MNT and VNT routing Multi-hop Technology d-hoc Networks ll hops are wireless ll nodes are mobile Military pplications rmy leet of warships pace pplications group of pathfinders group of satellites ommercial pplications Users with hand-held devices Why d oc Networks? ase of deployment peed of deployment ecreased dependence on infrastructure 2 1
The hallenges for Multi-hop Networks hannel ontention reduces channel efficiency ow do you reduce packet-level contention? Route omputation message intensive ow do you design scalable routing protocols? Qo upport applications do not like fluctuations What mechanisms are needed for Qo support on onehop and Qo support on the full multi-hop route? 3 hallenge 1 hannel ontention hannel ontention Reduces efficiency of channel Needs to be alleviated ontention in ingle op Networks 802.11 Medium ccess (M) protocol ontention in Multi op Networks (proposed extensions of 802.11) Quick xchange ast orward 4 2
ontention in ingle-hop Networks 802.11 802.11 mode (istributed oordination unction) M/ RT/T optional 802.11 P mode (Point oordination unction) Time ccess point polls users to transmit No contention Very few companies market products for P ender RT T T Receiver RT Request to send T lear to send 802.11 ontention in Multi-hop Networks elf-contention (the main type of contention in multi-hop networks) ontention between packets of same transport connection ntra-stream contention ontention caused by packets of the same stream at different nodes nter-stream contention ontention between T packet stream and packet stream T stream (TP or UP) TP T stream destination source ontention for shared media destination prior M solutions [u et. al., nfocom 03] source stream ontention for shared media prior M solutions (none) elf-contention is best resolved at the M layer because elf-contention arises in the M layer Requires no changes to widely deployed transport protocols 802.11 is an evolving standard and is amenable to changes 3
Routing or packet switched network oal path selection orwards the datagrams (packets) from source to destination destination address in packet determines next hop routes may change during session xample of routing and forwarding routing algorithm local forwarding table header output value0100 3 link 0101 2 0111 2 1001 1 value in arriving packet s header 0111 3 2 1 4
Routing lgorithm classification lobal or decentralized information? lobal all routers have complete topology, link cost info link state algorithms ecentralized router knows physicallyconnected neighbors, link costs to neighbors iterative process of computation, exchange of info with neighbors distance vector algorithms tatic or dynamic? tatic routes change slowly over time ynamic routes change more quickly periodic update in response to link cost changes Routing in Mobile d oc Networks (MNT) ormed by wireless hosts which may be mobile Without (necessarily) using a pre-existing infrastructure Routes between nodes may potentially contain multiple hops May need to traverse multiple links to reach a destination Mobility causes route changes ynamic topologies 5
looding for ata elivery broadcasts data packet P to all its neighbors ach node receiving P forwards P to its neighbors P P P P P P equence numbers used to avoid the possibility of forwarding the same packet more than once P P P P P Node does not forward the packet looding for ata elivery dvantages implicity May be more efficient than other protocols when rate of information transmission is low enough that the overhead of explicit route discovery/maintenance incurred by other protocols is relatively higher this scenario may occur, for instance, when nodes transmit small data packets relatively infrequently, and many topology changes occur between consecutive packet transmissions Potentially higher reliability of data delivery ecause packets may be delivered to the destination on multiple paths 6
looding for ata elivery isadvantages Potentially, very high overhead ata packets may be delivered to too many nodes who do not need to receive them Potentially lower reliability of data delivery looding uses broadcasting -- hard to implement reliable broadcast delivery without significantly increasing overhead roadcasting in 802.11 M is unreliable n our example, nodes and may transmit to node simultaneously, resulting in loss of the packet in this case, destination would not receive the packet at all Why is Routing in MNT different? ost mobility link failure/repair due to mobility may have different characteristics than those due to other causes Rate of link failure/repair may be high when nodes move fast New performance criteria may be used route stability despite mobility energy consumption 7
