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
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. 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
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. 3 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
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. 4 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 = max v k (v) n = number of nodes in the network 1-neighbors or “neighbors” of v 2-neighbors of v v
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. 5 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
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. 6 Interference / Collisions Interference on node b (“Hidden terminal problem”) a b c a b a b cd 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…
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. 7 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.
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. 8 Asynchronous Slot Assignment 213 ii 2134 2134 213 ii 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 2 2 . Having the same frame size at all nodes ensures that overlapping time slots remain the same . When 2 2, we show [JDC’07] that all nodes obtain a conflict-free slot within 2 log 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.
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. 9 ASAND – Basic Approach Select random slot Transmit beacon at Conflict Listen for slots Obtain slot Report conflicts between neighbors YESNO Ready Nonready
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. 10 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.
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. 11 ASAND – Conflict Reporting 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
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. 12 Problem: Chains of Conflict Reports a g b e c h f d t+t+ a g b e c h f d t+2 a g b e c h f d 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 f d t+3
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. 13 Upon detecting a collision at time t; report the collision at t+ with probability p report. otherwise, listen for collisions again at t+ (with prob. 1 p report,). If a collision is detected again at t+ , then report it at t+2 with probability 2 p report … In general, after c consecutive collisions, report with probability c p report … A slot conflict is always reported after at most 1/ p report frames. To obtain a slot, a node has to transmit 1/ p report times in that slot. Remedy: Probabilistic Conflict Reporting Example: (p report = 0.2)
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. 14 ASAND Protocol v node i: xwiu At algorithm termination, a node discovers its neighbors and a local schedule of their channel access times:
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. 15 An OPNET Implementation of ASAND
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. 16 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 b u ( j) = 1. Guarantees that each transmitting node can detect other transmissions during its current slot. Collision Detection u v
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. 17 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 X = id u Y = id v l = |id u | = log n
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. 18 Collision Detection – OPNET Implementation
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. 19 The Effect of Collision Reporting Probability, p report
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. 20 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 [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 , October Research with OPNET Program at RPI: Model files also available at the OPNET Contributed Models depot: Fikret Sivrikaya -