bandwidth channel variations, data errors/losses, and terminal capacity heterogeneity .... recover lost source data symbols from fewer received code symbols [8].
Unequal and Interleaved FEC Protocol for Robust MPEG-4 Multicasting Over Wireless LANs Abdelhamid Nafaa, Toufik Ahmed and Ahmed Mehaoua University of Versailles – CNRS-PRiSM Lab., 45 av. des Etats-Unis, 78035 – Versailles – France {anaf, tad, mea}@prism.uvsq.fr
Abstract — Robust video streaming over Wireless Local Area Networks (IEEE 802.11) faces many challenges, including bandwidth channel variations, data errors/losses, and terminal capacity heterogeneity. This is worsen by the TCP/IP architecture that does not offer any quality of service (QoS) guarantees to demanding applications such as video streaming. In order to improve error resilience and user-perceived video quality, a novel error control protocol, called “Unequally Interleaved Forward Error Correction” (UI-FEC), is proposed. UI-FEC is particularly efficient for adaptive MPEG-4 video multicast over Wireless LAN. The proposed protocol is composed of (1) a coordinated unequal and interleaved MPEG-4 data protection mechanism, that gracefully degrades video quality at receivers while minimizing the overall link bandwidth consumption; (2) an adaptive MPEG-4 video fragmentation and encapsulation protocol for a higher wireless link utilization. key words: MPEG-4, multicast, IEEE 802.11, FEC, interleaving.
I. INTRODUCTION Multimedia streaming over wired networks, such as the Internet, has been popular now for quite some time. However, with the development of broadband wireless networks, attention has only recently turned to delivering video over wireless networks. At the same time, the ISO/IEC MPEG-4 [1] multimedia content creation, management and distribution framework is foreseen to be an important component of many forthcoming wireless IP services. In this paper, we focus on the Wireless Local Area Network (WLAN), which can operate at high enough bit rates to allow transmission of high quality video data. Specifically, we investigate the MPEG-4 multicast communications over IEEE 802.11b wireless LAN [2], though the ideas that we present are applicable to other wireless networks or multimedia streams as well. The wireless channels are typically much nosier and have both multi-path and shadowing fades, thus making the bit error rate very high. In this work we aim at application level QoS control. Error control is one of the most popular application-level approaches dealing with packet loss and delay in the multimedia communications over bandwidth limited fading wireless channels. Error control is used to avoid the packet loss across the error prone networks. It consists of Forward Error Correction (FEC), retransmission such as ARQ (Automatic Repeat ReQuest), and error concealment in the source and destination. In our previous
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works [3], [4], we addressed the MPEG-4 wireless communication from an application point of view. However, the intrinsic wireless link characteristics involve multiple unpredictable bursts errors that are usually uncorrelated with the instantaneous available bandwidth. The resulting packet losses and bit errors can have devastating effect on multimedia quality. To overcome residual BER (Bit Error Rate), error resilience of Video/Audio codec is usually required. Moreover, error control techniques such as FEC and ARQ are necessary to ensure high-quality media transmission. Of these two error control mechanisms, FEC has been commonly suggested for real-time applications due to the (1) strict delay requirements of media streams and (2) its proven scalability in multicast communications. As pointed out in this paper, typically the multicast communication over the wireless LAN involves a high bit error rate, which usually occurs through correlated and adjacent packets losses. In this case, the classical FEC approaches [4], [5], can be inefficient since they involve an excessive bandwidth usage. Actually, such approaches use FEC to protect consecutive original data packets that is not effective against burst packets losses, and then implies transmitting additional FEC packets to overcome the increasing BER. Hence, we propose a novel unequal interleaved error protection scheme (UI-FEC) for efficient support of MPEG4-based multicast communications over wireless IP networks. UIFEC provides a low-delay media packets interleaved protection scheme based on adaptive MPEG-4 video fragmentation. Thus, UI-FEC minimizes burst errors consequences, and hence the video distortion at receivers. In addition, we provide an adaptive FEC transmission based on efficient feedback for 1-to-n multicast communication scenario. The remainder of this paper is arranged as follows. Section II reviews and gives a background related to reliable video multicasting over wireless LANs. In Section III, we present the proposed UI-FEC scheme. Section IV is devoted to performance evaluation. Finally, we conclude our paper in Section V.
