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ASAND: Asynchronous Slot Assignment and Neighbor Discovery Protocol for Wireless Networks
Fikret Sivrikaya[Joint work with Costas Busch, Malik Magdon-Ismail, Bülent Yener]
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Wireless Network Graphs
Each node has a transmission range, which determines its neighbors
Representation of the network as a graph
same transmission power/range symmetric links undirected graph
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MAC (Medium Access Control) Protocols
Specify how nodes in a network access the shared communication channel.
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Interference / Collisions
Interference on node b(“Hidden terminal problem”)
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Interference on node b
a and b interfere and hear noise only
Packets that suffered collisions should be re-sent.
Ideally, we would want all packets to be sent collision-free, only once…
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Static Slot Assignment
Because of the hidden terminal problem, a node may contend with any neighbor in its 2-neighborhood for channel access.
A static slot assignment protocol should assign each node a time interval that is guaranteed to be collision-free in its 2-neighborhood.
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Asynchronous Slot Assignment
21 3 i
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We propose a new protocol that does not require global clock synchronization.
Each node locally discretizes its local time.
The number of slots in a time frame, called the frame size and denoted by , is set to 22
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Having the same frame size at all nodes ensures that overlapping time slots remain the same.
When 22, we show [JDC’07] that all nodes obtain a conflict-free slot within 2log n time slots, with probability at least 1−1/n.
The clock skew may have a small effect, which can be addressed by introducing safety gaps within slots to tolerate a given amount of clock drift.
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ASAND – Basic Approach
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Schedule nodes’ transmission times so that neighbor nodes do not transmit at the same time.
Repeatedly select a random time slot until it is collision-free in the 2-neighborhood.
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ASAND – Conflict Reporting
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The 2-hop neighbors u and v are unaware that they have selected conflicting time slots (their transmissions collide on w).
Having observed a collision in its local time t, node w transmits at time t+, creating a spurious conflict with both u and v.
This is called conflict reporting essentially reduces a conflict between hidden terminals to a conflict between neighbor nodes.
After t+, u and v will be forced to select new slots
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Problem: Chains of Conflict Reports
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Observing a collision
Transmitting
Chain of messages goes on indefinitely and makes the time slot useless in this neighborhood.
Nodes involved in the chain waste energy. Continuous collisions impose a problem for algorithm termination.
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Upon detecting a collision at time t; report the collision at t+ with probability preport. otherwise, listen for collisions again at t+ (with prob. 1preport,).
If a collision is detected again at t+, then report it at t+2 with probability 2preport…
In general, after c consecutive collisions, report with probability c preport… A slot conflict is always reported after at most 1/ preport frames. To obtain a slot, a node has to transmit 1/ preport times in that slot.
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Protocol ASAND (node i)
1: ready FALSE 2: clear 0 3: Ci[t] 0, t=0..-1 // Conflict counters 4: Ni[t] NULL, t=0..-1 // Neighbor ID list 5: ti 0 // local virtual time (slot counter) 6: while not ready-1 do 7: if clear = 0 then 8: i random(0..-1) 9: if ti = i then
10: send a beacon message 11: if not ready then 12: if sensed any other transmission then 13: clear 0 14: else if clear++ 1/preport then 15: ready TRUE 16: else 17: with probability Ci[ti]preport 18: send a conflict-report message 19: Ci[ti] 0 20: otherwise 21: listen the channel in slot ti 22: if received a garbled message then 23: Ci[ti] ++ 24: else if correctly received <beacon, j> then 25: Ni[ti] j 26: ti (ti + 1) mod
ASAND Protocol
20% 40% 60% 80% 100%
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At algorithm termination, a node discovers its neighbors and a local schedule of their channel access times:
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When a node transmits, it must be able to detect simultaneous transmissions of neighbor nodes.
Divide each slot into 4 log n +8 mini slots. Generate a binary sequence b of 4 log n +8 bits
based on unique node id.
Transmit in mini slot j only if bu( j)=1. Guarantees that each transmitting node can detect
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The Effect of Collision Reporting Probability, preport
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References & Further Information
[JDC’07] C. Busch, M. Magdon-Ismail, F. Sivrikaya and B. Yener, “Contention-Free MAC Protocols for Asynchronous Wireless Networks,” to appear in the Journal of Distributed Computing (JDC). Available at http://www.cs.rpi.edu/~sivrif/academic/papers/JDC_CFMAC.pdf
[DISC’04] C. Busch, M. Magdon-Ismail, F. Sivrikaya, B. Yener, “Contention-Free MAC protocols for Wireless Sensor Networks,” In proceedings of the 18th International Conference on Distributed Computing (DISC 2004), Amsterdam, The Netherlands, pp. 245--259, October 2004.
Research with OPNET Program at RPI: http://www.cs.rpi.edu/~sivrif/opnet/
Model files also available at the OPNET Contributed Models depot: http://www.opnet.com/support/cont_models.shtml