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Sub-100ns accuracy at cell sites over commercial WDM networks

In his ITSF 2020 presentation, Nir Laufer explained how robust timing with nanosecond precision can be achieved by delivering timing over PTP-optimized optical transport channels. He discussed tackling GNSS vulnerabilities – including the growing threat of jamming and spoofing attacks – by ensuring robust backup from ePRTC core clocks to the aggregation site.

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Sub-100ns accuracy at cell sites over commercial WDM networks

  1. 1. Sub-100ns accuracy at cell sites over commercial WDM networks Nir Laufer, senior director, PLM, Oscilloquartz, ADVA ITSF Nov 2020
  2. 2. © 2020 ADVA. All rights reserved.22 Common synchronization challenges – sub-1usec Mobile radio networks 1.1usec, 260nsec relative Financial trading 1usec Regulation (MIFID II) , HFT Power utilities 1usec - PMU, fault location Data center infrastructure 1usec Broadcast networks 1usec How to make sync delivery more accurate and more resilient?
  3. 3. © 2020 ADVA. All rights reserved.33 GNSS vulnerabilities and threats GNSS for timing Jamming and spoofing Obstruction Interference with transmitters at adjacent bands Ionospheric disturbance, solar activity GNSS segment errors
  4. 4. © 2020 ADVA. All rights reserved.44 Application area for OTC: Aggregation network Core time base network Single-digit number of locations for large operator ePRTC enabled, TE ≤ ±30ns Aggregation network Hundreds of locations for large operator PRTC enabled, TE ≤ ±100ns Feeders to end application Thousands of locations for large operator TE ≤ ±1100ns Optical timing channel Optical timing channel 70ns budget ePRTC ? 30ns budget
  5. 5. © 2020 ADVA. All rights reserved.55 Core time base sites - ePRTC GNSS Receiver Clock Combiner PPS/PPS+ToD GNSS antennas Core redundent grandmaster 10 Mhz BITS Sync-E PTP NTP Clock combiner GNSS receiver Carrier-grade fully redundant HW Multi band, multi constellation GNSS PTP+SyncE Backup from peer site Peer core site ePRTC with cesium backups Smart antenna Sync and GNSS assurance Advanced jamming and spoofing detection ePRTC ePRC cesium clock ePRC cesium clock
  6. 6. © 2020 ADVA. All rights reserved.66 WDM over long-distance routes Typically long-distance WDM networks use C-band due to low attenuation The “data” channels are sent over fiber pairs (RX/TX)
  7. 7. © 2020 ADVA. All rights reserved.77 OTC Timing over ... Timing over ... Optimization methods Time error Layer 3 (routed) Small packet size High (ms/µs) Layer 2 (switched) VLAN with high priority Middle (µs) Layer 1 (OTN) OTN buffer policing or inband transmission Low (µs/ns) Layer 1 (transparent WDM) Single fiber working Lowest (ns) Asymmetry
  8. 8. © 2020 ADVA. All rights reserved.88 OpticallinesystemIntermediatesiteNode without timing access Terminal west Terminal east Payload traffic Line terminal The extract length of each of the fibers is unknown The different between length of fibers can be a few tens of meters over long-distance connections -> creating significant asymmetry (~2.5nsec/m) The inline amplifiers are directional and add unknown delay which varies between generation of amplifiers, types and suppliers
  9. 9. © 2020 ADVA. All rights reserved.99 Calibration of asymmetry with GNSS Calibration with using GNSS is not sufficient • One possible solution is to use GNSS on both sides of the link in order to compensate the asymmetry • GNSS might not be sufficiently accurate (typically ~ 40-100nsec) • Installing GNSS antenna in a remote site might be challenging (e.g., indoor) • Increase installation costs • Initial calibration may not be valid when changes are made in the network (replacing amplifier, fixing fiber, etc.) Terminal TerminalROADM ROADM GNSS Master Probe GNSS
  10. 10. © 2020 ADVA. All rights reserved.1010 Timing device OTC regen ILA ROADM Terminal Cross-connect Terminal/ADM Card Optical timing channel (OTC) The optical timing channel (OTC) achieves high-accuracy time distribution by PTP (IEEE 1588) via Gigabit Ethernet over dedicated wavelengths on a single fiber OTC has minimal constant link asymmetry due to bidirectional transmission on a single fiber OTC is independent from other layers (IP, Ethernet, OTN, reconfigurable optical layer), practically eliminating dynamic time errors OTC does not change optical network engineering rules and is just an overbuild to the optical transport layer
  11. 11. © 2020 ADVA. All rights reserved.1111 OpticallinesystemIntermediatesite Node with timing access Terminal west Terminal east PTP over optical timing channel Payload traffic Timing device featuring High-accuracy PTP boundary clock type D BiDi transceivers OTC provides ample optical budget: Up to 30dB between two BiDi transceivers Dispersion is irrelevant at GbE line rate Line terminal
  12. 12. © 2020 ADVA. All rights reserved.1212 OpticallinesystemIntermediatesite Node with timing access redundant OTC Terminal west Terminal east PTP over optical timing channel Anticlockwise PTP over Optical Timing Channel Clockwise Two redundant OTCs running on two fibers OTC Regen T-BC T-BC T-GM T-GM Timing is handed over two OTC. Best master clock algorithm of client equipment decides which one to use. Both OTCs are active all the time thus avoiding rearrangement transients T-BC T-BC Redundant BC (type D)
  13. 13. © 2020 ADVA. All rights reserved.1313 Impact of chromatic dispersion can be efficiently compensated Same east-west wavelength Asymmetric delay results in a deterministic time error Chromatic dispersion creates asymmetric delay 100km of fiber with dispersion of 18ps/(nm km) l1 = 1.605µm l2 = 1.615µm 𝑐𝑇𝐸 ≈ 100𝑘𝑚 2 10𝑛𝑚 18𝑝𝑠 𝑛𝑚 𝑘𝑚 = 9000𝑝𝑠 = 9𝑛𝑠 50km 65km50km 65km Time error from chromatic dispersion: 19ns Constant and deterministic
  14. 14. © 2020 ADVA. All rights reserved.1414 50KM_aOSC OSCOSC OSC OSC Paragon Neo OSA 5430 5WCAOTC Patch-through master port slave port OSC OTC accuracy measurment 3GU/1605LGBE/1615V 3GU/1605L SFP/GBE/1605V GBE/1615V GBE/1615V MALPB-OSC-C MTP-OSC-C MTPB-OSC-C MTPB-OSC-C MTP-OSC-C MTPB-OSC-C 50KM_b 65KM_a 65KM_b OSA 5421 GBE/1310 GBE/1310 210 cm 𝑐𝑇𝐸 ≈ 100𝑘𝑚 2 ∗ −10𝑛𝑚 18𝑝𝑠 𝑛𝑚 𝑘𝑚 = − 9000𝑝𝑠 = − 9𝑛𝑠 𝑐𝑇𝐸 ≈ 130𝑘𝑚 2 ∗ −10𝑛𝑚 18𝑝𝑠 𝑛𝑚 𝑘𝑚 = 11700𝑝𝑠 = −11.7𝑛𝑠 |TE| within15nsec. |cTE| within 5nsec. Known (fixed) asymmetry is configured on the BC port
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