Virtual Segment: Segmentation Method for Store-carry-forward Routing Schemes

Shinya Yamamura1, Akira Nagata1, Masato Uchida2, Masato Tsuru2

1Kyushu Research Center, National Institute of Information and Communications Technology, AIM bldg 7F, 3-8-1, Asano, Kitakyushu-shi, 802-001, Japan

Tel: +81-93-512-3852, Email:{yamamura,nagata}@nict.go.jp 2Network Design Reserch Center, Kyushu Institute of Technology, AIM bldg 7F,3-8-

1,Asano,Kitakyushu-shi,802-001 , Japan Tel: +81-948-29-7652, Email:{m.uchida,tsuru}@ndrc.kyutech.ac.jp

Abstract: In general, communication services based solely on the store-carry-forward routing schemes suffer from the scalability problem, meaning that the buffer consumption and the delay time in message delivery increase linearly with the size of the service area. Our contribution aims at a practical framework to provide a wide service area of supporting large-size data exchanges among users or from/to the Internet in a high cost-performance way. The basic idea is to carefully divide the entire service area into some segments (in which a store-carry-forward routing scheme by vehicles could be effectively adopted) properly formed along the road, each of which can be regarded as a virtual hot-spot, and to persistently connect those segments by wired/wireless networks (i.e., by making short-cut). We have preliminarily evaluated our proposed framework in case that a simple epidemic routing scheme is used in each segment and confirmed our proposal could narrow the spread range of the epidemic routing and thus mitigate the huge delivery delay and the wasteful buffer consumption.

1. INTRODUCTION The emerging growth of communications could not be coped with by means of only expanding or upgrading the wired and/or wireless network infrastructures especially in rural areas due to their low cost-performance. For these areas, the short range but high speed and cost-effective wireless communications combined with the physical conveyance of data by moving vehicles would be an appropriate tool for asynchronously exchanging huge but non-real-time multimedia data. If utilized, this could provide a new network infrastructure that will help alleviate the digital divide in rural areas. Disruption Tolerant Networks (DTNs) technologies are designed to overcome limitations in connectivity or performance due to poor infrastructure. IRTF DTNRG and related research efforts have proposed some store-carry-forward routing schemes. Epidemic routing [1] [5] is a basic concept, and Throw Box [2], Data Mules [3], and Message Ferry [4] are examples of supporting infrastructures for accelerating the store-carry-forward routing schemes. In general, communication services based solely on the store-carry-forward routing schemes suffer from the scalability problem, meaning that the buffer consumption and the delay time in message delivery increase linearly with the size of the service area. To address the scalability issue, it may be useful to confine the propagation area by some means. Several approaches have been proposed, such as limiting the number of copies of messages by hop count, or controlling the buffer efficiency with an appropriate life time value carefully chosen by considering the expected propagation distance, although the required information is not always available at the message sender. In addition, the idea of combining the infrastructure

(e.g., the WiFi hot-spots) in the propagation area seems promising. Recently, for example, Banerjee et al. [6] examined the cost-performance trade-off of different designs which add a supporting infrastructure. This approach achieved high reachability using store-carry-forward routing scheme, but at the same time it brought the high consumption of buffer. Our contribution aims at a practical framework to provide a wide service area of supporting large-size data exchanges among users or from/to the Internet in a high cost-performance way. The basic idea is to carefully divide the entire service area into some segments (in which a store-carry-forward routing scheme by vehicles could be effectively adopted) properly formed along the road, each of which can be regarded as a virtual hot-spot, and to persistently connect those segments by wired/wireless networks, i.e., by making short-cut without the store-carry-forward routing. This could narrow the spread range of the store-carry-forward routing schemes being confined within a segment, which results in mitigating the huge delivery delay and the wasteful buffer consumption in such schemes. In our proposed system, by carefully setting the propagation distance of the store-carry-forward routing schemes (i.e. segment size), the amount of buffer consumption and the delivery delay time can be estimated and thus controllable. By introducing a kind of ARQ, our proposal also results in solid communication reachability higher than Data Mules and comparable to Message Ferry without its high running cost to operate the scheduled ferries. This is because the proposed system employs the vehicles, which are not controlled in routing by the system but are expected to move along the roads in a segment and to collaborate with our system, in contrast to Message Ferry. The communication reachability becomes higher by shortening the segment size. The rest of the paper is organized as follows: Section 2 presents a brief overview of the proposed system architecture, especially on a retransmit method within a segment to efficiently accelerate the performance. Our proposed system is preliminarily evaluated in case that a simple epidemic routing is used in each segment in Section 3. The proposed system is also compared with other popular narrowband communication media in the delivery delay time for a huge data exchange situation. Finally we conclude the paper in Section 4.

