（北京航空航天大學 100191）

摘要：移動Ad Hoc網絡是一種無中心實體的自組織無線網絡，具有組網靈活、易部署、健壯性強的特點，因此成為近年來無線網絡技術領域的研究熱點。本文對移動Ad Hoc網絡動態源路由協議進行了分析、介紹了幾種針對鏈路狀態預測、服務質量和多徑傳輸等特性改進動態源路由協議的方法，并且介紹了一個動態源路由協議在視頻傳輸中的應用示例。

關鍵字：移動自組織網絡、動態源路由、服務質量、多徑路由

Latest research on Dynamic Source Routing protocol of MANET: Analysis, Approach and Application.

Ke Yan、Xiling Luo

                                                     （Beihang University 100191）

Abstract: Mobile Ad hoc Network is a non infra-structured self-organizing network with the characteristic of flexible, easy deployment and robustness, thus became the hot research field of wireless network technology in recent years. In this paper, we presents an analysis of mobile ad hoc network dynamic source routing protocol (DSR), introduce several approach for improving DSR regarding link state detection, quality of service and multi-path transmission along with an example of DSR’s application in video streaming.

 

Keywords: MANET, dynamic source routing, quality of service, multi-path routing

 

Mobile wireless network can be classified as two types in term of structure. The first is called infra-structured network, which employs centralized fixed wireless units called base station, serving several mobile units within its wireless signal range. An infra-structured network may consist of several base stations, separately distributed and covering every corner of a certain area. As mobile units travel around the area, they may leave one base station's range into another station's. On such condition a "handoff" operation occurs between the new station and old one, enabling seamlessly communication with network for the mobile units. The typical application of infra-structured network includes cellular phone network and WLAN. The second type of mobile wireless network is mobile ad hoc network (MANET). Unlike infra-structured network, this type of network does not require a centralized base station. Mobile nodes in the network act as both mobile unit and base station. They communicate with each other either by direct connection or by multi-hop relaying through intermediate nodes.

Mobile ad hoc network have gain plenty focus during recent years since it can be applied in many rigorous situations, like tactical scenario, emergency rescue mission, conference, inter-vehicular communication and so on. The key point of running such network is to build stable routes between the mobile nodes, most concerned aspect of which is their efficiency and correctness. People have proposed many routing protocols meeting such criteria.

Routing protocol of MAENT can be categorized as table-driven and on-demand. Table-driven protocols run like most wired network protocols, each node maintains frequently updated routing table regardless of if routes are being requested. On the contrary, in on-demand protocols route discovery is only initiated when it’s needed.

Research on MANET protocols mainly focus on on-demand protocols. Dynamic Source Routing (DSR) protocol [1] is a simple and efficient protocol proposed by David B. Johnson and David A. Maltz in 1996. Recent years many optimized or enhanced routing protocol based on dynamic source routing protocol have been presented by researchers to the public.

This paper reviews the latest research on dynamic source routing protocol of mobile ad hoc network along with its application on specified tasks.

The remainder of this paper is organized as follows: In section 2 we'll have an overview of the DSR protocol, in section 3 we first make an analysis on the characteristic of DSR to point out the optimization and enhancement we may posed on it, then we presents the latest research result on DSR during the past months, in section 4 we’ll introduce an application case of DSR protocol, in section 5 we’ll make a conclusion and foresee the prospect of MANET research.

The dynamic source routing protocol is a simple, efficient protocol designed for mobile ad hoc network. It works in two phases: route discovery and route maintenance. When a source node wants to communicate with a target node, the route discovery phase is initiated. During the route discovery phase, the source node broadcasts a route request packet (RREQ). Each node receiving the RREQ refreshes it by adding itself into it and re-broadcasts it again. If the target node finally receives the RREQ, it sends a route reply packet (RREP) back to the source node by following the route contained in RREQ reversely. Each node along the route path stores the route into its route cache when receiving the RREQ packet. If a node detects that a path to the target was broken, the route discovery phase will be initiated again. Node maintenance is that each node maintains a route cache which contains all the routes the node has learned. When a node needs to send data, it first checks its route cache if there’s an existing route to the target. If no route’s found, the node will initiate the route discovery phase.

The most remarkable characteristic of DSR is its route cache strategy: every mobile node maintains a table consists of entries of routes. Regarding this characteristic, variations of optimization can be made to improve the routing performance, such as link life time prediction, path selection strategy optimization, link-state detection, quality of service insurance, multi-path routing, cross-layer designed routing and so on. Below we’ll introduce many approaches modifying DSR for better performance.

