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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [ Timing Primitives for 802.15.4b ] Date Submitted: [ 10 August 2004 ] Source: [ Robert Poor, Edward Hill ] Company [ Ember Corporation ] Address [ 313 Congress Street, Boston MA 02210 ]
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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Timing Primitives for 802.15.4b] Date Submitted: [10 August 2004] Source: [Robert Poor, Edward Hill] Company [Ember Corporation] Address [313 Congress Street, Boston MA 02210] Voice:[+1 617 951-0200], FAX: [+1 617 951-0999], E-Mail:[rpoor @ ieee . org] Re: [15-04-0239-00-004b-tg4b-call-proposals.doc] Abstract: [This document proposes extensions to IEEE 802.15.4-2003 in support of a mechanism for sharing a time base among devices in 802.15.4 devices.] Purpose: [This document is intended to encourage discussion within the IEEE 802.15.4 TG4b 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. Robert Poor, Edward Hill (Ember Corporation)
Timing Primitives for 802.15.4b Robert Poor <rpoor @ ieee . org> Edward Hill <edward.hill @ ember . com> Robert Poor, Edward Hill (Ember Corporation)
Motivation & Philosophy • Motivation: A method for sharing a time base among nodes in a 15.4 network is useful for: time stamping of events; decentralized synchronization among nodes for advanced power management; secure key distribution; routing algorithms and others. • Design principles: • Make as few changes to IEEE 802.15.4-2003 as possible. • Build upon existing mechanisms whenever possible. • Use Symbol Clock as the fundamental time base. • Provide timing primitives for the higher layers, which will in turn implement timing and synchronization algorithms. Robert Poor, Edward Hill (Ember Corporation)
Existing Mechanisms Several useful components already exist in 802.15.4-2003: • Timing: The MAC maintains TimeStamp, a symbol-rate clock with >= 20 bit resolution for stamping received beacon frames (c.f. 7.5.4.1 [p 151] and table 41 [p 77]). • Transmission: MCSP-DATA.confirm (msduHandle, status) message is passed from the MAC to the SSCS when an outgoing message passes from the MAC queue to the PHY (c.f. 7.1.1.2.1 [p 59]). • Reception: An MCSP-DATA.indication (SrcAddrMode, SrcPanID…) message is passed to the SSCS from the MAC when an incoming message is received by the MAC from the PHY. Robert Poor, Edward Hill (Ember Corporation)
Extending the Spec • Transmission: Add a TimeStamp argument to the MCPS-DATA.confirm() primitive to indicate the time at which the message was transmitted. • Reception: Add a TimeStamp argument to the MCPS-DATA.indication() primitive to indicate when the message was received. • Timing: The byte offset into the message at the moment TimeStamp is captured is implementation dependent, but is the same for transmission and reception. Robert Poor, Edward Hill (Ember Corporation)
Notes • A bit clock of 250KBPS is a symbol clock of 62.5KCPS (16 uSec), a 20 bit TimeStamp counter rolls over once every 16.7 seconds. • A 24 bit TimeStamp counter rolls over once every 4.47 minutes. • It is possible to use the existing beacon mechanism to create a distributed time base in a beaconed network (Beacon Node captures macBeaconTxTime and Beacon Sequence Numbers, other node replies with TimeStamp and BSN), but it is desirable to provide this functionality in a non-beaconed network. Robert Poor, Edward Hill (Ember Corporation)