2. SYSTEM ARCHITECTURE

2.1 Concept of Virtual Segment Figures 1 and 2 show a conceptual example and a schematic depiction of our proposed system, respectively. We model three types of communication nodes: a regular node representing each stationary or mobile communication user, a relay node representing a vehicle that traverses along roads and has the ability to store, carry, and forward messages from/to regular nodes nearby, and a base node (BN) representing a stationary gateway that communicates with and controls the relay nodes passing near to the BN and also stores the messages to be collected and delivered. Each node has a wireless device (e.g., 802.11g) and opportunistically communicates with others only in a (relatively short) communication range, while each BN also has a persistent connection to other BNs and the Internet. A Virtual Segment (VS) is a geographical region surrounded and delimited by one or more BNs along the roads, in which the message exchange service is likely to be provided to the users, i.e. regular nodes. A BN belonging to a VS appoints an appropriate vehicle moving into the VS to a relay node in order to collect messages from and deliver messages to regular nodes within the VS. When the relay node leaves from the VS at some exit BN, it transfers all the messages being carried to the BN and is dismissed by the BN from the role as relay node of the VS.

2.2 Design Issues Our proposed system seems to have three design issues. Firstly the optimal placement of BNs is a key issue to achieve a good trade-off between communication delay and cost, by

considering the vehicle traffic (relay nodes) as well as the distribution of users (regular nodes) and their demand. In this paper, however this issue has not been addressed; it will be addressed in the future work. The second issue is an efficient retransmission control method within a segment to ensure a high reachability (i.e. reliable delivery) and a low buffer consumption. A problem faced by the existing store-carry-forward schemes except for Message Ferry [4] is unreliable delivery or an unexpectedly large delivery delay time. The lack of the propagation area size information prevents a message sender from estimating the life time of the message, and thus from performing an efficient retransmission for undelivered messages. As the mobile nodes in the store-carry-forward schemes cannot guarantee to carry the messages to the next hop, a custodial transport described in IRTF RFC 4838, 5050 does not work well in such circumstances. The Message Ferry is a special in that its path way is controllable and scheduled, and thus in general, the ferry can be considered as a custodian. However, running a number of Message Ferry or adaptively changing the optimal routes of Message Ferry enforcements a highly cost-consuming operation.

Figure 1: Conceptual example of our proposed system.

Figure 2: Schematic depiction of our proposed system.

In our Virtual Segment scheme, vehicles passing through the roads in a given segment are used, instead of the scheduled Message Ferries, for collecting, carrying, and delivering

R1

Ｎ１

R1 R1

Relay Node

Regular NodeVirtual Segment

Base Node

Road

BN2

Core Network

ＢＮ１

Base Node

Ｓｐｅｅｄ fＤｉｓｔａｎｃｅ 1D

R1

Ｎ１

R1 R1

Relay Node

Regular NodeVirtual Segment

Base Node

Road

BN2

Core Network

ＢＮ１

Base Node

Ｓｐｅｅｄ fＤｉｓｔａｎｃｅ 1D

ＢＮ

ＢＮ

ＢＮ ＢＮ

ＢＮＢＮ

ＢＮ

ＢＮＢＮ

ＢＮ

Core Network

Virtual Segment 1

road

Virtual Segment 2

Virtual Segment 3 Virtual Segment 4

messages. Compensating for the luck of reliability caused by using vehicles not fully controlled, we introduce segment control signals to confirm the message exchanges between users (i.e. regular nodes) and BNs. Accompanied with the segment control signals, we also use the segment specific life time which is estimated by the distance of the segment. This combination brings more rapid retransmission control in turn saving the buffer consumption. The third issue relates to naming, addressing, and security, which involves inter-segment and the Internet routing and authentication. The core network provides both the persistent connectivity to the Internet and the short-cut connections among not only BNs in the same VS but also different VSs, and it may be composed of wired networks, long-distance wireless networks, or multi-hop wireless mesh networks as an infrastructure for the VS system. The problem is how to identify each node and how to manage which regular nodes reside or temporary exist in each VS. For example, each regular node in a segment should send a registration message to the BNs which manage the segment to make the regular node visible. The details of this issue have not been addressed in this paper and remain as future work.

