a link-lifetime based dynamic source routing

Journal of the Chinese Institute of Engineers, Vol. 33, No. 5, pp. 761-768 (2010)

A LINK-LIFETIME BASED DYNAMIC SOURCE ROUTING PROTOCOL (LTDSR) FOR MULTIMEDIA OVER MANETS

Guangwei Bai*, Jinjing Tao, and Hang Shen

ABSTRACT This paper proposes a link-lifetime based dynamic source routing protocol (LTDSR) to support Soft QoS for multimedia over MANETs, with a series of improved routing mechanisms. First, our research has shown that some routes held in the route cache may become invalid as time passes, due to node mobility. In this work, we introduce a path lifetime based route selection strategy by establishing link lifetimes for routes, so that no node may select already broken routes. A route discovery will be initiated before all the routes in the cache are broken. Second, considering that the routing protocol reacts mistakenly to the transmission failures due to congestion, a cross-layer design framework is explored to establish a shared linkstate table with nodes’ coordinate and velocity vector; on this basis, the link-state detection mechanism can identify the cause properly when packet loss occurs, and then coordinate with the routing protocol to avoid unnecessary route updating. Our simulation results demonstrate that the LTDSR improves the quality of multimedia playing significantly, in terms of decodable frame ratio, end-to-end frame delay and delay jitter. In addition, the route overhead is greatly reduced. Key Words: mobile ad hoc networks, multimedia, routing protocol, cross-layer design.

I. INTRODUCTION In multihop wireless mobile ad hoc networks (MANETs), mobile nodes cooperate to form a network without using any infrastructure such as access points or base stations. Instead, the mobile nodes forward packets for each other, allowing communication among nodes outside wireless transmission range. With the advance in wireless communication technologies and the dramatic growth in network-based multimedia applications, there is a tremendous demand for multimedia delivery over MANETs (Cranley and Davis, 2005). However, delivering multimedia over wireless networks is a very challenging task. On the one hand, multimedia delivery inherently has strict quality of service (QoS) requirements for bandwidth, delay, and delay jitter, which have a direct effect on *Corresponding author. (Tel: +86-25-83532156; Fax: +86-2558139500; Email: [email protected]) The authors are with the College of Electronics and Information Engineering, Nanjing University of Technology, No. 5, XinMofan Road, Nanjing 210009, China.

the user perceived quality of experience. Even though multimedia network applications are typically loss tolerant, the quality of reconstructed video or audio will be impaired, due to the dependency among video frames in the same group of pictures (GOP) at the encoder. On the other hand, a wireless network is a dynamic environment. The nodes’ mobility and the fundamentally limited bandwidth of the wireless medium, together with other characteristics in wireless networking such as high error rates, unstable and dynamic changes of wireless channel condition, and channel competition, combine to create significant challenges for routing protocols operating in an ad hoc network (Bai et al., 2007). QoS routing protocols play a major part in a QoS mechanism for wireless real-time multimedia networking applications, since it is their task to find which nodes, if any, can serve an application’s requirements. In MANETs, node mobility may cause link failures frequently; in turn, the corresponding route held in the route cache becomes invalid; this causes large delays and dropped packets as data are sent down these broken routes. It is obvious that all route records

