Adaptive Error Control for Fine-Granular-Scalability ...

Adaptive Error Control for Fine-Granular-Scalability Video Coding over IEEE 802.11 Wireless LANS Xiaofeng Xu+, Mihaela van der Schaar*, Santhana Krishnamachari†, Sunghyun Choi‡, Yao Wang+ + Polytechnic University Brooklyn, NY 11201, USA

*

†

Philips Research USA Briarcliff Manor, NY 10510, USA

Abstract– Robust streaming of video over the IEEE 802.11 Wireless LANs (WLANs) poses many challenges, including coping with bandwidth variations and data losses. To address these challenges, we evaluate and compare various error control strategies in this paper, namely, Medium Access Control (MAC) layer Forward Error Correction (FEC), MAC retransmission, and application-layer FEC, combined with FGS scalable video coding to achieve reliable communication over the IEEE 802.11 WLANs. Moreover, we assess the performance of Fine-Grained Loss Protection (FGLP) for different multipath channel conditions. Furthermore, the performance of cross-layer strategies such as application-layer FEC with MAC retransmissions are also determined. 1. INTRODUCTION Currently, most wireless local area networks (WLANs) are predominantly used for data transfer. However, the higher bandwidth provided by new WLAN technologies such as IEEE 802.11a and IEEE 802.11g will ultimately lead to their increasing use for multimedia applications. Furthermore, the emerging IEEE 802.11e MAC standard will provide Quality-of-Service (QoS) for various applications. However, due to the error-prone, unpredictable characteristics of wireless links, the usage of WLANs for video transport poses many challenges, such as bandwidth variation and data losses. There have been many studies discussing the robust transmission of video over wireless networks. In [1-2], an Unequal Error Protection based Reed-Solomon (RS) codes and a hybrid Automatic Repeat Request (ARQ) are proposed for video packet transmission for WLANs. In [3], an application layer adaptation is combined with the link layer packetization to achieve robust wireless video transmission based on the packet loss ratio, frame type, frame delivery deadline, etc. Multicast and unicast realtime video streaming over wireless LANs is presented in [4], where the video multicast is formulated as an optimization of a maximum regret cost function across the multicast user space. However, most papers available in the literature solve the problem of bandwidth adaptation and robustness to packet-losses at the application-layer, without considering protection mechanisms provided by the lower network layers, such as the physical and MAC layers. In this paper, we investigate the performance of the FEC and ARQ protection mechanisms available at the MAC, and application layer for the robust and efficient transmission of Fine-Granular-Scalability (FGS) coded video [8] over IEEE 802.11 WLANs. FGS has been used in this paper because it provides fast adaptation to bandwidth variations as well as inherent resiliency and complexity scalability properties that are essential for efficient transmission over errorprone wireless networks. Nevertheless, the analysis and investigation performed in this paper for the various protection mechanisms can be easily extended, and will provide similar results when applied to alternative layered video coding schemes (e.g., data

EG Technology Atlanta, GA 30318, USA

‡

Seoul National University Seoul 151-744, Korea

partitioning, spatial scalability, temporal scalability, and waveletcoded video). Besides, by comparing the different protection mechanisms available in the different network layers, we also develop a cross-layer protection strategy for maximizing the received video quality by dynamically selecting the optimal combination of application-layer FEC and MAC retransmission based on the channel conditions. Our results show that using the adaptive error control for FGS in wireless networks provides robustness to losses and adaptability to varying network conditions. The rest of the paper is organized as follows. In Section 2, we present the multipath model that is used to simulate the wireless channel, and we also provide a brief review the IEEE 802.11 WLAN standards. In Section 3, the protection schemes available in different network layers are discussed, and the concept of Fine-Grained Loss Protection for the unequal protection of the FGS coded video is briefly presented. In Section 4, experimental results from various error control strategies are presented. The conclusions are drawn in Section 5. 2. OVERVIEW OF IEEE 802.11 IEEE 802.11 standard is becoming the most popular standard for WLANs. IEEE 802.11 standard and its extensions define the specification for the MAC and different physical layers. •

802.11a Physical Layer (PHY) The IEEE 802.11a PHY provides the interface between the MAC and the wireless medium [5]. Using Orthogonal Frequency Division Modulation (OFDM), the PHY transmits and receives data frames over a shared wireless medium. The PHY provides eight transmission modes having different modulation schemes and convolutional codes at the 5 GHz U-NII band, resulting in transmission bit-rates ranging from 6 to 54 Mbps. In this paper, the multipath channel model of [6] has been used. The channel is modeled as a tapped delay line, where the distribution of path amplitude is chosen to be Rayleigh and the average power of different taps declines exponentially with delay. Using the above channel model and a typical receiver model, the performance curves of Bit Error Rate (BER) vs. Signal-to-Noise Ratio (SNR) are obtained for different PHY modes of the 802.11a. Using these bit error values, and assuming random errors, the probability of error in a block of length L bytes is computed by:

