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. 

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. 

Figure 3-2. In IBSS mode, nodes can talk to any other node in range. A node with another network connection can provide gateway services

Note that an 802.11b radio must be set to work in either of these modes but cannot work in both simultaneously. Both modes support shared-key WEP encryption (more on that later). 

3.1.1 Access Point Hardware

Access points    are widely considered ideal for campus coverage. They provide a single point of entry that can be configured by a central authority. They typically allow for one or two radios per AP, theoretically supporting hundreds of simultaneous wireless users at a time. They must be configured with an ESSID (Extended Service Set ID, also known as the Network Name  or  WLAN Service Area ID , depending on who you talk to); it's a simple string that identifies the wireless network. Many use a client program for configuration and a simple password to protect their network settings. 

Most APs also provide enhanced features, such as the following:

 MAC address filtering. A client radio attempting access must have its MAC address  • listed on an internal table before being permitted to associate with the AP.   Closed networks. Usually, a client can specify an ESSID of " ANY " to associate with  • any available network. In a closed network, the client must specify the ESSID explicitly, or it can't associate with the AP.   External antennas.   •  Continual link-quality monitoring.  •  Extended logging, statistics, and performance reporting.  •

Other enhanced modes include dynamic WEP key management, public encryption key exchange, channel bonding, and other fun toys. Unfortunately, these extended modes are entirely manufacturer- (and model-) specific, are not covered by any established standard, and do not interoperate with other manufacturer's equipment. It should also be noted that, once a client has associated itself with an AP, there are no further restrictions imposed by the AP on what services the client can access. 

APs are an ideal choice for private networks with many wireless clients that exist in a confined physical space, especially on the same physical subnet (like a business or college campus). They provide a high degree of control over who can access the wire, but they are not cheap (the average AP at this writing costs between $800 and $1000). 

Another class of access point is occasionally referred to as a  residential gateway.  The Apple AirPort, Orinoco RG-1000, and Linksys WAP11 are popular examples of low-end APs. They are typically much less expensive than their commercial counterparts, costing between $200 and $500. Many have built-in modems, allowing for wireless-to-dialup access (which can be very handy, if Ethernet access isn't available wherever you happen to be). Most even provide Network Address Translation (NAT), DHCP, and bridging services for wireless clients. While they may not support as many simultaneous clients as a high-end AP, they can provide cheap, simple access for many applications. By configuring an inexpensive AP for bridged Ethernet mode, you can have a high degree of control over what individual clients can access on the wired network (see  Section 7.6  in  Chapter 7 ). 

Despite their high cost, APs have their place in building community wireless networks. They are especially well suited to remote repeater locations, due to their ease of configuration, low power consumption (compared to a desktop or laptop PC), and lack of moving parts. We'll go into detail on how to set up an AP in  Chapter 4 . 

3.1.2

If the goal of your wireless project is to provide public access to network services, the functionality high-end APs provide will almost certainly be overkill, particularly in light of their high cost. Luckily, with IBSS mode, AP hardware is entirely optional. 

Radios that are operating in IBSS mode can communicate with each other if they have the same ESSID and WEP settings. As stated earlier, a computer with an 802.11b card and another network connection (usually Ethernet or dialup) can serve as a gateway between the two networks. Add in DHCP and NAT services, and you effectively have a full-blown Internet gateway. As various free operating systems can provide these services and will run well on hardware that many people already have lying around in closets (e.g., 486 laptops and low-end Pentium systems), this mode of operation is an increasingly popular alternative to expensive APs. If you have host hardware available already, the low cost of making a gateway is very attractive (the cost of the average client radio card is $120, or about half that of a low-end AP). 

What is missing from a do-it-yourself gateway? Instead of the myriad access control methods that actual APs provide, the only out-of-the-box access control you have available is WEP. As we saw earlier, a shared key does little on its own for security, and it isn't appropriate in a public network setting anyway. So how can we provide network access and still discourage abuse by anonymous wireless clients? See  Chapter 5  and  Chapter 7 . 

