Transcript doc.: IEEE 802.15-01/440r2 Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: IEEE802.15.3: Guard Time in the 802.15.3

<September 2001>
doc.: IEEE 802.15-01/440r2
Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: IEEE802.15.3: Guard Time in the 802.15.3 MAC.
Date Submitted: 19 September, 2001
Source: Dr. William Shvodian Company: XtremeSpectrum
Address: 8133 Leesburg Pike, Suite 700, Vienna, Virginia 22182
Voice: +1.703.269-3047, FAX: +1.703.269.3092, E-Mail: [email protected]
Re: []
Abstract: This proposal describes the use of Guard Time for the 802.15.3 MAC.
Purpose: To provide an improvement to the current version of the 802.15.3 MAC
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.
Submission
Slide 1
<William Shvodian>, <XtremeSpectrum>
<September 2001>
doc.: IEEE 802.15-01/440r2
Overview
• This presentation describes the use of Guard
Time for 802.15.3
• It also describes Guard Time modifications for
static GTS slots
Submission
Slide 2
<William Shvodian>, <XtremeSpectrum>
<September 2001>
doc.: IEEE 802.15-01/440r2
Guard Time
Beacon
CAP
GTS0
GTS1
GTS2
GTS3
GTS4
Beacon
• In a TDMA system, Guard Time is needed between
slots to keep the transmissions from two stations
colliding.
• Each station synchronizes to the beacon, but clock
inaccuracies result in drift
• The maximum drift between the ideal time and a
station’s clock is:
MaxDrift 
Submission
Clock accuracy(ppm)
* superframeduration( sec)
1,000,000
Slide 3
<William Shvodian>, <XtremeSpectrum>
<September 2001>
doc.: IEEE 802.15-01/440r2
Guard Time Position
Beacon
CAP
GTS0
GTS1
GTS2
GTS3
GTS4
Beacon
• Guard time is time between adjacent slots
• Guard time can be defined as being at
– the beginning of the slot
Guard
– the end of the slot
– half at the beginning of the slot and half at the end of the G
slot.
– For Simplification, the basic Guard Time for each slot in
802.15.3 will be at the end of the slot. The slot time in the CTA
will denote the beginning of a GTS. Guard time occurs at the
end of the slot, before the start of the next GTS.
Submission
Slide 4
Guard
<William Shvodian>, <XtremeSpectrum>
G
<September 2001>
doc.: IEEE 802.15-01/440r2
Worst Case Timing (Transmission)
drift
drift
Ideal GTS n position
Ideal GTS n+1 position
Late Station’s Estimate of GTS position
Early Station’s Estimate of GTS position
SIFS
Guard Time
• The above picture illustrates the maximum offset between two
stations. The station with slot n has drifted the maximum time to the
right (late), while the station assigned to slot n+1 has drifted the
maximum amount to the left (early).
• In order to avoid transmission collisions or violating SIFS, the guard
time must be calculated to be two times the maximum drift plus SIFS
Submission
Slide 5
<William Shvodian>, <XtremeSpectrum>
<September 2001>
doc.: IEEE 802.15-01/440r2
Worst Case Timing (receive)
Early Station Transmits
Ideal GTS n position
Guard Time
Late Station’s Estimate of GTS position
Guard Time
Late Station’s Receive Window
– Since a station doesn’t know whether it is early or late, it must start
listening one full Guard Time before where it estimates the start of the
GTS slot to be and it must listen until the time that it estimates the
start of the next GTS slot to be - it must listen during the time that it
estimates to be Guard Time - on both sides of the GTS slot.
Submission
Slide 6
<William Shvodian>, <XtremeSpectrum>
<September 2001>
doc.: IEEE 802.15-01/440r2
Constant vs. Variable Guard Time
Beacon
CAP
GTS0
GTS1
GTS2
GTS3
GTS4
Beacon
• The minimum amount of guard time is a function of the
amount of time from the reference event to the event of
interest
• More guard time is actually needed at the end of a
superframe than the beginning because it is farther from
the beacon, which provides the reference timing. For
simplicity, the same guard time will be used for the entire
superframe in 802.15.3.
Submission
Slide 7
<William Shvodian>, <XtremeSpectrum>
<September 2001>
doc.: IEEE 802.15-01/440r2
Guard Time for static GTS slots
Beacon
CAP
GTS0
GTS1
GTS2
GTS3
GTS4
Beacon
• Static GTS slots allow a station to transmit in a superframe when it did
not receive the beacon correctly.
• If a device with static GTS slots receives the Beacon header
correctly, it can synchronize to the beacon header
• A device with static GTS slots does not need to correctly process
the CTAs in the beacon.
• If a station with static GTS slots does not correctly receive the beacon
header, additional guard time is needed.
• Dynamically changing the guard time to account for missed
beacons requires adding guard time at the start of a GTS and at
the end of the GTS and increases complexity.
• Fixed guard time can be used to account for the worst case –
maximum missed beacons. This is simpler and will be
recommended for 802.15.3
Submission
Slide 8
<William Shvodian>, <XtremeSpectrum>
<September 2001>
doc.: IEEE 802.15-01/440r2
Guard Time Calculation when Static
GTS allowed
• Guard time is calculated by the PNC
• Guard Time is a new field in the Piconet
Synchronization Parameters Information
Element.
