A Smoothing Algorithm for Time Slicing DVB-H Video Transmission with Bandwidth Constraints Pietro Camarda, Giovanni Tommaso Carone, Domenico Striccoli Politecnico di Bari - Dipartimento di Elettrotecnica ed Elettronica (DEE) Via E.Orabona, 4 - 70125 - BARI {camarda, d.striccoli}@poliba.it ,
[email protected]
ABSTRACT DVB-H transmission is assuming an ever growing importance for multimedia data delivery to hand-held terminals of reduced size and limited battery capacities. To minimize terminals power consumption, DVB-H systems adopt a time sliced transmission in bursts. In such a context, in this paper a new smoothing algorithm, the Smoothing Algorithm of the Burst (SAB), suitable for the transmission of multimedia streams with high bit rate variability, has been proposed and analyzed. Such an algorithm takes into account receiving buffer size, burst size and available bandwidth variability to reschedule data transmission, with the target to minimize the percentage of loss due to bandwidth and buffer limitations. Numerical results obtained by simulation show the improved performance of SAB compared with the usual unsmoothed data transmission in bursts. SAB can thus be utilized to transmit video streams with higher bit rate and better quality than actually considered in DVB-H systems.
Figure 1. Structure of a DVB-H system.
In Figure 2 the time slicing parameters are illustrated in detail.
Keywords DVB-H, Time Slicing, Smoothing, Available Bandwidth.
1. INTRODUCTION Digital Video Broadcasting (DVB) is actually one of the most significant technological challenges of our time. The natural evolution of the Terrestrial DVB (DVB-T) system is the broadcast diffusion of interactive TV on hand-held terminals like cellular phones, Personal Digital Assistants (PDAs), notebooks, etc. by almost totally exploiting the same Terrestrial DVB (DVB-T) network infrastructure [1]. The main challenge of the DVB for Handheld terminals (DVB-H) is to adapt the delivery of high quality multimedia streams to hand-held terminals, characterized by reduced size and weight, and limited battery capacities [2][3]. At data link layer, the DVB-H system adopts a Time Division Multiplexing (TDM) technique, known as “time slicing”, to optimize energy consumption and to facilitate the handover procedure. A Multi Protocol Encapsulation-Forward Error Correction (MPE-FEC) protocol is implemented to improve system tolerance towards noise [2]. At receiving side, DVB-H receiver includes components to manage time slicing and MPE-FEC, as illustrated in Figure 1. The time slicing technique transmits data in periodic “bursts” at higher bit rate if compared with a continuous data transmission. The burst size is the total amount of data in a burst. Bursts of different services are then time multiplexed into a continuous and uninterrupted data flow. The single receiver remains active only for the time fraction corresponding to the requested service burst duration, while it deactivates the reception circuitry during the “off-time” period. The terminal power consumption is thus reduced [3].
Figure 2. Time Slicing parameters.
The receiver should be able to receive and store data only during the burst duration, decoding and displaying them continuously in time without losses. This means that each burst size should be smaller than the memory available in the receiver. If this can be acceptable for multimedia streams with relatively low bit rate, it could be a serious limitation for high quality Variable Bit Rate (VBR) video streams and relatively small terminal buffer sizes. In fact, when the stream bit rate is high, the burst size could not store all necessary data needed in the decoding phase of the burst cycle duration (burst duration + off time). While if the VBR stream rate is relatively small, the available bandwidth during the burst duration could not be entirely exploited, with consequent bandwidth waste. Finally, available bandwidth fluctuations and reductions due to channel noise or interferences could seriously jeopardize data delivery without losses. In fact, when the channel bandwidth is low data stored into the client buffer could not be enough to guarantee the continuous playback at receiving side. In this paper, a video smoothing technique operating at transmission side is proposed and analyzed. The proposed smoothing algorithm exploits optimal data allocation of VBR video streams in bursts in
order to minimize video frame losses at receiving side. Losses can occur because of limited burst and buffer sizes and/or bandwidth reduction. Bandwidth resources, video data available during each burst and available buffer at the receiver are maximally exploited to store as many data as possible, in order to guarantee continuous playback without losses in each burst cycle . The rest of the paper is organized as follows. In section II the smoothing principles are reported. In section III the specific algorithm developed in this paper will be described and analyzed. In Section 4 its performance with respect to data losses will be compared with unsmoothed VBR data transmission in bursts. Numerical results will be derived for different buffer sizes, burst durations and available bandwidth conditions. Finally in Section 5 some conclusions will be given on the effectiveness of the proposed method.