Pem ( L) = 1 − (1 − pbm ) 8 L where Pbm is the BER of PHY mode m at a given channel SNR. •

(1)

802.11 MAC PCF The 802.11 WLAN standard allows different MAC mechanisms: distributed coordination function (DCF) and point coordination function (PCF) [5]. In the DCF mode, each station in the WLAN contends for the medium and relinquishes control. In the PCF mode, the point coordinator (PC) centrally coordinates the access to the wireless medium. Based on a poll-and-response protocol, the

central controller PC can control access to the shared wireless medium and eliminate contention among wireless stations. Clearly, the PCF mode is more suitable for video streaming applications and is employed for the evaluation presented in this paper. 3. ADAPTIVE PROTECTION STRATEGIES IN THE VARIOUS NETWORK LAYERS The protection of video content can be performed at different network protocol layers. In this section, we discuss different protection strategies at the MAC and application layers. 3.1. MAC Layer Protection • MAC-Level FEC Employing FEC at the MAC-level for improving the robustness to losses is currently under consideration in the IEEE 802.11 standard committee. Each MAC Protocol Data Unit (MPDU) or MAC frame consists of a MAC header, a variable length MAC Service Data Unit (MSDU), and a frame check sequence (FCS) as shown in Fig. 1. Both MAC header and FCS, which are 36 bytes long in total, contribute to the MAC overhead for a data frame. An MSDU can be split into (up to 12) multiple blocks that can be separately encoded using the (224,208) shortened RS encoder. Note that any RS block can correct up to 8 byte errors and that the protection offered by the MAC FEC is fixed for all packets, and hence unequal error protection cannot be provided for the various video layers. MAC Header Header FEC 32 16

MAC Body (N Blocks)

FCS FCS 4

FEC MSDU2 FEC --- MSDUN FEC MSDU1 208 16 208 16 --- 208 16 Figure 1: MAC frame format with the MAC FEC. •

MAC Automatic Repeat reQuest (ARQ) As shown in Fig. 2, the PC makes use of the priority inter-frame space (PIFS) to gain control of the medium. After seizing control of the medium, the PC starts a contention-free period (CFP). During a CFP, the PC sends a data frame and then expects a CF-ACK frame from the corresponding station within a short inter-frame space (SIFS) time.1 After receiving the CF-ACK frame, the PC sends the data frame to the next station. If the data frame is received in error or if the frame is not received at all, the PC starts the re-transmission of the data frame after a PIFS. If the data frame is received correctly, but the CF-ACK frame fails, then the PC waits for a SIFS time and re-transmits the data frame.

In 802.11a and .11b, a different retry-limit, specifying how many times a packet should be retransmitted before it is dropped, can be set for each packet. By varying the retry-limit, the various video packets/layers can be unequally protected. Also, note that the MAC ARQ is more effective for transmission of video over WLANs than the application layer ARQ because it incurs a much shorter delay. •

MAC Protection Performance Assuming that an L-byte frame body is transmitted using PHY mode m, the probability of a successful frame transmission is given by: m m m Pgood _ cycle ( L) = (1 − Pe,ack )(1 − Pe,data ( L))

(2)

where Pe,ack is the CF-ACK transmission error probability and Pme,data is the data frame transmission error probability. A CF-ACK frame also uses the same frame format as the data frame, but with zero frame body. Pme,ack and Pme,data can be calculated as follows:

(1 − Pe1 (3))(1 − Pem (38.75 + L )) (a ) Pem,data ( L ) =  1 m m N (1 − Pe (5.75))(1 − PRS ( 48))(1 − PRS ( 224)) (b)

1 − (1 − Pe1 ( 3))(1 − Pem (38.75)) ( a ) Pem,ack ( L) =  1 − (1 − Pe1 (5.75))(1 − PRSm ( 48)) (b)

where equations (a) and (b) correspond to the non-MAC-RS-coded and the MAC-RS-coded cases, respectively. Pe1(3) and Pe1(5.75) are the error probability of the PLCP header, 38.75 and 48 are the MAC and PHY overheads, Pem is the error probability of a non-RS-coded MPDU transmitted at PHY mode m with a 208 * N-byte frame body. The error probability of an RS-coded block equals:

PRSm ( n ) =

n

n

∑  i ( P  

i = t +1

m e

(4)

