An Adaptive Hybrid Routing Strategy (AHRS) Ahmed I. Saleh, Hesham Arafat, and Amr M. Hamed.

Dept. of Computer Eng. & Systems, Faculty of Engineering, Mansoura University, Egypt

Abstract

Mobile ad-hoc networks (MANETs) is a collection of wireless mobile nodes forming a temporary network without any fixed infrastructure or centralized administration. Although MANETs are easily deployed, they have several constraints such as; continuously changing topology, distributed operations, limitations of nodes radio interface, and limitations concerning battery energy. Recently, Ad-Hoc routing has become an important area of research due to the massive increase in wireless devices. Routing in MANETs is based on a cooperative multi-hop manner. However, due to the highly dynamic topology routing in MANETs is a true challenge. In this paper, we propose a new routing strategy for MANETs called Adaptive Hybrid Routing Strategy (AHRS). The basic idea of AHRS is to inform the network mobile nodes continuously with any changes in the network topology without flooding the network by a huge amount of control messages. AHRS is a hybrid routing strategy that can estimate failure time of links between network nodes through the historical information about link status. Accordingly, AHRS introduces not only the shortest available routes for data transmission, but also elects those reliable ones. AHRS uses no periodic routing advertisement messages but employs a special packet called ‘Carriage’ instead, thereby reducing the network bandwidth overhead and minimizing end-to-end transmission delay. AHRS has been compared against several well-known protocols, which are; DSDV, ZRP, AODV and DSR. Experimental results have shown that AHRS outperforms all competitive protocols as it introduces the minimal routing overheads, as well as a fast route delivery.

Keywords: MANET, Ad hoc, Routing protocols.

1. INTRODUCTION

Recently, with the dramatic progress of networking technologies and the vast growth of mobile wireless devices popularity, a lot of attention has been paid to Mobile Ad Hoc Networks (MANETs) [1]. A MANET is a collection of autonomous wireless Stations forming a temporary (short-lived) network without any fixed infrastructure where all nodes communicate with each other by forming a multi-hop radio network and maintain connectivity in a decentralized manner. In MANET, each Station acts both as a router and as a host, while the control of the network is distributed among the nodes [2]. If two Stations are not within transmission range of each other’s, they can communicate through one or more intermediate nodes, which work as routers, as depicted in Fig3. Due to the lake of any centralized administration, a MANET is easy to deploy and expand allowing the establishment of temporary communications without any pre-installed infrastructure. Accordingly, it becomes very suitable for several important application domains, in which it is not economically practical or physically possible to install an underlying infrastructure or the expediency of the situation does not permit such installation such as military, and emergency disasters (e.g., earthquake, flood, … etc.) [3].

It is a challenging task to choose a good route to establish the connection between a source node and a destination node so that they can roam around and transmit robust communication. Since each node can freely move around in MANETs, they have a dynamic topology triggered by nodes displacements, establishment of new nodes connections and nodes disconnections [4]. Accordingly, efficient routing protocols are required to guarantee a quick and reliable communication among Manet's nodes.

Unfortunately, conventional routing protocols are not appropriate for MANETs because of the temporary nature of the network links, and additional constraints on mobile nodes (i.e., limited bandwidth and power). This makes the classical routing algorithms fail to perform correctly since they are not robust enough to accommodate such a changing environment. Because of node mobility and the lake of an underlying infrastructure, frequent link failures may happen. Hence, each node in a

MANET should be able to find and maintain routes to other nodes whenever such routes are needed [5]. Although several MANETs routing algorithms had been proposed, they either fail to find a transmission route from source to destination nodes or presenting a much longer route than the optimal. Moreover, most of them suffer from control packet flooding, which results in a scalability problem. This happened because whenever a node requests a route, it sends a message that passes through potentially every node in the network. When the network is small, this is not a major concern. However, when the network is large, this can be extremely wasteful especially if the destination node is relatively close to the source.

This paper presents a new routing strategy for MANETs, which is called Adaptive Hybrid Routing Strategy (AHRS). The basic idea of AHRS is continuously inform the network Stations with the changes in the network topology without overloading the network with a huge amount of control packets. AHRS is hybrid as the routing information at each node is continuously updated using special roaming control packets called “Carriages”. AHRS has several salient properties that other conventional routing protocols do not have such as: (i) it uses no periodic routing advertisement flooding messages but employs Carriages instead, thereby reducing network bandwidth overhead. (ii) AHRS minimizes the mobility impacts on the routing efficiency due to the use of a set of roaming carriages that are continuously updating Stations routing tables. (iii) Battery power of network Stations is also conserved, not only by preventing packet flooding but also by Station’s sleeping while Carriages are away. (iv) AHRS minimizes the end-to-end delay of data transmission as it accurately guarantees both the shortest as well as the reliable path from the source to destination, since AHRS has the ability to predict failure time of links among the network Stations. AHRS has been compared against well-known protocols, which are; DSDV, AODV, DSR, and ZRP [6]. Experimental results have shown that AHRS outperforms these protocols introduces the minimal routing overheads and maximum throughput.

