Abstract—This paper proposes a link-level hybrid FEC/ARQ

based on an adaptive forward error correction (adaptive FEC) algorithm for wireless media streaming, in the hope that the media could be played on receivers with high quality. This model, which uses adaptive forward error correction mechanism at the application layer to adjust to the current network condition, can maximize the performance of wireless multimedia communication. Our simulation results and mathematical analysis show that the hybrid FEC/ARQ achieves better quality of media streaming comparing with FEC alone and ARQ alone, in terms of playable frame rate, reliability and real-time performance on the receiving side.

Index Terms—cross-layer design, link-level hybrid, FEC/ARQ, Wireless Media Streaming Performance

I. INTRODUCTION It is well known that multimedia data in its original,

uncompressed form would require a huge channel capacity for transmission. Thereby compression of great mass of multimedia data before their transmission is highly desirable. However, the compressed video streaming is variable bit rate (VBR) and bursty, which causes severe packet loss problem when the network congestion occurs. Some control mechanisms for the application layer have been proposed in the literature. For example, Z. MEI et al. suggested an adaptive FEC mechanism to improve the reliability and real-time performance of the video streaming [1]. However, many researches show that a poor control mechanism at the data link layer may also degrade the performance of wireless networks significantly. On the other hand, the real-time delivery of multimedia data over wireless links is a big challenge due to the unreliable and dynamic characteristics of wireless networks, limited bandwidth as well as high QoS requirements for multimedia application. The probability of packet loss during transmission over wireless networks is much higher than over wired links: while bit error rate (BER) varies from 10-8 to 10-6 for wired channels, it varies from 10-3 up to 10-1 for wireless channels [2], which are unacceptable for wireless multimedia application.

The purpose of this paper is, based on cross-layer design, to

This work is supported by the National Natural Science Foundation of China under Grant No. 60673185.

contribute towards a hybrid FEC/ARQ mechanism for wireless media streaming, in order that the media could be played on receivers with high quality.

The main idea is that a hybrid FEC/ARQ mechanism at the data link layer is introduced to reduce the round trip time (RTT) and the packet loss rate. The TCP-friendly rate control protocol is adopted to be compatible with other TCP protocols. The video data is encoded by the adaptive FEC algorithm at the application layer. As a result, we obtain the maximum number of playable frames and improve the reliability and real-time performance.

The remainder of this paper is organized as follows. Section II provides an overview of the related work and the state-of-the-art in this area. Section III proposes and discusses the data link layer hybrid FEC/ARQ based on an adaptive forward error correction (adaptive FEC) algorithm for wireless media streaming. Section IV presents the performance analysis and evaluation of the proposed mechanism in both analytic and simulation approaches. Finally, Section V concludes the paper.

II. RELATED WORK

The QoS guarantee of wireless real-time media streaming has been fairly well studied in the literature [1, 3-7]. Z. MEI et al. analyzed the structure of video stream, and presented a frame decoding model that takes into account the frame types and the dependence among frames in [1]. On this basis, an optimal algorithm was proposed to allocate the bandwidth resource between the source video data and the FEC data. C. Barakat et al., using long-lived TCP traffic, analyzed the hybrid FEC/ARQ at the data link layer over wireless links [3]. Q. Zhang et al. discussed the combination of the application layer, the transport layer and the data link layer based on a cross-layer design [4]. In [5, 6, 7], hybrid FEC/ARQ method was adopted. The number of data retransmission is limited to restraint the delay for improving the quality of transmission.

However, there are some drawbacks in all these methods and research findings explained above. The influence of the information from the data link layer on the quality of video transmission is not concerned in [1]. The reliability and real- time performance of media streaming transmission is not mentioned in [3], their focus is on long-lived TCP traffic. How

Performance Evaluation of a hybrid FEC/ARQ for Wireless Media Streaming Yong Jin, Guangwei Bai, Peng Zhang, Hang Shen, Junyuan Wang

Dept. of Computer Science and Technology, Nanjing Univ. of Technology, Nanjing 210009, China E-mail: njutjinyong@yahoo.com.cn, bai@njut.edu.cn

978-1-4244-1708-7/08/$25.00 ©2008 IEEE 90

to allocate bandwidth dynamically was not considered in [4，5, 6, 7].

In this paper, we analyze the hybrid FEC/ARQ mechanism, in the context of wireless media streaming at the data link layer and the application layer. We use analytical and simulation approaches to compare and investigate packet loss rate, end-to-end delay, achieved bandwidth of GOP and the number of decodable frames. The results show that our mechanism is capable of utilizing the available network resource and achieve good perceptual quality at the application layer and the data link layer.

