Documents
Resources
Learning Center
Upload
Plans & pricing Sign in
Sign Out

www.ieee802.org15pub200415-04-0446-01-004b-exp

VIEWS: 0 PAGES: 22

									                                                                     doc.: IEEE 802.15-04-0446-01-004b
 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 ]
Voice:[617-621-7517], FAX: [617-621-7550], EMail:[shao@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.




     Submissio                                      Slide 1
     n
                                                doc.: IEEE 802.15-04-0446-01-004b




            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




Submissio                          Slide 2
n
                                             doc.: IEEE 802.15-04-0446-01-004b


                              Outline
 • Motivation
 • Method
       – Basic Scheme
       – High Accuracy Options
 • Cases
       – Use beacon for synchronization
       – Use general packets for synchronization
 • Discussions
 • Message Sequence Charts




Submissio                        Slide 3
n
                                                                      doc.: IEEE 802.15-04-0446-01-004b


                              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



    Submissio                                       Slide 4
    n
                                              doc.: IEEE 802.15-04-0446-01-004b


                     Method Summarization
 •    Minimum changes to 802.15.4-2003

 •    Shared time-base distribution method

 •    Basic scheme for both beacon-enabled and non-beacon-
      enabled network

 •    Allow High Accuracy Options




Submissio                           Slide 5
n
                                      doc.: IEEE 802.15-04-0446-01-004b


                     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 received the
   synchronization packet. They then set local clocks to
   the timestamp carried in synchronization packet




Submissio                   Slide 6
n
                                          doc.: IEEE 802.15-04-0446-01-004b


                       Basic Scheme (cont’d)

              Sender                       Receiver
Step1: Get timestamp at T0
    T0
    T1
    T2

time                                      Step3: Receive packet
                                          at T2. Receiver sets local
Step2: Send Synchronization               clock to the timestamp
frame at T1                               carried in the packet




  Submissio                     Slide 7
  n
                                                  doc.: IEEE 802.15-04-0446-01-004b


                       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




Submissio                            Slide 8
n
                                              doc.: IEEE 802.15-04-0446-01-004b


                     High Accuracy Option (cont’d)

                Sender
 Step1 a: Get timestamp at T0               Receiver
        T0
        T1
        T2
        T3


Step1 b: Send Synchronization
frame at T1                                         Step 1 c: Receive
                                                    Synchronization packet
                                                    at T2 .Receiver sets local
     Step 2: Send real                              clock
     Timestamp, i.e. T1, at T3
                                  Step 3: Receiver adjusts
                                  local clock

    Submissio                     Slide 9
    n
                                                        doc.: IEEE 802.15-04-0446-01-004b


                     Implementation Choices
              Beacon Enabled                      Non-beacon enable
Use Beacon • Beacons are transmitted              • Beacons are transmitted
              regularly and periodically          occasionally and indeterminately.
              • Minimum modification              • Require assistant mechanism to
                                                  decide when to send synchronize
                                                  packet


Use           • Need to add Primitives and attributes
General       • Synchronization can be initiated by higher level
Packet




Submissio                              Slide 10
n
                                                doc.: IEEE 802.15-04-0446-01-004b


            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
 • Non-beacon WPAN
       – Beacons are transmitted occasionally and indefinitely
       – Require assistance to schedule beacon transmission for
         synchronization




Submissio                         Slide 11
n
                                            doc.: IEEE 802.15-04-0446-01-004b


              Case I : Interfaces and Parameters
 • Add symbol offset captured position attribute to MIB
   because this could be vendor specific
       – macSyncSymbolOffset
 • Receiver reports time to next higher layer
       – Timestamp can be carried in the beacon payload.
       – macBeaconTxTime in page 136, Table 71 stores the actual
         beacon transmitted time
       – MLME-BEACON-NOTIFY.indication is already specify in
         802.15.4-2003, Page 75 §7.1.5.1




Submissio                       Slide 12
n
                                              doc.: IEEE 802.15-04-0446-01-004b


Case II : Use General Packet As Synchronization Packet
 • Beacon-enabled and Non-beacon-enabled WPAN
       – Require higher layer to schedule synchronization packet
         transmission
       – More primitives are needed
 • Up to higher level to select configurations
       – Synchronization Frequency
       – Accuracy




 Submissio                        Slide 13
 n
                                               doc.: IEEE 802.15-04-0446-01-004b


               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




Submissio                          Slide 14
n
                                                         doc.: IEEE 802.15-04-0446-01-004b


            Discussion: Timestamp in Basic Scheme
 •    Timestamp carried in synchronization packet
       – A synchronization packet’s actual transmitting time can’t 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




Submissio                                 Slide 15
n
                                                 doc.: IEEE 802.15-04-0446-01-004b


            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



Submissio                           Slide 16
n
                                               doc.: IEEE 802.15-04-0446-01-004b



          Discussion: SNTP-Like High Accuracy Option

                Sender                     Receiver
 Step1 a: Get timestamp at T0
         T0
         T1
         T2

         T3

         T4

Step1 b: Send Synchronization                        Step 1 c: Receive
frame at T1                                          Synchronization packet
                                                     at T2 .
    Step 3: Sender sends
    feedback to reduce the       Step 2: Receiver send
    propagation error at T4      response to sender at T3
    Submissio                   Slide 17
    n
                                        doc.: IEEE 802.15-04-0446-01-004b


                         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




Submissio                    Slide 18
n
                                                              doc.: IEEE 802.15-04-0446-01-004b


                 MSC : Basic Scheme (Use General Packet)
                    SENDER                                              RECEIVER
   Next                      MAC                              MAC                       Next
High Layer                   Layer                            Layer                  High Layer

         MCPS-DATA.request
                                     Synchronization Packet
                                                                   MCPS-DATA.indication
                                                                   With timestamp




     Submissio                                 Slide 19
     n
                                                       doc.: IEEE 802.15-04-0446-01-004b


   MSC : High Accuracy Option (Use General Packet)

              SENDER                                             RECEIVER
                                                                                  Next
Next Higher            SENDER                          RECEIVER
                                                                                 Higher
  Layer                MLME                            MLME
                                                                                  Layer
       MCPS-DATA.request
                             Synchronization Packet
       MCPS-DATA.confirm                                    MCPS-DATA.indication
       (with time stamp)                                    (with time stamp)
       MCPS-DATA.request
                           Synchronization Follow Up
                                                           MCPS-DATA.indication
                                                           (with time stamp)




 Submissio                           Slide 20
 n
                                                           doc.: IEEE 802.15-04-0446-01-004b


                 MSC : Basic Scheme (Use beacon)

               SENDER                                              RECEIVER
Next Higher             SENDER                            RECEIVER                 Next
  Layer                 MLME                              MLME                  Higher Layer


                          Beacon
                          ( with Synchronize timestamp)
                                                            MLME-BEACON-NOTIFY.indication
                                                            (with time stamp)




   Submissio                            Slide 21
   n
                                                            doc.: IEEE 802.15-04-0446-01-004b


               MSC : High Accuracy Option (Use beacon)

                SENDER                                              RECEIVER
Next Higher              SENDER                            RECEIVER                 Next
  Layer                  MLME                              MLME                  Higher Layer


                           Beacon
                           ( with Synchronize timestamp)
                                                             MLME-BEACON-NOTIFY.indication
                                                             (with time stamp)
                           Synchronization Follow Up
                           (with macSyncTxTime)                MCPS-DATA.indication




   Submissio                              Slide 22
   n

								
To top