We consider a wireless ad hoc network with random access channel. We present a model that takes into account topology, routing, random access in MAC layer, and forwarding probability. In this paper, we focus on drawing benefit from the interaction of the MAC (governed by IEEE 802.11 or slotted Aloha) and routing by defining a new cross-layer scheme for routing based on the limit number of retransmission. By adjusting dynamically and judiciously this parameter in a saturated network, we have realized that both stability of forwarding queues and average throughput are significantly improved in linear networks with symmetric traffic: a gain of 100% can be reached. While in asymmetric topology network with asymmetric traffic, we achieve a better average delay (resp., throughput) for each connection without changing the average throughput (resp., delay). In addition, we show the efficiency of our new scheme in case of multimedia applications with delay constraint. A detailed performance study is presented using analytical and simulation evaluation.

A multihop wireless ad hoc network is a collection of nodes that communicate with each other without any established infrastructure or centralized control. Each of these nodes is a wireless transceiver that transmits and receives at a single-frequency band which is common to all the nodes. These nodes can communicate with each other, however, they are limited by their transmitting and receiving capabilities. Therefore, they cannot directly reach all of the nodes in the network as most of the nodes are outside of direct range. In such a scenario, one of the possibilities for the information transmission between two nodes that are not in position to have a direct communication is to use other nodes in the network. To be precise, the source node transmits its information to one of the nodes which is within transmission range of the source node. In order to overcome this, the network operates in a multihop fashion. Nodes route traffic for each other. Therefore, in a connected ad hoc network, a packet can travel from any source to its destination either directly, or through some set of intermediate packet forwarding nodes.

In recent years, an increased effort was consecrated
to cross-layer design, of ad hoc networks, where information is exchanged
between different layers. In wireless context where channel conditions and
network connectivity impose serious challenges, new cross-layer approaches are
needed to optimize performances. In fact, the knowledge of channel condition in
the physical layer may help on choosing the adequate number of retransmission
in the MAC layer which also depends on the application requirement and the
transport layer utilized. On the one hand, it is also profitable for routing
protocols to know the link state of lower layers to choose the best route
possible in terms of available bandwidth, stability of links, energy
consumptions, and so on. On the other hand, giving route information to the MAC
layer can be efficient to control packet retransmissions, as we will see in
this paper. In this case, we can achieve better QoS in terms of
end-to-end delay and throughput. Moreover, by using
cross-layer approaches, we can avoid network partitioning due to both link
degradation (and thus link breakage) and rapid energy expiration. Various
cross-layering approaches are analyzed in [

To study the network performances with the interaction of various parameters from different layers, we consider in this paper the framework of random access mechanism for the wireless channel where the nodes having packets to transmit in their buffers attempt transmissions by delaying the transmission by a random amount of time. This mechanism acts as a way to avoid collisions of transmissions of nearby nodes in the case where nodes cannot sense the channel while transmitting (hence, are not aware of other ongoing transmissions). We assume that time is slotted into fixed length time frames. In any slot, a node having a packet to be transmitted to one of its neighboring nodes decides with some fixed (possibly node-dependent) probability in favor of a transmission attempt. If there is no other transmission by the other nodes whose transmission can interfere with the node under consideration, the transmission is successful. We assume throughout that there is some mechanism that notifies the sender of success or failure of its transmissions. For example, the sources get the feedback on whether there was zero, one, or more transmissions (collision) during the time slot.

At any instant in time, a node may have two kinds of packets to be transmitted as follows.

Packets generated by the node itself. This can be sensed data if we consider a sensor network.

Packets from other neighboring nodes that need to be

In [

In this paper, we use a cross layer optimization
between MAC and network layer for routing. For a given path between a source
and a destination, each intermediate node computes a new limit number of
retransmission based on a specific algorithm. This parameter can be adjusted
easily by each node. Using this new routing, we achieve a better average delay
(resp., throughput) for each connection without changing the average throughput
(resp., delay). In extreme cases, a

In most recent literature, the tradeoffs between
throughput and delay have been investigate as a key measure of the network
performance. Several studies have first focused on wireless network stability
and finding the maximum achievable throughput. Stability problem was considerably studied for the Aloha [

In recent years, there has been a considerable effort
on trying to increase the performance of wireless ad hoc networks since Gupta
and Kumar [

The problem of optimizing the tradeoff throughput
delay can have a direct impact on the QoS of multimedia applications. The
previous mentioned literatures have mainly worked on analytical models.
Moreover, many papers have proposed some heuristics and new methods to improve
the quality of real-time streaming media over wireless networks. Since the
wireless links cause a lot of challenges due to the variation of channel
conditions, authors in [

In multihop networks case, the QoS problem is more
complicated as it includes additional factors like multihop transmissions,
variation of the channel conditions, link breakage, congestion. Papers in
[

In comparison to previous works, the following distinguishes our paper.

