Tutorial on Packet Time Metrics

Transcription

1 Power Matters. Tutorial o Packet Time Metrics Lee Cosart lee.cosart@microsemi.com ITS Microsemi Corporatio. COMPANY POPIETAY

2 Itroductio requecy trasport Oe-way: forward & reverse packet streams ca be used separately Asymmetry is irrelevat Stable frequecy eeded PC primary referece clock eeded GNSS/GPS atea cable compesatio/calibratio ot eeded GSM frequecy backhaul 50 ppb is example techology Time trasport Two-way: forward & reverse packet streams used together Asymmetry is critical Stable time ad frequecy eeded PTC primary referece time clock eeded GNSS/GPS atea cable compesatio/calibratio eeded LTE-TDD time/phase.5 µsec is example techology 204 Microsemi Corporatio. COMPANY POPIETAY Power Matters. 2

3 Testig Time Physical vs. Packet PPS Sigle Poit Measuremet Measuremets are made at a sigle poit a sigle piece of equipmet i a sigle locatio - a phase detector with referece - is eeded PPS UTC PPS PTC Network 0 s s s s Time Iterval Measuremet Software PPS Packet Dual Poit Measuremet Measuremets are costructed from packets time-stamped at two poits i geeral two pieces of equipmet, each with a referece, at two differet locatios are eeded Timestamp A Timestamp B PTC GigE Network UTC Probe PDV Measuremet Software 204 Microsemi Corporatio. COMPANY POPIETAY Power Matters. 3

4 Time Accuracy ad Stability equiremets PPS UTC PTC PPS PTC Time Accuracy MTIE G.8272 Time Iterval Measuremet Software Time Error: <=00s Time Stability TDEV Packet Network Limits MTIE is G.8 with 00 s maximum TDEV is G.8 exactly A: Time Error: <=00s C: Time Error: <=.µs G Microsemi Corporatio. COMPANY POPIETAY Power Matters. 4

5 Stability metrics for PDV Packet Selectio Processes Pre-processed: packet selectio step prior to calculatio Example: TDEVPDVmi where PDVmi is a ew sequece based o miimum searches o the origial PDV sequece 2 Itegrated: packet selectio itegrated ito calculatio Example: mitdevpdv Packet Selectio Methods x i mi x for i j i Miimum: Percetile: Bad: Cluster: x mi pct _ mea i i x x bad_ mea K j b m j0 b x m ja j i x j i w K i P, i i0, i 0 K i0, i for w K i 0 otherwise 204 Microsemi Corporatio. COMPANY POPIETAY Power Matters. 5

6 Packet Selectio Widows Widows No-overlappig widows ext widow starts at prior widow stop Skip-overlappig widows widows overlap but startig poits skip over N samples Overlappig widows widows slide sample by sample Packet Selectio Approaches e.g. selectig fastest packets Select X% fastest packets e.g. 2% Select N fastest packets e.g. 0 fastest packets i a widow Select all packets faster tha Y e.g. all packets faster tha 50μs 204 Microsemi Corporatio. COMPANY POPIETAY Power Matters. 6

7 G.8260 Appedix I Metrics. Packet Time Error Sequece Selected-Packet Time Error Sequece xt Etire PDV populatio Packet Selectio x t Selected subset with commo delay properties Stability Metric Estimated achievable performace Pre-processed packet selectio. Etire PDV populatio Stability metric with packet selectio Estimated achievable performace G.82600_I.4 Itegrated packet selectio. Packet Time Error Sequece Selected-Packet Time Error Sequece iltered-packet Time Error Sequece xt Packet Selectio x t Badwidth ilterig yt Stability Metric Metrics icludig pre-filterig. PC, P, PP: loor Packet Cout/ate/Percet PDV metrics studyig miimum floor delay packet populatio 204 Microsemi Corporatio. COMPANY POPIETAY Power Matters. 7

8 Power Matters Microsemi Corporatio. COMPANY POPIETAY Time Trasport: Two-way metrics Packet Time Trasport Metrics Normalized roudtrip: 2 r Normalized offset: 2 2 xoudtrip: 2 r xoffset: m m m 2 2 p p p 2 2 c c c mioffset: pctoffset: clusteroffset: Weighted average: a a w where 0 ɑ xtdisp mi/pct/clst time dispersio: xoffset {y} plotted agaist xoudtrip {x} as a scatter plot xoffset statistics: xoffset statistic such as mea, stadard deviatio, media, 95 percetile plotted as a fuctio of time widow tau

9 Time Trasport: Two-way packet delay orward Packet Delay Sequece #Start: 200/03/06 7:5: ,.47E ,.54E ,.23E ,.40E ,.47E ,.5E-6 #Start: 200/03/06 7:5: ,.47E-6,.E ,.54E-6,.09E ,.23E-6,.2E ,.40E-6,.3E ,.47E-6,.22E ,.5E-6,.05E-6 Packet Delay Sequeceeverse #Start: 200/03/06 7:5: ,.E ,.09E ,.2E ,.3E ,.22E ,.05E-6 Two-way Data Set Costructig f ad r from f ad r with a 3- sample time widow Times fµs rµs f µs r µs Microsemi Corporatio. COMPANY POPIETAY Miimum Search Sequece mioffset 2 2 Power Matters. 9

10 Time Trasport: Two-way metrics orward/everse PP orward PP everse PP Approaches: Based o both oe-way sequeces 2 Based o a sigle sequece costructed from both oeway sequeces e.g. offset Two-way MAE MAE of mioffset Commets: Kowledge of asymmetry ad latecy i both directios is critical 2 Offset is a fudametal two-way calculatio 3 Ideal fwd/rev packet: floor Ideal offset: zero Two-way MAE orward MAE everse MAE 204 Microsemi Corporatio. COMPANY POPIETAY Power Matters. 0

11 Offset Network Asymmetry Asymmetry i Wireless Backhaul Etheret wireless backhaul asymmetry ad IEEE 588 slave PPS uder these asymmetrical etwork coditios 6.0µs 0.5 µs/ div Symmetricom TimeMoitor Aalyzer; Etheret Wireless Backhaul; 2009/04/28; :37:0 mioffset vs mioudtrip Mi TDISP 2.0 µs 2.0 µs 0.5 µs/ div -.0 µs µs µs 0.0 hours 22.7 hours 588 Slave PPS vs.gps 204 Microsemi Corporatio. COMPANY POPIETAY Power Matters.

12 Network Asymmetry 50 km fiber SONET trasport Offset is 20.4 µsec which represets the 40.8 µsec differece betwee forward ad reverse oe-way latecies ev: 2.04 ms wd:.974 ms 204 Microsemi Corporatio. COMPANY POPIETAY Power Matters. 2

13 Coclusios Packet time trasport measuremets require commo time scale referece at both eds of the etwork beig studied GNSS at both eds is a way to do this Asymmetry is everywhere, asymmetry is ivisible to the IEEE 588 protocol, thus asymmetry has a direct bearig o the ability to trasport time precisely The offset calculatio is a direct measure of asymmetry There are two ways to assess time trasport: measurig a PPS referece at the ode beig studied ad 2 measurig a packet sigal at the ode beig studied Packet metrics for time trasport must use both forward ad reverse streams together rather tha separately as is the case for frequecy trasport Packet metrics for time trasport ca make use of much of the methodology used for packet frequecy trasport metrics 204 Microsemi Corporatio. COMPANY POPIETAY Power Matters. 3

14 Thak You Lee Cosart Seior Techologist Phoe: Microsemi Corporatio. COMPANY POPIETAY Power Matters. 4