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concerned should be erased from the route cache. There are two methods to identify whether a route should be removed from the route cache, or not. First, each route in the route cache may be assigned an attribute “RouteCacheTimeout”, e.g. RouteCacheTimeout may be set to 300 seconds (Johnson et al., 2007). If a route is not used when the corresponding RouteCacheTimeout expires, the route will be removed from the route cache. Second, a route may be deleted when a received route error message indicates that one of the links along the route is broken. The IEEE 802.11 standard defines the maximum retransmission times as 7 for small frames, and 4 for large frames. In this case, the link layer does not report the link failure to the routing agent until the retransmission limit is reached. Our investigation shows that the two policies above may degrade the utilization of the network bandwidth. Thus, it is necessary for MANETs to have an optimal routing and quality of service mechanism to support real-time multimedia delivery. For this purpose, we, in this work, introduce a route selection mechanism which is based on path lifetime. One of the challenging problems in MANETs involves how to distinguish whether data frame loss at the MAC layer has occurred due to node mobility or traffic congestion (Pandey et al., 2007), and then having the routing and transport protocols react properly. Unfortunately, most existing routing protocols for MANETs have a major design flaw in that they react to all frame loss as a sign of mobility, and then initiate an unnecessary route discovery or repair process, an expensive operation where a network-wide flood is generated to locate the destination node. As a result, quite an amount of bandwidth may be consumed. Both AODV (Jiang et al., 2007) and DSR (Johnson et al., 2007) view packet loss as an indication that all paths using the MAC layer destination as a next hop have failed. Similar behavior exists with routing protocols that use passive ACKs or HELLO messages to determine link availability. Consequently, the routing mechanism would misreact when packet loss occurs due to congestion. Therefore, it is desirable to explore a link-state detection mechanism to ensure that the routing protocol reacts to the congestion properly. This paper proposes a link-lifetime based dynamic source routing protocol (LTDSR) for multimedia over MANETs, with a series of improved routing mechanisms. We refer to our work as an enhanced version of DSR. The purpose is to explore a routing protocol with Soft QoS support. First, we introduce a path lifetime based route selection strategy by establishing a link lifetime for the route. The aim is to ensure that nodes choose a still connected route for packet transmission, and initiate route discovery before all the routes become broken. Second, a crosslayer design framework is explored to establish a

shared link-state table to provide node coordinate and velocity vector; according to the link-state table, the corresponding node can determine the cause of a lost frame properly, thus avoiding unnecessary route updating. Our simulation results show the LTDSR improves the performance of real-time multimedia streaming significantly. The remainder of this paper is organized as follows. Section II proposes a link-lifetime based dynamic source routing protocol (LTDSR) for multimedia over MANETs and discusses the details of its implementation. Section III involves thorough analyses and evaluation of the LTDSR performance in simulation methodology. Finally, section IV concludes this paper. II. LINK-LIFETIME BASED DYNAMIC SOURCE ROUTING This section focuses on the basic ideas of the link-lifetime based dynamic source routing protocol (LTDSR) and discusses the details of its implementation. We begin with a description of a path lifetime based route selection, followed by the link-state detection mechanism. 1. Path Lifetime Based Route Selection The expense of finding a link breakage is very high. One the one hand, the MAC layer can not realize a link breakage, until the packet retransmission limit is reached. On the other hand, since a link break induced by node mobility invalidates all routes containing this link, alternate routes have to be discovered for delivery of subsequent packets (on-demand routing). This new route discovery process incurs network-wide flooding of routing requests and extended delay for packet delivery. Although ad hoc routing protocols usually have their own route cache schemes, a route cache may contain inefficient or obsolete routes, resulting in a poor performance of real-time multimedia application. To avoid increased complexity of route caching schemes and trial failures of packet transmission, we suggest a novel routing strategy to prevent packets from transmitting over a path involving already broken links. The main idea is that a route is selected based on a path lifetime,T path, determined by the minimum link lifetime of the links over the path. And route requests for an original node of a packet may be flooded ahead of schedule to some extent. Tseng et al. presented a probabilistic model of route lifetime of a given routing path in a MANET based on the discrete-time, random-walk model (Tseng et al., 2003). First, the mechanism sets a length of time to detect link state and then compute a probability that a route remains alive; then it determines a

G. Bai et al.: A Link-Lifetime Based Dynamic Source Routing Protocol (LTDSR) for Multimedia over MANETS

reasonable value for t max (expected route lifetimes). In order to simplify the routing mechanism further, we, in this paper, apply a direction-prediction method, where the lifetime of a link is defined based on the current global position and velocity vector of the two neighboring nodes provided by GPS or other location systems. Let • (x i, y i) be the coordinates of node i; • (x j, y j) be the coordinates of node j; • (vx i , vy i) and (vx j, vy j ) be the velocity vector of nodes i and j respectively.

