MANET Literature Review

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Chapter 2

Routing and QoS in Mobile Ad hoc Networks

Literature Review

2.1 Ad hoc networks

Ad hoc networks are wireless networks without a fixed infrastructure, which are usually

assembled on a temporary basis to serve a specific deployment such as emergency rescue or

battlefield communication. They are especially suitable for scenarios where the deployment of an

infrastructure is either not feasible or is not cost effective. The differentiating feature of an ad

hoc network is that the functionality normally assigned to infrastructure components, such as

access points, switches, and routers, needs to be achieved by the regular nodes participating in

the network. For most cases, there is an assumption that the participating nodes are mobile, do

not have a guaranteed uptime, and have limited energy resources.

Before describing the types of approaches and example protocols, it is important to explain the

developmental goals for an ad hoc routing protocol so that the design choices of the protocols

can be better understood. Hence, the following are typical design goals for ad hoc network

routing protocols:

Minimal control overhead: Control messaging consumes bandwidth, processing resources, and

battery power to both transmit and receive a message. Because bandwidth is at a premium,

routing protocols should not send more than the minimum number of control messages they need

for operation, and should be designed so that this number is relatively small. While transmitting

is roughly twice as power consuming as receiving, both operations are still power consumers for

the mobile devices. Hence, reducing control messaging also helps to conserve battery power.

Minimal processing overhead. Algorithms that are computationally complex require significant

processing cycles in devices. Because the processing cycles cause the mobile device to use

resources, more battery power is consumed. Protocols that are lightweight and require a

minimum of processing from the mobile device reserve battery power for more user-oriented

tasks and extend the overall battery lifetime.


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Multihop routing capability. Because the wireless transmission range of mobile nodes is often

limited, sources and destinations may typically not be within direct transmission range of each

other. Hence, the routing protocol must be able to discover multihop routes between sources and

destinations so that communication between those nodes is possible.

Dynamic topology maintenance. Once a route is established, it is likely that some link in the

route will break due to node movement. In order for a source to communicate with a destination,

a viable routing path must be maintained, even while the intermediate nodes, or even the source

or destination nodes, are moving. Further, because link breaks on ad hoc networks are common,

link breaks must be handled quickly with a minimum of associated overhead.

Loop prevention. Routing loops occur when some node along a path selects a next hop to the

destination is also a node that occurred earlier in the path. When a routing loop exists, data and

control packets may traverse the path multiple times until either the path is fixed and the loop is

eliminated, or until the time to live (TTL) of the packet reaches zero. Because bandwidth is

scarce and packet processing and forwarding is expensive, routing loops are extremely wasteful

of resources and are detrimental to the network. Even a transitory routing loop will have a

negative impact on the network. Hence, loops should be avoided at all times.

2.2 QoS in MANETs

According to RFC2386, QoS is a set of service requirements to be met by the network while

transporting a flow. A flow is a packet stream from a source to a destination (unicast or

multicast) with an associated (QoS). The associated QoS could, in fact, be best effort. A

fundamental requirement of any QoS mechanism is a measurable performance metric. Typical

QoS metrics include available bandwidth, packet loss rate, estimated delay, packet jitter, hop

count and path reliability.

Analogous to todays Internet, ad hoc networks are being designed to provide best-effortservice(i.e. do not provide any guarantees regarding packet loss or delay, available bandwidth, jitter

etc.). In a best-effort service model, packets are dropped regardless of their importance. If a

packet is lost, the sender can simply retransmit the lost packet. This method is efficient for

applications that do not require bounds on packet delay or other QoS metrics. However, real-time

applications, such as video-on-demand (VoD), videoconferencing and Internet telephony have,


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are sensitive to packet loss and delay and may have minimum bandwidth requirements

Consequently, the best-effort service may not be suitable for these applications.

Technically, there are two ways in which QoS can be achieved: (1) over-provisioning and (2)

traffic engineering. Over provisioning utilizes the best-effort approach and simply increases the

available resources (e.g. bandwidth, buffers etc.). For example, network designers could simply

increase the capacity of a congestion link or network from 10 to 100 Mb. The second approach,

traffic engineering, tries to utilize resources efficiently and to make the network QoS-aware. This

could include additional service classes, admission control, resource reservations and so on. In

this paper, we focus on the traffic-engineering approach.

