The Expected Transmission Count

Published Date: 02 Nov 2017

In order to calculate the ETX for a link to a neighbour, we need to know the neighbour's idea of the link quality, i.e. the NLQ, as we can only determine the LQ ourselves, but we want to know ETX = 1 / (NLQ * LQ). So the link quality extensions to OLSR introduce a new kind of HELLO messages, which we call LQ HELLO messages. For each link listed in such a message, the originator of the messages also tells us the link quality. So, each neighbour puts the LQ values that it has determined in the message, which from our perspective are NLQ values. So, owing to the LQ HELLOs we now have all the information to calculate the ETX for each link between ourselves and one of our neighbours.

Let's again have a look at the total number of transmissions required for a route that consists of more than one hop, i.e. that is not a route to one of our neighbours. If we stick with the above example, we know ETX1 from the LQ HELLOs. But how do we learn ETX2 and ETX3? For this the link quality extensions to OLSR introduce a new kind of TC messages. TC messages are used in OLSR to tell the world, i.e. all other nodes in the MANET, which neighbours we have. We have extended TC messages to additionally carry information on how good the links to our neighbours are. We call this extended variant of TCs, analogously to LQ HELLOS, LQ TC messages.

So, with LQ HELLO messages we find out which neighbours we have and how good our links to them are and with LQ TC messages, we share this knowledge with all other nodes and all other nodes share their knowledge with us.

In this way each node in the network ends up knowing which links each other node in the MANET has and how good they are. Well, actually, it's a bit more complex than that, because of an optimization called multi-point relaying. But this is beyond the scope of this introductory text.

OLSR-ETX [22] is the extension to the OLSR in which the Expected Transmission Count is introduced. The ETX metric is very useful for selecting the path but it has a drawback of queue availability management and reduces the system performance [22]. Therefore, sometimes the multimedia packets like the video and audio packets suffer from loss, delay or jitter and the overall system performance will degrades as a result.

OLSR-ETX finds the paths that have the least expected number for the transmissions of the packets. It also makes sure that the packet is delivered to the destination and the acknowledgment of the received packet is obtained by the source.

To calculate the quality of link, ETX I have the formula as

ETX = 1 / (NLQ * LQ)

Where NLQ means neighbour link quality and LQ means link quality.

Thus, in the OLSR-ETX, the simple HELLO message is replaced by the LQ HELLO message with respect to the NLQ which is used to determine the link quality.

5.5 OLSR-ML (Minimum Loss)

OLSR-ML is an OLSR link quality extension with minimum loss metric. MPR selection is based upon link quality information from the neighbour. The best MPR is the one to route to any 2-hop neighbour. MPR selection in OLSR-ML is same as in the RFC-3626 because there is lack of traffic.

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Figure 5.: Re-mesh in OLSR-ML

The probability of successful transmission is based upon forward and reverse link delivery ratios. The delivery ratio is the probability that a data packet successfully arrives at the next hop. Expected probability of transmission is successful and acknowledge is the product of the probability of the forward delivery ratio and probability of reverse delivery ratio of the link:

Plink = df x dr

The best route of source is the highest probability of successful transmission of one source to another. i.e.; the minimum loss probability.

5.6 OLSR-MD (Minimum Delay)

The main idea behind the Minimum delay is to measure the link delay between the nodes. It is calculated through the Ad-Hoc network. Therefore, all calculations of routing tables are based upon each neighbouring nodes. Therefore OLSR-MD is the protocol with the route selection between the current node and the other nodes in the network which have the lowest sum of different transmission delays of all the links along the path.

Chapter 6

MPR Selection in OLSR

By making the use of the HELLO messages, the nodes of the OLSR get to know about the neighbours up to the two-hop neighbourhood. By using the information gained via the HELLO message, the Multi Point Relay (MPR) is chosen. In the conventional or the original OLSR, THE MPRs are chosen in accordance to the range of the two-hop neighbourhood but with the versions of the OLSR, this selection procedure has also changed to a great extent. Now not only the range is considered but also some other factors are taken into consideration. The main aim of the MPR is to reduce the traffic. However, selecting the MPR is still NP- complete problem. Even though the nodes have the complete or whole information about the network, still it is very difficult to find the most optimum MPR.

