Building Wireless Community Networks

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Building Wireless Community Networks Implementing the Wireless Web

By Rob Flickenger

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Chapter 1: Wireless Community Networks

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In recent times, the velocity of technology development has exceeded "blur" and is now moving at speeds that defy description. Internet technology in particular has made astounding strides in the last few years. Where only a few short years ago 56Kb modems were all the rage, many tech heads now find themselves complaining about how slow their company's T1 connection seems compared to their 6Mb DSL connection at home.

Never before have so many had free and fast access to so much information. As more people get a taste of millisecond response times and megabit download speeds, they seem only to hunger for more. In most places, the service everyone is itching for is DSL , or Digital Subscriber Lineservice. It provides high bandwidth (typically, anywhere from 384Kbps to 6Mbps) over standard copper telephone lines, if your installation is within about three miles of the telephone company's CO, or central office (this is a technical constraint of the technology). DSL is generally preferred over cable modems, because a DSL connection provides guaranteed bandwidth (at least to the telephone company) and thus is not directly affected by the traffic habits of everyone else in your neighborhood. It isn't cheap, ranging anywhere from $50 to $300 per month, plus ISP and equipment charges, but that doesn't seem to be discouraging demand.

Telephone companies, of course, are completely enamored with this state of affairs. In fact, the intense demand for high-bandwidth network access has led to so much business that enormous lead times for DSL installations are now the rule in many parts of the country. In many areas, if you live outside the perceived "market" just beyond range of the CO, lead times are sometimes quoted at two to three years (marketing jargon for "never, but we'll take your money anyway, if you like"). Worse than that, in the wake of widespread market consolidation, some customers who were quite happy with their DSL service are finding themselves stranded when their local ISP goes out of business.

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The Problem

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An obvious application for 802.11b is to provide the infamous "last mile" network service. This term refers to the stretch that sits between those who have good access to the Internet (ISPs, telcos, and cable companies) and those who want it (consumers). This sort of arrangement requires 802.11b equipment at both ends of the stretch (for example, at an ISP's site and at a consumer's home).

Unfortunately, the nature of radio communications at microwave frequencies requires line of sight for optimal performance. This means that there should be an unobstructed view between the two antennas, preferably with nothing but a valley between them. This is absolutely critical in long distance, low power applications. Radio waves penetrate many common materials, but range is significantly reduced when going through anything but air. Although increasing transmission power can help get through trees and other obstructions, simply adding amplifiers isn't always an option, as the FCC imposes strict limits on power. (See Appendix A for a copy of the FCC Part 15 rules that pertain to 2.4GHz emissions. We will return to this subject in detail in Chapter 7.)

Speaking of amplifiers, a related technical obstacle to wireless nirvana is how to deal with noise in the band. The 2.4GHz band isn't reserved for use solely by 802.11b gear. It has to share the band with many other devices, including cordless phones, wireless X-10 cameras, Bluetooth equipment, burglar alarms, and even microwave ovens! Using amplifiers to try to "blast" one's way through intervening obstacles and above the background noise is the social equivalent of turning your television up to full volume so you can hear it in your front yard (maybe also to hear it above your ringing telephone and barking dog, or even your neighbor's loud television...).

If data is going to flow freely over the air, there has to be a high degree of coordination among those who set it up. As the airwaves are a public resource, the wireless infrastructure should be built in a way that benefits the most people possible, for the lowest cost. How can 802.11b effectively connect people to each other?

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How ISPs Are Attempting a Solution

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Visions of license-free, monopoly shattering, high-bandwidth networks are certainly dancing through the heads of some business-minded individuals these days. On the surface, it looks like sound reasoning: if people are conditioned into believing that 6Mb DSL costs $250 per month to provide, then they'll certainly be willing to pay at least that much for an 11Mb wireless connection that costs pennies to operate, particularly if it's cleverly packaged as an upgrade to a brand name they already know. The temptation of high profits and low operating costs seems to have once again allowed marketing to give way to good sense. Thus, the wireless DSL phenomenon was born. (Who needs an actual technology when you can market an acronym, anyway?)

In practice, many WISPs are finding out that it's not as simple as throwing some antennas up and raking in the cash. To start with, true DSL provides a full-duplex, switched line. Most DSL lines are asymmetric, meaning that they allow for a higher download speed at the expense of slower upload speed. This difference is hardly noticeable when most of the network traffic is incoming (i.e., when users are browsing the Web), but it is present. Even with the low-speed upload limitation, a full-duplex line can still upload and download data simultaneously. Would-be wireless providers that build on 802.11b technology are limited to half-duplex, shared bandwidth connections. This means that to provide the same quality of service as a wired DSL line, they would need four radios for each customer: two at each end, using one for upstream and one for downstream service. If the network infrastructure plan is to provide a few (or even a few dozen) wireless access sites throughout a city, these would need to be shared between all of the users, further degrading network performance, much like the cable modem nightmare. Additional access sites could help, but adding equipment also adds to hardware and operating costs.