Research and development efforts are under way to enhance the Internet with QoS components

that will allow the transport of real-time data (e.g. digitized audio and video). However, these

enhancements may not be suitable for ad hoc networks. For example, current QoS routing

algorithms require accurate link state (e.g. available bandwidth, packet loss rate, estimated delay

etc.) and topology information. The time varying capacity of wireless links, limited resources

and node mobility make maintaining accurate routing information very difficult if not impossible

in an ad hoc networking environment. Thus, while providing QoS support in addition to

flexibility and mobility is a tremendously challenging task for the Internet as well as cellular

networks, in which the mobile node is only a single hop from a wired fixed infrastructure,

supporting QoS in ad hoc networks is an even more difficult challenge.

2.3 AD HOC NETWORKS APPLICATIONS

In this section, we present some applications of ad hoc networks [25]:

Conferencing:Mobile conferencing is without a doubt one of the most recognized applications.

Establishment of an ad hoc network is essential for mobile users where they need to collaborate

in a project outside the typical office environment.

Emergency Services: Responding to emergency situations such as disaster recovery is yet

another naturally fitting application in the ad hoc networking domain. During the time of

emergencies, several mobile users (policeman, firefighters, first response personnel) with


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different types of wireless devices need to not only communicate but also maintain the

connectivity for long periods of time.

Home Networking. The wireless computers at home can also create an ad hoc network where

each node can communicate with the others without taking their original point of attachment into

consideration. This approach is alternative to assigning multiple IP addresses to each wireless

device in order to be identified.

Embedded Computing Applications. Several ubiquitous computing [26] internetworking

machines offer flexible and efficient ways of establishing communication methods with the help

of ad hoc networking. Many of the mobile devices already have add-on inexpensive wireless

components, such as PDAs with wireless ports and Bluetooth radio devices.

Sensor Dust.This application can be considered a combination of ad hoc and sensor networks.

In hazardous or dangerous situations, it makes sense to distribute a group of sensors with

wireless transceivers to obtain critical information about the unknown site by the creation of ad

hoc networks of these sensors.

2.4 AD HOC NETWORKS ROUTING PROTOCOLS

Routing in wireless ad hoc networks is clearly different from routing found in traditional

infrastructure networks. Routing in ad hoc networks needs to take into account many factors

including topology, selection of routing path and routing overhead, and it must find a path

quickly and efficiently. Ad hoc networks generally have lower available resources compared

with infrastructure networks and hence there is a need for optimal routing. Also, the highly

dynamic nature of these networks means that routing protocols have to be specifically designed

for them, thus motivating the study of protocols that aim at achieving routing stability.

Figure 2.1 shows a simple example of the need for dynamic routing when a wireless topology

change occurs. In this case, routing provides the functionality to forward traffic from node A to

node F. As dynamics cause the achievable network topology to change (e.g., node movements,

wireless link failures), valid routes must be discovered and maintained in order to forward

network data to the desired destination, node F in this example. This capability is no different


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from the general goal of IP layer routing, but the underlying design assumption of wireless

interfaces and possibly mobile routing nodes presents increased technical challenges.

Figure 2.1 Dynamic routing in a changing topology

The traditional link-state and distance-vector algorithm do not scale in large MANETs. This is

because periodic or frequent route updates in large networks may consume significant part of the

available bandwidth, increase channel contention and may require each node to frequently

recharge their power supply. To overcome the problems associated with the link-state and

distance-vector algorithms a number of routing protocols have been proposed for MANETs.

These protocols can be classified into three different groups: global/proactive, on-

demand/reactive and hybrid. In proactive routing protocols, the routes to all the destination (or

parts of the network) are determined at the start up, and maintained by using a periodic route

update process. In reactive protocols, routes are determined when they are required by the source

using a route discovery process. Hybrid routing protocols combine the basic properties of the

first two classes of protocols into one. That is, they are both reactive and proactive in nature.

Each group has a number of different routing strategies, which employ a flat or a hierarchicalrouting structure.


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2.4.1 Proactive versus Reactive Approaches

Ad hoc routing protocols may generally be categorized as being either proactive or on-demand

(reactive) according to their routing strategy [50]. Proactive protocols require that nodes in a

wireless ad hoc network should keep track of routes to all possible destinations so that when a

packet needs to be forwarded, the route is already known and can be used immediately. Any

changes in topology are propagated through the network, so that all nodes know of those changes

in topology. Examples include destination-sequenced distance-vector (DSDV) routing [27],

wireless routing protocol (WRP) [28], global state routing (GSR) [29], and fisheye state

routing (FSR) [30].

