Cross-Layer Protocol Design and Optimization for Delay/Fault-Tolerant Mobile Sensor Networks IEEE Journal of Selected Areas in Communications, 2008 Yu Wang, Hongyi Wu, Feng Lin, and Nian-Feng Tzeng Presented by Hanjin Park (September 16, 2008) Computer Network Lab. 1 / 22

Contents Introduction Problem definition Solution – Cross-Layer Data Delivery Protocol for DFT-MSN Protocol Parameters Asynchronous Phase Synchronous Phase – Protocol Optimization Performance Evaluation Conclusion 2 / 22

Introduction Delay/Fault-Tolerant Mobile Sensor Networks(DFT- MSN) – Extremely low and intermittent connectivity Sparse network density and dynamic mobility – Without end-to-end connection Convention routing for MSN do not work effectively – Always end-to-end connected – limited buffer Save messages temporally to relay Store and forward A B E D F C / 22

Introduction Applications of DFT-MSN – Air quality sensor – Military sensor – Wildlife tracking ZebraNet at Princeton Univ. 4 / 22

Introduction Problem of DFT-MSN – Work aggressively in order to catch every possible opportunity for data transmission – But, working aggressively means DFT-MSN has power consumption problem – More important power than conventional MSN – Because the conventional MSN has stable connectivity and channel bandwidth – Tradeoff between link utilization and energy efficiency 5 / 22

Problem definition How to make efficient use of the transmission opportunities whenever they are available, while keeping the energy consumption at the lowest possible level? Cross-Layer data delivery protocol – Asynchronous phase / Synchronous phase – Optimization Layer 2 Data Link Layer One hop transmission Layer 3 Network Layer Routing 6 / 22

Preliminary Protocol Parameters – Nodal Delivery Probability Probability that a sensor can deliver message to sink Decision on data transmission is based on delivery probability i Sink k If there is no transmission during a certain period, reduce the delivery probability When Whenever sensor i transmits a data message to another node k, delivery probability will be updated 7 / 22

Preliminary Protocol Parameters – Message Fault Tolerance Multiple copies of the messages  create/maintain by diff sensor  redundancy Fault tolerant degree(FTD), amount of redundancy Data queue management j Sort the messages based on FTD. The smaller FTD, the more importantWhen a new Message come to queue with already full, then drop the bottom message in the queue If FTD of a message is larger than threshold, then drop it to reduce transmission cost Threshold = 0.57 M1,M2,M3,M4,M5,M6,M7 New Message M8Drop M5, M6, M8 M1,M2,M3,M4,M5,M6,M / 22

Proposed Scheme Cross-Layer Data Delivery protocol for DFT-MSN – Asynchronous Phase Sensor A turns on its radio and listens for a period of If channel idle, Node A transmits a preamble pkt. Preamble pkt. to grasp channel and inform its neighbor to prepare for receiving of RTS pkt. Send RTS pkt. RTS pkt. contains nodal delivery probability, FTD of msg, and length of contention window(W) Receive RTS. Only qualified receiver send back CTS. Qualified receiver means which nodes with higher delivery probability than sender and available buffer space. Receive CTS. Sender makes a neighboring table to control central arrangement for data transmission Enter Synchronous phase Collision may happens in two situations 1)Multiple nodes may try to grasp the channel by sending preamble pkt. 2)Multiple qualified neighbor nodes may reply with CTS pkts. simultaneously A B C D E 9 / 22 A B C D E

Proposed Scheme Cross-Layer Data Delivery protocol for DFT-MSN – Synchronous Phase After obtaining information of qualified receivers, Node A decides which of them are to be selected (outgoing message M’s FTD and receiver’s delivery probability) Send SCHEDULE pkt. SCHEDULE pkt. includes list of receiver’s IDs and corresponding FTD of the message. Receive SCHEDULE pkt. If a receiver finds its ID in SCHED pkt. then accept the following message M, insert M to its queue with FTD. Qualified receivers reply with an ACK pkt. At a specific time slot which predefined in the SCHED pkt. Collision Free After receiving the ACK pkt. the node A recalculates the FTD of its local copy of message M. A B C D E 10 / 22 A B C D E

Proposed Scheme Protocol Optimization – Periodic Sleeping Due to sparse connectivity of DFT-MSN, sensor nodes want to be mostly in the listening state  power management problem S cycles period si transmission cycles # of Msg with FTD smaller than F at node i total # of Msg at node i Sleeping period of node iMinimum sleeping period Threshold 11 / 22

