8/18/2015 mobile ad hoc networks coe 549 synchronization tarek sheltami kfupm ccse coe 1

23
06/15/22 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE http://faculty.kfupm.edu.sa/coe/tarek/co e549.htm 1

Upload: georgina-wilkins

Post on 24-Dec-2015

224 views

Category:

Documents


1 download

TRANSCRIPT

Page 1: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

04/19/23

Mobile Ad hoc Networks COE 549

SynchronizationTarek Sheltami

KFUPMCCSECOE

http://faculty.kfupm.edu.sa/coe/tarek/coe549.htm

1

Page 2: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

2

Outline Overview Key Issues Traditional approaches Fine-grained approaches Coarse-grained approaches

04/19/23

Page 3: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

04/19/23 3

OverviewDistributed wireless sensor networks need time synchronization for a number of good reasons, some of which are described below:1.Time-stamping measurements: data collection applications of sensor networks often require that sensor readings from different sensor nodes be provided with time stamps in addition to location information2.In-network signal processing: Time stamps are needed to determine which information from different sources can be fused/aggregated within the network3.Localization: TDoA-based ranging techniques used in node localization require good time synchronization4.Cooperative communication: Some physical layer multi-node cooperative communication techniques, which have the potential to provide significant energy savings, but require tight synchronization

Page 4: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

04/19/23 4

Overview5. Medium-access: TDMA-based medium-access schemes also

require that nodes be synchronized so that they can be assigned distinct slots for collision free communication

6. Sleep scheduling: synchronization is needed to coordinate the sleep schedules of neighboring devices, so that they can communicate with each other efficiently

7. Coordinated actuation: Advanced applications in which the network includes distributed actuators in addition to sensing require synchronization

Page 5: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

04/19/23 5

Key Issues The clock at each node consists of timer circuitry, often

based on quartz crystal oscillators The clock is incremented after each K ticks/interrupts of

the timer Practical timer circuits, particularly in low-end devices, are

unstable and error prone Crystal oscillators, such as those used in digital watches,

experience a shift in frequency as a function of temperature

A model for clock non-ideality

Where f0 is ideal frequency, ∆f is the frequency offset, df is the drift in the frequency, rf(t) an additional random error process and fi(t) is the instantaneous oscillator frequency

Page 6: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

04/19/23 6

Key Issues.. Frequency Drift is undesired change in frequency caused

by component aging and environmental changes Frequency Offset is the difference between a measured

frequency and the nominal frequency

Assuming t=0 as the initial reference time, the clock reads time Ci(t) at time t as:

Page 7: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

04/19/23 7

Key Issues.. Frequency drift and random error terms may be neglected to

derive a simpler linear model for clock non-ideality:

Where is the clock offset at the reference time t = 0 and βi the clock drift

The more stable and accurate the clock, the closer is to 0, and the closer βi is to 1

A clock is said to be fast if βi is greater than 1, and slow otherwise

Manufactured clocks are often specified with a maximum drift rate parameter ρ, such that 1− ρ ≤ βi ≤1+ ρ

Motes, typical sensor nodes, have ρ values on the order of 40 ppm (parts per million), which corresponds to a drift rate of ±40μs per second

Page 8: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

8

Ordering of events

“” is a partial ordering[ total ordering: for any two

events a,b (ab) either a b or b

aparting ordering: a,b can be concurrent ]

04/19/23

Page 9: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

9

DefinitionThe relation on the set of events of a

system is the smallest relation satisfying the following 3 conditions:1) if a,b events in same process, and a comes before b, then a b

2) if a is sending a message and b is receipt of same message by a different process, then a b

3) if a b and b c, than a c assume a a if a b and b a, than a,b are concurrent04/19/23

Page 10: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

10

Logical Clock Ci =logical clock (counter) at

process i C[b] = reading of Cj when event b

occurs at process j Clock condition for any events a, b

IF a b THEN C[a] < C[b]

No relationship to physical time

04/19/23

Page 11: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

11

Clock Condition

If a and b are events in process i and a comes before b, then Ci[a] < Ci[b]

If a is the sending of a message by process i and b is the receipt of that message by process j, then

Ci[a] < Cj[b]

C1

C2

If a b THEN C[a] < C[b]can be satisfied if the following conditions hold:

04/19/23

Page 12: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

12

Note: Ci[a] < Cj[b] a b

Note: a,b concurrent Ci[a] = Cj[b]

Note: Ci[a] = Cj[b] a,b concurrent

04/19/23

Page 13: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

04/19/23 13

Traditional Approaches

Lamport’s Algorithm Provides a consistent ordering of all events in a

distributed system Labeling each event x with a distinct time stamp Lx,

such that: Lx ≠ Ly ∀ unique events x & y If event x precedes event y in within a node Lx <

Ly If x transmitting and y receiving at two different

nodes Lx < Ly Drawbacks: Doesn’t provide true causality, assume

the true time of event x is indicated as Tx; then, while it is true that Tx<Ty Lx<Ly, it is not true that Lx<Ly Tx <Ty

Page 14: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

04/19/23 14

Traditional Approaches..