III. MODEL OF HYBRID FEC/ARQ In this paper, we use both Hybrid FEC/ARQ at the data link

layer and adaptive FEC at the application layer. At the application layer, the adaptive FEC algorithm is used to be adapting to the fluctuation of network condition and to allocate optimally network bandwidth. At the data link layer, the hybrid FEC/ARQ mechanism is used to reduce the RTT and packet loss rate, in order to meet the high QoS requirement of wireless media streaming.

A. RS Encoder

To meet the real-time performance requirement of video transmission, we use the forward error correction (FEC) technology. The source video stream is encoded by the RS(n,k) encoder. Note that the number of packets generated from the video frames is denoted by k, the number of redundant packets which are made from k source packets is denoted by n-k, and n is the total number of data packets transmitted. If any k or more packets are received, then all the source packets can be completely reconstructed. Let P(n,k) denote the probability that a k-packet video frame is successfully transmitted with n-k redundant FEC packets, which is given by formula (1).

( ) ( )∑ −==

−n

kl

lnlnl ppknP )1(, (1)

Where p denotes the average packet loss rate.

B. Adaptive Forward Error Correction Algorithm

According to [1], there are five parameters used to define the GOP structure, that is, NF represents the number of frames in a GOP; NP represents the number of P-frame in a GOP; SI represents the size of I-frame; SP

represents the size of P-frame; SB

represents the size of B-frame. In this work, we describe the wireless network condition

with four parameters. Let Pf be the frame loss rate at the data

link layer; Nmax be the number of maximal retransmission; tRTT be the round trip time; Rcur be the achieved bandwidth of

sending a GOP. The adaptive FEC mechanism focuses on how to select a

proper transmission strategy, for a given GOP structure and the wireless network condition.

Hence, the foregoing problems can be described as

∑

∑ ∑∈

∈ ∈

=

≤+=

Dfgop

Df Df

CUR

fEfE

SRffecfSizeFDBWts

][max][

][][),(.. (2)

Where D represents the set of frames to be sent in a GOP; F represents the allocation scheme of FEC data; E[fgop] represents the number of decodable frames in a GOP; E[f] represents the correctly decodable rate of a frame; BW(D, F) represents the full bandwidth of current transmission mechanism; Size[f] represents the size of a frame; fec[f] represents the FEC data allocated to a frame; S represents the size of a data packet.

The frame dependency relationships for P, B, and I frame are used to compute the playable rate for each type of frames. According to formula (1) and [1], we have four expressions for the playable rates of I-frame, P-frame, as well as the playable rates of B-frame prior to P-frame and B-frame after P-frame respectively, see formula (3), (4), (5) and (6).

))(),((][ IkInPIE = (3)

1

[ ] [ ] * ( ( ), ( ))i

i j jj

E P E I P n P k P=

= ∏ (4)

1

[ ] [ ] * ( ( ), ( )) * ( ( ), ( ))i

ij l l ij ijl

E B E I P n P k P P n B k B=

= ∏ (5)

2

1

[ ] [ ]* ( ( ), ( )) * ( ( ), ( ))PN

ij l l ij ijl

E B E I P n P k P P n B k B=

= ∏ (6)

A frame decoding model is proposed in [1] based on analyzing the structure of video stream, the video frame types and the dependence among MPEG frames.

Let’s take IBBPBBPBBPBBPBB in every GOP as an example. The frame decoding dependency sequence is given by I0P1B11B12P2B21B22P3B31B32P4B41B42B01B02. On the sender side, a series of consecutive MPEG frames of a video sequence are sent, starting with an I-frame. I-frames will be encoded with a higher amount of redundancy than P-frames, which in turn will be encoded with a higher amount of redundancy than B-frames. We can find a solution of formula (2) according to frame decoding dependency sequence. First, we find the set of all combinations of sending frames. Second, the FEC bandwidth should be allocated according to frame decoding dependency sequence. Finally, we will obtain the combination of the sending frames which is the maximum number of decodable frame by computing number of decodable frames respectively and the result how to allocate the bandwidth.

However, it is not concerned that the influence of the variation of wireless network condition at the data link layer on the end-to-end delay and achieved bandwidth in the algorithm. Obviously, the performance of the mechanism will be improved if we combined some mechanism at the data link layer with the algorithm.

C. The model of Hybrid FEC/ARQ In this section, the hybrid FEC/ARQ mechanism is

proposed to improve the foregoing algorithm in performance. The basic idea of our proposal is as follows. As shown in Fig. 1, the source video frames on the sender

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Fig. 1 The hybrid model FEC/ARQ

side are encapsulated to data packets in size of S, by using adaptive FEC mechanism at the application layer. This work focuses on the data transmission at the data link layer.

