Dynamic bandwidth allocation and buffer dimensioning for supporting video-on-demand services in virtual private networks

Computer Communications 23 (2000) 1410–1424 www.elsevier.com/locate/comcom

Dynamic bandwidth allocation and buffer dimensioning for supporting video-on-demand services in virtual private networks L. Zhang*, H. Fu Network Technology Research Center, School of Electrical and Electronic Engineering, Block S2, Nanyang Technological University, Singapore, Singapore 639798

Abstract This paper focuses on the optimization of network bandwidth allocation and buffer dimensioning for transporting pre-stored MPEG video data from the source to the playback destination across virtual private network (VPN). This is one of the most important issues in the support of video-on-demand (VoD) service over VPN. This paper provides a novel scheme in the dynamic allocation of bandwidth to segments of video using the ABR mode. The dynamic bandwidth allocation mechanism is based on a new concept, called playback tunnel obtained from the traffic characteristics of the pre-stored MPEG video trace, to determine the optimum of transmission bandwidth as well as the optimum of buffer capacity to ensure that the playback buffer neither underflows nor overflows. The proposed scheme is tested with real-life MPEG video traces. The obtained results have shown its significant performance improvement in terms of the capacity of playback buffer, the start-up playback delay, network utilization and the network multiplexing gain. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Multimedia networking; Quality of service; Network bandwidth allocation; Network multiplexing gain; Video-on-demand; Virtual private networks; Internet

1. Introduction Currently, there is significant interest in the deployment of virtual private networks (VPN) across the IP backbone [1]. Comparing with the existing private networks, VPN is a cost-effective alternative for constructing WAN to connect offices, individual users, and more importantly, the third parties. On the other hand, VPN is more flexible and convenient than the traditional networks in its ability to provide remote entry for any authorized user with Internet access. As shown in Fig. 1, the ISP (Internet Service Provider) architecture and its associated protocols to support VPN has been developed by IETF (Internet Engineering Task Force) [2]. Three major issues have been considered in Ref. [2], that is the opaque packet transport, data security and quality of service (QoS) guarantee. The development of broadband network technology such as ATM and WDM is able to provide more bandwidth with less latency for transporting multimedia data over the Internet. On the other hand, the development of IPv6 provides an alternative based on the differentiated service mechanism for QoS supporting over the Internet [3,4]. The differentiated service format defined in IPv6 includes the following * Corresponding author. Tel.: 1 65-792-0415; fax: 1 65-790-4508. E-mail address: [email protected] (L. Zhang).

two important components [3,4]. The first component is a 4bit priority field in the IPv6 header used to identify and discriminate, the traffic types. The second component is a flow label to enable the labeling of packets that belong to particular traffic flows for which the sender might request special handling, such as non-default QoS or real-time traffic. In the current situation, it may be difficult for the IP router to select the priority class based on the data packet content when the information data are encrypted. By contrast, VPN has the advantage that the class of service can be stated outside the VPN envelope of the IP packet. Hence, VPN is more flexible for supporting multimedia services by either reserving certain amount of bandwidth for mission-critical traffic or forwarding the data as the best-effort traffic using priority. It is generally believed that the multimedia services over the Internet including VPN are the most challenging applications. As video-ondemand (VoD) is one of the most demanding multimedia services being widely used in many areas such as entertainment and distance learning, the support of VoD service in VPN is becoming one of the most important issues related to the multimedia services in VPN. One of the most important technical issues to support VoD services is the transport of MPEG video data from source to playback destination [5–12]. The issues related to the admission control for transporting MPEG video

0140-3664/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S01 40-3664(00)0018 6-9

L. Zhang, H. Fu / Computer Communications 23 (2000) 1410–1424

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the playback time can be determined by the following equation bmin …d† ˆ

max

d#n#N 2 1

F n

nX 11

xi 2

iˆ1

d X

! xi ;

…1†

iˆ1

where xi …i ˆ 1; 2; …; N† is the video data in bytes contained in frame i. On the other hand, for a given playback buffer capacity, B, the maximum transmission bandwidth, bmax(d,B), to guarantee no buffer overflow can be determined by bmax …d; B† ˆ Fig. 1. VPN over IP backbone.

