Copper SFP modules
Standard Copper SFP modules give a bad WR timing performance
White Rabbit devices should follow the 1000BASE-BX10 standard and use 1310/1490nm pairs with a single LC fibre-optic connector.
However, for connecting PC network cards to a switch or applications where the high timing resolution of WR is not needed, Gigabit Ethernet copper SFP's may be used. Data transfer will work as expected, but the timing part of White Rabbit will be severely impacted.
The copper SFPs use the 1000BASE-T standard and the same module type can be used on both sides (switch and node).
We have tried the following modules (with a large impact on timing performance)
Brand | Type | Remarks |
---|---|---|
Axcen | AXGT-R1T4-05I1 | AV (10/2015) |
Avago | ABCU-5710RZ | TW |
Bel | SFP-1GBT-06 | BR (10/2015) |
Cisco | MGBT1 | TW AV (10/2015) |
Cisco | GLC-T | RW |
Why Copper SFPs give bad WR timing performance
Fiber SFPs contain only the laser driver, photodiode receiver and some simple AGC logic. Whatever comes to the LVDS input of such an SFP, goes out to the fiber preserving exactly the same protocol and timing (+/- the jitter).
Copper SFPs are on the other hand, not only transceivers. They convert the 8b10b protocol used in 1000Base-X (SFP interface) to a much more complex 1000Base-T physical layer protocol. Driving a CAT5/6 copper directly with 1.25 GBaud signal as in 1000Base-X is not possible.
Figure 1: unlike fiber SFPs, a copper implementation of an SFP retimes
and buffers data*
The logic inside a copper SFP is essentially two different PHYs (X and T) bridged with GMII interface. Conversion between the protocols involves retiming the data, so packets are preserved but reference frequency encoded in 1000Base-X carrier is lost. This is because the PHY inside an SFP is clocked from the SFP's internal oscillator. GBIC (SFP) standard does not foresee an external clock input for SFPs, so there are no SFPs on the market that could work synchronously to an external clock.
Moreover, WR over copper requires additional calibration logic which is not present in any copper SFP. So if someone wants sub-ns over a gigabit copper link, he must design his own, non-standard hardware.
Passive SFP copper cables
Passive SFP copper cables or direct attach cable (DAC) actually may give good timing performance as they feature no re-clocking. This is just 4-pieces of coax, AC-coupled, for TX+/- and RX+/-, with SFP-connectors at each end. Please let us know if you have any measurement results or experience with these.
Example
Further reading
- Improving IEEE 1588 synchronization accuracy in 1000BASE-T systems, R.Greenstreet & A.Zepeda, ISPCS 2015.
Erik van der Bij, Tom Wlostowski, Benoit Rat - 21 March 2017