Communicating Embedded Systems (Krief/Communicating Embedded Systems) || Quality-of-Service Routing...

Chapter 2

Quality-of-Service Routing in Mobile Ad Hoc Networks

2.1. Introduction

Ad hoc networks were introduced to provide connectivity between nodes in environments where it would be impossible to install an infrastructure (due to prohibitive costs or time constraints), or where existing facilities have been destroyed or taken out of service following an accident or earthquake, for example. The applications that can benefit from the capabilities offered by ad hoc networks are numerous, the most widespread being military applications, emergency and rescue operations, personal exchanges, and leisure uses. The strategic importance of these networks has led to a large number of research ventures in the field in the last 10 years.

The characteristics of ad hoc networks (particularly node mobility, the weakness of radio signals, collisions between nodes, and battery lifetimes), and the need to fulfill quality of service (QoS) requirements, make it very difficult to design and deploy these networks. In particular, the deployment of real-time multimedia applications (demanding in terms of time constraints, bandwidth, and

Chapter written by Zoubir MAMMERI.

30 Communicating Embedded Systems

packet loss rate) on ad hoc networks creates many challenges. Among the numerous problems encountered, we are interested in that of QoS routing. That is to say, protocols that allow us to select and maintain routes between sources and destinations for data exchange, while providing QoS guarantees (essentially in terms of delay and bandwidth).

The problem of QoS routing with two or more QoS metrics is known to be NP-complete even in wired networks [WAN 96]. The characteristics of wireless networks in general and of ad hoc networks in particular, make this problem even more complicated. Providing and maintaining QoS in ad hoc networks is difficult due to frequent changes in topology caused by the nodes’ movements and degradation in the quality of wireless communications (due to either the signal carrying properties of air, or competition between neighboring nodes transmitting at the same frequencies). Maintaining the link-state information necessary for route selection becomes almost impossible below a certain node speed, since as soon as a route is found, it is immediately lost. As a result of these factors, QoS has become an important field in the last 10 years.

The first works on network routing focused on the mechanisms that need to be implemented in order to allow nodes to communicate without infrastructure. Various forms of best effort routing protocols were proposed [HAA 97, IWA 99, JAC 97, JOH 94, KO 98, PER 94, PER 99]. Eventually, work became oriented towards the extension to best effort routing to fulfill QoS and security requirements.

Communication under QoS constraints has been the subject of much scrutiny during the 1980s and 90s, in particular concerning local industrial networks, field buses, and wired networks (such as asynchronous transfer mode (ATM)). These projects were based on wired networks, which have certain properties that facilitate routing. In particular, physical links between devices are stable, link capacities are known, data sources are known in advance, and even the characteristics of the data traffic can be known. Such information is not available in ad hoc networks. The consequence is that routing in mobile ad hoc networks is more complicated than in wired networks. The physical links are very temporary in ad hoc networks due to node

QoS Routing in Mobile Ad Hoc Networks 31

movements and the weak capacity of the signals. In ad hoc networks, the path between two nodes changes frequently. Ad hoc networks are created on the fly and the number of nodes is not known in advance. Depending on a node’s movement, it is constantly discovering and losing neighbors. The number of sources is almost always unknown in ad hoc networks.

Routing protocols are characterized by the QoS metrics taken into account to select paths, the mechanisms for propagating and maintaining link-state information, the routing and path selection strategies, whether or not multiple paths are used towards the same destination. Among the known QoS metrics (bandwidth, delay, jitter, loss rate, reliability, and availability), QoS routing essentially takes bandwidth and delay into consideration. The aim of this chapter is to present the problem of QoS routing and the main approaches towards design of routing protocols that have been proposed for mobile ad hoc networks. Although security is a crucial issue in mobile ad hoc networks, it is not developed in this chapter due to lack of space, and instead we concentrate on bandwidth and delay considerations.

The chapter is structured as follows: in section 2.2, we present some characteristics of ad hoc networks, their limits, and the challenges posed by them. Section 2.3 presents the problem of QoS routing and expected properties of the protocols within a general framework before considering this issue for the special case of ad hoc networks. Section 2.4 presents the general principles behind solutions to the problem of “best effort” routing. Section 2.5 presents the main solutions to the routing problem considering delay and bandwidth requirements. Examples of approaches to estimating bandwidth and delay and approaches to resource reservation are presented.

2.2. Mobile ad hoc networks: concepts, characteristics, challenges

2.2.1. Concepts and basic principles

A Mobile Ad hoc NETwork, or MANET, consists of a set of nodes (personal digital assistants (PDAs), laptops, embedded electronics in cars, robots, etc.) which communicate among themselves by radio


links. The nodes move with speeds that depend on the context of the application. We speak of ad hoc networks or networks without infrastructure, as there are no special devices that control or administer access to the communication channel, in contrast to cellular networks where communications pass along equipment installed in advance (i.e. base stations). Each node behaves simultaneously as a source and a destination for data and as a router.

Each node discovers its neighbors when they transmit frames (notably special frames called beacons), which indicate their presence. In certain cases, nodes can be supplied with a mechanism (such as a global positioning system (GPS)) which allows them to find out their geographical position.

When two nodes sense each other’s presence, they update their routing table to indicate that a physical link is available between them. When a node no longer receives signals from one of its neighbors, it considers the link that existed between them to be broken. When a node no longer receives signals from any of its neighbors, it concludes that the ad hoc network no longer exists. If it moves, it continues to listen for any signals that it may receive in order to participate in the creation or extension of a network. Each node detects its neighbors (which change in time) according to the signals that it receives. We say that ad hoc networks are self-creating, self-organizing, and self-administrative.

In contrast with wired networks where the topology is fixed, the topology of an ad hoc network changes whenever nodes move. The addition or removal of nodes is transparent, without intervention of a dedicated “supervisor” or “controller” node.

It is evident that path searching becomes difficult or impossible if the dynamics of the network are such that the propagation of link-state information signaling a change in topology takes longer than the time between two changes in topology. The works on routing assume that the changes in topology respect the principle of combinatorial stability [CHA 01]: the changes are slow enough to allow link-state information to propagate.


2.2.2. Limits and challenges

Ad hoc networks provide flexible means of communication that can be rapidly deployed without a prearranged infrastructure. These networks are characterized by several limits that have a considerable impact on the design and management of this class of networks:

– node mobility: nodes can move, leading to frequent changes in the topology of the network. Consequently, it is difficult or impossible to have a global view of the network topology that would be shared by all nodes and which reflects the current state of the network;

– limited bandwidth: when nodes transmit within the same frequency band, they generate collisions. This influences the bandwidth used for data transmission;

– fluctuations in link capacity: in addition to the problem of collisions, in a wireless environment the quality of signals fluctuates due to many factors (distance between communicating nodes, presence of obstacles that stop or reflect signals, etc.);

– unforeseeable delays in transfer: one of the consequences of the fluctuation in the connection capacity is that it is difficult or impossible to determine an accurate transfer delay;

– varying collision rate and link capacity according to node position: as the nodes move, we can find more nodes in some zones than in others. In low-density zones, the nodes do not disturb each other too much (there are few collisions). In high-density zones, the nodes disturb each other frequently and the collisions greatly reduce the available bandwidth;

– increased error rates: the transmission error rate in wireless networks in general and in ad hoc networks in particular, is high. Retransmitting, if this technique is used, only serves to exacerbate the situation in the case of a loaded network;

– synchronization difficulties: when nodes appear and disappear in the network independently of each other and no preliminary infrastructure is required, it is difficult to coordinate decisions, in particular concerning the allocation of bandwidth;


– limited batteries in certain cases: the battery can be an important criterion to be considered in order to enable each node or group of nodes to remain active for as long as possible;

– other limits: storage or processing capacity may also need to be considered.

In order to take the above limitations into account, various challenges should be considered. In particular, the following issues have been investigated:

– access methods (medium access control) adapted for ad hoc networks;

– routing: elaborating efficient routing strategies while taking account of the characteristics of ad hoc networks and applications (this is the only issue which will be developed in this chapter);

– precise and low-cost node location techniques;

– mobility management and node position prediction;

– transport protocols adapted for ad hoc networks;

– resource allocation techniques in highly mobile environments without centralized control;

– providing QoS guarantees;

– middleware to facilitate/automate the configuration of nodes in order to adapt to their environment;

– cross-layer design approaches so that the network layer can listen to current transmission conditions at the MAC (media access control) sub-layer and the physical layer;

– network self-organization methods despite unpredictable node entries and exits;

– optimization of power consumption;

– interconnection with other networks, such as cellular networks;

– discovery of services according to node positions;


– security of applications and routing in environments where entry into the network is difficult to control;

– effective models for analyzing the performance of ad hoc networks;

– improving antenna quality, reducing the weight or size of embedded equipment.

2.2.3. MAC protocols for ad hoc networks

There are two main types of protocol for managing access to the transmission channel (the air) in MANETs: contention-based methods and ordered access methods.

With contention-based methods, the nodes compete to transmit their packets. Since their emergence, ad hoc networks have essentially used the MAC IEEE 802.11 protocol in an ad hoc mode, because it is the most widely available technology. These methods lead to degradations in performance under high loads, as the number of collisions increases with the number of packets to send. As we will see later (section 2.5.3.2) we can, against some difficulties, make reservations even with a contention-based method.