t ij =

(rvx2 + rvy2)γ 2 – (rvxd y – rvyd x) 2 – (rvxd x + rvyd y) , rvx2 + rvy2

where rv x = vx i – vxj; rv y = vy i – vyj; d x = x j – x i; d y = y j – y i. To obtain the path lifetime, we extend the route reply (RREP) message to include coordination (x, y), velocity vector (vx, vy) and T path (see Fig. 1). On receiving a route request (RREQ) message, the destination node (D) creates an RREP and fills the coordinates (x, y), velocity vector (vx, vy) and Tpath (initiated zero). When a node receives an RREP, it first calculates the link lifetime, and then determines Tpath. Thus, the path expired time (T pet) is the current time (T curr) plus Tpath. When the source node (S) has data to send, it checks its route cache to see if an unexpired route is available. If so, it uses the route for data sending immediately. Otherwise it broadcasts an RREQ. The latency of the first RREP received in response to an RERR is 3.5 ms (Maltz et al., 1999). In consideration of node synchronization, we set the path invalid threshold value to 4 ms. So, S initializes a route discovery process, if the residual path lifetime of all paths is less than 4 ms, that is, T pet – T curr < 0.004. This helps to improve the packet delivery and delay for the subsequent packets. 2. Link-State Detection Mechanism When an intermediate node forwards a data packet based on the source route, it, at first, detects whether any broken link exists along the path to the destination. If yes, it sends a route error (RERR) message back to the source. Before sending RERR, the node salvages the packet and modifies the route information for the packets for which the next-hop link is the broken link in its Network Interface Queue, instead of discarding the data packets, if the node has an alternative route to the packets’ destination in its

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The transmission range of nodes is γ . Then, the expected link lifetime (t ij) for the link between nodes i and j is defined as follows (Jung et al., 2007):

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Route Cache. Although the DSR receives a notification of frame transmission failure from the MAC layer, it does not distinguish whether the failure is due to link breakage (mobility), or due to congestion. Multimedia traffic, due to its heterogeneity and bursty nature, will lead to more congestion than data traffic does. As a result, it suffers from increased overhead and decreases the efficiency and performance of wireless multimedia networks. In this work, a link-state detection (LSD) mechanism is thus used to ensure that routing protocols react properly to congestion. Depending on the path lifetime, we can not determine the time of link breakage due to node mobility on topology dynamics. To compensate for inaccuracy in path lifetime, we further establish a linkstate table for every node, and meanwhile update node coordinate and velocity by an event-driven method. Another important issue is how the routing protocol gets the next-hop information, without producing too much extra overhead. Traditionally, wireless network architecture has been divided into hierarchical layers, based on the OSI architecture paradigm of computer networks. It is not possible for the layered network structure to provide node information for routing operating at the network layer. Recently, performance optimizations for multimedia delivery over MANETs suggest the breaking of OSI hierarchical layers, which has come to be known as “cross-layer design” (Srivastava et al., 2005). In this work, we introduce a cross-layer design framework to provide information for the routing protocol (see Fig. 2). A shared link-state table is established between the MAC layer and the network layer. The MAC layer provides node coordinate and velocity by ACK frame for the network layer. The link expired time (T let) is T curr plus t. Therefore, the routing protocol can

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determine whether the next-hop node moves away, or not. The original IEEE802.11 ACK frame (IEEE Std 802.11-1999) is shown in Fig. 3. We define a new control frame using reserved Subtype (1001), named EACK (see Fig. 4), which is similar to ACK, but it adds the nodes’ coordinate and velocity vector. When node i apperceives its velocity or direction changes, it will notify the last-hop j using EACK in the next acknowledgement. Node j receives the EACK, and then updates the corresponding entry. Next, we present a link-state detection mechanism. Whenever node i wants to forward a data packet and determines that the next-hop node j is unreachable, node i searches its link-state table to see whether node j is still within the transmission range. LSD can conclusively determine whether the next-hop j is still within the transmission range of node i, if j still exists in the link-state table and the link lifetime (tij) has not expired. The complete LSD mechanism is shown in Fig. 5. The intermediate node does not send RERR to update the

If transmission failure to node j then search neighbor state table If node j exists in table then If Tcurr < Tlet then notify LTDSR that node j is congested Else delete the entry of node j notify LTDSR that node j has moved away End Else notify LTDSR that node j has moved away End End

Fig. 5 Link-state detection mechanism

transmitting route for S, if LSD determines the transmission failure is due to congestion. This mechanism extends the utilization of the existing route and reduces expensive route discovery processes, enhancing the network bandwidth utilization.

