffer is fully utilized even if the channel bandwidth is further increased. In addition, it is found that FD_STA(3) and FD_STA(4) do not require to store 15 min of video data in the buffer because the number of patching channels is limited by the bandwidth at the receiver. Therefore, the waiting time of FD_STA(3) and FD_STA(4) would be longer than FSPS_STA. On the other hand, in Fig. 12(a), it can be found that the video data stored in the buffer is not decreasing because the SB protocol dominates the buffer requirement that is limited by DSBMAX. In our algorithm, the optimal DSBMAX is found such that the receiver buffer is fully utilized and the waiting time is minimized.

When the number of channels is varied, the optimal number of patching channels such that the start-up latency is minimized is shown in Fig. 13. Since, FSPS_STA only requires the receiver at most listen to 2 streams regardless the number of patching channels allocated to the system, the patching channels is increasing when the number of channels is increased. However, in FD_FSPS, because the patching channels are directly proportional to the receiver I/ O requirement, we see that the number of patching channels is bounded by the bandwidth at the receiver. It is also found that, in SB, 3 patching channels are enough to minimize the waiting time. Thus, the bottleneck in FD_SKY is actually the buffer size. The performance is limited even if the receiver I/O bandwidth is infinity. In this case, the only way to reducing the waiting time is to increase the buffer size. The waiting time is shown in Fig. 14 when the buffer size is changed from 5 to 80 min. We assume that 16 channels (both broadcasting and patching) are available for the video. As we expect, a large buffer can reduce the start-up delay. However, in FSPS_STA, the waiting time is constant even if the buffer size is beyond 15 min. The reason is that if the buffer size is large enough, the delay is only depended on the number of channels available. In FD_STA(4), the result shows that the waiting time is unchanged. As mentioned

Fig. 12. The buffer requirement of FSPS and FD_FSPS.

W.-F. Poon et al. / Computer Communications 26 (2003) 1698–1708

Fig. 13. The number of patching channels allocated to minimize the waiting time.

before, if the bandwidth at the receiver is bounded, the buffer may not be fully utilized. From the result in Fig. 12, FD_STA(4) only requires 4.3 min of video data stored in the buffer. It means that the waiting time cannot be further reduced unless more channels are allocated. For SB with our proposed schemes, the waiting time is only less than 5 s that is insensitive to the customers if the buffer size can store 20 min of video data. The benefit from our schemes is not obvious if a larger receiver buffer is used. In Fig. 15, we compare the waiting time of different broadcasting protocols to determine the feasible solution to a specific VoD environment. For the HB and FB protocols, we assume that unlimited buffer is installed. To calculate the results for STB and SB, only 15% of the whole video can be stored at the client. In order to clearly illustrate the result, the waiting time is plotted in log scale. For a near VoD system, the customers are expected to wait from 5 to 10 min before watching the video. From the result, we can observe that if the receiver has a large buffer, HB is very efficient protocol. 4 broadcasting channels can reduce the delay to 4 min. However, if the server and clients cannot efficiently manage many video streams at the same time, FB is a better approach on condition that the receiver bandwidth is sufficient. It is shown that the startup latency of FB is less

Fig. 14. The waiting time against the buffer requirement with FSPS and FD_FSPS.

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Fig. 15. The waiting time of various broadcasting schemes.

than 4 min with 5 video streams. To provide a VoD service in which the delay is less than 30 s, as mentioned in Section 2, both HB and FB are not applicable. The former requires the server to handle many videos streams and the latter needs a huge bandwidth at the receiver. Therefore, to support a small delay VoD service in a small buffer environment, STB and SB are the only feasible solutions. It can be observed that FD_SKY(4) can serve the customers within 6 s with 16 video channels. If the receiver can only download 3 channels at the same time, FSPS_SKY should be used. Under the same bandwidth requirement of FD_SKY(4), the start-up delay of FSPS_SKY is only about 7 s. However, since it is still an open question to support interactive functions in SB. If the system is going to provide VCR functions, STB is the suitable choice but the delay is longer. For example, 20 video streams are used to reduce the waiting time less than 30 s in FD_STA(4).

6. Conclusions In this paper, we have considered various efficient broadcasting protocols under different VoD environments. For a near VoD system, HB and FB are able to use 5 broadcasting channels to serve the customers within 5 min. However, both schemes need a huge buffer installed at the clients. In order to support a small delay VoD system, we found that STB and SB are the only feasible solutions but more broadcasting channels are required. In view of this, a scheme called FSPS has been proposed to further reduce the start-up latency of STB and SB. In FSPS, the first segment of the video is divided into small sub-segments. When a new request arrives, the customer receives the missing subsegments from the patching channels to guarantee a continuous playback. The result showed that the start-up latency of STB could be substantially reduced from 514 to 69 s with 14 channels. From the FB protocol, it has been found that the waiting time could be greatly reduced if the receiver can listen to many channels simultaneously. Because FSPS simply requires the customers to download data from 2/3 channels

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in the STB/SB scheme, if the receiver bandwidth requirement is relaxed, it is expected that the waiting time can be further minimized. Thus, another technique called FD_FSPS has also been proposed. In FD_FSPS, the FB protocol is only applied in the first segment so that the buffer requirement can be bounded. The results we concern are the performance of FD_FSPS when the bandwidth at the receiver is limited. It has been shown that if the receiver can download data from 5 channels at the same time, the waiting time is smaller than 5 s with 16 channels under the SB protocol. From the results, we can see that SB with our proposed schemes is most efficient to provide a small delay VoD service. However, it is difficult to implement VCR operations. If interactive functions are also supported by the system, STB will be a better choice. In future, we will study how to provide VCR functions in SB. We will consider the system requirements such as extra buffer and network bandwidth, to support interactive functions.

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Acknowledgements The work of J. Feng was supported by the City University of Hong Kong under Grant A/C 7001127. The work of W.-F. Poon and K.-T.Lo was supported by the Centre for Multimedia Signal Processing, Department of Electronic and Information Engineering, The Hong Kong Polytechnic University.

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