(1)) n −i (1 − Pem (1)) i

The probability that the L-bytes video frame is successfully received given a retry-limit R and using PHY mode m is given by: m m R +1 Pgood _ cycle ( L, R ) = 1 − [1 − Pgood _ cycle ( L)]

(5)

3.2. Application-Layer FEC The IEEE 802.11 implementation discards the entire MAC frame when errors are detected: because of this, if RS coding is applied within a single packet at the application layer, the erroneous packet will not be available for error detection or correction at the application layer. Therefore, RS coding at the application layer is applied across packets using an interleaver [7]. As shown in Fig. 3, k video packets each of length L bytes are buffered at the interleaver and (n-k) parity packets are generated by the RS encoder. Therefore, n-k determines the protection level. The probability of error after RS decoding is given by: n −k n   m n −i m i Pr = 1 − ∑  ( Pgood (1 − Pgood _ cycle ) _ cycle ) i =0  i 

k information packets

video packet

Figure 2: MAC retransmission strategy. n-k redundancy packets 1

In this paper, we consider only the downlink (e.g., from a video server to a video client via the PC) video streaming, which is more typical situation with the video streaming.

(3)

Figure 3: Application-layer RS coding.

(6)

Bit Plane1

Bit Plane2

Bit Plane3

The performance of the adaptive FEC strategies at the applicationlayer has been investigated by determining the maximum PSNR that can be obtained given the optimal RS code selection from the following RS codes for the various FGS layers (base- and enhancement-layers): (63,30), (63,47), (63,59), and (63,63). For comparison, the PSNR performance obtained when no FEC is used is also portrayed. As can be seen from Fig. 6, the usage of application-layer FEC is especially important for improving the video quality performance for poor channel conditions. Furthermore, the adaptive application-layer FEC combined with scalable/prioritized coding of the video can ensure graceful degradation across a large range of channel conditions. 40 35

PSNR(dB)

3.3. Fine Grained Loss Protection The FGS framework consists of a non-scalable base-layer and a fine-granular enhancement-layer. The base-layer bit-rate is selected to provide a low, but guaranteed level of quality, and hence it should have a stronger protection against packet losses than the enhancement-layer. Furthermore, the unequal protection concept can be applied not only between the base- and enhancement-layers, like in most alternative scalable coding techniques [7], but also prioritization within the enhancement layer. Taking advantage of the fine-granularity of FGS, a higher level of protection can be assigned to the more significant bit-planes. This concept is referred to as Fine Grained Loss Protection (FGLP) [8]. As shown in Fig. 4, the enhancement-layer is partitioned into an arbitrary number of finegrained sub-layers having different levels of protection. In [8], FGLP has been implemented using application-layer FEC, but other protection strategies, like the MAC ARQ employed in this paper, could also be employed (see Section 4).

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Optimal Curves

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redundancy packets

Figure 6: Optimal FEC selection for various channel conditions. •

3.4. Priority Queuing Another form of providing unequal error protection at the MAClayer is by employing priority queuing (see Fig. 5). By setting the various FGS-layers in different queues that are delivered sequentially to the MAC driver, unequal protection can be provided to the different video layers. In our transmission scheme, the baselayer video packets of a GOP are transmitted in the high-priority queue, followed by the video packets in the first FGS enhancement layer, etc. As long as a higher priority queue is not empty, that queue can deliver its packets first. If a packet in any of the queues exceeds its playback time, it will be discarded. For FGS, a more significant bit-plane can be assigned to a higher priority queue. Furthermore, different MAC retransmission limits or application-layer FEC can be assigned to the different queues. This priority queuing strategy has been employed for the experiments presented in the next section.

Adaptive Application-Layer FEC versus MAC FEC In our investigation, we have also compared the performance of the application-layer and MAC FEC. The results are shown in Fig. 7. For good channel conditions, the optimal application-layer FEC outperforms the MAC FEC, but for moderate and poor channel conditions, the MAC FEC leads to a considerably better performance. The explanation behind these results is that the bitlevel protection provided by the MAC FEC is more effective in combating serious transmission errors than the packet-level protection offered by the application-layer FEC. 40 35

PSNR(dB)

Figure 4: Fine Grained Loss Protection.

30 25

Optimal Curves

20

MAC

15 23

Enhancement Layer

.. .

... I

P

B

...

