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This submission proposes a Distributed Contention Access Scheme to optimize the WPAN network throughput by addressing limitations in the current CSMA/CA mechanism. It highlights the need to improve the CAP Scheme to cater to busy systems and a large number of nodes for efficient data transmission. The analysis shows the drawbacks of the current backoff scheme and suggests a variable backoff approach to enhance channel access and reduce collisions, ultimately boosting system throughput. The proposal aims to encourage discussions and further development in IEEE P802.15 WPAN standards.
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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Distributed Contention Access Scheme] Date Submitted: [Sep 5, 2008 ] Source:[Paul Dixon, Pei Liu, Liang Li ] Company: [Hisilicom Technology Co. Ltd, Vinno Technologies Inc. ] Address: [Nan Tian Bld., No.10 Xinxi. Rd Shang-Di Information Industry Base, Hai-Dain District, Beijing,China ] Voice:[+1 469 229 5374], E-Mail:[pdixon@huawei.com, liupei@hisilicon.com, liangli@vinnotech.com] Re: [ IEEE 802.15.4e ] Abstract: [The requirement to upgrade CSMA performance for busy systems] Purpose: [To encourage discussion.] Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15. Paul Dixon, Pei Liu, Hisilicon.
The Features of WPAN Communication • low data rate and small packets from a single node • Large and variable number of nodes • CSMA/CA mechanism: slotted and unslotted • It is necessary to enhance the CAP Scheme to optimise the network throughput • We must consider the feasibility of optimising the CAP Scheme (based on the CAP model and calculation method of throughput) Paul Dixon, Pei Liu, Hisilicon.
The Current Scheme The current scheme works only over a small range of load As the number of competitors for access to the channel varies, many potential problems arise There can be large swings in throughput and efficiency Limits on the number of Backoffs and Retires are easily exceeded Paul Dixon, Pei Liu, Hisilicon.
The Current Scheme A simple assessment of parameters quickly identifies the dangers. The optimum range of back off values varies with the number of competitors for access For efficient operation over the full range of loads, back off limits must be variable Paul Dixon, Pei Liu, Hisilicon.
Back Off The current scheme assigns a random backoff count for attempts to gain access to the channel. This count takes a random value between 0 and 2 ^ be - 1 According to the default parameters, be starts with a value 3 and increments on each backoff to a maximum value of 5 Paul Dixon, Pei Liu, Hisilicon.
Back Off The default of the current scheme permits a maximum of 4 Backoffs before a declaring a Channel Access Failure Simulation demonstrates the fragility of this limit as Channel Access Failures can occur with as few as two competitors if there is an intense burst of activity Paul Dixon, Pei Liu, Hisilicon.
Back Off Using the Backoff Period as the unit of time Assuming the average random number to be mid value, four backoff periods corresponds to 44 units of time Typical period of a Transmission is 15 to 25 periods, depending on size Paul Dixon, Pei Liu, Hisilicon.
Collisions In a busy system, collisions reduce the throughput by causing periods of time to be lost during these collisions In terms of Channel Access Failures, collisions do not seem to as great a problem as Backoffs However collisions need to be minimised to enhance throughput Paul Dixon, Pei Liu, Hisilicon.
Conclusion As the system becomes busier with a larger number of competitors attempting to push the system to the limits of its throughput capacity, a new analysis of the channel access method is required This must address the assignment of the backoff variable as well as limits placed on the number of Backoffs and Retries Paul Dixon, Pei Liu, Hisilicon.
Analysis If we are to cater for n, a larger number of competitors, where the average transmission period is 20 units, then if each device could be made to cycle in sequence, it is clear that the wait of a device for its next turn could be of the order of 20n. This defines the backoff period that must be considered typical and catered for by the duration of each backoff and limit on the number of Backoffs permitted Paul Dixon, Pei Liu, Hisilicon.
Analysis Equally with n competitors, the probability of two devices beginning their CCA sequence at the same time and proceeding to collide is a function of the number od devices, n, and the range of backoff variables available Paul Dixon, Pei Liu, Hisilicon.
Analysis Simulation reveals a number of results, some of which are not immediately and intuitively obvious The number of Backoffs and Retries rises with throughput but is not dependent on the number of devices competing for that throughput Backoffs up to 50 and Retries up to 10 are typical of an optimised strategy Paul Dixon, Pei Liu, Hisilicon.
Distributed Contention Access Distributed CAP is proposed to optimise the CAP scheme for: • The channel model within CAP; And • The calculation method of network throughput; And • The performance of network throughput; And • Efficiency of total energy utilisation of network Paul Dixon, Pei Liu, Hisilicon.
optimisation of network get the time parameters calculate the back off number get the network parameters optimisation calculate successful CCA probability The Distributed Contention Access Scheme (1) • The model of CAP optimisation Paul Dixon, Pei Liu, Hisilicon.
The Distributed Contention Access Scheme (2) • The detail CAP optimisation steps: • Get the number of devices in the network using the CAP channel model. • Calculate the back off number to get optimal network throughput. • Revise the device number in CAP period based on the result of 2nd step Paul Dixon, Pei Liu, Hisilicon.
Get the parameters of a cycle Calculate throughput and average cycle length Calculate optimal back off number Calculate the probability of successful CCA Estimate the device number based on average successful CCA probability Revise the device number in the every successful CCA cycle Optimize the network The Distributed Contention Access Scheme (3) • Procedure of optimal contention access Paul Dixon, Pei Liu, Hisilicon.
Simulation Results of the CAP Optimisation (1) • Supposed: • Some devices transmit fixed size packets to the coordinator simultaneously, and • Each device can use the distributed CAP for optimisation. • The initial number of devices is 5, and 80s later, some additional devices are connected into the network • Other conditions strictly follow the ones in CSMA/CA procedure in 802.15.4 std Paul Dixon, Pei Liu, Hisilicon.
Simulation Results of the CAP Optimisation (2) • Case 1: 15 additional devices become active Paul Dixon, Pei Liu, Hisilicon.
Simulation Results of the CAP Optimisation (3) • Case 2: 35 additional devices become active Paul Dixon, Pei Liu, Hisilicon.
Simulation Summary • By CAP optimisation using Distributed Contention Access, the throughput of network will reach a new stable optimal value after short-lived fluctuation, even though the device number changes sharply • By the optimisation of CAP and throughput, the efficiency of total energy utilisation of WPAN is improved. Paul Dixon, Pei Liu, Hisilicon.