Download What is the beacon collision problem? - Mentor

November 2007
doc.: IEEE 802.11-07/2813r1
Analyzing the Beacon Collision
Probability in Mesh Networks
Date: 2007-11-12
Authors:
Name
Company Address
Michelle Gong Intel
Corporation
Submission
Phone
2200 Mission College
Santa Clara, CA 95054
USA
Slide 1
email
+1-408-765-7994 [email protected]
Michelle Gong (Intel)
November 2007
doc.: IEEE 802.11-07/2813r1
What is the beacon collision problem?
• In a mesh network, each MP sends out a beacon
periodically
• When two MPs are hidden from each other, their
beacons may collide
• Depending on their clock drifts, their beacons may be
colliding for a prolonged period of time, causing peer
link timeout, discovery difficulties, and wasting
medium time
MP-A
MP-B
MP-C
Submission
Slide 2
Michelle Gong (Intel)
November 2007
doc.: IEEE 802.11-07/2813r1
In a mesh network, what is the beacon
collision probability?
• Before we try to solve the beacon collision problem,
let’s first examine whether it is indeed a problem
Submission
Slide 3
Michelle Gong (Intel)
November 2007
doc.: IEEE 802.11-07/2813r1
The beacon collision problem is approximately
modeled as the birthday paradox
• This is a very rough model that intends to give us some
intuition regarding the beacon collision problem
• Two levels approximation are utilized
– A beacon interval is divided into discrete time slots, each of which
is twice the beacon duration
– Taylor series approximation
collision
Beacon interval
Submission
Slide 4
Michelle Gong (Intel)
November 2007
doc.: IEEE 802.11-07/2813r1
In a group of 23 or more randomly chosen people,
there is more than 50% probability that some
pair of them will have the same birthday.
• Because the beacon interval is divided into n time slots, the
probability of a beacon falling into any of the time slot is
1/n
• Assuming there are k hidden MPs in a two-hop network,
the probability of no more than one beacon falling into a
time slot is:
n!
n k ( n  k )!
• The probability of beacon collision is therefore:
 k ( k 1)
2( n  k )
1 e
Submission
Slide 5
Michelle Gong (Intel)
November 2007
doc.: IEEE 802.11-07/2813r1
The beacon collision probability can be high when the
beacon interval is short and there are many hidden nodes
•
Beacon transmission time: 250us
Beacon interval = 100ms
Probability of Beacon Collisions (%)
100
90
80
70
60
50
40
30
20
10
0
0
5
10
15
20
25
30
35
40
Number of Hidden MPs
Submission
Slide 6
Michelle Gong (Intel)
November 2007
doc.: IEEE 802.11-07/2813r1
Increasing the beacon interval can significantly
reduce the beacon collision probability
Beacon interval = 1s
Probability of Beacon Collisions (%)
30
25
20
15
10
5
0
0
5
10
15
20
25
30
35
40
Number of Hidden MPs
Submission
Slide 7
Michelle Gong (Intel)
November 2007
doc.: IEEE 802.11-07/2813r1
The beacon collision probability is small when the
beacon interval is 10 seconds
Beacon interval = 10s
Probability of Beacon Collisions (%)
3.5
3
2.5
2
1.5
1
0.5
0
0
5
10
15
20
25
30
35
40
Number of Hidden MPs
Submission
Slide 8
Michelle Gong (Intel)
November 2007
doc.: IEEE 802.11-07/2813r1
The root causes for the beacon collision problem are
hidden nodes, fixed delivery time, and broadcast
• Hidden MPs: Hidden nodes are the result of the
network topology. The hidden node problem cannot be
solved without changing the network topology
• Beacons are sent periodically at TBTTs: Replace the
beacon with an action frame that does not contain
timing information and is not sent at a fixed time
– Only feasible for MPs that do not need 11s synchronization
support and do not go into power save mode
• Beacons are broadcast: Replace the beacon with an
unicast frame
– Only feasible for maintaining peer link connections
– Only feasible for MPs that do not need 11s synchronization
support and do not go into power save mode
Submission
Slide 9
Michelle Gong (Intel)
November 2007
doc.: IEEE 802.11-07/2813r1
Beacon frames are essential for the operation of
synchronization and power management
• Beacon frames carry TSF timing information, which is
the base for the synchronization operation
• Beacon frames are sent at fixed time so that MPs in PS
know when to wake up and exchange packets with each
other
• Many existing 802.11 devices including mesh APs already
deployed today are not SW upgradeable to be 802.11s
compliant if an action frame must contain the TSF timing
information
Submission
Slide 10
Michelle Gong (Intel)
November 2007
doc.: IEEE 802.11-07/2813r1
In summary, the beacon collision problem cannot
be completely avoided but it can be mitigated
• When the beacon interval is small, i.e. 100ms, the
beacon collision probability can be high
• Increasing the length of the beacon interval and
reducing the size of the mesh network reduces the
collision probability
• The questions are:
– Do you think the beacon collision probability is tolerable for your
mesh networks? Is < 1%, < 0.1%, or < 0.01% tolerable?
– Should we define a feature to mitigate this beacon collision
problem or should we define a notification method when beacons
start to collide (implementers can decide how to act upon it)?
– Do you think 11s should reuse the existing beacon frame type or
define a new action frame to replace the beacon?
Submission
Slide 11
Michelle Gong (Intel)
November 2007
doc.: IEEE 802.11-07/2813r1
Straw Poll 1
• Do you think the beacon collision probability is
tolerable for your mesh networks?
• > 1% is tolerable:
• < 1% is tolerable:
• < 0.1% is tolerable:
• < 0.01% is tolerable:
Submission
Slide 12
Michelle Gong (Intel)
November 2007
doc.: IEEE 802.11-07/2813r1
Straw Poll 2
• What should we do about this beacon collision
problem?
• Define a feature to mitigate this beacon collision
problem
• Define a notification method when beacons start to
collide (implementers can decide how to act upon it)
• Do nothing
Submission
Slide 13
Michelle Gong (Intel)
November 2007
doc.: IEEE 802.11-07/2813r1
Straw Poll 3
• Do you think 11s should reuse the existing beacon
frame type or define a new action frame to replace the
beacon?
– Reuse the existing beacon frame type
– Define a new action frame type
Submission
Slide 14
Michelle Gong (Intel)