On-demand protocols only attempt to build routes when desired by the source node so that the

network topology is detected as needed (on-demand). When a node wants to send packets to

some destination but has no routes to the destination, it initiates a route discovery process within

the network. Once a route is established, it is maintained by a route maintenance procedure until

the destination becomes inaccessible or until the route is no longerneeded. Examples include ad

hoc on-demand distance vector routing (AODV) [23], dynamic source routing (DSR) [24],

and Cluster Based Routing protocol (CBRP) [31].

Proactive protocols have the advantage that new communications with arbitrary destinations

experience minimal delay, but suffer the disadvantage of the additional control overhead to

update routing information at all nodes. To cope with this shortcoming, reactive protocols adopt

the inverse approach by finding a route to a destination only when needed. Reactive protocols

often consume much less bandwidth than proactive protocols, but they will typically experience

a long delay for discovering a route to a destination prior to the actual communication. However,

because reactive routing protocols need to broadcast route requests, they may also generate

excessive traffic if route discovery is required frequently.


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2.4.2 Clustering and Hierarchical Routing

Scalability is one of the important problems in ad hoc networking. Scalability in ad hoc networks

can be broadly defined as the networks ability to provide an acceptable level of service to

packets even in the presence of a large number of nodes in the network. In proactive routing

protocols, when the number of nodes in the network increase, the number of topology control

messages increases nonlinearly and they may consume a large portion of the available

bandwidth. In reactive routing protocols, large numbers of route requests to the entire network

may eventually become packet broadcast storms. Typically, when the network size increases

beyond certain thresholds, the computation and storage requirements become infeasible. When

mobility is considered, the frequency of routing information updates may be significantly

increased, thus worsening the scalability issues.

One way to address these problems and to produce scalable and efficient solutions is hierarchical

routing. Wireless hierarchical routing is based on the idea of organizing nodes in groups and then

assigning nodes different functionalities inside and outside a group. Both the routing table size

and update packet size are reduced by including in them only part of the network. For reactive

protocols, limiting the scope of route request broadcasts also helps to enhance efficiency.

The most popular way of building hierarchy is to group nodes geographically close to each other

into clusters. Each cluster has a leading node (cluster head) to communicate with other nodes on

behalf of these clusters. Examples of hierarchical ad hoc routing protocols include zone routing

protocol (ZRP) [32] and zone-based hierarchical link state (ZHLS) routing protocol [33].


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2.5 Review of Ad hoc Proactive Routing Protocols

This section presents brief descriptions for several existing proactive routing protocols.

2.5.1 Dynamic Destination-Sequenced Distance-Vector Routing (DSDV)

The Destination-Sequenced Distance-Vector (DSDV) Routing Algorithm [27] is a proactive hop-

by-hop distance vector routing protocol, which is based on the idea of the classical Bellman-Ford

Routing Algorithm with certain improvements. Every mobile station maintains a routing table

that lists all available destinations, the number of hops to reach the destination and the sequence

number assigned by the destination node. The sequence number is used to distinguish stale

routes from new ones to avoid the formation of loops. The stations periodically transmit their

routing tables to their immediate neighbours. A station also transmits its routing table if asignificant change has occurred in its table from the last update sent. The update is both time-

driven and event-driven.

The routing table updates can be sent in two ways:

A full dump where the full routing table is sent to the neighbours (which could span many

packets);

An incremental update where only those entries from the routing table that have had a metricchange since the last update are sent (and these must fit in a single packet).

If there is space in the incremental update packet, then those entries whose sequence number has

changed may be included. When the network is relatively stable, incremental updates are sent to

avoid extra traffic and full dumps are relatively infrequent. In a fast-changing network,

incremental packets can grow large so full dumps will be more frequent.

Each route update packet, in addition to the routing table information, also contains a unique

sequence number assigned by the transmitter. The route labeled with the highest (i.e. most

recent) sequence number is used. If two routes have the same sequence number then the route

with the best metric (i.e. shortest route) is used. Based on past history, the stations estimate the

settling time of routes. The stations delay the transmission of a routing update by settling time so

as to eliminate those updates that would occur if a better route were found very soon.


