Download Sep 2008 doc: 15-08-0631-00-004e-Distitbuted

Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
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:[[email protected], [email protected], [email protected]]
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.
Submission
Slide 1
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
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)
Submission
Slide 2
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
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
Submission
Slide 3
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
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
Submission
Slide 4
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
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
Submission
Slide 5
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
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
Submission
Slide 6
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
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
Submission
Slide 7
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
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
Submission
Slide 8
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
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
Submission
Slide 9
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
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
Submission
Slide 10
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
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
Submission
Slide 11
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
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
Submission
Slide 12
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
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
Submission
Slide 13
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
The Distributed Contention Access Scheme (1)
• The model of CAP optimisation
optimisation of network
get the time parameters
calculate the back off number
get the network parameters
optimisation
calculate successful CCA probability
Submission
Slide 14
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
The Distributed Contention Access Scheme (2)
•
The detail CAP optimisation steps：
1. Get the number of devices in the network
using the CAP channel model.
2. Calculate the back off number to get
optimal network throughput.
3. Revise the device number in CAP period
based on the result of 2nd step
Submission
Slide 15
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
The Distributed Contention Access Scheme (3)
• Procedure of optimal contention access
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
Submission
Slide 16
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
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
Submission
Slide 17
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
Simulation Results of the CAP Optimisation (2)
normallized throughput
normallized throughput
• Case 1: 15 additional devices become active
simulation time
simulation time
(a) Optimal throughput
Submission
(b) 15.4's throughput
Slide 18
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
Simulation Results of the CAP Optimisation (3)
normallized throughput
normallized throughput
• Case 2: 35 additional devices become active
simulation time
simulation time
(b) 15.4's throughput
(a) Optimal throughput
Submission
Slide 19
Paul Dixon, Pei Liu, Hisilicon.
Sep 2008
doc: 15-08-0631-00-004e-Distitbuted-Contention-Access-Scheme
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.
Submission
Slide 20
Paul Dixon, Pei Liu, Hisilicon.