Vol.23 No.4 JOURNAL OF ELECTRONICS (CHINA) July 2006

PROXY-BASED PATCHING STREAM TRANSMISSION STRATEGY IN MOBILE STREAMING MEDIA SYSTEM1

Liao Jianxin Lei Zhengxiong Ma Xutao Zhu Xiaomin (State Key Laboratory of Networking and Switching Technology, Beijing University of Posts and Telecommunications,

Beijing 100876, China)

Abstract A mobile transmission strategy, PMPatching (Proxy-based Mobile Patching) transmission strat-egy is proposed, it applies to the proxy-based mobile streaming media system in Wideband Code Division Multiple Access (WCDMA) network. Performance of the whole system can be improved by using patching stream to transmit anterior part of the suffix that had been played back, and by batching all the demands for the suffix arrived in prefix period and patching stream transmission threshold period. Experimental results show that this strategy can efficiently reduce average network transmission cost and number of channels consumed in central streaming media server.

Key words Mobile streaming media system; Proxy-based Mobile Patching (PMPatching) transmission strategy; Patching stream; Batching

I. Introduction The mobile streaming media technology allows

mobile terminal users to view continuous video & audio clip after it is compressed and placed on the network server while it is being downloaded, with-out the necessity of downloading the whole file first. The mobile streaming media service is drawing on more and more attention[1]. Despite the fact that the promotion of the mobile streaming service is limited by the current bandwidth, as the third generation (3G) network advances gradually, the mobile streaming service will become one of the main-stream services in the 3G network.

Transmission strategy is one of the pivotalest strategies in the streaming media system. It deter- 1 Manuscript received date: June 30, 2005;

revised date: December 12, 2005. Supported by: (1) Specialized Research Fund for the Doctoral Program of Higher Education (No. 20030013006); (2) National Specialized R&D Project for the Product of Mobile Communications (Develop-ment and Application of Next Generation Mobile Intel-ligent Network); (3) Development Fund Key Project for Electronic and Information Industry (Core Service Plat-form for Next Generation Network); (4) Development Fund Project for Electronic and Information Industry (Value-added Service Platform and Application System for Mobile Communications); (5) Development Fund Project for Electronic and Information Industry (Con-tent-based Integrated Charging Platform for Telecom-munication Networks); (6) National Specific Project for Hi-tech Industrialization and Information Equipments (Mobile Intelligent Network Supporting Value-added Data Services). Communication author: Lei Zhengxiong, born in 1978, male, Ph.D. candidate. State Key Laboratory of Net-working and Switching Technology, Beijing University of Posts and Telecommunications, Beijing 100876, China. lzxathh@sohu.com.

mines when and which streaming media program the central streaming media server will transmit. The existing strategies include pyramid strategy[2], sky-scraper strategy[2], FCFS (First Come First Serve) strategy[3], batching strategy[4], patching strategy[5], piggybacking strategy[6], dynamic batched patching strategy[7], etc.

All the above strategies are designed on the supposition that the backbone network supports multicast, so they are not applicable for the Mobile Streaming Media System (MSMS) whose network only supports unicast. Zhou[8] proposes a transmis-sion strategy specifically for the system whose backbone network only support unicast. B. Wang[9] presents Unicast Patching (UPatch) transmission strategy specifically for the system whose network only supports full unicast. However, those two strategies have not taken into account very small memory space of mobile terminals in MSMS, so they are not applicable for MSMS.

Until now, no transmission strategies that apply to MSMS have been proposed. In this paper, we will present an innovative PMPatching transmission strategy that is applicable for MSMS.

The rest of this paper is organized as follows: Section II briefly introduces the network structure of MSMS. Section III proposes PMPatching transmis-sion strategy in MSMS. Section IV evaluates the performance of PMPatching strategy. Section V summarizes and concludes the paper.