across networks can be divided into two categories: (1) transport of live real-time MPEG video [13–26]; and (2) transport of pre-stored MPEG video [27–29]. In the first category, the re-negotiation constant bit rate (RCBR) [22– 24] and re-negotiation deterministic variable bit rate services (RED-VBR) [25,26] are the well-known approaches. Both RCBR and RED-VBR focus on the admission control for transporting real-time MPEG video utilizing two important properties of MPEG video: (1) compressed video traffic usually exhibits burstiness over multiple timescale [30]; and (2) quality control factor (Q-factor) contained in the MPEG format [31] which gives the possibility of graceful degradation of QoS. Both schemes represent different tradeoff in QoS and achievable network utilization. The differences is that RED-VBR builds the re-negotiation on the top of a deterministic variable bit rate model and, by contrast, RCBR builds the re-negotiation on the top of a constant bit rate model. RED-VBR is more efficient but RCBR is simple for its implementation. In the second category, the store-and-forward (SAF) is a wellknown approach [27,28] in which the MPEG video data is first transported from the server to the central office (CO) in bursts. Then the video data is buffered in CO before delivering to the playback destination in real-time. The other scheme for supporting VoD services is called constant rate transmission and transport (CRTT) scheme [29]. The basic concept of the CRTT scheme is transporting a large amount (but not all) of video data to a playback buffer on the user side before the playback is commenced. The transport of MPEG video data in CRTT is using the CBR mode but it also requires a playback buffer with extreme large capacity and involves long delay before starting of playback, called start-up delay. The key issue of CRTT is to determine the required transmission bandwidth as well as the capacity of playback buffer to guarantee neither underflow nor overflow during the playback. Considering that a MPEG video sequence of N frames is played back at a fixed rate of F frames per second; the CRTT scheme first sets up a fixed start-up delay d, then the minimum required transmission bandwidth, bmin(d), to guarantee no buffer underflow during

min

0#n#N…B† 2 1

F n11

"

nX 11 iˆ1

xi 2

d X

# xi 1 B ;

…2†

iˆ1

where ( N…B† ˆ max n :

n X

) xi 1 B , C ;

iˆ1

where C is the total video data in bytes to be played back. Hence, the minimum required capacity of playback buffer to guarantee no overflow is given by Bmin ˆ min{B : D…B† ± f};

…3†

where  D…B† ˆ {d : d # d…B†; (  d…B† ˆ max d :

d X

bmin …d† # bmax …d; B†};

…4†

) xi # B :

…5†

iˆ1

Comparing to the live real-time video data, the traffic characteristics of pre-stored MPEG video data is completely known as a priori. This paper provides a novel scheme, called the dynamic bandwidth allocation (DBA) scheme, for dynamically allocating bandwidth and sizing the playback buffer in the transport of pre-stored MPEG video data from the source to the playback destination to support VoD services. In the proposed DBA scheme, the pre-stored MPEG video sequence is divided into M segments. The transport of these segments across networks is using ABR mode in which the allocated bandwidth is varied from the segment to segment, to match the variable video frame sizes contained in segments. The dynamic bandwidth allocation mechanism is based on a new concept, called the playback tunnel to determine the optimum transmission bandwidth as well as the optimum buffer capacity to ensure that the playback buffer neither underflows nor overflows. The performance with the DBA scheme is tested with real-life MPEG video traces and compared to the CRTT scheme. The obtained results have shown that the DBA scheme greatly outperforms the CRTT scheme.

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Fig. 2. Transport of pre-stored MPEG video data from server to playback destination.

frames contained in the mth segment is l…m† 2 l…m 2 1†: Then video data in bytes contained in the mth segment is given by

2. Dynamic bandwidth allocation and buffer dimensioning Fig. 2 shows a diagram of transporting pre-stored MPEG video data across VPN. On the server side, an MPEG video sequence consisting of N frames is packetized into C bytes and divided into M segments with a corresponding length of L1 ; L2 ; …; Lm ; …; LM bytes, respectively. The bandwidth allocated to the mth segment …m ˆ 1; 2; …; M† is bm bytes/ s, then the transmission time for the mth segment is Lm =bm : On the user side, the playback buffer has capacity of B bytes. B(t) represents the buffer occupancy at time t. A(t)