•

Base Layer

GOP

Figure 5: Priority queuing. 4. SIMULATIONS AND DISCUSSION In our simulations, we investigate the transmission of MPEG-4 FGS-coded bitstreams using various protection strategies. For the experiments, we use a CCIR resolution video sequence coded at a frame rate of 25Hz. The total available channel data rate for this particular stream is 6Mbps. We present the results while using a fixed PHY mode equal to 5, i.e., 24 Mbps, and a video packet length of 2080 bytes. •

Optimal Application Layer FEC

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Figure 7: Decoded picture quality (PSNR) of MPEG-4 FGS using application-layer FEC versus MAC FEC.

Low

High

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Adaptive Application-Layer FEC and MAC-Layer ARQ We now investigate a cross-layer protection strategy combining the application-layer FEC and MAC-layer ARQ. For a better understanding of the interaction between these various protection strategies, the results have been generated with and without the application-layer FEC. The retransmission limit is changed adaptively in the range of R = 0 to 8 to maximize the video quality performance. Figure 8(a) shows the PSNR performance for different retransmission limits, assuming that no application-layer FEC is used. From the figure, it can be concluded that MAC-layer ARQ is essential for achieving a graceful degradation over a large range of SNR channel conditions. For low SNRs, the channel BER is large, and hence using the maximum allowed number of retransmissions improves the link reliability. When the channel SNR is higher than 26.5 dB, note that there is only limited performance gain compared with R=1. This is because when the channel is good, the number of retransmissions is low anyhow, and hence the maximum

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PSNR(dB)

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5. CONCLUSION In this paper, we focus on the robust and efficient transmission of FGS video over WLANs. We specifically address the recent WLAN standard, IEEE 802.11a, which offers high bit rates, enabling the transmission of delay sensitive audio/visual (AV) traffic. We presented and investigated various error control strategies, namely, MAC FEC and ARQ, application-layer FEC and priority queuing, combined with FGS scalable video coding to achieve reliable communication over IEEE 802.11 WLANs. The results indicate that the adaptive cross-layer FEC-ARQ protection strategies lead to a significantly improved visual performance, and the protection strategies in the different protection layers should be optimized jointly for the optimal performance. REFERENCES [1] Y. Wang and Q-F. Zhu, “Error Control and Concealment for Video Communications: A Review,” Proceedings of the IEEE, vol. 86, no.5, pp. 975-997, 1998. [2] D. Qiao and K.G. Shin, “A Two-Step Adaptive Error Recovery Scheme for Video Transmission over Wireless Networks,” in Proc. IEEE INFOCOM ’2000, pp. 1698-1704, June 2000. [3] Y. Shan and A. Zakhor, “Cross Layer Techniques for Adaptive Video Streaming over Wireless Networks,” in Proc. IEEE Int. Conference on Multimedia and Expo (ICME) ’2002, August 2002. [4] A. Majumdar, et al, “Multicast and Unicast Real-Time Video Streaming over Wireless LANs," IEEE Trans. on Circuits and System for Video Technology, vol. 12, pp. 524-534, June 2002. [5] D. Qiao and S. Choi, “Goodput Enhancement of IEEE 802.11a Wireless LAN via Link Adaptation,” in Proc. IEEE ICC ’01, Helsinki, June 2001. [6] N. Chayat, “Tentative Criteria for Comparison of Modulation Methods,” IEEE P802.11-97/96, September 1997. [7] K. Stuhlmuller, M. Link, and B.Girod, “Scalable Internet Video Streaming with Unequal Error Protection,” in Proc. Packet Video Workshop’99, April 1999. [8] M. van der Schaar and H. Radha, “Unequal Packet Loss Resilience for Fine-Granular-Scalability Video”, IEEE Trans. on Multimedia, December 2001.

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Figure 9: Decoded picture quality (PSNR) of MPEG-4 FGS using different retry limits.

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retransmission limit is not used. Therefore, increasing the retransmission limit R above 1 would not result in a significant improvement in the PSNR performance. Note that these results are expected, since an increasing number of retransmissions always result in the better video performance. Unlike in the FEC case that leads to a constant overhead irrespective of loss, ARQ overhead happens only when errors occur. Consequently, even though a higher retransmission limit R leads to a reduced transmitted enhancement-layer rate if the previously discussed priority queuing is used, retransmitting the more important packets is important, since the lesser significant packets are discarded anyhow if an error occurs in the lower-layer packets of a video frame. The transmission of the lesser significant packets is hence wasting bandwidth unnecessarily. However, it should be noted that, in practical implementations, the MAC retransmission limit should be selected based on the delay constraints of the application. Figure 8(b) presents the video performance when both applicationlayer FEC and MAC ARQ are simultaneously employed. The application-layer FEC utilizes different RS codes: (63,30) for the base-layer, (63,47) for the first enhancement-layer queue and (63,63) for the second enhancement-layer queue. Note that at poor channel conditions (SNR