II.
BACKGROUND AND MOTIVATIONS: VIDEO MULTICASTING OVER WIRELESS LANS
A. Reliable Communication over Varying Channels In order to reliably communicate over packet-erasure channels, it is necessary to exert some form of error control. Two classes of communication protocols are used in practice to reliably communicate data over packet networks: synchronous and asynchronous. Asynchronous communication protocols, such as ARQ operates by dividing the data into packets and appending a special
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error check sequence to each packet for error detection purposes. The receiver decides whether a transmission error occurred by calculating the check sequence. For each intact data packet received in the forward channel, the receiver sends back an acknowledgement. While this model works very well for data communication, it is not suitable for multimedia streams with hard latency constraints. The maximum delay of the ARQ mechanism is unbounded, and in the case of live streaming it is necessary to interpolate late-arriving or missing data rather than insert a delay in the stream playback. In synchronous protocols (i.e. FEC-based protocols), the data are transmitted with a bounded delay but generally not in a channel adaptive manner. The FEC codes are designed to protect data against channel erasures by introducing parity packets. No feedback channel is required. If the number of erased packets is less than the decoding threshold for the FEC code, the original data can be recovered perfectly. ARQ-based schemes are not appropriate for the multicast case for three reasons: (1) ACK explosion, that scales with the multicast group size; (2) for significant loss rate, each user will require frequent packet retransmissions that probably are useless for the other multicast users; and (3) unbounded data transmission delays. Hence, using a FEC-based error control seems to be more appropriate for real time multicast communication.
addressed by many researchers. Most of the reliable multicast protocols propose the use of ARQ [6][7]. However, the use of a simple ARQ for reliable multicast transmission towards a large group may cause a high retransmission rate at sender even if each receiver has a low error rate. Furthermore, ARQ introduces unbounded data transmission delays, which is discordant with realtime video distribution. Besides, FEC for multicast streaming of different characteristic have been extensively studied in the literature [5]. In a multicast scenario, to tackle the problem of heterogeneity and to ensure graceful quality degradation, the use of multi resolution-based scalable bitstream has been previously suggested in [7]. In wireless communications, the loss of successive packets is correlated. A packet loss may usually be followed by a burst of loss, which significantly decreases the efficiency of FEC schemes. In order to reduce burst loss, redundant information has to be added into temporally distant packets, which introduces even higher delay. Hence, the repair capability of FEC is limited by the delay budget. In this paper, we introduce a novel technique that combines reliability and efficiency advantages of interleaved FEC coding. Our proposal is based on an adaptive MPEG-4 streams fragmentation coordinated with a low-delay unequal interleaved protection, which improve the burst packet loss tolerance while minimizing the overall bandwidth consumption. To the best of our knowledge, there has been no work proposing an adaptive FEC over unequal interleaved media packets.
