[ntp:questions] Re: IEEE 1588 support in NTP?

Ulrich Windl Ulrich.Windl at RZ.Uni-Regensburg.DE
Wed Nov 2 15:30:34 UTC 2005

martin.burnicki at meinberg.de (Martin Burnicki) writes:

> 3.) By the NIC hardware. You need a time stamp unit (TSU) which listens on the 
> data lines between the MAC and the PHY. The TSU contains a pattern matching 
> unit which detects the pattern of the desired packet type in the serial bit 
> stream on those data lines. If such a pattern is detected then the pattern 
> matching unit must capture a time stamp of a high speed counter which is also 
> part of the TSU. Since this method takes time stamps when a packet goes on 
> the wire or arrives on the wire is also capable to eliminate the latency due 
> to network collisions. 
> The driver must be able to read those time stamps from the TSU and either 
> assign them to the packets or pass them up the protocol stack to the 
> application which then assigns the time stamps to the packets.
> The TSU can be implemented in a programmable logic device like an ASIC or 
> This method is the most accurate, but it requires a NIC with a TSU, and a 
> kernel driver which supports the TSU.
> The problem here is that most commonly used NICs have the MAC and the PHY in a 
> single chip, and the data lines on which the TSU must listen is not 
> accessible outside the chip. 
> You must also be aware that the timing accuracy is significantly degraded if 
> the PTP packets go through switches or routers which don't have a special 
> handling for such packets, so packets may be queued which introduces again
> an 

It would be an interesting experiment to echo those PTP packets
1) directly via an X-cable
2) via some switch, hub, etc.


Ulrich Windl

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