Proposed Scheme Protocol Optimization – Collision Avoidance During RTS Transmission Node i dynamically select a listening period between 1 to  reduce overall collision prob. node with a lower delivery probability to have a better chance to grasp the channel  high channel efficiency The larger (maximum listening period), the less likely collision will happen, but less link utilization(power efficiency problem) Find a minimum which keeps the collision prob.( ) under predefined threshold ( ) node i node j node i node j listen Send preamble pkt. simultaneously preambleRTS time slot 12 / 22

Proposed Scheme Protocol Optimization – Collision Avoidance During CTS Transmission RTS pkt. has CW for that receiver can select randomly from the CW No fixed CW, adaptively changes the size of CW for efficiency Two schemes to optimized CW 1.Minimizing overall collision probability 2.Minimizing collision probability for nodes with high delivery probabilities* (Appendix 2-A, 2-B) node j node i listen RTS contains common CW size (W) preambleRTS node k listen CW of node j CW of node k CTS time slot Send CTS pkt. simultaneously 13 / 22

Proposed Scheme Protocol Optimization – 1) Minimizing overall collision probability Node i send RTS to its neighbor( ) Every qualified neighbor randomly selected a time slot between 1 and Probability that every CTS pkt.s is transmitted in a collision-free Overall collision probability Find node i neighbor of node i ( ) 14 / 22

Proposed Scheme Protocol Optimization – Summary For channel efficiency, – Lower NDP is given a larger sleeping period because of lack of delivery probability – To get the channel easily, Lower NDP, smaller listening period To reduce collision probability – Higher NDP, more likely selected as relay node. Higher Delivery Prob.Lower Delivery Prob. Sleeping periodshorterlonger Listening period (preamble/RTS) longershorter Contention Window (CTS) longershorter 15 / 22

Performance Evaluation OPT – Proposed protocol that employs all optimization schemes – (sleeping period), (maximum listening period) – (contention window) NOOPT – Basic protocol without parameter optimization NOSLEEP – Similar to OPT, except that nodes do not perform periodic sleeping ZBR – Differs from OPT only in the message transmission scheme – ZebraNet’s history-based scheme 16 / 22

Performance Evaluation (# of sinks) More sink, fewer hops NOSLEEP – low delay, but highest energy consumption ZBR – low delay? 17 / 22

Performance Evaluation (# of nodes) More node  limited bandwidth, queue size  most msg are drop  lower delivery ratio ZBR’s transmission control is bad More node  delay decrease (better chance to meet nodes with HDP) 18 / 22

Performance Evaluation (Max queue size) Bigger queue size, increase delivery ratio Queue size does not affect the number of data transmission, and power consumption Because of waiting in queue, delay slightly increased 19 / 22

Performance Evaluation (Avg. speed) More speed, delivery ratio is increased? More speed, power consumption is decreased slightly due to less transmission More speed, delay is decreased (better chance to meet nodes with HDP) 20 / 22

Conclusion DFT-MSN – Low and intermittent connection – No guarantee end to end connection – Limited buffer / store and forward Cross-Layer data delivery protocol – Asynchronous phase, synchronous phase – Employ layer 2’s information for working related layer 3 Nodal delivery probability, Message fault tolerance Optimization – Sleeping period – Maximum Listening period – Contention window 21 / 22

Discussion The lack of mentions about – Definition of some notation No consideration about – Impact of threshold – Parameters’ sensitivity – Synchronization – Recursive transmission 22 / 22

Q & A Thank you 23 / 22

Reference [1] Y. Wang and H. Wu, “The Delay Fault Tolerant Mobile Sensor Network(DFT-MSN) : A New Paradigm for Pervasive Information Gathering,” IEEE Trans. Mobile Computing, vol. 6, no. 9, pp , [2] 24 / 22

Appendix 1 Message Transmission Process ij M M7 M M1 M 25 / 22

Appendix 2-A Protocol Optimization – 2) Minimizing overall probability for nodes with high delivery probability Node with high delivery probability(DP) are better candidates as dada relay nodes, so high collision prob. Reduce collision probability according to DP If sender is node i, RTS of sender(CW( ), maximum DP of neighbors( ), DP of sender( ) Qualified neighbor node j receive RTS, select a timeslot from 0 to to transmit CTS The larger (receiver DP), close to whole contention window W, otherwise if is small, it may have few choices. sink node i node j neighbor node 26 / 22

Appendix 2-B Protocol Optimization – 2) Minimizing overall probability for nodes with high delivery probability(cont.) :prob. that node k doesn’t choose slot s :prob. that no other node select slot s except node j  neighbor node j ‘s collision probability (delivery probability ) Find slot s 27 / 22