2.3 Segment Control Signals and their specific life times Automatic Retransmission reQuest (ARQ) on segment basis is introduced. The segment control signals give a fine timer control for retransmission, because each base node (BN) is a custodian and arranged in a distance so as that the round-trip-time of a message between a regular node and the BN is predictable. The delivery notification signal combined with a fine life time control enables more efficient retransmission control. Two signals used between regular nodes and base nodes via relay nodes are introduced. One is the base-node-confirm-report confirming whether a message from a regular node is reached a BN or not. This message is required by the regular node and created by the BN. The other is the regular-node-confirm-report confirming whether a message from a BN is reached a destination regular node or not. This message is required by the BN and created by the regular node A Segment Management Extension (SME) shown in Fig.3 is a function block of the bundle layer handling the above signals. The SME conceals the VS management over the bundle layer to the application layer and provides fine retransmission control within a segment. In the following, those two signals are briefly explained in the simplest case illustrated in Fig.2 assuming a single relay node passing through a road between two BNs. We assume each node has its own identifier EID (Endpoint IDentifier). Note that Mansy et al. [7] investigates a detailed retransmission mechanism with data fragmentation in carrying messages between two nodes along a road via a vehicle passing through the road. This mechanism can be used in transferring a message between a regular node and a base node. However, in our scheme, since multiple BNs and multiple regular nodes in a VS should be combined by relay nodes, a retransmission mechanism with data fragmentation becomes more complex. Algorithm for the base-node-confirm-report • When a vehicle (mobile node) R1 meets BN1, i.e., appears in the WLAN area of BN1

which is an entrance point of the VS, R1 advertises its EID to BN1 after establishing a wireless connection.

• The SME of BN1 appoints R1 to a relay node in the VS allowing R1 to communicate with any regular node around the roads in the VS. Appointing a relay node is to configure the needed information to communicate with each regular node in the VS, such as interface information, IP address, and routing information used by bundle layer. The SME sets the configuration of the wireless interface or/and routing information on a bundle daemon via API.

• When regular node N1 wants to send a message to some destination, it stores the message on a buffer in SME. The buffer status of the message is set to ”none-sending”. Relay node R1 moves along the road to BN2. N1 monitors its wireless interface and when R1 meets N1, i.e., R1 reaches the WLAN range of N1, N1 starts to communicate with R1. If N1 has a message stored with status of “none-sending” or “transmit” then N1 transfers the message and sets the status to “transmitted”. N1 holds the message and sets the base-node-confirm timer for future retransmission. The timer value is set to the estimated round-trip-time of a message between BNs in the VS plus some margin.

Figure 3: Segment Control Signal flow.

• When R1 meets base node BN2, which is an exit point of the VS, it transfers the carrying messages to BN2. BN2 forwards a received message to a base node in another segment or to the Internet via the core network (CN) if the message is destined for some node not in the same VS, or stores it in its buffer to transfer it to another relay node later if the message is destined for some node in the same VS.

• BN2 dismisses R1 as a relay node. Dismissing the relay node is to clear the buffer, to delete the routing information, the identification, and WLAN interface information, etc.

• BN2 creates the base-node-confirm report and sends it to the BN1 via the permanent CN. • Both BN1 and BN2 store the base-node-confirm report on the buffer. The buffer status is

set to “non-sending”. The base-node-confirm report is transferred to another relay node (R2 for example) that appears in the segment, which will behave in the same way as R1 except for carrying the base-node-confirm report to N1.

• When R2 meets N1, the base-node-confirm report is delivered to N1 and N1 deletes the original message stored in the buffer and cancels the Base-node-confirm timer.

• If the Base-node-confirm timer is expired before the corresponding Base-node-confirm report is reached, the SME of the N1 sets the status of the buffer to “transmit”.

Algorithm for regular-node-confirm-report • On BN1, when a message destined for N1 is reached, it is buffed with status of “non-

sending”. • When R1 entering the VS meets BN1 and is appointed to a relay node, BN1 tries to

transfer the messages stored with status of “non-sending” or “transmit” to R1. Then BN1 changes the buffer status to “transmitted” and sets the Regular-node-confirm-timer for future retransmission. The timer value is set to the estimated round-trip-time of a

Application Layer

Bundle Layer

Ｔｒａｎｓｐｏｒｔ Ｌａｙｅｒ

SME

Source Node (Regular Node)

daemon

Intermediate Node (Relay Node) Intermediate Node (Base Node)

Destination Node

4.Regular-node-confirm report

2.Base-node-confirm report

Base-node-confirm timer

1.Message

(request)3.Message

(response)