Speaking of access points, where exactly should they be placed? Naturally, the antennas should be located wherever the greatest expected customer base can see them. Unless you've tried it, I guarantee this is trickier than it sounds. Trees, metal buildings, chain link fences, and the natural lay of the land make antenna placement an interesting challenge for a hobbyist, but a nightmare for a network engineer. As we'll see later, a basic antenna site needs power and a sturdy mast to mount equipment to, and, preferably, it also has access to a wired backbone. Otherwise, even more radio gear is needed to provide network service to the tower.

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How Cooperatives Are Making It Happen

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The difficulties of a commercial approach to wireless access exist because of a single social phenomenon: the customer is purchasing a solution and is therefore expecting a reasonable service for their money. In a commercial venture, the WISP is ultimately responsible for upholding their end of the agreement or otherwise compensating the customer.

The "last mile" problem has a very different outlook if each member of the network is responsible for keeping his own equipment online. Like many ideas whose time has come, the community wireless network phenomenon is unfolding right now, all over the planet. People who have been fed up with long lead times and high equipment and installation costs are pooling their resources to provide wireless access to friends, family, neighbors, schools, and remote areas that will likely never see broadband access otherwise. As difficult as the WISP nightmare example has made this idea sound, people everywhere are learning that they don't necessarily need to pay their dues to the telco to make astonishing things happen. They are discovering that it is indeed possible to provide very high bandwidth connections to those who need it for pennies—not hundreds of dollars—a month.

Of course, if people are going to be expected to run a wireless gateway, they need access either to highly technical information or to a solution that is no more difficult than plugging in a connector and flipping a switch. While bringing common experiences together can help find an easy solution more quickly, only a relatively small percentage of people on this planet know that microwave communications are even possible. Even fewer know how to effectively connect a wireless network to the Internet. As we'll see later, ubiquity is critical if wide area wireless access is going to be usable (even to the techno über-elite). It is in everyone's best interest to cooperate, share what they know, and help make bandwidth as pervasive as the air we breathe.

The desire to end this separation of "those in the know" from "those who want to know" is helping to bring people away from their computer screens and back into their local neighborhoods. In the last year, dozens of independent local groups have formed with a very similar underlying principle: get as many people as possible connected to each other for the lowest possible cost. Web sites, mailing lists, community meetings, and even IRC channels are being set up to share information about extending wireless network access to those who need it. Wherever possible, ingeniously simple and inexpensive (yet powerful) designs are being drawn up and given away. Thousands of people are working on this problem not for a personal profit motive, but for the benefit of the planet.

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About This Book

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The ultimate goal of this book is to get you excited about this technology and arm you with the information you need to make it work in your community. We will demonstrate various techniques and equipment for connecting wireless networks to wired networks, and look at how others "in the know" are getting their neighborhoods, schools, and businesses talking to each other over the air. Along the way, we will visit the outer limits of what is possible with 802.11b networking, how to stretch the range to miles and ways of providing access for hundreds. If your budget won't allow for all of the networking gear you need, we'll show you how to build some of your own.

Through the efforts of countless volunteers and hobbyists, more bits are being moved more cheaply and easily than at any other time in history. There is more happening in the wireless world right now than is practical to put down on paper. Get online and find out what others in your area are doing with this technology (extensive online references are provided throughout this book and in Appendix A).

I hope you will find this book a useful and practical tool on your journey toward your own wireless utopia.

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Chapter 2: Defining Project Scope

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What do you want to accomplish? As a sysadmin, this is a question I ask whenever a user comes to me with a new request. It's easy to get wrapped up in implementation details while forgetting exactly what it is you set out to do in the first place. As projects get more complex, it's easy to find yourself "spinning your wheels" without actually getting anywhere.

The most common questions I've encountered about 802.11b networking seem to be the simplest:

How much does it cost?
How far will it go?
Can I use it to [ fill in the blank ] ?

Of course, these questions have pat theoretical answers, but they all have the same practical answer: "It depends!" It is easiest to explain how people have applied wireless to fit their needs and answer these questions by way of example.

People are using 802.11b networking in three general applications: point-to-point links, point-to-multipoint links , and ad-hoc (or peer-to-peer) workgroups. A typical point-to-point application would be to provide network bandwidth where there isn't any otherwise available. For example, suppose you have a DSL line at your office but can't get one installed at your house (due to central office distance limits). If you have an unobstructed view of your home from your office, you can probably set up a point-to-point connection to connect the two together. With proper antennas and clear line of sight, reliable point-to-point links in excess of 20 miles are possible (at up to 11Mbps!).

One common way of using wireless in a point-to-multipointapplication is to set up an access point at home to let several laptop users simultaneously browse the Internet from wherever they happen to be (the living room couch is a typical example). Whenever several nodes are talking to a single central point of access, this is a point-to-multipoint application. But point-to-multipoint doesn't have to end at home. Suppose you work for a school that has a fast Internet connection run to one building, but other buildings on your campus aren't wired together. Rather than spend thousands getting CAT5 or fiber run between the buildings, you could use an access point in the wired building with a single antenna that all of the other buildings can see. This would allow the entire campus to share the Internet bandwidth for a fraction of the cost of wiring, in a matter of days rather than months.

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Hardware Requirements

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The total cost of your project is largely dependent on your project goals and how much work you're willing to do yourself. While you can certainly spend tens of thousands of dollars on outdoor, ISP-class equipment, you may find that you can save money (and get more satisfaction) building similar functionality yourself, with cheaper off-the-shelf hardware.