hallenge 2 Route omputation ey challenge in Routing in Multi-hop Wireless Networks The destination may be anywhere in the network Routing Layer olutions for ingle-hop wireless networks Macro Mobility Mobile P Micro Mobility ellular P Routing Layer olutions for Multi-hop wireless networks Query the whole network Proactive Routing Reactive Routing ynamic ource Routing (R)/d-hoc On emand istance Vector (OV) end broadcast only in the right direction (coped roadcast) Location ided Routing (LR) xplore only few select paths Zone Routing Protocol (ZRP) xplore only a single path reedy Perimeter tateless Routing (PR) 15 Proactive routing (Tabledriven) Uses periodic route updates to maintain routing tables an either be link state or distance vector Mobility is treated as link change We introduce 1.V (ighly ynamic estination-equenced istance-vector Routing).. Perkins and P. hagwat, ighly dynamic destination sequenced distance-vector routing (V) for mobile computers, in M OMM 94, 1994, pp. 234 244. 2.R (lobal tate Routing) T.-W. hen and M. erla, lobal tate Routing New Routing cheme for d-hoc Wireless Networks, n Proceedings of 98, tlanta,, un. 1998, pp. 171-175. 8
Table-driven, Proactive Link-tate routing table estinatio next n 10 routing table estinatio next n link-state Link-eq istance 8 (,) (,) 10 8 (,) (,) 8 routing table routing table estinatio n next estinatio n next 7 Table-driven, Proactive istance-vector estinatio n estinatio n routing table next next update estinatio n update next 10 8 8 estinatio n routing table estinatio n next update next routing table routing table estinatio n next estinatio n 7 update next estinatio n next 9
V estination equence istance Vector Routing M 1996, simulated but not implemented Uses modified ellman-ord lgorithm, distance vector based, table driven Route settling time and route may not converge equence number derived from dest. node is used to keep the routing table up-to-date V est next hop seq time 2 14 100 2 18 110 2 20 130 routing table est next hop seq time 1 14 100 2 18 110 2 20 130 routing table periodically sends its own route information to neighbors until stable 10
V est next hop seq time 4 20 210 4 22 200 3 40 150 routing table est next hop seq time 3 20 210 3 22 200 3 40 150 routing table est next hop seq time 1 20 210 1 22 200 2 40 150 routing table ind the path according to the Routing Table (hop by hop) R Link tate lgorithm ach node periodically sends its own route information to neighbors ach node notifies neighbors only when its route information changes neighbors then notify their neighbors if necessary Using the Link tate lgorithm to create the routing table 11
R link-state Link-eq istance (,) 10 (,) 8 (,) 6 (,) 10 link-state Link-eq istance (,) 5 (,) 10 (,) 8 link-state Link-eq istance (,) 5 (,) 10 periodically sends its own route information to neighbors until stable R hortest Path Router-Table path order (,), (,), (,), (,),,, (,),, (,) (,) (,),,,,,,, ind the path according to the Routing Table (source to any hosts) 12
Proactive routing rawbacks nefficient if there is few demands for routes, and instability if there is high mobility Lots of signaling traffic ll of the routes may never be used Reactive routing (On-demon) New nodes are added when needed No periodic route updates ind route when needed by the source node aching will help to improve the performance We introduce 1.R (ynamic ource Routing ) ONON,.., N MLTZ,.. ynamic source routing in ad hoc wireless networks. n Mobile omputing, T. mielinski and. orth, ds. luwer cademic Publishers, 1996, ch. 5, pp. 153 181. 2.OV (d oc On-emand istance Vector Routing Protocol ) harles. Perkins and lizabeth M. Royer. d hoc On-emand istance Vector Routing, n Proceedings of the 2nd Workshop on Mobile omputing ystems and pplications (WM 99), New Orleans, L, ebruary,1999, pp.90-100. 13
Reactive routing (On-demon) lood the whole network in search of the destination estination or node knowing route to destination responds Two phases Routing discovery Route Request and Route Reply Routing maintenance R ynamic ource Routing MU 1996, simulated and implemented in 1999 n extension of P, it uses options field in P Two phases routing discovery and routing maintenance aching and other features are used 14
R When node wants to send a packet to node, but does not know a route to, node initiates a route discovery ource node floods Route Request (RRQ) ach node appends own identifier when forwarding RRQ Route iscovery in R Y Z M N L Represents a node that has received RRQ for from 15
Route iscovery in R roadcast transmission Y [] Z M N L Represents transmission of RRQ [X,Y] Represents list of identifiers appended to RRQ Route iscovery in R Y [,] [,] Z M N L Node receives packet RRQ from two neighbors potential for collision 16
Route iscovery in R Y Z [,,] [,,] M N L Node receives RRQ from and, but does not forward it again, because node has already forwarded RRQ once Route iscovery in R Y Z [,,,] M [,,,] N L Nodes and both broadcast RRQ to node ince nodes and are hidden from each other, their transmissions may collide 17