Lm ˆ

l…m† X

xi ; m ˆ 1; 2; …; M:

…6†

iˆl…m 2 1† 1 1

where xi is the video data in bytes contained in the ith video frame …i ˆ 1; 2; …N†: The arrival stream A(t) can be expressed as

8 0 > > > > m 21 > X > > < Li 1 bm ‰t 2 T…m 2 1†Š A…t† iˆ1 > > > M >X > > > : Li

t#0 T…m 2 1† , t # T…m†; m ˆ 1; 2; …M

…7†

t . T…M†

iˆ1

and D(t) represent the cumulated video data that have been transported to the playback buffer at time t and the cumulated video data that have been played back at time t, respectively. The video sequence is played back at a fixed rate of F frames per second. The relationship between A(t), D(t) and B(t) is shown in Fig. 3, where we assume that the delay and delay jitter introduced by the ATM network for A(t) is negligible. This is because this paper focuses on the performance at the admission control level which is normally evaluated in a much larger time-scale than that used for the evaluation of network delay and delay jitters. T(m) is the cumulated time for Pm transporting the previous m segments, i.e. T…m† ˆ iˆ1 …Li =bi †: Let l(m) be the frame sequence number …0 # l…m† # N† of the last video frame contained in the mth …m ˆ 1; 2; …M† segment, therefore, the total number of video

Fig. 3. The relationship between arrival stream A(t), playback stream D(t) and buffer occupancy B(t).

L. Zhang, H. Fu / Computer Communications 23 (2000) 1410–1424

Pm 2 1

where the term iˆ1 Li represents the cumulated video data that have been transported to the playback buffer during time period ‰T…0†; T…m 2 1†Š: The term bm ‰t 2 T…m 2 1†Š represents the video data contained in the mth segment that have been transported to the playback buffer using an allocated bandwidth bm during time period ‰T…m 2 1† , t # T…m†Š: As the MPEG video is played back at a fixed rate of F frames per second, the playback stream D(t) can be described as

D…t† ˆ

8 0 > > > > …t 2 > Xd†F > > < x

0,t#d i

d , t # …N=F† 1 d

iˆ1

> > > N > X > > > : xi

;

…8†

max

0#t#…N=F† 1 d

{B…t†}:

Li 1 bm ·‰t 2 T…m 2 1†Š 2

ˆ

max

{B…t†} (m 2 1 X

d#t#T…M†

Li 1 bm ‰t 2 T…m 2 1†Š 2

iˆ1

…t 2 Xd†F

) xi ;

iˆ1

…13†

0,t#d d , t # T…M†

…t 2 Xd†F

for d # t # T…M†;

xi $ 0;

and Eq. (10) gives Li

t 2 T…m 2 1†

…9†

t . …N=F† 1 d

(10)

iˆ1

m ˆ 1; 2; …M:

T…M† , t # …N=F† 1 d

iˆ1

iˆ1

mX 21

0#t#…N=F† 1 d

t#0

To guarantee no playback buffer underflow, it is required that B…t† $ 0: From Eq. (9), we can obtain

bm $

B$

and the maximum required bandwidth for the mth segment

8 0 > > > > m 21 > X > > > Li 1 bm ‰t 2 T…m 2 1†Š > > > iˆ1 > > > > mX …t 2 21 < Xd†F Li 1 bm ‰t 2 T…m 2 1†Š 2 xi B…t† ˆ > > iˆ1 iˆ1 > > > > …t 2 M > Xd†F X > > > L 2 xi i > > > iˆ1 iˆ1 > > : 0

xi

(12)

P where k ˆ …t 2 d†F and y…k† ˆ kiˆ1 xi : Likewise, to guarantee no playback buffer overflow, the buffer capacity B must satisfy

max

B…t† ˆ A…t† 2 D…t†;

iˆ1

m ˆ 1; 2; …; M;

From Eq. (9), we can obtain

P 2 d†F xi where d is the start-up playback delay, and term …tiˆ1 represents the video data in bytes that have been played back during time period [d,t]. Obviously, the playback buffer occupancy, B(t), is given by