If you simply want to connect your laptop to someone else's 802.11b network, you'll need only a client card and driver software (at this point, compatible cards cost between $50 and $200). Like most equipment, the price typically goes up with added features, such as an external antenna connector, higher output power, a more sensitive radio, and the usual bells and whistles. Once you select a card, find out what the network settings are for the network you want to connect to, and hop on. If you need more range, a small omni-directional antenna (typically $50-$100) can significantly extend the roaming range of your laptop.

If you want to provide wireless network access to other people, you'll need an access point (AP). This has become something of a loaded term and can refer to anything from a low-end "residential gateway" class box (about $200) to high-end, commercial quality, multi-radio equipment ($1000+). They are typically small, standalone boxes that contain at least one radio and another network connection (like Ethernet or a dialup modem). For the rest of this book, we'll use the term access point to refer to any device capable of providing network access to your wireless clients. As we'll see in Chapter 5, this can even be provided by a conventional PC router equipped with a wireless card.

While a radio and an access point can make a simple short range network, you will more than likely want to extend your coverage beyond what is possible out of the box. The most effective way of extending range is to use external antennas. Antennas come in a huge assortment of packages, from small omnidirectional tabletop antennas to large, mast-mounted parabolic dishes. There isn't one "right" antenna for every application; you'll need to choose the antenna that fits your needs (if you're trying to cover just a single building, you may not even need external antennas). Take a look at Chapter 6 for specific antenna descriptions.

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Hot Spots

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The IEEE 802.11b specification details 11 possible overlapping frequencies on which communications can take place. Much like the different channels on a cordless phone, changing the channel can help eliminate noise that degrades network performance and can even allow multiple networks to coexist in the same physical space without interfering with each other.

Rather than attempting to set up a single central access point with a high- gain omnidirectional antenna, you will probably find it more effective to set up several low-range, overlapping cells. If you use access point hardware, and all of the APs are connected to the same physical network segment, users can even roam seamlessly between cells. Figure 2-1 shows an example of using multiple APs to cover a large area.

Figure 2-1: Using non-adjacent channels, several APs can cover a large area

As detailed in the specification, 802.11b breaks the available spectrum into 11 overlapping channels, as shown in Table 2-1.

Table 2-1: 802.11b channel frequencies
Channel	Frequency (GHz)
1	2.412
2	2.417
3	2.422
4	2.427
5	2.432
6	2.437
7

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Potential Coverage Problem Areas

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While a spectrum analyzer (and an engineer to operate it) is the ultimate survey tool, such things don't come cheap. Fortunately, you can get a lot of useful information using a good quality client radio and software. Take a look at the tools that come with your wireless gear (Lucent's Site Monitor tool, shown in Figure 2-2, which ships with Orinoco cards, is particularly handy). You should be able to get an overview map of all networks in range and which channels they're using.

Figure 2-2: Lucent's Site Monitor tool shows you who's using 802.11b in your area

Other (non-802.11b) sources of 2.4GHz radio emissions show up as noise on your signal strength meter. If you encounter a lot of noise on the channel you'd like to use, you can try to minimize it by moving your access point, using a more directional antenna (see Chapter 6), or simply picking a different channel. While you always want to maximize your received signal, it is only usable if the ambient noise is low. The relationship of signal to noise is critical for any kind of communications. It is frequently abbreviated as SNR, for signal to noise ratio. As this number increases, so does the likelihood that you'll have reliable communications. (For fine examples of low SNR, kindly consult your local Usenet feed.)

Of course, no known technology can determine the SNR of the actual data you're transmitting or receiving. In the end, you still have to figure out for yourself how to pull signal from the noise once it leaves the Application layer of your network.

To sum up: be a good neighbor, and think about what you're doing before turning on your own gear. The radio spectrum is a public resource and, with a little bit of cooperation, can be used by everyone to gain greater access to network resources.

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Topographical Mapping 101

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As you roll out wireless equipment, you'll find yourself looking at your environment in a different way. Air conditioning ducts, pipes, microwave ovens, power lines, and other sources of nastiness start leaping into the foreground as you walk around. By the time you've set up a couple of nodes, you will most likely be familiar with every source of noise or reflection in the area you're trying to cover. But what if you want to extend your range, as in a several-mile point-to-point link? Is there a better way to survey the outlying environment than walking the entire route of your link? Maybe.

Topographical surveys have been made (and are constantly being revised) by the USGS in every region of the United States. Topo (short for topographical) maps are available both on paper and on CD-ROM from a variety of sources. If you want to know the lay of the land between two points, the USGS topos are a good starting point.

The paper topo maps are a great resource for getting an overview of the surrounding terrain in your local area. You can use a ruler to quickly gauge the approximate distance between two points and to determine whether there are any obvious obstructions in the path. While they're a great place to start assessing a long link, topographical maps don't provide some critical information: namely, tree and building data. The land may appear to cooperate on paper, but if there's a forest or several tall buildings between your two points, there's not much hope for a direct shot.