LTDSR [3], standing for link-life time based dynamic source routing, is a novel protocol proposed by Guangwei Bai and Jinjing Tao. The LTDSR mainly focus on two aspects of the routing procedure: link-lifetime based route selection and link state detection.

In order to eliminate the affection of link failure caused by node movement, the author employs a link life time prediction method, which measures life time of a link by the current location and relative speed of two neighboring nodes.

The method introduces a new parameter called T_path, which can be calculated using node coordinate, velocity of two neighboring nodes. The route reply message (RREP) is then extended to include coordinate, velocity vector and T_path parameter of current node. RREP originates from the destination node and is relayed reversely through the route to the source node. Each node receiving the RREP should calculate the T_path with parameters mentioned above and then adds the result into the RREP. Thus, in the circumstance that a node is assigned the task of relaying data packets, it can verify the availability of route entries in its route cache by measuring each route entry’s life time thus selects a route with more reliability.

Besides, the author also proposed a link state detection mechanism to resolve the problem of nodes not aware of the MAC frame transmission failure caused by congestion. Firstly an extent of ACK message (EACK) of MAC layer was defined, added the nodes coordinate and velocity vector parameter. Then a link state table was build using the EACK message and certain algorithm within each node. Every time a transmission to next hop node failure occurs, current node can view its neighbor’s state table for the next hop node’s state to decide if the failure is caused by its congested or moved away.

A simulation was finally performed, basing on variation cases, compares the proposed LTDSR protocol with the original DSR protocol. The simulation result shows that the LTDSR achieves considerable enhancement in all aspect including link failures, route discovery frequency, packet loss rate, route overhead and normalized packet load.

NPR [5], proposed by R. Nikolaev, B. Gupta, is an evolved routing protocol based on the MP-DSR protocol proposed by B. Li Chan, mainly focus on providing quality of service ensured multi-path routing.

         The MP-DSR [4] is a QoS based routing protocol, whose main feature is finding disjoint route paths which enables multi-path routing. The problem concerning MP-DSR is under rigorous conditions, it’s not possible to find required number of disjoint paths to meet a given reliability requirement thus lead to failure of route discovery.

         The NPR breaks the constraints of utilizing only disjoint path in MP-DSR to elaborate the problem MP-DSR may bring. The author proposed a link reliability computation model based on link availability model, path reliability model and end-to-end reliability model to compute the reliability for both single link and composite link. A new schema of discovering single/multi path that satisfies some required standard is also employed.

A simulation was performed at last to make a compare between the NPR protocol and DSR protocol showing that the NPR protocol gained better performance on the metric of number of hop.

RA-MDC[6], standing for Routing Aware Multiple Description Coding, proposed by Yiting Liao and Jerry D.Gibson, is an employment of mobile ad hoc network in video streaming task. The authors combined mobile ad hoc network with multiple description coding algorithm to gain more robust and effective video encoding and streaming over lossy network.

Multiple description coding is a promising technique for video transmission over a lossy network. It encodes video stream into descriptions and with part of them the video can be reconstructed with acceptable video quality. Higher quality will be gained if more description were delivered over the lossy network. Generally, descriptions are transmitted via multiple different paths, which is referred to as MDC with multiple path transport. This MDC over multi path method can reduce the possibility of simultaneous loss of bundle descriptions thus can increase the quality of decoded video.

However, such method is facing the problem of video quality degradation due to frame mismatch caused by loss of the key reference frame. The author of RA-MDC resolved this problem by utilizing a multi-path dynamic source routing protocol called SMR.

The route discovery and maintenance in dynamic source routing both provides feedback concerning the network conditions. These feedback messages can be used to estimate the packet losses in the network and to adapt the video coding accordingly. The author implemented a packet loss estimation method based on the feedback message and then architected a coherent video streaming system as Figure 1.

Figure 1. System architecture

         During the transmission of video stream via mobile ad hoc networks, the packet loss estimation model keep estimating packet loss states and adjust video encoding parameters with the estimation result thus better video streaming performance will be gained.

The RA-MDC was evaluated by the author using Qualnet simulator and shows a result that achieves gains in average PSNR of up to 2.3 dB and gains in a perceptual quality metric RSNR_r,f up to 13.6 dB, which indicates fewer bad-quality frames for most users over the network.

It has been years since the mobile ad hoc network concept was originally introduced, variations of routing protocols have been introduced by researchers and variations of optimization have been posed on them. The three novel protocols introduced in this paper have been verified respectively by their author and proved to be more efficient than the original dynamic source routing protocol. Regarding the research trends on mobile ad hoc network showed by paper publications, we can foresee that in the near future, researchers should concentrate more on secure communication and application of MANET in business or engineering area.