Materiales y métodos

This chapter presented, in detail, the design of the proposed data dissemination system for opportunistic networks. From the literature review in chapter 2, we highlighted the limitations of existing approaches which were taken into account in our design. We have set up three main objectives our system aims to meet, which are do not rely on mobile node cooperation, to be able to employ the mobility characteristics of users, and to be a decentralized system. To simplify our design, we have presented a set of assumptions at the start of this chapter. Since most of the proposed approaches rely on mobile nodes cooperation, we have taken a different approach which employs stan- dalone fixed wireless devices to relay data objects instead of using mobile nodes. The RDD design composed of four parts, which are data object management, user prefer- ences management, data dissemination protocol, and the repository placement. Firstly, in data object management, we have defined how data objects created, what form of data objects the system can deal with, and how the system manages data objects within the device buffers. Secondly, in user preference management, we have defined how the system organizes user channel subscriptions. Thirdly, in data dissemination, we have defined the dissemination protocol which is responsible for pushing data objects between devices. Finally, we have defined the placement strategy which is used by the system to place the repository and for this purpose we have defined social betweenness, a placement strategy which employs the knowledge of the user’s preferences as well as the geographical location in determining the suitability of a location.

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Chapter 5

RDD Evaluation and Results

This chapter presents the analyses of results in order to qualify the benefits of using the RDD system for data dissemination in opportunistic networks. The study was carried out using the evaluation model presented in chapter 3. First, the system performance metrics are presented which were used in different tasks in the evaluation process. Next, details of how the benchmark datasets created are presented. Before presenting the results we justify the validity of the simulation experiments. The evaluation pro- cess starts by comparing the repository location strategies presented in chapter 4. Next, using the best location strategy, RDD is evaluated by comparing network data deliv- ery rate and redundancy rate with that of the epidemic protocol in identical network scenarios. Finally a conclusion of the evaluation experiments is presented.

5.1

Performance Metrics

Many metrics are commonly used to measure the performance of network protocols. This section presents the metrics used in our evaluation process.

LetDT be the set of data objects created in the entire duration of the simulation time (denoted as T), i.e. time period (0, T), whereDT = {d1, d2, d3, ..dNd}. Let Dt(m)

refer to the subset of DT carried by a given mobile node m at time t, and let Dt(r) refer to the same for a repository noder at timet. The data objects carried by a mo- bile nodem or similarly repository are placed into subsets based on whether they are

available to be forwarded to further nodes or not. For a nodemthe maximum number of data objects are allowed to be saved by the node is defined as the buffer size for

m. Figure 5.1 shows an overview of data objects management within a node itself and when two nodes encounter.

Received List: The listDrc

t (m)contains all data objects which have beenreceivedby a nodemin the time period[0, t]. As shown in Figure 5.1, a data objectdis added to

Drc

t (m)only due to the occurrence of aSend event.

• Send event (event 11 and 12 in Figure 5.1) is responsible for pushing a data object from a nodes(either a mobile node or a repository) to a nodemwhich is interested in that data object, i.e. ifsencountersmand there existsd∈ Dwt

t (s) (whereD_twt(s)includes all data objects carried bysat timet which are waiting to be sent to other nodes) such thatc(d) ∈S(m), anddhas not already seen by

m,( i.e. d /∈ D_trc(m)) thens pushes d tom and it is added to D_trc(m). Every time a data objectd is added toDrc

t (m), the data objects inDtrc(m)are sorted by expiration time of data objects in ascending order, i.e. data objects with the earliest expiration timesx(d)are listed first.

Figure 5.1: Data object status, showing movement between buffers and lists when two nodes encounter each other .

5.1 Performance Metrics 83

Waiting list : The listDwt

t (m)includes all data objects carried by mat timet which are waiting to be sent to other nodes. The listDwt_t (r)includes all data objects carried byr at timetwhich are waiting to be sent to other nodes. Note that any data objects

d∈Dwt_t (m)ord∈D_twt(r)are removed whenx(d)< t.

As shown in Figure 5.1, a data objectdbelongs to the listD_twt(m)for mobile node

mdue to the occurrence of one of the following:

• A generating event (event 0 in Figure 5.1) is responsible for generating a new data object by the node. I.e. ifmgenerates a new data objectdwith channelc(d)

and time to livettl(d)at timet, (i.e. ct(d) = t), thendis added toD_twt(m)and

x(d)is defined in Equation (4.1).

• Receive and relay event (event 3 in Figure 5.1) follows a send event and is re- sponsible for copying a data object from a node’s received list to it’s waiting list provided the node willing to relay. That is ifmhasd∈D_trc(m)andmis willing to relay thendis copied toDwt

t (m)providedx(d)< t.

• Relay only event (event 10 in Figure 5.1) and is responsible for pushing a data objectdfrom a nodemAto another nodemB which is not interested in the data object (i.e. c(d) ∈/ S(mB)) but is willing to provide relay. That is if mA has

d ∈ Dwt

t (mA) and encounters mB who is willing to relay and d /∈ Dtwt(mB) thenmApushdtomB anddis added toDtwt(mB).

A data objectdbelongs to the listDwt

t (r)for a repository r only due to the occur- rence of theRelay only event:

• Relay only event(event 10 in Figure 5.1) is responsible for pushing a data object

dfrom nodem to another node. I.e. ifm hasd∈Dwt_t (m)and encountersrand

d /∈Dwt

Every time a data objectdis added to Dwt

t (m), the data objects inDtwt(m)sorted by expiration time of data objects in ascending order, i.e. a data object with earliest expir- ation timex(d)listed first. Although we have assumed that buffer size is unlimited, any data objectsd∈D_twt(m)ord∈D_twt(r)are removed from the buffer whenx(d)< t.

In addition to these two lists, there are other two counters (sent anddelivered) which have been defined for analysis purposes.

1. TheSentcounter, counts all data objects that have been sent by a node (mobile or repository) using (relay-only and send events) during the simulation regardless of the subscription of their destinations. Cs

T(m) is a counter for the number of data objects has been sent by a node m regardless of their destinations and

Cs

T(r)is a counter for the number of data objects has been sent by a repositoryr regardless of their destinations.

1. TheDelivered counter, which counts all data objects that have been sent suc- cessfully to a target, i.e. C_Td(m)is a total number of all data objects received by

mwherec(d)∈S(m).

The metrics used in our evaluation process are presented now, while the follow- ing metrics are (technically) only defined for RDD, but used for EPR too. These metrics are as follows:-

1. Data Delivered(DD): This metric gives an indication of the ability of the protocol to deliver data to its destinations, i.e. how many messages successfully delivered to in- terested nodes.DDis defined as the total number of messages have been successfully delivered to its destinations for all mobile nodes in the entire duration of the simulation time and is defined by Equation (5.2).

DD=

|M| X

m∈M