The USGS also provides DOQs (or Digital Orthophoto Quadrangles) of actual aerial photography. Unfortunately, freely available versions of DOQs tend to be out of date (frequently 8 to 10 years old), and recent DOQs are not only expensive but also often aren't even available. If you absolutely must have the latest aerial photographs of your local area, the USGS will let you download them for $30 per order and $7.50-$15 per file. You will probably find it cheaper and easier to make an initial estimate with topo maps and then simply go out and try the link.

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Chapter 3: Network Layout

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In many ways, 802.11b networking is very much like Ethernet networking. Assuming you want to connect your wireless clients to the Internet, you'll want to provide all of the usual TCP/IP services, such as Domain Name Service (DNS) and Dynamic Host Configuration Protocol (DHCP), that make networking so much fun. To the rest of your network, wireless clients look just like any other Ethernet interface and are treated no differently than the wired printer down the hall. You can route, rewrite, tunnel, fold, spindle, and/or mutilate packets from your wireless clients just as you can with any other network device.

Presumably, no matter how many wireless clients you intend to support, you will eventually need to "hit the wire" in order to access other networks (such as the Internet). How do packets find their way from the unbridled freedom of the airwaves to the established, hyper-interconnected labyrinth of the Internet? This chapter describes what you need to know to do that.

As with any network supporting different physical mediums, network bridges must exist that are capable of exchanging data between the various network types. A wireless gateway consists of a radio card and a network card (usually Ethernet). In the case of 802.11b, radios participating in the wireless network must operate in one of two modes: BSS or IBSS .

BSS stands for Basic Service Set. In this operating mode, a piece of hardware called an access point (AP) provides wireless-to-Ethernet bridging. Before gaining access to the wired network, wireless clients must first establish communications with an access point within range. Once the AP has authenticated the wireless client, it allows packets to flow between the client and the attached wired network, effectively acting as a true Layer 2 bridge, as shown in Figure 3-1. A related term, ESS (or Extended Service Set), refers to a physical subnet that contains more than one AP. In this sort of arrangement, the APs can communicate with each other to allow authenticated clients to "roam" between them, handing off IP information as the clients move about. Note that (as of this writing) there are no APs that allow roaming across networks separated by a router.

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Wireless Infrastructure: Cathedral Versus Bazaar

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Figure 3-1: In BSS (or ESS) mode, clients must authenticate to a hardware access point before being able to access the wired network

IBSS stands for Independent Basic Service Set and is frequently referred to as ad-hoc or peer-to-peer mode. In this mode, no hardware access point is required. Any network node that is within range of any other can commence communications if they agree on a few basic parameters. If one of those peers also has a wired connection to another network, it can provide access to that network. Figure 3-2 shows a model of an IBSS network.

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Vital Services

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A network can be as simple as a PPP dialup to an ISP, or as grandiose and baroque as a multinational corporate MegaNet. But every node on a multimillion dollar network in Silicon Valley needs to address the same fundamental questions that a dialup computer must answer: who am I, where am I going, and how do I get there from here? In order for wireless clients to easily access a network, the following basic services must be provided.

The days of static IP addresses and user-specified network parameters are thankfully far behind us. Using DHCP (Dynamic Host Configuration Protocol), it is possible (and even trivial) to set up a server that responds to client requests for network information. Typically, a DHCP server provides all the information that a client needs to begin routing packets on the network, including the client's own IP address, the default Internet gateway, and the IP addresses of the local DNS servers. The client configuration is ridiculously easy and is, in fact, configured out of the box for DHCP in all modern operating systems.

While a thorough dissection of DHCP is beyond the scope of this book, a brief overview is useful. A typical DHCP session begins when a client boots up, knowing nothing about the network it is attached to except its own hardware MAC address. It broadcasts a packet saying, effectively, "I am here, and this is my MAC address. What is my IP address?" A DHCP server on the same network segment listens for these requests and responds: "Hello MAC address. Here is your IP address, and by the way, here is the IP address to route outgoing packets to, and some DNS servers are over there. Come back in a little while and I'll give you more information." And the client, now armed with a little bit of knowledge, goes about its merry way. This model is shown in Figure 3-3.

Figure 3-3: DHCP lets a node get its network settings dynamically and easily

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Security Considerations

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Although the differences between tethered and untethered are few, they are significant. For example, everyone has heard of the archetypal "black-hat packet sniffer," a giggling sociopath sitting on your physical Ethernet segment, surreptitiously logging packets for his own nefarious ends. This could be a disgruntled worker, a consultant with a bad attitude, or even (in one legendary case) a competitor with a laptop, time on his hands, and a lot of nerve. Although switched networks, a reasonable working environment, and conscientious reception staff can go a long way to minimize exposure to the physical wiretapper, the stakes are raised with wireless. Suddenly, one no longer needs physical presence to log data: why bother trying to smuggle equipment onsite when you can crack from your own home or office two blocks away with a high-gain antenna?

Visions of cigarette smoking, pale skinned über-crackers in darkened rooms aside, there is a point that many admins tend to overlook when designing networks: the whole reason that the network exists is to connect people to each other! Services that are difficult for people to use will simply go unused. You may very well have the most cryptographically sound method on the planet for authenticating a user to the system. You may even have the latest in biometric identification, full winnow and chaff capability, and independently verified and digitally signed content assurance for every individual packet. But if the average user can't simply check her email, it's all for naught. If the road to hell is paved with good intentions, the customs checkpoint must certainly be run by the Overzealous Security Consultant.