II. Mobile Streaming Media System (MSMS)

Fig.1 depicts an MSMS in WCDMA network. The system is comprised of such devices as

streaming media server, GGSN (Gateway GPRS

516 JOURNAL OF ELECTRONICS (CHINA), Vol.23 No.4, July 2006

Support Node), SGSN (Serving GPRS Support Node), streaming media proxy, Radio Network Controller (RNC), Node B and mobile terminal (ME).

Fig.1 MSMS in WCDMA network

In the version R5 of WCDMA, all the interfaces, including Gi, Gn, Iu and Iub in Fig.1 support IP protocol, so WCDMA network can be seen as a subnet of Internet; an ME can be seen as a mobile host, with its own IP address to get addressed and receive IP packets. In order to support streaming media service, the mobile terminal needs to install a streaming media player and reserves some playing memory space.

The network environment of MSMS exhibits 3 salient features as follows: the core network only supports unicast; the radio channel connecting to the mobile terminal only supports unicast; and the memory space of the mobile terminal is very lim-ited.

The bit rate of the streaming media program can be up to 1.5Mbps. The enhanced technology HSDPA (High Speed Downlink Packet Access) of the WCDMA network can meet the requirement of the bit rate. The discussion of this paper is also based on the supposition that the system supports HSDPA.

III. PMPatching Strategy in MSMS Suppose there are N Constant-Bit-Rate (CBR)

streaming media programs in the streaming media server. For program i, the length is Li (s), the bit rate is bi (bits/s), the length of prefix stored in the proxy is vi (s) and the access probability is pi. We assume that the proxy cache size is S (bits). The minimum memory space that all mobile terminals can support is m (bits) and each mobile terminal can receive at most 2 video streams simultaneously from two channels.

UPatch strategy[9] does not consider very limited memory space of mobile terminals, the third charac-ter of MSMS as mentioned earlier, so it does not adapt to the MSMS. We will propose an advanced strategy, PMPatching strategy, based on UPatch strategy with the consideration of three characters of MSMS.

PMPatching strategy will delay the sending of suffix of a program from server to proxy as much as possible on the premise that the two conditions are met: the mobile terminal can play video continu-ously without interruption and the mobile terminal will not continue to save streaming media data after its memory space is filled with streaming media data in order to prevent losing data.

If m/bi≥vi, PMPatching strategy is almost the same as UPatch strategy. A patching stream thresh-old Pi (s) for each program i is confirmed and the whole time axis is divided into several partitions, named Pi domains by a time segment whose length is vi+Pi. For any request for program i arriving in a Pi domain, prefix in proxy is transmitted immedi-ately to the terminal that sends the request. As shown in Fig.2, suppose the starting time of a Pi domain is TPi, all the requests for program i arriving in segment [TPi, TPi+vi+Pi] will be batched together and only one unicast stream of suffix [vi, Li], marked as b stream is sent from streaming media server to proxy at time TPi+vi. Suppose a request for program i arrives at time t in segment [TPi+vi, TPi+vi+Pi], a patching stream in segment [vi, t−TPi] of suffix, marked as p stream besides the b stream is sent from streaming media server to mobile terminal at time t+vi.

However, PMPatching strategy is very different from UPatch strategy because the length of Pi do-main should be smaller than m/bi so as to prevent overflow of memory space of mobile terminals.

If m/bi<vi, UPatch strategy is not applicable for the MSMS. The patching stream cannot be used be-cause of the limited memory space of mobile termi-nals. Therefore, the transmission costs can be re-duced only by batching all the requests for program i that arrive in a time segment whose length is m/bi. That time segment is defined as an m domain whose starting time is the arrival time of a request for pro-gram i.

As when m/bi ≥ vi, the whole time axis is divided into several partitions by m domain according to the arrival time of requests for program i. The starting time of the current m domain is marked as Tm as shown in Fig.3. When a new request for program i arrives at time t, if t ≤ Tm+m/bi, the request will be-long to the current m domain. Otherwise, the request will not belong to the current m domain and a new m domain whose starting time Tm is t will be created. The rest can be deduced by analogy. So the requests for program i that arrive at different time fall into different m domains.

