I disagree about it not being a useful model for teaching stuff. In particular as a facilities based ISP, you have to care about what the OSI layer 1 is, becaus

I disagree about it not being a useful model for teaching stuff. In particular as a facilities based ISP, you have to care about what the OSI layer 1 is, because ultimately an ISP is built out of real world physical things. The article seems to skip over concerns about layer 1. But if you have no understanding of how things are engineered at layer 1, or of the possible failure modes, all sorts of bad things can happen. Or you'll end up buying the wrong thing, or getting bamboozled by equipment vendors once they realize you're not paying attention to the layer 1. People not understanding layer 1 are how things happen like an ISP sending out somebody to try to install a 10GbE 1310nm/LX circuit to a piece of equipment that was shipped with a multimode SFP+ in it. Engineering of large complex backbone systems encompasses the overlap between layers 1 and 2. For instance all of the optical engineering that goes into a inter-city DWDM system is "layer 1" - but those same DWDM chassis have 10/100GbE short reach layer 2 transport circuits hanging off of them, to connect to your own ISP's or customers' ISPs' network equipment. Then the controller card in your DWDM system is likely running some form of embedded Linux and has a 100BaseTX/1000BaseT interface in it with an IP address in a management network, it can have an snmpd on it that exposes a set of OIDs to poll by network monitoring gear, it runs an sshd on it it, etc. So you've got layer 3 and layer 4/5 stuff going on in that same 10RU DWDM box which needs to be taken into account in your network design. Maybe you have a system somewhere which periodically retrieves a text dump of the entire configuration of the DWDM box, whatever its equivalent of a 'show run' is, and stores it in git? That's all layers 4-7. In the modern all-ethernet world, tons of possible configuration parameters for exactly how the 10 or 100GbE handoffs are set up on each side. And various configuration parameters. Ever encountered somebody who mistakenly ordered a 10GbE WANPHY circuit when it should have been LANPHY, or vice versa? That's an example of why an understanding of the underlying layers of what you're working on is important. Only once you have a good solid grasp of layers 1 and 2 can you begin to properly architect your network at layer 3. One thing that I do try to repeat as often as possible, if helping to train NOC and junior neteng people, is that things get very blurry between how software is set up at layers 4-7. The same piece of software that might be doing things at what the OSI model would call layer 4 may also have a end user GUI/presentation/operator interface that would be defined as layer 7. I do not think that mentally imagining rigid definitions between layers 4-7 is very useful these days. I personally think that as a method of understanding network architecture, the first level of layers 1-3, and 4, are much more useful than the blurry "4-7" model.