Ordered access methods are generally based on Time Division Multiplexing Access (TDMA). Each node is assigned a set of time slots in order to transmit its data without causing a collision. The main difficulties in using these methods come from the need for reliable synchronization between the nodes (synchronization of clocks and of slot reservation records). TDMA-based routing protocols are essentially intended for networks with few nodes and few displacements. With TDMA, all nodes transmit within the same frequency band. Consequently, a node cannot transmit if one of its neighbors is transmitting or receiving from its other neighbors. In order to increase the bitrate offered to the nodes, the CDMA-over-TDMA method can be used. This consists of dividing time into slots (the TDMA principle) and assigning a spreading code to each node which is different from that of its immediate neighbors, and eventually also of its two-hops neighbors, in order to allow for the problem of


hidden stations. By transmitting in the same slot but with different codes, neighboring nodes avoid disturbing each other. This is the Code Division Multiple Access (CDMA) principle.

2.2.4. Node mobility and location

As previously mentioned, the topology of an ad hoc network changes according to the movements of the nodes. In order to efficiently select and maintain paths between source and destination nodes, the routing function should have recent information about the mobility parameters and the node positions, but this knowledge comes at a cost. The more precisely the node positions are to be known, the more control packet exchanges are needed in order to obtain this information.

The manner in which the topology of an ad hoc network changes can affect the performance of the routing algorithms. It is necessary to properly master the node mobility model in order to properly choose a routing algorithm [BAI 03, CAM 02, WIL 01]. Node position information is obtained by use of a node location service or a mobility model.

Location services are essentially used to locate nodes while the network is in use (i.e. in operational mode). Mobility models are essentially used to determine node positions during the simulation or network performance analysis step. They can be used in operational mode to estimate node positions without the use of a location service (the location service has a cost, which sometimes needs to be reduced).

2.2.4.1. Locating mobile nodes Generally, the location of people, vehicles, targets, etc., is required

for conducting operations at the right place and time (allowing firefighters to act quickly in the case of an accident or fire, for example).


More specifically, when we are interested in routing, knowledge of the node locations is useful or necessary for the selection of the most appropriate paths to fulfill QoS requirements.

Recent years have seen the emergence and development of various location techniques and technologies fulfilling different requirements in terms of location precision, energy consumption, security, availability, and cost [HIG 01]. Today GPS is the most popular location system.

2.2.4.2. Mobility models When we wish to analyze the performance of a routing algorithm,

some assumptions are made with regards to node mobility. We can either use the tracks of node movements as they are observed, or use synthetic mobility models that mathematically specify the rules of node mobility.

Mobility models use parameters linked simultaneously to the nodes themselves (i.e. the initial position, speed and acceleration, changes in direction, duration of a movement, frequency of changes in movement and time-span between two consecutive movements), but also to the environment in which they are moving (spatial dimensions, for example, a square with sides of 1,000 m in length, the signal transmission range, obstacles in the paths).

For about 10 years, many mobility models have been proposed to take into consideration the diverse specifications of node mobility in ad hoc networks. These models can be grouped into two classes: mobility models for individual nodes and models for groups of nodes. As far as we know, the most complete survey on mobility models is that of Camp et al. [CAM 02].

2.3. QoS routing: general considerations

QoS routing is a special case of conventional routing, which takes into account the QoS requirements of flows depending on the availability of resources [CHE 98, PAU 02]. For example, choosing a route guaranteeing a minimum bit rate of 500 kb/s and end-to-end


delay below 100 ms. In addition to satisfying QoS constraints, routing algorithms also have the objective of optimizing network resources.

Works on QoS routing started in the 1980s, and generally concerned algorithms for guaranteeing QoS in ATM networks, particularly for CBR (constant bit rate) and rt-VBR (real-time variable bit rate) traffic. In the 1990s, QoS routing was included in the Inserv, DiffServ and MPLS (multiprotocol label switching) architectures, allowing support of multimedia and real-time applications in internet networks [CRA 98]. QoS routing is also at the heart of wireless networks, such as: UMTS, IEEE 802.11 networks, ad hoc networks, and sensor networks.

2.3.1. Functions of routing protocols

The functions realized by routing protocols can be grouped into three groups: collection and dissemination of link-state information, path selection, and path maintenance.

2.3.1.1. Collecting and disseminating link-state information Each node has local information that indicates particularly: the

state of the packet queues, the available bandwidth (as seen or estimated by the node), the waiting time in the queues, the level of processor use, etc. According to the strategy for disseminating link-state information, periodically or on-demand, each node broadcasts its local information to its neighbors, to all nodes or to certain nodes (for example, the nodes located k hops away or the nodes located in a given geographical zone). This dissemination of link-state information allows all nodes to have a view of the global state of the network. As exchanges of link-state information cannot be instantaneous, the global state is an approximate state. In particular, in the context of ad hoc networks, the global state is not accurate due to node mobility.

Minimizing the cost of the link-state information dissemination function is of paramount importance in the design of routing protocols, particularly those intended for large-scale networks. Indeed, this function should meet two objectives: 1) minimizing the overhead in terms of bandwidth used for the dissemination of link-state


information and processing time, and 2) providing a view of the state of the network that is as accurate as possible at each instant. These two objectives are contradictory, as the more one refreshes the link-state information (to have an up-to-date overview) the more resources are consumed.

Regarding the dissemination of information, the protocols are also classified into distance vector protocols and link-state protocols.

2.3.1.2. Path selection The selection of paths is based on link-state information available

at each node. The selection can be done by the source which selects the path then communicates it to the intermediate nodes, or hop-by-hop (i.e. each node decides on its own which node to choose for the next hop in the path).

We can select a single path between each couple <source, destination> or also choose several redundant paths. We can also only select paths fulfilling performance requirements (particularly providing QoS guarantees).

From an algorithmic point of view, this function is the most complex and the most attractive for researchers. Indeed, it has been shown that QoS-aware path-searching problem is NP-complete as soon as we consider two or more types of non-correlated QoS constraints (for example, delay and bandwidth, delay and availability, etc.) [WAN 96]. In general, we use heuristics to determine the paths that meet several QoS criteria.

2.3.1.3. Route maintenance and repair In a network the topology changes as a function of the node

dynamics (nodes booting up, shutting down, moving, and breaking down) and of the physical links that connect them (breakage in communication links, installation of new lines, etc.). In particular, in the case of ad hoc networks, while a source is transmitting its data packets, the intermediate nodes, and also the source and destination, may move and so the path that connects the source and the destination may change several times during data transfer.


The objective of routing protocols is to find paths and maintain connectivity between nodes despite changes in the topology. Two complementary functions fulfill this objective: maintenance and repair of paths. There are many strategies for implementing these functions according to whether the protocol is reactive or proactive, with or without resource reservation, etc.

The simplest form of path maintenance consists of sending refresh packets to signal to all intermediate nodes that the path is still being used by the source. If after a fixed time no refresh packet has been received by a given node, or no data packet has been relayed by the same node for this particular path, the node concludes that the path is broken or that the source has stopped participating. Following this detection, the node deletes the entry associated with the broken/stopped path from its routing table and releases the resources that were allocated to this path.

In many routing protocols, path repairs are undertaken in two ways: initiated by the source (when a path is broken, the source is informed so that it can put in a path discovery request) or with the participation of all the intermediate nodes (each intermediate node that detects a lost link attempts to repair the paths concerned).

In certain (rare) routing protocols, at the path discovery stage for the path from a source to a destination, several (redundant) paths are selected and stored in the routing tables. If the path in use is lost, an alternative path is chosen (without having to initiate the route discovery process). If all paths are lost, the path discovery procedure is rerun.

2.3.2. Classification of routing protocols

There are numerous routing protocols used in networks in general. One way to understand and master them (considering analysis or implementation) is to categorize them, in particular from the following perspectives:

– the nodes where decisions are made (source routing, hop-by-hop routing or hierarchical routing);


– the number of data recipients (unicast, multicast or anycast routing);

– path redundancy (with or without redundancy);

– administrative management (intra and interdomain routing).

2.3.2.1. Source routing, hop-by-hop routing, or hierarchical routing Routing protocols are often grouped into three classes: by source,

distributed (or hop-by-hop), and hierarchical.

In source routing, each node has a global view of the network state and selects the path to be used according to this view and to the destination of the packets. Once the path has been selected, the node signals to the others that a path has been selected. In general, this type of routing is not adapted to ad hoc networks. In effect, the use of this type of routing means that a data source imposes the path followed for all the connecting nodes, which contradicts the principle of node autonomy in ad hoc networks.

In hop-by-hop routing, each node knows only the next node (next hop) in order to reach the destination, and chooses the next hop towards different destinations according to its view of the state of the network. The drawback is that the (independent) nodes can have inconsistent views, which can lead to loops. It is worth noting that hop-by-hop routing is the most widely used in networks in general and in ad hoc networks in particular.

Hierarchical routing is especially useful for large-scale networks. The network is structured on several hierarchical levels, the lowest levels (leaves) correspond to physical nodes. These nodes are organized into groups, which form the second hierarchical level, and following this we build groups of groups, etc. Each node representing a group knows the information relating to its group and sends it out to the leaders of the other groups. At each hierarchical level, we can have an adapted routing algorithm. The main advantage of this type of routing is its scaleability. Its drawback is that the link-state information is aggregated, which can lead to a loss of precision on the real state of the physical nodes. This type of routing is used in large-scale ad hoc networks.


2.3.2.2. Unicast, multicast and anycast routing In unicast routing, a single transmitter and a single receiver are

concerned with the data. This is the most widely implemented type of routing, for which we know how to guarantee QoS in the majority of cases. Only this type of routing will be developed further in this chapter.

Applications, such as videoconferencing, shared workspace, and distributed interactive simulation, require multicast routing, as there are almost always many receivers for the same flow. The receivers of a flow form a group. Multicast routing consists in finding trees for which the root is the source and the leaves are the members of the multicast group so as to optimize the use of resources and fulfill the QoS constraints in order to serve all the receivers in the group.