…t 2 Xd†F

segment to guarantee no playback buffer underflow can be obtained from Eq. (11), that is 3 2 mX 21 y…k† 2 L i 7 6 7 6 iˆ1 7; 6 bm;min ˆ max 6 T…m 2 1†F 1 1#k#l…m† 6 …k=F† 1 d 2 T…m 2 1† 7 7 7 6 7 6

t . …N=F† 1 d

iˆ1

mX 21

1413

; m ˆ 1; 2; …; M; for d # t # T…M†: …11†

Hence, the minimum required bandwidth bm,min for the mth

to guarantee no playback buffer overflow is given by 7 6 mX 21 7 6 7 6 B 1 y…k† 2 L i 7 6 7 6 iˆ1 7; 6 min bm;max ˆ 7 6 T…m 2 1†F 1 1#k#p…m† 4 k=F 1 d 2 T…m 2 1† 5 m ˆ 1; 2; …; M;

(14)

where k ˆ …t 2 d†F: Combining Eqs. (12) and (14), the bandwidth bm allocated to the mth segment must satisfy …15† bm;min # bm # bm;max to guarantee that the playback buffer neither underflows nor overflows.

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4. for m ˆ 1:M 5. calculate bm,min over ‰T…m 2 1†; l…m†Š; using Eq. (12); 6. calculate bm,max over ‰T…m 2 1†; p…m†Š; using Eq. (14); 7. select bm between bm,min and bm,max according to the network conditions; 8. output the mth segment Lm with bm; 9. Set T…m† ˆ T…m 2 1† 1 Lm =bm ; 10. end

Fig. 4. Playback tunnel.

3. Tunnel for dynamic bandwidth allocation Fig. 4 illustrates the three curves, i.e. arrival stream A(t), playback stream D(t) and the maximum amount of cumulated video data, D…t† 1 B; which can be transported to the playback buffer at time t. The shadow area between D(t) and D…t† 1 B is defined as a playback tunnel. Obviously, to guarantee that the playback buffer neither underflows nor overflows, the arrival stream A(t) must be driven within the tunnel boundary. For example, as shown in Fig. 4, the mth segment has three cross points of p(m), a(m) and l(m) with the curves D(t) 1 B, A(t) and D(t), respectively. The slope of the line from a…m 2 1† to p(m) represents the maximum bandwidth bm,max that can be allocated to the mth segment to guarantee no buffer overflow, if and only if the line from a…m 2 1† to p(m) is under the curve D…t† 1 B all the time in the mth segment period of T…m 2 1† , t , T…m†. Likewise, the slope of line from a…m 2 1† to l(m) represents the required minimum bandwidth bm,min to the mth segment to guarantee no buffer underflow, if and only if the line from a…m 2 1† to l(m) is above the curve D(t) all the time in the mth segment period of T…m 2 1† , t , T…m†. Hence, the bandwidth bm allocated to the mth segment can be dynamically determined by the slope of the line from a…m 2 1† to a(m) which must be located within the angle between bm,max and bm,min. On the other hand, the allocation of bm can be improved by the following two approaches. When playback buffer capacity B increases, the tunnel becomes wider and more selective space for bm is available. On the other hand, when segment size Lm is reduced, the angle between bm,max and bm,min becomes wider which also provides more selective space for bm. The bandwidth bm allocated to the mth segment can be determined by the following procedure. PROCEDURE for Dynamic_Bandwidth_Allocation 1. select Lm, m ˆ 1; 2; …; M; 2. set l(m), p(m) corresponding to Lm, m ˆ 1; 2; …; M; 3. T…0† ˆ 0;

Clearly, at the beginning of the first segment, b1, b1,min and b1,max are all starting from 0 as shown in Fig. 4. Then b1 can be chosen as the slope of the line from 0 to a(1), which is the allocated between the lines from 0 to p(1) and from 0 to l(1) according to the network conditions. Likewise, for the second segment, the allocated bandwidth b2 can be determined as the slope of the line from a(1) to a(2), where a(1) is determined by the first segment, and so on. Our further research has shown that the optimum bm can be determined using a middle tunnel DBA approach, in which the bm is always determined as the slope of the line in the middle of the playback tunnel. The technical details and performance evaluation is reported in Ref. [34]. We note that the performance of the proposed scheme is certainly affected by the segmentation of the video data to be played back. This is because the playback tunnel reflects the content of the video stream. The optimum segmentation is also one of the very important topics under our studies [35]. 4. Performance evaluation For illustrative purposes, the performance with the proposed DBA scheme is only evaluated for a simple case where MPEG video sequence is divided into M segments of equal length, i.e. each segment contains fixed video data of L bytes, i.e. L1 ˆ L2 ˆ … ˆ LM ˆ L ˆ C=M: In this case, the minimum bandwidth allocated to the mth segment to guarantee no playback underflow can be P buffer 21 L in Eq. (12) with a obtained by replacing the term m iˆ1 i new term …m 2 1†L; i.e.   y…k† 2 …m 2 1†L max ; bm;min ˆ T…m 2 1†F 1 1#K#l…m† k=F 1 d 2 T…m 2 1† m ˆ 1; 2; …; M:

