Christian Lazo R. Roland Glöckler Sandra Céspedes U. Manuel Fernández V.
Instituto de Informática Instituto Electrónica Departamento T.I.C. E.T.S.I. Telecomunicación Universidad Austral de Chile Universidad Austral de Chile Universidad ICESI Universidad de Vigo General Lagos Nº 2086 General Lagos Nº 2086 Calle 18 Nº 122, Pance Lagoas - Marcosende s/n Casilla 567, Valdivia, Chile Casilla 567, Valdivia, Chile Cali, Colombia CP 36200, Vigo, España clazo@uach.cl rolandglockler@uach.cl scespedes@icesi.edu.co mveiga@det.uvigo.es
ABSTRACT
The Mobile Ad-hoc NETworks (MANET) consist of a spontaneous association of a group of nodes that dynamically change their position and exchange data between each other, regarded as autonomous network segments with flat address schemes. However, its study has shown the benefits obtained by interconnecting them to fixed network segments and Internet. This article will revise by means of using a simulation tool, the behavior of the data transmissions between the fixed network segment, a reactive gateway and the IPv6 MANET, whose nodes show a high degree of mobility, such as vehicles in an urban environment.
KEYWORDS: MANET, Hibrid Network ,Internet, IPv6.
I. INTRODUCTION
The Mobile Ad-hoc NETworks (MANET) consist of a spontaneous and non-coordinated association of a group of nodes that dynamically change their position and exchange data between each other via wireless connections, without the participation or help of any external fixed network infrastructure.
The routing protocols used in MANETs have as main objective the maintenance of a communication between a pair of nodes (origin-destination) even when the position and speed described by their nodes change. When these are not directly connected, intermediate nodes forward the packets.
Currently the behavior of many of these routing protocols is investigated and some of them are in the process of standardization of the IETF (Internet Engineering Task Force).
The protocols that already have been reached experimental RFC (Request For Comments) status are DYMO [2], OLSR [3], AODV [4], DSR [5] and TBRPF [6].
The MANET routing protocols deliver the necessary mechanisms to exchange data packets inside the network reach. In order to send packets toward external networks, such as a fixed network or Internet, some network component has to act as gateway and has to provide the mechanisms for the
interconnection with that domain [7]. The means to provide this connectivity for the gateway device generally is a wireless connection toward the MANET and a second connection to the external network segment.
The operation mode of the gateway interconnecting the two networks can be active, reactive or hybrid. However, the performance impact of these interconnection options is not clear yet [8], [9] since it is affected by several external factors, such as the mobility degree of the nodes, the hop count between the gateway and the communicating node, the routing protocol used inside the MANET and the partitioning degree of the network, among other factors.
The present article documents the results obtained with a simulation tool in the analysis of the behavior of data transmissions between a fixed network segment, a reactive gateway and an IPv6 MANET, composed by nodes with a high degree of mobility, just as the vehicles of an urban environment.
The outline of this work is the following. Section 2 discusses about the gateway strategies for the interconnection of the MANET and Internet, in the section 3 different mobility models are revised, in the section 4 the used routing protocol is described, in section 5 the network pattern implemented in the tests is explained and finally in section 6, the conclusions finalize the paper.
II. CONNECTION STRATEGIES
Although, the MANETs initially were conceived as solution for isolated networks, it could be seen that when interconnecting them to fixed networks (Internet) its development potential grew and the number of possible applications considerably increased. MANETs that are connected to fixed networks are also known as hybrid networks [7], [10]. In this type of networks, one or more nodes of the MANET additionally connect by means of any of its connected interfaces to a fixed network segment. This node is called gateway and it is the one that provides a path to Internet.
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A MANET node has to be informed about the available packet forwarding gateways in order to be able to send or receive traffic from Internet. To carry out this process there are three proposed outlines [8] that operate similarly as a standard routing protocol.
A. Proactive Gateway
In this case the gateway floods the network periodically with messages indicating its existence and availability. In this case the nodes know permanently the gateway that can be used for information forwarding toward Internet. Using this outline, a low latency is achieved, but on the cost of a high bandwidth consumption inside the MANET.
B. Reactive Gateway
In this scheme the gateway plays a more passive role. Only when a node tries to send a packet to Internet, the process of requesting information of the present gateway is carried out.
C. Hybrid Method
In the hybrid approach a gateway informs their presence in proactive form only to the neighboring nodes (one distance hop). The more distant nodes are configured using the reactive mechanism described above.
III. MOBILITY MODELS
When we speak of mobility we should consider that the form, speed and trajectory of the movement described by the nodes, influence directly in the partitioning degree and connectivity level that the network achieves. Some of the measures of these variables are:
• Network Partition: indicates the number of network segments that exist for a group of nodes in a certain time (Fig. 1.).
• Space dependence: is the measure of how two nodes are dependent on each other in the movement; if two nodes have a similar speed and the same direction, then those nodes have high space dependence.