We use a multilayer model that can track the retransmission effects on the performances. Then, in symmetric networks, it is possible to prove the efficiency of any methods that control the retransmission limit.

Encouraged by previous studies done in WLAN on the retransmission limit, we remarked that an optimization of this latter through a given path in multihop networks can increase the available bandwidth. It is a kind of congestion control that can be applied to multimedia streams.

The distributed cross-layer scheme that we propose besides of its novelty and efficiency is characterized by its simplicity. It does not need external information to the node itself, but a local decision can be taken with the help of routing information from the network layer and load estimation if needed.

The rest of the paper is organized as follows. In
Section

We model the ad hoc wireless network as a set of

Network layer
handles the two queues

We assume a channel access mechanism only based on a
probability to access the network, that is, when a node

The model of
Figure

Network layer and
MAC layer of node

We summarize the parameters and notations used in this paper for a general network topology.

(1) MAC layer notations are as follows.

(2) Network layer notations are as follows.

The maximum
number of the transmissions

In this section, we propose a new dynamic

the length of the path in number of hops; and

its position in the path in terms of the number of hops that separates it from the source.

Consider that each node has a default value of the
maximum number of transmissions set to

Under this scheme, we aim to give more chance of success to packets that had come near to their destination. It rather means that we need to avoid as much as possible loosing packets near their destination so that waste of bandwidth throughout a path becomes lower. In other terms, we expect to reduce the number of wasted slots in each connection.

Normally, the way of choosing good

In this
section, we evaluate the performance of the dynamic scheme for a symmetric
linear and an asymmetric network. We specify and detail our dynamic scheme for
choosing

Our purpose by
studying the linear network case is to understand the advantage of the new
dynamic scheme so that we can understand the efficiency of the method on the
asymmetric case. Also, the linear symmetric network is simple to study
[

We will mainly study the stability-throughput issue with the new scheme. In addition, the probability of success and the average number of transmissions bring additional material to understand what is happening in the network. Later, in the asymmetric network section, the end-to-end delay gives more information about stability of connections; while in the symmetric linear network, the stability of a node is equivalent for all nodes, thus the variation of a node stability informs about delay variation.

General expressions for performance evaluation in
general networks were already derived in our paper [

Now, to determine

We use a simple method that gives a dynamic value of

This method maintains an average

Practically, for each packet transmission, a node

As an exterior observer point of
view, the

For example, when

We draw some
numerical results of the above formulas using the previous parameters

Average number
of transmission

This remarkable amelioration is mainly due to the following facts.

The dynamic scheme privileges the forwarded packets that come near the destination. It is better to encourage these packets to reach their destination, otherwise, the network will suffer more wasted bandwidth.

The flow of packets from each source is limited on the first hops of each connection. If the network cannot support transporting more packets on a connection, it is better to limit the flow of new entering packets in the network. This is a load moderating issue.

As a consequence, Figure

Stability region
for the linear case with static and dynamic

We distinguish two states of the network for two
contention degrees when we use the dynamic scheme. These are shown on Figure

Average
Probability of success for the linear case with static and dynamic

Average
throughput for the linear case with static and dynamic

(i) The

(ii) The

These two states can merge to one state for some
values of

We study the impact of

(1)

Gain ratio of
dynamic case compared to the static one versus

(2)

Gain ratio of
dynamic case compared to the static one versus

Gain ratio of
dynamic case compared to the static one versus

Gain ratio of
dynamic case compared to the static one versus

(3)

Consider an asymmetric static wireless network with 11
nodes as shown in Figure

Ad hoc wireless network.