Route iscovery in R Y Z [,,,,M] M L N Node does not forward RRQ, because node is the intended target of the route discovery Route iscovery in R estination on receiving the first RRQ, sends a Route Reply (RRP) RRP is sent on a route obtained by reversing the route appended to received RRQ RRP includes the route from to on which RRQ was received by node 18
Route Reply in R Route Reply can be sent by reversing the route in Route Request (RRQ) only if links are guaranteed to be bidirectional To ensure this, RRQ should be forwarded only if it received on a link that is known to be bi-directional f unidirectional (asymmetric) links are allowed, then RRP may need a route discovery for from node Unless node already knows a route to node f a route discovery is initiated by for a route to, then the Route Reply is piggybacked on the Route Request from. f 802.11 M is used to send data, then links have to be bi-directional (since ck is used) Route Reply in R Y RRP [,,,,] Z M N L Represents RRP control message 19
ynamic ource Routing (R) Node on receiving RRP, caches the route included in the RRP When node sends a data packet to, the entire route is included in the packet header hence the name source routing ntermediate nodes use the source route included in a packet to determine to whom a packet should be forwarded ata elivery in R Y T [,,,,] Z M N L Packet header size grows with route length 20
R Optimization Route aching ach node caches a new route it learns by any means When node finds route [,,,,] to node, node also learns route [,,] to node When node receives Route Request [,,] destined for node, node learns route [,,,] to node When node forwards Route Reply RRP [,,,,], node learns route [,,] to node When node forwards ata [,,,,] it learns route [,,,] to node node may also learn a route when it overhears ata packets Use of Route aching When node learns that a route to node is broken, it uses another route from its local cache, if such a route to exists in its cache. Otherwise, node initiates route discovery by sending a route request Node X on receiving a Route Request for some node can send a Route Reply if node X knows a route to node Use of route cache can speed up route discovery can reduce propagation of route requests 21
Use of Route aching [,,,,] [,,,] [,,],[,,] [,,,] M [,] [,,] N L Z [P,Q,R] Represents cached route at a node (R maintains the cached routes in a tree format) Use of Route aching an peed up Route iscovery [,,,,] [,] [,,,] [,,] [,,,] When node Z sends a route request for node, node sends back a route reply [Z,,,] to node Z using a locally cached route [,,],[,,] RRQ [,,,] RRP Z M N L 22
Use of Route aching an Reduce Propagation of Route Requests [,,,,] [,] [,,,] [,,] [,,,] [,,],[,,] RRQ [,,,] ssume that there is no link between and Z. Route Reply (RRP) from node limits flooding of RRQ. n general, the reduction may be less dramatic. Z RRP M Y N L Route rror (RRR) Y RRR [-] Z M N L sends a route error to along route --- when its attempt to forward the data packet (with route ) on - fails Nodes hearing RRR update their route cache to remove link - 23
ynamic ource Routing dvantages Routes maintained only between nodes who need to communicate reduces overhead of route maintenance Route caching can further reduce route discovery overhead single route discovery may yield many routes to the destination, due to intermediate nodes replying from local caches ynamic ource Routing isadvantages Packet header size grows with route length due to source routing lood of route requests may potentially reach all nodes in the network are must be taken to avoid collisions between route requests propagated by neighboring nodes insertion of random delays before forwarding RRQ ncreased contention if too many route replies come back due to nodes replying using their local cache Route Reply torm problem Reply storm may be eased by preventing a node from sending RRP if it hears another RRP with a shorter route 24
d oc On-emand istance Vector Routing (OV) [Perkins99Wmcsa] R includes source routes in packet headers Resulting large headers can sometimes degrade performance particularly when data contents of a packet are small OV attempts to improve on R by maintaining routing tables at the nodes, so that data packets do not have to contain routes OV retains the desirable feature of R that routes are maintained only between nodes which need to communicate OV Route Requests (RRQ) are forwarded in a manner similar to R When a node re-broadcasts a Route Request, it sets up a reverse path pointing towards the source OV assumes symmetric (bi-directional) links When the intended destination receives a Route Request, it replies by sending a Route Reply Route Reply travels along the reverse path setup when Route Request is forwarded 25
