SiFT: An efficient method for Trajectory BasedForwarding

A. Capone, L. Pizziniaco, I. FilippiniDipartimento di Elettronica e Informazione

Politecnico di MilanoMilano, Italy

Email: {capone, pizziniaco}@elet.polimi.it,ilario.filippini@gmail.com

Abstract-Trajectory Based Forwarding (TBF) is a newapproach for routing in ad hoc wireless networks. It assumesthat nodes know their position and, similarly to source routing,requires the source node to encode a trajectory into the packetheader. However, trajectories are just geometrical lines andthe routing process does not require specifying forwardingnodes. As a matter of fact, forwarding nodes are dynamicallyselected while packets cross the network according to theirposition with respect to the trajectory. Therefore, this newapproach is particularly suitable for application scenarios wherenetwork topology is fast varying, due to node mobility (e.g.inter-vehicular networks) or to energy management schemes(e.g. sensor networks), whereas the stability of the trajectoriesis guaranteed by the physical characteristics of the servicearea (roads, building aisles, etc.). In this paper we propose anew TBF scheme that exploits broadcast transmissions at theMAC layer and does not require maintaining a list of activeneighbours positions in every node. We consider piecewise linesconnecting source node to destination area and we extend theapproach to the multicast case defining trajectory-trees.

Keywords: ad hoc networks, wireless sensor networks, vehicularnetwork, routing, multicasting, trajectory based forwarding.

I. INTRODUCTIONWireless ad hoc networks have attracted the attention of

the networking research community in the last years. Afteran initial phase during which general solutions for ad hocnetworking have been proposed, current research activities arefocusing on specific application scenarios whose characteris-tics can be exploited to improve efficiency and reliability. Themost promising applications include pervasive networks andvehicular networks.

Pervasive networks have the goal of providing computingand communication services all the time, everywhere, transpar-ently to the user, using many low-cost small devices embeddedin the surrounding physical environment. Networks nodes arebattery operated and have limited computation and memorycapabilities. Therefore, energy management schemes must beconsidered to extend network lifetime. Such schemes allownodes to switch to low-activity states during which they do notreceive/forward packets. As a result, even if most of the nodeshave fixed positions the network topology changes frequently.In this scenario packet routing is not an easy task [1], [2],

0-7803-9206-X/05/$20.00 ©2005 IEEE

M. A. Garcia de la FuenteTelecom Unit

ROBOTIKER - TECNALIAZamudio, Spain

Email: mgarcia@robotiker.es

even if location systems can be exploited and location-basedrouting schemes can be adopted [3], [4].

Another interesting scenario of ad hoc networking is thatof vehicular networks that allow cars to exchange informa-tion. With vehicular networks it is possible developing newapplications able to improve road safety, as well as to providetools for traffic management, mobile office and passengerentertainment. In this case, network nodes have no energyconstraint, nevertheless network topology chances frequentlydue to node mobility. Moreover, information provided bypositioning systems, such as GPS, GALILEO or other positionsystems [5], can be exploited for routing.

Both in case of pervasive and vehicular networks, routingpaths, defined as sequence of forwarding nodes, are unstabledue to topology changes, while geographical routes, definedas lines, are quite stable due to the physical characteristics ofthe service area.

Trajectory Based Forwarding (TBF) [6] exploits this basicobservation proposing a routing scheme that, similarly tosource routing, requires the source node to encode a geograph-ical line, referred to as trajectory, into the packet header. Sincethe sequence of forwarding nodes is not specified, packets arerouted hop-by-hop according to node positions with respect tothe trajectory.The forwarding schemes proposed in [6] and in [7], [8]

are based on point-to-point transmissions. Once received thepacket, each forwarding node selects among its neighbors thenode which is in the best position with respect to the trajectoryand then send it the packet. Obviously, the information ofneighbor positions is essential for the neighbor selectionalgorithm. Therefore, it must be kept updated together withthe information on neighbors' status (active, nonactive). For-warding schemes differ according to the neighbor selectionalgorithm and the trajectory representation.