In  Chapter 5 , we'll build a Linux-based wireless gateway from scratch. In  Chapter 7 , we'll examine one method of extending the gateway to provide different classes of service, depending on who connects to it.

3.2

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. 

3.2.1

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

In a wireless environment, DHCP is an absolute necessity. There isn't much point in being able to wander around without a cable if you need to manually set the network parameters for whatever network you happen to be in range of. It's much more convenient to let the computers work it out on their own (and let you get back to more important things, like IRC or "Quake III Arena"). Since DHCP lets a node discover information about its network, one can get "online" without any prior knowledge about that particular network's layout. This service demonstrates a condition that network administrators have known for years: users just want to get online without knowing (or even caring) about the underlying network. From their perspective, it should just work. DHCP makes this kind of magic possible. 

From a network admin's perspective, the magic isn't even terribly difficult to bring about. As long as you have exactly one DHCP server running on your network segment, your clients can all pull from a pool of available IP addresses. The DHCP server manages the pool on its own, reclaiming addresses that are no longer in use and reassigning them to new clients. 

In many cases, a wired network's existing DHCP server serves wireless users with no trouble. It sees the wireless node's DHCP request just as it would any other and responds accordingly. If your wired network isn't already providing DHCP, or if your wireless gateway isn't capable of L2 bridging, don't worry. We'll cover setting up the ISC's  dhcpd  server in Linux in  Chapter 5 . 

3.2.2 DNS

My, how different the online world would be if we talked about sending mail to This e-mail address is being protected from spambots. You need JavaScript enabled to view it   or got excited about having just been  64.28.67.150 'd. DNS is the dynamic telephone directory of the Internet, mapping human friendly names (like oreillynet.com  or  slashdot.org ) to computer friendly numbers (like the dotted quads above). The Internet without DNS is about as much fun and convenient as referring to people by their Social Security numbers. 

Much like DHCP, your network's existing DNS servers should be more than adequate to provide name resolution services to your wireless clients. However, depending on your particular wireless application, you may want to get creative with providing additional DNS services. A caching DNS server might be appropriate, to reduce the load on your primary

DNS servers (especially if you have a large number of wireless clients). You might even want to run separate DNS for your wireless hosts, so that wireless nodes can easily provide services for each other. 

3.2.3 NAT

In order for any machine to be reachable via the Internet, it must be possible to route traffic to it. A central authority, the IANA (Internet Assigned Numbers Authority, http://www.iana.org/ ), holds the keys to the Internet. This international body controls how IP addresses are parceled out to the various parts of the world, in an effort to keep every part of the Internet (theoretically) reachable from every other and to prevent the accidental reuse of IP addresses in different parts of the world. Unfortunately, due to the unexpectedly tremendous popularity of the Net, what was thought to be plenty of address space at design time has proven to be woefully inadequate in the real world. With thousands of new users coming online for the first time every day, the general consensus is that there simply aren't enough IP addresses to go around anymore. Most ISPs are increasingly paranoid about the shortage of homesteading space, and they are loath to give out more than one per customer (and, in many cases, they won't even do that anymore, thanks to the wonders of DHCP). 

Now we see the inevitable problem: suppose you have a single IP address allocated to you by your ISP, but you want to allow Internet access to a bunch of machines, including your wireless nodes. You certainly don't want to pay exorbitant fees for more address space just to let your nephew get online when he brings his wireless laptop over once a month. 

This is where NAT can help you. Truly a mixed blessing, NAT (referred to in some circles as "masquerading") provides a two-way forwarding service between the Internet and another network of computers. A computer providing NAT typically has two network interfaces. One interface is connected to the Internet (where it uses a real live IP address), and the other is attached to an internal network. Machines on the internal network use any of IANA's thoughtfully assigned, reserved IP addresses and route all of their outgoing traffic through the NAT box. When the NAT box receives a packet bound for the Internet, it makes a note of where the packet came from. It then rewrites the packet using its "real" IP address and sends the modified packet out to your ISP (where it winds its way through the rest of the Internet, hopefully arriving at the requested destination). When the response (if any) comes back, the NAT box looks up who made the original request, rewrites the inbound packet, and returns it to the original sender. As far as the rest of the Net is concerned, only the NAT machine is visible. And as far as the internal clients can tell, they're directly connected to the Internet. Figure 3-4  shows a model of a NAT configuration. 