Regular-node-

confirm timer

Virtual Segment

(store-carry-forward )

Core Network

(none store-carry-forward)

Application Layer

Bundle Layer

Ｔｒａｎｓｐｏｒｔ Ｌａｙｅｒ

SME

daemon

Application Layer

Bundle Layer

Ｔｒａｎｓｐｏｒｔ Ｌａｙｅｒ

SME

daemon

Application Layer

Bundle Layer

Ｔｒａｎｓｐｏｒｔ Ｌａｙｅｒ

SME

Source Node (Regular Node)

daemon

Intermediate Node (Relay Node) Intermediate Node (Base Node)

Destination Node

4.Regular-node-confirm report

2.Base-node-confirm report

Base-node-confirm timer

1.Message

(request)3.Message

(response)

Regular-node-

confirm timer

Virtual Segment

(store-carry-forward )

Core Network

(none store-carry-forward)

Application Layer

Bundle Layer

Ｔｒａｎｓｐｏｒｔ Ｌａｙｅｒ

SME

daemon

Application Layer

Bundle Layer

Ｔｒａｎｓｐｏｒｔ Ｌａｙｅｒ

SME

daemon

message between two BNs in the VS plus some margin. BN1 also makes the list of messages transferred to the relay node and sends the list to the other BNs within the same segment.

• When R1 meets N1, the message destined for N1 is delivered and N1 creates the regular-node-confirm report and sends it to BN1 via R1 (or other relay nodes).

• When R1 meets BN2, BN2 confirms that R1 successfully delivers the message to N1. Then the message is deleted from all the buffers of the BNs within the same segment. The regular-node-confirm report for the message is also forwarded to the BNs.

• If the Regular-node-confirm timer is expired, the SME of the BN1 sets the status of the buffer to “transmit”.

3. SYSTEM EVALUATION

3.1 Evaluate for comparing the Store-carry-forward schemes. We evaluated the proposed system by simple computer simulations to examine its effectiveness compared with an existing store-carry-forward routing scheme, Epidemic routing (EP) [1]. Note that, in EP approach of our simulation, each BN acts as a stationary node that contributes to message storing and forwarding. The simulation model has been made based on that of Data Mules [3]. The parameters used in the simulation are field related ones, nodes related ones, and simulation ones. The field related parameters involve area size, block size, communication range, and base node arrangement strategy. The nodes related parameters involve the number of regular nodes, the number of relay nodes, relay node speed, message sending interval, and bundle life time. And the simulation conditions involve store-carry-forward strategy, the number of trials, expire time of simulation, and measurement item. In each simulation setting, we calculate the following two metrics for evaluation: one is the delay time, which is the average elapsed time of the message round trip between an arbitrary pair of nodes, and the other is the buffer consumption which is the average number of buffers that are used for the message round trip between an arbitrary pair of nodes. We change the number of blocks composing the entire simulation field as a simulation parameter, and 100 trials with different random seeds were done per each parameter setting. The simulation model of the entire filed composed of Virtual Segments is illustrated in Figure 4 and the parameters are shown in Table 1. The block is a simulator’s conceptual unit that shows assumed wireless communication range to compute the message exchange among the each node. Initially the given number of regular nodes and relay nodes are randomly distributed on the field. Then each relay node moves around along the road randomly and independently during one simulation. More precisely, each relay node chooses one of four neighbour blocks (up, down, right, left) and moves to it every unit time. All nodes in the same block can exchange a message each other. This type of moving pattern is known as random walk [8].The detailed action of this simulation is described below.

Table 1: Simulation Parameters

Wireless network range 200m Relay speed 10m/s The number of total nodes 20% of total block The number of relay nodes 10% of total blocks The number of base nodes 4 in each segment

Store-carry-forward strategy EP • The Regular node, the relay node, and the base node decrease their lifetime of all

messages in their buffers and if the lifetime reached to zero, then the messages are removed from the buffers.

• The position of all the relay nodes is updated every simulation time.

• The contents that exist in the buffer of the regular nodes, the base nodes, and the relay nodes in the same block are synchronized.1

• If the message reaches to the destination node, then the message is removed from the buffer.

VS The VS strategy is same as the EP strategy except for the following. • If the destination of the message is not in the belonging segment, then the BN forward the

message to the other BNs that come in the given segment and removes the message from its buffer.

• The relay node checks the area number every time it moves and if the area number is changed then the all messages are removed from the buffer.