k
k
0
0
Funder ( k ) = ∑ f i ; F 'over ( k ) = b + ∑ f i
(1)
where fi is the i-th frame size (in bits). These two curves are calculated without taking into account bandwidth limitations and the slotted transmission in bursts. The SAB calculates also the cumulative transmission plan S (k ) as the sum of the bits s i to be transmitted in the i-th frame time of the smoothed schedule , until the k-th frame time : k
S (k ) = ∑ si
(2)
0
As obvious, is has to be:
2. SMOOTHING ISSUES As known, VBR video traffic is characterized by a high bit rate variability, self-similarity and burstiness over different time scales [4][5]. To reduce traffic high variability, that would bring to a waste of bandwidth resources and frequent data losses, smoothing techniques have been introduced. Exploiting a buffer at receiving side, smoothing algorithms reschedule transmitted data according to a transmission plan that avoids receiving buffer overflows and underflows [6][7] [8]. The transmission plan results into a series of Constant Bit Rate (CBR) pieces whose number and size depend on the particular adopted algorithm. Several smoothing techniques have been developed in literature, minimizing peak rate, or bit rate variability among CBR pieces [9]. They have been developed for stored video, when the whole bit rate behavior of the stream to be smoothed is known in advance. The same smoothing principles can be adopted for real-time traffic, where only a small portion of a stream bit rate is known for smoothing [10] [11]. Other algorithms take also into account available bandwidth constraints to further optimize the smoothed transmission plan, minimizing frame losses [12][13] [14]. The main purpose of this work is to exploit a smoothing algorithm for video stream transmission, that takes into account the DVB-H characteristics and available bandwidth. The smoothed video transmission plan considers time slicing limitations, i.e., data transmission that is not continuous in time. Since data can be sent only during burst durations, immediately followed by inactivity time periods (off-times), the algorithm will try to fully exploit each burst capacity to schedule enough data for continuous playback without losses also during off-times at receiving side. The proposed algorithm takes into account also available bandwidth limitations. This last condition is very useful in practical DVB-H applications, where channel bandwidth can fall down because of interferences during service transmissions or noise.
3. THE SMOOTHING A LGORITHM OF THE B URST
(SAB) The smoothing algorithm proposed in this paper, called Smoothing Algorithm of the Burst (SAB), exploits some basic principles of the Minimum Variability Bandwidth Allocation (MVBA) algorithm developed in [8], suitable for continuous transmission, but adapting it to a slotted transmission of video data in bursts. This algorithm can be applied for real-time video transmission, where only a portion of the video stream is known and thus subject to smoothing in bursts. Given the receiver smoothing buffer size b, the SAB calculates the buffer underflow and overflow curves at k-th discrete frame time, as explained in [8]. The basic time unit is the time interval necessary for the transmission of the single video frame, that is, (1/25) sec for PAL and (1/30) sec for NTSC. The underflow and overflow curves are respectively :
Funder (k ) ≤ S (k ) ≤ F 'over (k ) ∀k . To take into account the burst transmission typical of time slicing in DVB-H, the SAB considers the maximum burst capacity of BU bits, representing the stream information contained within the burst. Obviously, BU varies with the burst size. The upper bound
F 'over (k ) is thus modified by SAB in such a way that the maximum cumulative data amount the DVB-H terminal can receive does not exceed BU along the entire burst cycle : Fover ( k ) = min { F ' over ( k ), BU } with F 'over (k ) given by (1). Exploiting the MVBA principles [7], the SAB calculates the maximum bit rate cmax necessary to transmit data without experimenting a buffer overflow, but only in the burst duration period [Tbi , Tbs ] , where the initial buffer fill level in Tbi is
