4 ms·

Interesting. CERN has white rabbit for sub-ns timing. Needs special hardware but is now IEEE 1588 (PTP) "High Accuracy" profile. I wonder how this deals with t

by nimish 5y ago

Interesting. CERN has white rabbit for sub-ns timing. Needs special hardware but is now IEEE 1588 (PTP) "High Accuracy" profile. I wonder how this deals with the numerous retimers and DSPs required for really high speed ethernet.

macroblocks 5y ago
I’ve been working on fairly large scale broadcast television back-end systems which have been using 10/25/100G SMPTE 2110 IP for video flows instead of HD-SDI, and those have been timed with PTP for a while (SMPTE 2059-2 standard, which is a PTP profile). The GPS locked master clocks are custom hardware (ex: https://evertz.com/products/5700MSC-IP https://evertz.com/products/5700MSC-IP), but a lot of the edge devices like the video playout servers are standard X86 hardware with Ubuntu Linux, with PTP delivered in-band over the 10G/25G ports to the device (same link as the video/audio flows). (… Which I realize is not as accurate as the newer White Rabbit related PTP update you refer to - but still odd that the original article referenced NTP and not even regular PTP which has been in use for a while and seems pretty close to their claims even before the more recent enhancements)
- nimish 5y ago
  Regular PTP is old hat as you say. ~5ns though I thought needs some real thought, and picosecond sync needs a combo of PTP + SyncE + link compensation, which is now the "high accuracy" profile. It's all fairly simple in the end if the hw has the tools but good luck buying SyncE or 1588 without $$$.
zbrozek 5y ago
In a simple 100 mbps network where I needed good synchronization, I used the RXC pin of the MII bus as input to a PLL+VCXO and explicitly configured the PHYs in the link to be master-->slave in the desired clock distribution direction. PTP provided phase. Across a small network (a couple switch layers) I was seeing about ~20 ns of time uncertainty. Clocks were locked within about 2 ppb. Worked well enough for my needs. In hardware that's actually designed to do this, I think it's called "synchronous ethernet" but you can totally duct tape it into pedestrian hardware the way I did.
- Nextgrid 5y ago
  Out of curiosity, do you have any write-up or some documentation about this? Can't imagine a scenario where I'd need this any time soon but it sounds very interesting.
- nimish 5y ago
  Yeah I think SyncE is what telecom RANs use to get within 5g reqs on timing. White Rabbit operates like 1588 + SyncE + a compensation model for the fiber link. It's pretty slick. It does need L2-L1 coordination but the ethernet MAC usually has no idea of what's happening on the L1 (it speaks "medium independent interface" or *MII). The CERN people warn against using Base-T since the PHY needs to do pretty complex signal processing which would likely destroy the syntonization. They then use basic fiber links, but 100g, 400g+ have a gearbox and DSP and FEC in the way.

by nimish 5y ago

macroblocks 5y ago

I’ve been working on fairly large scale broadcast television back-end systems which have been using 10/25/100G SMPTE 2110 IP for video flows instead of HD-SDI, and those have been timed with PTP for a while (SMPTE 2059-2 standard, which is a PTP profile). The GPS locked master clocks are custom hardware (ex: https://evertz.com/products/5700MSC-IP https://evertz.com/products/5700MSC-IP), but a lot of the edge devices like the video playout servers are standard X86 hardware with Ubuntu Linux, with PTP delivered in-band over the 10G/25G ports to the device (same link as the video/audio flows). (… Which I realize is not as accurate as the newer White Rabbit related PTP update you refer to - but still odd that the original article referenced NTP and not even regular PTP which has been in use for a while and seems pretty close to their claims even before the more recent enhancements)

nimish 5y ago
Regular PTP is old hat as you say. ~5ns though I thought needs some real thought, and picosecond sync needs a combo of PTP + SyncE + link compensation, which is now the "high accuracy" profile. It's all fairly simple in the end if the hw has the tools but good luck buying SyncE or 1588 without $$$.

zbrozek 5y ago

In a simple 100 mbps network where I needed good synchronization, I used the RXC pin of the MII bus as input to a PLL+VCXO and explicitly configured the PHYs in the link to be master-->slave in the desired clock distribution direction. PTP provided phase. Across a small network (a couple switch layers) I was seeing about ~20 ns of time uncertainty. Clocks were locked within about 2 ppb. Worked well enough for my needs. In hardware that's actually designed to do this, I think it's called "synchronous ethernet" but you can totally duct tape it into pedestrian hardware the way I did.

Nextgrid 5y ago
Out of curiosity, do you have any write-up or some documentation about this? Can't imagine a scenario where I'd need this any time soon but it sounds very interesting.
nimish 5y ago
Yeah I think SyncE is what telecom RANs use to get within 5g reqs on timing. White Rabbit operates like 1588 + SyncE + a compensation model for the fiber link. It's pretty slick. It does need L2-L1 coordination but the ethernet MAC usually has no idea of what's happening on the L1 (it speaks "medium independent interface" or *MII). The CERN people warn against using Base-T since the PHY needs to do pretty complex signal processing which would likely destroy the syntonization. They then use basic fiber links, but 100g, 400g+ have a gearbox and DSP and FEC in the way.