The remainder of this paper is organized as follows. In Sect. 2, we introduce related work about routing in ad-hoc networks. In Sect. 3, we present our routing strategy. In Sect. 4, we detail and discuss the results obtained on the NS2 Simulator [7]. We conclude the paper in Sect. 5.

2. Previous Efforts

The routing protocols of MANETs fall into three major categories [8], which are illustrated in table 1. Table 1, Routing categories

Routing categoryDescription

Proactive routing [9]Rely on the periodic broadcast of topology information, where each node periodically broadcasts its routing table to all nodes within its transmission range. All such protocols essentially always provide up-to-date routing information, which can be used to forward data packets through the network towards their destination. In spite of their short response time to set up a route from the source to destination, periodic routing updates require a remarkable amount of network overhead due to the flooding of exhaustive non-productive control packets used to maintain a network overview at each node. Moreover, most of the established routes information is never used, wasting the network resources. The best-known example of a proactive ad hoc routing protocol is the Destination Sequenced Distance Vector (DSDV) protocol [10, 11].

Reactive routing [6]The idea is to only search for paths through the network if data packets need to be transmitted. A source node establishes a route to the destination only on demand, when a route to destination is requested. Protocols fall into such category do not guarantee a short waiting time before the transmission of data packets. This is simply because the source node does not know which neighbor to be selected as the next hop for the packet due to the unpredicted change in the network topology. Therefore, discovering and maintaining a route using reactive routing protocols is extremely difficult, as the source node has to find the route to the destination on the fly. Two examples of reactive routing protocols are Dynamic Source Routing (DSR) protocol and Ad Hoc on Demand Distance Vector (AODV).

Hybrid routing [9]Try to exploit the advantages of proactive routing protocols, i.e. the fast delivery of packets, as routing information is always available, and of reactive routing protocols, i.e. the reduced costs in terms of network

overhead for state maintenance. A typical example is the Zone Routing Protocol (ZRP).

In [9, and 10] DSDV is a proactive table driven routing protocol. Each node maintains a table listing all the other nodes it has known either directly or through some neighbors. Every node has a single entry in the routing table. The entry will have information about the node’s IP address, last known sequence number and the hop count to reach that node. Along with these details, the table also keeps

track of the next hop neighbor to reach the destination node, the timestamp of the last update received for that node. The DSDV update message consists of three fields, Destination Address, Sequence Number and Hop Count. Each node periodically broadcasts out its entire routing table. The sequence number as new attribute guarantees loop-freedom. It makes it possible for the mobile to distinguish stale routes from new ones and that is how it prevents loops. DSDV can only handle bidirectional links.

In [12 and 13] protocol AODV enables multi-hop routing between the participating mobile nodes wishing to establish and maintain an ad-hoc network. AODV is a reactive protocol based upon the distance vector algorithm. The algorithm uses different types of messages to discover and maintain links. Whenever a node wants to find a route to another node, it broadcasts a Route Request (RREQ) to all its neighbors. The RREQ propagates through the network until it reaches the destination or the node with a fresh enough route to the destination. Then the route is made available by uncasing a RREP back to the source. The algorithm uses hello messages (a special RREP) that are broadcasted periodically to the immediate neighbors. These hello messages are local advertisements for the continued presence of the node, and neighbors using routes through the broadcasting node will continue to mark the routes as valid. If hello messages stop coming from a particular node, the neighbor can assume that the node has moved away, mark that link to the node as broken, and notify the affected set of nodes by sending a link failure notification (a special RREP) to that set of nodes.

In [10, 11, and 15] DSR is a reactive routing protocol i.e. determines the proper route only when packet needs to be forwarded. For restricting the bandwidth, the process to find a path is only executed when a node requires a path (On-Demand Routing). In DSR, the sender (source, initiator) determines the whole path from the source to the destination node (Source-Routing) and deposits the addresses of the intermediate nodes of the route in the packets. DSR was developed for MANETs with a small diameter between 5 and 10 hops and the nodes should only move around at a moderate speed. DSR is based on the Link-State Algorithms, which mean that each node is capable to save the best way to a destination. In addition, if a change appears in the network topology, then the whole network will get this information by flooding. The DSR protocol is composed of two main mechanisms that work together to allow discovery and maintenance of source routes in MANET. Route Discovery: When a source node S wishes to send a packet to the destination node D, it obtains a route to D. This called Route Discovery. Route Discovery used only when S attempts to send a packet to D and has no information of a route to D. Route Maintenance: When there is a change in the network topology, the existing routes can no longer be used. In such a scenario, the source S can use an alternative route to the destination D, if it knows one, or invoke Route Discovery. This is called Route Maintenance.

