,
, and
?
Multimedia Laboratory
Department of Computer Science and Engineering
University of California at San Diego
La Jolla, CA 92093-0114
E-mail: f sramanat, venkat, vin g
@cs.ucsd.edu
Abstract. In order to provide efficient frame loss guarantees for video communication over ATM-like fast packet switched networks, we propose a simple to implement, yet effective, strategy called Frame-Induced Packet Discarding
(FIPD), in which, upon detection of loss of a threshold number (determined by an application’s video encoding scheme) of packets belonging to a video frame, the network attempts to discard all the remaining packets of that frame. Performance simulations are shown to demonstrate the efficacy of the FIPD strategy; networks employing FIPD exhibit close to two-fold increase in the number of video channels that they can support.
Advances in networking are making it feasible to support a wide spectrum of video services over fast packet-switched networks [5, 10, 8, 12]. An important problem in supporting video communication over computer networks is Quality of Service (QoS) management, which refers to strategies for allocating network resources so as to guarantee real-time video playback [1, 4, 7]. The design of a novel QoS management strategy targeted at supporting video applications efficiently over ATM-like fast packet-switched networks constitutes the subject matter of this paper.
In order to support the QoS needs of video applications, integrated networks must offer guarantees of minimum bandwidth, and maximum end-to-end delay, delay jitter, media loss, and asynchrony. Among the above, bandwidth, delay and loss guarantees can be regarded as fundamental, since they have to be provided by the network layer 1 , possibly by efficiently allocating the network resources, such as processing time and buffer space at network switches [4]. Delay-jitter and synchronization guarantees, on the other hand,
?
Current address of Harrick M. Vin: Department of Computer Sciences, University of Texas,
Austin, TX 78712-1188
1
This is assuming that media losses at the network layer cannot be compensated for at higher layers using retransmission-based schemes, mainly due to the real-time nature of video.

can be supported at higher layers of the network architecture (using bandwidth, delay and loss guarantees provided by the network layer) [11]. Furthermore, if the buffering at the network switches is bounded, violation of either the bandwidth or the delay constraint at the network layer induces buffer overruns at the switches, resulting in media losses 2 .
Hence, if the buffers at network switches are appropriately sized, network-layer QoS management strategies only need to explicitly control buffer overruns at the switches.
At the network level, QoS requirements are both expressed and enforced in terms of transport units, such as packets (or cells in ATM terminology). The QoS requirements of video applications, on the other hand, are generally in terms of video frames, each of which may be composed of multiple packets. Merely using the frame-level QoS requirement of a video application as the packet-level QoS requirement at the network level may not be sufficient. To see why, consider a scenario in which video frames are encoded using the DPCM algorithm [13]. Although it achieves a high degree of compression, the DPCM algorithm is highly susceptible to losses; loss of even one of the packets of a frame will result in the loss of that entire frame. Suppose each video frame is composed of 100 packets, and that an application requests for a 1% bound on the frame loss. If the network were to provide a 1% bound on the packet loss instead, in the pathological case, one packet could be lost for every consecutive 100 packets transmitted by the application, thereby resulting in the loss of each and every video frame. Of course, a straightforward way in which the network can guarantee the application-requested 1% bound on the frame loss is by providing a 0
:
01% bound on the packet loss. However, this constitutes an overly stringent demand on the network, potentially leading to its under-utilization.
Based on the above reasoning, we advocate that future integrated networks must implement mechanisms that can exploit video application specified hints to better utilize the network resources. Doing so will not unduly complicate the network design; rather, such an approach will be mutually beneficial to the video application and to the network provider: from the application’s perspective, such an approach relieves the application of mapping its requirements to the network domain, and from the network’s perspective, such a design enables the network to provide exactly what the application requires and no more, thereby optimally utilizing its resources.
In order to satisfy frame-level QoS requirements of video applications, in this paper, we propose a simple to implement, yet effective, packet discarding strategy called
Frame-Induced Packet Discarding (FIPD). Section 3 elaborates upon the FIPD strategy.
Section 4 outlines an admission control procedure that we have developed for networks employing the FIPD strategy. Section 5 presents trace-driven performance simulations that we have carried out for evaluating the practical utility of FIPD. Finally, Section 6 concludes the paper.
2 Since errors during transmission over fiber-optic networks are negligible, we assume that the communication links are reliable and hence, buffer overruns at the switches are the only cause of packet loss.

