IEEE 802.15 <subject>

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Project IEEE P802.15 Working Group for Wireless
Personal Area Networks (WPANs) Submission Title
Explanation and Revision of Previous Time
Synchronization Proposal to IEEE 802.15.4b Date
Submitted August 23, 2004 Source Huai-Rong
Shao, Hui Dai and Jinyun Zhang Company
Mitsubishi Electric Research Labs Address 8th
Floor, 201 Broadway, Cambridge, MA 02139
Voice617-621-7517, FAX [phone],
EMailshao_at_merl.com Re Response to call for
proposal of IEEE 802.15.4b, Doc Number
15-04-0239-00-004b. Abstract Further
explanation and revision of shared time-base
distribution methods proposed in
15-04-0313-01-004b Purpose Proposal to IEEE
802.15.4b Task Group Notice This document has
been prepared to assist the IEEE P802.15. It is
offered as a basis for discussion and is not
binding on the contributing individual(s) or
organization(s). The material in this document is
subject to change in form and content after
further study. The contributor(s) reserve(s) the
right to add, amend or withdraw material
contained herein. Release The contributor
acknowledges and accepts that this contribution
becomes the property of IEEE and may be made
publicly available by P802.15.
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Time Synchronization Proposal-Explanation and
Revision for Shared Time-Base Distribution
Methods Proposed in Document 15-04-0313-01

• Huai-Rong Shao, Hui Dai and Jinyun Zhang
• Mitsubishi Electric Research Laboratories

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Outline

• Proposal Summary
• Motivation and Objectives
• Method
• Basic Scheme
• High Accuracy Options
• Cases
• Use beacon for synchronization
• Use general packets for synchronization
• Discussions
• Message Sequence Charts

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Proposal Summary

• Minimum changes to 802.15.4-2003
• Add macSyncSymbolOffset attribute to MIB
• Shared time-base distribution method
• Basic scheme for both beacon-enabled and
  non-beacon network
• Allow High Accuracy Options

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Motivation and Objectives

• Time synchronization is important
• Maintain superframe/slot synchronization among
  devices
• Fine-tuned coordination of wake/sleep duty cycles
  to reduce power consumption
• Preserve the event orders
• Time synchronization is also important to
  security protocols since the clock reading is
  often used for encryption key generation
• Loop free routing (Robert Poor said)
• Objective
• A shared time-base distribution method with
  minimum modifications to 802.15.4-2003
• Simple, Efficient and Flexible
• Performance Consideration
• Accuracy
• Overhead

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Basic Scheme

• Step 1 At T0 , sender obtains a timestamp
• Step 2 At T1 , a synchronization packet
  containing timestamp is sent on wireless channel
• Step 3 At T2 , receivers receive the
  synchronization packet. They then adjust local
  clocks according to the timestamp carried in
  synchronization packet

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Basic Scheme (contd)
Sender
Receiver
Step1 Get timestamp at T0
T0
T1
T2
Step3 Receive packet at T2. Receiver adjusts
local clock according to the timestamp carried
in the packet
time
Step2 Send Synchronization frame at T1
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High Accuracy Option

• Step 1 Basic Scheme
• Step 2 Sender sends a second frame to receiver
• The second frame contains the actual transmitting
  time of synchronization packet
• Step 3 Receiver adjusts the local clock again
  for higher accuracy

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High Accuracy Option (contd)
Sender
Receiver
Step1 a Get timestamp at T0
T0
T1
T2
T3
Step1 b Send Synchronization frame at T1
Step 1 c Receive Synchronization packet at T2
.Receiver adjusts local clock
Step 2 Send real Timestamp, i.e. T1, at T3
Step 3 Receiver adjusts local clock
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Implementation Choices
Beacon Enabled Nonbeacon-enabled
Use Beacon Beacons are transmitted regularly and periodically Minimum modification Using Ad Hoc Beacon Minimum modification Synchronization frequency is implementation specific
Use General Packet Need to add more primitives and attributes Synchronization can be initiated by higher level, assistant mechanism is needed Need to add more primitives and attributes Synchronization can be initiated by higher level, assistant mechanism is needed
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Case I Use Beacon As Synchronization Packet