In [15 and 16] ZRP is a hybrid protocol, which is a combination of the both proactive and reactive protocols. If a packet's destination is in the same zone as the origin, the proactive protocol using an already stored routing table is used to deliver the packet immediately. If the route extends outside the packet's originating zone, a reactive protocol takes over to check each successive zone in the route to see whether the destination is inside that zone. This reduces the processing overhead for those routes. Once a zone is confirmed as containing the destination node, the proactive protocol, or stored route-listing table, is used to deliver the packet.

3. The Proposed Adaptive Hybrid Routing Strategy (AHRS)

AHRS is a new routing strategy for MANETs, which adapts to the dynamics of MANET environment. AHRS aims to; (i) introduce up to date network map (e.g., topology) to each node with the minimal overheads, which achieves scalability, (ii) guarantee a fast delivery of the shortest and reliable path between the source and destination due to proactive behavior of AHRS, (iii) maintain the path during the transmission by delivering alternative paths in case of route failure, which becomes an important issue especially for applications that transmit huge amount of data after route establishment,

such as audio and video streaming. Table 2 depicts the used terminologies through the rest of the paper.

Table 2, AHRS Terminologies

Symbol Description

S Station represents mobile node in MANET.

C Carriage is a packet(s) traverse through the MANET.

RRC Route Request Carriage.

SB Station Board.

CC Carriage Cargo.

PT Parking Time.

TSk(Si)Time Stamp when Station i data entry were collected. It's

also reflects the time of carriages visiting station.

BC(Si, TSk) Battery Capacity of Station i at Time Stamp k.

LSSi→j(TSk) Link Signal Strength in direction from Station i to Station j.

NMP Network Mapping Process.

RSP Route Setup Process.

TD Time of Desertion.

The basic idea of AHRS is to continuously inform the network Stations with the changes in the network topology for presenting up to date view of the network topology without overloading the network by a huge amount of control messages. A group of Carriages travel through the network, visiting network Stations, and exchange experience of network topology. Updating Station’s view of the current network topology is accomplished by updating the Station Board (SB) by the Carriage Cargo (CC) to insure the most converged view of network topology with the real network topology.

Definition 1: Station Board (SB): it is a 3D table represents the current and historical Station knowledge of the MANET topology. SB represents the known MANET nodes for the station and the link between them. SB is built through knowledge exchange with neighbors' stations, and from knowledge exchange with visiting Carriages.

Definition 2: Carriage Cargo (CC): it is a 2D table represents the current knowledge of the MANET topology. CC represents the knowledge carriage earned through visiting network Stations, and merge its cargo with the visited SBs.

Definition 3: Neighbor Station: it is a station N is neighbor to Station S, when N is in the transmission range of S.

Fig. 1 illustrates the structure of Station Board, which consists of several layers. Each layer is represented by a two dimensional table of neighboring cells. Those cells represent the Link Signal Strength (LSS) among network Stations. It is also important to mention that LSS from Si to Sj at time stamp TSk (denoted as; LSSij(TSk)) may differ from LSSji(TSk). AHRS supports both bidirectional and unidirectional links, which are illustrated in Fig. 8. When a Carriage reaches a Station, Station records LLS with its neighbors, station’s Battery Capacity BC (expressed by the remaining time until Station battery drained), and Time Stamp TS, which refers to the time Station's record updated, and also indicates the time at which the Carriage visits the Station. Each time a Carriage visits a Station, a new layer is added to the visited station SB, which reflexes a snapshot of network topology based on

       

 

 

 

 

 

 

           

           

            

            

       

 

 

 

 

 

 

           

           

            

            

 

   

    

    

S1 S2 ... SN

S1

S2

...

BC

SN

TS

… … … … …

… … … … …

 … … … … …

 … … … … …

Neighbors

TS

Stati

ons

TSk(Si)BC(Si, TSk)LSSi→j(TSk)

SB H

istorical Snapshot at TSk

Most U

p to Date B

oard

Figure 1, Station Board Structure

S1 S2 S3 S4

S1

BC TS

  0.8 0 0.5 450 10:45

S1 Entry in SB S2 and S4 are in the transmission range of S1

SB Entry Example

The Signal Strength of the Link from S1 to S2 is 80%.The Signal Strength of the Link from S1 to S4 is 50%.There is no link from S1 to S3.BC= 450S: S1 is power on (alive) for 450S.S1 Entry is estimated and recorded at TS= 10:45 AM.

this Carriage experience that gained through its surfing the network. So that each time a Carriage visits a Station, the Station enhances its ability to estimate Life Time between Stations and routes failures.

As depicted in Fig. 2, AHRS is divided into two main processes; (i) Network Monitoring Process (NMP), and (ii) Route Setup Process (RSP). NMP is the process of monitoring the network topology changes through navigating Carriages between network Stations and keep updating their SBs. Using its SB, a Station can estimate a large number of valid routes to the other pre-defined Stations. The more recent of the SB the more map of routes converged with whole or portion of real network topology.

RSP is the process of estimating the available route(s) from source to destination stations using the information recorded in the source’s SB. In case the source’s current SB has insufficient information (has no route to destination), the source creates a special Carriage called Route Request Carriage (RRC), which surfs network to search for route to destination Station. In addition, RSP responsible for setting up route and maintaining transmission.

