802.1AS: Time-of-day synchronization (interworking...
Transcript of 802.1AS: Time-of-day synchronization (interworking...
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IEEE 802.1AS PlenarySan Diego, CA
1July 18, 2006
802.1AS:802.1AS:
TimeTime--ofof--day synchronizationday synchronization((interworkinginterworking layer?)layer?)
Thoughts by David V James
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IEEE 802.1AS PlenarySan Diego, CA
2July 18, 2006
Cascaded TOD synchronizationCascaded TOD synchronization
bridge[0]
bridge[1]
bridge[2]
Wall-clock distribution model
1Gb Ethernet 100Mb Ethernet 802.11EPON
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IEEE 802.1AS PlenarySan Diego, CA
3July 18, 2006
Cascaded TOD synchronizationCascaded TOD synchronization
bridge[0]
bridge[1]
bridge[2]
Cascaded adjacent-synchronization hierarchy
Legend:clock masterclock slave
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IEEE 802.1AS PlenarySan Diego, CA
4July 18, 2006
802.1as:802.1as:
What are theWhat are theinterworkinginterworking parameters?parameters?
When bridging across 802 networks…
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IEEE 802.1AS PlenarySan Diego, CA
5July 18, 2006
uniqueID
InterworkingInterworking layerlayerWhat are the basic parameters?
fractionsseconds
timeOfDay
superSecs
leapSecs
(UTCOffset)
grandMasterPrecedence
(epochNumber)
pri
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IEEE 802.1AS PlenarySan Diego, CA
6July 18, 2006
identstrat lo uniqueID
InterworkingInterworking layerlayerWhat are the extended parameters?
fractionseconds
timeOfDay
secondsHi
extraSecs
(UTCOffset)
grandMasterPrecedence
tech
(Oops!)
(subNs)(epochNumber)fractionLo
hi variance
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IEEE 802.1AS PlenarySan Diego, CA
7July 18, 2006
802.1as:802.1as:
TimeTime--sync on EPONsync on EPONwhich already does which already does ““almostalmost”” thisthis
A possibility for illustration purposes…
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IEEE 802.1AS PlenarySan Diego, CA
8July 18, 2006
EPON synchronizationEPON synchronizationCouple into the existing protocol!
Head end Homes
t1 t1 t1 t1 t1
t1
{t0, t1}
We want t0, butwe only have t1!
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IEEE 802.1AS PlenarySan Diego, CA
9July 18, 2006
Basic conceptsBasic concepts
• The grand master can observe both times– Measured at the same time.– But, at almost any snap-shot time is acceptable.
• There is no need for distinct 1588 packets!– Lower level protocols already provide synchronization,
since that facility is needed for other purposes.– That mechanism already exists (32-bit, 16ns ticks).
• But, the interworking interface should be defined…
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IEEE 802.1AS PlenarySan Diego, CA
10July 18, 2006
802.1as:802.1as:
TimeTime--sync on 802.11sync on 802.11which already does which already does ““almostalmost”” thisthis
The “beacon” provides a time …
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IEEE 802.1AS PlenarySan Diego, CA
11July 18, 2006
802.11 synchronization802.11 synchronizationCouple into the existing protocol!
t2 t2 t2 t2 t2
t2
{t0, t2}
We want t0, butwe only have t2!
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IEEE 802.1AS PlenarySan Diego, CA
12July 18, 2006
802.1AS:802.1AS:
TimeTime--ofof--day synchronizationday synchronizationpointpoint--toto--point Ethernet linkspoint Ethernet links
High-level principles…
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IEEE 802.1AS PlenarySan Diego, CA
13July 18, 2006
local
add
global
AdjacentAdjacent--station synchronizationstation synchronization
local offsetadd
global
Offset value adjustments
stationA: master stationB: slave
10 ms
10 ms
10 ms
offset
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IEEE 802.1AS PlenarySan Diego, CA
14July 18, 2006
Minimal HW design modelMinimal HW design model
PHY
MACclientglobalTime
rxStrobe txStrobe
txrx FIFOFIFO
convert
Notes:Rate matching FIFOsare not within our scope.