Anycast routing is used in environments such as mirror websites. It consists of selecting a path to one receiver among a set of receivers (e.g. searching one website among n mirror sites). This type of routing has so far been investigated least in the literature.

2.3.2.3. Redundant paths and disjoint paths In some applications (particularly the deployment of rescue teams),

the availability of communication facilities is the most important issue rather than abundant bandwidth or low delays. In other words, the crucial issue is connectivity.

For fault tolerance (and, therefore, also availability of communication paths), certain protocols do not select a single path, but several: a main path and one or several alternative paths. We run the path search procedure once, producing several paths, and as long as there remains at least one operational path the procedure is not rerun: we change the path only in the case of a broken connection.

2.3.2.4. Intradomain and interdomain routing The path used by a flow can cross a single domain (a business

network, for example) managed by a single authority or cross several domains managed by independent authorities.


Intradomain routing is concerned with finding paths within one domain depending on policies or rules local to this domain.

Regarding interdomain routing, this has the objective of finding paths in several domains while taking account of the management policies or rules of these domains, which are not the same (and which can even be conflicting for security reasons, for example). Interdomain routing was not developed within the ad hoc networks framework, as it is assumed that there are no domains to administer. Each node is assumed to be autonomous and, therefore, independent of the others in order to form a domain with them.

2.3.3. Expected routing protocol properties

Various properties may be expected from routing protocols in order to be applicable to mobile ad hoc networks [COR 99]:

– low delay in path discovery procedure;

– minimum number of hops in order to form paths;

– (statistical) guarantees of QoS metrics;

– high network throughput;

– rapid reconfiguration in case of broken paths;

– weak (even restricted) loss rate due to incorrect routing;

– high network utilization rate and fair link utilization;

– maximum connectivity rate (a node should have access to paths in order to reach all desirable destinations);

– minimum lifespan, during which the batteries of certain nodes or of all nodes do not run out;

– minimum overhead (in terms of control packets, central memory, and central processing unit (CPU)).

In addition to the above properties, which are quantifiable, other properties may also be considered:


– distribution: decisions should be taken in a distributed manner, as the centralized solutions have often not been adapted in ad hoc networks;

– data security and routing function security;

– adaptation to imprecision of routing information: in ad hoc networks the flows are often unpredictable. Certain nodes can randomly connect, disconnect, and move. As a consequence, the protocol must be capable of evaluating the precision (and credibility) of the metric information that it uses;

– scaleability: networks include more and more nodes, and therefore, it is important that the routing protocol used is capable of scaling without affecting either the QoS delivered or the global performance of the network.

The criticality of each of the properties mentioned depends on the context of use. Certain properties contradict others; this is particularly the case for the number of hops and the level of node use. Anyone who wants to choose a routing protocol will have to make a trade-off between certain properties.

2.3.4. QoS routing problems

In order to solve a routing problem, the network considered is modeled by a directed graph G(V, E) where V is the set of vertices and E is the set of edges. Each physical node is modeled by a vertex. Each wired or radio link between two neighboring nodes, u and v, is represented by an edge (u, v). Each edge is assigned a weight, w(u, v), expressed with one or several QoS metrics. Figure 2.1 shows an example of a network with nine nodes, where the metrics are jitter, bandwidth, and error rate.

The topology of the graph changes dynamically as a function of the arrivals of new nodes into the network, node or link breakdowns, node mobility, etc. The weights of the edges also change as a function of the resource usage level at each link. The graph is constructed (from the link-state information) each time that a node wishes to select a complete or partial path.


The QoS routing algorithms solve the Multi-Constraint Path (MCP) problem which can be expressed as follows [KUI 02, MAS 06].

3

(1 ms, 100 kb/s,

10 %)

(3 ms, 500 kb/s,

1 %)

(2 ms, 100 kb/s,

5 %)

(4 ms, 1 M b/s, 2 %)

(2 ms, 1 Mb/s, 10 %)

(25 ms, 1 Mb/s,

2 %)

(10 ms, 500 kb/s,

10 %)

(1 ms, 200 kb/s,

15 %)

(2 ms, 500 kb/s,

5 %)

(10 ms, 1 Mb/s, 7 %) (10 ms, 1 Mb/s, 5 %)

s

1

2

4

5

6

7

d

Link-state = (jitter, bandwidth, error rate)

Figure 2.1. Example of a graph associated with a network

Each link, denoted (u, v), between two nodes, u and v, is characterized by a weight ),( vuw , which is a vector with m components (m being the number of QoS metrics):

[ ]),(),..,,(),,(),( 21 vuwvuwvuwvuw m= . The general routing problem consists of finding a path P from a source s towards a destination d satisfying condition [2.1] for all the constraints Li expressed on the m QoS metrics.

( ) iiPvu

defi LvuwPw p),()(

)( ∈→Ω= , i=1, …,m. [2.1]

For example L1 = 100 ms and L2 = 1 Mb/s and L3 = 95% (availability). The Ω function and the p operator have different meanings depending on the QoS metrics used. For example, condition [2.1] is written:


– iPvui

defi LvuwPw ≤= ∑

∈→ )(),()( if the Li metric is the delay;

– ( ) iiPvu

defi LvuwPw ≥=

∈→),(min)(

)( if the Li metric is the bandwidth;

– ( ) iiPvu

defi LvuwPw ≤Π=

∈→),()(

)( if the Li metric is availability.

A route-fulfilling condition [2.1] is called a feasible route. For a couplet <source, destination>, we can find many routes satisfying condition [2.1]. If, in addition to fulfilling the QoS constraints, it is necessary to optimize another criterion (for example, the number of hops), the problem becomes a Multi-Constraint Optimal Path (MCOP) problem. According to the number and types of demands, the routing problem can be an optimization problem, a constraint satisfaction problem, or both.

All problems of routing with a single metric (to be optimized or guaranteed) can be solved by direct use (possibly with slight adaptations) of the Dijkstra and Bellman-Ford algorithms.

If we consider several constraints or optimizations simultaneously (therefore, several demands), the problem of routing may become NP-complete. The reader interested by these problems can refer to [CHE 98], which provides the list of QoS routing problems, as well as their class (NP-complete or not). The reader will find the proof of NP-completeness of certain routing problems in [WAN 96].

Sometimes, the routing problem with constraints on m QoS metrics is solved by suboptimal algorithms that function on the following principle: an optimal algorithm is used to find paths fulfilling a single metric, then the paths found are filtered in order to keep only those that optimize a second metric, then a third metric is applied to the subset of paths that have passed the second filter, and so on, until all metrics have been considered.


Another idea is to “combine” independent metrics into a mixed metric, and to implement the selection on this mixed metric or use this mixed metric as a criterion to optimize in addition the metrics that are to be guaranteed. For example, Xiao [XIA 04] proposed a composite metric given by equation [2.2] to select a path with a delay bound by Dmax and a cost bound by Cmax.

⎟⎠⎞

⎜⎝⎛ −∗⎟

⎠⎞

⎜⎝⎛ −=

CmaxPC

DmaxPDPM )(1)(1)( [2.2]

where D(P) and C(P) denote the delay and cost of path P.

2.4. Best-effort routing protocols in MANETs

QoS routing protocols are extensions to best-effort routing protocols, which do not explicitly take QoS into account. We will, therefore, begin by giving an overview of approaches to the best-effort routing problem before considering QoS.

2.4.1. Criteria for routing protocol classification

The limitations of ad hoc networks mentioned in section 2.2.2, (i.e. limited bandwidth, limited battery life, increased error rates, collisions, etc.) make routing in ad hoc networks a complex problem. Numerous best-effort protocols have been proposed over the last decade.

Protocol classification is very useful for understanding and analyzing them, and for better understanding the context in which they can be deployed. No protocol can fulfill all the requirements simultaneously. Many studies have been published on routing protocol synthesis and classification [MUR 04]. These studies give rise to the following (not mutually exclusive) commonly used criteria for categorizing routing protocols (Figure 2.2):

– strategies for discovering and updating paths;

– network structure;


– use of geographical position information;

– use of past or future information;

– link and path selection criteria.

2.4.1.1. Strategies for discovering and maintaining paths Routing protocols can be categorized into three classes according

to the strategies used for selecting paths and updating the routing tables:

– proactive protocols (also called table-driven protocols): each node keeps a table that indicates the next hops needed to reach the other nodes in the network. The paths are, therefore, prepared in advance. Periodical exchanges are activated to maintain the existing paths, discover new ones, or repair those that have been broken due to node mobility or shutdown. The advantage of this type of protocols is that the time required to select a required path is negligible as the paths are already known. The drawback is that it is necessary to maintain paths that may never be used. DSDV (Destination Sequenced Distance-Vector) and OLSR (Optimized Link-State Routing) are the two best known proactive protocols;

– reactive protocols (also called on-demand protocols): the path-discovery procedure is initiated only when a source needs a path to send its data. The path establishment request sent by the source is relayed closer and closer until the destination, or an intermediary node, which already knows a path to this particular destination, responds. The advantage of this type of protocols is that only the paths used are stored in the tables and kept up to date. The drawback is that the path-selection time can be significant and may not be suited to applications that need to transmit data urgently. DSR (Dynamic Source Routing) and AODV (Ad hoc On-demand Distance Vector) are the two best known reactive protocols;


Routing protocols in

MANETS

Route discovery and maintenance

Signal quality

Pro-active Protocols

Reactive protocols

Hybrid protocols

Path and link selection criteria

Energy saving

Link stability

Shortest path

QoS requirements

Future (prediction)

Past

Protocol class Distance vector

Link-state

Timing Information

Position and geographical information

None

Position, speed..…

Network topology

Flat

Hierarchical

Zone-based

Figure 2.2. Classification of routing protocols


– hybrid protocols: the protocols of this category combine the advantages of the two aforementioned categories. For a given node, we use a proactive protocol to keep the paths up to date with the nodes located at n hops (or a certain distance Dist) and a reactive protocol for more than n hops (or Dist).