Figure 3-4. Using NAT, several computers can share a single "real" IP address

The IANA has reserved the following sets of IP addresses for private use (as outlined in RFC 1918,  http://rfc.net/rfc1918.html ): 

10.0.0.0 - 10.255.255.255 172.16.0.0 - 172.31.255.255 192.168.0.0 - 192.168.255.255

These are addresses that are guaranteed never to be used on the Internet. As long as your internal machines use IP addresses in any of these three ranges, your traffic will not interfere with any other host on the Net. As an added bonus, since the reserved IP address traffic isn't even routed over the Internet, you effectively get a free firewall for all of your NAT'd hosts. 

NAT is handy but isn't without its drawbacks. For example, some services may not work properly with some implementations of NAT. Most notably, active FTP sessions fail on some NAT boxes. Another big disadvantage to NAT is that it effectively makes the Internet a read-only medium, much like television. If you can have only outbound traffic (to web servers, for example) and traffic from the Internet can't reach your machine directly, then you have no way of serving data and contributing back to the Net! This doesn't prevent you from using two-way services like IRC and email, but it does preclude you from easily running services where Internet users connect to you directly (for example, running your own web server from behind a NAT isn't trivial). 

Despite these drawbacks, NAT is an invaluable tool for allowing throngs of people to access Internet resources. In  Chapter 5 , we'll build a Linux gateway that will do NAT for you and handle almost every popular form of Internet traffic you care to throw at it (including active FTP). 

Of course, if you're lucky enough to have a ton of live IP address space, feel free to flaunt it and assign live IPs to your wireless clients! Naturally, most people (and, indeed, their laptops) are unprepared for the unbridled adrenaline rush of using a live IP address without a firewall.

But if you have that many real IPs to throw around, you must be used to living large. Just don't worry when you find your clients spontaneously rebooting or suddenly serving 0-dAy W@r3z. It's all part of the beautiful online experience. 

3.3 Security Considerations

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.   [ 1]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? 

As the story goes, a major computer hardware manufacturer once found a new "employee" sitting in a previously unoccupied cube. He had evidently been there for three weeks, plugged into the corporate network and happily logging data before HR got around to asking who he was.  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. 

Let's take a look at the tools we have available to put controls on who can access what.

3.3.1 WEP 

The 802.11b specification outlines a form of encryption called  wired equivalency privacy , or WEP.  By encrypting packets at the MAC layer, only clients who know the "secret key" can associate with an access point or peer-to- peer group. Anyone without the key may be able to see network traffic, but every packet is encrypted. 

The specification employs a 40-bit, shared-key RC4 PRNG   algorithm from RSA Data [ 2]Security. Most cards that talk 802.11b (Agere Orinoco, Cisco Aironet, and Linksys WPC11, to name a few) support this encryption standard. 

Pseudo-Random Number Generator. It could be worse, but entropy takes time. Although hardware encryption sounds like a good idea, the implementation in 802.11b is far from perfect. First of all, the encryption happens at the link layer, not at the application layer. This means your communications are protected up to the gateway, but no further. Once it hits the wire, your packets are sent in the clear. Worse than that, every other legitimate wireless client who has the key can read your packets with impunity, since the key is shared across all clients. You can try it yourself; simply run  tcpdump  on your laptop and watch your neighbor's packets just fly by, even with WEP enabled. 

Some manufacturers (e.g., Agere and Cisco) have implemented their own proprietary extensions to WEP, including 128-bit keys and dynamic key management. Unfortunately, because they are not defined by the 802.11b standard, there is no guarantee that cards from different manufacturers that use these extensions will interoperate (and, generally speaking, they don't). 

