Performance Evaluation of Multimedia Applications over an OLSR-based Mobile Ad Hoc Network using OPNET

Patrick Sondi, Dhavy Gantsou and Sylvain LecomteUniv Lille Nord de France, F-59000

Lille, France UVHC, LAMIH, F-59313

Valenciennes, France CNRS, UMR 8530, F-59313

Valenciennes, France {patrick.sondiobwang, dhavy.gantsou, sylvain.lecomte}@univ-valenciennes.fr

Abstract— Mobile ad hoc network architecture allows cheap and easy deployment of network services almost anywhere and at anytime. Besides Internet access which is a best-effort service, multimedia applications are the most requested. They usually impose stringent quality of service constraints in terms of metrics including bandwidth, end-to-end delay, jitter and loss rate. In this paper, we evaluate the performance of a QoS extension of the OLSR protocol on voice communication using the OPNET simulator. The network model can represent various locations like railway stations, Campus University or traffic jam. We describe the performance end users involved in voice communication session can expect, even in presence of other traffics, like file transfer between other users in the same network.

Keywords-Multimedia ad hoc networks; QoS routing; OLSR; OPNET

I. INTRODUCTION The advances in the convergence of fixed, mobile and IP

networks have popularized ubiquitous Internet and multimedia services access. Mobile ad hoc network (MANET) architecture is a good alternative for deploying wireless local area networks (WLAN) in locations like railway stations, Campus University or public places in the heart of city town. Internet access, phone calls, video streaming and videoconference could then be proposed as additional services in these WLAN. To that end, the main hurdle to overcome is that of providing efficient quality of service support in mobile ad hoc networks. Indeed, MANETs are characterized by link instability due to its wireless nature, and frequent topology changes due to mobility of the nodes. The lack of a centralized coordination of the network implies that the nodes have to compete in order to access the shared medium. Despite all these difficulties, several works have addressed the issue of implementing QoS support routing in MANET. A survey reporting most of the main contributions is proposed in [1]. A recent work [2] mentions the deployment of mobile phone connections in Africa using mobile ad hoc networks by a Swedish company TerraNet. This example brings the confirmation that using MANET to support multimedia

applications is feasible. Such initiatives could be generalized even in developed countries, especially in the locations where private WLAN can propose multimedia services without relying on main ISPs’ or mobile phone operators’ infrastructures.

In this paper, we evaluate the performance of voice communications over mobile ad hoc networks using network scenarios that characterize precisely the locations mentioned previously. Using the the OPNET simulator, we evaluate the quality perceived by the end users of the OLSR QoS extension proposed in [3]. Indeed, basing on indicators like jitter, end-to-end delay, Mean Opinion Score (MOS) and loss rate for which the IUT-T G 1010 provides the thresholds guaranteeing good quality for voice communication, we are able to check if MANET fits for multimedia applications. We also investigate the variations in the performance due to both mobility and background best-effort traffic occurring in the neighborhood of users involved in a voice communication session. The rest of this paper is organized as follows: in section II, we present a brief survey of work related to evaluation of multimedia applications over mobile ad hoc networks. Section III provides a brief description of the simulation model of the OLSR QoS extension in OPNET. We describe the network scenarios and the traffic used for performing evaluations in section IV. Section V presents the main results, while VI concludes the paper.

II. RELATED WORK The growing demand for various services access by

mobile users, including mobile phone connections, radio or TV, and the Internet, has lead to works investigating efficient and cheaper deployment solutions. Among these works, there are as well studies on infrastructure-based solution [4] as experiments using mobile ad hoc networks. Some of the latter are performed on test-bed using a few number of real portable devices [5], [6]. The authors of [5] present good results on voice communication performance. The evaluations in [6] use a traffic flow which is not related to a specific multimedia application, but which requires similar network resources. The main problem with such test-bed is the lack of information on the scalability of the evaluated

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solutions since the number of nodes involved is limited by material constraints. The network simulator NS2 was used in [6], but only few results were derived from that simulation and the authors did not mention whether they could characterize a specific multimedia application. Similar works have investigated the transmission of voice traffic over 802.11b-based ad hoc networks [7], [8]. But the performance results they obtain cannot be extended to 802.11g-based MANET which may support considerably higher throughput. The designers of the QoS extension of the Ad Hoc On-Demand Distance Vector (AODV) routing protocol claim to provide good voice communication support , but relying on simulation scenario involving only sixteen nodes [9]. Moreover, the size of the network area and the range used almost limits any route length to at most two hops. Finally, having regard to both the node number, and the network density they used for simulation, one cannot predict whether their QoS extension does scale or not. A similar approach [3] targeting the Optimized Link State Routing (OLSR) protocol has proposed a QoS extension of the latter. Performance evaluations carried out show that the proposed QoS extension suites better than native OLSR for supporting voice communication. The network scenarios are a 25-nodes and a 100-nodes MANETs, thus proposing results as well as for low as for high density networks. However, the impact of mobility was not analyzed, and the details about the quality of individual voice session including jitter, end-to-end delay, MOS and loss rate experienced by two end users in communication were not presented.

