This chapter investigates the overheads, i.e., the control packets generated by MDSDV.
Each routing protocol uses a number of control packets and has its own strategy to build routes. MDSDV uses five types of control packets to maintain its routes as discussed in section 5.3. In this chapter, we investigate the control packets that are generated and used by MDSDV. Specifically, we investigate all control packets in subsection 6.2.1, Full Dumps in subsection 6.2.2, Update Packets in subsection 6.2.3, Error Packets in subsection 6.2.4, Hello Messages in subsection 6.2.5, and Failure Packetsin subsection 6.2.6. We conducted all our simulation experiments using the Network Simulator NS-2 [33] (version 2.30).
Although most researchers use the number of control packets to measure the over-head, there are other metrics that can be used to measure overhead (e.g., number of control bytes, memory overhead, energy overhead). The total number of control bytes transmitted is one of the metrics. It includes not only the bytes in the routing con-trol packets, but also the bytes in the header of the data packets [73]. The memory overhead can be described as the size in bits of all the data structures used by the routing protocol [98]. The energy overhead is the energy level associated with each transmission and the power spent by receivers. All these measurements show similar behaviours to the number of control packets, so we use that metric.
While this chapter focuses solely on MDSDV overheads, chapter 7 and 8 also report on overheads. Chapter 7 compares the Normalized Routing Load (NRL) of MDSDV and DSDV in a number of scenarios, and Chapter 8 compares the NRL of MDSDV, AODV, and DSR.
6.1 Simulation Environment
Our simulation environment uses similar traffic and mobility models to [26][27][60].
The evaluation is based on the simulation of 50 nodes forming a network over a 670 x 670 square meter area. Nodes move according to the widely used random waypoint model [11][15][63]. In this model, each node begins the simulation by remaining stationary for pause time seconds. It then selects a random destination in the 670m x 670m space and moves to that destination at a speed distributed uniformly between 0 and some maximum speed. Upon reaching the destination, the node pauses again for pause time seconds, selects another destination, and proceeds there as previously de-scribed, repeating this behaviour for the duration of the simulation which is 200 sec-onds. The Distributed Coordination Function (DCF) of IEEE 802.11 [24] for wireless LANs is used as the MAC layer protocol. We fix the number of nodes at 50 nodes where each node has a 250 meter propagation radius. Meanwhile, we varied the pause time and speed of nodes to illustrate the impact of mobility and speed on the number of control packets generated by MDSDV. We run our simulations varying the pause times from 0, 50, 100, 150 and 200 simulated seconds obtaining a range of scenarios that span continuously moving nodes to static ones. We varied the maximum node speed among values 1, 5, 10, 15, 20 and 25 m/s.
The traffic is generated by 10 Constant Bit Rate (CBR) sources spreading the traffic among all nodes. The sending rate was set to 4 packets per second, and the data packet’s size was set to 512 bytes. Each data point represents an average of thirty runs with identical traffic models, but different randomly generated mobility scenarios.
Results are based on simulation of 30 runs, and the error bars represent the 95% con-fidence interval of the mean. Table 6.1 lists the parameters used for the simulations.
Parameter Value
Simulator NS-2
Simulation time 200 seconds
Area of the network 670m x 670m
Number of nodes 50 nodes
MAC layer IEEE 802.11
Transmission range 250 m
Pause time 0 , 50, 100, 150, and 200 seconds Maximum speed of nodes 1, 5, 10, 15, 20, and 25 m/s.
Mobility model Random waypoint
Traffic type CBR (UDP)
Number of data sources 10 Sources
Packet size 512 byte
Transmission rate 4 packets/second
Bandwidth 2 Mb/s
Link failures models Link failure detection method of MAC layer and TimeOut of beacon packet
Number of runs per data point 30
Table 6.1: Simulation parameters used to evaluate the control packets generated by MDSDV
6.2 Simulation Results
6.2.1 Control Packets
In this subsection, we analyze the total control packets that are generated and trans-mitted by MDSDV. From Figure 6.1 and Table B.1, we observe that the number of control packets transmitted in static networks (pause time 200) at all speeds is very similar, whereas it increases as the mobility increases.
Figure 6.1: Control Packets as a function of Pause Time
Also, the other parameter of mobility (speed of nodes) has an impact on the number of control packets, where the number of control packets increases as the speed increases.
From the first row of table B.1, we see that in highly dynamic networks (Pause time 0), MDSDV transmits at low speed (1 m/s) only about 27% of control packets that are transmitted at high speed (25 m/s). This is because nodes move slowly at low speed. As a result, topology changes happen rarely. In other words, nodes do not discover new neighbours frequently and hence do not need to unicast Full Dumps frequently. Also nodes do not discover broken links frequently and hence do not need to broadcast Error Packets frequently. Thus, the number of Full Dumps and Error Packets is reduced. It is interesting that the number of control packets transmitted at
high mobility (Pause time 0) with low speed (1 m/s) is similar to the number of control packets transmitted at low mobility (pause time 200) with high speed (25 m/s).
Moreover, we observe that the 4653 control packets transmitted in high mobility (pause time 0) at high speed (25 m/s) is four times greater than the 1274 control packets transmitted in high mobility (pause time 0) at low speed (1 m/s).
6.2.2 Full Dumps
Figure 6.2 and Table B.2 show the number of Full Dumps unicasted during the sim-ulation. When any node receives any type of control packet from a new neighbour, it responds by unicasting a Full Dump to that neighbour as described in Chapter 5.
We found that the number of Full Dumps is very low and similar in medium mobility (pause time 100 sec) and low mobility (pause time 200 sec) at all speeds, whereas it increases as the mobility increases and the speed increases.
This is because Full Dumps are unicast only when discovering new neighbours. This happens rarely at low mobility and at low speeds, and happens frequently at high mobility.
Figure 6.2: Full Dumps as a function of Pause Time
6.2.3 Update Packets
Figure 6.3 and Table B.3 show that neither the mobility nor speed of nodes has an impact on the number of Update Packets. This is because Update Packets in MDSDV are time-triggered only, i.e., there are no event-triggered updates. As a result, the number of Update Packets will be very similar in both dynamic and static networks at all speeds.
Figure 6.3: Update Packets as a function of Pause Time
6.2.4 Error Packets
Figure 6.4 and Table B.4 show the number of Error Packets that are broadcast during the simulation. As shown in Figure 6.4, number of Error Packets increases as the mo-bility increases. Also, the number of Error Packets increases as the speed increases.
This is because Error Packets are broadcast when broken links are discovered, and the probability of links breaking increases as the mobility increases.