The Flooding Time Synchronization Protocol Miklós Maróti Branislav Kusy Gyula Simon

The Flooding Time Synchronization Protocol

Miklós Maróti Branislav Kusy Gyula Simon

Ákos Lédeczi

SenSys’04, November 3–5, 2004, Baltimore, Maryland, USA.

Outline

Introduction Uncertainties In Radio Message Delivery Clock draft Time synchronization approach Conclusion

Introduction

describe the Flooding Time Synchronization Protocol (FTSP) for Wireless Sensor Network

designing the FTSP, our goals were to achieve network-wide time synchronization with error in the micro-second rang

The algorithm compensates for utilizing the concepts of MAC layer time-stamping and skew compensation with linear regression

The algorithm was implemented on Mica/Mica2 platforms running the TinyOS operating system

Uncertainties In Radio Message Delivery

Send Time—Depending on the system call overhead of the operating system and on the current processor load

Access Time—message stay s the radio device buffer and waiting for access to the transmit channel

Interrupt Handling Time—the delay between the radio chip raising and the microcontroller responding to an interrupt. This time is mostly less than a few microsecond (waiting for the microcontroller to finish the currently executed instruction)

Transmission Time—depending on the length of the message and the speed of the radio

Propagation Time—is highly deterministic ,This time is less than one microsecond (for ranges under 300 meters)

Encoding Time—for the radio chip to encode and transform a part of the message to electromagnetic waves

Byte Alignment Time—because of the different byte alignment of the sender and receiver and can computed from bit offset and speed of the radio


Variability in Packet Delay

software MAC propagationTX RX software

sender receiver

ALL DELAYS ARE VARIABLE !

Senderuncertainty

222

UCUCUC RPSError

Propagationcertainty

Receiveruncertainty

Clock Skew

- cause by clock drift

- variations in crystal oscillation frequency

- due to environmental variation (temperature , humidity) or

manufacturing

- frequency difference

- clock offset

Error of time synchronization

Host B

Host A

Time

T2

T1

T3

T4

T2=T1+offset+ propagation delay

T4=T3- offset+ propagation delay

propagation delay = (T2-T1) + (T4-T3) /2

drift clock = (T2-T1) - (T4-T3) /2

Variability in clock offset

sender-receiver synchronization

Variability in clock offset

BAttRD

41

t4

BAtD 4

Real Time

Ideal clock

Node B clock

Node A clock

t1

BAtD 1

Local node time

Relative drift

222241

BA

tt

UCUCUC RDRPSError

Time Synchronization Techniques

Sender Receiver

NIC

Physical Media

NIC

Propagation Time

Receiver

NICI saw itat t=4 I saw it

at t=5

Reference Broadcast Synchronization (RBS)

Reference Broadcast Synchronization (RBS) - A transmitter broadcast a reference packet to two receivers

- Each receiver records the time that the reference was

received , according to its local clock.

- receivers exchange their observations each other

- suitable only for low precision demands


Reference Broadcast Synchronization (RBS) - advantage

. Completely eliminating transmitter side uncertainties

- disadvantage

. Number of message exchanges is high

. For n node . 1node broadcast , n-1 message exchange for sync

result network traffic overhead and high energy consumption


Timing-sync Protocol for Sensor Network (TPSN)

- the TPSN algorithm first create a spanning tree of the network

- level discovery phase

. Create a hierarchy topology

. Each node is assigned a level

. A node is selected as the root node (level 0)

. Root broadcast level_discovery package which contain

identifier and level of the sender

. Neighbor receive the package then assigned itself a level plus1

and subsequent message received are discard


Timing-sync Protocol for Sensor Network (TPSN) . Node then rebroadcast level_discovery message

. Until all nodes are assigned a level



Level Discovery Phase

Timing-sync Protocol for Sensor Network (TPSN)

