Welcome to time-pi Discussions!

[geerlingguy]

If something is not directly related to the Time Pi hardware or software, but still in the realm of Raspberry Pis, timing, PTP, NTP, GPS/GNSS, etc., please feel free to open a discussion here.

[charlie-cobb]

Hey Jeff!

Glad to see that someone is tackling the world of PTP timing on Raspberry Pis. I tried building my own about a year ago and time-pi would've been a lot of help!

I lead the Cellular team at Meter, which is a product to bring cell service indoors from the major US cell carriers at a fraction of the cost of traditional Distributed Antenna Systems (DAS). We use CBRS small cells (which we call Cellular APs) to broadcast cell signal inside venues. A single Cellular AP can cover 5k-10k square feet, and we have a few deployments with tens to hundreds of them deployed in a building.

CBRS is a TDD frequency, which requires the use of PTP to keep everything in sync. Today, we use off-the-shelf PTP hardware connected to Meter switches that are not time-aware, but are performant enough that we don't have sync issues.

We're in the process of building our next generation networking hardware, and have made the decision to build our own primary source clock (GM) to make Cellular installs cheaper, faster, and more efficient. It'll be PoE powered and IP66 rated so we can either stick it in a window or outdoors. Our next generation security appliances and switches will also be time aware with hardware timestamping support.

We're planning on running the majority of the stack off the linuxPTP project. I'm curious if you've run into any limitations with the number of PTP clients supported by LinuxPTP? Our current gen PTP unit has a 64 client limit, but I think it's a software licensing thing, not a hardware limitation. We haven't done enough digging yet to see if that's true. Also curious if you've had any gotcha's with the linuxPTP project that are mentionable!

[geerlingguy]

I've heard of hundreds of clients supported by modest hardware (like low end servers), or dozens for a Raspberry Pi (it's a limit of how many packets the CPU can handle, mostly).

But this is honestly one thing I want to do some more testing of—I've been working on the Switchberry with the folks at OCP-TAP, and the goal there is to have a nice small scale deployable switch + timing grandmaster for tens or dozens of clients.

[charlie-cobb]

Cool, good to know. Switchberry looks like a cool project. Have you experimented with setting up boundary clocks using LinuxPTP or mainly grandmasters?

[geerlingguy]

@charlie-cobb mainly grandmasters at this point. Though I plan on brushing up my knowledge of transparent/boundary clock configuration and performance with the Switchberry, as it should be able to perform both roles with the Microchip switch chip on it.