In order to provide efficient frame loss guarantees for video communication, we propose a simple to implement, yet effective, strategy called Frame-Induced Packet Discarding
(FIPD). This strategy attempts to exploit the inevitability of loss of a frame on a video channel
3 whenever a subset of the packets constituting that frame is lost. The subset of packets (of a frame) whose loss invalidates the frame, henceforth referred to as Packet
Resiliency, depends on the encoding scheme employed. In the simplest case, as in the basic DPCM algorithm, loss of even one of the packets of a frame may result in the loss of that entire frame. In a more general case, such as when forward error correcting schemes are employed [3], the loss of upto a certain threshold number of packets can be tolerated before the corresponding frame is lost. A switch that implements the FIPD strategy forcibly discards all the packets constituting a video frame whenever it detects the loss of more than the threshold number of packets of that frame. The network bandwidth and buffer space released by the packets thus discarded can be reallocated to other video channels, thereby permitting networks employing the FIPD strategy to admit and service a larger number of video channels as compared to those that only consider packet-level QoS strategies.
One of the attractive features of the FIPD strategy is the ease of its implementation:
In order to selectively discard packets, network switches must identify frame boundaries, by recognizing packets that constitute the same video frame. A straightforward way is for the video source itself to associate a “frame-identifier” bit with each packet, and to flip this frame-identifier bit for successive frames (for instance, the frame-identifier bit could be 0 for the first frame, 1 for the second, 0 for the third frame, and so on). If at least one packet of each frame is guaranteed loss-free transmission at all switches enroute to the destination, each of those switches can easily identify frame boundaries. However, for implementing this scheme, one buffer must be reserved per-channel at each switch.
To avoid this overhead of explicit buffer reservation at switches, we propose a simple bit-switching technique (so named because of its resemblance to the “label-switching” technique used in ATM networks for routing cells between input and output links). In this technique, as before, each video source distinguishes between packets of successive frames that it transmits by flipping the frame-identifier bit. However, in addition, for each video channel, each switch en route maintains the frame-identifier of the last frame whose packets it forwarded to the next switch, and flips this value for all the packets of the next frame that it is able to forward on the same video channel. This way, packets of frames that arrive one after another at each switch are guaranteed to have toggled values of the frame-identifier, and hence, each switch can correctly identify frame boundaries.
In an ATM network, the cell-loss priority (CLP) bit in the cell header can serve as a frame-identifier. Alternatively, the AUU bit, which is slated for inclusion in the payload type field of the ATM cell header to enable AAL5 compatible adaptation layers to reassemble fragmented frames at destinations, can also be used as the frame-identifier.
Clearly, FIPD is simplest to implement when intra-frame encoding schemes (such as JPEG) are employed. When inter-frame encoding schemes (such as MPEG) are employed, information present in one video frame (I-frame) may be used for reconstructing
3 A video channel is a source to destination network connection for video communication.

subsequent video frames (P and B frames), and hence, a single video frame loss at the network may, in fact, trigger multiple frame losses as seen by the application. In this case, it is simplest to regard a group of related frames as a meta-frame and to implement the FIPD strategy by considering all packets within a meta-frame as “related”. Alternatively, if the network can distinguish between different types of frames (e.g. I, P, and B frames in the case of MPEG), the switches can implement different packet resiliencies for different frames (for instance, the packet resiliency for an I-frame may be much higher than that of a B-frame).
For video channels whose packet resiliency exceeds one, each switch must also maintain a count of the packets dropped for each frame. Since packet losses can occur at different switches at different times and none of the switches may have global information about the other switches, the packet resiliency must be partitioned amongst all the network switches en route to the destination. Each of the switches can independently monitor packet loss at its buffers, and discard frames as and when the per-switch packet resiliency bound is violated.
In order to analytically quantify the performance of FIPD so as to obtain fractional frame losses that can be guaranteed to video channels, we have developed a finite state, discrete time markov chain model of the FIPD strategy. The fractional frame loss thus computed can serve as the criterion for admission control at the network.
In this procedure, when an application request for a video channel is received by the network, an underlying routing mechanism yields a candidate path for the video channel. The video channel is established only if: (i) each of the switches along the path determines that the channel can be admitted without violating the loss guarantees provided to all the existing channels serviced by the switch, and (ii) the frame loss for the new channel aggregated over all the switches does not exceed the maximum loss tolerance of the channel. Assuming video sources to be markovian [2], the formulation of the FIPD-based admission control procedure at a switch consists of:
1. Modeling the switch and the video channels multiplexed through it as a discrete time, finite state, markov chain: Each state in the markov chain encapsulates the buffer occupancy at the switch and the activity modes of all the channels - each channel can be in one of three modes: an A CTIVE mode when it is transmitting a video frame, an I NACTIVE mode in between frame transmissions, and a H OLD mode when the frame transmitted over the channel is discarded by the switch, Hence, each state
S i where b i in the markov chain is represented by a quadruple: is the buffer occupancy at the switch,
N
A i
; hb i
N i
I
, and
N
H i
; N
A i
; N
I i
; N i
H i
, are the number of channels that are A CTIVE , I NACTIVE , and on H OLD , respectively.
2. Computation of the frame loss probability at the switch and its comparison with frame loss tolerances of each of the channels: The frame loss computation requires the identification of all possible states of the markov chain in which buffer overruns will occur, and the determination of the probabilities of occurrence of each of these overflow states. The computed frame loss probability, if it is within the bounds of all