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2.5.2 The Wireless Routing Protocol (WRP)

The Wireless Routing Protocol (WRP) [28] is a proactive distance-vector routing protocol. Each

node in the network maintains a distance table, a routingtable, a link-cost table and a message

retransmission list. is a simple path, a loop or invalid. Storing predecessor and successor in the

table enables loops to be detected.

The link-cost table contains the cost of the link to each neighbour of the node and the number

of timeouts since an error-free message was received from that neighbour.

The message retransmission list (MRL) contains information to let a node know which of its

neighbours has not acknowledged its update message and to retransmit update message to that

neighbour.

Nodes periodically exchange routing tables with their neighbours using update messages as well

as on link changes. The nodes present on the response list for the update message (formed using

the MRL) are required to acknowledge the receipt of the update message. If there is no change in

the routing table since last update, the node is required to send an idle Hello message to ensure

connectivity. On receiving an update message, the node modifies its distance table and looks for

better paths using the new information. Information is sent back to the original nodes about any

new paths found so that their tables can be updated. The routing table is also updated if the newpath is better than the existing path.

The distance table of a node x contains the distance of each destination node y via each

neighbour z of x. It also contains the downstream neighbour of z through which this path is

realized.

The routing table of node x contains the distance of each destination node y from node x, the

predecessor and the successor of node x on this path. It also contains a tag to identify if the entry


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2.5.3 Global State Routing (GSR)

Global State Routing (GSR) [29] is similar to DSDV in that it takes the idea of link state routing

but makes an improvement by reducing the flooding of routing messages.

In this algorithm, each node maintains a neighbor list, a topology table, a next hop table and a

distance table.

The neighbour list of a node contains the list of its neighbours (all nodes that can be heard by

it).

The link state information for each destination is maintained in the topology table together with

the timestamp of the information.

The next hop table contains the next hop to which the packets for each destination must be

forwarded.

The distance table contains the shortest distance to each destination node.

The routing messages are generated on a link change as in all link state protocols. When it

receives a routing message, the node updates its topology table if the sequence number of the

message is newer than the sequence number stored in the table and it then reconstructs its routing

table and broadcasts the information to its neighbours.

2.5.4 Fisheye State Routing (FSR)

Fisheye State Routing (FSR) [30] is an improvement of GSR. The large size of update messages

in GSR wastes a considerable amount of network bandwidth, so to reduce this, FSR takes an

approach where each update message does not contain information about all nodes. Instead, it

exchanges information about closer nodes more frequently than it does about farther nodes, thus

reducing the update message size. In this way, each node gets accurate information about near

neighbours and accuracy of information decreases as the distance from the node increases.


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Even though a node does not have accurate information about distant nodes, the packets are

routed correctly because the route information becomes more and more accurate as the packet

moves closer to the destination.

2.5.5 Optimized Link State Routing protocol (OLSR)

The Optimized Link State Routing Protocol (OLSR) is developed for mobile ad hoc networks. It

operates as a table driven and proactive protocol, thus exchanges topology information with

other nodes of the network regularly. The nodes which are selected as a multipoint relay (MPR)

by some neighbor nodes announce this information periodically in their control messages.

Thereby, a node announces to the network, that it has reachability to the nodes which have

selected it as MPR. In route calculation, the MPRs are used to form the route from a given node

to any destination in the network. The protocol uses the MPRs to facilitate efficient flooding of

control messages in the network. OLSR inherits the concept of forwarding and relaying from

HIPERLAN (a MAC layer protocol) which is standardized by ETSI.

Optimized Link State Protocol (OLSR) is a proactive routing protocol, so the routes are always

immediately available when needed. OLSR is an optimization version of a pure link state

protocol. So the topological changes cause the flooding of the topological information to all

available hosts in the network. Optimized Link State Routing (OLSR) protocol, which is natively

based on a simple hop-count metric for the route selection process. Based on such metric, OLSR

exploits Dijkstras algorithm to find optimal paths across the network. Being a proactive

protocol, routes to all destinations within the network are known and maintained before use.

Having the routes available within the standard routing table can be useful for some systems and

network applications as there is no route discovery delay associated with finding a new route.

