[e2e] A not-end-to-end question
braden at isi.edu
Thu Mar 27 15:32:36 PDT 2014
I am pondering a question that is sort of anti-end-to-end. But since I
set up this list in the first instance, I figure I have the right to
abuse it ;-)
There is a community of electrical power engineers who are reworking the
power transmission system, starting by instrumenting it with measurement
devices called Phasor Measureent Units or PMUs. A PMU samples the
electrical state at a particular point ("bus") at O(100) times a second
, encapsulates the sample in a frame of ~100 bytes, and sends it (in
general) towards one or more control cemters Each frame carries an
absolute timestamp, currently using GPS clocks at each PMU. The frames
are passed downstream to a data sink, an application program running
usually in a control center computer.
This PMU data transmission problem requires high availability and
controlled latency. Just throwing away packets as we commonly do in the
Internet does not work here.
There are several proposals, eg MPLS, to solve this problem. However, I
have been pondering the question: isn't this a nearly perfect
application for Integrated Services and RSVP? Didn't we solve this
problem more than 15 ears ago?
Is there any difference in principle between streaming audio/video data
and streaming PMU data?
The major argument against Intserv and RSVP has always been with scaling
up to Internet sizes. However, the network delivering PMU data will not
suffer from a scaling problem. The population of PMUs is expected to
grow, but unlikely to exceed O(10,000), so we can put quite reasonable
bounds on state in each router. Furthermore, at lest one well known
router vendor implements RSVP and Integrated Service, I believe,
although I do not know the details.
One remaining obstacle might be the lack of down-sampling (if you don't
know what this means, never mind). but I suspect this will be more a
theoretical than a practical problem in the real world. I guess another
issue is whether a token bucket is adequate and appropriate
for modeling PMU data streams, but I suspect that it is OK.
I hope that other refugees from the IntServ development will comment on
this. What have I overlooked?
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