(16)

Likewise, the maximum bandwidth that can be allocated to the mth segment to guarantee no playback buffer overflow can also be obtained from Eq. (14), i.e.  bm;max ˆ

max

T…m 2 1†F 1 1#k#p…m†

m ˆ 1; 2; …; M:

 B 1 y…k† 2 …m 2 1†L ; k=F 1 d 2 T…m 2 1† (17)

L. Zhang, H. Fu / Computer Communications 23 (2000) 1410–1424

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Table 1 Characteristics of MPEG video traces Type-name

Mean rate (bytes/frame)

Maximum rate (bytes/frame)

Minimum rate (bytes/frame)

Standard Div (bytes/frame)

Cov. (bytes/frame)

Movie-Lambs MTV-1 MTV-2 Episodes-Mr. Bean Sports-Soccer Movie-Star Wars Talk-show Cartoon-Asterix

913.9440 3075.5280 2472.5616 2205.8880 3138.6960 1164.1000 1817.1024 2793.5904

16,778.0016 28,650.0000 31,425.9984 28,634.0016 23,787.0000 15,602.0000 13,346.0016 18,422.0016

36.0000 45.9984 60.0000 42.9984 369.0000 34.0000 260.0016 38.0016

1399.4496 2882.8224 2681.6496 2580.4368 2657.5728 1612.8000 2064.9360 2518.0032

1:9585 × 106 8:3108 × 106 7:1912 × 106 6:6586 × 106 7:0627 × 106 2:6013 × 106 4:2640 × 106 6:3404 × 106

Real-life MPEG video traces with different contents including Movies, Sports, News, Talk show, MTV, several Episodes and Cartoon are used in the performance evaluation. These MPEG video traces are decoded and extracted into individual frames using a public-domain software Berkeley MPEG version 1.3 1 with the following parameters. • • • • • • • • •

Encoder input: 384 × 288 pixels Color format: YUV (4:1:1, resolution of 8 bits) Quantization value: I ˆ 10; P ˆ 14; B ˆ 18 Pattern: IBBPBBPBBPBB GOP size: 12 Motion vector search: ‘Logarithmic’/’Simple’ Reference frame: ‘Original’ Slices: 1 Vector/range: half pixel/10

• Frame rate: 25 frames/s • Video length: 40,000 frames The characteristics of video traces are shown in Table 1. The following numerical results are obtained based on an MPEG-1 video trace “Movie-Lambs” [32], as shown in Table 1. The video trace of N ˆ 40; 000 frames is divided into M ˆ 32 equal segments. The video trace is played back at a rate of F ˆ 25 frames per second. The measured realtime playback video in frames and the corresponding cumulated playback stream in frames are shown in Figs. 5 and 6, respectively. Fig. 7 shows the measured arrival stream A(t) versus the playback stream D(t) where the equal segmentation is used. The zoom-in of Fig. 7 is shown in Fig. 8. It can be seen that when the bandwidth allocated to A(t) is chosen using Eq.

Fig. 5. The playback of MPEG video trace “Movie-Lambs”.

1 MPEG traces available via anonymous FTP fromftp://ftp-info3.informatik.uni-uerzburg.de/pub/MPEG/.

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Fig. 6. The cumulated playback video data of “Movie-Lambs”.