3CarriageArrival

Figure 2, Basic idea of AHRS

Carriage

Creation

NMP

2

Route Found?

SearchRoute

s

Routes Map

RSP

s

YDeliver Route

1

2

NSend RRC

3

3Traverse

Route Found?

4

YDeliver Route

5N

Deliver Route Not Found

1

Build

Routes

3.1. Network Monitoring Process (NMP)

NMP is a proactive process, as it introduces in advance information of network map, which Stations can use to build the available routes between available Stations before a route requested. AHRS introduces a quick route delivery when a route is requested, as NMP continuously monitors, maintains, and predicts availability and failure of network routes.

Network Monitoring Process(NMP)

Receive updates from neighbor Stations

Merge local SB with updates, and record BC, LSS, TS

Figure 3, NMP.

Start

Station hooks network

Build Empty SB

Forward Carriage to the neighbor Station with oldest TS

Stop

Carriage Arrives Station

Carriages Crash

Combine Crashed Carriage

N

Y

Create Carriage

Out of date SB

SB Updated?

N

Load CC with SB fresh entries

Collect neighbors' SBs updates

Merge local SB with updates and record BC, LSS, TS

RRCS→D?N Y

PT+ζ, and Wait for PT

Y

Build Routes to D

Route to D Found?

Load CC with SB fresh entries

Station S?Y

N

Send RRC back to SY

RRC(HtL)=0?Y

NN

RRC(HtL)-1

Forward RRC to Station with earliest TS

Merge local SB with CC and Send it to neighbor Stations

Set PT=0

As illustrated in Fig. 3, NMP is triggered by the presence of a carriage in the Station by either a Carriage Creation, or Carriage Arrive Events the Station. Station creates Carriage in two cases, which are; (i) when a Station hooks up the network, or (ii) when Station's SB becomes out of date. A Station can detect that its SB becomes out of date, when Carriages stop visiting the Station for a while denoted as; TD. While a significant change has occurred in the perimeter of the Station. Such significant change is detected through estimating Links Life Times with neighbors by identifying several failures of links with neighbor Stations.

AHRS considers two type of Carriages, which are; Normal Carriage (NC) and Route Request Carriage (RRC). The former surfs the network to exchange network topology information among the network Stations. NCs move to the Station with latest visited time by other Carriages (oldest TS). On the other hand, RRC is a special Carriage the surfs the network searching for a fresh route from a source Station to certain destination Station. Normal Carriages move to the Station with earliest visited time by other Carriages (Earliest TS).

As soon as a Carriage reaches a Station Si, it merges its local SB entries with CC of the incoming Carriage. After the merging process, the local SB of Si is containing the freshest entries in each SB and CC. Then, Si sends its local SB to neighbor Stations in its range. Each neighbor Station merges its SB with the sent SB from Si (the station that received the carriage) and sends back the freshest entries in its SB only. When Si receives back all updates, it merges its SB with the updates collected from the neighboring Stations. Then, Si records its Battery Capacity (BC), Links Signal Strengths (LSS)

between Si and each neighbor, and the current Time Stamp (TS) to its SB. TS represents the time of recording Station's entry in its SB to differentiate between up to date and out of date SB entries. Hence, it indicates the freshness of SB entry. The merge of local SB, neighbors' SBs, with CC of the incoming Carriage creates an up to date SB converged with the current real network topology.

Definition 5: Route Request Carriage (RRCS→D): Normal Carriages role limited to transfer network topology information between Stations. In opposite to normal Carriages, RRC is an uncrushable purpose oriented Carriage, where it travels through the network with zero parking time to search for route(s) from specific source Station S to destination Station D, beside exchange network topology experience with the visited Stations during RRC journey. Each RRC has Hop-to-Live(HtL) is the number of hops RRC can jump to Station searching for route to D before it returns back to S. HtL depends on S's SB size, in other word, number of known Stations for S.

In case Carriage visiting Si is a RRC sent by source Station (S) searching for route to destination Station (D). If Si is S, RRC stops forwarding; else, Si loads the RRC Cargo with its local SB fresh entries and tries to build a map of routes to (D). Whenever a fresh route(s) to D is available, RRC returns to S with the enough information to establish a route to D. On other hand, if Si fails to find a valid route to D, it decreases the received RRC's Hop-to-Live (HtL) by 1, and forwards it to the Station with the earliest TS of its neighbors. Such station with the earliest TS has the most recent information about network topology with high probability to find information leads to a route to D. In case RRC's HtL becomes 0, RRC moves backward informing S that there is no route to D, which indicates that D is either turned off or in an isolated region of the network

Definition 6: Parking Time (PT): PT is the time Carriage waits before leaving the Station. PT reflects Carriage velocity of traversing through the network .Initially, Carriage PT is set to zero, when carriage created. Each time Carriage jumps to Station without affecting its SB (no updates in network topology, indicating that the movement of Carriage is faster than network topology change), Station increases Carriage's PT by ζ seconds else PT is set to 0. ζ depends on network dynamics, where ζ is long for static network, and short for dynamic network.