In this paper we introduce a novel TBF scheme: SiFT (Sim-ple Forwarding over Trajectory). Differently from previouslyproposed TBF schemes, SiFT is based on broadcast transmis-sions and does not require neighbor positions and states, sincethe forwarding decision is shifted from the transmitter to thereceiver. Moreover, we extend the forwarding scheme to thecase of multicast considering a tree of trajectories and a simple

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method to duplicate packets at crossing points.In Section II we describe SiFT, compare it with previously

proposed schemes, and analyze critical issues related to thehidden tenninal problem. In Section III we discuss possibleinteraction between multiple access and forwarding mecha-nisms. In Section IV we present some simulation results onthe performance of SiFT. Finally, in Section IV we give someconcluding remarks.

LI. SiFTDifferently from previously proposed TBF schemes, SiFT

uses broadcast instead of point-to-point transmissions. Wire-less transmissions are broadcast in nature and allow reachingpossibly all active neighbors at the same time. Moreover,the forwarding decision is shifted from the transmitter to thereceiver. Each node that receives the packet takes the decisionif forwarding it or not based only on its own position, the trans-mitter position and the trajectory. This greatly reduces controloverhead introduced by the protocol and energy consumption.Once received a packet, each node sets a timer according to

its position with respect to the trajectory and the transmitter:

Tout = aDtDi (1)

where a is a constant, Dt is the distance of the node fromthe trajectory and DL is the distance from the last node thattransmitted the packet. If a copy of the packet, forwardedby another node, is received before the timer expires, thetimer is stopped and the packet is deleted from the forwardingqueue. Otherwise, the packet is passed to the Medium AccessControl (MAC) layer for transmission when the timer expires.Therefore, the node with the minimum Tout will forward thepacket. It is the node in the best position since it far from thelast the node and close to the trajectory.

Packets include into the header the trajectory and thecoordinates of the last node that forwarded the packet. Theoriginal source identifier, a sequence number, and a hop countare included as well. Each node maintain a list of recentlyreceived packets (source ID and sequence number) to avoidcycles.The idea of using node position only is basically the

same exploited by GeRaF (Geographic Random Forwarding)[9], [10]. However, differently from SiFT, GeRaF considersboth packet forwarding and medium access. It is based ona contention resolution scheme that allows to select the for-warding node, and on a busy tone transmitted on anotherchannel that allows to avoid the hidden terminal problem.Moreover, GeRaF does not use trajectories but the positionof the destination.

SiFT can potentially be adopted with any MAC scheme,even if its performance depends on the characteristics of theMAC scheme considered. Note however, that this forwardingapproach is quite robust to transmission error and collisionssince, if the node in the best position does not correctly receivethe packets, other nodes may be able to take on the role offorwarding node. Moreover, in the unlucky case that no node

is able to receive the packet, the transmitting node can detectthe problem and retransmit it.

SiFT can use any line as trajectory. However, in this paperwe consider piecewise lines. Similarly to source routing, theoverhead added to code the trajectory depends on the numberof pieces of the line, and previous pieces can be removed fromthe packet header by forwarding nodes along the trajectory.A. Admissibility stripDue to limited transmission ranges, some additional nodes,

far from the trajectory, could also forward the packet if theyare not able to listen to the transmission of the node in thebest position. For example, let us consider the scenario inFig. 1. Suppose that node A broadcasts the packet to all itsneighbors. C is the node in the best position that calculates theshortest Tout. Therefore, when its timer expires it forwards thepacket. When nodes B, and D receive the copy of the packetforwarded by C, they stop timers and delete the packet fromthe forwarding queue. However, since nodes E and F are out ofthe coverage range of node C, they do not receive the packet.When the timers expire also node E and F forward the packet.

-o .0_

-_, 0 _

Fig. 1. Admissibility strip

This effect may lead to duplicated packets travelling inthe network along parallel trajectories at a distance from theoriginal trajectory at least equal to the transmission range.Even if this is not a completely negative effect (it could beexploited to improve reliability), SiFT eliminates or limits itintroducing a distance threshold (admissibility strip): if the dis-tance from the trajectory is longer than the threshold, e.g. thetransmission range, then the node refrains from participatingto the forwarding process.

Figure 2 shows two snapshots obtained by simulation withthe same trajectory but different sets of active nodes. Thedistance threshold has be set to the transmission range.