The routing overhead generated, while generally greater than that of a reactive protocol, does not

increase with the number of routes being created [34],[35].


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2.6 Review of Ad hoc Reactive Routing Protocols

Reactive protocols take a lazy approach to routing. In contrast to proactive routing protocols, all

up-to-date routes are not maintained at every node, but instead the routes are created as and when

required. When a source wants to send to a destination, it invokes the route discovery

mechanisms to find the path to the destination. In this section several typical reactive (on-

demand) routing protocols are introduced.

2.6.1Ad Hoc On-demand Distance Vector Routing (AODV)

Ad hoc on-demand distance vector (AODV) routing [23] adopts both a modified on-demand

broadcast route discovery approach used in DSR [24] and the concept of destination sequence

number adopted from destination-sequenced distance-vector routing (DSDV)[27].

When a source node wants to send a packet to some destination and does not have a valid route

to that destination, it initiates a path discovery process and broadcasts a route request (RREQ)

message to its neighbours. The neighbors in turn forward the request to their neighbour until the

RREQ message reaches the destination or an intermediate node that has an up-to-date route to

the destination. Figure 2.2(a) illustrates the propagation of the broadcast RREQs in an ad hoc

network.

(a)RREQ messages propagation


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In AODV, each node maintains its own sequence number and a broadcast ID. Each RREQ

message contains the sequence numbers of the source and destination nodes and is uniquely

identified by the source nodes address and a broadcast ID. AODV utilizes dest ination sequence

numbers to ensure loop-free routing and use of up-to-date route information. Intermediate nodes

can reply to the RREQ message only if they have a route to the destination whose destination

sequence number is greater or equal to that contained in the RREQ message.

So that a reverse path can be set up, each intermediate node records the address of the neighbor

from which it received the first copy of the RREQ message, and additional copies of the same

RREQ message are discarded. Once the RREQ message reaches the destination (or an

(b)RREP message sent back to sourceFigure 2.2 Route discovery in AODV

intermediate node with a fresh route) the destination (or the intermediate node) responds by

sending a route reply (RREP) packet back to the neighbour from which it first received the

RREQ message. As the RREP message is routed back along the reverse path, nodes along this

path set up forward path entries in their routing tables (Figure 2.2(b)).

When a node detects a link failure or a change in neighbor-hood, a route maintenance procedure

is invoked:

If a source node moves, it can restart the route discovery procedure to find a new route to the

destination. If a node along the route moves so that it is no longer contactable, its upstream

neighbour sends a link failure notification message to each of its active upstream neighbors.

These nodes in turn forward the link failure notification to their upstream neighbours until the

link failure notification reaches the source node.


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2.6.2 Dynamic Source Routing (DSR)

Dynamic source routing (DSR) [24] is an on-demand routing protocol for wireless ad hoc

networks. DSR is based on the concept of source routing, in which a source node indicates the

sequence of intermediate routes in the header of a data packet. Like other on-demand routing

protocols, the operation of DSR can be divided into two procedures: route discovery and route

maintenance.

(a)Building Record Route

(b)RREP Propagation

Figure 2.3 Route discovery in DSR


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Each node in the network keeps a cache of the source routes that it has learned. When a node

needs to send a packet to some destination, it first checks its route cache to determine whether it

already has an up-to-date route to the destination. If no route is found, the node initiates the route

discovery procedure by broadcasting a route request message to neighbouring nodes. This route

request message contains the address of the source and destination nodes, a unique identification

number generated by the source node, and a route record to keep track of the sequence of hops

taken by the route request message as it is propagated through the network. When an

intermediate node receives a route discovery request, it checks whether its own address is

already listed in the route record of the route request message. If not, it appends its address to the

route record and forwards the route request to its neighbours. Figure 2.3(a) illustrates the

formation of the route record as the route request propagates through the network.

When the destination node receives the route request, it appends its address to the route record

and returns it to the source node within a new route reply message. If the destination already has

a route to the source, it can use that route to send the reply; otherwise, it can use the route in the

route request message to send the reply. The first case is for situations where a network might be

using unidirectional links and so it might not be possible to send the reply using the same route

taken by the route request message. If symmetric links are not supported, the destination node

may initiate its own route discovery message to the source node and piggyback the route reply on

the new route request message. Figure 2.3(b) shows the transmission of route record back to the

source node.