26 Route Requests in OV Z Y Represents a node that has received RRQ for from M N L Route Requests in OV Represents transmission of RRQ Z Y roadcast transmission M N L
27 Route Requests in OV Represents links on Reverse Path Z Y M N L Reverse Path etup in OV Node receives RRQ from and, but does not forward it again, because node has already forwarded RRQ once Z Y M N L
28 Reverse Path etup in OV Z Y M N L Reverse Path etup in OV Z Y Node does not forward RRQ, because node is the intended target of the RRQ M N L
Route Reply in OV Y Z M N L Represents links on path taken by RRP Route Reply in OV n intermediate node (not the destination) may also send a Route Reply (RRP) provided that it knows a more recent path than the one previously known to sender To determine whether the path known to an intermediate node is more recent, destination sequence numbers are used The likelihood that an intermediate node will send a Route Reply when using OV not as high as R new Route Request by node for a destination is assigned a higher destination sequence number. n intermediate node which knows a route, but with a smaller sequence number, cannot send Route Reply 29
orward Path etup in OV Y Z M N L orward links are setup when RRP travels along the reverse path Represents a link on the forward path ata elivery in OV Y T Z M N L Routing table entries used to forward data packet. Route is not included in packet header. 30
Timeouts routing table entry maintaining a reverse path is purged after a timeout interval timeout should be long enough to allow RRP to come back routing table entry maintaining a forward path is purged if not used for a active_route_timeout interval if no data is being sent using a particular routing table entry, that entry will be deleted from the routing table (even if the route may actually still be valid) Link ailure Reporting neighbor of node X is considered active for a routing table entry if the neighbor sent a packet within active_route_timeout interval which was forwarded using that entry When the next hop link in a routing table entry breaks, all active neighbors are informed Link failures are propagated by means of Route rror messages, which also update destination sequence numbers 31
Route rror When node X is unable to forward packet P (from node to node ) on link (X,Y), it generates a RRR message Node X increments the destination sequence number for cached at node X The incremented sequence number N is included in the RRR When node receives the RRR, it initiates a new route discovery for using destination sequence number at least as large as N estination equence Number ontinuing from the previous slide When node receives the route request with destination sequence number N, node will set its sequence number to N, unless it is already larger than N 32
Link ailure etection ello messages Neighboring nodes periodically exchange hello message bsence of hello message is used as an indication of link failure lternatively, failure to receive several M-level acknowledgement may be used as an indication of link failure Why equence Numbers in OV To avoid using old/broken routes To determine which route is newer To prevent formation of loops ssume that does not know about failure of link - because RRR sent by is lost Now performs a route discovery for. Node receives the RRQ (say, via path --) Node will reply since knows a route to via node Results in a loop (for instance, ---- ) 33
ummary OV Routes need not be included in packet headers Nodes maintain routing tables containing entries only for routes that are in active use t most one next-hop per destination maintained at each node Multi-path extensions can be designed R may maintain several routes for a single destination Unused routes expire even if topology does not change lustering / ierarchical void drastic network-wide changes mall clusters converge fast May use hybrid design ompromised Method Reduce disadvantages and promote advantages On theory better but MO it s a bit too complicated and smart, the rule of the real world is The simpler the better. We introduce 1. ZRP (Zone Routing Protocol), Z., N PRLMN, M. The performance of query control schemes for the zone routing protocol. n Proceedings of the OMM 98 onference on ommunications rchitectures, Protocols and pplications (ept. 1998). 34
ZRP Zone Routing Protocol ornell 1998, simulated only Zone based and hybrid of proactive and reactive Proactive intra-zone and reactive inter-zone ach node has complete topology up to k hops or destination outside k hops, query forwarded to the boundary nodes Problem ow to decide the appropriate radius of the zone ZRP roup ach node has complete topology up to k hops, and become a group order Node The edge node in a group iscovery Process 1. When node wants to send data to node, f is in the group, send it lse, send Route Request to order Node 2. When receives a Route Request f is in the group, send Route Reply to source node lse, send Route Request to order Node 35
