FebruarMarchy 2009 IEEE P802.11p5doc.: 802.11-09/0274r01
IEEE P802.11
Wireless LANs
Date: 2009-02-19
Author(s):
Name / Company / Address / Phone / email
Justin McNew / Kapsch TrafficCom | TechnoCom Mobility Solutions / 2035 Corte del Nogal, Suite 105, Carlsbad, CA 92011 / 760-438-5115 x175 /
Insert the following new subclause 7.3.2.b at the end of subclause 7.3.2:
7.3.2.b Timing information element (TIE)
The TIE format is shown in Figure 7-95a2.
The Timing information element, shown in Figure 7-95a2, contains fields describing the source of time associated with an external clock (external time source), an estimate of the offset between thethat external clock and the TSF timer, and the standard deviation of the error inof the offset estimate. This information may be used by a STA to align its own external clock to that of another STA.
Element ID / Length / ContentTiming capabilities / TTOE / TTOE-STD / reservedOctets 1 1 1 10 5 0 - 239 Variable (16- 255)
Figure 7-95a2—Timing information element format
The content field, shown in Figure 7-95a3, contains subfields describing the source of time as well as estimates of the offset and standard deviation of error of TSF timer values versus the time maintained by an external clock. This information can be used by a receiving STA to calculate an offset to its internal TSF timer to estimate the current time of the external clock on the transmitting STA.
Timing capabilities / TTOE / TTOE-STD / reservedOctets: 1 10 5 0 - 239
Figure 7-95a3—Content field format
The subfields of the content field are described as follows:
− The Timing capabilities field, shown in Figure 7-95a34, is a mandatory subfield used to inform receivers of indicate the transmitter’sa STA’s source of external time and information as well as the current condition of the its timing estimator at the transmitter. Bits 0-2 shall be set to 0 if there is no source of external time associated with the Timing information. Bits 0-2 are set to 0b001 if UTC is the external source. Other values for bits 0-2 are reserved. Bit 3 shall be set to 1 if this external time source is currently available and being used to estimate time, and 0 if not. Bits 4–7 are reserved..
bits / descriptionb0 / b1 / b2 / b3 / b4 / b5 / b6 / b7
0 / 0 / 0 / No standardized external time source
0 / 0 / 1 / External time source is UTC [ed: add this reference: ITU-R TF.460-4, and insert the associated pointer here]
010 - 111 / Reserved
0 / External time source is not currently available (time may continue to be estimated based on this source, e.g. if UTC is being derived from GPS the coverage may be sporadic)
1 / External time source is available and is currently being used to estimate time
0000 – 1111 / Reserved
Time Source / Availability of Time Source / Reserved
Bits: 0-2 3 4-7
Figure 7-95a34—Timing capabilities subfield format
− The TSF timer offset estimate (TTOE) subfield is mandatory. This subfield contains a two’s complement integer in nanoseconds which, when added to the transmitting tTimestamp present in the same transmitted frame gives the receiving STA’s an TSF timer value, gives the STA’s best estimate of the transmitting STA’s external time sourceat the instant the first bit of the Timestamp in the frame is transmitted from that STA’s antenna. The tTimestamp is derived from the TSF timer as defined in 11.6a.
− The TSF timer standard deviation subfield (TTOE-STD) is mandatory. This subfield contains an unsigned integer in nanoseconds giving an estimate ofthat defines the standard deviation of the error in the transmitting STA’s estimate of external time, including biases that have not been calibrated outin the TTOE. A STA sets the TTOE-STD field to theWhen it is not meaningful, the TTOE-STD subfield shall be set to its maximum value (240-1) to indicate that it has no meaningful error information to provide for the estimate of the external time source.. If TTOE-STD is set to this value, TTOE should not be used to estimate external time. Note that once any lower variance source of time becomes available, that source and its error variance can be adopted as initial conditions for the estimator. The value of the TTOE-STD, i.e. the error in the estimate of external time (or equivalently, the error in TTOE), is generally a function of various environmental and implementation factors and may vary with time.