(16), the playback stream D(t) is always below the arrival stream A(t), so that the underflow of playback buffer is certainly avoided. Figs. 9 and 10 show the measured bandwidth allocation and the playback buffer occupancy. In Fig. 9, it can be seen

that the bandwidth allocated to video segments changes by jumps, i.e. using the ABR mode. On the other hand, comparing Fig. 9 with Fig. 5, it also shows that the allocated bandwidth using the DBA scheme matches the real-life video playback trace very well.

Fig. 7. Arrival stream A(t) vs. playback stream D(t).

L. Zhang, H. Fu / Computer Communications 23 (2000) 1410–1424

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Fig. 8. Zoom-in of the arrival stream A(t) vs. the playback stream D(t).

Fig. 11 shows the minimum capacity required for the guarantee of no playback buffer overflow versus the number of segments being used for transporting video data. It can be seen that the required buffer capacity decreases when the number of segment increases. This can be explained using the playback tunnel diagram shown in Fig. 4. When the capacity of playback buffer is fixed, smaller segment size corresponds to a wider angle between bm,max and bm,min. On

the other hand, if the angle between bm,max and bm,min is fixed, then the small segment size corresponds to less buffer capacity.

5. Impact on network characteristics From the network point of view, there are two important

Fig. 9. Bandwidth allocation using DBA scheme.

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Fig. 10. Playback buffer occupancy.

parameters commonly used for performance evaluation: (1) traffic burstiness; and (2) network multiplexing gain. The traffic burstiness is defined as a ratio of the peak rate to the average rate which indicates the grade of difficulty to satisfy the QoS requirement. The traffic burstiness of the MPEG video traces used for the performance evaluation is shown in Table 2. It can be seen that the video streams using DBA scheme have lower burstiness comparing to the original video streams without using DBA scheme. For example, the original video trace “Movie-Lambs” has a burstiness of 18.3578, by contrast, for the same video trace using DBA scheme, the burstiness is reduced to 4.2039. On the other hand, the burstiness of the multi-

plexed stream is significantly reduced from 5.8182 to 1.9953. Consider that W independent MPEG video streams are multiplexed in the process of transporting data across the network. Then the network multiplexing gain for these W MPEG video streams is defined as 0 1 B C B pW C C × 100 GˆB 1 ˆ B C W X @ A Pw

…18†

wˆ1

where PW is the aggregate bandwidth required to satisfy a given QoS requirement (say, no loss) for the multiplexed

Table 2 Traffic burstiness MPEG video traces

Movie-Lambs MTV-1 MTV-2 Episodes-Mr. Bean Sports-Soccer Movie-Star Wars Talk-show Cartoon-Asterix Multiplexed stream

Mean rate (bytes/frames)

Peak rate (bytes/frames)

Traffic burstiness

Original

With DBA

Original

With DBA

Original

With DBA

913.9424 3:0755 × 103 2:4726 × 103 2:2059 × 103 3:1387 × 103 1:1641 × 103 1:8171 × 103 2:7936 × 103 1:781 × 104

913.8810 3:0749 × 103 2:4718 × 103 2:2051 × 103 3:1377 × 103 1:1641 × 103 1:8170 × 103 2:7935 × 103 1:7575 × 104

16,778 28,650 31,426 28,634 23,787 15,602 13,346 18,422 102,292

3842 12,576 10,631 7,969 10,929 4029 4394 8185 35,068

18.3578 9.3156 12.7097 12.9806 7.5786 13.4021 7.3447 6.5944 5.8182

4.2039 4.0899 4.3009 3.6139 3.4831 3.4612 2.4181 2.9300 1.9953

L. Zhang, H. Fu / Computer Communications 23 (2000) 1410–1424

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Fig. 11. Playback buffer capacity vs. segment number.

video stream and Pw …w ˆ 1; 2; …W† is the peak rate of the aggregate load for each individual stream [33], hence, PW P represents the total aggregate bandwidth required wˆ1 w to satisfy the QoS for all MPEG video streams, if these W MPEG video streams are transported individually. Therefore,

the statistical multiplexing gain thus defined represents the fractional reduction in the aggregate bandwidth requirement needed in comparison of without multiplexing. It quantifies the potential utilization improvement that can be achieved by ABR mode over CBR mode. The following numerical

Fig. 12. Multiplexing of eight different, independent and unprocessed Videos.