According to RFC 3580 [8], the MTU of the data packet is defined as 1500 bytes. In this case, one IP packet is encapsulated to one link-level (LL) frame. So, the packet loss rate is equal to the LL frame loss rate. At the data link layer, each LL frame is divided into K link-level (LL) transmission units. A LL transmission unit may be a bit, a byte, or any other data unit. To the K transmission units, we add N-K redundant units decoded by block code FEC, e.g. Reed-Solomon [9], which is very well suited for error protection against LL frame loss [10]. N is called the length of the code, K its dimension, and K/N its rate. A LL frame is decoded if we correctly receive K or more units of the frame at the receiver. To be delivered to the destination, the K units of a LL frame have to be well decoded. We denote Pt by the successful link level frame transmission rate and obtain it from formula (1), which is expressed by formula (7).

lNu

N

Kl

lut PP

lN

KNP −

=∑ −= )1)((),( (7)

Where Pu denotes the loss rate of LL transmission units, which is decided by formula (9).

2 max(1 ) (1 ) (1 ) (1 )Nt u u u u u u uP P P P P P P P= − + − + − + + −

max 11 NuP += −

(8)

So, we have 1 1max max

lnln(1 ) ftN N

PP

uP e e+ +−

= = (9)

Here we determine Pf based on a series of simulation experiments.

According to formula (7) and (9), we can calculate the LL frame loss rate, that is

* 1f tP P= − (10)

Where Pf* denotes the LL frame loss rate, which will be used

in the analytical study in section IV. If one LL transmission unit does not arrive at the receiver

successfully, the error recovery mechanism at the data link layer will resort to ARQ for the retransmission of the unit. Let Nmax (Nmax=0, 1, 2, 3…) denote the maximum number of retransmission. Retransmission at the data link layer clearly improves the correctly received frames rate. Nmax = 0 means

that there are no retransmissions and that ARQ is disabled. When Nmax is larger than 0 and the LL transmission unit did not get through the wireless link, ARQ assumes that the unit cannot be correctly recovered.

The receiver acknowledges each LL frame, either with a positive ACK or a NACK. On receiving a NACK, the sender retransmits the corresponding frame immediately.

In addition, we use the TCP-friendly rate control protocol to ensure the fairness in channel bandwidth allocation and the smoothness of sending rate at video application to adjust to the variation of wireless channel. Padhye et al. [11] suggested that the available bandwidth of the TCP-friendly rate control is decided by equation (11)

( )( / )

2 3 23 1 323 8

bytes sSRp pt t p pRTT RTO

=+ +

(11)

Where S is the packet size; tRTT is the round trip time; p is the packet loss rate on the interval [0, 1.0]; tRTO is the TCP retransmission timeout threshold. R provides an upper bound of the network bandwidth.

We suppose that LL transmission units are lost independently of each other with probability p (Bernoulli model). The error process is only applied to data packets. ACKs of ARQ are supposed to cross the wireless link without being dropped given their small sizes. The focus is on how to improve performance of the reliability and to meet the real-time performance quality of media streaming transmission.

IV. PERFORMANCE EVALUATION

In this section, we use NS-2 to simulate, analyze and evaluate the performance of both the data link layer protocol (Hybrid FEC/ARQ) and the application layer algorithm (adaptive FEC). The simulation environment is illustrated in Table I.

The video traffic trace used in the experiment is a clip from the movie “StarWarsIV” [12], with a GOP structure of IBBPBBPBBPBB. We study separately the effects of FEC, ARQ and hybrid FEC/ARQ on the streaming performance. The metrics of interest at the data link and transport layers include packet loss rate, end-to-end delay, achieved bandwidth of GOP as well as the number of decodable frames.

A. Performance with FEC alone In this case study, we only consider the FEC mechanism at

the data link layer, by setting Nmax = 0. Fig. 2 compares the packet loss rate between the simulation

and the FEC alone mechanism at the data link layer. The packet loss rate is obviously decreased as using FEC alone mechanism. However, the drawback of FEC is that it consumes some extra bandwidth to transmit the redundant information.