LIAO et al. Proxy-based Patching Stream Transmission Strategy in Mobile Streaming Media System 517

Fig.2 PMPatching stategy

(m/bi ≥ vi) Fig.3 PMPatching stategy

(m/bi<vi) For any request for program i that arrives in m

domain, proxy sends segment [0, m/bi] of the prefix immediately to the mobile terminal that sends the request. At the stopping time of the m domain, namely, at time Tm+m/bi, proxy sends segment [m/bi, vi] of the prefix to all terminals that had sent re-quests for program i in the m domain. At time Tm+vi, proxy gets suffix from the server by a unicast stream, marked as m stream and sends it to all mobile ter-minals that had sent requests for program i in the m domain.

IV. Performance Evaluation Suppose the arrival of the request for streaming

media program complies with the Poisson process. The average request rate for program i is ,iλ the network transmission cost on the server-proxy path and on the proxy-terminal path is cs (per bit) and cp (per bit) respectively, and the bit rate bi of program i is 1.5Mbps. Since the memory space of the mobile terminal is very small, we suppose that the memory space of the mobile terminal can accommodate 0.4, 0.8 or 1.2 minutes streaming media data in the simulated experiment. To evaluate the performance of the PMPatching strategy, it is compared with the following transmission strategy: without the help of proxy, server unicasts program to mobile terminal directly. This is shortened as Unicast strategy. 1. Average transmission cost

When m/bi ≥ vi, like UPatch strategy, average transmission cost[8] in a unit time segment for deliv-ering program i is

( / 2)1 ( )

2s i i i i

i p i i ii i i

c L v PC c L bv Pλ λ

λ⎛ ⎞− + ⎟⎜ ⎟= +⎜ ⎟⎜ ⎟⎜ + +⎝ ⎠

(1)

Because m is limited, that is different from UPatch strategy, the length of Pi domain must not be longer than m/bi, that is, 0 ≤ Pi ≤ m/bi−vi. Pi should make the average transmission cost minimal, so it is

{ , // , /

i ii

i i i i

x m b v xP m b v m b v x≥ += − < + (2)

where x is( )21 ( ) 2 1 .i i i i i i iv L vλ λ λ λ+ + − −

When m/bi<vi, the average number of requests for program i in an m domain is 1 / .i im bλ+ Only a single suffix of [vi, Li] is transmitted from server to

proxy for these requests. For every request, the whole program whose length is Li will be transmit-ted from proxy to mobile terminal. So average transmission cost in a unit time segment for deliver-ing program i is

( )1 /

s i ii p i i i

i i

c L vC c L bm b

λλ

⎛ ⎞− ⎟⎜ ⎟= +⎜ ⎟⎜ ⎟⎜ +⎝ ⎠ (3)

When using Unicast strategy, for each request, a unicast stream will be generated to transmit the whole program. Number of requests for program i in unit time segment is ,iλ so average transmission cost in a unit time segment for delivering program i is

iiipsi bLccC λ)( += (4)

Obviously, there is no difference between trans-mission cost from proxy to terminal under PMP atching strategy and that under Unicast strategy. The difference only exists in transmission cost on the backbone network from server to proxy. So we will only compare the backbone transmission cost under the two strategies.

The saving of the backbone transmission cost under PMPatching strategy in contrast to that under Unicast strategy is

/ 21 ,(1 ( ))

CSaving( )

,/

2i i i i

ii i i i i

i ii i i

i i i

L v P m vL v P b

imL vL L v

1 m b b

λλ

λ

⎧⎪ − +⎪ − ≥⎪⎪ + +⎪⎪=⎨⎪⎛ ⎞⎪ − ⎟⎪⎜ − <⎟⎜⎪ ⎟⎜⎪ ⎟+⎝ ⎠⎪⎩

(5)

2. Average number of channels in streaming media server

Upatch strategy does not develop how to calcu-late average number of channels in streaming media server, we will work out its expression.