2.4.1.2. Network structure Certain ad hoc networks are of small size and do not present any

scaling problems. Others are large or very large (thousands of nodes) and require certain precautions for addressing nodes and managing link-state information in order to permit scaleability. Routing protocols are, therefore, grouped in two categories:

– protocols based on a flat topology: all the nodes are at the same level and the functions for dissemination of information and path searching are the same for all nodes;

– protocols based on a structured topology: the nodes are structured according to their geographical distribution, the functions that they carry out, or other criteria. This allows the size and number of packets related to path searches and updates to be reduced. There are two types of structured topologies: hierarchical structure and structured by zones. In a hierarchical structure, each cluster, or group, of logical or physical nodes of level i passes by a node of level i+1 to send and receive link-state information. The functions for dissemination of information and path searching are the same for nodes of the same level in the hierarchy, but not for nodes belonging to different levels. For partitioning by geographical zones, the nodes’ geographical positions are used to partition them into clusters. HSR (Hierarchical State Routing) and FSR (Fisheye State Routing) are the best known hierarchical protocols.

2.4.1.3. Using geographical information A routing protocol makes decisions according to the topology of

the network. Certain protocols use topologies independent of the nodes’ physical positions, while others (called location-aided protocols) use geographical position information (supplied by a location server, such as GPS, or by a position estimation function


using a node mobility model) to guide its decisions. In effect, knowledge of the area in which a destination is located allows path-discovery packets, or data packet expeditions, to be targeted and directed. GPSR (Greedy Perimeter Stateless Routing), LAR (Location-Aided Routing) and ZHLS (Zone-based Hierarchical Link-state routing) are the best known geographical protocols.

2.4.1.4. Using past or future information To make decisions regarding path selection, each node must use

information relating to the network state. Routing protocols are distinguished by the temporal (past or future) of the information used:

– use of past information: protocols of this category collect link-state information of the network and then use this information (which, therefore, reflect the history of the network) to make decisions. Unfortunately, in an ad hoc network where the nodes move at medium or high speed, there is a risk that information about earlier topologies may no longer be useful for selecting the next path, as the topology will have changed since the last update of this information;

– use of future information (prediction): in order not to have to continuously update link-state information that becomes out of date before even being used, certain routing protocols use methods for estimating link-state information to predict the future topology of the network. The estimated information is used to decide which path to select. The prediction accuracy, and therefore, the accuracy of the path selection decisions, depends largely on the power of the mobility model for the nodes.

2.4.1.5. Criteria for link and path selection Various criteria can be used to assist the path selection:

– balancing the load distribution between the nodes (balancing the number of paths that pass through the nodes in order to avoid congestion of certain nodes or running down their battery);

– signal quality (by prioritizing links where the signals are of high quality, we can avoid transmission errors, and therefore, also retransmissions);


– link stability (by prioritizing stable links, we can avoid using paths that are constantly being modified);

– minimum path length;

– optimizing energy consumption (energy may be a critical issue, and it is necessary to optimize its use);

– QoS guarantees.

2.4.2. Presentation of routing protocols

QoS routing protocols are extensions to basic protocols, such as DSDV, OLSR, AODV, ZRP (Zone Routing Protocol), HSR, and LAR. We give a brief presentation of these protocols to help the reader understand their extensions for QoS support. Currently, there are only three protocols that have reached the status of IETF standard: AODV, OLSR, and DSR. AODV and DSR are very similar; therefore, DSR will not be discussed here.

2.4.2.1. DSDV protocol DSDV was one of the first protocols proposed for routing in ad hoc

networks [PER 94]. It is a proactive protocol, which is an extension to the Bellman-Ford algorithm where each node maintains a routing table that includes the distance (in number of hops) and the next hop needed to reach each of the other nodes in the network. When a node detects that a destination is not reachable, it sets the number of hops to that destination to ∞. The routing tables are exchanged between neighbors periodically, and also in the case of major changes, to the topology of the network. The tables are exchanged completely (in this case, we refer to a total update) or partially (in this case, we refer to an incremental update) according to the importance of updates. When a node transmits its routing table to its neighbors, it includes in the packet a sequence number. When a node receives a packet including the routing table of a node, which may have been relayed by other nodes, it decides, based on the sequence number, whether or not it needs to update its own table. The sequence numbers avoid loops in the path.


2.4.2.2. OLSR protocol OLSR belongs to the proactive protocols, which have introduced

specific techniques for reducing link-state information transmissions [JAC 97]. In 2003, it became an IETF standard [CLA 03]. It is adapted to high-density networks.

OLSR is based on the use of multipoint relays. Each node periodically exchanges information with its neighbors to calculate the set of its multipoint relays (MPR set). This set contains the minimum of nodes one-hop away, which allow us to reach all the nodes that are two-hops away. The calculation of the minimum set of multipoint relays is an NP-complete problem, and so OLSR uses heuristics for the calculation. Each node knows its MPR set and also knows the set of other nodes that have chosen it, in their turn, to be a member of their MPR set. When a node receives a link-state update packet, it retransmits this packet only if it is part of the MPR set of the transmitter node. Thanks to the reduction in the number of nodes that relay the link-state update packets, the performance of the network is optimized.

2.4.2.3. AODV protocol AODV is the most popular reactive routing protocol [PER 99]. In

2003 it became the first protocol to achieve the status of IETF standard [PER 03a].

When a source wants to transmit data and does not have a path towards the destination, it initiates a path discovery procedure, broadcasting a path search request to its neighbors. This request is relayed until an intermediary node (which already knows of a path towards the destination), or the destination, receives the request and sends a reply. When a node relays a request to its neighbors, it saves the identity of the node from which it received this request in its table (this allows us to reconstruct the reverse path right up to the source). As the same request may reach an intermediary node or the destination by several paths, this intermediary node or the destination sends its reply along the reverse path (towards the source). Each time the node or destination finds that the path associated with a request is shorter than that which it already knows, the shortest path is chosen.


AODV associates a unique sequence number with each request to avoid loops in paths. Furthermore, AODV includes a recipient sequence number into its responses so that the freshest path (i.e. the most recent path known to the recipient) is always chosen. Each node changes the path towards a destination automatically whenever it receives a packet with a destination sequence number that is larger than that contained in its routing table.

2.4.2.4. ZRP protocol ZRP was one of the first hybrid protocols to attempt to combine the

advantages of proactive and reactive routing [HAA 97]. Each node sees the network as being composed of the zone in which it is located (the routing zone) and the rest of the network. The routing zone of a node, I, is defined as the set of nodes located no more than r hops far from node i. The efficiency of the ZRP protocol depends on the zone radius, r: a large (small) value for r would lead to a performance almost identical to that of a reactive (proactive) protocol. It also depends on the density of the network. The more nodes there are, the more overlaps there would be between routing zones.

In ZRP, a proactive routing protocol, IARP (Intra-Zone Routing Protocol) is used for the exchange of link-state information with the neighboring nodes located up to r hops away, and a reactive routing protocol, IERP (Inter-Zone Routing Protocol) is used for exchanges with the nodes located further than r hops away. The IARP protocol allows us to maintain paths towards all destinations within the routing zone, to make them available to the sources that want to transmit to the interior of this zone. When a source wants to sent data towards a destination outside the routing zone, the IERP protocol launches the path search procedure. A request is broadcast by the source towards the nodes surrounding the routing zone of this source. Each node that receives the request checks whether the destination is located in the routing zone; if it is, it sends a reply. If not, it broadcasts the request to the nodes surrounding its own zone.

2.4.2.5. HSR protocol HSR is a proactive protocol that uses the principle of

hierarchization of the nodes in the network [IWA 99]. HSR may use


either physical partitioning (for example, nodes are grouped according to their geographical positions) or logical partitioning (for example, nodes are grouped according to their functions). Level 0 (called the physical level) is the level of the physical nodes. The nodes at level 0 are grouped into clusters and each cluster chooses a cluster head. The cluster heads of the nodes at level 0 form the nodes at level 1. The principle of grouping is reiterated to form the nodes at level N.

The nodes belonging to the same physical cluster exchange their link-state information by broadcasting within the group. The information broadcasted within a physical group is incorporated (aggregated) by the cluster head of the group, which subsequently broadcasts them to the cluster heads belonging to the same level 1 group as it. Each cluster head, on receiving link-state information from other cluster heads, broadcasts it to the members of the group that it represents and so on, until the entire network has received the link-state information. By grouping the nodes, we can reduce the link-state information exchanged between the nodes in relation to a flat network structure.

2.4.2.6. LAR protocol LAR is a reactive protocol [KO 98] where the path search and path

update functions are similar to those of DSR. LAR was one of the first protocols to use information on the nodes’ physical (geographical) location for routing decisions to reduce the overhead due to broadcast requests. The LAR protocol assumes that each node knows its own position thanks to a GPS receiver. When a source wants to search for a path, it defines two zones to reduce the broadcasting of its request: an assumed zone and a requested zone.

The assumed zone represents the zone where the destination is assumed to be located when that the request is generated. The current position of the destination is estimated by the source according to the destination’s previous location and behavior. If the source has no information on the destination’s history, the assumed zone corresponds to the entire search space. The assumed zone is generally a circle centered around the estimated location of the recipient and the radius is chosen in order to maximize the probability of finding a path.