B. IEEE 802.11b WLAN Architecture At this point, we would like to highlight important WLAN features that influence the design of multicast media streaming III. UI-FEC: A NOVEL FEC PROTOCOL FOR WIRELESS VIDEO system. As in the wired Ethernet, corrupted packets are dropped at MULTICAST the link layer (i.e. packets with bit errors are unavailable to multimedia applications). Wireless communications are A. Reed Solomon Codes complicated by the inability of radio transceivers to detect In coding theory, an error is defined as a corrupted symbol in collisions as they transmit data, and by the possibility of devices an unknown position while an erasure is a corrupted symbol in a outside the network to interfere with network transmissions [2]. known position. For now, the focus will be on the erasure Note that in unicast communication all wireless data frames are correction capability only. RS code is very well suited for error acknowledged by the receiver. Furthermore, each retransmission protection against packet loss, since it is a maximum distance introduces an additional latency due to collision avoidance routine separable code. This means that no other coding scheme can triggering. In case of multicast or broadcast traffic, the data packets recover lost source data symbols from fewer received code are not acknowledged, and hence not retransmission is performed symbols [8]. The aim of Reed-Solomon (RS) codes is to produce on the MAC/Logical link layer; this mode of communication at the sender n blocks of encoded data from k blocks of source reduces transmissions delays while making communications less data in such a way that any subset of k encoded blocks suffices at reliable. the receiver to reconstruct the source data. C. Error Erasure for Wireless Multicast Streaming There are two challenges for video streaming over wireless LANs: 1) fluctuations in channel quality and 2) high bit-error rates compared with wired links. In addition to wireless channel-related challenges, when there are multiple clients (e.g. multicast scenario), we have the problem of heterogeneity among receivers. In fact, each user will be affected by unpredictable factors such as co-channel interference, adjacent channel interference, propagation path loss, shadowing and multipath fading. This changing condition will directly influence the measured bit error rate for each receiver. In our case, and within wireless multicast scenario, the transmission will be less reliable but involves an important gain in data control overhead reduction due to the absence of the MAClevel acknowledgements and retransmissions. Furthermore, the data packets transmission delays will be significantly reduced. In the following, we will exploit those facts to design an efficient error control scheme. D. Related Works There has been a substantial amount of prior work in the area of reliable video streaming. The single-user scenario has been
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B. Adaptive MPEG-4 Video Fragmentation and Encapsulation In this research, we focus on the simple profile of MPEG-4 standard in error resilient mode, where data partitioning (DP) is enabled. DP provides MPEG-4 video stream partitioning into independently decodable entities. Since the MPEG-4 simple profile is based on a single video object coding, it is much suitable for transport over wireless IP networks. In fact, using the simple profile allows: (1) a low decoder complexity at the mobile terminal and (2) a simplified transport-level synchronization management. In our study we used only one video object (VO) and one video object layer (VOL). Each VOL consists of many video object planes (VOP), and each VOP consists of many video packets (VP). Note that, a VOP is the equivalent of a Frame in the MPEG1/MPEG2 coding domain. The video packet approach in MPEG-4 is based on providing periodic resynchronization markers throughout the bitstream. Since the coded MPEG-4 bitstream involves a high dependency between adjacent MPEG-4 video packets. An appropriate video Fragmentation/Encapsulation rules are
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=
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Figure 1: Correlation between packet size and link utilization.
Now, it is readily realized that the maximum of the utilization function is obtained through resolving for S. Thus, VPsize is determined for each VOP to be transmitted based on overall loss rates experienced at receivers (i.e. aggregated loss rates). The fragmentation/encapsulation presented here, allows better wireless link utilization depending on the actual channel loss rate and the current VOP size. Moreover, this scheme minimizes the dependency between adjacent RTP packets, which mitigate the dependency of MPEG-4 decoder on any lost packet.
C. Adaptive FEC over Unequally Interleaved Packets Figure 2 illustrates a sample trace we obtained using an 802.11 AP (Access Point) and mobile stations. We emphasize the packet loss behavior when multicasting video over wireless links. The AP was sending multicast MPEG-4 video packets and each receiving station was recording the sequence number of the correctly received packets. Each point represents the packet loss rate
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Multicast receivers heterogeneous losses 1
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Figure 2: Correlated multicast receivers packet losses.
As the wireless link fluctuation occurs usually through adjacent packets losses, we propose to use an interleaved packetlevel FEC protection. As depicted in Figure 3, the FEC packets protect temporally scattered RTP packets in order to cope with the wireless sporadic packet losses. This will lead to better error erasure efficiency at the client side. Burst error
RTP media Packets
oh + 1 ≤ S ≤ MTU
Where, oh represents the overhead involved by the successive underlying protocols encapsulations. In Figure 1, U is plotted against S for VOPSize=10000 and oh=40 at four different loss rates Lr = {0, 0.05, 0.2, 0.3} respectively. 0.95
measured at each receiver (averaged over 10 packets). It should be noted that, this experience reveals an important number of adjacent packet losses as consequence of the wireless burst error. Furthermore, the packet loss rate is different at each receiver depending on its proximity to the AP (i.e. location-varying channel conditions).