To simplify the following discussion, the chan-nels to transmit b stream, m stream and p stream are named b channel, m channel and p channel respec-tively.

When m/bi ≥ vi, the system comprises b channels and p channels. For program i, the lifecycle of the b channel is ,i iL v− so the maximal number of b channels in system is the maximal number of Pi domains existing in the period whose length is

.i iL v− The length of Pi domain is vi+Pi, so the maximal number of b channels is ( ) /( ) .i i i iL v v P⎡ ⎤− +⎢ ⎥ Based on CallGap algorithm[10], the average number of b channels is ( ) /( 1/ ) .i i i i iL v v P λ⎡ ⎤− + +⎢ ⎥

For any request for program i arriving in seg-

518 JOURNAL OF ELECTRONICS (CHINA), Vol.23 No.4, July 2006

ment [TPi+vi, TPi+vi+Pi], system generates a p stream and occupies a p channel. Fig.4 shows when the p channel is generated.

Fig.4 The time to generate a p channel

As shown in Fig.4, the starting time of the Pi domain is TPi. Suppose a request for program i ar-rives at time t and in segment [TPi+vi, TPi+vi+Pi], system will generate a p stream at time t+vi and the p stream will last for a period whose length is t−TPi−vi. Obviously, all the p streams that are gen-erated in a Pi domain only exist in segment [TPi+2vi, TPi+2vi+2Pi]. Suppose that segment consists of 2n little segments whose length is .δ Assume δ is ex-tremely small and the p channels appearing in the little segment exist in the whole little segment. In No.i (0 )i n≤ < little segment [TPi+2vi+i ,δ TPi+ 2vi+(i+1) ]δ of the segment [TPi+2vi, TPi+2vi+Pi], the number of p channels is ( / 2 ) ,iiδ δ λ+ and the total number of p channels ever existing in the seg-ment [TPi+2vi, TPi+2vi+Pi] is 2( 3 ) / 4.in n δλ+ In No.i (0 )i n≤ < little segment [TPi+2vi+2Pi−i ,δ TPi+2vi+2Pi−(i−1) ]δ of the segment [TPi+2vi+Pi, TPi+2vi+2Pi], the number of p channels is / 2,iiδλ and the total number of p channels ever existing in the segment [TPi+2vi+Pi, TPi+2vi+2Pi] is 2(n +

) / 4.in δλ Therefore, the total number of p channels ever existing in the segment [TPi+2vi, TPi+2vi+2Pi] is ( 2 ) / 2,2

in n δλ+ and the average number in the segment [TPi+2vi, TPi+2vi+2Pi] is / 4 / 2.i i iPλ λ δ+ Since δ is extremely small, the average number in the segment is / 4.i iPλ These p channels will be generated in any Pi domain and the length of Pi do-main is vi+Pi. Based on CallGap algorithm[10], the average number of p channels in the whole time axis is 2 /( 1/ ) / 2i i i i iP v Pλ λ+ + .

The average number of channels for program i in streaming media server is sum of the average num-ber of b channels and p channels, that is:

2( ) /( 1/ ) /( 1/ ) / 2i i i i i i i i i iL v v P P v Pλ λ λ⎡ ⎤− + + + + +⎢ ⎥ (6)

Thereinto value of Pi is determined as Eq.(2). When m/bi<vi, patching streams can not be used,

the system only comprises m channels. For program i, the lifecycle of the m channel is .i iL v− As when m/bi ≥ vi, the maximal number of m channels in sys-tem is the maximal number of m domain existing in the period whose length is .i iL v− The length of m domain is m/bi, so the maximal number of m chan-

nels is ( ) / .i i iL v b m⎡ ⎤−⎢ ⎥ Based on CallGap algo-rithm[10], the average number of channels for pro-gram i in streaming media server is