B. Multicast transmissionsSiFT scheme can be adapted also to the multicast case using

trajectory-trees.The forwarding algorithm has been modified to include

packet duplication when it reaches tree-splitting points. In thebasic forwarding scheme, nodes calculate the distance from thetrajectory considering lines orthogonal to trajectory segments(Figure 3). If the node is close to more than one segment,the shortest distance is considered. Previous segments of the

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Fig. 2. SiFT: snapshots of different forwarding paths.Fig. 4. SiFT: snapshot of multicast forwarding.

piecewise line are removed from the packet header beforeforwarding it.

Fig. 3. Tree-based Forwarding.

With trajectory-trees, nodes close to a tree-splitting pointhave to calculate the distance from different branches of thetree. Also in this case the shortest distance is considered andonly the remaining part of the tree branch is included inthe packet header. Nodes close to different branches of thetree will encode different remaining trajectories in the header.Even if the transmission of a node close to another branchis received, the packet is not deleted from the queue and itis transmitted when the timer expires. On the contrary, if thetransmission of another node on the same branch is receivedthe packet is deleted. This allows packet copies to follow thetree along different trajectory branches.

Figure 4 shows a snapshot obtained by simulation with asimple trajectory-tree with two branches. Forwarding nodesare indicated with a darker circle in the picture.

III. MULTIPLE ACCESS

SiFT behavior is somehow related to the multiple accessscheme adopted at the MAC layer. Several multiple accessschemes can be considered. Obviously, MAC schemes thatprovide a reliable broadcast service guarantee the best perfor-mance since all neighbors receive the packet [11]. However,in this paper we focus on simpler schemes based on CSMA(Carrier Sense Multiple Access) in order to show that SiFT iseffective also with MAC layers with no quality guarantee.

A. Classical CSMAThe simplest way of implementing SiFT on CSMA is that

of keeping the two protocols at network and MAC layersrespectively, without any interaction other than the send andreceive service primitives. This could be easily achievedimplementing SiFT on top of the broadcast transmission modeof IEEE 802.11 [12] or the CSMA mode of ZigBee [13].When a timer expires, SiFT sends the packet to the MAC

layer for transmission. According to CSMA scheme, the MAClayer waits for the channel to be available and transmitsthe packet. No feedback from neighbors is adopted and nocollision avoidance scheme is implemented.

Obviously, the ideal behavior of SiFT we described inprevious section may be affected by the interaction withCSMA. Due to limited transmission range, collisions can occur(hidden terminal problem). However, as we observed above,the forwarding scheme is quite robust to collisions.

Moreover, when the channel is busy, several timers ofpotential forwarding nodes may expire, leading to a collisionwhen the channel becomes available. This effect is depicted inFigure 5, case a), where the timers of two nodes expire whilethe channel is busy, and their transmissions then collide.

Finally, more than one node may forward the packet whenthe timer expires while the first node is forwarding it, as shownin Figure 5, case b). Note that this multiple-forwarding effectmay be quite relevant when many timers are shorter than thepacket transmission time. In the extreme case all timers are

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Packetreceived r Timer A

Packetforwarded

r Timer B I collision

a)

Packet Packetreceived Timer A forwarded

Timer B Packet) forwarded

b)

set according to:

T.t = 0.01D + 0.00193 [s] (2)

where 0.00193 is the packet transmission time.In Figure 6 we show the delivery time over the diagonal

trajectory versus the interference traffic. We observe that thedelivery time with the network carrier sense is always higherthan that obtained with the classical CSMA. The differencebecomes remarkable at high loads. In fact, when traffic in-creases, the network carrier sense mechanism stops timersmore often to prevent collisions and packet duplications. Quitesurprisingly, however with the classical CSMA, collisions donot affect remarkably the delay performance.

Packetreceived Timer A

PacketFL forwarded

Timer B PacketI=_ ~ deleted

C)Tirnersstopped

Fig. 5. SiFT operation with classical CSMA, case a), and with networkcarrier sense, case b).

very short, the forwarding mechanism is basically equivalentto flooding within the admissibility strip.

B. Network Carrier Sense

To reduce these effects, we can increase the interaction ofthe network and MAC layers making the carrier sense signalavailable also at the network layer. Therefore, SiFT can stoptimers when the channel is busy, as shown in Figure 5, casec). As a result, the collision probability reduces and packetmultiple-forwarding is less likely to occur.

