Oscillator Impact on PDV and Design of Packet Equipment Clocks. ITSF 2010 Peter Meyer

Transcription

1 Oscillator Impact on PDV and Design of Packet Equipment Clocks ITSF 2010 Peter Meyer

2 Protocol Layer Synchronization When deployed and inter-connected within the packet network the packet equipment clocks will allow frequency, phase and time to be transferred over the packet network Different types of packet equipment clocks (PEC) PEC-M the input is physical timing and the output is packet timing signal PEC-B the input is a packet timing signal and the output is a packet timing signal PEC-S the input is a packet timing signal and the output is a physical timing signal PRS/PRC PRTC PEC-B PEC-B PEC-B PEC-B PEC-M PEC-B PEC-B PEC-B PEC-S [Page 2]

3 Frequency & Time Transfer over PSN Two approaches A PSN may be inserted between the server and client, that is not aware of protocol layer synchronization packets (e.g. IEEE ) The PSN has on-path support where each switch / router is aware of protocol layer synchronization packets (e.g. IEEE ) [Page 3]

4 PEC Model & Generic Requirements

5 Protocol Layer EC Functional Model ITU-T G.8263 (draft) Annex includes a functional model of a PEC-S packet-based clock Local reference PEC-S PEC differs from traditional EC with introduction of a packet selection block has been included Packet Timing Signal Packet Selection Time Scale Comparator Low Pass filter Local Time scale Oscillator Output Clock The PLL filters the network wander with a low pass filter This means the PLL acts as a high pass filter for the local XO [Page 5]

6 PEC-S Functional Model: Packet Selection & Low Pass Filter Goal of the packet selection block is to select from all the input packets to the packet equipment clock a certain subset that are the least affected by the packet switched network These packets would thus best reflect the timing signal at the transmitter Both the packet selection block and the low pass filter function to remove noise from the packet timing signal to faithfully re-create the timing source The cleaned timing signal can then be used to discipline the local oscillator Eliminate Noise from Packet Timing Signal [Page 6]

7 Equipment Clock Specifications Definition of EC Jitter & Wander Generation Jitter & Wander Transfer Jitter & Wander Tolerance Holdover Transients Freerun Oscillator dominant factor in meeting parts of the specification Wander Generation (both MTIE & TDEV) Holdover Stability (both constant & variable temperature) Freerun Accuracy Packet EC Model [Page 7]

8 Oscillator-Dependent EC Characteristics Wander Generation The amount of wander generated by the EC when locked to an ideal reference Oscillator noise measured in the time domain using MTIE & TDEV metrics Holdover Stability The stability of an EC when after losing lock to its input reference Oscillator drift due to ageing, temperature, voltage and other effects measured in the frequency domain Freerun Accuracy The accuracy of an EC without using an input reference Oscillator error due to all error sources in the frequency domain [Page 8]

9 Example: Oscillator Requirements for Stratum 3E Looking at Stratum 3E EC, with a focus on the oscillator, yields the following requirements to be met by the oscillator specification Other ECs (Stratum 3, SMC, etc.) would have similar requirements Requirements Free-run Frequency Accuracy ±4.6 ppm Wander Generation MTIE & TDEV masks specified in ITU-T G.812 Type III & Telcordia GR CORE Stratum 3E, using 1 mhz clock bandwidth Holdover Stability ± 1 ppb/day at constant temperature (1.16x10^-5 ns/s^2) 10 ppb over temperature range [Page 9]

10 Design Considerations of Packet Equipment Clock

11 PEC Design Considerations Trade-off between PDV noise (LPF) and XO noise (HPF) Effects of XO on packet selection Possible PEC characteristics & XO requirements Before Filtering t After Filtering t [Page 11]

12 Trade-off Between PDV and XO PDV and XO noise can be shown on a frequency spectrum plot Network PDV has wide frequency spectrum Ramp test case has 12 uhz fundamental frequency On/Off test case has 139 uhz fundamental frequency XO has increasing magnitude at low frequency XO TCXO Network PDV 0 OCXO Low Pass Filter Value XO f Where to place the loop filter? May be governed by wander generation of XO No specification defined for PEC Common specification may not apply for mobile backhaul 0 PDV < TCXO LPF OCXO TCXO > HPF Network PDV f [Page 12]

13 Ramp (TC13) and Square (TC14) Fundamental Frequency 2158 tau: 139 uhz Square TC s tau: 12 uhz Ramp TC mhz 10 mhz 1 mhz 0.1 mhz 1.00E E-06 TDEV (s) 1.00E E E E E E E E E E E E+06 Observation time (s) [Page 13] E1, TDEV, G.823, SEC E1, TDEV, G.8261, EEC Option 1 T1, TDEV, T1.101, OC-N Sync Ref T1, TDEV, G.824, Sync Reference T1, TDEV, T1.101, DS1 Sync Ref E1, TDEV, G.823, Sync PDH T1, TDEV, G.824, Option 2 SEC T1, TDEV, G.8261, EEC Option 2 T1, TDEV, T , SONET Ref