Loss rate
primordial for maintaining an acceptable perceived video quality at receivers when some packets are lost. The IETF standard for transporting MPEG-4 video over IP allows conveying several VPs in the RTP payload [9]. This can cause an excess overhead due to VP headers redundancy. Moreover, the loss of a single RTP packet will involve the loss of several adjacent VPs, which leads to devastating consequences at decoder side. Hence, we propose to encapsulate a single VP per RTP packet. In this context, a large VP size is essential for wireless bandwidth saving. Thus, the RTP media packet must not exceed the MTU (Maximum Transfer Unit) in order to avoid a too costly IP-level packet fragmentation. In other hand, using a large VP will affect the packet loss rate due to wireless networks error prone character. Previous studies have proved that the packet size influences the measured packet loss rate [10], [11]. Therefore, in this work, we try to find the optimal VP size that maximizes the bandwidth utilization according to the instantaneous wireless link conditions. Let VOPsize be the VOP size, let S be the packet size in bytes, let oh be the overhead in bytes and Lr be the current packet loss rate. The utilization, U, i.e. the proportion of the bandwidth consumed by the application, which is used for actual video data multicasting (including the redundancy to be transmitted), will then be given by (1):
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Figure 3: Scattered media packets protection.
In order to maintain the synchronization between FEC and media packets both of them are conveyed to the same multicast address. This is achieved through assigning different port numbers for FEC stream and protected media stream. Hence, the demultiplexing is achieved without ambiguity. All information concerning both the media and FEC sessions can be signaled at session initiation using SDP message or other out of band means. Within UI-FEC, the time is divided into transmission rounds. Each transmission round corresponds to the transmission of ULP Block (Unequal Loss Protection Block, or UB). An UB consists of n = k + h packets (see Figure 5). A transmission round ends when the sender transmit the last packet of an UB.
At this point we define the interleaving factor (i) as the Sequence Number (SN) difference between each two successive protected RTP packets in the UB. The interleaving factor is fixed for all k protected RTP packets belonging to the same UB. When i = 1, the interleaved protection is not applied and, hence, the FEC packets protect adjacent RTP packets. m ∀ j ,1 ≤ j ≤ k − 1 : i = UB m j + 1 . SN − UB j . SN
(2)
m m m and , ∀j ,1 ≤ j ≤ k − 1, ∀l ,1 ≤ l ≤ k − 1 : UB m j +1 .SN − UB j .SN = UBl +1 .SN − UBl .SN
(3)
Here, UBjm.SN represents the sequence number of the jth media packet belonging to the mth UB. Figure 4 summarizes the UI-FEC scheme working for an interleaving factor i = 3. For synchronization considerations the protected media data of a given UB must belong to the same VOP. In other words, each VOP is transmitted as an integer number of UBs. After applying FEC, the media packets are transmitted in their initial order (i.e. according to the sequence number order). Note that, delay
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introduced by FEC interleaving has not important consequences, since it is applied over a single VOP. The induced delays can be resolved through an initial buffering time at the server. This novel interleaving over a single VOP (i.e. video frame) will be used to provide an adaptive and unequal FEC for efficient support of MPEG-4-based multicast communications over wireless IP networks.
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Figure 4: UI-FEC scheme.
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Figure 5: UB structure details.