q = S (Tbi ) − Funder (Tbi ) . That is: cmax =
F (k ) − ( Funder (Tbi ) + q ) min over k − Tbi
Tbi +1 ≤k ≤Tbs
(3)
Let us note that q represents the bit amount not consumed for playback from the previous burst cycle (if any), while the information on the burst capacity is contained in Fover (k ) . Similarly, the SAB calculates the minimum bit rate cmin necessary to transmit video data without experimenting a buffer underflow in [Tbi , Tbs ] :
F (k ) − ( Funder ( Tbi ) + q) cmin = max under Tbi +1 ≤k ≤Tbs k − Tbi
(4)
Also Tbi and Tbs are supposed an integer multiple of a frame time. The calculation of the transmission plan is performed like MVBA, exploiting (3) and (4). The SAB takes into account time slicing limitations by considering that in [Tbi , Tbs ] smoothed data can enter and leave the receiving buffer, while during the off time period
Tbs , Tcycle , until the end of the burst cycle Tcycle , video data can not enter the buffer, since during the off-time the receiver is not active. During off-time data can only leave the buffer. The corresponding cumulative transmission plan S (k ) in Tbs , Tcycle will thus be an
horizontal segment S ( k ) = S (Tbs ) and the corresponding smoothed bit rate at receiving side will be null. An example of the cumulative transmission plan in a burst cycle is given in Figure 3. The SAB verifies the existence of a transmission plan S in each burst cycle , exploiting the MVBA algorithm in
[Tbi , Tbs ]
with cmax
calculated for the burst duration by applying MVBA (line 7), but taking into account bandwidth limitation (lines 8 and 9). In line 11, the horizontal segment of S is traced for the off-time period. Control on total losses within the burst cycle is performed in lines 13 and 14. The algorithm is then applied along the total number of burst cycles N burstTOT , until the end of the stream transmission. ,
and cmin given by (3) and (4) respectively, and ending the cumulative transmission plan with a horizontal segment
S = S (Tbs )
in
Tbs , Tcycle . If the transmission plan crosses the Funder curve, losses will occur due to buffer or burst size limitations. Specifically, the losses represent the quantity of bit that are not available for playout. The SAB is also able to quantify such losses, as the distance between Funder and S after the cross. An example of this can be given in Figure 3, where total losses (in bits) are quantified as the distance between Funder and S on the extreme right of the figure, in the 143-th frame time. If available bandwidth in the k-th frame time, indicated by Bw ( k ) , is consistently low during the transmission of a burst, the SAB takes into account this limitation by introducing a constraint on the maximum CBR segment slopes characterizing the cumulative smoothed transmission plan. In fact, a bandwidth limitation translates into a maximum bit rate of each CBR segment. Bandwidth limitations are obviously considered only during a burst duration, since during the off-time there is no data transmission. Let us note that a limitation on the maximum slope of S (k ) for each Tbi ≤ k ≤ Tbs means that the cumulative transmission plan is closer to the Funder curve, thus increasing the buffer underflow probability during off-time. This is obvious, since if there is any bandwidth constraint that limits the total amount of data in a burst, these could not be sufficient to guarantee a continuous stream playback during the entire off-time period. The receiving buffer could become empty before the end of the off-time period; a buffer underflow thus occurs. 3
x 10
2.5 over
Bits
2
1.5
DEFINE BU , b , q = 0 , N burst = 1 ; REPEAT FOR k from Tbi to Tcycle k k Funder ( k ) = ∑ f i ; Fover ( k ) = min b + ∑ f i , B ; Bw ( k ) 0 0
5 6 7
END CALCULATE S ( k ) in [Tbi , Tbs ] through MVBA
8
IF S ( k ) - S ( k − 1) > Bw ( k )
9 10 11
END
12
S ( k ) = S ( k − 1) + Bw ( k ) S ( k ) = S (Tbs ) ∀k ∈ Tbs , Tcycle FOR k from Tbi to Tcycle IF S ( k ) < Funder ( k )
13
TotalLoss ( k ) = Funder (k ) − S ( k )
14 15 16
END END
{
}
17
q = max ( S (Tcycle ) − Funder (Tcycle )) ,0
18
N burst = Nburst + 1 ;
19
UNTIL N burst = NburstTOT ,
Figure 4. the SAB in its formal presentation.