The performance of the protocol depends majorly on the MPR. OLSR-ML (Minimum Loss) selects the MPR on the basis of link quality information that is a node in the 1-hop neighbourhood is chosen as a MPR if it has best route to the 2-hop neighbourhood. On the other hand in the OLSR-MD (Minimum Delay) the focused area is the link delay.The whole concept of OLSR is based on the Multipoint Relay (MPR). MPRs in OLSR are used to form a route from source to the destination. Its selection eliminates the overhead of flooding messages in the network to some extent by minimizing the redundant transmissions of the packet in the same area or region.

Node

1-hop neighbour

2-hop neighbour

MPR

X

P, Q, R

A, B, C

R

Y

T, S, V

D, E, F

S

Table 6.1: Table of MPR selection in OLSR

In the conventional way of selecting the MPR, the node selects those neighbours as their MPR which covers the farthest or the unreached node in the two-hop neighbour. This procedure is very effective but sometimes it is very inefficient because "good quality" links may be sometimes "hidden" to other nodes in the network and thus we may not get the best route for our packet to get it transmitted.

In the table, the node X chooses R as its MPR and the node Y chooses S as its MPR. However it may happen that other node like Q would be better than the MPR selected. Thus, the new versions of the OLSR not only considers the range as a factor to choose the MPR but also some other factors like minimum delay, minimum packet loss etc. This table shows how the MPR is selected in the OLSR routing protocol on the basis of different scenarios and conditions.

This helps to select the optimum MPR based on more than one factor of consideration. The versions of the protocol not only changes the selection criteria of the MPR but also differs in the functionalities some or the other way. In some cases the packet loss is maximum but the delay is minimum while in some cases the delay is much more but the packet delivery is assured. Thus each version has its own pros and cons. We can make use of these protocol version based on the scenario on which it is being used.

CHAPTER 7

Result and Conclusion

On the basis the simulation performed on all the simulation performed on all the versions of the OLSR, following results were obtained which showed their advantages and disadvantages very clearly. Here are some of the screen shots to make understand how the simulation environment was run and what were the results that were considered to be simulation outcomes.

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Fig 7.1: Generation of Beacons in the simulation environment

Fig 7.1 shows the generation of the beacons in the simulation environment. The beacons generated show the active and inactive nodes in the network. The simulator timer was set for 50 seconds. This means that within 50 seconds, the node would start sending the beacons and start functioning.

The transfer of information for selecting the MPR was also seen during the simulation process.

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Fig 7.2 MPR selection information

Fig 7.2 shows small dots within the nodes. The nodes are numbered for convenience. These dots represent the transfer of information for the MPR selection. Each node shares the information after a periodic time set earlier and thus the transfer of the topological information takes place.

This is the most important feature of the proactive protocols in which the information or we can say that the topological information is shared among the nodes in regular interval of time. In OLSR and its version also, this feature is common.

Apart from the transfer of the topological information, the packet drop and transfer of data was also observed in the simulation environment.

Some packets were also dropped during the transfer of information and packets from one node to other node.

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Fig 7.3 Packet drop and beacons

The third figure shows how the beacons that represent the active and the inactive nodes, are generated while the information was being passed from one node to the other node. The simulation process not only shows the transfer of the packets but also the dropping of the packets very clearly in it.

7.1 RESULT

7.1.1 End to End Delay

End-to-end delay refers to the time taken for a packet to be transmitted across a network from source to destination. The result shown here is obtained for OLSR, OLSR-ML, OLSR-MD and OLSR-ETX using the X graph and plotting the individual files of the trace file form NS2 obtained.

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Figure 7.4: End-To-End Delay

The results from the simulation are generally drawn in two ways. One via the X graph and another method is upon the Excel Sheet. However, the result from the X graph is considered to be the correct one as it gives the exact outcome.