of the existing channels, represents the lowest frame loss guarantee that is offered by the switch to the new channel requesting admission.
A detailed formal derivation of the admission control procedure is presented in
[9]. The following example serves to illustrate this FIPD-based admission procedure:
Consider a simple switch with a buffer capacity
B =
1 packet. Suppose that the switch is servicing one video channel, at which time a second video channel makes a requests for admission. Let the transmission rate of each video channel be 1 frames/slot (a
100 slot represents the service time of a packet at the switch) and average frame size be
10 packets/frame. In a feasible state
S i
= hb i
; N i
A
; N i
I
; N i
H i of the markov chain for the switch, it must be the case that: (i) the buffer occupancy b i at the end of a slot is
0 immediately following the servicing of the sole packet that can be in the buffer; (ii) the number of channels in the three modes, A CTIVE , I NACTIVE and H OLD must together equal the total number of channels, i.e.,
N
A i
I H
= N =
2, and (iii) the
+ N i
+ N i number of channels
N
H i in H OLD mode can be at most 1 (Only one frame is discarded every time a buffer overrun occurs, because the buffer size is 1 and there are atmost two active sources). Under these constraints, the states of the markov chain that are feasible are: h
0
;
0
;
1
;
1 i
, h
0
;
0
;
2
;
0 i
, h
0
;
1
;
0
;
1 i
, h
0
;
1
;
1
;
0 i
, and h
0
;
2
;
0
;
0 i
. Of these, the only state in which buffer overrun can occur (the condition for which is b i
+ N i
A
> B
) is h
0
;
2
;
0
;
0 i
. Assuming the video sources to be markovian, the transition probabilities between states of the markov chain can be determined, as shown in Figure 1. Using the transition probabilities so computed, the probabilities of the individual states of the markov chain can be obtained. The expected fractional frame loss is derived from the probability, 0
:
0018 of the overflow state h
0
;
2
;
0
;
0 i
. Since each source generates a frame once every 100 slots on an average, assuming frame losses to be equally likely on both the video channels, we derive the frame loss probability to be
0
:
0018 1 100
2
=
0
:
0904.
Hence, the switch admits the second video channel only if the tolerable fractional frame loss specified by the applications on each of the two channels is at least 0
:
0904.
In order to evaluate the practical utility of FIPD, we have carried out extensive simulations using commonly available JPEG [6] and MPEG video traces. The target environment comprises of a network of 300 Mbps, ATM switches, each with a buffer capacity of 500, 53-byte packets.
Our first experiment serves to validate the necessity for employing FIPD at network switches. In this experiment, JPEG-encoded video channels (video frames being generated at the rate of 24 frames/second) are multiplexed through an ATM switch that implements First-Come-First-Served (FCFS) scheduling. Figure 2(a), which depicts the performance when packet resiliency is one packet/frame, illustrates that in the absence of FIPD the discrepancy between packet loss and frame loss fractions is large; when the packet loss fraction is 10%, the corresponding frame loss fraction is four times as large.
Similar discrepancies between packet loss and frame loss fractions were also observed for other video encoding schemes: MPEG, hierarchical JPEG, and run-length encoding, thereby demonstrating the requirement for FIPD-like QoS management strategies for enforcing application-level frame loss guarantees.

