[Nml-wg] Links

[Freek Dijkstra]

John Vollbrecht commented on physical links:

> To me the physical link is the basic element.  Virtual or logical
> links may exist but are built by "relations" with physical links.

When I think about network I often also reason in this way. However,
I suggest not to _enforce_ that virtual links are built by relations 
with physical links, but only _allow_ it.

For example, it should be possible to describe all links in the GLIF 
community (http://www.glif.is/publications/maps/). Remember that these 
links are *not* direct physical links. Most of them are OC-192 circuits 
provided by large networks such as T-Systems and Global Crossing. I do 
not want to _enforce_ people to first describe those underlying physical 
networks, although I most certainly like to _enable_ people to do so.


Chris Tracy responded to my comment about link "capacity":

>> 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..

Agreed, it is a lot of detail, and I certainly don't advocate that we 
force people to publish these details. I just say that we should be 
explicit what capacity we are referring to (payload only or with 
encoding/headers -- think IP MTU 1480 bytes = Ethernet MTU = 1500 bytes).

The capacity I'm talking about would include the headers, not only the 
payload. (to describe the capacity of the payload, you must first define 
the payload as a channel (= logical link) inside this link, and then 
specify the capacity of that channel as property of that channel.

As for this specific case, I don't see a compelling use case (yet) to 
describe FEC, but I would advocate a technology-independent model which 
allows one to describe this as two different sublayers, with two 
different link capacities (with a GFP-based adaptation between the two 
sublayers).

Regards,
Freek