• Beacon provides natural synchronization
  embodiment in beacon-enabled network.
• Use of beacon can minimize modification to
  802.15.4-2003
• Beacon-enabled WPAN
• Beacons are transmitted periodically
• Nonbeacon-enabled WPAN
• Ad hoc Beacons can be used
• Association requires Beacon anyway
• Synchronization frequency is implementation
  specific

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Case I Interfaces and Parameters

• Add symbol offset captured position attribute to
  MIB because this could be vendor specific
• macSyncSymbolOffset
• No other modification
• Timestamp can be carried in the beacon payload.
• macBeaconTxTime (page 136, Table 71) stores the
  actual beacon transmitted time
• MLME-BEACON-NOTIFY.indication is already
  specified in 802.15.4-2003, Page 75 7.1.5.1

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Case II Use General Packet As Synchronization
Packet

• Beacon-enabled and Nonbeacon-enabled WPAN
• Require higher layer to schedule synchronization
  packet transmission
• More primitives are needed
• Up to higher level to select configurations
• Synchronization Frequency
• Accuracy

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Case II Interface and Parameters

• Add symbol offset captured position attribute to
  MIB because this could be vendor specific
• macSyncSymbolOffset
• Receiver reports timestamp to next higher layer
• Add timestamp argument to MCPS-DATA.indication,
  page 60 7.1.1.3
• Sender reports the actual Tx time to next high
  layer
• Add timestamp argument to MCPS-DATA.confirm, page
  59 7.1.1.2
• OR Add macSyncTxTime to MIB to record the
  synchronization packet transmission time

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Discussion Timestamp in Basic Scheme

• Timestamp carried in synchronization packet
• A synchronization packets actual transmitting
  time cant be carried in itself. Because actual
  transmitting time is captured during packet
  transmission according to 802.15.4-2003.
• A timestamp different from actual transmitting
  time is put into synchronization packet at MAC
  layer or above.
• The error can be possibly reduced by estimation.
• Detailed error analysis and estimation are
  presented at the previous proposal
• 15-04-0313-01-004b-enhancements-to-ieee-802-15-4

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Discussion Other High Accuracy Options

• SNTP-like Method
• Step 1 Basic Scheme
• Step 2 Receivers send response frame to the
  sender
• Response frame contains received timestamp and
  the previous sync frame sequence number
• Step 3 Sender sends back calculation to reduce
  the propagation error
• Problem
• Too much overhead for WPAN
• It mainly focus on reducing propagation error,
  which is NOT the major error source in 802.15.4

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Discussion SNTP-Like High Accuracy Option
Sender
Receiver
Step1 a Get timestamp at T0
T0
T1
T2
T3
T4
Step 1 c Receive Synchronization packet at T2 .
Step1 b Send Synchronization frame at T1
Step 3 Sender sends feedback to reduce the
propagation error at T4
Step 2 Receiver send response to sender at T3
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Conclusion

• Works with both absolute clock and relative clock
• Totally compatible with existing mechanisms
• Minimum changes to 802.15.4-2003
• Can be easily extended to multi-hops cases

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MSC Basic Scheme (Use beacon)
SENDER
RECEIVER
RECEIVER MLME
Next Higher Layer
Next Higher Layer
SENDER MLME
Beacon ( with Synchronize timestamp)
MLME-BEACON-NOTIFY.indication (with time stamp)
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MSC High Accuracy Option (Use beacon)
SENDER
RECEIVER
RECEIVER MLME
Next Higher Layer
Next Higher Layer
SENDER MLME
Beacon ( with Synchronize timestamp)
MLME-BEACON-NOTIFY.indication (with time stamp)
Synchronization Follow Up (with macSyncTxTime)
MCPS-DATA.indication
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MSC Basic Scheme (Use General Packet)
SENDER
RECEIVER
MAC Layer
Next High Layer
Next High Layer
MAC Layer
MCPS-DATA.request
Synchronization Packet
MCPS-DATA.indication With timestamp
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MSC High Accuracy Option (Use General Packet)
SENDER
RECEIVER
RECEIVER MLME
Next Higher Layer
Next Higher Layer
SENDER MLME
MCPS-DATA.request
Synchronization Packet
MCPS-DATA.indication (with time stamp)
MCPS-DATA.confirm (with time stamp)
MCPS-DATA.request
Synchronization Follow Up
MCPS-DATA.indication (with time stamp)