In case the visited Carriage is a Normal Carriage (NC), if Si (e.g., the station that receives the carriage) finds that, the merging operation does not change or update its local SB entries, it slows Carriage motion by stopping Carriage for a period of time PT. Each time a Carriage visits a Station without updating local SB, which indicates that there is no topology change; the hosting Station increases Carriage's PT by ζ second. After PT passes, Station collects updates from neighbor Stations, and merge updates with its local SB, then load the Carriage with local SB fresh entries, and forward Carriage to neighbor Station with oldest TS. On the other hand, if the incoming NC updates the SB's Si, Si immediately sends the Carriage to the next Station and sets its PT to zero.

In highly dynamic MANETs, network topology changes frequently, so that Carriages needs to move faster through the network to inform Stations with network topology changes. Vice versa in static MANETs, Carriages should slow down where network topology infrequently change. AHRS has the ability to control the overall Carriages’ movement velocity adapting with network topology changes, and accordingly minimizing overhead of mapping network. However, slowing down the Carriage speed (by increasing the time the carriage stays in the Station) increases the opportunities of Carriage crash. A Carriage Crash occurs when a normal Carriage jumps to a Station while another normal Carriage is currently staying in the same Station. At this situation, the Station at which the crash takes place combines the crashed Carriages by merging their Cargos into one Carriage that holds the most up to date Cargo. After crashing, the new generated Carriage starts its journey around the network.

Generally, a Station has a permission to create a new Carriage if it does not receive any Carriage of any type during a per-estimated time period called “Time of Desertion” and is denoted as; TD. TD is an essential issue that hardly affects the Network Mapping and consequently future routing decisions. Accordingly, it should be tuned carefully. Table 3 summarizes the impacts of short and long TD as well as the methodology implemented by AHRS.

Definition 4: Time of Desertion (TD): TD is the period of time that network Stations wait without Carriages' visit before creating a Carriage to update SB and to surf network and share topology information with other Stations.

Table 3, TD value issues

Issue Description

Short TDIf the TD made short then the creation of large number of Carriages, which may not be needed especially in static MANET, will overhead network.

Long TDIf the TD made long then the Network Map in Stations may be out-of-date to take the optimal routing decisions especially in highly dynamic MANET.

Proposed TD

determination strategy

The length of TD is set to the average expected change time of Station perimeter, which is the average estimated Link Life Time (LLT) of all links between the Station and its neighbor Stations. LLT and TD estimation will be discussed in section 3.1.3.

Generally, NMP is triggered by a Carriage arrival. To save the Station’s battery as well as minimizing the additional overheads, while a Station is waiting for next NMP initiation (e.g., a new Carriage arrival); it goes in a Station Sleep Mode (SSM), and stops collecting information about neighbors. Most Proactive strategies overloads the network, due to the periodical broadcasting of topology information. NMP overcomes the additional overheads for re-building and maintaining the network map by using a Triggered Selective Casting (TSC) method to share topology information instead of periodical broadcasting.

Definition 5: Triggered Selective Casting (TSC): it is a method of sharing network topology information by collecting topology information from Station's neighbors only when a Carriage visits Station (Triggered to Carriage Arrival) instead of collecting it periodically. Also Selective as the new topology information collected is sent to the neighbor which is not visited lately by a Carriage instead of broadcasting "flooding" collected topology information to all neighbors in perimeter.

NMP casting method is a triggered, as the exchange of network topology information between Station and neighbors happens only if a carriage exists at the station. AHRS casting method is a selective, as Station sends the existed Carriage, which loaded by the most up-to-date network topology information, to certain Station, which has oldest TS. The privilege of triggered selective casting method is to minimize network overhead during NMP.

3.2. Carriage Propagation Delay

In this section, we estimate the time required to propagate network information topology among network Stations through the traverse of Carriages, and time required for Carriage to complete one journey around the network until it returns to its original Station.

Assuming the network covers a circular area of radius R, the density of N-Stations distribution, denoted as; ρ, can be determined by (1):

ρ= Nπ R2 (1)

Where N is the total number Stations, and R is the radius of network area. Therefore, the number of stations within range of any Station S can be estimated by (2):

NS=ρ × π rS2= N

π R2 × π rS2=

N r S2

R2 (2)

Where rs is the radius of Station range. Therefore, the number of Bytes transferred between a Station and its neighbors within its range due to network topology information exchange, denoted as; BS, can be estimated using (3):

BS=2× ¿ (SB )× N S=2× ¿ (SB ) ×N rS

2

R2 (3)

Where Size of (SB) is the size of Station Board in Bytes. The time consumed for exchanging the network topology information among a Station and its neighbors within its range due to one carriage visit can be calculated by (4):

T ( BS )=BS

BW+PT (4)

Where BW is network bandwidth measured by Byte/sec, and PT is Carriage Parking Time. For the first Carriage visit let PT=0, then T(BS) can be calculated by (5):

T ( BS )=2× ¿ (SB )BW

×N rS

2

R2 (5)

One carriage needs to jump N-hops through N Stations to propagate network topology information to each Station in the network. If a number CN of Carriages surf among network Stations, then the number of visits for each Carriage to (6) can express complete Carriage journey and returns to its origin:

CVN=NCN

(6)

Therefore, eq. (7) gives the time required for Carriage to complete its journey across the network.