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Fig. 13. Multiplexing of eight different, independent and ESD Videos.

results are obtained by multiplexing W ˆ 8 independent MPEG video traces, as shown in Table 2, for a given QoS requirement to guarantee that the playback buffer neither overflows nor underflows.

Fig. 12 shows the aggregated bandwidth of the multiplexed original MPEG video traces. Likewise, Fig. 13 shows the aggregated bandwidth of the multiplexed MPEG video traces in which each video trace is transported

Fig. 14. Comparison of DBA scheme and CRTT scheme.

L. Zhang, H. Fu / Computer Communications 23 (2000) 1410–1424

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Fig. 15. Zoom-in of Fig. 14.

using DBA scheme. The network multiplexing gain for both cases can be obtained from Eq. (18), that is GOriginal ˆ 42:0918; GDBA ˆ 80:1478: It shows that the DBA scheme can significantly improve the network multiplexing gain from 42.0918 to 80.1478. The significant improvement in terms of traffic burstiness and network multiplexing gain has demonstrated that the DBA scheme has also another important feature to reduce the network transmission costs. 6. Comparison of DBA scheme and CRTT scheme This section presents a comparison of performance between the proposed DBA scheme and the CRTT scheme proposed by Ross and McManus [29] in which a large amount (but not all) of video data is pre-transported to the playback buffer before the playback is commenced. CBR mode is used in CRTT but it requires the playback buffer with extremely large capacity. The performance with CRTT scheme is described by Eqs. (1)–(4). Fig. 14 shows the arrival stream and playback stream for both CRTT scheme and DBA scheme with a start-up delay of 0.48 s. A zoom-in of Fig. 14 is shown in Fig. 15. It can be seen that the arrival stream under DBA scheme matches the playback stream much better than that under CRTT scheme. For example, as shown in Fig. 15, the playback stream has a turning point from high speed to low speed at the 73rd

frame. As the allocated bandwidth used in CRTT is CBR, which cannot be changed until the completion of the transmission, it can be seen that from the turning point of the 73rd frame onwards, the gap between the playback stream and the arrival stream of CRTT scheme increases significantly. In contrast, the DBA scheme is able to re-adjust the bandwidth at the 506th frame, which is the beginning of the following segment. Therefore, buffer occupancy under the DBA scheme can be flexibly controlled. Fig. 16(a) and (b) shows the buffer occupancy for both the CRTT and DBA schemes with a start-up delay of 0.48 and 40 s, respectively. It can be seen that the DBA scheme is able to significantly reduce the required buffer capacity even for a small start-up delay. In contrast, the buffer occupancy under CRTT scheme is significantly large even for the large start-up delay up to 40 s. For example, when M ˆ 32 and d ˆ 0:48 s; the buffer occupancy under the CRTT scheme reaches the peak value of 22, 962, and 720 bytes at the frame of 15,992. However, the peak value of the buffer occupancy under the DBA scheme is only 640,992 bytes at the frame of 26,864. The zoom-in of Fig. 16(a) is shown in Fig. 17. Fig. 18(a) and (b) shows a comparison of the allocated bandwidth for both the DBA and CRTT schemes with a start-up delay of 0.48 and 40 s, respectively. The bandwidth allocation under the CRTT scheme is constant, but increasing the start-up delay can reduce its required bandwidth. On the other hand, the allocated transmission bandwidth under the DBA scheme is varied from segment to segment, in which the advantage is the ability to achieve higher network multiplexing gain.

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Fig. 16. Receiving buffer occupancy for DBA and CRTT schemes.

7. Conclusion This paper presents a novel scheme for dynamically allocating bandwidth and buffer dimensioning to transport pre-recorded MPEG video stream across the network to support the VoD service. The new approach combines both the advantages of using ABR mode to match the MPEG video characteristics as well as to achieve higher network multiplexing gain.

Using the playback tunnel, the bandwidth allocation can be dynamically determined based on the network traffic condition and also both the underflow and overflow of the playback buffer can be avoided. The performance is evaluated with real-life MPEG video traces. The obtained results have demonstrated that the proposed scheme is not only able to match the playback stream very well but also able to significantly improve the performance by comparing with the CRTT scheme.

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Fig. 17. Zoom-in of the Fig. 16(a), d ˆ 0:48s:

Fig. 18. Bandwidth allocation for DBA and CRTT.

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