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Holdover Challenges in NIC Based Boundary Clocks

Eyal Cohen, Silicom, eyalc@silicom.co.il, July 2021

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Storyboard

  • Section 1, Timing Synchronization NIC
  • Section 2, Selected Holdover Issues

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Acknowledgements

  • H.B.

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What is it that we promote

  • Enhancement of standards for telco profiles in general
  • … and specifically to fit timing synchronizing NIC

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Section 1�Timing Synch’ NIC

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What are the pieces

  • High performant NIC
  • Time synchronization complex:
      • SDPLL
      • OCXO
      • Stamper PHY
      • GNSS receiver

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PCIe Gen4 x16

STS2

PCIe form factor

Full Height, Half Length, Single Slot

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BMC

SFP+

SFP+

SFP+

SFP+

PTP/1588

SyncE

QSFP+

GNSS

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What are the pieces (cont.)

  • How different from other timing synchronization solutions?

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  • A 1588 aware switch – From 1588 / SyncE point of view, STS works similarly

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  • A time server / PRTC device – STS is a PRTC device on a NIC, typically with more ports

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Where does it operate, Baseband Nodes

  • Full timing support (FTS)
  • Partial timing support (PTS)
  • [Block diagram of FTS]

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What are the challenges

  • Working in an open environment
    • The ORAN application is the owner.
    • Synchronizing virtual entities (a topic in itself).
  • Many ports
    • Many potential up/down events on each port
    • More than one NIC on a server
    • Many holdover schemes, in T-BC use case (slave ports >1)

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What are the challenges

    • Many holdover schemes, in T-BC use case (slave ports >1)

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Baseband Node

NIC, master ports

NIC master ports

NIC, slave ports

To Backhaul

To Fronthaul

1588

SyncE

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G.8275.1

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What a holdover is

Once two independent clocks are synchronized, they will start walking and straying away from one another

Quality of oscillator affects holdover performance

Holdover is important for business continuity

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Section 2�Selected Holdover Issues

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Problem Statement

  • When PTP or SyncE recovery fail, only part of it is standardized, while C1 and C3 cells are no defined

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SyncE

PTP

Required Behavior

Governing standard

 

A

B

C

D

1

Down

Up

?

n/a

2

Up

Down

No holdover to PLL; Yes holdover to physical layer frequency, SyncE, whilst QL is ok (Class A, B); Class (D, C) for further study

G.8273.2

3

Down

Down

?

Sec. 7.4.2.1 in G.8273.2, for further study

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Up

Up

Full timing support (FTS) 

G.8271.1

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SyncE Up, PTP Down

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  • We are in class C; left for further study. Should still comply to the above.

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SyncE

PTP

Required Behavior

Governing standard

 

A

B

C

D

2

UP

Down

No holdover to PLL; Yes holdover to physical layer frequency, SyncE, whilst QL is ok (Class A, B); Class (C, D) for further study

G.8273.2 (table below)

MTIE limits [ns]

Observation Interval τ [s]

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1 <= τ <= 100

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100 <= τ <= 1000

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SyncE Down, PTP Up

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  • This is the tricky one. What can used to inspire a definition?
    • G.8265.1 PTP frequency Synchronization? No mcast
    • G.8275.2 PTP phase / time synchronization? No mcast
  • It is a holdover situation – even if master clock class is lower – and should fall back to local oscillator

 

SyncE

PTP

Required Behavior

Governing standard

 

A

B

C

D

1

Down

Up

?

n/a

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SyncE Down, PTP Down

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  • Relying solely on local oscillator
  • What we suggest is to define a framework in the spirit of G.8271.2: max|TE| within 1100ns (1.1us) confines; 4h / 8h holdover (depending on oscillator quality)

 

SyncE

PTP

Required Behavior

Governing standard

 

A

B

C

D

3

Down

Down

?

n/a

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For Further Study

  • Case of two (or more) slave ports
    • Either SyncE and PTP recovering clocks
  • APTS and jittery satellite frequency
  • Transitions:

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SyncE

PTP

1

Down

Up

2

Up

Down

3

Down

Down

4

Up

Up

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THANK YOU

www.silicom-usa.com

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Backup

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Who Are We?

Founded in 1987

A public company NASDAQ

HQ, R&D and manufacturing facilities are in Israel, with offices in the US and Denmark

33 years of experience in networking and connectivity

300 employees

Collaboration with strategic partners

Over 300 clients worldwide

FPGA design experts both HW and IP cores

Intel SOC design house

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Global Customers

Cyber Security

Cloud

Data Centers

SD-WAN / NFV

Web 2.0

Telcos/ Service Providers

Storage

More than 400 active design wins with over 300 customers, including many market leaders

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