4. NUMERICAL RESULTS
Transmission plan
1 F 0.5
0 0
DEFINE Tbi , Tbs , Tcycle ;
2 3 4
To test the SAB effectiveness, we first present its cumulative transmission plan applied on a single VBR video stream, “The silence of lambs”, MPEG-1 codified, of length 5.000 video frames, as reported in Figure 5. The SAB is applied, considering a receiving buffer size of 200 kB, a burst duration of 120 ms, an off-time of 5,8 s. The burst size, representing the amount of video information in a burst, has been fixed to 188,81 kB. These are all standard DVB-H parameters, as reported in [3]. No bandwidth limitation is considered in this example.
6
F
1
20
40
60
under
80
Frame time [k]
100
120
140
Figure 3. Transmission plan with losses.
In Figure 4 the SAB is formally presented. Lines 1 and 2 set the main parameters: times of burst beginning ( Tbi ) and end (Tbs ), the burst cycle duration Tcycle , the burst size BU and the buffer size b . The buffer queue q is initially null for the first burst to be transmitted ( N burst = 1 ). After the definition of the Funder , Fover and available bandwidth Bw curves (lines from 4 to 6) the transmission plan S is
As it can be seen from Figure 5, the SAB transmission plan (continuous black curve) consists on high sloped segments, representing data transmitted during bursts, followed by horizontal segments representing the absence of transmitted data during off-times. Black arrows point out data losses occurring whenever the SAB curve crosses the buffer underflow bound (red curve), thus pointing out losses due to buffer underflows. In this case, losses happen because of the burst size that limits the maximum amount of video data to be transmitted. As previously explained, it derives that the total amount of data contained in a burst, never exceeding the burst size, are not sufficient to guarantee the continuous data playback outside the receiving buffer. Let us note that this happens independently from the buffer size. In this case, the buffer will receive less data than necessary due to the burst limitation. To test the SAB performance, we calculate the percentage of lost bits for different buffer sizes, during the transmission of 5.000 video frames of “The silence of lambs”, MPEG-1 codified, smoothed by SAB. Available bandwidth limitations are not considered. We repeat the same evaluation for the unsmoothed transmission of the same video stream, in which video frames are sent in bursts according to the
DVB-H standard, but without taking into account the receiving buffer size. Each burst is filled in such a way to guarantee the continuous stream playback for the whole burst cycle. At receiving side, like the smoothed case losses can occur for different reasons. Firstly, for buffer size limitations as explained for the smoothed case. Secondly, losses can occur for buffer overflow. In fact, even if the burst size is sufficient to store video frames for playback in a burst cycle, the receiving buffer could not be able to store them all because of its limited size. This situation is not present for SAB transmission because of the smoothed transmission plan that avoids buffer overflows. The comparison between smoothed and unsmoothed transmission is illustrated in Figure 6. 7
4.5 x 10 4 3.5
Bits
3 2.5 2 1.5 1
for buffer overflows and underflow; this last are still due to burst size limitation. In Figure 7 the comparison between the SAB and the unsmoothed video transmission in bursts has been performed by calculating the percentage of lost bits with different burst sizes. Let us remember that an increased burst size means an increased off-time correspondingly, because of the time multiplexing of the same number of DVB-H services [3]. The buffer size is fixed to 200 kB, and the lost bits have been calculated on the same piece of video stream, of length 5.000 frames and without any available bandwidth constraint. The SAB always performs better than the uns moothed transmission for each considered burst size. In particular, losses increase with burst size for the unsmoothed transmission. This happens because a greater burst size contains more useful data that the receiving buffer could not store accordingly; losses for buffer overflow thus increase. As regards SAB, for smaller burst sizes bit losses are mainly due to the limited capacity of the burst to contain useful data. After the minimum loss percentage, reached when burst and buffer sizes almost coincide, losses increase with burst size. This happens because, as previously explained, when burst size increase, the off-time increase accordingly. Since the buffer size remains unchanged, received data in a burst cycle are not suffic ient to guarantee continuous playback during the whole off-time , thus increasing the frequency of buffer underflows.