To throw more kerosene on the burning WEP tire mound, a team of cryptographers at the University of California at Berkeley have identified weaknesses in the way WEP is implemented, effectively making the strength of encryption irrelevant. With all of these problems, why is WEP still supported by manufacturers? And what good is it for building public access networks? 

WEP was not designed to be the ultimate "killer" security tool (nor can anything seriously claim to be). Its acronym makes the intention clear: wired equivalency privacy. In other words, the aim behind WEP was to provide no greater protection than you would have when you physically plug into your Ethernet network. (Keep in mind that in a wired Ethernet setting, there is no encryption provided by the protocol at all. That is what application layer security is for; see the tunneling discussion later in this chapter.) 

What WEP does provide is an easy, generally effective, interoperable deterrent to unauthorized access. While it is technically feasible for a determined intruder to gain access, it is not only beyond the ability of most users, but usually not worth the time and effort, particularly if you are already giving away public network access! 

As we'll see in  Chapter 7 , one area where WEP is particularly useful is at either end of a long point-to-point backbone link. In this application, unwanted clients could potentially degrade network performance for a large group of people, and WEP can help not only discourage would-be link thieves, but also encourage them to set up more public access gateways. 

3.3.2 Routing and Firewalling

The primary security consideration for wireless network access is where to fit it into your existing network. Regardless of your gateway method (AP or DIY) you need to consider what services you want your wireless users to be able to access. Since the primary goal of this book is to describe methods for providing public access to network services (including access to the Internet), I strongly recommend setting up your wireless gateways in the same place you

would any public resource: in your network's DMZ or outside your firewall altogether. That way, even in a complete breakdown of security precautions, the worst that any social deviant will end up with is Internet access, and not unrestricted access to your private internal network. 

This configuration, as shown in  Figure 3-5 , leaves virtually no incentive for anyone to try to compromise your gateway, as the only thing to be gained would be greater Internet access. Attacks coming from the wireless interface can easily log MAC address and signal strength information. In IBSS mode, this is an even greater deterrent. As the would-be attacker needs to transmit to carry out an attack, they give away not only a unique identifier (their MAC address), but also their physical location! 

Figure 3-5. Place your wireless gateways outside of your private network!

Assuming that all wireless connectivity takes place outside of your private network, what happens when you or your friends want to connect from the wireless back to the inside network? Won't other wireless users be able to just monitor your traffic and grab passwords and other sensitive information?  Section 3.3.3  addresses this potential problem. 

3.3.3 Encrypted Tunnels

Application layer encryption is a critical technology when dealing with untrusted networks (like public-access wireless links, for example). When using an encrypting tunnel, you can secure your communications from eavesdroppers all the way to the other end of the tunnel. 

If you're using a tunnel from your laptop to another server, would-be black hats listening to your conversation will have the insurmountable task of cracking strong cryptography. Until someone finds a cheap way to build a quantum computer (and perhaps a cold fusion cell to power it), this activity is generally considered a waste of time. In  Figure 3-6 , a web server providing 128-bit SSL connections provides plenty of protection, all the way to your wireless laptop. SSL provides application layer encryption. 

Figure 3-6. WEP only encrypts to the gateway, exposing your traffic to other wireless users and anything after the wire. Tunnels protect your traffic from end to end

SSL is great for securing web traffic, but what about other network services? Take this typical scenario: You're at work or at home, merrily typing away on your wireless laptop. You want to retrieve your email from a mail server with a POP client (Netscape Mail, Eudora, fetchmail, etc.). If you connect to the machine directly, your email client sends your login and password "in the clear." This means that a nefarious individual somewhere between you and your mail server (either elsewhere on your wireless network, or even "on the wire" if you are separated by another network) could be listening and could grab a copy of your information en route. This login could then not only be used to gain unauthorized access to your email, but in many cases also to grant a shell account on your mail server! 