The requested zone defines the zone where the path search must be carried out and where the path search request will, therefore, be broadcast. If the requested zone is small, there will be fewer request retransmissions, but the probability of finding a path is also reduced. The LAR protocol works in two modes: LAR1 and LAR2.

In the LAR1 mode, the source begins by determining the smallest rectangle that contains it, as well as the assumed zone, and includes the coordinates of the four corners of the rectangle in the request. When a node receives a request, it rejects it if it is not located in the rectangle indicated in the request. If it is in the rectangle, it sends a response if it already knows of a path to the destination, otherwise it broadcasts the request to its neighbors. If the destination receives the request, it replies, also including its current position in the response (and occasionally its speed and direction, which will allow the source to refresh its position information for the destination). If the source does not receive any response within a fixed time, it broadcasts a new request with new assumed and requested zones.

With the LAR2 mode, the source calculates the distance Ds which separates it from the destination and places this distance in its request, along with its current coordinates. When a node i receives a request directly from the source, it calculates its distance Di in relation to the destination. If Ds + δ < Di (δ is an adjustable tolerance parameter to take account of the error in estimating the position of the destination), it discards the request. If not, it broadcasts the request to its neighbors, replacing Ds with Di. When a node j receives a request coming from another node i (which is not the source), it rejects it if it has already received it. Otherwise, it calculates Dj, its distance in relation to the destination. If Di + δ < Dj then it rejects the request. Otherwise, it replaces Di with Dj in the request and broadcasts it to its neighbors.

2.5. QoS routing in MANETs

QoS routing in ad hoc networks involves an additional level of complexity in comparison with QoS routing in wired networks, due to the characteristics of this type of networks (section 2.2.2). The key points for the design of QoS routing protocols are essentially tied to:


– signaling QoS requirements (in particular, using control packet extensions to convey the values of the QoS parameters);

– estimating QoS metrics;

– exchanging information linked to QoS metrics;

– resource management (particularly reservation) to fulfill QoS requirements while taking account of the node mobility;

– reaction speed of the protocol in the case of broken links, to minimize time intervals where QoS is no longer provided;

– continuity of service using location prediction models to anticipate broken paths and to continue to deliver the data to the mobile nodes without any significant degradation in QoS;

– minimizing the overheads of the protocol;

– scaleability.

Many (a few tens of) algorithms have been proposed to solve the problems of QoS routing in ad hoc networks [BHE 06, CHA 01, ZHA 05]. Certain algorithms are considered original, others are more often extensions to these original algorithms to take account of particular aspects, particularly in terms of optimizing criteria or problem-solving techniques (genetic algorithms, fuzzy logic, etc.).

The diversity of these contexts (QoS requirements, node mobility, energy constraints, etc.) makes a quantitative comparison of these protocols difficult or impossible. Our objective here is to give the main aspects of proposed protocols, which we have deemed important (Table 2.1).

2.5.1. Approaches for QoS routing

The classifications proposed in the literature for studying routing protocols provide the following criteria relevant for the understanding and analysis of routing protocols:

– type of routing: as in the case of best-effort routing, QoS routing can be reactive, proactive, or hybrid. For example, AQOR, OLMQR,


OQR, QoS-AODV, TBP, and TDR are reactive protocols, QOLSR is a proactive protocol, and CEDAR is a hybrid protocol. It is worth noticing that almost all QoS routing protocols are reactive. Indeed, as the QoS constraints are only known once a flow arrives, QoS route maintenance by a proactive protocol is not effective. The only exception where a proactive routing can be used is in the case of critical traffic for which routes have been set aside in advance even if they are cut off before being used [HUG 03];

– resource management: there are three management strategies for fulfilling QoS requirements:

- with explicit resource reservation, - without any resource reservation, - with packet marking like in DiffServ, but there is no reservation

of resources at the moment that the flow arrives. During the packet forwarding, the highest priority is given to packets with the strongest QoS constraints. The packet marking strategy is rarely used in ad hoc networks, and consequently, we will not develop it further. We will return to the first two reservation strategies;

– coupling between routing and QoS management mechanisms:

- without coupling: QoS route searching is general and supposes that there are other resource allocation mechanisms, admission control, supervision and maintenance of QoS. INSIGNIA [LEE 00] and SWAN [AHN 02] are two examples of protocols in this category,

- with coupling: QoS management mechanisms are a part of the routing protocol. Protocols such as AQOR, BR, CEDAR, OLMQR, OQR, PLBQR, QoS-AODV, TBP, and TDR are based on this coupling principle,

– dependence of the routing protocol on measures and estimations carried out by the MAC sublayer:

- with dependence: the routing function uses measures and estimations (particularly bandwidth and delay) resulting from the MAC sublayer in order to determine the QoS-aware routes. Examples of this class include AQOR, BR, CEDAR, OLMQR, OQR and TDR,

- without dependence: the network layer uses only information exchanged between network layer entities to make decisions. In other


words, this class of protocols does not require that MAC sublayer provides mechanisms for estimating parameters relevant to QoS monitoring. TBP, PLBQR, and QoS-AODV are examples of protocols belonging to this class.

2.5.2. Resource reservation

Resource reservation implies different forms of resources: bandwidth, memory, and the central unit. In this chapter, we consider only bandwidth. As we mentioned earlier, routing protocols may or may not use resource reservation. It is important to emphasize that the efficiency of reservation or non-reservation depends on the context of use (essentially the mobility of the nodes, the QoS constraints, and characteristics of the flows).

2.5.2.1. QoS routing with resource reservation In wired networks, resource reservation is the main solution used

to fulfill QoS requirements. It involves, depending on the nature of the flows, determining the QoS requirements to which each node (from the source to the destination) must comply with. The necessary resources (memory buffer, bandwidth, and CPU) are then reserved. A node that accepts a flow after having executed an admission test reserves the resources necessary to meet the flow QoS requirements. A hard-state (strict) or soft-state (flexible) reservation can be made.

The “hard-state” approach is limited to critical applications. The resources are explicitly reserved for the incoming flow and are not released until the source declares that it has finished transmitting data. If a part of the path can no longer guarantee the QoS, a new path is searched for and new reservation is made. This approach is only realistic for networks where the topology and link capacities are fixed (or change very rarely). It is almost unusable in ad hoc networks.

The “soft-state” approach is used to fulfill QoS requirements of applications that accept that from time to time their QoS demands will not be met. On each node, we make reservations that become invalid after time ∆tthreshold. If within time ∆tthreshold a node does not see any data packet coming from a source for which it has reserved some


resources, it cancels the reservations. This approach is usable in the context of ad hoc networks. When a node moves, certain links are broken and resource reservations for these links are automatically cancelled. According to the path repair method used, other nodes are selected and resources reserved for these nodes.

2.5.2.2. QoS routing without resource reservation The characteristics of ad hoc networks (mobility and fluctuations

in link capacity) mean that the reservation of resources sometimes has little chance of producing the expected results (i.e., not fulfilling the QoS requirements). It can also be complex to implement or may generate a significant overhead. Rather than explicitly reserving resources, certain routing protocols, such as [KAZ 02, MUN 02], opt for an optimistic approach to QoS without any reservation. Each node carries out some measurements and estimations for some metrics, such as the available bandwidth and the transmission delay, on each link. These measures serve to assist path selection. The assumption is that, while a path has enough resources, the source chooses it (and begins to transmit data in the case of reactive protocols) hoping that this path fulfills the QoS requirements for as long as possible. If the QoS observed deviates from this expectation for a certain amount of time, a new path is set up.

Before choosing a path, the source checks whether certain links in this path are not already in use; if they are in use, the source does not choose them. There is a type of self-censorship of sources when they know that the links are already in use. The first flow to arrive is, therefore, the best served.

2.5.3. Examples of reservation methods

In this section, we present two of the most widely cited reservation methods. The first method is synchronous and operates on top of TDMA. It is included in several routing protocols, such as BR, OQR, and OLMQR. The second method is asynchronous and operates on top of the IEEE 802.11 protocol in DCF (distributed coordination function) mode.


2.5.3.1. Synchronous reservation As the use of TDMA imposes synchronization of node clocks (for

example, via a GPS), the bandwidth reservation techniques are only adapted to networks of smaller size and with low node mobility (otherwise synchronizing the reservations becomes very hard).

In the reservation methods on top of TDMA, time is divided into frames of equal length [LIN 99, ZHU 02]. Each frame S is divided into slots s1, s2, …, sM. The set of slots allocated to a node i to transmit its data and the data that it is relaying, is denoted TSi and the set of slots where node i or its neighbors receive packets (node i can be the destination, a relay or may not be concerned by these packets, but it may not interfere during the transmission of the packets) is denoted RSi.

While a node i is transmitting to a node j in the slot sk, node j cannot transmit (as it must receive) and no neighbor of i or j may transmit within this time slot, otherwise there would be a collision. The problem is finding the sets TSi and RSi for each node i in the network, whether or not there are data to be transmitted.

To allocate r slots to a path P from a source s to a destination d, each node i along the path must find r slots to transmit to its neighbors and these slots do not interfere with the transmitting slots. The problem in calculating the maximum bandwidth on a path has been shown to be NP-complete. Consequently, the solutions proposed are pseudo-optimal. Various heuristics have been proposed to solve this problem [LIN 99, ZHU 02].

