A modified power saving mode in IEEE 802.11 distributed coordinator function
Introduction
With the rapid development of wireless communication technology, computing and communication anytime, anywhere and to anybody else is made possible by providing the wireless networking facility to all kinds of mobile devices [19]. These wireless devices are often powered by batteries in order to achieve mobility and flexibility. Furthermore, transmitting and receiving data through the wireless radio transceiver is costly from the energy consumption perspective, especially when a random-access wireless radio channel is shared among many users. Due to the constraint of weight and capacity, batteries can provide only a finite amount of energy. Unless there is a significant break through in battery technology, research on power-efficient method is always worthwhile.
As one of the most popular wireless technology, Bluetooth [1] is characterized by its inexpensiveness and high availability, but it is only considered as a cable-replacement alternative because of a relatively small bandwidth and short communication range. With the growing demand for bandwidth, IEEE 802.11 based wireless LAN [5] is gaining an important role in the development of wireless network. With the increased transmission range and high speed transmission capability, IEEE 802.11 based wireless LAN leads to a high energy consumption [17] which contributes to as much as 20% of the total energy consumption of a laptop computer equipped with wireless network interface [2], [3]. However, as far as handheld devices are concerned, the power consumption of communication is even more significant, because typically handheld devices are not equipped with hard disk and the screen is much smaller than laptop computers [4].
Understanding of the power consumption model of mobile radio used in wireless devices is important for efficient design of communication protocols. A wireless interface can be in transmit state, receive state, idle state (any of these three states is considered as an active mode), or in doze state (also known as power saving (PS) mode). As denoted in IEEE 802.11 [5], a station is fully powered in active mode and is able to transmit and receive packets at any time or wait in an idle state. According to Ref. [6], it is found that even in the idle state significant power is consumed (about 20 times more than the PS mode) while listening to the transmission of other stations and monitoring the channel, which is considered as a major power wastage [10]. The basic idea in power saving is to put the node in PS mode as long as possible without affecting the network performance and connectivity [[7], [8], [15]].
The power saving mechanism specified for Distributed Coordinated Function (DCF) is a power-efficient mechanism operating in a totally distributed manner. Time synchronization is required to decide the start and end time of each Beacon Interval, as well as the Announcement Traffic Indication Message (ATIM) window, which is a common awake period for all the nodes within the Independent Basic Service Set (IBSS). The ATIM window is utilized to announce the pending data packets using small control frames. During the rest of beacon period all nodes that have transmitted ATIM announcement and have received the ATIM ACK will have to contend for the medium using standard DCF access procedure to send their multicast/broadcast or unicast data packets. In this paper, we introduce a scheduling mechanism at the end of ATIM window by having a node act as the centralized scheduler to send out the Time Synchronization Function (TSF) beacon along with explicit transmission ordering information, which can organize the data packet transmission phase to achieve the benefit of centralized protocol. The proposed protocol takes advantage of centralized scheduler. However, all nodes operate under the distributed mode and no infrastructure is required.
The rest of the paper is organized as follows. Section 2 presents a detailed overview of existing PS mode specified for DCF of IEEE 802.11 and Section 3 is a review of related power saving mechanisms for wireless network. The proposed modification of IEEE 802.11 DCF PS mode is presented in Section 4. Simulation results and performance comparison are shown in Section 5. The analysis and discussion are given in Section 6. Conclusion will be addressed in Section 7.
Section snippets
Power saving mode specified for IEEE 802.11
In the IEEE 802.11 [5], [9] PS mode specified for DCF, it is assumed that all nodes in the same IBSS are fully connected. In such a single-hop ad hoc network scenario, time synchronization is easily achieved by periodically sending TSF beacon. Transmission time is therefore divided into beacon intervals. At the start of each beacon interval, each node in the PS mode periodically wakes up for a duration known as ATIM window and TSF beacon is sent at the beginning of each ATIM window. As a result
Related work
Several power management protocols have been proposed for wireless LAN. SPAN [11] is a power saving technique that adaptively elects a group of coordinators from all nodes in a network and rotates them in time. The coordinators always stay active and perform multihop packet routing within the ad hoc network. Non-coordinators follow the power saving mechanism in IEEE 802.11. Nodes buffer the data packets while in PS state and announce these packets during ATIM window. In SPAN, modification is
Proposed modification of IEEE 802.11 DCF power saving mode
In this section, we show how the existing IEEE 802.11 DCF PS mode is modified to achieve better performance. In order to implement the scheduling mechanism, we modified the function of the beacon packet in traditional IEEE 802.11 PS mode. A successfully transmitted beacon packet will be received by all nodes within the IBSS. Considering this characteristic of the beacon transmission, it is very natural to utilize the centralized schedule through the beacon packets.
It is assumed that nodes are
Simulation of proposed modification of IEEE 802.11 DCF power saving mode
We evaluate the performance of the proposed modification to IEEE 802.11 DCF PS mode using simulation programs written in C++. The simulator follows closely both the protocol details of the PS Mode of 802.11 (802.11 PSM) and our proposed algorithm (S-PSM) including beacon generation, ATIM window and contention based DCF access procedure [16].
We have simulated the wireless LAN environment in which all nodes are within transmission range. This implies that the time synchronization is not a problem
Analysis and discussion on the simulation
In the simulation, we used various packet generation rates and the ATIM window sizes to compare the performance between IEEE 802.11 PS mode and our proposed algorithm.
The ratio of average sending attempt per received packet is a very important metric that shows the status of channel contention of data packets. It is clear that the collision probability of ATIM packets is increased when the number of nodes is increased (given that all nodes have equal probability of sending and data packets are
Conclusion
In this paper, a modified power saving mechanism is proposed based on the PS mode of IEEE 802.11. Simulation results show that up to 70% of the total energy can be saved compared to that in PSM of IEEE 802.11. Probability of collision is greatly reduced by a schedule that insures the data transmission to be contention-free. So our proposed algorithm is more scalable when the number of nodes in an IBSS is increased. The proposed mechanism maintains the network performance under different
Acknowledgements
The authors would like to thank Dr Sukumar Nandi, Department of Computer Science and Engineering, Indian Institute of Technology, Guwahati North-Guwahati, India, for his insightful suggestions, comments and advices on various aspects of this work.
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