Preface

My first exposure to the subject of wireless LANs was perhaps in 1999 or 2000, and at the time I was impressed with one simple fact: this is complex technology. Of course, at the time I knew absolutely nothing about wireless local area networks (LANs), so you might argue that by virtue of its very newness combined with my complete ignorance of the subject, it should have seemed complicated, but it was much more than just a little different. Nothing about it seemed familiar. It was like learning a new language, or better yet, like learning a new alphabet. There were so many new terms; for one small example among many, I could choose SSID, for which I did not even know what the acronym stood for, much less what it really meant. And even once I had learned that SSID stood for "Service Set Identifier," I hadn't gained much ground, since it just further exposed the depths of my ignorance, since I didn't know what a "service set" was, or more importantly why it would be necessary or useful for a service set to have an identifier.

These new terms were not only unfamiliar, there was also no easy or obvious way to map them back to something with which I was already familiar. Wireless LANs have virtually no common technological ground with wired LAN technologies. I used to joke that if you gave engineers the task of designing the most complex LAN MAC^[1] sub-layer protocol that they could imagine, they would come up with the Fiber Distributed Data Interface (FDDI). Now that I know much more about wireless LANs, I would change the joke to say that they would come up with IEEE 802.11, which is the official designation of the family of wireless LAN standards described hereinafter.

^[1] The Medium Access Control sublayer is the lower half of the Data Link layer (which is layer 2 in the OSI Reference Model), and which will be discussed in considerable detail in this book.

However, I do not want to imply that the technology is complex for complexity's sake it really is extremely complicated to design a set of physical layer modulations and an associated set of Medium Access Control (MAC) procedures that result in a usable wireless LAN (WLAN) technology. The WLAN protocols that are the subject of this book are the result of nearly 10 years of standards work focused on creating an IEEE standard WLAN technology. The area is evolving rapidly, and it is clear that the current WLAN standards are far from the last word on the subject. There is no end in sight for the evolution of the IEEE 802.11 standards.

While I was busy with other projects at work in 2000 and 2001, wireless LAN products were proliferating, being adopted at an ever-increasing, seemingly feverish, pace. Apparently, there was a pent-up demand that was being served, even by first- and second-generation products that were expensive and didn't work as smoothly as they might have. Throughout 2000 2003, one of the only bright spots in the high-tech economy was the wireless LAN sector. Despite the recession in those years, the growth rate of this market was still in the 20 50% range (depending on whose market analysis studies you read).

Over the last five years, the price of wireless LAN hardware has dropped by about an order of magnitude, and continues to drop. The products are much more robust today, easier to install and use, and in general, much better overall. However, even with the higher prices, poor integration, difficult installation, and marginal performance under anything other than ideal conditions, a nontrivial number of early adopters jumped on the WLAN bandwagon well before there were viable, reliable, easy-to-use standards-based products at reasonable prices. The rate of increase in importance of wireless LANs is such that the book I wrote in 1999 (IP Fundamentals) makes no mention of WLAN technology, but today I am writing an entire book devoted to that single topic.

The WLAN market did not really take off significantly until there was a standard in place that had the backing of a large consortium of vendors, who could leverage higher volumes to help drive down costs. Before the ratification of the first IEEE 802.11 standard in 1997, there were pre-standard wireless LAN products, such as those based on HomeRF, for example. There were also numerous purpose-built wireless communications systems using technologies that could be seen as direct forbears of the modern WLAN protocols. Due to a combination of factors, perhaps partially due to the expense of the equipment and the limited ease of use due to minimal integration into PC operating systems of the day, the early pre-standard products were expensive and were not widely perceived as user-friendly. However, the users who managed to get wireless LANs installed and operating were delighted with their newfound freedom and flexibility, unshackled from the wires that had constrained them in the past.