The graph shown here clearly shows the performance of the OLSR, OLSR-ML, OLSR-MD and OLSR-ETX. Out of all the four protocols, the performance of the OLSR-ETX was seen to be very efficient in various scenarios. The QoETX that is the queue based OLSR-ETX is very much efficient in terms of the packet delay and loss. However, I could not implement or include QoETX in my thesis but from the extension of the OLSR-ETX, I came to know a bit about the QoETX. The most isolating feature of ETX is utilizing the queue matrix that gives cost of the fuzzy link that can be used to route the packets from one node to another. Here are some results obtained through the X graph. Some of them are:

7.1.2 Average Throughput

The Average throughput is the throughput which shows the average bandwidth of the protocol. Below are the results obtained from the trace file used along with the AWK scripts.

Protocol

Average Throughput(kbps)

OLSR

7.9

OLSR-ML

27.78

OLSR-MD

245.75

OLSR-ETX

283.04

Table 7.: Average Throughput

It was observed that OLSR had the minimum bandwidth of 7.9 kbps and OLSR-ETX had the maximum bandwidth of 283.04 kbps. Thus the average throughput of OLSR was minimum and that of OLSR-ETX was maximum

Normalize Routing Load

The NRL is the load offered on the protocol under the given scenario. Below are the results obtained from the trace file used along with the AWK scripts.

Protocol

Normalized Routing Load

OLSR

0.062

OLSR-ML

0.063

mOLSR-MD

0.094

OLSR-ETX

0.052

Table 7.: Normalized Routing Load

The normalized routing load for OLSR was 0.062 and that for the OLSR-ETX was 0.052 which was much lesser than the OLSR. However for the other two routing protocols the normalized routing load were more than the original OLSR protocol too.

Packet Delivery Ratio

Protocol

PDR

OLSR

0.58

OLSR-ML

0.53

OLSR-MD

0.72

OLSR-ETX

0.73

Table 7. : PDR

Based on all the observations and simulations I conclude with certain issues and results as:

The performance of the OLSR and its modifications OLSR-ML, OLSR-MD and OLSR-ETX performed better than the original OLSR under all the conditions. In this simulation OLSR-ETX and OLSR-ML showed the minimum end-to-end delay out of the four protocols.

Throughput of the OLSR-MD was a bit high and that of OLSR-ETX was very high than the original OLSR.

Normalized Routing Load of OLSR-MD was very high due to ETX extensive packets flooding and minimum of the NRL was in OLSR-ETX.

Packet Delivery Ratio of OLSR-MD and OLSR-ETX was better than original OLSR. As in our result the PDR for OLSR-MD was 0.72 and that of OLSR-ETX was 0.73. However the PDR for the OLSR was 0.58 which is much less than the OLSR-MD and OLSR-ETX.

7.2 CONCLUSION

The OLSR-MD and OLSR-ETX performed better than the original OLSR and the results were incomparable as the performance was very high.

Original OLSR drawbacks of low bandwidth and Throughput can be overcome by OLSR-MD and OLSR-ETX as these have minimum delay and extended packets that perform better under all the scenarios. The above mentioned scenarios clearly reflect the advantages and some disadvantages of the OLSR protocol. OLSR is designed so as to work independently that is in a completely distributed manner and does not rely or depend on any particular central control or entity

I have presented several situations of OLSR by the help of the table. This makes it very convenient to study further that what are the areas where we can still do some work regarding the improvement of the protocol and also it reflects some special scenarios where the protocol proves to be the best when compared to other protocols for the MANET. OLSR-ML (Minimum Loss) selects the MPR on the basis of link quality information that is a node in the 1-hop neighbourhood is chosen as a MPR if it has best route to the 2-hop neighbourhood. On the other hand in the OLSR-MD (Minimum Delay) the focused area is the link delay.

From the study and simulation process I came to the conclusion that OLSR-ETX was the best among all other versions of OLSR taken under consideration. The dijkistra algorithm used is one of the best suited algorithm for OLSR to find the shortest path to the destination.

FUTURE SCOPE

The protocol is based on the dijkistras algorithm to find the shortest distance to the destination node. However we can also try to implement the same with the help of other algorithms like kruskal’s or prim’s algorithm. This again will depend on the scenario of the network being concerned. Also the OLSR-MD can also be made efficient by minimizing the packet loss that occurs mostly in it.