Einstein’s cat

Now we all know about Schrodinger’s cat – the weird cat that is both dead and alive. Well, many of us do – that is a pretty famous cat. Somewhat less known is Einstein’s cat. When Einstein was asked to explain what is wireless, in the days when wireless was very new and mysterious, he is quoted as saying:

“The wireless telegraph is not difficult to understand. The ordinary telegraph is like a very long cat. You pull the tail in New York, and it meows in Los Angeles. The wireless is exactly the same, only without the cat.”

This quote implies that a wireless communication system is the same as a wired communication system, except that the wires have been eliminated. This notion was widespread among people designing and building communication networks, especially in the earlier days, and seems quite firmly entrenched to this day. The thinking here is that when one wants to migrate parts of a wired network to wireless, all one has to do is to replace a piece of Cat 5 cable (yes, another cat, of a different color …) by a wireless link. So for example all the communication protocols and network protocols which have been in use in wired networks can be used unchanged on a mixed wired/wireless network. Furthermore, the network manager, who is someone knowledgeable about wired networks but has typically no knowledge or experience of wireless, will now design and maintain the mixed network. After all, we are only replacing a Cat 5 by a wireless link – just a pair of radios!

The idea that a wire can be simply replaced by by a wireless radio link can, and often does, result in problems due to the fundamentally different characteristics of wired and wireless systems. The wired environment is basically fixed and unchanging. Once the Cat 5 cables have been laid out (if this is done properly) nothing much happens. So the behavior of the wired link is highly predictable and controllable.

A wireless link on the other hand is changing with time, usually randomly, is highly unpredictable, and not fully controllable. Once the radio waves leave the safety and comfort of the transmitter and go on their merry way, they interact with the outside environment. They get reflected, refracted, and scattered (multipath), they meet and join other radio waves (interference), until they eventually get to the receiver. Because the outside environment is constantly changing and unpredictable, so is the wireless link.

These fundamental differences between the characteristics of wired and wireless systems means that Einstein’s statement is misleading. A wireless system is not the same as a wired system in which the wire has been eliminated. It is quite different, and therefore designers and managers of mixed wireless/wired networks need to be “wireless literate” if they are to do their job properly. This, unfortunately, is not usually the case. We will discuss some specific examples in future postings, and introduce some of the basic concepts of wireless literacy.


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