0.8900
0.8100
0.8911
State: <0,1,1,0>
Probability: 0.1709
0.0100
0.0010
0.1000
0.0990
0.0900
0.0900
0.0100
0.9000
0.0989
0.0100
0.900
State:
Probability:
<0,1,0,1>
0.0090
State:
State: <0,0,1,1>
Probability: 0.0090
<0,2,0,0>
Probability: 0.0018
0.0219
0.0001
State: <0,0,2,0>
Probability: 0.8092
0.9780
Fig. 1. State transition diagram for the markov chain of a switch of buffer capacity
B =
1 packet servicing two video channels each with frame rate of 0
:
01 frames/packet and frame size of 10 packets; circles represent the feasible states and the weighted arcs represent feasible state transitions of the markov chain, with the weights indicating the associated probabilities of occurrence. The shaded state, h
0
;
2
;
0
;
0 i
, is the only overflow state in the markov chain.
Figure 2(b) illustrates the effectiveness of FIPD. By limiting the distribution of packet losses amongst fewer frames, FIPD reduces the fraction of frames that are rendered unusable during transmission. In the process, FIPD bridges the gap between frame loss and packet loss fractions; the slight difference that is observable in Figure
2(b) between frame loss and packet loss fractions is attributed to variations in video frame sizes, introduced by JPEG compression.
By judiciously discarding all the packets of a lost frame, FIPD not only reduces the frame loss fraction but also releases network bandwidth and buffer space for new channels. Hence, a switch that employs FIPD is able to service a greater number of video channels than a switch that does not. Figure 3 illustrates this marked increase in servicing capacity of a switch employing FIPD, for a frame loss tolerance of 0.15.
Figure 2 also illustrates that the effectiveness of FIPD depends on applications’ frame loss tolerances. Some applications may require very high-quality transmission, e.g., medical imaging, and may have low frame loss tolerances. For such applications, the performance gains yielded by FIPD are low; in the limit when an application cannot tolerate any frame loss, FIPD becomes ineffective. In general, video applications possess moderate to high tolerances. Moreover, loss tolerances also increase with the rate of video display: whereas at a frame rate of 10 frames/second, loss of 3 frames every second is likely to be perceived as lack of motion and hence, deemed intolerable, at a frame rate of 30 frames/second, the same fractional loss may be tolerable. Larger an application’s loss tolerance, greater is the number of frames that can be discarded during

Without FIPD, Frame loss vs. Channels
Without FIPD, Packet loss vs. Channels
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40 50 60 70 80
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With FIPD, Frame loss vs. Channels
With FIPD, Packet loss vs. Channels
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0 10 20 30
(b)
40 50 60 70 80
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Fig. 2. Performance of a switch servicing multiple JPEG video channels: (a) In the absence of
FIPD, the frame loss fraction is much different from the packet loss fraction; (b) Implementing
FIPD at the switch bridges the gap between the frame loss and packet loss fractions. The packet resiliency used in this experiment is 1 packet/frame.
transmission; the performance benefits accruing from FIPD accumulate with each such discard; hence, FIPD is more effective at higher tolerances.
Packet resiliency may also vary, depending upon both the robustness of the encoding scheme, and upon the application. As the packet resiliency of a video channel increases, a switch has to wait for a longer duration before deciding to discard a frame (in the extreme case, when the packet resiliency equals the frame size, the switch is not able to discard any frame). The concomitant delay in detection of frame loss reduces the buffer

60
40
20
0
0
140
120
100
80
With FIPD
Without FIPD
10 20 30 40 50
Video frame rate (frames/sec.)
Fig. 3. Increase in number of video channels admitted at a switch when FIPD is employed; a fractional frame loss tolerance of 0
:
15 and packet resiliency of 1 packet/frame are used in this simulation.
space and bandwidth savings that accrue from FIPD. Hence, higher the packet resiliency, lower is the effectiveness of FIPD. Figure 4 demonstrates the drop in effectiveness of
FIPD as packet resiliency increases, for the case when video frames are transmitted immediately following their digitization.
Interestingly, our simulations reveal that the choice of packet resiliency also depends upon the transmission patterns of video frames: the frequency and distribution of packet losses is influenced by whether video frames are transmitted as and when they are digitized, or whether the video sources voluntarily shape their traffic by distributing transmission of the packets of each frame within the playback duration of a frame. In the former case, packet losses are bursty: only a small fraction experience large losses and a large fraction of frames experience few packet losses. On the other hand, in the latter case, packet losses are distributed more uniformly amongst all the frames. Figure
5 contrasts the packet loss distributions that occur in the above two cases, for the same frame loss fraction (0.15) and the same packet resiliency (150 packets/frame). In the absence of traffic shaping, over 63% of frames experience no packet loss at all, whereas when traffic shaping is in effect, only 0.3% of frames experience no loss, but over 70% of frames lose as many as 50 to 150 packets/frame. In the former case, owing to the relatively infrequent occurrence of packet loss, a packet resiliency of 150 packets/frame may be acceptable. However, owing to increased frequency of packet losses, we infer that video sources that resort to traffic shaping must specify lower packet resiliencies.
Variations in frame sizes too influence the performance of FIPD. When the average frame size is high, packet losses are more bursty. Moreover, since the effectiveness of
FIPD is directly related to the bandwidth freed up by frame discarding, larger the sizes of video frames, greater are the bandwidth savings and hence, greater the effectiveness of FIPD. Figure 6 corroborates this observation. In this experiment, video frame sizes are doubled and the frame rates halved, so as to retain the same per JPEG-channel bandwidth as before. As expected, the decrease in frame loss fraction is greater when the frame size is larger.