Bandwidth reservation with TDMA presents numerous problems in the context of ad hoc networks, particularly:

– clock synchronization: to reserve time slots, the set of frames must be identical as seen by all the nodes. For this, all the nodes must have access to the same global time. This assumption is hard to meet or could incur significant costs for networks on a large scale (large number of nodes or large node mobility space);


– collision of reservations: the set of nodes can split into two or more groups (or networks) following node displacements. The nodes of a group g1 no longer communicate (due to signal range limitations) with those in another group g2. In this case, the nodes of each group synchronize themselves in order to determine the beginning of the time frames and reserve the slots in relation to these times. If now, following the node displacements, the groups g1 and g2 are joined together again to form a single network, the reservations made separately in the two groups no longer work (this is the problem of reservation collisions [BOU 08]).

2.5.3.2. Asynchronous slot reservation methods Asynchronous reservation methods bring solutions to avoid the

difficulties of using synchronous reservation methods. These methods are based on extensions to MAC IEEE 802.11 to make reservations on wireless links. One of the most widely cited methods in the literature is that proposed by Manoj et al. [MAN 04], which uses the RTMAC protocol. This protocol was also proposed by Manoj et al. [MAN 02] and allows the reservation of slots. We begin by presenting the RTMAC protocol. Following this, we will see how it is used to reserve bandwidth along a path.

RTMAC protocol for reservation on a link: the RTMAC (Real-Time MAC) protocol is an extension to the IEEE 802.11 DCF protocol for reserving slots for real-time and periodic traffic (CBR).

In addition to the best effort packets exchanged via the DCF protocol, reservation packets for real-time flow are used: ResvRTS, ResvCTS, and ResvACK which are equivalent to the RTS, CTS, and ACK packets used in the IEEE 802.11 protocol to reserve some bandwidth in the competition-free window. In order to reduce the delays in access to the reservation packets for real-time traffic, these packets have the highest priority (associating them with waiting times before attempting the transmission, shorter than those used by non-real-time packets).

Time is divided into frames (called super-frames) of fixed length for all the nodes, but no mechanism for synchronizing time frames of


different nodes is required. Slots are reserved (depending on required bandwidth) in the time frames. The duration of a slot is twice the maximum delay in propagation along the link. To send data on a link, the source must reserve a sufficient number of slots and should mark the start time of the first slot and the end time of the last slot in relation to the beginning of its time frame. As the source flow is supposed to be periodic, the reservation remains valid for all the time frames, which repeat cyclically until the reserved slots are explicitly released.

The RTMAC principle is as follows: when a node A wants to reserve k slots on the link to its neighbor B, it starts by determining in its own super-frame (i.e. its Network Allocation Vector, or NAV, according to the IEEE 802.11 terminology) the position where it can place the slots to be reserved. Node A determines (in relation to the start time of its super-frame) trdA the start time of the first slot to be reserved, and trfA the end time of the last slot (trfA = trdA + k). It constructs a ResvRTS packet including the two relative times (trdA and trfA) and sends it to B. When B receives the ResvRTS, it checks (according to the reservation state of its super-frame) if it can make the required reservation. If it accepts the reservation, it sends a ResvCTS packet including the same relative times of reservation. Each of B’s neighbors that receives the ResvCTS updates its reservation table (so as not to interfere with node A when it wants to use the reserved slots).

When node A receives a ResvCTS, it broadcasts a ResvACK to inform its neighbors (which have not received the ResvCTS) so that they can also update their reservation tables. If the reservation is successful, node A uses the reserved slots to transmit its data packets (RTData), which will be acknowledged by B with the RTACK packets. If node B cannot reserve the slots requested, it rejects the ResvRTS and decides whether or not to return a negative response to A. Node B, which knows – thanks to its super-frame – the reservations of its other neighbors, does not respond to A if it considers that its response could result in a collision, otherwise it returns a ResvNCTS (reservation refused) to A. According to the case (obtaining a ResvNCTS packet or timeout), node A can decide whether or not to resubmit its reservation


request, changing the place of the slots. At the end of the real-time flow, node A initiates the process of releasing the slots. For this, three priority packets (ResvRelRTS, ResvRelCTS, and ResvRelACK) are used to explicitly release the slots reserved at nodes A, B and all their respective neighbors. The principle of packet exchange is the same as for ResvRTS, ResvCTS, and ResvACK. Just in case, if node B, which has reserved some slots for A, does not see any real-time data packet pass by which originating from A, it initiates the slot release process instead of A.

Contrary to synchronous reservation methods, with RTMAC reservations are not synchronized on a global timescale common to all nodes but on relative times. For the transmitter A (or the receiver B respectively), the reserved slots begin at trdA (trdB) from the time when it has transmitted (received) the ResvRTS packet. Given the relative reservation start and end times included in the ResvRTS packet sent by node A and the duration of the transmission and propagation of a ResvRTS packet, node B can calculate its own reservation start and end times.

End-to-end reservation protocol: Manoj et al. [MAN 04] have proposed a method for extending the DSR protocol to establish the paths on demand to fulfill bandwidth constraints by making slot reservations along the path between a source and a destination of a real-time periodic flow. Here we are interested only in slot reservation. This reservation is made in three steps: feasibility test, assigning the slots, and reserving the slots:

– feasibility test: each node i, which receives a path request transmitted by its neighbor node i’, tests (basing itself on reservations already made in its super-frame) whether there are still enough free slots to satisfy the request to establish the path. So that node i can accept the request, it must find free slots that are compatible with the free slots of node i’ so that node i can send data packets and node i’ can receive them during the selected slots. If the test is positive, it broadcasts the request to its neighbors, including its reservation table to the request. Thus the request conveys the reserved slots and free slots of all the nodes which have relayed it. The number of free slots of node i can be higher than that requested by the source. In other


words, node i offers ranges of slots and lets the destination node choose, taking account of the set of nodes;

– slot assignment: when a request arrives at the destination node, such a request includes all the reservation tables of the nodes that have relayed it. The destination node now knows the free and busy slots in all the intermediate nodes, as well as the number k of consecutive slots to be reserved. It can, therefore, determine the k slots that each intermediate node must effectively reserve (the authors call this operation bandwidth allocation). As several slot assignment solutions are possible, heuristics can be used to choose the solution that optimizes certain criteria;

– bandwidth reservation: if the destination node finds a slot allocation solution for the different links forming the path conveyed by the request, the slots allocated to each node are placed in a response packet and this response is transmitted to the source along the reverse path. Each node that receives the response checks that the slots that have been assigned to it are still free. If the test is positive, the RTMAC protocol is called to reserve the slots effectively. If the RTMAC protocol confirms the reservation, the node transmits the response along the reverse path. When the response arrives at the source node, the reservation is successfully established. If an intermediate node cannot reserve the slots that have been assigned to it (as its state has changed between the time it agreed to relay the request and the time it received the response) or if the destination node does not find a slot assignment solution, the path discovery fails.

2.5.4. Estimation models

As has been emphasized above, QoS routing protocols need information on the QoS metrics to make decisions, and this information is obtained with the aid of measures and estimates. QoS metric estimation models are of paramount importance in QoS routing: the more accurate the estimations, the more appropriate the routing decisions. There is no ideal estimation model, especially in ad hoc networks, as an accurate knowledge of the QoS metrics is impossible due to the intrinsic characteristics of ad hoc networks. For this reason, various estimation models have been proposed. Most of


these models are based on the EWMA (Exponentially Weighted Moving Average) estimation, which is well known in statistics and used particularly for congestion control in TCP.

We give the overview of some models for estimating bandwidth and delay, which seem to us to be representative of the works on QoS routing in ad hoc networks. Due to lack of space, we will not discuss the pros and cons of these models.

2.5.4.1. Estimation of available bandwidth One of the main difficulties in selecting paths that fulfill QoS

requirements is in determining the bandwidth available for accepting new flows. Various solutions have been proposed going from the simplest (and, therefore, least realistic) to the most complicated (and, therefore, difficult to implement).

The solutions differ according to whether the underlying MAC technique allows bandwidth reservations or not. Estimation models have been proposed mainly for TDMA and CSMA/CA protocols. Given that the MAC IEEE 802.11 protocol can operate according to different policies (DCF, EDCA, HCF, etc.) different models can be used in order to better take into account the properties of the underlying MAC protocol.

Cansever et al.’s reservation method [CAN 99] In CSMA/CA-based networks, the bandwidth available for a node

depends both on the flows that pass by the node, and on the flows that pass by its neighbors (as they share the same channel). The bandwidth available for node i can be estimated by:

∑ ∑∑∈ ∈∈

−−=i ji Nj Nk

jkNj

ijii llCBw [2.3]

Ci denotes the maximum capacity (in bits per second) of the channel used by node i. lij denotes the capacity consumed by the flow from i to j (this flow corresponds to the data packets relayed by node i as well as the data packets for which i is the source). Nx denotes the set of neighbor nodes of node x. The difficulty in using this method is


linked to the fact that each node has to have knowledge of the flows relayed by its neighbors.

Kazantzidis and Gerla’s method [KAZ 02] This method is suited to networks using the MAC IEEE 802.11

protocol in DCF mode. It is based on the use of measures captured at the data-link layer and subsequently communicated to the network layer. The LLC sub-layer measures the rate of use of the queue of frames to be transmitted (this rate is called u). The MAC sub-layer measures the packet throughput (denoted Throughp) for each link.

