MAC Throughput Improvement Using Adaptive Contention Window

Author(s): Chun‐Liang Lin, Wei‐Ting Chang, Min‐Huei Lu

Abstract
HomePlug AV (HPAV) is a standard developed by HomePlug Powerline Alliance (HPA) for power line communication. In HomePlug AV, it uses a technology named Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) to reduce collision happened in network. However, when network nodes increase, the contention window number may not be wide enough. It will cause collision probability to increase. In this paper, we introduce a new idea of adaptive contention window which will produce suitable contention window under actual network environment. Our method only requires the information of CSMA/CA parameters. It means that one doesn’t need to correct the original CSMA/CA procedure but substitutes old parameters by the new ones. Simulation experiments conducted in the network simulator NS3 show that compared with HomePlug AV, our method promotes throughput significantly when the node number increases.

Introduction
Power-Line Communication (PLC) is developed rapidly in recent years. Because it doesn’t need to build additional transmit channel and exists almost everywhere even in the backward areas, it is mostly recognized as the solution of “last mile” to the current network communication. Since power line was not originally built for information transmission, there are several problems. One of these is the data collision. When channel transmits multiple packets at the same time, packets will collide naturally causing packets to be destroyed. The station has to retransmit packets which will cause throughput decrease in the end. Thus, HomePlug AV [1] uses CSMA/CA [2] to avoid collision happening. The principle is as follows: each time the station tries to send package, it will
detect channel’s status. If the channel is idle, the station sends the packets. Otherwise, it goes into the contenti onmode. This method can avoid too many stations transmitting at a time hence to effectively reduce the chance of data collision.

A PLC architecture consists of head end (HE) and user modem. HE erects at the transformer side and one side is connected to the transmission backbone, like Ethernet. Another side is connected to the power line to receive request from PLC users. The user modem connects to the communication equipment to communicate through the power line. If a PLC modem accesses the Internet, it will receive packets from the head end through the power line.

Although HomePlug AV standard can guarantee high throughput when the number of nodes is very limited, when the node number increases, the value of contention window setting in HomePlug AV will be big which leads throughput to decrease accordingly. In [3], information theory and data mining technique were applied for network traffic profiling. Some research tasks focused on enhancing throughput via the revised MAC layers [3] [4]-[8]. Among which, the authors of [3] proposed an adaptive contention window mechanism for HomePlug AV and verified their designs through experiments. They have conducted an experiment to reach the best successful transmission number in a beacon period. That is, if the number of successful transmits is less than the optimal one, it means that the current contention window size is inappropriate. It will then change it in the next beacon period. In 2011, the authors of [4] found the optimal contention window value for the situation of different nodes. Both of them show that throughput can be improved in an efficient manner if the size of contention window is optimized.

Even though previous research can achieve the desired effect, most of them require information of all stations. That means that they need another bit to transmit that information to each node. Or, they have to modify the protocol. In this research, we propose a new strategy of an adaptive contention window mechanism that doesn’t change the original CSMA/CA procedure or require station’s information as a basis of modifying the contention window size. Our correction factor includes the CSMA/CA parameters. Soft experiments at the network simulator, NS3, show the improved effect in throughput.

Source:

Journal: Journal of Computer and Communications
DOI: 10.4236/jcc.2015.31001

(PDF)
Paper Id: 53493 (metadata)

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