The two primary concerns when dealing with wireless clients are these:

Who is allowed to access network services?
What services can authorized users access?

As it turns out, with a little planning, these problems can be addressed (or neatly sidestepped) in most real-world cases. In this section, we'll look at ways of designing a network that keeps your data flowing to where it belongs, as quickly and efficiently as possible.

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Summary

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In order to maintain maximum compatibility with available 802.11b client hardware and yet still provide responsible access to the Internet, we can apply a combination of inexpensive hardware and freely available software to strike an acceptable balance between access and security.

In the following chapters, we'll see how to set up basic wireless access to your existing wired network. We will then build a workable method for providing wireless services to your local community, for minimal cost, while promoting community participation and individual responsibility.

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Chapter 4: Using Access Points

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As we discussed in Chapter 3, an access point is a piece of hardware that connects your wireless clients to a wired network (and usually on to the Internet from there). As with any piece of bridging hardware, it has at least two network connections and shuffles traffic between them. The wireless interface is typically an onboard radio or an embedded PCMCIA wireless card. The second network interface can be Ethernet, a dialup modem, or even another wireless adapter.

The access point hardware controls access to and from both networks. On the wireless side, most vendors have implemented 802.11b access control methods (like WEP encryption keys, "closed" network ESSIDs, and MAC address filtering). Some have added proprietary extensions to provide additional security, like public key encryption. Many access points also allow control over what the wired network can send to the wireless clients, through simple firewall rules.

In addition to providing access control, the access point also maintains its own network connections. This includes functions like dialing the phone and connecting to an ISP on demand, or using DHCP on the Ethernet interface to get a network lease. Most access points can provide NAT and DHCP service to the wireless clients, thereby supporting multiple wireless users while only requiring a single IP address from the wire. Some support direct bridging, allowing the wired and wireless networks to exchange data as if they were physically connected together. If the access point has multiple radios, it can bridge them together with the wire, allowing for a very flexible, extendable network.

Another important service provided by APs is the ability to "hand off" clients as they wander between access points. This lets users walk around a college campus, for example, without ever dropping their network connection. Current AP technology only allows roaming between access points on the same physical subnet (that is, APs that aren't separated by a router). Unfortunately, the roaming protocol was left unimplemented in the 802.11 spec, so each manufacturer has implemented their own method. This means that hand-offs between access points of different manufacturers aren't currently possible.

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Access Point Caveats

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One feature that is in high demand (among users trying to go for distance) is the ability to bridge over the air to another access point. Allegedly, the Intel 2011 can do AP-to-AP bridging (as can the Linksys WAP11 after a firmware upgrade). But reports from the field seem to indicate shaky performance at best, as of this writing. Normally, APs don't talk to each other over the air; they're designed to talk to client cards. So on a long distance point-to-point link, you'll need to either use a client PC router to talk to an AP or use two routers in IBSS mode (with no AP). See the information in Section 7.1 in Chapter 7 if you're interested in long-distance point-to-point links. By the time this book makes it to press, the manufacturers should have their firmware in better shape (we hope).

You should also seriously consider how to fit APs into your existing wired network. Even with WEP encryption and other access control methods in effect, AP security is far from perfect. Because an access point is, by definition, within range of all wireless users, every user associated with your access point can see the traffic of every other user. Unless otherwise protected with application layer encryption, all email, web traffic, and other data is easily readable by anyone running protocol analysis tools such as tcpdump or ethereal. As we saw in Chapter 3, relying on WEP alone to keep people out of your network may not be enough protection against a determined black hat.

In terms of establishing a community network, access points do provide one absolutely critical service: they are an easy, standard, and inexpensive tool for getting wireless devices connected to a wired network. Once the wireless traffic hits the wire, it can be routed and manipulated just like any other network traffic, but it has to get there first.

Wireless access points that are on the consumer market today were designed to connect a small group of trusted people to a wired network and lock out everyone else. The access control methods implemented in the APs reflect this philosophy, and if that is how you intend to use the gear, it should work very well for you. For example, suppose you want to share wireless network access with your neighbor but not with the rest of the block. You could decide on a mutual private WEP key and private ESSID and keep them a secret between you. Because you presumably trust your neighbor, this arrangement could work for both of you. You could even make a list of all of the radios that you intend to use on the network and limit the access point to only allow them to associate. This would require more administrative overhead, as one of you would have to make changes to the AP each time you wanted to add another device, but it would further limit who could access your wireless network.

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The Apple AirPort Base Station

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The Mac AirPort is a tremendously popular access point. It looks like a slick, retro-futuristic prop from "War of the Worlds," and it is very portable and rugged. While designed for use with the Mac platform, it works very well as a general purpose access point (and you don't even need a Mac to configure it; see the next section). As I write this, the AirPort sells retail for about $299. What does that get you?

Direct Ethernet bridging
DHCP/NAT
56K dialup modem port
User-definable ESSID
Roaming support
MAC address filtering
WEP encryption

What doesn't it get you? It has only one radio (actually, an embedded Orinoco Silver card) and no external antenna connector. But this isn't much of a problem, because the internal Silver card itself has an external connector. See http://homepage.mac.com/hotapplepi/airport/or http://www.wwc.edu/~frohro/Airport/Airport.htmlfor examples of how to add your own antenna.

