CSMA/CD detects collisions in the transmitting station through the carrier sensing that enables the transmitting station to detect transmissions from other stations [2]. Upon detection of the collision, the transmission process is stopped and the station waits for a random interval of time before resending the frame again [7]. Generally, CSMA/CD is a medication to improve the efficiency of the original CSMA by shortening the required time to resend a frame after collision detection.
The objective of this simulation experiment is to analyse the performance of bus and hub based CSMA/CD network protocols in OMNET++ simulation models. When several devices in a communication line share the same physical connection, the type of network is referred to us bus topology. Figure 1 show the bus topology in a communication network [5]. If one stations transmit on the bus, the other stations are alerted and if more than one station transmits simultaneously, a collision occurs [1]. Therefore, CSMA/CD technique allows more than one station to transmit on the bus without collision.
For hub network configuration however, several devices are connected to one central point. Some devices use a switch or a router instead of a hub [8]. Figure 2 show hub network topology [6]. When a packet is received it the hub, it is broadcasted to all other devices connected to the hub thus all of them receive the package at the same time with the possibility of collision [3][4]. However, CSMA/CD prevents occurrence of any collision.
In this lab, four simulations are conducted using the bus and hub network configurations. From the given parameters, simulation results are obtained and analyzed. The total load was divided by the number of hosts to determine the inter-arrival time of the packets.
A client-server application is implemented using the CSMA/CD simulation model. Here, the main server is represented by host 0 while the clients are represented by all the other remaining terminals. When a request is send by the client in form of Ethernet packets, the server gives a reply. Normally, the total traffic flows in the system is two times the number of generated packets by all the network terminals. Table 1 shows the simulation parameters.
|
Simulation parameters |
Value |
|
Rate of transmission (R) |
10 Mbits/ second |
|
Packet length (L) |
Server: 1000B and Client: 200B |
|
Simulation length |
800 seconds |
|
Positions |
0,50 and 100 |
Table 1: Simulation parameters.
The previously developed OMNET++ simulation models found in the ELEC3500 folder were utilized in this experiment. By selecting a model in the run window, the model was run both in the Hub and Bus configurations mode. In cases of switched Lan selection, the line simulation initial time was reduced.
For SIMULATION I, from the traffic load values given, results for both bus and hub models were obtained. They included received channel collision and utilization, number of collisions and packet delay versus traffic lad values. The packet inter-arrival time was varied to vary the traffic load. For the hub model, server statistics and any other available terminal was used. For the bus model, server statistics, statistics of two different terminals together with a terminal at the middle and the end of the bus were utilized to obtain simulation results.
For SIMULATION II, the results of simulation I were obtained using different traffic loads and terminal numbers.
SIMULATION III, was conducted using the bus mode only to determine the effects of the bus length such as delay variability and propagation delay. The delay variabilities were compared from the vector plots.
SIMULATION IV was conducted to determine the effects of packet length on collisions and delay using the bus model by changing the size of the requested packet from the client. Two different client packets were used to determine the collision numbers and delay vector plots from the server and one of the bus terminals.
Load vs delay and load vs collisions delay plots for both hub and bus configurations are represented in the figures 3,4,5 and 6.
From the plotted graphs, there is much similarities between the hub and bus network configuration as compared to the differences. The response is almost linear for both cases.
The shorter the bus segment length the higher the performance. On the other hand, longer bus segment length leads to lower performance. This is because a shorter length permits first transmission of data packages thus minimizing collision and package loss.
The receive link utilization varies non-linearly with the traffic intensity due to package losses and high collision possibility that occurs at high traffic intensity.
A packet is generally a single unit that is transferred over time. A small packet length will take a shorter time to be transmitted thus resulting to less number of collisions as compared to a packet with longer length size. A larger packet also inhibits other devices from using the bus and it also requires larger memory size.
The delay variabilities values of the bus and hub networks are approximately the same as they represent the time that a packet takes to be transmitted from one station to another.
Generally, it is difficult to achieve 100% efficiency using bus based Ethernet LAN because of the possible collisions and package loss that may occur when the other devices sharing the line transmit data simultaneously. Also, in case of a line breakdown, the systems will not access the network.
Conclusion
CSMA/CD minimizes the number of collisions both in the hub and bus network configurations. It is generally and improvement of the initial CSMA protocol. The simulation results show more similarities in the performance of both bus and hub configurations. With the inclusion of the CSMA/CD, both the performance of the bus and hub configurations improve significantly.
References
[1] U. Black, Data communications and distributed networks. New Delhi, India: Prentice-Hall of India, 2008.
[2] S. P. H. and M. S. K., “Performance Analysis of CSMA, MACA and MACAW Protocols for VANETs”, International Journal of Future Computer and Communication, pp. 129-134, 2014.
[3] R. Moraes, F. Vasques and P. Portugal, “Survey of Real-Time Communication in CSMA-Based Networks”, Network Protocols and Algorithms, vol. 2, no. 1, 2010.
[4] S. Alumur, B. Kara and O. Karasan, “Multimodal hub location and hub network design”, Omega, vol. 40, no. 6, pp. 927-939, 2012.
[5] T. Wang, B. Zhang, Z. Yao and H. Mouftah, “Network coding based adaptive CSMA for network utility maximization”, Computer Networks, vol. 126, pp. 31-43, 2017.
[6] M. Löblich and S. Pfaff-Rüdiger, “Network analysis”, International Communication Gazette, vol. 73, no. 7, pp. 630-647, 2011.
[7] P. Gonçalves, J. Oliveira and R. Aguiar, “A study of encoding overhead in network management protocols“, International Journal of Network Management, vol. 22, no. 6, pp. 435-450, 2012.
[8] H. Singh, “Comparison of CSMA Based MAC Protocols of Wireless Sensor Networks”, International Journal on AdHoc Networking Systems, vol. 2, no. 2, pp. 11-20, 2012.
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