To prevent this, you can use the tunneling capabilities of SSH. An SSH tunnel works like this: rather than connecting to the mail server directly, we first establish an SSH connection to the internal network that the mail server lives in (in this case, the wireless gateway). Your SSH client software sets up a port-forwarding mechanism, so that traffic that goes to your laptop's POP port magically gets forwarded over the encrypted tunnel and ends up at the mail server's POP port. You then point your email client to your local POP port, and it thinks it is talking to the remote end (only this time, the entire session is encrypted).  Figure 3-7  shows a model of an SSH tunnel in a wireless network. 

Figure 3-7. With an SSH tunnel in place, your otherwise insecure conversation stays private

With the tunnel in place, anyone who tries to monitor the conversation between your laptop and the mail server gets something resembling line noise. It's a good idea to get in the habit of tunneling anything that you want to keep private, even over wired networks. SSH tunneling doesn't have to stop at POP connections either. Any TCP port (SMTP, for example) can easily be set up to tunnel to another machine running SSH, almost anywhere on the Internet. We'll see an example of how to do that in  Chapter 7 . For a full discussion of the ins and outs of this very flexible (and freely available) tool, I highly recommend O'Reilly's  SSH: The Definitive Guide , by Daniel J. Barrett and Richard E. Silverman. 

3.4 Summary

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. 

You can use these modes of operation in conjunction with each other (and with other wired networking techniques) to extend your network as you need it. It is very common, for example, to use a long distance wireless link to provide access to a remote location, and then set up an access point at that end to provide local access. 

2.1 Hardware Requirements

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. 

2.1.1 Site Survey

The most efficient wireless network consists of a single client talking to a single access point a few feet away with absolutely clear line of sight between them and no other noise on the channel being used (either from other networks or from equipment that shares the 2.4GHz spectrum). Of course, with the possible exception of the home wireless LAN, these ideal conditions simply aren't feasible. All of your users will need to "share the airwaves," and more than likely they won't be able to see the access point from where they are located. Fortunately, 802.11b gear is very tolerant of less than optimal conditions at close range. When planning your network, be sure to look out for the following: 

 Objects that absorb microwave signals, such as trees, earth, brick, plaster walls, and  • people 

 Objects that reflect or diffuse signals, such as metal, fences, mylar, pipes, screens, and  • bodies of water   Sources of 2.4GHz noise, such as microwave ovens, cordless phones, wireless X-10  • automation equipment, and other 802.11b networks 

The more you can eliminate from the path between your access points and your clients, the happier you'll be. You won't be able to get rid of all of the previous obstacles, but you should be able to minimize their impact by working around them.  

2.2 Hot Spots

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 2.442 8 2.447 9 2.452 10 2.457 11 2.462

The channels are spread spectrum and actually use 22MHz of signal bandwidth, so adjacent radios will need to be separated by at least five channels to see zero overlap. For example, channels 1, 6, and 11 have no overlap. Neither do 2 and 7, 3 and 8, 4 and 9, or 5 and 10. While you will ideally want to use non-overlapping channels for your access points, in a crowded setting (such as a city apartment building or office park) this is becoming less of an option. 

You stand a better chance at saturating your area with usable signals from many low-power cells rather than a single tower with a high-gain antenna. As your individual cells won't need a tremendous range to cover a wide area, you can use lower power (and lower cost) antennas, further limiting the chances of interfering with other gear in the band. For example, you could use as few as three channels (such as 1, 6, and 11) to cover an infinitely large area, with no channel overlap whatsoever. 

The worst possible case would involve two separate busy networks trying to occupy the same channel, right next to each other. The further you can get away from this nightmare of collisions, the better. Realistically, a single channel can easily support fifty or more simultaneous users, and a fair amount of channel overlap is tolerable. The radios use the air only when they actually have something to transmit, and they retransmit automatically on error, so heavy congestion feels more or less like ordinary net lag to the end user. The sporadic nature of most network traffic helps to share the air and avoid collisions, like playing cards shuffling together into a pack. 