ZRP Neighbor Table of node border x x x o o o 將 RRQ 送往自己管理的 order 群組的領導 表示藍色節點所管轄的中間節點 表示藍色節點所管轄的邊界節點 表示藍色節點管轄之外的其他節點 ZRP Neighbor Table of node border O x X O X Routing Table of Path order (,),, 將 RRP 由原路徑送回 群組的領導 表示藍色節點所管轄的中間節點 表示藍色節點所管轄的邊界節點 表示藍色節點管轄之外的其他節點 36
37 ZRP Path order (,),, Routing Table of 群組的領導群組的領導群組的領導群組的領導 表示藍色節點所管轄表示藍色節點所管轄表示藍色節點所管轄表示藍色節點所管轄的中間節點的中間節點的中間節點的中間節點 表示藍色節點所管表示藍色節點所管表示藍色節點所管表示藍色節點所管轄的邊界節點轄的邊界節點轄的邊界節點轄的邊界節點 表示藍色節點管轄表示藍色節點管轄表示藍色節點管轄表示藍色節點管轄之外的其他節點之外的其他節點之外的其他節點之外的其他節點 ZRP node border O x X O X Neighbor Table of 群組的領導群組的領導群組的領導群組的領導 表示藍色節點所管轄表示藍色節點所管轄表示藍色節點所管轄表示藍色節點所管轄的中間節點的中間節點的中間節點的中間節點 表示藍色節點所管表示藍色節點所管表示藍色節點所管表示藍色節點所管轄的邊界節點轄的邊界節點轄的邊界節點轄的邊界節點 表示藍色節點管轄表示藍色節點管轄表示藍色節點管轄表示藍色節點管轄之外的其他節點之外的其他節點之外的其他節點之外的其他節點 f not the estination
VNTs vs. MNTs VNT consists of vehicles to form a network which is similar to a Mobile d oc Network (MNT). owever, there are following differences between these two networks. Vehicles mobility Vehicles move at high speed but mobility is regular and predictable Network topology igh speed movement makes network topology dynamic No significant power constraint Recharging batteries from vehicle Localization Vehicles position estimate accurately through P systems or on-board sensors 75 eatures of VNTs The characteristics of VNTs can be summarized after comparing with the MNTs. ynamic topology Nomadic nodes with very high speed movement cause frequent topology variation Mobility models Vehicles move along original trajectories completely different from typical MNT scenarios nfinite energy supply Power constraint can be neglected thanks to always recharging batteries Localization functionality Vehicle can be equipped with accurate positioning systems (P and LLO) integrated by electronic maps 76 38
Preliminaries in Routing Protocols (1/2) n VNTs, the following characteristics affect the design of the routing protocols. igh-speed node movement requent topology change hort connection lifetime especially with multihop paths Traditional topological routing protocols for ad hoc networks are not suitable. 77 Preliminaries in Routing Protocols (2/2) n the following, the routing protocols in the recent years that address the characteristics of VNTs are presented with three basic categories Position-based Routing eocasting Routing roadcast Routing MNT Table-driven, Proactive On-demon, Reactive lustering / ierarchical 78 39
coped roadcast (Location ided Routing) Obtain the location and speed and direction of motion of the destination using P ased on the time elapsed since last update, restrict the direction of route query flood ssues Requires P support source Request Zone etermined by source dest 1000x1000 region Number of nodes 15, 30, 50 Various tx range 200, 300, 400 and 500 Routing packets/data packet looding LR Number of nodes 79 Position-based Routing The routing decision at each node is based on the destination s position contained in the packet and the position of the forwarding node s neighbors. ssumption Position-based routing does not keep global network information but requires information on physical locations of nodes. very vehicle has an on board P (or some other type of positioning service) for routing. 40
xploring a single path (reedy Perimeter tateless Routing) ach node learns the location of all its neighbors reedy mode forwarding (default) pick the neighbor closest to the destination Perimeter mode forwarding f a node itself is closest to the destination, it then tries to forward the packet around the void ssues Requires P support void x Node x s void with respect to destination 81 Motivation ource routing vs. routing table op-by-hop routing table small number of entries ource routing high bit overhead lobal protocol annot scale nd-to-end route maintenance looding Overhead proportional to network size & mobility 41
PR nswer is location! Local state neighbor table ll need to know is all my neighbors locations #tate proportional to density instead of #destinations Local protocol Route discovered and fixed locally Overhead independent from network size & mobility 10/8/2002 537 ensor Networks 83 ssumptions ll wireless routers know their own positions ources can determine the locations of destinations local directory service onus location-based communication make directory service unnecessary To handle void Perfect radio model identical, circular radio range. nodes are roughly in a plane. 10/8/2002 537 ensor Networks 84 42