In this paper, we mainly evaluate the performance of their QoS extension on a medium density ad hoc network. We complete the study by integrating as well mobility as the impact of the presence of best-effort traffic. We also analyze the characteristics of each voice communication session in details.

III. THE OPNET MODEL OF THE OLSR QOS EXTENSION OPNET’s network R&D solutions are the industry’s

leading software for network modeling and simulation (see www.opnet.com). The OPNET modeler and wireless packages provide ready-to-use components and tools like:

Tools for modeling the network environment by defining its geographical location, area size, terrain;

Node models representing wireless workstations with a detailed implementation of each layer implied in the network communications;

Process models of the main network vendor or standard protocols from application layer to physical layer, including routing protocols for MANET;

Tools for modeling mobility scenario, including predefined itineraries in specific locations;

Tool for modeling and deploying application traffic; Tools for generating statistics and reports on the

functioning of all the components during simulation. All these tools are preconfigured with default values, thus they can be easily used even by beginners. It is also obvious to define custom node or process models, making it easy for researchers to test new solutions and obtain their

performance before real implementation. The main difficulty is the great number of parameters that may be varied from one simulation to another. To allow accurate comparison of the solutions, the authors should use default values or publish the custom values they used during their simulations. The package provides an implementation of the OLSR protocol compliant with the RFC 3626 [10] as a process model which is invoked by the routing manager component of WLAN stations. Any other MANET routing protocol can be invoked that way. The main changes introduced in the OLSR process model by [3] to obtain the QoS extension can be summarized as follows:

Each node x generating a HELLO message adds the current time tgen in the header of the packet containing the message. Since the nodes are synchronized according to the simulation time, each node y receiving the message at trcv can compute the end-to-end delay on the link with the sender using: = − × _ _ (1)

The variable MTU_size allows reporting the delays to a common reference for a correct link comparison.

The respective delays Di measured and the packet size Si of the corresponding messages are cumulated during an observation period observ_t. Each time in the corresponding period, at the reception of the nth message, the average data rate experienced on the link with the sender can be estimated using: = ∑ ==1∑ ==1 (2)

When observ_t is reached, all the variables are reinitialized for the corresponding link.

The sender x also adds its geographic coordinates in the HELLO messages. When node y receives the message, basing on its own coordinates, it is then able to compute both the distance dxy and an approximate value for the delivery probability P on the link with the sender using the equation in [11]:

= ⎩⎪⎨⎪⎧ 1 − 4

2 <2 − 42 4 ℎ ℎ .

(3)

The parameter R is the range of transmission. The QoS extension keeps the same source code than

native OLSR, except for data structures representing the links and the network graph that are modified in order to integrate the metrics values for B, D and P.

These metrics values are then used in algorithms proposed for multi-metric multipoint relay (MPR) selection and routes computation.

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IV. NETWORK SCENARIOS

A. Network model

Figure 1. 50 nodes on a 1000mx1000m area.

The network is modeled by a square area of 1000mx1000m. 50 workstations equipped with 802.11g wifi cards and running the QoS OLSR extension with the default parameters proposed for OLSR obtain random positions in the network area. We can therefore distinguish two kinds of scenarios at this step:

In the first, there are no mobile nodes in the network area. The simulation results related to this scenario are entitled QoS_OLSR_50nodes_1000x1000_ BGFTP and QoS_OLSR_50nodes_1000x1000.

In the second, all the nodes are mobile with speed ranging from 0 to 15 m/s and pause time ranging from 10 to 100 s, except those involved in a voice communication or FTP session. These latter are not mobile and have a black circle surrounding them on figure 1. The idea is to maintain these nodes at a constant distance from each other to insure that they will never have one-hop communications. The corresponding scenarios are: QoS_OLSR_50nodes_1000x1000_BGFTP_MOBILE and QoS_OLSR_50nodes_1000x1000_MOBILE.

The network proposed can be assimilated to locations where there are pedestrians, roads where pass communicating cars, a tram in which passengers are using their mobile phone equipped with 802.11g wifi card, people in the park or waiting for a bus. Such location can be a campus University, a railway station, downtown, or simply disaster areas and battlefields.

B. Network traffic used for evaluations For evaluation purpose, we chose a voice communication

application. First, because it is a multimedia application that requires multiple QoS constraints, including the bandwidth or data rate, the end-to-end delay and the loss rate. Second, because the criteria that allow evaluating the quality level of a voice communication session are also clearly defined, notably in the ITU-T G 1010 (See TABLE I.). To this, we can add the MOS value which varies from 1 (worst) to 5 (best). Calls on cellular phones have a MOS of 3.8 for example. The codec used is G.711 (64 kbps encoding bitrate) which resorts to the Pulse Code Modulation (PCM) technique. Voice communication is deployed in the four

scenarios presented in section IV.A. In each case, the voice communication session is established between the nodes Mobile_35 and Mobile_45 (See figure 1).