tickTimer
(firmware computations)
txTimerxTime
tickTimer properties:Increment at <= 10 nsRestarts every second
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IEEE 802.1AS PlenarySan Diego, CA
15July 18, 2006
preference
Frame formatFrame format
destinationAddress
Grand-master selection
sourceAddressprotocolType
662
subType1syncCount1hopsCount1
1uniqueID8
timeOfDay10
12
localTime4deltaTime4
fcs4
Cable-delay&crystal-rate calibration
reserved
totalRates4
Time distribution
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IEEE 802.1AS PlenarySan Diego, CA
16July 18, 2006
Clock calibration modelClock calibration model
availableClock
Conceptual clock-generation model
convertToCommon
multiply
addition
timeOfDay
(1.0 + rating)
offsetTime
localTimer
timeOfDay
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IEEE 802.1AS PlenarySan Diego, CA
17July 18, 2006
Adjusting Adjusting timeOfDaytimeOfDay valuesvaluesSnapshot value distributions
snapshotA
myTimeOfDay
snapshotB
timeOfDay
offsetTime += (timeOfDay – snapshotB) – cableDelay;
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IEEE 802.1AS PlenarySan Diego, CA
18July 18, 2006
Computing Computing cableDelaycableDelay valuesvaluesBased on localTimer reference values…
snapshotA
localTimer
snapshotB
localTime
cableDelay = ((snapshotB - localTime) + deltaTime)/2;
deltaTime
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IEEE 802.1AS PlenarySan Diego, CA
19July 18, 2006
local
add
global
CableCable--delay parametersdelay parameters
aTx2
local offsetadd
global aRx2
bRx2
bTx2
Snapshot value distributions
stationA: master stationB: slave
aTx1aRx0-bTx0 offset
bTx1bRx0-aTx0
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IEEE 802.1AS PlenarySan Diego, CA
20July 18, 2006
Calibrating rate values (100ms)Calibrating rate values (100ms)Based on localTimer reference values…
snapshotA
localTimer
snapshotB
localTime
myRating = (localDiff – snapDiff)/snapDiff;localDiff = localTime[n+10] – localTime[n];snapDiff = snapshotB[n+10] – snapshotB[n];
totalRates
myRating
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IEEE 802.1AS PlenarySan Diego, CA
21July 18, 2006
Uncompromised precisionUncompromised precision
timeOfDaydeviation
time
2 μs
5 ns
470 ms 480 ms 490 ms 500 ms 510 ms 520 ms 530 ms460 ms
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IEEE 802.1AS PlenarySan Diego, CA
22July 18, 2006
GrandGrand--master selection protocolmaster selection protocolGrand-master precedence comparisons
MinimumValue
hopsCount += 1thisPrecedence
MinimumValue
hopsCount += 1thisPrecedence
Grand-master
Clock-slave
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IEEE 802.1AS PlenarySan Diego, CA
23July 18, 2006
802.1AS:802.1AS:
Keeping it simple (KISS)Keeping it simple (KISS)——focus on the fundamentalsfocus on the fundamentals1588 Instrument legacy is interesting, but…
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IEEE 802.1AS PlenarySan Diego, CA
24July 18, 2006
Minimal packet typesMinimal packet types
Required 1588 messages:1) Sync2) Follow_Up3) Announce4) Pdelay_Req5) Pdelay_Resp6) Pdelay_Resp_Follow_Up
Possibly required 1588 messages:7) management messages (only those that are needed)8) signaling messages (only those that are needed)>>
Periodic 10ms transmissions
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IEEE 802.1AS PlenarySan Diego, CA
25July 18, 2006
No timing dependenciesNo timing dependencies
Send every 10ms
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IEEE 802.1AS PlenarySan Diego, CA
26July 18, 2006
Normal timeNormal time--ofof--day day ““integersintegers””
nanosecondsseconds
(IEEE 1588)
OR
ticks
(EPON)
16 ns
fractionsseconds
>150 years <250 ps
(NTP RFC-1305, SNTP RFC-2030)secondsHi
epochNumber
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IEEE 802.1AS PlenarySan Diego, CA
27July 18, 2006
TIME FOR QUESTIONSTIME FOR QUESTIONS
TBD
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IEEE 802.1AS PlenarySan Diego, CA
28July 18, 2006
802.1AS:802.1AS:
TimeTime--ofof--day synchronizationday synchronization((interworkinginterworking layer?)layer?)
Backup slides
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IEEE 802.1AS PlenarySan Diego, CA
29July 18, 2006
pref
port
GrandGrand--master precedencemaster precedence
stationID (byte swapped EUI-64)
1394 precedencepreferred
1394 precedence(larger)
eui64 hops port
transmitted values
uniquenessGM precedence(smaller)
snsl eui48 hops pl
portsystem uniqueness ageSTP precedence(smaller)
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IEEE 802.1AS PlenarySan Diego, CA
30July 18, 2006
Rate adjustmentsRate adjustments
Compute nearest neighbor errors – Based on adjacent baseTimer information
• Cumulative values are computed– Rate differences are added in a cascaded fashion
• The grand-master “timer” is assumed to be correct• Rate changes after grand-master changes
– Saving rate offsets complicates the protocols– Could degrade the new-grand-master accuracy
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IEEE 802.1AS PlenarySan Diego, CA
31July 18, 2006
Backup slides forBackup slides forResidential Ethernet:Residential Ethernet:
TimeTime--ofof--day timer synchronizationday timer synchronization
Maintained by David V James
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IEEE 802.1AS PlenarySan Diego, CA
32July 18, 2006
RERE--SG basic requirementsSG basic requirements
• Cheap via simplicity – Delayed snapshots– Periodic symmetric transmissions
• Cheap and precise– Limited to snapshot capture accuracy– Minimal grand-master handover transients
• Cheap and robust– Single-phase grand-master selection
• Cheap and responsive– Short rogue-frame lifetimes
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IEEE 802.1AS PlenarySan Diego, CA
33July 18, 2006
House reference clockHouse reference clock
802.11e
Ethernet
802.11e
1394 1394
Room #1 Room #2
Ethernet
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IEEE 802.1AS PlenarySan Diego, CA
34July 18, 2006
FIFO
Precise time synchronizationPrecise time synchronization
talker FIFO
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IEEE 802.1AS PlenarySan Diego, CA
35July 18, 2006
TimeTime--ofof--day format optionsday format options
fractionsseconds
nanosecondsseconds
>150 years <250 ps
(IEEE 1588)
OR(…)
ticks
(EPON)
16 ns
(NTP RFC-1305, SNTP RFC-2030)
(+epoc)
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IEEE 802.1AS PlenarySan Diego, CA
36July 18, 2006
Basic requirementsBasic requirements
• KISS (keep it simple, stupid) – Delayed snapshot processing– Periodic symmetric transmissions– Etc., etc.
• NTP (RFC-1305) and SNTP (RFC-2030)– Definition of the 64-bit time-of-day value
• For a detailed summary, see:– http://dvjames.com/esync– dvjTimeSync2005Dec12.pdf (or later revision)
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IEEE 802.1AS PlenarySan Diego, CA
37July 18, 2006
timeOfDay
flexTimer
myOffset
flexOffset
timeOfDay
flexTimer
myOffset
flexOffset
Template pictureTemplate picture
timeOfDay
flexTimer
myOffset
flexOffset
grand-master