Route maintenance uses route error messages and acknowledgement messages. If a node detects

a link failure when forwarding data packets, it creates a route error message and sends it to the

source of the data packets. The route error message contains the address of the node that

generates the error and the next hop that is unreachable. When the source node receives the route

error message, it removes all routes from its route cache that have the address of the node in

error. It may initiate a route discovery for a new route if needed. In addition to route error

message, acknowledgements are used to verify the correct operation of links.

To reduce the route search overhead, an important optimization is allowing an intermediate node

to send a route reply to the source node if it already has an upto-date route to the destination.


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2.7 Review of Ad hoc Hybrid Routing Protocols

The characteristics of proactive and reactive routing protocols can be integrated in various ways

to form hybrid networking protocols. Hybrid networking protocols may exhibit proactive

behavior given a certain set of circumstances, while exhibiting reactive behavior given a

different set of circumstances. These protocols allow for flexibility based on the characteristics

of the network.

2.7.1 Distributed spanning trees based routing protocol (DST)

In DST [36] the nodes in the network are grouped into a number of trees. Each tree has two types

of nodes; route node, and internal node. The root controls the structure of the tree and whether

the tree can merge with another tree, and the rest of the nodes within each tree are the regularnodes.

Each node can be in one three different states; router, merge and configure depending on the type

of task that it trying to perform. To determine a route DST proposes two different routing

strategies; hybrid tree-flooding (HFT) and distributed spanning tree shuttling (DST).

In HTF, control packets are sent to all the neighbours and adjoining bridges in the spanning tree,

where each packet is held for a period of time called holding time. The idea behind the holding

time is that as connectivity increases, and the network becomes more stable, it might be useful to

buffer and route packets when the network connectivity is increased over time. In DST, the

control packets are disseminated from the source are rebroadcasted along the tree edges. When a

control reaches down to a leaf node, it is sent up the tree until it reaches a certain height referred

to as the shuttling level. When the shuttling level is reached, the control packet can be sent down

the tree or to the adjoining bridges. The main disadvantage of the DST algorithm is that it relies

on a root node to configure the tree, which creates a single point of failure. Furthermore, the

holding time used to buffer the packets may introduce extra delays in to the network.


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2.7.2 Zone routing protocol (ZRP)

In ZRP [37], the nodes have a routing zone, which defines a range (in hops) that each node is

required to maintain network connectivity proactively. Therefore, for nodes within the routing

zone, routes are immediately available. For nodes that lie outside the routing zone, routes are

determined on-demand (i.e. reactively), and it can use any on-demand routing protocol to

determine a route to the required destination. The advantage of this protocol is that it has

significantly reduced the amount of communication overhead when compared to pure proactive

protocols. It also has reduced the delays associated with pure reactive protocols such as DSR, by

allowing routes to be discovered faster. This is because, to determine a route to a node outside

the routing zone, the routing only has to travel to a node which lies on the boundaries (edge of

the routing zone) of the required destination. Since the boundary node would proactivelymaintain routes to the destination (i.e. the boundary nodes can complete the route from the

source to the destination by sending a reply back to the source with the required routing address).

The disadvantage of ZRP is that for large values of routing zone the protocol can behave like a

pure proactive protocol, while for small values it behaves like a reactive protocol.

2.7.3 Zone-based hierarchical link state (ZHLS)

Unlike ZRP, ZHLS [38] routing protocol employs hierarchical structure. In ZHLS, the network

is divided into non-overlapping zones, and each node has a node ID and a zone ID, which is

calculated using a GPS. The hierarchical topology is made up of two levels: node level topology

and zone level topology, as described previously. In ZHLS location management has been

simplified.

This is because no cluster-head or location manager is used to coordinate the data transmission.

This means there is no processing overhead associated with cluster-head or Location Manager

selection when compared to HSR, MMWN and CGSRprotocol s. This also means that a single

point of failure and traffic bottlenecks can be avoided. Another advantage of ZHLS is that it has

reduced the communication overheads when compared to pure reactive protocols such as DSR

and AODV.


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In ZHLS, when a route to a remote destination is required (i.e. the destination is in another zone),

the source node broadcast a zonelevel location request to all other zones, which generates

significantly lower overhead when compared to the flooding approach in reactive protocols.