GuardTime (MaxLostBeacons 1) * MaxDrift* 2  SIFS
Submission
Slide 9
<William Shvodian>, <XtremeSpectrum>
<September 2001>
doc.: IEEE 802.15-01/440r2
Guard Time Overhead
(function of superframe duration)
ppm
Clock Acuracy
Superframe duration us
drift
25
25
25
25
25
25
25
0.000025 0.000025 0.000025 0.000025 0.000025 0.000025 0.000025
7812.5
15625
31250
62500
125000
250000
500000
0.195313 0.390625 0.78125
1.5625
3.125
6.25
12.5
number of missed beacons
drift guard time (us)
SIFS
Total guard time
OH per GTS
4
4
1.953125 3.90625
17
17
18.95313 20.90625
0.24%
0.134%
4
7.8125
17
24.8125
0.079%
4
15.625
17
32.625
0.05%
4
31.25
17
48.25
0.04%
4
62.5
17
79.5
0.03%
4
125
17
142
0.03%
2.43%
6.07%
12.13%
24.26%
60.65%
121.30%
242.60%
0.79%
1.99%
3.97%
7.94%
19.85%
39.70%
79.40%
0.52%
1.31%
2.61%
5.22%
13.05%
26.10%
52.20%
0.39%
0.97%
1.93%
3.86%
9.65%
19.30%
38.60%
0.32%
0.80%
1.59%
3.18%
7.95%
15.90%
31.80%
0.28%
0.71%
1.42%
2.84%
7.10%
14.20%
28.40%
GTSs
17
10
25
50
100 Total OH
250
500
1000
1.34%
3.35%
6.69%
13.38%
33.45%
66.90%
133.80%
• Setting Guard Time based on superframe
duration yields reasonable overhead.
Submission
Slide 10
<William Shvodian>, <XtremeSpectrum>
<September 2001>
doc.: IEEE 802.15-01/440r2
Guard Time Overhead
(fixed to 142s regardless of superframe duration)
ppm
Clock Acuracy
Superframe duration us
drift
25
25
25
25
25
25
25
0.000025 0.000025 0.000025 0.000025 0.000025 0.000025 0.000025
7812.5
15625
31250
62500
125000
250000
500000
0.195313 0.390625 0.78125
1.5625
3.125
6.25
12.5
number of missed beacons
drift guard time (us)
SIFS
Total guard time
OH per GTS
4
1.953125
17
142
1.82%
4
3.90625
17
142
0.909%
4
7.8125
17
142
0.454%
4
15.625
17
142
0.23%
4
31.25
17
142
0.11%
4
62.5
17
142
0.06%
4
125
17
142
0.03%
18.18%
45.44%
90.88%
181.76%
454.40%
908.80%
1817.60%
9.09%
22.72%
45.44%
90.88%
227.20%
454.40%
908.80%
4.54%
11.36%
22.72%
45.44%
113.60%
227.20%
454.40%
2.27%
5.68%
11.36%
22.72%
56.80%
113.60%
227.20%
1.14%
2.84%
5.68%
11.36%
28.40%
56.80%
113.60%
0.57%
1.42%
2.84%
5.68%
14.20%
28.40%
56.80%
0.28%
0.71%
1.42%
2.84%
7.10%
14.20%
28.40%
GTSs
17
10
25
50
100 Total OH
250
500
1000
• Setting Guard Time to worst case (142s)
regardless of superframe duration causes
unreasonable overhead for short superframes.
Submission
Slide 11
<William Shvodian>, <XtremeSpectrum>
<September 2001>
doc.: IEEE 802.15-01/440r2
Guard Time relationship to
Channel Time Requests and CTAs
• Station requests channel time which includes one SIFS per
frame (and per ACK if applicable)
• PNC adds drift part of guard time, but not SIFS. This simplifies
calculations for the requesting device and for the PNC if the
PNV combines or divides GTS slots.
Frame 1
SIFS
Ack SIFS
Frame2
SIFS Ack SIFS
Frame 3
SIFS
Ack SIFS
drift
Channel Time Request
Guard
Time
Slot n Start Time in CTA n
Slot n+1 Start Time in CTA n+1
Submission
Slide 12
<William Shvodian>, <XtremeSpectrum>
<September 2001>
doc.: IEEE 802.15-01/440r2
End of transmission
• A transmitting device shall ensure that transmission will end one
guard time before the start of the next GTS slot based on the
transmitting stations time reference.
• If Immediate ACK is used, the transmitting device shall not
transmit a frame unless there is sufficient time for the frame, a
SIFS, the ACK and the Guard Time
Frame+SIFS+ACK+Guard Time
Frame 1
SIFS
Ack SIFS
Frame2
SIFS Ack SIFS
Frame 3
SIFS
Ack SIFS
Guard
Time
Slot n Start Time in CTA n
Slot n+1 Start Time in CTA n+1
Submission
Slide 13
<William Shvodian>, <XtremeSpectrum>
<September 2001>
doc.: IEEE 802.15-01/440r2
Guard Time for 802.15.3
• Fixed Guard time for all GTS slots
broadcast in the Beacon
• New Field in the synchronization
Information Element
– GuardTime
Submission
Slide 14
<William Shvodian>, <XtremeSpectrum>