The measure of available bandwidth (denoted Bwi,j) between two neighboring nodes i and j is defined by:

jiji ThroughpuBw ,, *)1( −= [2.4]

The throughput of link <i, j> can be calculated using measures captured on the time taken to transfer a packet. It expresses the effective data transmission bandwidth on link <i, j>. The throughput for a packet is defined as follows:

ionTtransmissACKTreceptionPacketSizeughpPacketThro

−= [2.5]

where PacketSize is expressed in bits, Ttransmission is the transmission time of the packet and TreceptionACK is the time when the transmitter of the packet receives the acknowledgment. Using the operational parameters of the IEEE 802.11 protocol, the PacketThroughp is measured by:

∑=

++++= R

rrbtROHtCEttranstqueuet

PacketSizeughpPacketThro

1

_*)___(_

[2.6]

where t_queue denotes the waiting time at the MAC sub-layer before the transmission of the packet, t_trans the duration of the transmission of the packet, t_CE the collision avoidance time, t_OH the overhead


time (for example, the time taken by the RTS/CTS packets), R the number of retransmissions of the packet following non-reception of the ACK and t_br the back-off time (the time to wait before attempting retransmission).

The throughput per packet given by equation [2.6] depends mainly on three variable factors: the packet size, the waiting time, and the number of retransmissions (which depends in its turn on the load on the network).

In order to make the throughput of a link relatively independent of these factors, a window of 32 measurements is considered to have a statistical value of the throughput per packet. This gives rise to the following formula for calculating the throughput per packet:

)32...1,(, == kughpPacketThrostatThroughp kji [2.7]

where the function stat is generally the average function.

Let FMDuration be the duration of the window during which the last 32 measurements have been made, and FreeTime the period where the LLC sub-layer had no frame to send during the measurement window. Then we have:

FMDurationFreeTimeu −= 1 [2.8]

Now, thanks to equation [2.4], we can determine the amount of available bandwidth. The amounts of available bandwidth calculated separately by the nodes are exchanged between the nodes.

2.5.4.2. Delay estimation

Probe model for estimating the end-to-end delay This is one of the simplest models (but also generally the least

effective). Each node i sends probe packets periodically or occasionally to measure the round-trip time RTTi,k between itself (node i) and any other node k. Node i takes one half of the last measured RTT as its estimate for the delay Di,k in reaching node k.


Chen’s end-to-end delay estimation model [CHE 99] Chen et al. have proposed a model allowing the estimation of the

end-to-end delay or of the available bandwidth for the TBP protocol. We present it here in the case of the delay. Instead of keeping the estimated delay in reaching node j in its routing table, each node i keeps the variation in the delay using the following estimation model:

oldji

Newji

oldji

Newji DDDD ,,,, )1( −∗∗−+∆∗=∆ βαα

[2.9]

where NewjiD ,∆ (respectively old

jiD ,∆ ) denotes the new (respectively old) value estimated for the variation in the delay for the path going from node i to node j, New

jiD , ( oldjiD , ) denotes the new (old) value estimated

for the delay in the path going from node i to node j, α (α < 1) and β (β > 1) are two adjustable parameters. The advantage in keeping the variation in a metric rather than the value of the metric itself allows us to recognize which are the stable and, therefore, the best paths that are able to provide QoS guarantees and to continue to provide it (if needed) for the duration of the flow.

Romdhani and Bonnet’s model for estimating the delay on a link Romdhani and Bonnet [ROM 05] have proposed a model for

periodically estimating the delay on each outgoing link, every T time units. Each node i estimates the mean delay Dmean to transmit a packet on the link that it is using by:

1)1( −∗+−= jmean

jreal

jmean DDD αα [2.10]

where j denotes the number of the update period for the link delay, j

realD the last real delay measured for the last packet successfully transmitted on the link, j

meanD the last mean delay estimated and α is a weighting parameter. To take account of the activity on the link, the parameter α is defined as follows:

TTT idle /)( −=α [2.11]


where Tidle denotes the time of inactivity (no traffic on the link) during an interval of T time units.

Probabilistic model for link-delay estimation Some research, such as [SHE 01], uses probabilistic models to

determine the delay in crossing a node. The models proposed are often tied to CSMA/CA operation with or without a reservation mechanism. For each node, we take account of all the flows that cross it, as well as the flows that cross its neighbors. By making hypotheses on the flow distribution (generally according to Poisson’s distribution), we can determine a delay bound so that the node can successfully transmit a packet where this delay bound is met at a certain probability. The SIFS and DIFS values, and the mean number of retransmissions and the back-off window appear directly in the delay calculation formula.

2.5.5. Presentation of the main QoS routing protocols

Several protocols have been proposed since 1999 to fulfill the QoS demands (particularly bandwidth and delay) of mobile ad hoc networks [BHE 06, CHA 01, ZHA 05]. As mentioned previously, “best-effort” routing protocols have been extended to take account of QoS requirements. The QoS path discovery principle is almost identical for all protocols in a class (reactive or proactive).

In QoS reactive protocols, the path-discovery procedure is launched by the source (with a few exceptions) specifying its QoS requirements.

Upon receiving a request, each node checks if it has already relayed a request with the same parameter values as the ones included in the received request. If the check is negative, then the node tests (applying the bandwidth and delay estimation models) if it can satisfy the demand. If the test is positive, it broadcasts the request to its neighbors.

When a request arrives at the destination, and the destination can meet the QoS requirements, a path has been found. Several requests may arrive at the destination. In this case, the destination can choose


the best path. In the same way, if several responses arrive at the source, the source can choose the best path or use the first path found.

In Table 2.1 we have summarized the main characteristics of the protocols that are the most widely studied and most innovative in terms of QoS routing. For each protocol, the following are indicated: the type of routing, the QoS metrics taken into account, the use or not of resource reservation methods, whether or not the authors of the protocol have proposed estimation models for metrics, and some remarks on the operation of the protocol.

2.6. Conclusion

Ad hoc networks provide interesting opportunities to allow mobile devices to exchange data with other mobile devices (either in their immediate vicinity or further away), and also to use their neighbors as relays for accessing distant internet sites without prior infrastructure. To allow mobile nodes to transmit data with QoS constraints (particularly delay and bandwidth) various problems should be taken into consideration to comply with these needs.

Among the problems encountered, this chapter focuses on the problem of QoS routing in ad hoc networks. This problem is difficult to solve, given the diversity of QoS requirements and the characteristics of ad hoc networks (particularly frequent changes in topology, energy constraints, and limitations in available bandwidth).

The number of routing protocols for MANETs, which take account of the constraints in delay and bandwidth, is very high considering the number proposed in the framework of wired networks. It is commonly accepted that it is difficult or impossible to deploy real-time multimedia applications with hard QoS constraints over MANETs. Only applications that can accept that QoS may not be met from time to time can be deployed on mobile ad hoc networks. This is only considering the particular cases where we can deploy applications with hard QoS constraints, for example, the case of a group of soldiers or robots that move together and have a global, common objective.


Protocol Type Guaranteed QoS metrics

Reservation of resources

Model for estimating

metrics Remarks

AQOR [XUE 03]

Reactive Bandwidth and delay Yes Yes (1)

BR [LIN 99]

Reactive Bandwidth Yes No (2)

CEDAR [SIN 99]

Reactive, hierarchical

Bandwidth Yes No (3)

D-LOAR [SON 03]

Reactive (15) No Yes (4)

ODRP [ZHA 05]

Hybrid Delay and number of

hops Yes Yes (5)

OLMQR [CHE 02]


OQR [LIN 01]


PLBQR [SHA 02]

Reactive and geographical Delay No No (8)

QOLSR [MUN 02]

Proactive Bandwidth and delay No No (9)

QoS-AODV

[PER 03b] Reactive Bandwidth

and delay Yes No (10)

QoS-ASR [LAB 02]

Reactive Bandwidth and delay No No (11)

SBSR [AGA 05]

Reactive Delay and reliability No No (12)

TBP [CHE 99]

Reactive Bandwidth and delay

Yes/no (16)

Yes (13)

TDR [DE 02]

Reactive and geographical

Bandwidth and number

of hops Yes No (14)


(1) In AQOR (Ad hoc Qos On-demand Routing), the destination supervises the QoS. AQOR operates with IEEE 802.11 DCF.

(2) BR (Bandwidth Routing) operates with TDMA.

(3) CEDAR (Core Extraction Distributed Ad hoc Routing) propagates link-state information only when the available bandwidth increases slowly but indubitably and when the bandwidth decreases rapidly. It uses a subset of nodes (called dominant nodes) to manage QoS.

(4) D-LOAR (Delay Load-Aware On-demand Routing) is an extension of AODV. It uses the IEEE 802.11 protocol DCF. It takes account of the node load. If the queue at the MAC level of a node is more than 80% occupied, the node rejects any new path discovery request.

(5) In ODRP (On-demand Delay-constrained Routing Protocol), if the shortest path known to the source does not fulfill the delay requirements, the path-discovery phase initiated by the source fails and the destination launches the procedure in the reverse direction in order to find a path that fulfills the delay.

(6) OLMQR (On-demand Link-state Multipath QoS Routing) operates with CDMA-over-TDMA and searches for several paths that comply collectively with the QoS requirements.

(7) OQR (On-demand Qos Routing) is an improvement on the BR protocol.

(8) PLBQR (Predictive Location-Based QoS Routing) is one of the rare source-routing protocols. It uses a node position prediction model to limit broadcasting of requests. Furthermore, the nodes only relay a path discovery request when they are closer to the destination than the node from which they received the request.

(9) QOLSR (Qos OLSR) is an extension of OLSR.

(10) QoS-AODV (QoS extension to AODV) is an extension of the AODV control packets for QoS support.

(11) QoS-ASR (QoS-Adaptive Source Routing) allows the consideration of other criteria that are to be optimized: lifespan, stability, and path congestion.

(12) SBSR (Segmented Backup Source Routing) allows the discovery of a primary path and secondary paths. As soon as the current path is broken, there is a switch to one of the remaining available secondary paths.

(13) TBP (Ticket-Based Probing QoS routing protocol) uses the concept of a ticket (right or price to pay) to reserve resources.