• Temporary dependence: is the measure of dependency between two nodes when their instantaneous speed is evaluated in magnitude and direction.
Considering their characteristics, MANETs are suited to give a connectivity solution to different scenarios and situations, therefore, each one of these scenarios should be represented with the pattern that comes closest to its reality at the moment of being evaluated by simulation. Among the main mobility models used by the investigation community we can mention [11]:
A. Random Waypoint
This mobility model is the most used one at investigation level. The nodes move at random speed whose values are in the range between 0 and [V_max] and select a random destination. After arriving at the destination the node stops for
a time period [Pause] and again selects a next destination and a new speed.
Fig. 1. Network partitions during the simulation.
B. Rpgm
This group mobility model is used mainly to simulate the communication in battle fields, where a group leader moves at a speed [V_lider] and the rest of the team moves next to him with similar speed and direction. The direction and speed of the rest of the team is adjusted periodically to the variations that the leader of the group executes. This model is characterized by high space dependence.
C. Freeway/Highway
The Freeway/Highway models are well suited to evaluate the behavior of the MANETs in vehicular network environments on freeways or highways. In this approach, three levels of speed (slow, medium and fast) are defined. Each level represents the displacement rails of the vehicles inside the freeway. The vehicles can change their displacement rail and consequently their speed in a gradual way, i.e., in order to pass from slow speed to fast, they have to pass through the medium speed first.
D. Manhattan Grid
The mobility pattern called Manhattan Grid is used to evaluate the behavior of MANETs in urban centers. This model allows representing pedestrians and vehicles moving in the streets of the city. Therefore, the pattern defines a grid where the lines and columns represent the streets, and the intersections the corners. In this model, nodes can move in a random way with medium speed [V_med]. When arriving to an intersection and under certain random probability, the nodes can stop and rotate in any direction (left - right) or continue on the same street (Fig. 2.).
This model imposes clear geographical restrictions on the node displacement, increasing this way the probability of partitions inside the network, just as indicated in Figure 1. On the other hand, it has the advantage of allowing the development of simulations with a high degree of similarity to the scenario found in urban environments. It also offers a LAZO ET AL.
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very high degree of space and time dependence because of the displacement pattern that the nodes describe.
Fig. 2. Graphical network representation.
IV. ROUTING IN MANET
In the last years, many proposals of routing protocols for MANETs have been developed and presented for standardization [2], [3], [4], [5], [6]. Additionally, many comparisons have been carried out between them [12], [13], [14]. However, the consent is that the reactive protocols show the best performance and, therefore, have reached a better acceptance inside the IETF, specifically the case of the protocol Ad Hoc on Demand Distance Vector AODV[4].
Extensions have been developed for this protocol with the purpose of offering multiple paths (Routing Multipath) inside the MANET [15] and conectivity to Internet to the MANET domain [8].
A. Ad-Hoc On Demand Distance Vector
The AODV proposal bases on a reactive (or on-demand) protocol, based on distance vector routing that avoids the count-to-infinity problem and routing loops. For route control purposes it uses sequence numbers for each destination, this way it only maintains routes toward the nodes with those it maintains active communications. The routing table in the nodes keeps each entry for a while limited, so when this time expires, the routes are eliminated and a new process of route search should be started for the destination. Each entry is associated with a sequence number that is used to verify how new the route is in order to avoid loops.
B. Route Discovery
The route discovery process in AODV is initialized when a source node needs a route toward a destination node. If the sender does not find the path in their routing table, it increases the sequence number and floods the network with a ROUTE REQUEST message (RREQ). A neighboring node that receives the message checks if the message is duplicated, in which case it terminates. If the message is not a duplicate, then the node should create or update the inverse route toward
the node that transmitted the RREQ with the new sequence number . A node that receives the RREQ message can generate a ROUTE REPLY message (RREP) if it is the destination node or an intermediate node that previously knows the path to the destination node. The RREP message will be sent directly to the source node through the path discovered by the RREQ messages, after updating the corresponding sequence number. The intermediate nodes that receive a RREP create or update the route of the neighbor that sends it and the direct route toward the originator of the message of RREP. If the node that received the message RREQ cannot generate a RREP, then it forwards the RREQ message to its neighbors, with an updated sequence number.
C. Route Maintenance
In order to maintain the connection among the source and destination node, the nodes with active routes send periodical modified RREP messages with a TIME TO LIVE field equal to one (TTL=1) or HELLO messages. These messages are only received by the direct neighbors and they have the purpose of updating the route table. If the HELLO messages have not been received from a neighboring node for some time, it is assumed that this device is no longer available, therefore, the route is discarded and it is deleted from the routing table.
In the case that a destination cannot be reached or a loss of connectivity with a neighboring node is detected, a ROUTE ERROR message (RERR) is generated and the route discovery process is started again.