Out of the box, the AirPort will try to get a DHCP lease from the Ethernet and start serving NAT and DHCP on the wireless with no password. Yes, by simply plugging your new toy into your LAN, you have eliminated all of the hard work that went into setting up your firewall. Anyone within earshot now has unrestricted wireless access to the network you plugged it into!

While this could be handy as a default configuration (say, at a conference or other public access network), this probably isn't what you want. To change the defaults, you'll need configuration software.

If you have a Mac handy, you are in luck. The AirPort Admin utility that ships with the AirPort is excellent (although only for Mac OS 9 or later). Apple has gone out of their way to make the whole AirPort system easy to set up, even for beginners. If you don't own a Mac, you have a couple of options. It turns out that the innards of the AirPort are virtually identical to the Orinoco RG-1000 (previously, the Lucent Residential Gateway). That means that the RG configuration utility for Linux (called

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Chapter 5: Peer-to-Peer (Ad-Hoc) Networking

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In traditional wired networks, those responsible for the existence of the network can exert a high degree of control over what happens on their wires. Through border firewalls, proxies, packet filters, and clever routing, the ultimate network content available to an individual node can be manipulated to an almost infinite degree.

The rules are very different when the wires are taken away. Anyone with an 802.11b card can effectively generate whatever sort of packet they like and send it out to anyone within range. As long as nodes can agree on a common method of communications, any number of peer-to-peer networks can be created to exchange data, in a way that makes it prohibitively difficult for a single entity to impose any sort of restriction on the flow of that data.

IBSS mode is one of the most liberating aspects of the 802.11b protocol. It effectively makes expensive access point hardware entirely optional, relying instead on each node of the network to maintain its own communications. Instead of a centralized model, where all clients must be within range of an access point in order to participate, IBSS allows any node to talk to any other node within earshot. If one of those nodes happens to be a gateway to the Internet, it can not only provide access-point-like services but also do the sorts of things that any self-respecting gateway can do: route packets, throttle bandwidth, act as a firewall, etc.

While IBSS refers to a specific mode of 802.11b operations, you will also encounter a couple of other similar (but loaded) terms in your travels: ad-hoc and peer-to-peer. A few vendors have implemented their own "AP-less" mode of operations that, while occasionally providing some interesting features, don't conform to the IEEE standard and will work only with their own hardware. These proprietary modes are referred to in product literature as ad-hoc or peer-to-peer mode. Cards that claim 802.11b compliance may support these additional proprietary modes, but they must also support true IBSS communications.

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Building a Wireless Gateway with Linux

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To a Linux machine, the wireless card appears to be just another Ethernet device. The wireless driver in the kernel provides a network device (e.g., eth0) that can do all of the things any other network device can do. The rest of the system is completely unaware that communications are happening over radio. If you have ever built a firewall with Linux, much of this section should seem familiar to you.

If you haven't built a firewall with Linux, I highly recommend building one with old fashioned Ethernet to get familiar with the process. O'Reilly's Building Internet Firewalls covers the specific networking issues involved in much greater detail than I have space for here. Another excellent document to work through is the Firewall and Proxy Server HOWTO at http://www.linuxdoc.org/HOWTO/Firewall-HOWTO.html.

Most 802.11b cards on the market today are PCMCIA devices. From a design and manufacturing standpoint, this is an excellent idea, because it simplifies the production line and helps keep costs down. At the time of this writing, wireless cards cost anywhere from $75-$200, with the average hovering around $120. Don't be fooled by their small size; these tiny cards are capable of sending a signal several miles with the proper antennas.

Obviously, to set up a machine as a wireless gateway, you need a computer with at least one PCMCIA slot. Although the most common computers that support PCMCIA are laptops, a desktop or rack mount box with a PCMCIA converter card works just fine. Many vendors (Cisco and Agere, for example) are selling PCMCIA to PCI or ISA converters specifically to fit wireless cards into desktop machines.

As long as the chipset on the PCMCIA converter is supported under Linux, and the wireless card itself has Linux drivers, your wireless card should work fine. It isn't absolutely necessary to use an Orinoco wireless card with an Agere converter, for example.

If you have any doubts about whether your hardware is supported under Linux, be sure to consult the current Hardware HOWTO at

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Chapter 6: Wide Area Network Saturation

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You have an access point. Your laptop is humming merrily along. While working at home or the office is more flexible than ever, you find yourself wondering what it would take to get a signal across the street, at your favorite coffee shop.

Or maybe you live in an area where you're on the perpetual "we'll get back to you" list for broadband services like DSL and cable modems, and you're ready to make it happen now. With the right equipment, enough participants, and the cooperation of the lay of the land, you can make broadband Internet access a reality in your neck of the woods.

Whatever your motivations, you are looking for a way to extend 802.11b beyond the listed 300-meter limit. This is not only possible and completely legal, it's also a lot of fun. You first need to figure out what your target coverage area is, and what resources you need to make it happen.

While extending your private network an extra block or two (or even several miles, with the proper antennas) may be interesting for you, it does nothing for those around you except generate more noise in the band. Most people will find it prohibitively expensive to rent tower space and set up network access for themselves, for wherever they happen to be in town. This has led to the fascinating phenomenon of the cooperative wireless network.