Let

Values of the
dynamic maximum number of transmissions for

Nodes | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 10 | 11 |
---|---|---|---|---|---|---|---|---|---|

Conn. | |||||||||

Conn. | |||||||||

Conn. | |||||||||

Conn. | |||||||||

Conn. |

Values of the
dynamic maximum number of transmissions for

Nodes | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 10 | 11 |
---|---|---|---|---|---|---|---|---|---|

Conn. | |||||||||

Conn. | |||||||||

Conn. | |||||||||

Conn. | |||||||||

Conn. |

Stability region

Secondly, as the nodes

Stability region

Throughput of the dynamic
(

Thirdly, the end-to-end delay of a
connection gives a global vision on the stability of nodes that forms this
connection. Precisely, it is mainly affected by the waiting time on the
forwarding intermediate queues. In Figure

Delay of the
dynamic (

Delay of the
dynamic (

What about the gain percentages of our scheme compared
to the static one? On one hand, we can observe from the presented Figures

Throughput of the
dynamic (

Delay of the dynamic (

Here, we start our discussion on the observations of previous figures of the asymmetric network. In summary, the dynamic scheme has better performances when some of the source nodes collaborate by forwarding packets and when these kinds of sources are well distributed in the network. In fact, there are two properties that help a connection to get a good performance:

a connection must include within its intermediate nodes a source node of another connection;

the source node of a connection must not forward packets.

The first one
ameliorates the delay and the second one the throughput. These two properties
are jointly found in connections

When one of these two properties is not found, then
three

maintains unchanged one performance criterion (throughput or delay) and ameliorates the other one;

deteriorates one performance criterion and ameliorates the other one;

maintains unchanged the performance.

Here, we consider only three connections

Stability region

Throughput of the
dynamic (

Delay of the
dynamic (

These observations correspond to the second

If the average queue size exceeds a given
threshold, then go to the second step, else do nothing. The average waiting
time in the queue can be another criteria to decide whether to use the

Choose judiciously a connection (according to its data type and if it is not yet chosen) between those traversing it.

Apply the reset of

In our example, the

Stability
region

Throughput
of the dynamic (

Delay of
the dynamic (

After we have seen in general the performances of the scheme, here we study the performances for multimedia applications with delay constraint. Delay, jitter, and packet loss are the main factors impacting audio quality in interactive multimedia applications. In ad hoc network, the audio packets transmitted from a source to a destination can encounter variable delay while crossing the intermediate nodes. In order to play the receiver stream, an application must buffer the packets and play them out after a certain deadline to get again a periodic stream at the application level. Packets arriving after their corresponding deadline are considered lost and are not played out. For that, we need to fix a delay limit needed for some type of application.

Let

From Figure

The effective
throughput of the dynamic (

The effective
throughput of the dynamic (

The effective
throughput of the dynamic (

The effective throughput
of the dynamic (

In this paper, we have presented a new cross-layer scheme using the limit number of retransmission parameter in a saturated ad hoc network, so a dynamic routing can be achieved. The advantage of this scheme consists of the following.

It is a simple distributed cross-layer scheme that can be implemented easily on each node.

It does not need external information about neighboring or other things, but the decision of setting the limit number of retransmission can be taken locally at each node with the help of routing information from the network layer.

The limit number of retransmission has a direct impact on the end-to-end throughput and delay in the saturated network case.

It mainly controls packets flow by allocating priority to some of them using route information. It is thus efficient in congestion situations.

An advanced configuration of the scheme is
possible by also adjusting the step parameter

The performance evaluation study using analytical and
simulation tools has shown that in the case of symmetric linear networks the
scheme significantly improves the stability and the throughput for all
transmission probabilities. We have also studied the impact of several parameters
such as the maximum length of connections and seen that we take benefit from
large connections. On the other hand, asymmetric networks performances are
directly related to the topology and the neighboring distribution. However, we
have identified two properties that a connection must have to get both the
throughput and the delay ameliorated. If one of these is not presented, then
connection performance can be classified on one of 3

Moreover, it will be interesting to apply our results in this paper to study its exact behavior using the IEEE 802.11 DCF operation. In fact, an extension of our model presented in this paper is sufficient to study analytically the problem, and some simulations can track the real functioning of the network. It is always possible to enhance the scheme according to the applications need or according to the complexity that we allow. It can be also integrated with other schemes. A content-based dynamic retransmission can be one of the directions to enhance the scheme for video streaming over wireless. In fact, based on the priority of multimedia packet, we can adjust the retransmission limit locally in each node. Therefore, we get a cross-layer-based APP/NETWORK/MAC layers. Improving the QoS of multimedia traffic in multihop ad hoc networks is a real issue, specially, if we need to work on with analytical multilayer model. This is an open issue since high interactions and correlations may exist between different layers in the same node and different nodes in the same network.