Location-based ommunication -based rom to What is the reading of sensor 125.111.1.5? Rely on unreliable individual sensors Location-based rom location to location What is the virus density in south terminal of airport? ndividual sensors NOT important Local coordination ensors in interested area aggregate data ensor-base comm. end aggregated result to base station 10/8/2002 537 ensor Networks 85 xchange Location Proactive, periodic beaconing Waste bandwidth in static network Piggyback location in data packet On-demand beaconing efore send, request for all neighbor locations if not known More efficient in static sensor networks 10/8/2002 537 ensor Networks 86 43
reedy orwarding Optimal path fficient O(1) on sorted neighbor table The denser a network, the more likely to work losest to 10/8/2002 537 ensor Networks 87 ut We till Need to olve Void specially in network that is deployed in a ad hoc fashion 10/8/2002 537 ensor Networks 88 44
Right-and Rule o around the perimeter of a void! 10/8/2002 537 ensor Networks 89 ssembling PR Together Maintenance ll nodes maintain a single-hop neighbor table hop links to construct planar graph t source mode = greedy ntermediate node greedy fails reedy orwarding Perimeter orwarding greedy works have left local maxima greedy fails 10/8/2002 537 ensor Networks 90 45
xample d e z a c f x b 10/8/2002 537 ensor Networks 91 Location-ased Multicast (LM) (3/4) cheme 1 ox orwarding Zone When a node receives a eocast packet, it will forward the packet to its neighbors if it is within a forwarding zone; otherwise, it will discard the packet. ource/ink eocast Region orwarding Zone 92 46
Location-ased Multicast (LM) (4/4) cheme 2 ON orwarding Zone Unlike scheme 1, cheme 2 does not have a forwarding zone explicitly. oecast packet that should be forwarded is based on the position of the sender node and receiver node. (X,Y ) (X,Y ) ource/ink (X,Y ) (X,Y ) Multicast Region (X,Y ) Y X Z (X,Y ) 93 roadcast Routing n nter-vehicle ommunication ystems (V),broadcasting is an efficient method to spread messages. The reasons of occurring broadcast storm n a broadcasting network, the situations of contentions and collisions often take place if an efficient broadcasting scheme is not used. The result incurred by broadcasting is called broadcast storm. 47
roadcast torm n VNTs, broadcast is used for disseminating the traffic information etour route ccident alert onstruction warning etc ome messages will be periodically broadcasted by roadside unit (RU) for several hours or even some days. The problem of broadcast storm in VNT is more serious than that in MNT roadcast Routing Message issemination deal solution Minimum onnected ominating et, which minimizes packet rtx and preserves network connectivity. Realistic solutions trade-off between robustness and redundancy. The important concern in designing a broadcast scheme in VNT. ow to design broadcast algorithm to efficiently transmit messages to the target nodes. To design a broadcast algorithm to make the desired vehicles to receive the message as soon as possible. 48
our roadcasting trategies ifferent broadcasting strategies to select the forwarding nodes Probability-based Location-based Neighbor-based luster-based roadcast Routing 1. Probability-based given P determines the decision, for example depending on the number of copies a node has received. The strategy is often dynamic. P = probability distribution function 49
roadcast Routing Probability-based a r P = 0.8 a r P = 0.5 o rw a rd in g N o d e c h o o se roadcast Routing Location-based The selection criterion is the amount of additional area that would be covered by enabling a node to forward. ome proposal also computes position prediction as useful input information. 50
roadcast Routing Location-based ar Target ar wants to turn right orwarding Node choose roadcast Routing Neighbor-based node is selected depending on its neighbors status (for instance, the status concerns how a neighbor is connected to the network). 51
roadcast Routing Neighbor-based Target ar ollect the information of neighbors ar orwarding Node choose roadcast Routing luster-based Nodes are grouped in clusters represented by an elected cluster-head. Only cluster-heads forward packets. Nodes in the same cluster share some features (e.g., relative speed in VNTs). Reclustering on-demand or periodically. 52
roadcast Routing luster-based luster-eader ateway-node luster-eader orwarding Node choose V Vehicle-ssisted ata elivery in Vehicular d oc Networks ing Zhao uohong ao epartment of omputer cience & ngineering The Pennsylvania tate University n Proceedings of nnual oint onference of the omputer and ommunications ocieties (NOOM), pp.1-12,2006. 106 53
V 107 V 1. Transmit through wireless channels as much as possible. 2. f the packet has to be carried through certain roads, the road with higher speed should be chosen. 3. ynamic path selection should continuously be executed throughout the packet forwarding process. 108 54
V 109 55