0.5 0 0
30
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3000
Frame Time [k]
3500
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4500
5000
25
Figure 5. SAB cumulative transmission plan.
SAB Unsmoothed
Lost bits [%]
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60 Unsmoothed SAB
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Figure 7. Lost bits vs burst size for SAB and comparison with unsmoothed transmission.
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Buffer size [kB]
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Figure 6. Lost bits vs buffer size for SAB and comparison with unsmoothed transmission.
The burst size has been fixed to 188,8 kB. As reported in Figure 6, in both cases losses decrease with buffer size increase, since a higher buffer capacity obviously better prevents from overflow and underflow situations. Nevertheless, SAB (blue curve) experiments minor losses than the unsmoothed transmission (red curve), due to its capacity to redistribute transmitted data to further prevent buffer underflows and overflows. It can be noted that the SAB curve has an asymptotic behavior after a buffer size of about 400 kB. This means that for buffer size of 400 kB or more, the only losses are due to buffer underflows deriving from the burst size limitation, despite the optimized smoothed transmission plan. For smaller buffer sizes, bit losses occur for buffer limitation that does not guarantee a fluid transmission. Bit losses of the unsmoothed transmission include losses
In Figure 8 the SAB performance without available bandwidth limitation is compared with the one obtained introducing available bandwidth constraints. The lack of available bandwidth has been simulated by forcing the available bandwidth to randomly fall down into burst durations (the lack of available bandwidth for the single transmitted service is not relevant during off-times, when no data arrive to the receiver). Comparisons between the SAB and the unsmoothed transmission, both with bandwidth constraints, have been omitted for brevity, but results are quite similar to those obtained in absence of bandwidth limitations (see Figures 6 and 7). The red curve of bit losses with bandwidth constraints has been obtained by averaging losses occurred in 20 different simulations, each one obtained by randomly varying the available bandwidth profile . As expected, bit losses increase with available bandwidth limitation respect to the optimal SAB transmission plan without bandwidth constraints. This is due to an increase of buffer underflow situations, even if the smoothed transmission plan is optimized to minimize them. It has to be remembered (see Section 3) that a bandwidth constraint limits the total amount of data in a burst during the burst time, and these are generally not sufficient to guarantee continuous playback during the subsequent off-time in a cycle.
[2] 8.5
[3]
SAB withtout bandwidth constraints SAB with bandwidth constraints
8
[4]
Lost bits [%]
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[5]
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4.5 160
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Burst size [kB]
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Figure 8. SAB performance with and without bandwidth constraints.
5. CONCLUSIONS In this paper a new smoothing algorithm, the Smoothing Algorithm of the Burst (SAB), is proposed for the optimization of VBR high quality video transmission that exploits DVB-H time sliced bursts. The proposed algorithm redistributes data sent by transmitter into a transmission plan that takes into account the burst size, the terminal buffer size and eventually available channel bandwidth fluctuations during bursts transmission. Numerical results prove the improved SAB performance if compared with the standard DVB-H unsmoothed transmission. The SAB ability to regulate transmitted data flow to minimize losses makes it particularly suitable for transmission of video streams with higher bit rate than actually considered in DVB-H systems.
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ETSI EN 302 304: "Digital Video Broadcasting (DVB); Transmission System for Handheld Terminals (DVB-H)". ETSI TR 102 377: "Digital Video Broadcasting (DVB); DVB-H Implementation Guidelines". M.W. Garrett, W. Willinger, “Analysis, Modeling and Generation of Self-Similar VBR Video Traffic” ACM Computer Communications Review, Vol. 24, No. 4, pp. 269-280, October 1994.
ETSI EN 300 744: "Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television".
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