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III. PERFORMANCE EVALUATION In this section, we use NS-2 (Fall and Varadhan) to simulate, analyze and evaluate the performance of the proposed link-lifetime based dynamic source routing (LTDSR) protocol for multimedia streaming in a MANET environment. The existing DSR (Dynamic Source Routing) is selected as a reference in our performance comparison. Two case studies were designed and conducted, with variation in node moving speed and the number of cross traffic sessions. In order to eliminate the impact of randomness, ten scenarios were generated randomly to run for each simulation experiment. The presented results are the average of the ten runs. For multimedia traffic, we use a medium qualify MPEG4 video clip from the movie Star Wars IV (Video Traces Research Group), coded at 25 fps with Group of Pictures (GOP) size of 12. The mean bit rate is 77.72 kbps and the peak value is 938 kbps. The bit rate of the cross traffic (CBR) session is 12kbps. The network consists of 60 nodes distributed in a rectangular region with 1200 meters by 800 meters. The random waypoint model is used to model mobility. The length of interface queue is 50. The standard IEEE802.11 is used in MAC. The metrics for performance evaluation of the LTDSR for supporting the multimedia application are as follows. • Decodable Frames Ratio: We use this metric rather than packet delivery ratio, because it is more indicative of the quality of media play. It considers the dependency among different frames. For example, if an I-frame is undecodable on the receiving side, all other frames belonging to the same GOP cannot be decoded. • Average End-to-End Frame Delay: We only calculate the delay of the decodable frames. • Average Frame Delay Jitter: It only considers decodable frames. • Link Failures: The number of link failures reported during the simulation (per second). • Route Discovery Frequency: The total number of route discoveries initiated per second. • Packet Loss Rate: The number of packets which

fail to arrive at the destination, divided by the total number of packets sent by the source. • Route Overhead: The total times control packets are transmitted by all nodes per second. • Normalized Packet Load: The total times control packets are transmitted by all nodes are divided by the total number of packets received by the destination node. 1. Case Study 1: Varying Mobility Figures 6 and 7 illustrate the eight performance metrics as a function of mobility in experiment I. We only consider the continuous mobility case. To change the mobility level of nodes, the maximum speed is varied from 5 m/s to 25 m/s. There are 5 cross traffic sessions (non-multimedia) in the scenarios. Figure 6 shows the performance of LTDSR and DSR from the application layer, in terms of decodable frame ratio, frame delay and delay jitter. Fig. 6(a) shows the result of decodable frame ratio versus the mobility, which has a significant impact on quality of media play. We can observe tremendous improvement of decodable frames ratio using LTDSR, compared with DSR. Even at a high moving speed of 25 m/s, the quality of video playing using LTDSR remains good (over 90%), while the decodable frames ratio using DSR drops to 66%, causing a poor quality of video playing. The main reason is that the linkstate detection mechanism prevents the LTDSR from initializing unnecessary route discovery processes, enhancing the bandwidth utilization. In addition, the path lifetime based route selection also helps to increase the packet delivery ratio. Figures 6(b) and (c) plot the average frame delay and delay jitter of the two routing protocols respectively. The delay jitter is an important QoS metric for wireless real-time multimedia streaming. As the node moving speed increases, both the frame delay and delay jitter tend to be increased. But LTDSR greatly decreases the frame delay and delay jitter. We determine that a better quality of video play can be obtained by using LTDSR than by using DSR. The link-state detection mechanism reduces the route overhead

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efficiently, which alleviates channel competition, thus LTDSR improves frame delay and delay jitter. Figure 7 shows the performance of LTDSR and DSR from the network layer. Fig.7(a) presents the average number of link failures per second during the simulation. This metric reflects the number of RERR messages generated. As the node moving speed increases, the link failures become more frequent. However, the link-state detection mechanism in LTDSR ultimately identifies transmission failures in cases where congestion prevents frame transmission. Thereby, LTDSR produces less RERR than DSR does. Figure 7(b) illustrates the simulation result of the route discovery frequency versus the mobility. The frequency of routing discovery for LTDSR is less than that for DSR. As the node moving speed increases, the link breakages become more frequent. Therefore, the route discovery frequency shows a growth tendency. For the traditional routing protocol, a frame dropped at the MAC layer indicates that the corresponding link is broken; hence, S tries to use any other route to D that it happens to know, or triggers a new route discovery process to find a route for subsequent packets to D. Whereas, the link-state detection mechanism in LTDSR helps to distinguish a packet dropped due to congestion; as a result, unnecessary RERR and route discovery processes are eliminated. As shown in Fig. 7(c), the packet loss rate for both LTDSR and DSR increases with the mobility level. The packet loss rate using DSR rises sharply, while the packet loss rate using LTDSR rises slowly. Even at a high mobility (25 m/s), the packet loss rate using LTDSR is less than 9%. On this basis, we determine that LTDSR can commendably be adapted for