Another advantage of ZHLS is that the routing path is adaptable to the changing topology since

only the node ID and the zone ID of the destination is required for routing. This means that no

further location search is required as long as the destination does not migrate to another zone.

However, in reactive protocols any intermediate link breakage would invalidate the route and

may initiate another route discovery procedure. The Disadvantage of ZHLS is that all nodes must

have a preprogrammed static zone map in order to function. This may not feasible in applications

where the geographical boundary of the network is dynamic. Nevertheless, it is highly adaptable

to dynamic topologies and it generates far less overhead than pure reactive protocols, which

means that it may scale well to large networks.

2.8 Ad hoc delay aware routing protocol

In recent years, MANET has emerged as one of the high growth applications of the wireless

communication technology. However, it is difficult to provide an end-to-end quality of service

(QoS) guarantee because of the error prone wireless channel, the changing topology, and the

energy constrained etc. Routing protocol design is one of the major challenges for wireless Ad

Hoc network research and implementation. Most existing ad hoc routing protocols do not take

the traffic load as a factor when making routing decision. The result is that the heavily loaded

intermediate nodes can be included in the route during the route request time and will be chosen

to relay the packet during the packet forwarding time. Longer transmission delay and higher

packet loss may happen due to the overloaded nodes along the route. These problems demand the

discovery of delay aware routing protocols alarmingly because implementation of a proper

routing methodology in ad hoc networks makes it efficient in terms of performance.


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2.8.1 Delay-aware multipath source routing protocol (DMSR)

DMSR is proposed by extending existing DSR to send packets simultaneously over multiple

paths. It also provide QoS support for realtime multimedia applications in wireless ad hoc

networks. Based on local information, node delay is calculated as metric for route path selection

in the DMSR protocol. The metric takes into account the number of the neighbor nodes of the

forwarding nodes, the channel busy time and the number of packets in the send buffer.

Simulation analyses and results show that the DMSR protocol can reduce the average end-to-end

delay and meet the demands of real time multimedia services.

The protocol includes two parts:

1) The accumulation delay is considered as the admission metric to choose the paths.

2) Node delay is considered as the metric to measure the end-to-end delay and determine the best

routing path.

DMSR adds maximum delay demand of current services and accumulation delay in route request

(RREQ). The maximum delay field remains unchanged and the accumulation delay field is

constantly increasing in route discovery process. DMSR takes the accumulation delay as the

metric of admission control. Then the intermediate nodes determine whether RREQ should be

transmitted according to the accumulation delay. The routing cache response function is disabledbecause the routing caches of intermediate nodes do not have enough information about end-to-

end delay. Protocol uses source routing in which only source nodes should have the information

about the paths to be followed. The packets are sent along the best shortest paths where the

subsequent nodes donot need anything special. It simply forwards the packets to next node

according to the source route in packet header. DMSR also considers the node delay factor in the

route discovery process. average end-to-end delay increases as the node load increases. When

the node load is light, the delay of I DSR is larger than original DSR protocol. This is because in

i-DSR protocol, the routing cache response function is disabled and the detecting of delay is a

waste of system resources when the network is idle. DMSR outperforms DSR and i-DSR in the

average end-to-end delay. The reason is the ability of the new metric to decrease the time waiting

in the send buffer and contending the channel. The multiple path approach can minimize the

route maintenance time and the frequency of route discovery.[39,40,41,42] .


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2.8.2 Power and Delay aware Temporally Ordered Routing protocol

Power and Delay aware Temporally Ordered Routing Algorithm (PDTORA), based on

Temporally Ordered Routing Algorithm (TORA) Protocol, where verification of power and

delay requirements is carried out with a query packet at each node along the path between source

and destination. Implementation of QoS routing protocols in ad hoc networks serves to fulfill the

purpose of reservation of sufficient resources along a route so as to meet the QoS requirements

of a flow. On the other hand, the QoS routing protocol should be able to find the path that

consumes minimum resources. QoS metrics vary from application to application. Major QoS

metrics for ad hoc networks are available bandwidth, cost, end-to-end delay, power, packet loss

ratio and so on.

The QoS metrics can be generally classified as, additive metrics, concave metrics and

multiplicative metrics. As a source-initiated on-demand routing protocol, TORA relies on a link

reversal algorithm and provides loop-free multipath routes to a specified destination. In this

approach, a node maintains the topology information involving its one-hop neighbors. During a

reconfiguration process following a path break, TORA has the unique property to limit the

control packets to a small region.

