RE: Fw: [RPRWG] CRC check in each node?
Reuvan,
In ATM networks only an HEC was required on the header, since
data and link integrity is provided by the underlying STM, SONET,
or PDH, transmission layer. Since ATM cells are asynchronous
to the SONET frame, SONET overhead frame checks providing the link integrity
did
not suffice to guard against misrouting. The ATM HEC is provided to
protect against misrouting. In SONET/ATM, the link error performance
is provided by the section, line, path overhead, and misrouting is
provided by the ATM-HEC. If the SONET frame is corrupted, but doesn't
affect the ATM header, then SONET error statistics are updated, but the
cell (with corrupted data) is still routed by the ATM network. ATM cell
payload integrity is performed by the ATM Adaptation Layer at the ends of
the VC.
This is analogous to what has been discussed regarding having
separate header/data integrity checks for RPR. Since GbE and 10GbE
do not have the SONET overhead equivalents, a similar check on the
entire data payload allows these transmission technologies to provide
similar performance monitoring capabilities in an RPR network. One
of the goals of RPR is to achieve SONET-like resilience for
packet ring networks.
The data CRC provides the link performance monitoring, while the
header check prevents misrouting. One could argue that the CRC used
to protect the data portion of the frame is strictly an RPR specific frame
integrity check that is independent of any user CRCs that also might be part
of the payload. In this case the RPR frame check is strictly used by the
RPR stations and is stripped when the frame is given to the client. The
client
would then have to rely on its own client-level frame CRCs in order to
validate
the payload. This is the paradigm used in ATM.
regards,
bob
Robert Castellano
Jedai Broadband Networks
rc@xxxxxxxxx <mailto:rc@xxxxxxxxx>
(732) 758-9900 x236
-----Original Message-----
From: owner-stds-802-17@xxxxxxxxxxxxxxxxxx
[mailto:owner-stds-802-17@xxxxxxxxxxxxxxxxxx]On Behalf Of Reuven Zeitak
Sent: Tuesday, July 03, 2001 1:47 AM
To: ieee 802.17 list
Subject: RE: Fw: [RPRWG] CRC check in each node?
Hi Everybody,
I have been following this interesting thread. I want
to rock the boat a bit here to make sure all points have
been considered:
Question: Is a CRC on data really what we want?
Consider:
ATM only has a HEC and does not protect data,
it is left to the application.
In some sense RPR is just an encapsulation of user data
and has to be agnostic to the data content. As it was pointed
out here, Users may want to keep on getting the data even if
there are link errors. Maybe the application has a sophisticated error
recovery method. Maybe it has none. Is it the RPR layer's job to
provide the application an error indication?
We clearly need some kind of BER measure (in the SONET jargon)
to verify if the link is alive. Isnt the HEC good enough for that?
HEC errors are to be discarded at the next node anyway, so that
there is no issue of double counting an error.
reuven
----------------------------------------------------------------------------
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Reuven Zeitak PhD
Modeling And Algorithms
Native Networks
2 Granit St., P.O.Box 7165
Petah Tikva, Israel
Tel: +972-3-920-2800 x 875
Fax: +972-3-9210080
reuven.zeitak@xxxxxxxxxxxxxxxxxx
www.nativenetworks.com
The Native Way = Ethernet Simplicity + SONET Reliability