Terremotos / Eric K. Noji

FAAC-Multipath protocol [91] has all the capabilities of FAAC protocol as discussed in chapter 3. The protocol is partially coupled with the Dynamic Source Routing (DSR) protocol and the primary route selection is like FAAC protocol. The protocol also assures the guaranteed throughput to the applications and it is evaluated with a single as well as multipath admission control protocols. In this chapter we have illustrated the functionality of multiple paths, backup route selection, backup route reliability and backup route maintenance of the protocol.

4.1.1 Route Discovery

The application agent specifies the requirements o f the data session in the form o f session request (SReq) packet and then passes on this SReq packet to the network layer. The source node stores this information and initiates the Route Request (RtRq). The RtRq propagates till the destination, and the destination unicast Route Reply (RtRp) to all routes found between source and destination. When the source receives multiple route replies, then it applies the local as well as neighbour’s capacity test either to grant or reject the admission o f data session. The protocol tests and maintains two routes for each data session all the time; one of the routes is called primary and the other one is called secondary. The primary route is selected for data transmission while secondary route is selected as a backup for the data session. If primary route fails either due to nodes mobility or congestion then the primary route is removed from cache and secondary route is selected as a primary route for data transmission. After this other untested route is tested and maintains as a secondary route for the data session. The two routes primary and secondary must be partially disjoint; means half o f the route. We have selected partially disjoint route condition because it is very difficult to find fully disjoint backup in MANETs. Therefore, we make our protocol more flexible, while selecting backup path. Our protocol can select a route as backup path whether it is fully disjoint or it share half of the nodes with primary route. We allow the backup route to share maximum 50% nodes with primary route, we choose maximum 50% so that the probability o f the concurrently route failure of both routes is not higher than 50%.

4.1.2 Selection of Backup routes

The protocol may find many routes from source to destination during route discovery process. During the route discovery process, the nodes capacities are not tested on these

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likely routes. The source stores all the received RtRps from destination and tests the capacity of routes from source to destination.

For backup route, local capacity is tested using Channel Idle Time Ratio (CITR) mechanism while neighbour’s capacity is tested passively using lower neighbours carrier sensing threshold. Contention Count (Ccoum) may underestimate or overestimate the capacity because FAAC-Multipath supports partially disjoint routes means that backup route and primary route may share some nodes as well Carrier Sensing Range (CSR), so instead o f Ccount, FAAC-Multipath will use Contention Difference (Cdiff). The Cdiff o f a node is the number of those carrier sensing neighbours excluding destination node which are on backup path but not on the current path of the data flow. CD is estimated as:

Cdiff=j CcountI — I C S N C l Rcurr \ { D } | (4.1)

Here Cdiif is representing the contention difference which will be the number o f nodes, Ccount is the Contention Count of the node on backup path. Carrier Sensing Neighbour (CSN) include those nodes which comes in the Carrier Sensing Region (CSR) o f the stated node, Rcun^ represent the current flow the data traffic and D is the destination o f the data traffic which is not included in Cdiff calculation. Figure 4-1 presents the illustration o f Cdiff.

^ Current path ► Backup path

c s

—

<D I

Figure 4-1 Calculation of Contention Difference (CD)

The small circles represent nodes and larger circle represents carrier sensing range o f node ‘h’. The solid and dotted arrows represent the current and backup route o f the flow respectively. According to formula of contention count (Ccoum) [32], node ‘h’ Ccoum is 3, but two nodes {d, e} are part of the current data traffic route, so the Cdiff o f the node ‘h ’ will be

4.1.3 Backup Route Reliability

The protocol uses partially disjoint routes so that the probability of concurrently routes failure may be minimized. The primary and backup routes can share half of the route nodes. The disjointness of the routes improves the reliability of the routes. The following formula finds the reliability of the backup route by comparing the available bandwidth with the required bandwidth of the data flow.

Cavail — Crsv > = Cdiff * Creq (4 .2 )

Here Cavaii is the available capacity at node, Qsv is the reserve capacity of the node, Qeq is the required capacity of the data session and Cdiff is the contention difference. On the basis of resource estimation, the node will decide the admission of new data session.

4.1.4 M aintenance of Backup route

FAAC-Multipath protocol has one primary and one tested backup route for each data session and data is always transmitted on primary route. Source node may have stored more untested backup routes for the same data session. The protocol switches the data session in three different situations: Firstly, the data session is switched from primary to secondary route whenever primary route fails. Secondly, the data session switches to secondary when primary route is not upholding the guaranteed throughput. Finally, the secondary route offers higher throughput than the primary route. Figure 4-2 shows, the operation of switching of data flow from primary to backup route due to the mobility of nodes of other data flow.

C5>->(6^-X3F-X3F.

CSR

C u rren t p a th Backup p a th

Figure 4-2 Explanation of Route Changes

Figure 4-2 shows the transmission of two data flows. The small circle represents the nodes, large circle represent the carrier sensing range of the nodes. Flow 1 use the route A-^B and flow 2 use the route S ^ 1 ^ 2 - ^ 3 —>4-^D. At the beginning, the nodes participating in the transmission of flows are out of carrier sensing range of each other. When the first flow nodes move into the carrier sensing range of nodes of second flow, then the both routes fail

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to satisfy the QoS requirements. Therefore the source of flow 2 switches the flow to alternate route S-^5—>6—>7—> 8 ^ 9 —>D so it can satisfy the requirements.

When route failure occurs at any node, then this node tried to recover the route locally. By local repairing, we mean that the error finding source node will search for the alternate route, which can fulfil the requirements o f that flow.

If the backup route is, cached at the source, discovered either during the route discovery of primary route or found opportunistically. Then the protocol conducts the testing of nodes resources on the stated route. The process is similar to the second step of the route discovery where the protocol unicast the Session Request (SReq) packet on the stated route. Local resources are tested using channel idle time ratio (CITR) mechanism and neighbour’s resources are tested passively. The capacity testing is passive, so it does not introduce overheads to the network. The source stores the backup path on the successful completion of the resource testing on that path. If the testing fails then the route is removed from untested backup route cache.

If all the cached backup routes are removed, then the source set off new route discovery for the same data session because if the current route fails, then there should be a tested backup/secondary route in the cache, there will be no need to stop the data traffic but just switch to tested backup route.