Fundamentals of the 802.11 protocol

This situation has been driven by the ubiquitous acceptance of wireless communications among laptop and handheld computers, as well as the massive presence of cellular communications. This works quite nicely if you happen to be working in the PC world or in a specialized area such as cellular handset design, but what if you are in the business of making specialized embedded systems doing anything else?

This article concentrates on the details of wireless protocol stacks that are useful to engineers that need to dig deeper into the technology than the typical application programmer. The goal is to allow you to use these stacks to their fullest potential in the specialized systems you are designing.

Unique qualities of wireless stacks
In a sense, the thought that we would need to tweak wireless protocol stacks defeats the whole idea of a protocol stack in the first place. Isn't the concept that the lower layers of a protocol stack can be treated like a black box, something that just works and can be safely ignored?

There are two rebuttals to that point of view. The first one is that it is there and therefore it must be understood. This is the "hacker" argument, using the original concept of hacking rather than the criminal sense that is finally beginning to fall out of favor with the press.

This is the engineering mindset that drives children of all ages to disassemble their toys to see what makes them tick. It is only when all mysteries have been solved that we can truly be comfortable with any new technology, especially one as fascinating as wireless communications.

The second rebuttal is more practical. If you are building communicating systems that must interoperate with systems from other sources it is required that you use standards to do that communicating. If your system is a general-purpose computer like a laptop it is quite likely that the generic tuning that is used for these standards will be quite sufficient for your needs.

But what if you have control of both sides of the communications? In this case it still may very well make sense to use a standard for communications, but it may be undesirable to use that standard as it comes off-the-shelf. In this case it is quite possible to tune the standard to be optimized for your particular application.

This type of standards modification has a long history in embedded systems. Serial ports have been built that used TTL signal levels rather than the 12v signaling required by the RS-232 standard because the application did not need the line distance afforded by the higher voltages. TCP/IP protocol stacks have been gutted because all that was needed was raw Ethernet connectivity.

But what are the unique qualities of wireless communication that must be conserved, and what can be stripped out in the name of dedicated-system efficiency? The unique characteristics of wireless protocol stacks that must be understood boil down to a few ideas.

Data formatting for unreliable communications
Wireless communications is unreliable. This is not a value judgment; it is instead a simple fact of life. A wired interface can send digital data in a very simple mapping between signals and the data itself. This voltage is a zero, that voltage is a one. Wireless communications must deal with the fact that data sent is quite likely to be munged in the process.

As a result, wireless systems will add redundancy in the data to allow error recovery. It is quite common for them to use specialized encoding to increase the Hamming distance between symbols (i.e. data values) within a data packet.

Resistance to interference
Another common feature in wireless data is the use of techniques that resist interference. The most common of these is some form of spectrum spreading. The concept is quite simple. Send data on a wide swath of spectrum. If one part of the spectrum is blocked, it will still get through on another part.

The simplest concept of interference is two signals broadcasting on the same frequency, but it turns out that this is just one of the possibilities for interference. There is a concept referred to as multipath that is often much more difficult to deal with. This is literally a signal interfering with itself.

It is caused by a signal traveling many different ways to get from the source to the receiver. Each of these paths has a different length, so the same message gets received multiple times. This single problem is probably the biggest one to overcome in any wireless communications system.

Adaptation
They say that the one constant is change. Nowhere is this truer than in a wireless communication system. The parameters of signal reception can change completely in a fraction of a second as signal paths change and interference sources appear and disappear.

Wireless protocol stacks must be able to adapt to these changes in real time and smooth the apparent communication path that is seen by applications software that is using that stack.

Solving these unique challenges has been what has allowed wireless data communications to break into the real world over the last few years. The following sections will examine some of the more popular protocol stacks that are currently being used and start setting the groundwork to allow us to modify these stacks for specific applications.