A review of: Understanding Packet Delivery Performance In Dense Wireless Sensor Networks

A review of:

Understanding Packet Delivery Performance In Dense Wireless Sensor Networks

Jerry Zhao & Ramesh Govindan

The First ACM Conference on Embedded Networked Sensor Systems (Sensys'03), November 2003.

    This paper talks about several experiments results based on three different kind of environments : an indoor office building , a habitat with moderate foliage , and an open parking lot. In order to address the problem of packet delivery performance in dense wireless sensor networks. Basically, the key point which the author tries to figure out is that by measuring at physical & MAC layers packet transmission to get understand of packet delivery in dense sensor networks.

    The motivation of this paper is that wireless sensor networks can be deployed in harsh environment by using low power radio (that means not much frequency diversity) and can be densely deployed. Hence, they try to get a quantitative understanding of packet delivery by physical-layer measurement and MAC layer measurement. So I am wondering why we need focus on packet delivery? As my own opinion, the main reason is that it is the very basic element for a wireless sensor network. Moreover, packet delivery radio determines energy efficiency and network lifetime, which is important for the real world design. Hence, by studying the packet delivery we can avoid the poor packet delivery situation , so that may help improve the performance of applications . Moreover , it is also very important for evaluating almost all communication protocols. As the paper mentioned before a low-power RF transceivers for multiple short hops is more energy efficient than a single hop over a long range , so according to these experiment results people can experimentally verify WSN design principles.

The authors use mica motes based on an experiment software to test the packet delivery performance under three different environments : the indoor environment (office building), the natural habitat and the empty parking lot . The key result from these experiment is the heavy-tailed distributions of packet losses . For example , in an indoor setting , half of the links experience more than 10% packet loss , and a third suffer more than 30% loss. There also has some interesting point in different layers . In the physical layer , receivers suffer choppy packet reception, in some case, gray area is 1/3 of the common range. In the MAC layer, the packet loss is heavy-tailed.50%-80% common energy is wasted to overcome packet collisions and environmental effects. About 10% of links exhibit asymmetric packet loss . This kind of phenomena can be caused by several wireless communication problems.

As my own knowledge of view, it may be caused by hidden node problem. Say , node A transmits to B, node C cannot hear it and transmits to B, so they both collision at node B. It may be caused by multipath problem. A radio signal is reflected by obstacles and parts of the signal may take different paths to the sink, hence confusing the receiver. Here's a link to illustrate it                                                                    

http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008019f646.shtml#summary

And also it may be caused by signal attenuation, etc. After get many tests data from the experiments . I can simply conclude what I have learned from these experiments in two aspects. One is that , by selecting a shortest path simply based on the geographic distance or hop count is not sufficient. And another one is that , nodes need to carefully select neighbors based on the measured packet delivery performance. And another important thing we can learn from their experiment result is that there's no specify relationship between signal strength and packet delivery performance. In other word , signal strength cannot estimate the link quality by itself, there may have other conditions to be concerned. About the gray area, is it possible for a sophisticated physical layer coding to mask it? According to their test , the answer is not necessary , because the SECDED has the lowest effective bandwidth.

    In a word , this paper performed experiments to understand packet delivery performance in dense sensor network deployments , quantify the prevalence of gray are. But still, many causes for the tested phenomena are not for sure , most of them are conjectures , guesses, etc.