Cristian’s Algorithm A node A sends a request to node B (which has the

reference clock) and receives back the value of B’s clock, TB

Node A records locally both the transmission time T1 and the reception time T2

Page 15: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

04/19/23 15

Traditional Approaches..

Cristian’s Algorithm.. There are many sources of uncertainty and delay, which

impact its accuracy: Send time – which includes any processing time and

time taken to assemble and move the message to the link layer

Access time – which includes random delays while the message is buffered at the link layer due to contention and collisions

Propagation time – which is the time taken for point-to-point message travel. While negligible for a single link, this may be a dominant term over multiple hops if there is network congestion.

Receive time – which is the time taken to process the message and record its arrival.

Page 16: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

04/19/23 16

Traditional Approaches..

Cristian’s Algorithm.. The simplest estimate is to approximate the message

propagation time as (T2 −T1)/2 If the processing delay is known to be I, then a better

estimate is (T2 − T1 − I )/2. More sophisticated approaches take several round-trip

delay samples and use minimum or mean delays after outlier removal.

Page 17: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

04/19/23 17

Fine-grained clock synchronization

Reference broadcast synchronization (RBS) Two nodes receive the beacon of one node in the same

broadcast area The two receivers record the local time when the

reference signal was received. The two receivers exchange this local time stamp

through separate messages

Page 18: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

04/19/23 18

Fine-grained clock synchronization..

Time-Sync Protocol for Sensor Networks (TPSN)

Page 19: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

04/19/23 19

Fine-grained clock synchronization..

Linear parameter-based synchronization

Assuming the same pair-wise message exchange as in TPSN for nodes A and B, we have that the transmission time T1 and reception time T4 are measured in node A’s local clock, while reception time T2 and transmission time T3 are measured in node B’s local clock. We therefore get the following temporal relationships:

Page 20: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

04/19/23 20

Fine-grained clock synchronization..

Linear parameter-based synchronization

Page 21: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

04/19/23 21

Fine-grained clock synchronization..

Flooding time synchronization protocol (FTSP)1. Interrupt handling time: This is the delay in waiting

for the processor to complete its current instruction before transferring the message in parts to the radio

2. Modulation/encoding time: This is the time taken by the radio to perform modulation and encoding at the transmitter, and the corresponding demodulation and decoding at the receiver

FTSP uses a broadcast from a single sender to synchronize multiple receivers

Each broadcast provides a synchronization point (a global–local time pair) to each receiver

Page 22: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

04/19/23 22

Coarse-grained clock synchronization

wireless sensor network system for structural-response data acquisition Assume that:

Page 23: 8/18/2015 Mobile Ad hoc Networks COE 549 Synchronization Tarek Sheltami KFUPM CCSE COE  1

04/19/23 23

References1. L. Lamport, “Time, Clocks, and the Ordering of Events in a Distributed

System,”Communications of the ACM, 21, 7, July 1978, 558–565.2. F. Cristian, “A Probabilistic Approach to Distributed Clock Synchronization,”

DistributedComputing, 3, 1989, 146–158.3. J. Elson, L. Girod, and D. Estrin, “Fine-Grained Network Time Synchronization using

Reference Broadcasts,” in Proceedings of the Fifth Symposium on Operating Systems Design and Implementation (OSDI), December 2002.

4. S. Ganeriwal, R. Kumar, and M. B. Srivastava, “Timing-Sync Protocol for Sensor Networks,” Proceedings of ACM SenSys’03, November 2003.

5. M. L. Sichitiu and C. Veerarittiphan, “Simple, Accurate Time Synchronization for Wireless Sensor Networks,” Proceedings of IEEE Conference on Wireless Communications and Networking (WCNC), March 2003.

6. M. Maroti, B. Kusky, G. Simon, and A. Ledeczi, “The Flooding Time Synchronization Protocol,” Proceedings of ACM SenSys, November 2004.

7. N. Xu, S. Rangwala, K. Chintalapudi, D. Ganesan, A. Broad, R. Govindan, and D. Estrin, “A Wireless Sensor Network for Structural Monitoring,” Proceedings of ACM Conference on Embedded Networked Sensor Systems (SenSys), November 2004