You may have total control over your own access points, but what about your neighbors? How can you tell what channels are in use in your local area?  

2.3 Potential Coverage Problem Areas

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.  

2.4 Topographical Mapping 101

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. 

Interestingly enough, MapQuest ( http://www.mapquest.com/ ) has recently started providing color aerial photos (in addition to their regular street maps) from GlobeXplorer ( http://www.globexplorer.com/ ). While there's little indication as to how recent their data is, it may be a good place to get a quick (and free) aerial overview of your local area. 

You can buy paper maps from most camping supply stores or browse them online for free at http://www.topozone.com/ . If you're interested in DOQs, go to the USGS directly at http://earthexplorer.usgs.gov/ . We'll take a look at some nifty things you can do with topo maps on CDRom and your GPS in  Chapter 6 .

continue to interoperate with older 802.11 DSSS hardware. The technology was intended to provide "campus" access to network services, offering typical usable ranges of about 1500 feet.

It didn't take long for some sharp hacker types (and, indeed, a few CEO and FCC types) to realize that by using 802.11b client gear in conjunction with standard radio equipment, effective range can extend to more than twenty miles and potentially provide thousands of people with bandwidth reaching DSL speeds, for minimal hardware cost. Connectivity that previously had to creep up monopoly-held wires can now fly in through the walls with significantly higher performance. And since 802.11b uses unlicensed radio spectrum, full- time connections can be set up without paying a dime in airtime or licensing fees.

While trumping the telco and cable companies with off-the-shelf magical hardware may be an entertaining fantasy, how well does 802.11b equipment actually perform in the real world? How can it be applied effectively to provide access to the Internet?

1.1 The Problem

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?

1.2 How ISPs Are Attempting a Solution

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 [ 2 ] 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.

Wireless Internet Service Providers. No, I didn't make that one up. 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.

Suppose that marketing has sufficiently duped would-be customers and claims to have enough tower sites to make network services at least a possibility. Now imagine that a prospective customer actually calls, asking for service. How does the WISP know if service is possible? With DSL, it's straightforward: look up the customer's phone number in the central database, figure out about how far they are from the CO, and give them an estimate. Unfortunately, no known database can tell you for certain what a given address has line of sight to.

As we'll see later, topographical software can perform some preliminary work to help rule out at least the definite impossibilities. Some topographical packages even include tree and ground clutter data. At this point, we might even be able to upgrade the potential customer to a "maybe." Ultimately, however, the only way to know if a particular customer can reach the WISP's backbone over wireless is to send out a tech with test gear, and try it.

So now the poor WISP needs an army of technically capable people with vans, on call for new installations, who then need to make on-site calls to people who aren't even customers yet. And if they're lucky, they might even get a test shot to work, at which point equipment can finally be installed, contracts signed, and the customer can get online at something almost resembling DSL. That is, the customer can be online until a bird perches on the antenna, or a new building goes up in the link path, or the leaves come out in the spring and block most of the signal (at which point, I imagine the customer would be referred to the fine print on that contract).

I think you can begin to see exactly where the bottom line is in this sort of arrangement. It's certainly not anyone's fault, but this solution just isn't suited to the problem, because the only entity with enough resources to seriously attempt it would likely be the phone company. What hope does our "wireless everywhere" vision have in light of all of the previously mentioned problems? Perhaps a massively parallel approach would help....

1.3 How Cooperatives Are Making It Happen

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 [ 3] 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.

GAWD, the Global Access Wireless Database, lists 198 public wireless access points at the time of this writing. Check out http://www.shmoo.com/gawd/ to add your own or search for one. 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.

It is worth pointing out here that ISPs and telcos are in no way threatened by this technology; in fact, Internet service will be in even greater demand as wireless cooperatives come online. The difference is that many end users will have access without the need to tear down trees and dig up streets, and many others may find that network access in popular areas will be provided gratis, as a community service or on a cooperative trust basis, rather than as a corporate commodity.

1.4 About This Book

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.