IV. SIMULATION RESULTS

To evaluate the performance of SiFT we implemented itwithin Omnet++ simulation engine with the Mobility Frame-work [14] (snapshots in Figures 2 and 4 have been obtainedwith the same simulator).We considered a square area lOOOmxlOOOm with 100 active

nodes uniformly distributed with a transmitting range of 263m.In the area we drawn a grid of 9 horizontal and 9 verticalstraight trajectories, and placed sources on top and on theright and destinations on the opposite side. Moreover, weconsidered a diagonal trajectory from the left-top corner tothe right-bottom one. All statistics have been collected onthe diagonal trajectory, while Poisson-distributed interferencetraffic has been generated on the others. The admissibility-stripwidth has been set equal to the transmission range.The packets generated have all the same length equal to 192

bit. The channel bit-rate is equal to 100 Kb/s, while timers are

1E

1.4

12,

JL 1 \

j_ OB - \ Network Carmer SenseiOF-0.5

04

Classical CSMA

im 20 2 1 0.2 01 005 0025 00125 0.0025T,.fiIM.PA..4

Fig. 6. Delivery time.

In order to see the effect of collisions we can give a lookto the average number of hops per delivery reported in Figure7. Due to collisions the node in the best position to forwardthe packet may be unable to correctly receive it. As a result,the number of hops is slightly higher with the classical CSMAthan with the network carrier sense.

0.25

- 8.2Classical CSMA

3.15

& 05.

I E Network Carrier Sense

70100 20 2 1 02 0.1 0.o5 0o25 00120 0o0or25

Tr.ffi. Pftyw

Fig. 7. Number of hops per delivery.

The effect of multiple-forwarding can be appreciated con-sidering the average number of forwarding nodes per delivery.Ideally, the number of forwarding nodes should be exactlyequal to the number of hops if only one node per hop forwards

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the packet. This is actually the case observed with the networkcarrier sensing mechanism. However, with classical CSMAthe network layer may pass the packet to the MAC layerwhen other neighbors are already forwarding it, increasing thenumber of forwarding nodes, as shown in Figure 8.

14

3 12 - ts Classical CSMA6- ~~/128

E Network Carrier Sense= 4-

2-

100 20 2 02 01 005 0.02 0,0125 D0E025Traffic IMsglsecl

Fig. 8. Number of forwarding nodes per de

Finally, we evaluated the number of hops forof the node density. Results are reported inobserve that, as expected, the number of hops c

number of nodes in the area increases. The nfor the two approaches, classical CSMA andsense, is almost the same, even if the multeffect increases the number of forwarding nodeCSMA.

0

Iaz2

8.1 -Classical CSMA

8-

7.9~~~~ '''.7 .9-,

7.8-Network Carrier Sense

7.7 -

7.6

7.5 -

7.450 75 100 150 300

Number of nodes

Fig. 9. Number of hops per delivery with different

V. CONCLUSION

Trajectory-based forwarding is a promisingfor routing in wireless ad hoc networks whchighly variable but constrained by the possib]positions of nodes and the physical characteristronment. Interesting applications include pervand vehicular networks.

livery.

different valuesFigure 9. We

In this paper we have proposed SiFT, a new trajectory-basedforwarding scheme that shifts the forwarding decision fromthe transmitter to the receiver. This allow a more effective andefficient operation since, differently from previously proposedschemes, nodes do not need to maintain a list of activeneighbors and their geographical coordinates.We have shown that SiFT is able to work even on top of very

simple medium access schemes like CSMA. We evaluated itsperformance with the classical CSMA protocol and with anenhanced version that exploits the carrier sense signal alsoat the network layer. Even if the enhanced scheme allowsto achieve a more effective operation limiting the number ofcollisions and packet duplications, the delay performance ofthe classical scheme is even better than that of the enhancedone. This suggests that SiFT could be easily adopted withstandard MAC protocols like IEEE 802.11 and ZigBee.

ACKNOWLEGEMENTThis work has been partially supported by the European

Network of Excellence EURO-NGI (http://www.eurongi.org/)under the Specific Research Project VNET (Vehicular NET-works).

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