Stefan,
The effect of reduced latency is an application question requiring knowledge of the architecture. As my slides showed, you can’t estimate the improvement
from the PHY alone. Likely it is not 2x, but some smaller gain.
Some thoughts:
For an individual control message, you could reduce the entire transmission from 84 octets (672 bits or 67.2 us @ 10 M) to 39 octets (312 bits or 31.2
us @ 10 M), saving 36 us.
In a braking example:
120 km/hr, 33 m/s, 33 um/us
33 um/us * 36 us = 1188 um = 1.2 mm (reduced travel before brake command is applied)
Of course, real architectures are more complex.
If you had cut-through, then the forwarding decisions along the path would be made before we reached the pad field in the frame. So the 36 us would only
be saved at the last receiver. Cut-through should know the destination before forwarding (20 octets or 16us).
Store and forward time would increase latency based on the number of hops multiplied by the packet size.
If there were packets in progress along the path, you could be delayed by the maximum packet time at each hop. Maybe your brake packet could have the
highest priority, but the lower priority transmission may already have begun.
You could have pre-emption to limit the effect of the lower priority traffic. Then you’d have up to 127 octets / 102 us on each hop before preemption
(minimum fragment is 64 octets).
The network could be scheduled, but unless the brake command is the only traffic, it will have to wait for its turn in the schedule. Maybe there are
4 brake command (one to each wheel) that are sent in a periodic schedule, then one of them has to be last in the schedule.
Possibly the brakes should all be applied at once or the vehicle twists. Maybe this is more important than absolute lowest latency. You might then consider
accurate time synchronization to each wheel and also bounding the latency of the commands. Various methods are possible to gain this determinism.
Regards,
David D. Brandt
Senior Principal Engineer
Rockwell Automation - Advanced Technology
1201 South Second Street
Milwaukee, WI 53204-2496
414.382.4309
Hi David,
thanks for your presentations.
Especially the discussion of the reduced frame size is very interesting.
some points/remarks/question:
-
I think for (at least) some of the applications the overall transmission delay of the message is as well very important (control applications).
Any additional input on how reduced frame size would influence this would be interesting as well.
-
For the control applications as well we have to consider that the overall latency of the network is
bigger than the individual transmission time (number of hops or better number of switches)
-
For calculating this overall latency: Can anyone (maybe of the semiconductor suppliers) give an assumption of the time a 10/100/1000Mbps store’n’forward switch would
need for a frame (only the switching from MAC to MAC)? Is this significantly different between the speed grades? Is this different for a cut-through switch?
(Or is the dominant factor the “store” part of the frame (which is nearly equal to the individual transmission time per link) and switching is negligible?)
Regards,
Stefan
PS: unfortunately I am not sure if I can join this evening…
------------------------------------------
Stefan Buntz
Mercedes-Benz Cars Development, Daimler AG
Group Research & Advanced Engineering
Safeguarding Hard & Software
HPC: U059 – Dep.: RD/EEQ
Phone:
+49 731 505-2089
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------------------------------------------
George,
Here are the following:
Industrial Automation Bit Error Rate
brandt_082216_10SPE_01_adhoc
Reduced Minimum Frame Size
brandt_082216_10SPE_02_adhoc
I am also enclosing a third submission, updated from the 802.24 version as requested.
Industrial Automation and Emerging Single-pair Ethernet
brandt_082216_10SPE_03_adhoc
Regards,
David D. Brandt
Senior Principal Engineer
Rockwell Automation - Advanced Technology
1201 South Second Street
Milwaukee, WI 53204-2496
414.382.4309
The IEEE website administration isn’t set up yet, so for right now, the presentations will have to go via the reflector.
The attached two contributions are offered to help guide and focus our work – one with acting chair’s comments on par, CSDs and the process, and one on draft objectives for tomorrow.
Peter Jones will be chairing our ad hoc, and I believe that we have presentation requests from David Brandt:
Industrial Automation Bit Error Rate
brandt_082216_10SPE_01_adhoc
Reduced Minimum Frame Size
brandt_082216_10SPE_02_adhoc
David – if you can forward these to the reflector, it will make the distribution easier.
See you all tomorrow morning.
-george
George Zimmerman, Ph.D.
President & Principal
CME Consulting, Inc.
Experts in Advanced PHYsical Communications
george@xxxxxxxxxxxxxxxxxxxx
310-920-3860
George Zimmerman, Ph.D.
President & Principal
CME Consulting, Inc.
Experts in Advanced PHYsical Communications
george@xxxxxxxxxxxxxxxxxxxx
310-920-3860
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