For FEC efficiency reasons [5], it is suitable to keep the number of information packets (k) as high as possible with few redundant packets. Moreover, a higher number of information packets lead to better granularity, which allows adjusting the redundancy rate according to match channel conditions. As an example, if a VOP to be transmitted is 10 Kbytes in size a packet payload size (VP size) of one kilobyte will result in ten RTP packets. This makes the precision at which the error protection can be applied be in units of 10% (i.e. one packet of redundancy will make the transmission resilient against packet loss rates up to 10%, two packets of redundancy can withstand up to 20% losses, and so on). In our case, we take a minimum of 10 information packets for each UB. This is insured using an appropriate l ≥ 10 according to video bitstream bitrate (i.e. according to the mean VOP size). For a fixed l, the interleaving factor (i) is then stated following (4) and (5): VOPSize i = l ∗ VPSize
(4)
Here, the VOPSize is obtained from the VOP header, while the VPSize is fixed by the MPEG-4 data partitioning used according to formula (1). Note that, for a given VOP (V1) the interleaving factor (i) represents the number of UBs constituting V1. Moreover, i is fixed for UBs belonging to the same VOP. Also, the different UBs belonging to the same VOP can contain a different number of media packets (k). From this fact, the number of k original packets per UB is derived from formula (4) as follows: VOPSize k = i ∗ VPSize
(5)
The interleaving factor (i) is tightly dependent on VOP size. Since the different VOPs of an MPEG-4 video sequence have a different size, the interleaving factor (i) will scale along with the VOP size. It is clear that an intra-coded VOP (VOP-I) is larger than a predicted VOP (inter-coded VOP-P or VOP-B). Basically, a large inter-coded VOP (VOP-P or VOP-B) convey a lot of motion vector and error prediction information. Otherwise, the larger the coded VOP is, the more it codes a highly changeful scene with different texture, and the more it involves an important video distortion when lost. As a consequence, and within our proposed UI-FEC, the interleaving is applied differently based on VOP size, and hence
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p ∗ k
(6)
Note that, the amount of redundancy introduced is computed once per each VOP to be transmitted depending on the current packet loss rate and the video fragmentation parameters. Since the
successive UBs have different structures, we propose a simple signaling of required parameters for UB decoding. We transmit within FEC stream (i.e. FEC header) the base RTP sequence number (BSeq) of the UB’s protected RTP packets, the UB size (n), the redundancy amount (h), and the interleaving factor (i). Thus, both interleaving and redundancy are adjusted to match the overall measured BER.
IV. PERFORMANCE EVALUATION UI-FEC is evaluated with the Network Simulator v2 using various network configurations. We emphasize UI-FEC robustness for wireless multimedia communication.
A. Simulation model The video multicasting applications are considered for the simulation. We take as simulation scenario a MPEG-4 multicast session. In this case and within our proposal, the MPEG-4 multicast server generates FEC packets that protect interleaved RTP media packets. While, within classical approach, the FEC protection is applied to adjacent RTP media packets. It should be noted, that both approaches are evaluated for the same network configurations. In our simulation, we have used an MPEG-4 coded Soccer sequence as an example of high activity video traffic (MPEG-4 video parameters are depicted in Table 1). We choose a highly changeful video sequence in order to highlight the unequal interleaved protection efficiency. Figure 6 represents the VOP size trace of Soccer. VOP size trace of Soccer
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VP 1 VP 4
VOP relevance, which provides a smooth video quality degradation. Actually, within a video sequence, the different VOPs are unequally interleaved, protected, and then transmitted based on their relevance and without any previous classification scheme. The adaptive FEC scheme proposed in this paper is based on systematic RS (n, k) codes, where n, k and i are reassigned for each VOP to be transmitted based on the VOP relevance and the current channel loss rate. When using k packets in a UB, transmitting h FEC packets will provide an erasure resiliency against a packet loss rate of h/k. Therefore, it is easy to calculate the amount of redundant packet (h) using the packet loss rate (p) and the number (k) of original media packet of current UB to be transmitted.
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Figure 6: VOP size trace of Soccer. M PEG-4 v ide o sequence
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Table 1: Video features.
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We use a combined wired/wireless network simulation models for evaluating and comparing our proposal with the classical approach. The MPEG-4 multicast server is located in the wired network; it multicasts the stream to receivers located in the wireless LAN We include constant-bit-rate (CBR) traffic over UDP in order to load the network differently each time so that we obtain further information about UI-FEC behavior.