T C=T ( BS ) ×CVN=2 × ¿ (SB )BW

×N r S

2

R2 × NCN

T C=2 ׿ (SB ) × N 2 ×r S

2

BW × R2× CN

(7)

As illustrated in equation (7), the maximum time required to propagate network topology information across network is directly proportional to the total number of network Stations (e.g., network size), and inversely proportional to number of surfing Carriages..

3.3. Link Failure Estimation

Any station Si can detect a direct link failure to a neighboring station Sj (e.g., Sj moves faraway Si a distance exceeds the considered transmission range of Si or Sj is powered off). While, a station S detects a link failure to one of non-neighbors' stations only if S is informed through a carriage loaded with link status. However, in order to ensure a quick response under the challenge of frequent changes in the network topology, it is important to provide a methodology to estimate accurately the link failure. Hence, the most suitable route for data delivery can be chosen accordingly. To accomplish such aim, through this section, a simple but effective strategy that proximally estimates Links lifetime using the historical copies of SB will be introduced.

To estimate the Link Lifetime between two stations Si and Sj at Station S, we consider the last N historical entries of station board (SB) for time stamps TS1→N (S). The greater number of historical SB's copies (N) leads to more accurate expectation of stations movement, on other hand more memory space needed to store these copies (N is set to 5 in our experimental simulation). Then the amount of change in Time Stamps ∆TS(S) of Si and Sj can be given by (8, and 9):

∆ TS ( S i )=∑k=2

N

(TSk ( S i)−TSk−1 ( Si ))N−1

(8)

∆ TS ( S j )=∑k=2

N

(TSk (S j)−TSk−1 ( S j ) )N−1

(9)

In addition, the change in Battery Capacity ∆BC(S) of Si and Sj can be estimated by (10) and (11):

∆ BC ( Si )=∑k=2

N

( BC ( Si ,TSk ( S i ))−BC (S i , TSk−1 ( Si )) )N −1

(10)

∆ BC ( S j )=∑k=2

N

(BC (S j ,TSk (S j ))−BC (S j , TSk−1 ( S j )))N−1

(11)

The existence of the link depends on the presence of both sides. Link Life Time LLTi→j (∆BC, t) only due to the depletion of stations' batteries capacity can be estimated as in (12, and 13):

LLT i → j (∆ BC (S i ) ,t )=BC ( S i , t )×∆ TS ( Si )∆ BC ( S i)

(12)

LLT j→ i (∆ BC (S j ) , t )=BC ( S j , t )×∆ TS ( S j )∆ BC ( S j )

(13)

Moreover, eq. (14) can give the change in signal strength of the link from Si to Sj:

∆ LSS i→ j=∑k=2

N

( LSSi → j (TS k ( S i) )−LSS i → j (TSk−1 ( S i) ))N−1

(14)

It is noted that; the value of ∆LSSi→j can be negative for diverging stations. By other words, as Si and Sj are moving away from each other, the strength of link between them is getting weaker. In this case, at a certain time t, Link Life Time from Si and Sj, denoted as; LLTi→j (∆LSSi→j, t), can be estimated by (15):

LLT i → j ( ∆ LSSi → j ,t )=LSSi → j (t )×∆ TS (S i )∆ LSSi → j

(15)

On the other hand, if the value of ∆LSSi→j is positive, the stations are converging then diverging. On the other words, Si and Sj are moving toward each other and the strength of link between them is getting stronger till reaches the maximum value. Then, Si and Sj moves away from each other until the strength of link between them is vanished. In this case, at certain time t, eq. (16) introduces Link Life Time LLTi→j (∆LSSi→j, t) only due to the gradient of Link Signal Strength:

LLT i → j ( ∆ LSSi → j ,t )=(2 max ( LSSi → j )−LSS i → j ( t ) )×∆ TS ( Si )∆ LSSi → j

(16)

Generally, the link from Si to Sj exists, as long as their Battery Capacities are not depleted and Sj is in transmission range of Si. From the foregoing, Link Life Time of the link from Si to Sj at time t given by (17):

LLT i → j ( t )=min ( LLT i → j (∆ BC ( S i ) ,t ) , LLT i → j (∆ BC ( S j ) , t ) , LLT i→ j ( ∆ LSSi → j ,t )) (17)

