copyright © 2007 opnet technologies, inc. confidential - restricted access: this information may...

Copyright © 2007 OPNET Technologies, Inc.

CONFIDENTIAL - RESTRICTED ACCESS: This information may not be disclosed, copied, or transmitted in any format without the prior written consent of OPNET Technologies, Inc. Used with permission of the Author.

ASAND: Asynchronous Slot Assignment and Neighbor Discovery Protocol for Wireless Networks

Fikret Sivrikaya[Joint work with Costas Busch, Malik Magdon-Ismail, Bülent Yener]

Rensselaer Polytechnic Institute


CONFIDENTIAL - RESTRICTED ACCESS: This information may not be disclosed, copied, or transmitted in any format without the prior written consent of OPNET Technologies, Inc. Used with permission of the Author. 2

Outline

Introduction Wireless Networks

Terms & Notations

MAC Protocols & Slot Assignment

Asynchronous Slot Assignment Basic Approach

Problems & Remedies

ASAND Protocol

OPNET Modeler Implementation of ASAND

Simulation Results & Conclusion



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



Basic Terms & Notations…

k-neighborhood of a node v:

k(v)

k-neighborhood size of a node v:

k(v) = |k(v)|

max k-neighborhood size (in the network):

k = maxv k(v)

n = number of nodes in the network

1-neighbors or

“neighbors” of v

2-neighbors of v

v



MAC (Medium Access Control) Protocols

Specify how nodes in a network access the shared communication channel.

Three main classes:

Contention-based Contention-free Limited-contention

Slot Assignment

Assign each node a time interval for channel access.

Mainly used for contention-free MAC protocols.

Existing approaches require a common view of time by all nodes, i.e. global time synchronization.

frametime slot



Interference / Collisions

Interference on node b(“Hidden terminal problem”)

a

b

c

a b

ab

c d

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…



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.



Asynchronous Slot Assignment

21 3 i

21 3 4 21 3 4

21 3 i

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

.

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.



ASAND – Basic Approach

Select random slot

Transmit beacon at

Conflict

Listen for slots

Obtain slot

Report conflictsbetween neighbors

YES NO

Ready

Nonready



ASAND – Basic Approach

w

u

v

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.



ASAND – Conflict Reporting

w

v

u

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



Problem: Chains of Conflict Reports

a

g

b

e

c

h

fd

t+

a

g

b

e

c

h

fd

t+2

a

g

b

e

c

h

fd

t

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.

a

g

b

e

c

h

fd

t+3



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.

Remedy: Probabilistic Conflict Reporting

20% 40% 60% 80% 100%

u

v

w

Example: (preport = 0.2)



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%

u

v

w

vnode i:

x wi u

At algorithm termination, a node discovers its neighbors and a local schedule of their channel access times:



An OPNET Implementation of ASAND



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

other transmissions during its current slot.

Collision Detection

u v

(2)01id00id uuub



Collision Detection

1 shift log n

shift = log n + 1

shift = log n + 2

log n + 3 shift 2log n + 2

shift = 2log n + 3

(2)01id00id uuub X = idu

Y = idv

l = |idu| = log n



Collision Detection – OPNET Implementation



The Effect of Collision Reporting Probability, preport



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

Fikret Sivrikaya - [email protected]

copyright © 2007 opnet technologies, inc. confidential - restricted access: this information may...

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