Evaluation action Average delay time • Make the simulation field with given parameters. • Decide the initial position of the regular node, the base node, and the relay node. • Choose the source and the destination nodes randomly from the regular nodes that have

been created in above step and place a message destined for the destination node to on the bundle layer of the source node.

• The simulator is excuted according to the store-carry-forward strategy that mentioned above and is continued to until a bundle layer received the report that will be returned from the destination node to the source node. The delay time is calculated by converting this round trip time in the simulation into an actual one that is defined by taking into consideration of the block size and the relay node speed.

• The average delay time is calculated by taking the average of all trials. • The position of all nodes is rearranged at each trial.

Figure4: Simulation model of the entire filed composed of Virtual Segments (VSs).

Average buffer consumption • The buffer consumption is calculated by summing the total number of entries of buffer

that has been registered to all nodes during the passage of the above delay time. The average buffer consumption is calculated by taking the average all trials.

Figure 5 shows the performance variations according to density of BNs with a constant density of total nodes which is also included BNs themselves and with a constant simulation field. The value of 0 of x axis shows EP (i.e. non segmentation). Only less than 1% density of BNs exhibited approximate four times performance to none segmentation case. This is

1 This action simulates the flooding type routing. In VS, we could select the more comfortable routing scheme, however at this time we have used the flooding type routing for VS to confirm the effectiveness of segmentation briefly.

Simulation Field Segment

Base Node

Block

Main line

Branch line

Segment

because, in VS approach, the BNs divide the entire filed into some segments, which confine the need of data carry by relay nodes within a narrower segment.

Figure5: The performance variation according to density of BNs.

Figure 6 shows the performance variations according to the density of total nodes with a constant density of BNs. A ration of the regular nodes and the relay nodes is same. The average delay time is improved according to increasing the density of total nodes, at the same time buffer consumption is also increased. In VS, the average delay time is drastic improved on increasing the density of relay nodes, and confirmed the EP worked only within a distributed the narrow range segment. Concerning to the buffer consumption, the VS keeps the low value and the influence of the density of node is not obviously.

Figure 6: The performance variation according to the density of total nodes with a constant density of

BNs.

Figure 7 depicts the performance variation according to the ration of regular nodes and relay node with the condition of the same density of total nodes and filed size. According to this measurement we can find the average delay time is drastic improved for both EP and VS in the case of the relay nodes ration of the total number is about 20%. In EP, according to the average delay time is improved, the buffer consumption is also increased. On the contrast，in VS, buffer consumption is almost constant.

3.2 Evaluate for comparing to exiting narrowband communication medium. We also evaluated the performance of VS by comparing the existing narrowband communication media in the situation of the large file transfer, and found that the VS could reduce the communication time by relatively a small number of relay nodes. We show the effectiveness of VS comparing the communication time in case of that a communication device is used with narrow band channel. The life time is estimated as the duration time when the relay node moves from the one side of the BN to other side of BN. In Figure 2, if we

0

5000

10000

15000

20000

25000

0.00 0.20 0.40 0.60 0.80 1.00

Dnsity of total nodes

Ave

rage

dela

y tim

e (

second)

epidemic

vs

0

500

1000

1500

2000

2500

3000

0.00 0.20 0.40 0.60 0.80 1.00

Density of total nodes

Num

ber

of

buff

ers

epidemic

vs

0

5000

10000

15000

20000

25000

0 0.01 0.02 0.03 0.04 0.05

Density of BN (Number of BNs/Total Number of Blocks)

Ave

rage

Dela

y Tim

e (

second)

0

500

1000

1500

2000

2500

0 0.01 0.02 0.03 0.04 0.05

Density of BN (Number of BNs/Total Number of Blocks)

Num

ber

of

buff

ers

assume that regular node N1 wants to access the Internet, in this simple case, the average delay time d is estimated as next formula:

d ＝ ＋ , w twhere is the average wait time, and t is propagation delay time. The average wait time is calculated the average of the interval time of the relay nodes appears on VS. The occurrence interval is assumed to be the exponential distribution .Thus average of the occurrence interval is calculated as

w w

)(tf tetf λλ −=)(λ/1 . The propagation delay time t is

calculated as the sum of the moving time from the N1 to the BN2 and the communication time from the BN2 to the Internet. The communication time from the BN2 to the Internet is omitted because its value is too small comparing to the moving time of the relay. The moving time is taken from the distance D1 between N1 and BN2 and the relay node speed (see Figure2). At last an approximate formula of the average delay time is shown as bellow.

f

fDd //1 1+= λ Figure 8 estimates the average delay times of various communication media and the VS system. The condition parameters of the VS system are assumed as follows. The average speed of relay node = 10m/sec, f λ＝10 unit/hour , communication range = 200m, data transmission speed = 10Mbps.