- synchronization phase

. Using a sender-receiver synchronization . Use two-way message exchange . A node initiates synchronization by sending a pulse message that including the node’s level and local time . B node that receives and reply an acknowledgement that including the original time stamp . Calculate the drift clock and propagation delay and synchronizes itself with the receiving node

time synchronization techniques


Synchronization Phase


A

Send at T3Recv at T4

T4 = T3 + DELAY- OFFSET

Send at T1 Recv at T2

T2 = T1 + DELAY + OFFSET

B

OFFSET = {(T2-T1)-(T4-T3)}/2

DELAY = {(T2-T1)+(T4-T3)}/2

Timing-sync Protocol for Sensor Network (TPSN) - advantage

. better Performance than RBS

. More minimum error than RBS

- disadvantage

. Tow-way communication that higher communication load

. Limits its accuracy


When a node collects enough reference point , it estimates the offset and skew

Flooding Time Synchronization Protocol

root

Time stamp

Root ID Sequence

number

Broadcast a single radio message time stamp

Collect reference point

Synchronization message format

local time-stamp

global time-stamp[global , local ] time-stamp

A

Multi-hop time synchronization - FTSP utilize reference points to perform synchronization

- a reference points are generated by sending and receiving

periodic broadcast message

- a reference points contains a pair of global and local time stamp

- When a node collects enough reference point , it estimates the

offset and skew

- the newly synchronized node can then broadcast

synchronization messages to another node

- offset and skew calculated by linear regression


synchronization message format - consists three fields

- timestamp , contains the global time estimate of the transmitter

- rootID , ID of the root

- SeqNum, sequence number set and incremented by the root

when a new synchronization round , used to handle redundant

synchronization message


Managing redundant information - in a dense network a receiver may receive several message

from different node in a short time interval

- due to limited resources , subset of the message must be

selected to create reference point

- in the Mica2 implementation an eight-element regression table

stores the reference point used to calculate the regression line

- to aid message filtering for more beneficial to store reference

point

. Each node maintains a highestSeqNum variable and

MyRootID


The root election problem - election process is needed to provide a root after startup - to perform global synchronization that only one root is needed - nodes may fail or the network disconnected or no node can play the role of root - FTSP utilizes a simple election process base on unique node ID

- when a node can’t receive new time synchronization message for ROOT_TIMEOUT number of message broadcast periods then declares itself to be the root (myrooID=myID) - ROOT_TIMEOUT = NUMENTRIES_LIMIT = times *Periodically re-sync


Managing redundant information (cont.)


Heartbeats = numbers of message broadcast

NUMENTRIES_LIMIT = times *Periodically re-sync

60 Mica2 motes deployed in a 5x12 grid Each mote can communicate with neighbors only ID1 is smallest id When ID1 is fail then ID2 will replace ID1 The period of time re-synchronization was 30 sec for 60 nodes

Experimental

ROOT_TIMEOUT=6

Experimental

A: at 0:04 all motes were turned on B: at 1:00 the root with ID1 was switched off, ID2 becomes the

new root C: between 2:00 and 2:15 randomly selected nodes were reset

one by one with 30 second period D: at 2:30 the motes with odd node IDs were switched off (half

of the nodes are removed) E: at 3:01 the motes with odd node IDs were switched back on

(100% new nodes were introduced) F: at 4:02 the experiment was ended.

Experimental

The nodes were switched on at the time (0:04), but during the next 3 minutes (0:07) no node was synchronized because none of them declared itself to be the root.

Then many nodes timed out and became the roots of the network, which was the reason why the average and maximum synchronization errors soared for 10 minutes untill the election process has completed (0:17) and only a single root remained (ID1).

Complete synchronization has been achieved in 14 minutes (at 0:18)

Experimental

When the root ID1 was switched off that global time had not been updated

until each node timed out and declared itself to be the root. The election process again resulted in a single root (ID2) the error stayed low during this time because nodes did not

discard their old offset and skew estimates and the new root was broadcasting its estimation of the old global time

Election process finished at 1:06 (in 6 minutes)

Experimental

TPSN eliminates the send, access, interrupt handling, encoding, decoding and receive time errors

The authors of TPSN algorithm implemented both TPSN and RBS on the Mica platform using a 4 MHz clock for time-stamping, and compared the precision of the two algorithms. The resulting average errors for a single hop case for two nodes are 16.9µs and 29.1µs for the TPSN and RBS algorithms,

Multiple time-stamps with linear regression are used to estimate clock skew and offset. The average error of the algorithm for a single hop case using two nodes was 1.48µs

FTSP also utilizes less network resources than either RBS or TPSN

Conclusion

The Flooding Time Synchronization Protocol Miklós Maróti Branislav Kusy Gyula Simon

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