( ) ( )1i i i iL v m b λ⎡ ⎤− +⎢ ⎥ (7)

When using Unicast strategy, for program i, the average number of channels in streaming media server is

ii Lλ (8) Thus the saving of the number of channels for

program i in the streaming media server under PMPatching strategy in contrast to that under Uni-cast strategy is

() ( )

( )

1 ( ) /( 1/ )/( 1/ ) / 2 /

/NSaving( )

1 ( ) /( / 1/ ) //

i i i i i2

i i i i i i i

i i

i i i i i i

i i

L v v PP v P L

m b vi

L v m b Lm b v

λλ λ λ

λ λ

⎧ ⎡ ⎤⎪ − − + +⎪ ⎢ ⎥⎪⎪ + + +⎪⎪⎪ ≥=⎨⎪⎪⎪ ⎡ ⎤− − +⎪ ⎢ ⎥⎪⎪ <⎪⎩

(9) 3. Simulation results

Suppose the bit rates of all programs are the same, m can be denoted by the length of the stream-ing media data that the memory space of mobile teminals can store. To make the discussion about simulation results perspicuous, we will use minute (min) as unit of m in the following text.

Fig.5(a) plots the saving of the backbone trans-mission cost under PMPatching strategy in contrast to that under Unicast strategy, CSaving (i), versus prefix size when Li is 120min, iλ is 1/min and m takes different values, Fig.5(b) plots the CSaving (i) versus prefix size when Li is 120min, m is 0.8min and iλ is given different values.

Fig.5 CSaving(i) versus prefix size

Fig.6(a) plots the saving of the number of chan-nels in the streaming media server under PMPatch-ing strategy in contrast to that under Unicast strategy, NSaving(i), versus prefix size when Li is 120min,

iλ is 1/min and m takes different values, Fig.6(b) plots the NSaving(i) versus prefix size when Li is 120min, m is 0.8min and iλ is with different values.

LIAO et al. Proxy-based Patching Stream Transmission Strategy in Mobile Streaming Media System 519

Fig.6 NSaving(i) versus prefix size

We observe from Fig.5 and Fig.6 that when the memory space of mobile terminals, the request arri-val rate for the program and the prefix for the pro-gram stored in proxy increases, PMPatching strategy saves more backbone transmission cost and channels in streaming media server.

PMPatching strategy results in substantially saving of the backbone transmission cost and the number of channels in the streaming media server over Unicast strategy. For instance, when the length of the prefix vi is 24min, 20% of the length of the program, the memory space of mobile terminals m can hold 0.4min long streaming media data and the request arrival rate iλ is 10/min, PMPatching strat-egy can save backbone transmission cost by 84.1% and number of channels by 84.0% over Unicast strategy.

Furthermore, when prefix size is very small or even 0, PMPatching strategy can reduce transmis-sion cost and number of channels significantly. For instance, when vi is 0, m is 0.4min and iλ is 1/min, PMPatching strategy can save backbone transmis-sion cost by 28.5% and number of channels by 28.3% over Unicast strategy.

It is obvious that PMPatching strategy adapts to the current MSMS perfectly. For mobile terminals with little memory space, an important feature of MSMS, PMPatching strategy can save transmission cost and number of channels substantially even when length of the prefix is very small.

V. Conclusions In this paper we have proposed an innovative

PMP atching transmission strategy considering the salient features that the backbone network and the access network of the MSMS only supports unicast and that the memory space of mobile terminals is very limited and worked out the calculation for-mula for the average network transmission cost and the average

number of channels under this strategy. The experi-mental results prove that PMPatching strategy is fully applicable for the mobile streaming media sys-tem. It can dramatically cut down the network transmission cost and the number of channels in the streaming media server, and it also shows important practical value in the deployment and research of the mobile streaming media system.

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