a network environment of high mobility. The comparison of route overhead is shown in Fig. 7(d). The number of control packets increases with the mobility level. LTDSR achieves a remarkable reduction in routing overhead at all speed levels. By using link-state detection mechanism, LTDSR reduces routing overhead dramatically compared with DSR, thus enhances the network bandwidth utilization. In addition, the path lifetime based route selection contributes to increasing the number of packets delivered. The result of normalized packet load (Fig. 7(e)) shows a similar tendency with route overhead. With the packet delivery rate increases, the slope of normalized packet load is less than that of the route overhead. As a result, LTDSR shows a high efficiency of performance protocol, compared with DSR. 2. Case Study 2: Varying Number of Cross Sessions Figures 8 and 9 show the eight performance metrics as a function of varying sessions in experiment II. We vary the number of cross traffic sessions with 12 kbps from 5 to 10, in order to compare performance as offered load increases. The maximum speed of node is fixed as 10 m/s. Figure 8(a) shows the simulation result for decodable frame ratio. As the number of cross traffic sessions increases, the collision probability for data packets increases significantly, reflecting a decrease in the decodable frame ratio. The simulation result demonstrates tremendous improvement of decodable frames ratio using LTDSR, compared with DSR. At a large number of cross traffic sessions, the decodable

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frames ratio using DSR is less than 63%, while that using LTDSR is still over 85%. According to this result, we determine that LTDSR can profitably be adopted for use in poor network environments. The reasons are that the link-state detection mechanism and path lifetime based route selection save the limited bandwidth, and thus enhance the packet delivery. From Figs. 8(b) and (c), the average end-to-end frame delay and delay jitter increase with increase of the number of cross traffic sessions. As the number of cross traffic sessions increases, the collision probability for data packets sent increases significantly. However, the average frame delay and delay jitter using LTDSR show a tremendous improvement. The main reason is that the link-state detection mechanism ultimately distinguishes packet loss due to congestion, which decreases the transmission of route control messages, thus, the LTDSR can alleviate network collisions and improve the performance in terms of frame delay and delay jitter. Figure 9(a) presents the average number of link failures per second during the simulation. As the number

of sessions increases, the link failures increase. The link-state detection mechanism in LTDSR successfully distinguishes packets dropped due to congestion from those dropped due to mobility. Therefore, LTDSR efficiently reduces the initialization of route discovery processes. Figure 9(b) illustrates the simulation result for route discovery frequency. Similar to Fig. 7(b), LTDSR initializes fewer route discoveries, compared with DSR. When the number of cross traffic sessions increases, the network becomes more congested. However, with the link-state detection mechanism, the LTDSR avoids unnecessary route discovery processes. As shown in Fig. 9(c), the packet loss rate increases as the offered load increases. However, LTDSR achieves a remarkable improvement in packet loss. This indicates that congestion is only temporary for multimedia over MANETs. The path will resume the transmission of packets in a short time. Two schemes in LTDSR greatly help to reduce the overhead and to improve network bandwidth utilization. The performance of route overload is shown in

