[Nml-wg] More UML....

[Chris Tracy]

Hi Freek,

> GMPLS defines it in Bytes per second, and I don't see a good reason to 
> deviate (although I also wondered why on earth they picked this strange 
> convention).

Regarding bits vs bytes:

The original definitions of the RSVP TSPEC/RSPEC objects [1] used
floats to represent rates and the use of IEEE single-precision
floating point seems to have carried over to GMPLS.  Now floats are
used in both the RSVP messages as well as the OSPF TE LSAs.  :-)

No reason for choosing bytes vs bits is given in RFC 2212, but I would
assume it is to allow for a larger range of bandwidth values to be
represented as accurately as possible when using 32-bit floats since
precision decreases as the exponent increases.

[1] see RFC 2210, RFC 2212, and RFC 2215 (circa 09/1997)

> To me it's much more important to be very clear what this attribute 
> means. For example, the actual transmission rate of 1 Gbit/s Ethernet is 
> 1.25*10^9 Baud, which due to the 10b/8b encoding correlates to 1.00*10^9 
> bit/s data rate.
> 
> In this case, I'd say that the capacity of the physical layer link is 
> (1.25*10^9)/8 byte/s and the capacity of the Ethernet layer link is 
> (1.00*10^9)/8 byte/s.

Similarly, in the case of DWDM systems, the physical layer on the TRIB
side for 10GigE might be ((1.00*10^10)*66/64) bits/s but higher on the
LINE side if g.709 FEC is being used...(which according to some
descriptions of g.709 makes the line rate ~11.095 Gbps for a 10GigE w/
g.709 overhead and FEC [2]).  I can understand it being useful to know
which portions of the path may or may not have FEC capabilities, but
it is a lot of detail to try and capture..

As an example, on the DRAGON network we have 10GigE waves across our
core, some of which have FEC (using transponders) and some that don't
(because they are alien waves coming from DWDM XFPs passing through
our optical core).  To further the example, there is an "enhanced" FEC
(EFEC) mode supported by one of the commonly used g.709 chips used by
many vendors [3] -- how it works exactly and whether or not the
overhead is different, who knows?  On our 10G transponders, we have
the option to either disable FEC, enable SFEC or enable EFEC, so of
course we use EFEC everywhere.  ;-)

[2] http://www.innocor.com/pdf_files/g709_tutorial.pdf
[3] http://link.aip.org/link/?OPEGAR/42/3456/1

> This is incorrect -- you may connect two ends of a fiber to the same 
> device. Why anyone would want to do that is a different matter, but it 
> is possible. The underlying idea is correct though.

Such a thing can be very useful for debugging and testing circuits --
we often put copper CAT5 cables between two Ethernet ports to loop
traffic around a portion of the network, such that each port is set to
untagged but associated with a different VLAN.  We typically do not
put a single piece of fiber between the Rx/Tx of a single transceiver
though, but some people routinely do this for testing purposes,
especially if you are just blasting packets down the line to see if
they come back to you...(e.g. you are using ixia/spirent boxes which
don't care about arp or you configure fake arp entries to force
packets to go out a particular NIC).

-Chris

> 
> 
> Regards,
> Freek
> _______________________________________________
> nml-wg mailing list
> nml-wg at ogf.org
> http://www.ogf.org/mailman/listinfo/nml-wg

-- 
Chris Tracy
Mid-Atlantic Crossroads (MAX)
Office phone: 301.314.6655
GPG key: 0xB3B9C93D