Three major functions performed by TORA are: establishing, maintaining and erasing routes.

Route establishment function is initiated, when a source node requires a path to a specific

destination, to which it does not possess a directed link. During this process, a destination

oriented Directed Acyclic Graph (DAG) is established using a query / update mechanism.

The extension of TORA protocol is the power and delay aware modification (PDTORA) where

the nodes in the network which do not satisfy to the QoS requirements of maximum delay and

minimum power levels, are eliminated from the route of communication, during query phase.

Each intermediate node on receipt of the query packet determines whether to forward it or not,

depending on the QoS requirements. At the destination, an update packet is generated.[43,44].


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2.8.3 Delay Aware AODV-Multi-path routing protocol

In Delay Aware AODV-Multi-path (DAAM). Multiple node disjoint paths are set up during a

single route discovery, and the delay of each route is recorded during the route discovery.

Whenever a data packet arrives at the routing layer from the application layer, the type of data

contained in the packet is classified according to the ToS field in the IP header and a delay

request is assigned accordingly. This is then used to determine if a new discovered route is

suitable for a certain traffic type.

DAAM, shows remarkable improvements of end-to-end packet delay and delay variation of

voice and video traffic, routing overhead, and packet delivery fraction of video traffic compared

to AODV, DSR, OLSR End-to-end delay was the main design criteria and DAAM not only

shows the lowest end-toend delay in all scenarios for both traffic types, but also all end-to-end

delay is lower than the ITU-Ts maximum recommendation of 400 ms. Although the P.D.F.s for

voice traffic are in some cases lower for DAAM than that of OLSR, the delay is improved to an

acceptable level and delivered packets are thus expected to be usable.

Routing overhead of DAAM is the lowest due to efficient route discovery and management.

Once routes are established, route rediscovery is only necessary if all known routes to a

destination fail.

A potential drawback of the current design is that route delay information might not always be

up to date. Link delay however, is continually monitored and evaluated, as mentioned. This

functionality is not optimal, but quite efficient for its simplicity.

A more optimal solution would be to notify all sources that use a link in an active route of the

exact change in delay, if the links delay has changed. Each source can then update the delays of

all routes that use the link.[45,46].


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2.8.4 Link Delay aware Routing protocol

The rationale behind the LDAR model is based on the observation that the link delay

experienced by a packet comprises of three types of delays, namely, processing delay, queueing

delay and transmission delay. In a MANET, the processing is typically the delay incurred due to

the processing of the packet in the node. The queuing delay is the time spent by the packet in the

interface queue. The transmission delay of the packet depends on the behaviour of the 802.11

MAC protocol.

The 802.11 MAC layer in a MANET node provides the CSMA/CA media access mechanism. In

order to estimate the total delay for the next packet, we modified the Linux kernel and Madwifi

driver for Atheros-based wireless interface to compute the processing, queueing and transmission

delays. The time taken by the network and the data link layers of the TCP/IP stack at the sender

node to process the packet were taken as the processing delay of the packet. This is measured by

keeping track of timestamp of the packet when it enters the network layer and before it enters the

MAC interface queue.

The queuing delay is the time spent by the packet in the 802.11 MAC interface queue. The

estimated delay values are sent up to LDAR for dispersion throughout the network. LDAR uses a

modified Optimized Link State Routing (OLSR) protocol with ETX extension5. In the standard

OLSR with ETX extension, each node periodically broadcasts HELLO messages that are used to

maintain information about the 2-hop neighborhood around each node. The HELLO packets are

used as probe packets for the ETX computation. The 2-hop neighborhood information is used in

determining the multipoint relay (MPR) set of each node.

The Topology Control (TC) messages containing ETX information about the links around each

node are efficiently dispersed in the network through the MPRs. Upon receipt of the TC

message, each node computes in a centralized fashion the shortest paths to all other nodes in the

network and updates its routing tables. In the LDAR design, the LDAR module periodically

requests the IOCTL extension module for the most recent link delay estimates and disperses the

link delay estimates to the rest of the network using the modified TC message. In addition, the

shortest path routing in LDAR uses link delay estimates as the link metric to compute routes to

other nodes in the network. [47]


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