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