0

5000

10000

15000

20000

25000

0 0.2 0.4 0.6 0.8 1

Ration of Ｒｅｌａｙ Nodes

Ave

rage

Dela

y Tim

e (

second)

Figure 7: The performance variation according to ration of relay nodes.

Figure 8: Comparing the performance of communication media in the average delay time.

epidemic

vs

0

200

400

600

800

1000

1200

1400

1600

0 0.2 0.4 0.6 0.8 1

Ration of Ｒｅｌａｙ Nodes

Num

ber

of

buff

ers

epidemic

vs

0

100

200

300

400

500

1 2 3 4 5 6 7 8 9 10 11 12

Ave

lage

Dela

y T

ime (

min

ute

s)

2M

10M

100M

Cell phone(PDC/line）

phone（analogue）

phone（ＩＳＤＮ）

ＡＤＳＬ

FTTH

Cell phone(PDC/packet）

cdmaone

W-CDMA

PHS

MF(1K) MF(5K) MF(10K)

0

100

200

300

400

500

1 2 3 4 5 6 7 8 9 10 11 12

Ave

lage

Dela

y T

ime (

min

ute

s)

2M

10M

100M

Cell phone(PDC/line）

Cell phone(PDC/packet）

phone（analogue）

cdmaone PHS

phone（ＩＳＤＮ）

ＡＤＳＬ MF(1K) MF(5K) MF(10K)W-CDMA

FTTH

4. CONCLUSIONS In this paper, we have discussed the need for segmentation method accelerating the store-carry-forward routing schemes and proposed a practical framework that we call Virtual Segment. Although we have confirmed the potential of the VS approach from the macro view point, many issues should be addressed for operation and implementation in practice such as the optimal placement of BNs and the addressing issues. We need to evaluate the VS in more realistic situations and improve the buffer consumption within a segment by enhancing the retransmission algorithm with predicting and optimally sharing the relay nodes in the VS. The use of multiple relay nodes to deliver a huge message by fragmentation is also needed. Adopting the erasure coding and/or the network coding should be addressed in future work. While the unicast message delivery is considered in this paper, multicasting and broadcasting over multiple VSs may be useful for some applications and should be implemented in a cost-efficient and secure manner. In general, however, multicasting and broadcasting can be a double-edged sword with regard to security attacks such as DOS, which are difficult to prevent in the conventional store-carry-forward routing schemes. On the other hand, in our VS approach, each BN can be a security gateway to a segment to manage the security on messages to/from the segment. REFERENCES [1] A.Vahdat and D.Becker (2000). Epidemic routing for partially connected ad hoc networks. Technical Report CS-2000-06. [2] W.Zhao, Y.Chen, M.Ammar, M.Corner, B.Levine, and E. Zegura (2006). Capacity Enhancement using throwboxes in mobile tolerant network. SCS Technical report GIT-CSS-06-04. [3] R.Shah, S.Roy, S.Jain and W.Brunette (2003). Data MULEs: Modelling a Three-Tier Architecture for Sparse Sensor Networks. In Proceedings of the First IEEE, 2003 IEEE International Workshop on Sensor Network Protocols and Applications. Pp. 30-41. [4] W.Zhao and M.Ammar (2003). Message Ferrying: Proactive Routing in Highly-partitioned Wireless Ad Hoc Networks. In Proceedings of Ninth IEEE Workshop on Future Trends. Pp. 308-314. [5] T.Matsuda and T.Takine (2008). (p,q)-Epidemic Routing for Sparsely Populated Mobile Ad Hock Networks. Selected Areas in Communications, IEEE Journal, vol.28. Pp. 783-793. [6] N.Banerjee, M.D.Corner, D.Towsley and B.N.Levine (2008). Relays, Base Stations, and Meshes: Enhancing Mobile Networks with Infrastructure. In Proceedings of Mobicom’08. Pp. 81-91. [7] A.Mansy, M.Ammar, and E.Zegura (2007).Reliable roadside-to-roadside data transfer using vehicle traffiic. In Mobile Adhoc Sensor Systems, 2007. MASS 2007. Pp1-6. [8]T.Camp, J.Boleng, and V.Davies (2002). A survey of mobility models for ad hoc network research. In Wireless Communications & Mobile Computing (WCMC), vol. 2. Pp483-502.