RE: [RPRWG] Gandalf - question on Framing
A couple of comments:
Disgression: Note that once a packet with good HEC but bad
FCS hits a bridge it has to be dropped. 802.1D specifies
that a packet with bad FCS cannot be forwarded.(6.3.5 of
802.1D 1999). Therefore this HEC while useful in a single
ring, does not allow rings of rings to be built and
transport TDM, or TDM bridging into an Ethernet Mesh network
unless we change 802.1D.
>> The whole RPR frame header is useful only in a ring, so it is not strange
that the HEC has value only in the ring. Errors may occur in the ring and
our mandate is to provide an optimal solution for packet rings.
There are other ways of interconnecting rings with equipment that does not
discard frames because of FCS errors, and in those cases HEC is useful.
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For the fun of it, I will debate it now. This is really
more of a thought experiment then anything else.
Assume for the moment that we are trying to transport
a DS-1. Depending on the efficiently you want to achieve
you might pack from 1 to 16 DS-1s in a packet. Allowing
for some overhead per packet (including a sequence
number) you get the following approximate packet sizes (in bits)
# of DS-1s Size
1 416
4 992
16 3296
Fiber links have error rates in the range of 1E-15 to 1E-12
at the begining of life dropping over time. A SONET link is
speced to have an error rate of better than 1E-10 worst case.
In reality, customers get cranky if it is worse than 1E-12.
So if a bit error occurs and kills the entire packet
then the packet is dropped and the box reconstructing the
TDM stream sees a bit error rate that is "size" times larger.
So the bit error rate degrades by 2 to 3 orders of magnitude.
But this still leaves the system with a bit error rate far better
than a copper DS-1. I believe the acceptable limit for BER in
a DS-1 circuit is somewhere in the 1E-3 to 1E-6 range
and so we meet that limit.
Now, this can be mitigated somewhat by doing any of the following
1) The box repeats the last good DS0 sample.
2) Play some tricks with trans-coding the samples to minimize
the number of bits changed (on average) from sample to sample.
So the bit error rate can be protected to some degree. Some people
feel passionately about the need for this feature and Gandalf
supports it.
>> Packet networks will carry higher rates than DS-1. IETF is in advanced
stages of defining STS-n transport over packet networks (PWE3 working
group), RPR will have to carry these packets as well.
Even for DS-1s, the result of a single bit error is very different from the
result of 16 frames of errors in a burst, even if the average BER is the
same.
Leon