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Fig. 9(d). The number of control packets increases with the increment of the offered load. However, LTDSR achieves a remarkable overhead reduction, due to the link-state detection mechanism and path lifetime based route selection. Fig.9(e) presents the performance of normalized packet load. This metric has a tendency similar to route overhead. With packet delivery rate incense, the slope of normalized packet load is less than that of route overhead. IV. CONCLUSIONS Real-time multimedia application over mobile ad hoc networks is particularly challenging due to the QoS requirements of multimedia applications and the unreliability of the wireless medium. In this paper, we propose the LTDSR protocol for multimedia in mobile ad hoc networks. First, we present a path lifetime based route selection to prevent packets being transmitted over already broken paths. Second, we propose a cross-layer design framework to provide the node information to the network layer, and then introduce a link-state detection mechanism to avoid both unnecessary RERR transmission and route discovery processes. The results demonstrate that LTDSR significantly improves the performance of multimedia streaming over MANETs, in terms of decodable frame ratio, delay and delay jitter. Meanwhile, LTDSR decreases route overhead. As a result, we believe that the mechanisms in LTDSR are feasible for multimedia application over MANETs. ACKNOWLEDGMENTS The authors would like to thank the anonymous reviewers who provided excellent and thorough feedback that improved the quality of this paper. The authors gratefully acknowledge the support and financial assistance provided by the National Natural Science Foundation of China under Grant No. 60673185; the Scientific Research Foundation for the Returned Overseas Chinese Scholars by the State Education Ministry of China under Grant No.[2007]1108, as well as “Qing-Lan” Project Foundation of Jiangsu Province, China under Grant No. [2007]2. NOMENCLATURE D LSD RREP RREQ RERR S T curr T path T pet

destination node link-state detection route reply route request route error source node current time path lifetime path expired time

REFERENCES Bai, G., Oladosu, K., and Williamson, C., 2007, “Performance Benchmarking of Wireless Web Servers,” Ad Hoc Networks, Vol. 5, No. 3, pp. 392-412. Cranley, N., and Davis, M., 2005, “Performance Evaluation of Video Streaming with Background Traffic over IEEE 802.11 WLAN Networks,” Proceedings of ACM WMuNeP’05, Montreal, Quebec, Canada, pp. 131-139. Fall, K., and Varadhan, K., 2000, “Ns Notes and Documents,” The VINT Project, UC Berkeley, LBL, USC/ISI, and Xerox PARC. Available: http://www.isi.edu/nsnam/ns. IEEE Std 802.11-1999, ISO/IEC DIS 8802-11, “Wireless LAN Medium Access Control (MAC) and Physical Layer Specifications,” LAN MAN Standards Committee of the IEEE Computer Society. Jiang, M., and Li, J., “Ad Hoc On-Demand Distance Vector (AODV) Routing,” Network Working Group, Available: http://www.ietf.org/rfc/rfc3561.txt. Johnson, D., Hu, Y., and Maltz, D., 2007, “The Dynamic Source Routing Protocol (DSR) for Mobile Ad Hoc Networks for IPv4,” Network Working Group, Available: http://www.ietf.org/rfc/rfc4728. txt. Jung, S., Lee, Yoon, S., Shin, J., Lee, Y., and Mo, J., 2008, “A Geographic Routing Protocol Utilizing Link Lifetime and Power Control for Mobile Ad Hoc Networks,” Proceedings of MobiHoc, Hong Kong, China, pp. 25-32. Maltz, D. A., Broch, J., Jetcheva, J., and Johnson., D. B., 1999, “The Effects of on Demand Behavior in Routing Protocols for Multihop Wireless Ad Hoc Networks,” IEEE Journal on Selected Areas in Communications, Vol. 17, No. 8, pp. 1439-1453. Pandey, M., Pack, R., Wang, L., Duan, Q., and Zappala, D., 2007, “To Repair or Not to Repair Helping Ad Hoc Routing Protocols to Distinguish Mobility from Congestion,” Proceedings of IEEE INFOCOM, Anchorage, Alaska, USA, pp. 2311-2315. Srivastava, V., and Motani., M., 2005, “Cross-Layer Design: A Survey and the Road Ahead,” IEEE Communications Magazine, Vol. 43, No. 12, pp. 112-118. Video Traces Research Group, Available: http://trace. eas.asu.edu/index.html. Yu-Chee Tseng, Yueh-Feng Li, and Yu-Chia Chang, 2003, “On Route Lifetime in Multihop Mobile Ad Hoc Networks,” Mobile Computing, IEEE Transactions, Vol. 2, No. 4, pp. 366-376. Manuscript Received: Aug. 20, 2009 Revision Received: Mar. 10, 2010 and Accepted: Apr. 10, 2010