TABLE I. QOS CONSTRAINTS FOR VOICE COMMUNICATION (IUT-T G 1010 RECOMMANDATION)

In order to simulate background traffics which could be the Internet session of surrounding users for example, and analyze their impact on the voice session, we introduce a file transfer session between the nodes Mobile_2 and Mobile_21. The scenarios involving background traffic are identified by the BGFTP suffix.

V. SIMULATION RESULTS

Figure 2. Impact of background traffic and mobility on number of hops.

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The results presented here represent fifteen minutes of the functioning of the MANET. In all the scenarios, voice communication (VC) or FTP traffic, if any, starts after the 100th second and the sender stops generating packets 200 s before the end of the simulation. We collected a great number of results, but we only present the most significant.

A. Number of hops As shown on figure 2 (a), the presence of background

traffic almost have no impact on the routes length, while mobility increases the number of hops of up to 25%. The average number of hops for VC traffic in this scenario is 2.

B. MOS, Jitter and end-to-end delay values

Figure 3. MOS value in all scenarios.

The MOS factor varies between 3.6 and 3.7 in the basic scenario, and it keeps this range of values even when background traffic occurs during the voice communication session (figure 3 a). Mobility has a very bad impact on this factor since its value falls to 3.3 in the scenario including mobile nodes as shown on figures 3 (b) and 3 (c). The jitter on voice communication traffic is less than 1 ms for the basic scenario without mobility.When the nodes are mobile, we can see on figure 4 (a) that the jitter increases up to 3.5 ms. This value stays under the required threshold. For all the scenarios the end-to-end delay is close to 60 ms. In mobility scenario (figure 4 b), a great number of packets reach delays between 80 ms and 100 ms, but only two packets are above the threshold of 150 ms while remaining under the limit of 400 ms.

Figure 4. Jitter and end-to-end delay in all scenarios.

C. Traffic sent and traffic received The FTP traffic sent is fully received as well as in the

basic scenario as in the scenarios involving nodes mobility (figure 5 a). Figure 5 (b) shows that there is a slight difference in the amount of traffic sent in different scenarios. The basic scenario has the most amount of traffic correctly sent, and the mobile scenario without background traffic has the least one. From figure 5 (b) and figure 5 (c), we derive a loss rate of 1% for all scenarios, except the one involving mobility of the nodes. From figure 5 (d), we observe that the scenario implying mobile nodes without background traffic reaches a loss rate of 3%.

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Figure 5. Traffic sent and received in all scenarios

Basing on the results presented in sections V.B and V.C, we can conclude that the proposed QoS extension of OLSR evaluated in this paper allows supporting voice communication in MANET, even in presence of background traffic and in situations where most of the nodes in the network are mobile. In this latter case, the values of the different metrics used to evaluate the quality of service provided by the application reach the threshold values recommended by the IUT-T G 1010. The quality of the communications is close to the minimum acceptable in this scenario. Indeed, the MOS value reaches 3.3 that is close to the MOS value 3.0 characterizing a voice quality level which is perceived as annoying by the end user.

D. Impact of mobility on the routing protocol behavior In this section we are interested in the behavior of the

routing protocol, namely the OLSR QoS extension, in situation where the nodes are mobile.

Figure 6. Impact of mobility on the routing protocol behavior

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Figure 6 (a) shows that in mobility scenario the number of nodes acting as multipoint relays (MPR) in the network tends to be lower than in scenario without mobility. Usually, it is admitted that when the nodes move, the periodicity of HELLO and TC messages should be lower in order to refresh more frequently the neighborhood information.

Figure 6 (b) shows that even when these periodicities are kept to the default value, in mobility scenario the nodes receive more than the double of the routing traffic normally received in the same network with no mobility profile. Thus, introducing more routing traffic by diminishing the periodicity may lead to overhead in the network. We also observe on figure 6 (c) that there are lots of changes in the neighborhood at the beginning, but this tends to stabilize with the topology organization.

VI. CONCLUSION In this paper, we pointed out the benefits of using mobile

ad hoc networks to provide multimedia services including Internet access, mobile phone connections and videoconference. We evaluated the performance of the OLSR QoS extension proposed in [3], using a voice communication application in various scenarios. The results of the simulations performed using OPNET show that this QoS extension allows deploying efficiently voice communication over MANET and maintaining good quality to voice sessions, even when other traffics occur or when nodes are mobile. We also pointed out the impact of mobility on the behavior of the QoS extension, notably the fact that it increases the routing traffic received by each node.

ACKNOWLEDGMENT The present research work is supported by: the International Campus on Safety and Intermodality in Transportation, the Nord-Pas-de-Calais Region, the European Community, the Regional Delegation for Research and Technology, the Ministry of Higher Education and Research,

the National Center for Scientific Research, and the university research program of OPNET Technologies Inc. The authors gratefully acknowledge the support of these institutions.

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