Fig. 3 illustrates the variation of the achieved bandwidth of a GOP, as a function of the FEC parameter “N”, in case of delay with 10ms, 100ms and 200ms. The amount of redundan-

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Table I: Parameter settings

Parameters Value Simulation time 50sec

Topology size 800m*800m

Number of mobile nodes 50 Traffic type MPEG-4 video Packet size 1500bytes Transport protocol UDP Route protocol AODV MAC protocol IEEE802.11 Pause time of mobile nodes 0sec Motion model Random waypoint Traffic model Spread randomly Number of sessions 1 Maximal speed 10m/sec T 0.01sec NF 15 NP 4 SI 20bytes SP 12bytes SB 3bytes Nmax 1~5

Fig. 2 Packet loss rate, FEC alone

Fig. 3 Achieved bandwidth of GOP, FEC alone, Pu=0.01

cy depends on “N”. In all cases, the LL transmission unit loss rate Pu is set to be 0.01. Clearly, there is a significant improvement in the achieved bandwidth of GOP, as “N” is increased, up to N=15. Beyond this point, the achieved bandwidth drops slowly, reflecting that the decay of the achieved bandwidth is decided by K/N. This behavior was also demonstrated in [3]. On the other hand, the link utilization is less than 100% as using FEC.

In this work, we compute the link utilization without consideration of redundancy data resulting from the FEC.

Fig. 4 Packet loss rate, ARQ alone

Fig. 5 Achieved bandwidth of GOP, ARQ alone, Pu=0.01 B. Performance with ARQ alone Next, we examine the effect of ARQ alone on media

streaming transmission for N=K (=6). Fig. 4 compares the packet loss rate between the simulation

and the ARQ alone mechanism at the data link layer. The packet loss rate is obviously decreased, as the value of Nmax is increased. When the quality of the channel is getting poor, the packet loss rate is increased. The larger Nmax is , the lower the packet loss rate.

Fig. 5 shows the achieved bandwidth of GOP plotted as a function of Nmax. Clearly, the achieved bandwidth of GOP is always increasing with Nmax, even though we are dealing with an extreme case where the delay D is large and the packet loss rate p is high. The reason is that the ARQ mechanism reduces the packet loss rate much more than it increases the end-to-end delay. As a result, the performance is improved.

C. Performance with the model of Hybrid FEC/ARQ In this section, we discuss the performance of wireless

media transmission using hybrid FEC/ARQ (the data link layer) with adaptive FEC (the application layer). Fig. 6 shows three performance of hybrid FEC/ARQ comparing with the simulation results and analytical results using FEC alone and ARQ alone.

By comparing Fig.2 and Fig.4, as well as by investigating Fig.6 (a) and Fig.6 (b), we clearly notice that the ARQ alone mechanism is able to achieved good performance. This good performance of ARQ is a surprising result, but it seems to be logical since FEC always consumes extra bandwidth, whereas ARQ consumes extra bandwidth only when LL transmission units are lost.

One idea could be that we only use FEC for LL frames to be retransmitted, instead of original LL frames. Next, we explore this mechanism by simulation and analytical approach.

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Our studies show that the FEC mechanism performs better when the packet loss rate is larger than 0.01, since ARQ alone will result in a lot of retransmissions. However, too much FEC redundant data may not produce an optimal value of the number of decodable frames.

Fig.6 (c) indicates the number of decodable frames in the simulation results, FEC alone, ARQ alone and hybrid FEC/ARQ with time. We observe that all of the FEC alone, ARQ alone, and hybrid FEC/ARQ improve the number of decodable frames significantly. But at 3.641808 sec, the number of decodable frames of the simulation, FEC alone and ARQ alone are lower than the one of the hybrid FEC/ARQ, which is 15. On the other hand, the benefit from the hybrid FEC/ARQ (Nmax is 3) is very high. The hybrid FEC/ARQ (Nmax is 3) with time manages to keep the number of decodable frames at 15 after 3.641808 sec, suggesting that the hybrid FEC/ARQ (Nmax is 3) is the best solution to maximize the number of decodable frames in lossy networks than the simulation approach, FEC and ARQ at the data link layer.

(a) Packet loss rate

(b) End-to-End Delay

(c) Number of Decodable frames

Fig. 6 Performance parameters with the simulation, ARQ alone, FEC alone and hybrid FEC/ARQ, N=8, K=6, Nmax=3

In this section, the performance with the link-level hybrid FEC/ARQ is superior to one with the simulation approach, FEC and ARQ at the packet loss rate, end-to-end delay, achieved bandwidth of GOP as well as the number of decodable frames.

V. CONCLUSION In this paper, a link-level hybrid FEC/ARQ mechanism was

proposed for supporting the wireless multimedia application. We analyze the performance of the hybrid model based on an adaptive FEC. We apply the hybrid FEC/ARQ mechanism at the data link layer to decrease the round trip time and the packet loss rate. Our investigations in both analytic and simulation approaches demonstrate that the hybrid FEC/ARQ mechanism provides a significant improvement in the reliability and the real-time performance of wireless media streaming transmission, with adapting to the fluctuation of the wireless network state dynamically.

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