The single best piece of advice I can give you on your journey to the ultimate network (whether public or private) is to fight the urge to blindly go it alone. Get people from your local neighborhood involved. Call a general meeting of interested parties. Find other people in your area who have similar goals, and get your resources together. If there aren't any, join the development lists of any of the major community network groups (see Appendix A) and ask around. Chances are, others have done (or are contemplating doing) what you want to do, and they'll probably be more than happy to share their experiences.

As the number of people interested in wireless network access increases, a public access network stands to benefit from access to more vantage points, both figurative and physical. While you might not have direct line of sight to a place you want to talk to, your neighbor might. And for complimentary network access, they might just be willing to let you install some equipment and use their house as a repeater. Wireless bandwidth costs only electricity and equipment, not telephone or cable company charges. This kind of massively parallel, cooperative arrangement is what makes a high speed wireless wide area network possible. However, I can only give you the technical details; the social details are left as an exercise to the reader.

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Topo Maps 102: Dealing with Geographical Diversity

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If you want to stretch your signal farther than just across the street, you're going to have to consider exactly what lies between the points of your network. In Chapter 2, we looked at using USGS topographic maps and DOQs to estimate how the land lies between two arbitrary points. In addition to paper and online topographic maps, CD-ROM versions have been around for a few years. While typically geared toward hikers and outdoor enthusiasts, they have a lot to offer the aspiring wide area network engineer.

I have evaluated two popular commercial topo packages, Topo! by National Geographic, and DeLorme's TopoUSA 2.0. Figure 6-1 shows a Topo! rendering of Sebastopol, California. While they're both packed with features, only a few are directly applicable to helping analyze land between two points.

Figure 6-1: A Topo! rendering of Sebastopol, CA

Here are some points to keep in mind when evaluating topo software for link analysis:

The software should provide cross-section views of a route or drawn trail. This is probably the feature used most often in trying to figure out if the land will cooperate (see the next section for examples).
Almost any self-respecting topo software package includes the ability to mark up the maps with points and text. The ability to import lat/lon data and translate them into data points is handy if you know a site only by its latitude and longitude.
If you intend to use a GPS with your software (see the next section), make sure your topo software supports your GPS hardware. Both packages I evaluated support most popular GPS hardware and the NMEA data standard (which virtually all modern GPS receivers speak).
Both packages I currently use are for Windows only. There are lots of Linux mapping packages floating about that render USGS DOQs and DEMs, but, like a lot of Linux software, it may take a bit of fiddling to get them going. Check out the mapping resource links at the major community wireless sites (see Appendix A), as this is an area of rapid development in the open source community.

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Antenna Characteristics and Placement

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While I am not a radio frequency (RF) engineer, I have had a lot of practical experience setting up 802.11b networks. There isn't nearly enough room here for a full examination of the nuances of radio frequency communications. For more authoritative sources, be sure to check out the great resources in Appendix A—notably, the fantastic publications put out by the American Radio Relay League (AARL), an association of amateur radio operators. Radio is an entire field of study unto itself.

Antenna selection has a tremendous impact on the range and usability of your wireless network. Ironically, the design of almost every external 802.11b card puts the antenna in the worst possible orientation: sideways and very close to the laptop (or desktop). In this position, the radiation pattern is almost straight up and down! Not only does this drive half your signal into the table, it leaves your poor, underpowered radio susceptible to interference from the computer itself.

The one notable exception to this state of affairs is Apple's built-in AirPort card. They've thought enough to include an internal antenna connector that runs up the LCD panel. This is an excellent design with much better range, although it does preclude adding an external antenna. It looks like IBM is the first to play copycat with their i Series ThinkPads as well.

You will see a tremendous difference in signal strength by attaching a small omnidirectional external antenna to your client card and orienting it properly. Which way is properly? That depends on your environment. Try every possible position, with your signal strength meter open. I've put mine on top of my monitor, below the desk, sideways, on the table behind me, and even slung over my shoulder. The best orientation of your antenna is always in the position in which it receives the best signal, so don't be afraid to move the antenna around.

If you're in a pinch without an external antenna, you can watch the wonders of RF by opening up your strength meter and tilting your laptop sideways. Watch that signal bar grow. Go for the green! Learn to type sideways! Better yet, redesign your network to extend your range, and always pack a spare external antenna.

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Power Amps and the Law

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Frequently, when people think of extending range, they immediately think of using amplifiers (I suppose it's only natural; you have an amplifier for your home stereo, why not an amplifier for your network?). Good amplifiers that work in the microwave range have several nontrivial technical obstacles to overcome:

Amplifiers blindly amplify everything that they're tuned to, both signal and noise. A greater signal won't help you if the noise in the band is increased as well, because the signal will just get lost (like shouting to your friends at a concert).
802.11b radio communications are half duplex; they send or receive, but never both at the same time. An amplifier attached to the antenna line will have to detect automatically when the radio is sending and quickly switch the amp on. When it's finished, it has to quickly cut it off again. Any latency in this switching could actually impair communications or, worse, damage the radio card.
Amplifiers can help a bit on receive by adding some pre-emphasis, but they are really meant for transmitting. This means that if you only have an amp on one end of a link, the other end may be able to hear you, but you may not hear them. To make amps effective, you'll need them on both ends of the link.
All amplifiers require power to operate. This means adding a DC injector to your antenna feed line or using an external adapter. This further drives up the cost of your node and makes yet another device that you have to provide power for.