One specific area in which early products came up short was software drivers, especially for client products. It is considerably easier today to use a wireless LAN card with, for example, Windows XP. You simply insert the card, and the operating system auto-detects the hardware, loads the correct drivers, initializes the card, and you're off and surfing. In earlier days, it was a challenge to get products with drivers that were stable, and that could support a range of operating systems (not just different flavors of Windows [e.g., 95 / 98 / NT / 2000 / Me / etc.], but also Linux and MacOS).

Again, this is a complex technology. The best advice I can give you is to try to suppress your natural tendencies to want to map the world of wired LANs that you already understand to the new world of WLANs. The wireless LANs are just different, and ultimately stand apart. WLAN standards, more than any others with which I am familiar, are unique, and have to be understood on their own terms.

Eventually, I began paying much closer attention to WLANs. Now that I have had a chance to synthesize some level of understanding for myself, I hope to be able to share that with a wider audience. In this book, I hope to give the reader a feel for why WLANs need to be so very different from and so much more complex than wired LANs. Fundamentally, the conclusion I came to was that WLANs are more complex because they have to be. It's as simple as that.

In the course of my dealings with other IEEE 802.11 Working Group members, I have come to fully appreciate that I do not know everything there is to know about this subject. However, you can rest assured that whatever is in this book is as accurate as I could make it as of the time it was written. It took me a long time to digest the standard(s) (no criticism to the IEEE 802.11 WG, but these documents aren't easy to read!), and I really didn't "get it" until I had the chance to play with some real hardware and put all my knowledge to some actual use. Besides real products, I also have had access to wireless LAN diagnostic tools, such as packet sniffers, which helped me to see what was really going on "under the covers."

I cannot claim to have top-to-bottom knowledge of wireless LAN technology. My knowledge base is centered at the Data Link and Network layers, and I will be the first to admit that I am no expert in Physical layer modulation schemes. In my judgment, this is arcane knowledge (another of my subjective assessments, but I think it would be difficult to convince WLAN users that they needed to understand this stuff any more than Ethernet users could be convinced that they need to understand Differential Manchester encoding). If this sort of thing is what you are looking for, you will find just enough of it here for me to be able to tell you how the various schemes differ, and more importantly, what they are used for. If you want more details on information theory, coding theory, and so forth, there are many excellent books on those subjects. I will make sure that the reader understands the terms FHSS, DSSS, CCK, PBCC, and OFDM (and more) and will be able to tell them apart. I'm not even going to tell you what those mean now. Read Chapter 3, Speeds and Feeds, for that information….

I will cover the Physical layer technologies in sufficient detail to explain what they do, but I won't try to convince you that I am intimately familiar with their inner workings. I know what they do, and I think that's all that most people need to know (heck, if this stuff works smoothly enough, most users won't ever even need to wonder how it works).

When I needed to learn more about WLANs, I found that the best reference was usually the standards themselves, despite their being written less than clearly. The IEEE 802.11-1999 standard is a large enough document that even experienced network software engineers needed "handbooks" to help them find their way through it. At the time, however, WLAN technology was just beginning to mature into a consumer-level technology that needs some basic treatment for mass audiences. By now, that transition is well under way. WLAN-enabled products are hot.

There are other excellent books on wireless LANs if you need to run right out and design your own wireless LAN implementation; however, books for users are few and far between. I hope to ease the acquisition of the new "language skills" that are necessary to become comfortable with this technology. To deploy and use WLANs, much of this knowledge is not required vendors try to make their products fairly bulletproof to keep their support costs down. However, if you need to set up and manage a WLAN, you'll want to know enough to be effective, without needing to drown in details. If you ever need to go behind the scenes and debug a problem, you will need to have the context that I provide in this book. It's difficult to fix a problem unless you can notice it. To do that, you need to know what the normal behavior is expected to be.

If you want to use products based on IEEE 802.11 and need to understand the terminology involved in the standard, so that you can use, deploy, or manage the technology with the minimum of confusion, then this is the book for you. Ultimately, it is my assertion that very few people really need to know about which radio frequencies are being used, or the ways that data is modulated onto those frequencies. I'll include this information for completeness, but I think that most readers can safely skip such sections.