V. MODEL EVALUATION
The experiment to evaluate the behavior of the data transmissions between the fixed network and the MANET segments through a Gateway was conducted by simulation with the tool Network Simulator (NS-2) [16].
In this section we will describe the different parameters of the scenario used in our simulation:
A. Simulation Scenario
The examined scenario contains 3 fixed nodes in Internet, 15 mobile nodes using the net protocol AODV+ [8] and a gateway that interconnects both domains. The MANET topology is distributed on a rectangular area of 800m x 500m.
The gateway works in reactive mode and is fixed to the center of the area (coordinates 400, 250). The mobile nodes move in random manner according to the mobility pattern “Manhattan Grid", with a medium speed of 20m/s (Figure 2). The mobile nodes and the gateway use omni-directional antennas offering a cover radius of 250m. The media access control is realized with the MAC layer 802.11. The fixed nodes in Internet and the gateway are interconnected by means of bi-directional connections at 5Mb/s and with 2 ms of latency.
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B. Network Traffic
The traffic load on the network is composed of two data flows crossing the gateway and passing between both domains. In the first case, the network traffic goes from two nodes of the MANET environment to two nodes in the fixed net (manet to Internet). In the second case, the flow direction is reverted (Internet to manet), maintaining the correspondence among the nodes. For both cases the data transmission is realized with TCP and UDP flows.
For the nodes involved in sending and receiving the data flows, their packet size and the time of beginning and terminating the transmissions stay constant during the execution of the group of simulations.
C. Used Metrics
For the comparison of the behavior of the data transmissions among both networks, the following metrics were used:
End-to-End Delay (Delay)
The end-to-end delay is defined as the time difference between the packet generation and the arrival at their destination.
Packet Delivery Fraction (PDF)
This value corresponds to the percentage of successfully received packets by the destination node. It is calculated dividing the number of packets received by the number of packets generated.
Normalized Routing Overhead (NRO)
This value gives a measure of the overhead introduced by the MANET routing protocol and it is calculated as the fraction of control packets over data packets.
The first two metrics have direct relationship with the quality of service (QoS) measures of the Best-Effort service used in Internet and the last one with the overhead introduced by the MANET routing protocol.
VI. CONCLUSIONS
Table I shows statistical values obtained by the measurement series. They are ordered first by flow type (TCP, UDP) and then by the origin-destination segment (Fixed-mobile or Mobile-fixed). It can be observed that the data flow direction has little influence, regarding that for similar flows in different directions the behavior is quite similar. The small differences can be explained by the routes stored in the gateway at the moment of the transmission.
For the TCP flows, smaller Delay and Jitter values and better PDF rates can be observed; in contrast, UDP traffic obtains higher Delay and Jitter values and a lower PDF rate, that is, a higher latency compared to TCP and a lower rate of successfully received packets.
On the other hand, it can also be indicated that the PDF and the NRO are directly related, independent of the type and
direction of the flows, mainly due to the work executed by the routing protocol (AODV), to carry out the packet forwarding.
TABLE I
MEAN STATISTICS OF THE DATA FLOWS
A. Delay
As can be seen in Figure 3(a), the behavior of the delay in UDP shows smaller instantaneous values that TCP, but a higher latency variation rate (Jitter) than the one shown by TCP figure 3(b), where the latency values are a lot more stable.
B. PDF
In general, the PDF is acceptable for both flows. Figure 4(b) shows the effect of the TCP error recovery mechanism and the relationship between the Jitter and the packet loss in the UDP flows which are produced by discarding some packets in the intermediate nodes.
C. NRO
Figure 5(b) shows that that TCP has smaller network overhead rates. This can be explained in the behavior of the protocol itself, since the ACK sent by TCP helps to maintain routes up to date. Therefore, in this type of transmissions a smaller number of HELLO packets are necessary from the routing protocol. This way it is TCP who helps to maintain the routes stable, phenomenon that doesn't happen with the UDP transmissions just as can be appreciated in Figure 5(a).
In summary we can indicate that the behavior of both data flows in different directions indicates that TCP achieves a better performance compared to UDP in a network of these characteristics. The main reason is the error recovery mechanism included in the protocol, that reacts on the segment failures of the mobile segment and that interacts with the routing protocol mechanisms.
TCP UDP
F-M M-F F-M M-F
Delay 0,206878 0,194848 0,463238 0,549002 Jitter 0,11299 0,14191 1,29870 1,32829 PDF. 98,04781 97,73402 93,28205 92,59437
NRO 0,292047 0,115367 0,690216 0,477686
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(a) Fig. 3. End-to-End Delay. (b)
(a) Fig. 4. Rate of Received Paquets. (b)
(a) Fig. 5. Routing Protocol Overhead. (b)
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ACKNOWLEDGMENT
This work was supported by the “Ministerio de Educación y Ciencia” (Spain) through the project TSI2006-12507-C03-02 of the “Plan Nacional de I+D+I”.
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