As a result, amplifiers that work well with 802.11b gear are expensive ($400+) and difficult to come by. But do you really need them? Using standard gear and high gain antennas, you can extend a point-to-point link to 25+ miles without amplifiers. Your money is probably better spent on high-quality directional antennas and cabling, and possibly even adding another node for further saturation.

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Chapter 7: Other Applications

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Thanks to the efforts of countless engineers, we have an open 802.11b standard. Now that hardware that adheres to this standard is in the hands of non-engineers, all sorts of interesting applications have been implemented. Thousands around the globe are pushing the capabilities of these inexpensive radios well beyond their intended limits. Standard client PCMCIA cards have been used to create point-to-point backbone links several miles apart. Not content with tiny, private networks, people are using inexpensive access points to create public networks that can support hundreds of simultaneous users. Even the popularized security shortcomings of 802.11b are being overcome by some careful planning and the proper application of open source software. Whether the IEEE committee intended it to be so or not, 802.11b has stumbled on the magic formula that makes the ultimate platform for hardware hackery: low cost, ease of use, ease of modification, and ubiquity.

In this chapter, we'll take a look at some wireless applications that demonstrate the enormous flexibility of wireless (and some that are just really cool!). Be warned that some of the examples in this chapter will certainly void warrantees and may damage your equipment if you're not careful. If you are ever unsure about how to proceed, ask around. Chances are, someone else has done what you're thinking of doing and can at least lend you their shared experience. The various wireless group mailing lists are a great resource for ideas and working out implementation details.

From a radio perspective, point-to-point links are very straightforward to set up. You should always follow more or less the same steps when evaluating the possibility of a link:

Establish that you have line of sight from end to end.
Measure the distance between the points and calculate the path loss.
Add the capabilities of your equipment to determine your link budget.

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Point-to-Point Links

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From a radio perspective, point-to-point links are very straightforward to set up. You should always follow more or less the same steps when evaluating the possibility of a link:

Establish that you have line of sight from end to end.
Measure the distance between the points and calculate the path loss.
Add the capabilities of your equipment to determine your link budget.
Go out and hook up your gear.

If you intend to make a long-distance point-to-point link, first find out the latitude, longitude, and altitude of each end point. You can find this by physically going to each site and marking the coordinates with a GPS, or you can estimate using topographical maps or software (see Chapter 6 for some examples of how to do this). With the coordinates and altitude of both sites, you can calculate a bearing and tilt angle, so you know roughly where to point the antennas on each end. A decent GPS can help here by giving you a bearing to and from each point. You should also check out the online wireless design CGIs at http://www.qsl.net/n9zia/wireless/page09.html for help with many of the calculations you'll need to perform.

Obviously, if you can see the other point through binoculars or a telescope, this is a good first step. Ideally, there should be very little on the ground between the two points. The closer the path is to an actual valley, the better. Take a look at Chapter 6 for details about how to calculate the path loss and link budget for your link. I've mentioned it before, but here it is again: keep your antenna cable as short as possible! On a long-distance point-to-point link, every few decibels count.

Now that you're ready to hook up your gear, the question remains: what gear do you want to use? That depends on your budget and how you plan to use the link. As we saw in Chapter 5, it is very simple to set up a Linux gateway in IBSS mode. This is probably the cheapest way to go for each end, but it presents some routing issues (most wireless cards disable Ethernet bridging, so you'll need to use masquerading to get packets to flow across the wireless link). If you already have a hardware access point, you can use it for one end of the link and a client card for the other, with the same configuration (i.e., masquerading on the client side). A recently released, low-cost hardware alternative is the Linksys WAP-11 access point. Linksys has released a firmware upgrade that allows this AP to communicate with others (of the same model) over the air, either in point-to-point or point-to-multipoint mode. You can download Version 1.4f.5 from their web site at

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The Pringles Can

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At the Portland Summit last June, Andrew Clapp presented a novel yagi antenna design. It used a bolt, metal tubing, washers, and PVC tubing to make an inexpensive "shotgun" yagi, either 18" or 36" long. While his antenna shows between 12 and 15dBi gain (which is impressive for such a simple design), it's also quite large. When we returned from Portland, some members of our local group and I realized that, if we were careful, we could fit a full wavelength inside a Pringles can, as shown in Figure 7-1. This would show a reduced total gain, but it would also make the entire antenna much more compact.

Figure 7-1: The complete antenna—it's just a can!

Here are the items you'll need to make a Pringles can antenna:

Part	Approximate cost
All-thread, 5 5/8" long, 1/8" OD	$1.00
two nylon lock nuts	$0.10
five 1" washers, 1/8" ID	$0.10
6" aluminum tubing, 1/4" ID	$0.75
A connector to match your radio pigtail (we used a female N connector)	$3.00
1 1/2" piece of 12-gauge solid copper wire (we used ground wire from house electrical wiring)	negligible
A tall Pringles can (any flavor, Ridges are optional)

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Redundant Links

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