(14) TDR (Trigger-based Distributed Routing) takes account of the quality of the signals. TDR allows a rapid re-routing while anticipating link breakages.

(15) Optimization of the time delay and number of hops.

(16) According to the option chosen for the operation of the protocol.

Table 2.1. Main routing protocols to consider delay and bandwidth constraints


As we know the requirements of the mobile elements in this group in advance, we can verify whether the application would be able to operate correctly (regarding timing) when deployed on these mobile elements.

There are no conclusive results that indicate the superiority of a routing protocol in relation to others for all criteria. Everything is a question of context of use. In research, many routing protocols have been proposed, discussed in abundance, and evaluated in simulation. Unfortunately, the deployment of these networks on a large scale remains at an experimental stage, and their impact on the market is still negligible. The research into general purpose ad hoc networks has had little impact on practice due to the sometimes antagonistic assumptions made in these publications, which attempt to find protocols that can be deployed on a large scale, fulfilling performance and security criteria, without being bound to a particular domain of application. The current trend is to act as in the case of wired networks, distinguishing two large classes: general ad hoc networks allowing connectivity without guaranteeing QoS (the “best-effort” approach), and ad hoc networks oriented towards specific applications (guiding vehicles, road traffic information, military domain, robotics, etc.) where we can demand a certain QoS depending on the target domain.

2.7. Bibliography

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[BHE 06] BHEEMARJUNA R. et al., “Quality of service provisioning in ad hoc wireless networks: a survey of issues and solutions”, Ad hoc Networks, 4:83-124, 2006.

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[CHA 01] CHAKRABARTI S., MISHRA A., “QoS issues in ad hoc wireless networks”, IEEE Communications Magazine, vol. 39, p. 142-148, 2001.

[CHE 98] CHEN S., NAHRSTEDT K., “An overview of quality-of-service routing for next generation high-speed networks: problems and solutions”, IEEE Network, vol. 12, p. 64-79, Nov/Dec, 1998.

[CHE 99] CHEN S., NAHRSTEDT K., “Distributed quality-of-service routing in ad hoc networks”, IEEE Journal on Selected Areas in Communications, vol. 17, p. 1488-1504, 1999.

[CHE 02] CHEN Y. et al., “On-demand, link-state multi-path QoS routing in a wireless mobile ad-hoc network”, European Wireless, Florence, Italy, Feb 25-28, 2002, p. 135-141.

[CLA 03] CLAUSEN T., JACQUET P., “Optimized link state routing protocol (OLSR)”, RFC 3626, IETF, Oct 2003.

[COR 99] CORSON S., MACKER J., “Mobile ad hoc networking (MANET): routing protocol performance issues and evaluation considerations”, RFC 2501, IETF, Jan 1999.

[CRA 98] CRAWLEY E. et al., “A framework for QoS-based routing in the internet”, RFC 2386, IETF, Aug 1998.

[DE 02] De S. et al., “Trigger-based distributed QoS routing in mobile ad hoc networks”, ACM Mobile Computing and Communications Review, vol. 6, p. 22, 2002.


[HAA 97] HAAS Z.J., “A new routing protocol for the reconfigurable wireless networks”, Int. Conference on Universal Personal Communications, San Diego, CA, Oct 12-16, 1997, p. 562–566.

[HIG 01] HIGHTOWER J., BORRIELLO G., “Location systems for ubiquitous computing”, IEEE Computer, vol. 34, p. 57-66, Aug 2001.

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[IWA 99] IWATA A. et al., “Scalable routing strategies for ad hoc wireless networks”, IEEE Journal on Selected Areas in Communications, vol. 17, p. 1369-1379, 1999.

[JAC 97] JACQUET P. et al., “Increasing reliability in cable free radio LANs: low level forwarding in HIPERLAN”, Wireless Personal Communications, vol. 4, p. 51-63, 1997.

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[KAZ 02] KAZANTZIDIS M., GERLA M., “End-to-end versus explicit feedback measurement in 802.11 networks”, Seventh International Symposium on Computers and Communications, Taormina, Italy, Jul 1-4, 2002, p. 429-434, .

[KO 98] KO Y.B., VAIDYA N.H., “Location-aided routing in mobile ad hoc networks”, ACM/IEEE Mobicom, Dallas, Texas, Oct 25-30, 1998,, p. 66-75.

[KUI 02] KUIPERS F. et al., “An overview of constraint-based path selection algorithms for QoS Routing”, IEEE Communications Magazine, p. 50-55, Dec 2002.

[KUI 05] KUIPERS F.A., VAN MIEGHEM P.F.A., “Conditions that impact the complexity of QoS routing”, IEEE/ACM Transactions on Networking, vol. 13, p. 717-730, 2005.

[LAB 02] LABIOD H., QUIDELLEUR A., “QoS-ASR: an adaptive source routing protocol with QoS support in multihop mobile wireless networks”, IEEE Vehicular Technology Conference, Vancouver, Canada, p. 1978-1982, 2002.


[LEE 00] LEE S.B., “INSIGNIA: an IP-based quality of service framework for mobile ad hoc networks”, Parallel and Distributed Computing, vol. 60, p. 374-406, 2000.

[LIN 99] LIN C.R., LIU J., “QoS routing in ad hoc wireless networks”, IEEE Journal on Selected Areas in Communications, vol. 17, p. 1426-138, 1999.

[LIN 01] LIN C.R., “On-demand QoS routing in multihop mobile networks”, IEEE INFOCOM’2001, Apr 2001, p. 1735-1744.

[MAN 02] MANOJ B.S., MURTHY S.R., “Real-time traffic support for ad hoc wireless networks”, IEEE International Conference On Networks, Singapore, Aug 2002, p. 335-340.

[MAN 04] MANOJ B.S., VIDHYASHANKAR V., Murthy S.R., “Slot allocation strategies for delay sensitive traffic support in asynchronous ad hoc wireless networks”, Journal of Wireless Communications and Mobile Computing, vol. 5, p. 193-208, 2004.

[MAS 06] MASIP-BRUIN X. et al., “Research challenges in QoS routing”, Computer Communications, vol. 19, p. 563-581, 2006.

[MUN 02] MUNARETTO A. et al., “A link-state QoS routing protocol for ad hoc networks”, 4th IEEE International Conference on Mobile and Wireless Communications Networks, Stockholm, Sweden, Sep 2002, p. 222-226.

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[PER 94] PERKINS C.E., BHAGWAT P., “Highly dynamic destination-sequence distance-vector routing (DSDV) for mobile computing”, ACM SIGCOMM, p. 234-244, 1994.

[PER 99] PERKINS C.E., BELDING-ROYER E.M., DAS D.R., “Ad hoc on-demand distance vector routing”, IEEE Workshop on Mobile Computing Systems and Applications, New Orleans, LA, Feb 25-26, 1999, p. 90-100.

[PER 03a] PERKINS C.E., BELDING-ROYER E.M., DAS D.R., “Ad hoc on-demand distance vector (AODV) routing”, RFC 3561, IETF, Jul 2003.


[PER 03b] PERKINS C.E., BELDING-ROYER E.M., “Quality of Service for Ad hoc On-Demand Distance Vector Routing”, Internet Draft, IETF, Oct 2003.

[ROM 05] ROMDHANI L., BONNET C., “A cross-layer on-demand routing protocol for delay-sensitive applications”, 16th IEEE Annual International Symposium on Personal Indoor and Mobile Radio Communications, Berlin, Germany, Sep 2005.

[SHA 02] SHAH S.H., NAHRSTEDT K., “Predictive location-based QoS routing in mobile ad hoc networks”, IEEE International Conference on Communications, New York, May 2002, p. 1022-1027.

[SHE 01] SHEU S.T., CHEN J., “A novel delay-oriented shortest path routing protocol for mobile ad hoc networks”, IEEE International Conference on Communications, Helsinki, Finland, 2001, p. 1930-1934.

[SIN 99] SINHA P., SIVAKUMAR R., BHARGHAVAN V., “CEDAR: a core-extraction distributed ad hoc routing algorithm”, IEEE Conference on Computer Communications (INFOCOM), New York, March 21-25, 1999, p. 202-209.

[SON 03] SONG J.H., WONG V., LEUNG V.C.M., “Load-aware on-demand routing (LAOR) protocol for mobile ad hoc networks”, IEEE Vehicular Technology Conference, Jeju, Korea, Apr 2003, p. 1753-1757.

[WAN 96] WANG Z., CROWCROFT J., “Quality-of-service routing for supporting multimedia applications”, IEEE Journal on Selected Areas in Communications, vol. 14, p. 1228-1234, 1996.

[WIL 01] WILSON J.W., The Importance of Mobility Model Assumptions on Route Discovery, Data Delivery, and Route Maintenance Protocols for Ad Hoc Mobile Networks, Virginia Polytechnic Institute and State University, Virginia, USA, Dec 2001.

[XIA 04] XIAO W. et al., “QoS routing protocol for ad hoc networks with mobile backbones”, IEEE International Conference on Networking, Sensing and Control, Taipei, Taiwan, Mar 2004, p. 1212-1217.

[XUE 03] XUE Q., GANZ A., “Ad hoc QoS on-demand routing (AQOR) in mobile ad hoc networks”, Journal of Parallel and Distributed Computing, 2003, 63:154-165.

[ZHA 05] ZHANG B., MOUFTAH H.T., “QoS routing for wireless ad hoc networks: problems, algorithms, and protocols”, IEEE Communications Magazine, p. 110-117, Oct 2005.


[ZHU 02] ZHU C., CORSON S., “QoS routing for mobile ad hoc networks”, IEEE INFOCOM, New York, Jun 2002, p. 958-967.

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