B. Results analysis The simulated MPEG-4 multicast session involves the transmission of 16411 RTP packets for 1322 UBs. Thus, we observed the loss of 952 RTP packets during the whole streaming session (i.e. the loss rate is around 5,8%). Our proposal recovered a total of 381 packets while the classical approach recovered only 140 packets. Figure 7 represents packet loss rates measured for each transmitted MPEG-4 VOP. The simulation reveals relatively high bit error rates due to (1) wireless channel burst errors and (2) absence of MAC-level retransmissions in multicast communications. Consider that the measured high loss rates are often provoked by temporally consecutive packet losses affecting the same VOP (due to physical layer burst errors). Besides, we observe that UI-FEC increases error resiliency at receivers’ side through recovering more RTP packets.
Furthermore, UI-FEC behaves better with a high bitrate video multicasting (i.e. video streams with a large mean VOP size) due to the use of a high interleaving factor. Within UI-FEC, we combine an adaptive FEC transmission with an unequal interleaved protection. Hence, the interleaved protection, proposed in this paper, lead to a better error resiliency at receivers. This reduces the redundancy transmission involving a better wireless bandwidth usage.
V. CONCLUSION This paper describes an error control protocol for robust MPEG-4 video multicast over wireless channels, called, unequal interleaved FEC. By combining the best features of MPEG-4 data partitioning, FEC and interleaving techniques, the proposed protocol is integrated to the transport layer and it is capable of achieving a high reliability and efficiency during video multicast transmission with a low latency cost and signaling overhead. The simulation results demonstrated that our protocol offers better wireless bandwidth utilization through video data interleaving, and graceful video quality degradation by means of adaptive forward error correction.
VI. REFERENCE
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ISO/IEC 14496-2, 01 “Coding of audio-visual objects -- Part 2: Video final committee draft”. Version 3: January 2001. [2] Information technology—Telecommunications and information exchange between systems—local and metropolitan area networksspecific requirements—Part 11: IEEE 802.11 standard, August 1999. [3] A. Nafaa, et al., “RTP4mux: a novel MPEG-4 RTP payload for multicast video communications over wireless IP” in Proc. PV 2003, 13th International Packet Video Workshop, Nantes, France, April 2003. [4] T. Ahmed, et al., “MPEG-4 AVO Streaming over IP using TCP-Friendly and Unequal Error Protection” in Proc. IEEE ICME’03, Baltimore, Maryland, July 2003. [5] N. Nikaein, et al., “MA-FEC: A QoS-based adaptive FEC for multicast communication in wireless networks” in Proc. IEEE ICC 2000, New Orleans, USA, June 2000. [6] S. Floyd, et al., “A reliable multicast framework for light-weight sessions and application level framing,” in Proc. ACM SIGCOMM ’95, Cambridge, MA, 1995. [7] A. Mjumdar, et al., “Multicast and unicast real-time video streaming over wireless LANs” IEEE Transaction on Circuits and System for Video Technologie, June 2002. [8] S. Lin, et al., Error Control Coding: Fundamentals and Applications, Prentice-Hall, Inc. Englewood Cliffs, 1983. [9] K. Yoshihiro al., "RTP payload format for MPEG-4 Audio/Visual streams", IETF proposed standard, RFC 3016, November 2000. [10] Paul Lettieri, et al.,” Adaptive frame length control for wireless link throughput, range, and energy efficiency” in Proc. IEEE INFOCOM'98, San Francisco, CA, 1998. [11] M. Johanson “Adaptive Forward Error Correction for real-time Internet video” in Proc. PV 2003, 13th International Packet Video Workshop, Nantes, France, April 2003.
Figure 8 illustrates the bandwidth consumption measured during 1 minute of MPEG-4 multicasting (i.e. for 1500 VOPs). It is clear that our proposal achieves better wireless bandwidth exploitation. The measured mean bandwidth saving is around 27 Kbps. This bandwidth saving is principally due to the interleaved protection and the provided error resiliency. We observed that UI-FEC efficiency is particularly noticeable when transporting video over error prone channels. In fact, the unequal FEC interleaving scheme provides better robustness in networks with a high BER and, consequently, a small MTU.
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