14 Ramp (TC13) TDEV 2158 tau: 139 uhz Square TC s tau: 12 uhz Ramp TC mhz 10 mhz 1 mhz 0.1 mhz 1.00E E E-07 TDEV (s) 1.00E E E E E E E E E E E E E+06 Observation time (s) [Page 14] E1, TDEV, G.823, SEC E1, TDEV, G.8261, EEC Option 1 E1, TDEV, G.823, C O Sync N F I PDH D E N T I 1 A mhz: L TC13, Square, TM2 0.1 mhz: TC13, Square, TM2

15 Ramp (TC13) MTIE 1.00E E E-06 MTIE (s) 1.00E E E E E E E E E E E E+05 Observation time (s) E1, MTIE, G.823, SEC E1, MTIE, G.8261, EEC Option 1 E1, MTIE, G.823, Sync PDH 0.1 mhz: TC13, Square, TM2 1 mhz: TC13, Square, TM2 [Page 15]

16 Relationship between PDV, XO and Clock Bandwidth

17 Wander Generation vs. Clock Bandwidth With a 10 mhz loop filter this oscillator has a low TIE and TDEV noise mhz Loop Filter Computed TDEV G.824 Envelop (T7/F5) G.824 Envelop (T6/F4) G.823 Envelop (T11/F9) G.823 Envelop (T13/F11) TDEV for 10 mhz Loop Filter TIE (ns) 0 TDEV (ns) Seconds x Observation interval (seconds) With a 1 mhz loop filter there is significantly MORE noise contributed by the oscillator A lower the loop filter will filter LESS oscillator noise Cannot keep lowering the loop filter to be more robust against PDV without increasing the cost of the equipment! TIE (ns) mhz Loop Filter TDEV (ns) Computed TDEV G.824 Envelop (T7/F5) G.824 Envelop (T6/F4) G.823 Envelop (T11/F9) G.823 Envelop (T13/F11) TDEV for 1 mhz Loop Filter [Page 17] Seconds x Observation interval (seconds)

18 Wander Generation vs. Clock Bandwidth Wander Generation MTIE 3 mhz, 1 mhz, 0.3 mhz & 0.1 mhz clock bandwidths 1 mhz 0.1 mhz results in 10x more 8000 s [Page 18]

19 Wander Generation vs. Clock Bandwidth Wander Generation TDEV 3 mhz, 1 mhz, 0.3 mhz & 0.1 mhz clock bandwidths 1 mhz 0.1 mhz results in >4x more 1000 s [Page 19]

20 Oscillator Selection Impact on Packet Selection

21 Packet Selection vs. Oscillator Cleaned packet timing signal used to discipline local oscillator Will the oscillator movement impact on the packet selection to reduce estimated performance If there was originally a stable floor delay, how does it appear to move based on a non-ideal local oscillator? What is inter-packet gap between selected packets and how should this be adjusted to match the non-ideal local oscillator? + = [Page 21] Packet Delay (Zoom) Oscillator Observed Packet Delay (Zoom)

22 Packet Selection vs. Oscillator: Histogram Two Oscillators Same Clock Bandwidth, Packet Selection, PDV Observation: FWPR is reduced [Page 22]

23 Packet Selection vs. Oscillator: MAFE Two Oscillators Same Clock Bandwidth, Packet Selection, PDV Observation: Frequency accuracy not greatly impacted for typical mobile backhaul application 3 ppb 1000 seconds 4 ppb 1000 seconds [Page 23]

24 Packet Selection vs. Oscillator: Packet Timing Signal MTIE Two Oscillators Same Clock Bandwidth, Packet Selection, PDV Observation: MTIE substantially impacted relative to synchronization performance requirements [Page 24]

25 Packet Selection vs. Oscillator & Clock Bandwidth Summary XO directly impacts wander generation conformance, a parameter defined in the time domain Absence of time domain characterization in XO makes component selection difficult Time domain is significantly impacted by oscillator selection vs. packet selection & clock bandwidth Lack of standard for PEC results in freedom to optimize clock bandwidth based on custom design choices Frequency domain performance is less impacted by oscillator selection vs. packet selection & clock bandwidth Specifically the mobile backhaul application (< 50 ppb accuracy) Target application is very forgiving of XO selection Lowest hanging fruit PEC for applications requiring only frequency accuracy, such as mobile basestation, are easier to design based on traditional XO characterization information [Page 25]

26 Thank-you for Your Time & Attention ITSF 2010