Originally posted on Jan 12, 2015
RF engineering and a layer hierarchy that extends all the way to the spectral authorities
In our day jobs we focus mainly on the physical layer of RF communications, and there is certainly enough challenge there for a lifetime of productive work. The analog and digitally modulated signals we wrestle with are the foundation of an astonishing worldwide expansion of communications.
Of course, the physical layer is just the first of many in modern systems. Engineering success often involves interaction with higher layers that are commonly described in diagrams such as the OSI model shown below.
The Open Systems Interconnection (OSI) model uses abstraction layers to build a conceptual model of the functions of a communications system. The physical layer is the essential foundation, but many other layers are needed to make communication efficient and practical. (Image from Wikimedia Commons)
The OSI model is a good way to build an understanding of systems and to figure out how to make them work, but sometimes we need to add even more layers to see the whole picture. A good example comes from a recent event that caught my eye.
Many news outlets reported that some hotels in one chain in the US were “jamming” private Wi-Fi hotspots to force convention-goers to use the hotel’s for-fee Wi-Fi service. The term jamming grabbed my attention because it sounded like a very aggressive thing to do to the 2.4 GHz ISM band, which functions as a sort of worldwide public square in the spectral world. I figured regulatory authorities such as our FCC would take a pretty dim view of this sort of thing.
As is so often the case, many general news organizations were being less than precise. The hotel chain was actually blocking Wi-Fi rather than jamming it. This is something that happens not at the physical layer—RF jamming—but a few layers higher.
According to the FCC, hotel employees “had used containment features of a Wi-Fi monitoring system” to prevent people from connecting to their own personal Wi-Fi networks. Speculation from network experts is that the Wi-Fi monitoring system could be programmed to flood the area with de-authentication or disassociation packets that would affect access points and clients other than those of the hotel.
It may not surprise you that the FCC also objected to this use of the ISM band, and the result was a $600,000 settlement with the hotel to resolve the issue. The whole RF story thus extends the OSI model to at least a few more levels, including the vendor of the monitoring system, the hotel management and—at least one layer above them!—the FCC itself.
I suppose you can insert some legislative and political layers in there somewhere if you want, but I’m happy to focus my effort on wrangling the physical layer and those near it. Keysight signal generators and signal analyzers are becoming more capable above the physical layer, with features such as Wireless Link Analysis to perform layer 2 and layer 3 analysis of LTE-FDD UL and DL signals.
In the end, I hope there are ways to resolve these issues and give everyone fair access to the unlicensed portions of our shared spectrum. I dread a situation in which a market emerges for access points or hotspots with counter-blocking technology and a resulting arms race that could leave us all without access.