Once a station hooks into the network, it immediately creates a Carriage and initiates the Network Monitoring Process. Then, the Station set TD due to current information in SB. When a Carriage visits Station, TD timer is reset. On the other hand, if TD without a Carriage, the station creates a new Carriage. TD can be calculated by (18):

T D ( t )=∑ (∀ j∈neighbors LLT i → j (t ) )

no .of neighbors (18)

Moreover, link existence from Si to Sj can be checked using Eq. (19):

Let t=TS ( Si ) …. The latest time link information (LSSi → j , BC ( Si )) been recorded

(TS (S i )+LLT i → j (TS ( Si ) )) – Current time={ +ve Link exist– ve Link broken (19)

Route Life Time for Source Station S to Destination Station D (RLTS→D) depends on the weakest link from S to D. RLTS→D can be estimated using (20):

RLT S → D=Min (∀S → D (LLT i→ j (t ))) (20)

3.4. Route Setup Process (RSP)

Due to Carriages traverse around the network, and updating SBs, each Station can build a large number of paths using its SB. Until now, our proposal behaves in a proactive manner. RSP guarantees delivering shortest route, and route alternatives in case of route failure.

As shown in Fig. 4 When Station S requests a route to Station D, RSP builds a map of fresh routes to D. If RSP finds a working route to D, S starts data session and begins data transmission. RSP can deliver shortest route with minimal number of hops, or reliable route with the longest lifetime. If a Carriage arrives S or current route from S to D fails, RSP chooses another best route available in routes map to D and redirect data transmission, until transmission completed.

Route Setup Process (RSP)

Build Map of Routes to D

Choose best available Route

Figure 4, RSP.

Start

Stop

Route Request to D

Route Fail?Y

Route(s) to DFound?

Y N

N

CarriageArrive?

Create and Send RRC

RRC Sent?Y

N

Y

RRC Back?

Y

N

N

Route to D Not Found

Start (Resume) Transmission

TransmissionCompleted?

Y

N

Route Found?Y

N

Convert RRC to Normal Carriage

`

AHRS behaves reactively; when a fresh route from S to D is not available using the local SB, the Station creates a RRC with configurable HtL. HtL depends on network dimension. RRC traverses through the network merging its Cargo with the visited SBs, and searching for fresh route(s) to D with in the visited Stations' SBs. When RRC finds a route from S to D, RRC begins its journey back to S, delivering the route(s) to S. S switches RRC into normal carriage, and sends it to traverse the network. S sets up the route and starts data transmission to D.

While S is waiting for RRC to return back, if a Carriage visits S, S updates its SB with the visiting Carriage Cargo, and builds routes map to D. If a fresh route to D found, S starts data transmission immediately instead of waiting RRC to return. While RRC is traversing, if RRC's HtL is decreased by one at each hop. If RRC's HtL reaches zero, RRC starts its journey back to S. When RRC returns to S without fresh route to D, RSP declares route to D not found. On other hand if RRC returns with new network topology information leads to a fresh route to D, S starts data transmission to D, and convert RRC to a normal Carriage that starts surfing the network to collect network topology information.

AHRS introduces both shortest route with the minimal number of hops to reach destination Station, as well as reliable route, which estimated to be alive along the communication duration or introduce alternative route in case of current route failure. Shortest route improve the quality of network communication.

Another scenario to tune and improve quality of network communication, when a source Station S needs to transfer a sequence of data packets to destination D, initially RSP delivers the shortest available route due to the on fly available topology information stored in SB. Then, S commences data transmission to D. During S to D transmission, if current route fails, or a Carriage visits S with new network topology information leads to a new shortest path, RSP delivers the shortest available route

due to the current state, and redirect communication through the new route until transmission completed.

4. Experimental Design and Results

In this section, AHRS will be evaluated against several types of efficient MANET routing protocols, which are; proactive routing protocol (DSDV), two on-demand reactive routing protocols (AODV & DSR), and hybrid routing protocol (ZRP) will be explained. The different evaluation metrics are illustrated in table 4.

Table 4, The considered performance metrics

Metric Description

Packet Delivery Fraction (PDF)

PDF is the ratio of data packets delivered to the destination to those generated by the sources. PDF is a measure for the routing protocol robustness as it measures the amount of packets the protocol able to deliver properly. PDF can be calculated by dividing the number of packet received by destination through the number packet originated from source. Hence, PDF can be calculated as; PDR= (Pp/PT)*100, where, Pp is the amount of data packets able to reach the destination node, and PT is the total amount of data packets delivered by the source during the simulation time.

Average End-to-End Delay (second)

This includes all possible delay caused by buffering during route discovery latency, queuing at the interface queue, retransmission delay at the MAC, propagation and transfer time. It is defined as the time taken for a data packet to be transmitted across an MANET from source to destination. Hence, D = (Tr –Ts), where Tr is receive Time and Ts is sent Time.

Through this section; a set of experiments have been conducted to measure the strength of the proposed routing strategy (AHRS) strategy against two well known protocols, which are; DSDV, AODV, DSR, and ZRP using the metrics defined above. The parameters that have been used in the following experiments are summarized in Table 5.