100
90
80
70
60
50
40
30
20
10
0
0.00
With FIPD
Without FIPD
0.10
0.20
0.30
(a)
0.40
0.50
0.60
0.70
0.80
JPEG Video frame loss fraction
100
90
80
70
60
50
40
30
20
10
0
0.00
With FIPD
Without FIPD
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(b)
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100
90
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50
40
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10
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0.00
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Without FIPD
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0.20
0.30
(c)
0.40
0.50
0.60
0.70
0.80
JPEG Video frame loss fraction
Fig. 4. Effect of increasing packet resiliency on the performance of FIPD: Figures (a), (b), and
(c) respectively represent the cases when the packet resiliency is 1, 150, and 250 packets/JPEG frame, out of a total number of approximately 650 packet per each frame. In this case, video frames are transmitted as and when digitized.

70
60
50
40
30
20
10
0
0 50 100 150 200 250
Packet loss per frame
(a)
70
60
50
40
30
20
10
0
0 50 100 150 200 250
Packet loss per frame
(b)
Fig. 5. Influence of video transmission pattern on the distribution of packet losses at a switch :
Figure (a) depicts the packet loss distribution when JPEG frames are transmitted as and when digitized; Figure (b) depicts the effect of traffic shaping on the packet loss distribution: packet losses are more uniformly distributed over all the frames. In both cases, the fractional frame loss is 0.15, and packet resiliency is 150 packets/frame, each frame comprising of 650 packets, on an average.
All of the above mentioned experiments point to the effectiveness of FIPD when employed at a switch that is directly connected to the video sources. Figure 7 depicts the performance of the FIPD strategy in a multi-hop network. The performance gain in this case is comparable with that observed for a single switch, illustrating that FIPD retains its effectiveness even in multi-hop networks.
Figure 8 illustrates the performance of FIPD for MPEG-encoded video channels.
In this experiment, different packet resiliencies are employed for I, P, and B frames, and frame discarding is adaptively triggered, reflecting the greater priority of I frames compared to P and B frames, and that of P frames over B frames. For a frame loss tolerance of 0.15, FIPD permits a switch to admit almost 60 additional MPEG channels.
Besides demonstrating that FIPD performs uniformly well for different media encoding schemes, this experiment also highlights the effectiveness of FIPD for different frame size distributions (MPEG channels exhibit more than 6:1 variations in frame sizes, as compared to the 3:1 variation exhibited by JPEG channels).
In this paper, we have argued that significant performance benefits will accrue if integrated networks can provide an application-specific service interface, as well as implement application-specific mechanisms that account for the diversities in media compression schemes. Towards this end, we have proposed a simple, yet effective, strategy called Frame Induced Packet Discarding, and have presented results of trace-driven simulations that demonstrate the performance gains that FIPD promises to offer.
1. J.T. Amenyo, A.A. Lazar, and G. Pacifici. Proactive cooperative scheduling and buffer management for multimedia networks. ACM/Springer-Verlag Multimedia Systems Journal,

Without FIPD, Frame Size: Double
With FIPD, Frame Size: Double
Without FIPD, Frame Size: normal
With FIPD, Frame Size: normal
1.00
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Fig. 6. Evaluation of the performance of a switch employing FIPD with increase in frame size:
The video frame size is doubled, and the frame rate is halved to 12 frames/second, so as to not alter the bandwidth required per video channel. As can be observed in the figure, greater the average frame size, greater is the reduction in frame loss fraction yielded by FIPD.
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Fig. 7. Evaluation of FIPD in a multi-hop network: The reduction in frame loss fraction resulting from deployment of FIPD in a multi-hop network is of the same order as that observed in the case of a single switch. In this experiment, the packet resiliency is 150 packets/frame, each frame comprising of 650 packets, on an average.
With FIPD
Without FIPD
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Fig. 8. Performance of FIPD strategy when MPEG encoded video channels are multiplexed through the network. The video frame rate is 24 frames/second and the packet resiliencies for I,
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