Insert the following new subclause 11.6a after subclause 11.6 and renumber as appropriate:
11.6a Time base synchronization Time advertisement
According 11.1.2 each STA maintains a TSF Timer,. T and the Ttimestamp field is included in the Timing Advertisement frame, and w. When a STA transmits the Timing Advertisement frame the Ttimestamp shall be set to the value of the STA’s TSF timer at the time that the data symbol containing the first bit of the Ttimestamp is transmitted to the PHY plus the transmitting STA’s delays through its local PHY from the MAC-PHY interface to its interface with the WM [e.g., antenna, light emitting diode (LED) emission surface].
11.6a.1 Advertising an external time source using the TSF Timer, Timing Advertisement frame and Timing information element (informative)
A STA may advertise an external time source when transmitting the Timing Advertisement frame by including the Timing information element. As defined in clause 7.3.2.b the Timing information element contains an estimate of the difference between its external time source and the Titimestamp included in the frame (using the TTOE field) as well as the standard deviation of the error inof that estimate (using the TTOE-STD field). A STA with an external time source can implement an estimator in a variety of ways, which are beyond the scope of this standard. The SME provides the Timing information element to the MLME when it requests the MLME to send a Timing Advertisement frame. When a Timing Advertisement frame is received by a STA its MLME reports the timestamp and Timing information element to the SME.
In order to support this Timing Advertisement frame and Timing information element, the MLME-GETTSFTIME.request and MLME-GETTSFTIME.confirm primitives were added to clause 10. (In order to accurately estimate the TTOE the MLME must be able to provide the current value of its TSF timer to the SME in a known or reasonably bounded time between the instant the SME requests the current value of the TSF timer and the instant the MLME reports it.) In order for the SME to request the transmission of a Timing Advertisement frame Tthe MLME-TIMING_ADVERTISEMENT.request primitive was also added to clause 10 and includes the Timing information element. Finally, the MLME-TIMING_ADVERTISEMENT.confirmindication primitive was added to clause 10 for reception of the frame and includes both the timestamp and Timing information element. using Timing Advertisement frames and TIEs
A STA choosing to synchronize to the external time base of another STA may do so using the TSF Timer primitives provided along with the information contained in the TIE. For example, a STA may choose to synchronize to Universal Coordinated Time (UTC) time being kept and advertised by another STA. UTC is defined in ITU-R TF.460-4 (1986) [ed: add this reference to the list of references and insert the associated pointer here] and can be obtained externally from many sources including GPS. GPS receivers typically provide a precise 1 pulse per second (PPS) UTC signal (with an error less than 100 nsec), and these precise 1 PPS signals can be used for timing and synchronization. A STA that chooses to synchronize to external time being advertised by one or more other STAs using Timing Advertisement frames may estimate external time as the value of its TSF timer plus any offsets estimated using the information contained in the TIEs (see 7.3.2.b) received from the other STAs. This estimate of external time can be used for many purposes including synchronization of RF channel changes.
STAs that have a source of external time can easily implement an estimator in a variety of ways, all of which are beyond the scope of this standard. STAs that do not have an external source of time (UTC or any other) may nonetheless estimate external time from Timing Advertisement frames containing a Timestamp and a Timing information element received from STAs that do have an estimate of time (UTC or any other). The details of how this can be accomplished are beyond the scope of this standard. The Timestamp field gives an estimate of the value of the TSF timer of the transmitting STA at the moment the first bits of the Timestamps were transmitted out the transducer (antenna). TIEs provide sufficient information to estimate offsets to the Timestamp value so that an estimate can be made of the external time source at the transmission time. The local timestamp attached by the receiving STA to the received Timing Advertisement frame gives an estimate of the local TSF timer value at the instant of reception of each Timing Advertisement frame. This information is sufficient for the receiving STA to produce an estimate (biased if the propagation delays from the transmitters to the receiver is not accounted for properly) of the external clock on the transmitting STAs. The details of how this is implemented are beyond the scope of this standard, but statistically unbiased (this is a bit difficult to achieve in these applications) and efficient estimators are preferred. Such estimators also produce estimates of the uncertainties of the parameters they estimate, and these uncertainties are included in the TIE as well.
Motion: Move to accept these changes to P802.11p D5.01 and instruct the editor to update the latest 11p draft to include these changes.
Moved:
Seconded:
YES:
NO:
ABSTAIN:
page 3 Justin McNew, Kapsch