Table 5, Simulation ParametersParametersValueRouting ProtocolsDSDV, AODV, DSR, ZRPMAC Layer802.11Packet Size512 bytesTerrain Size1000m * 1000mNodes100Transmission Range150, 100-300 m (interval of 50)Mobility ModelRandom way point [16]Data TrafficTCPNo. of Source10, 50Simulation Time900 sec.Maximum Speed0-60 m/sec (interval of 10)

4.1. Experiment 1: Packet Delivery Fraction (PDF)

As illustrated in Fig. 5, all protocols deliver almost all originated data packets (around 96-99.7%) when mobility is low and number of sources is low (10 sources). DSDV introduces the worst performance where it is periodical flood network with routing control packet, which may not needed especially in static environment. Vice versa, AHRS slows Carriages (represents Routing Overhead) movement with slow network topology change, and accelerates them in case of rapid network topology change. However, the packet delivery fraction starts to gradually degrading in the case of DSR and AODV when there is increase in number of sources (50 sources) and with the increase in speed of nodes. AHRS outperforms all other algorithms as the Carriages update the Stations' SBs with up-to-date routing information.

10 20 30 40 50 6095

96

97

98

99

100

Figure 5(A), PDF Vs Max Speed with 10 sources.

DSDV-10 DSR-10 AODV-10ZRP-10 AHRS-10

Maximuim Speed (m/s)

Pack

et Delivery Frac

tion (PDF) %

10 20 30 40 50 6092

93

94

95

96

97

98

99

100

Figure 5(B), PDF Vs Max Speed with 50 sources.

DSDV-50 DSR-50 AODV-50 ZRP-50AHRS-50

Maximuim Speed (m/s)

Pack

et Delivery Frac

tion (PDF) %

Fig. 6 shows that AHRS performs outstandingly better than other competitive protocols, when nodes' transmission ranges are variable, as AHRS supports unidirectional links.

100 150 200 250 30092

93

94

95

96

97

98

99

100

Figure 6, PDF Vs Max Node Range.

DSDV-50 DSR-50 AODV-50 ZRP-50AHRS-50

Maximuim Speed (m/s)

Pack

et Delivery Frac

tion (PDF) %

4.2. Experiment 4: Average End-To-End Delay (AETE))

As illustrated in Fig. 7, delay in reactive protocols is too much higher than proactive protocols and AHRS especially with the increase of network load (e.g., 40 sources). Over all in case of real time packet delivery, AHRS is the best choice. AHRS and proactive protocols deliver routes in advance; also, it represents alternative routes when route to destination fails. On the other hand, DSR produces more delay due to route caching. Although DSR can respond a route quickly, it yields a long delay when a route is rebuilt. This is because when source node receives RERR packet, it will try to find alternative routes from the route cache. If alternative routes are not available, the source node, then, will enter route discovery phase to find new routes.

10 20 30 40 50 600

0.050.1

0.150.2

0.250.3

0.35

Figure 7(A), AETE Vs Max Speed with 10 sources.

DSDV-10 DSR-10 AODV-10 ZRP-10AHRS-10

Maximuim Speed (m/s)

Aver

age En

d-To

-End

 Delay

 (Sec

onds

)

10 20 30 40 50 600

0.10.20.30.40.50.60.7

Figure 7(B), AETE Vs Max Speed with 50 sources.

DSDV-50 DSR-50 AODV-50ZRP-50 AHRS-50

Maximuim Speed (m/s)

Aver

age En

d-To

-End

 Delay

 (Sec

onds

)

5. Conclusion

This paper focuses on the routing problem of Ad hoc networks. A new scalable routing has been introduced, which is called An Adaptive-Reliable Hybrid Routing Strategy (AHRS). AHRS tries to continuously inform the network mobile nodes with the changes in the network topology without overloading the network by a huge number of control messages. AHRS is hybrid as the routing information at each node is continuously updated using Carriages;

AHRS deliver shortest routes in advance minimizing end-to-end delay. AHRS offers a number of potential advantages over conventional routing protocols such as; (i) it uses no periodic routing advertisement messages but used Carriages instead, thereby reducing network bandwidth overhead. (ii) AHRS minimizes the mobility impacts on the routing efficiency. (iii) Battery power is also conserved on the mobile hosts, both by not sending the advertisements and by not needing to receive them. (iv) AHRS minimizes the end-to-end delay of data transmission as it accurately guarantees the shortest path from the source to destination. (v) AHRS introduces reliable paths for critical communications where it estimated routes lifetime, and support unidirectional links.

In spite of its effectiveness, AHRS suffers from a relative bigger amount of routing cache requirements than AODV and DSR as each mobile node should maintain a 3D Station board. AHRS has been compared against well known protocols, which are; DSDV, ZRP, AODV and DSR. Experimental results have shown that AHRS outperforms competitive protocols in most cases as it introduces the minimal routing overheads.

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