Wireless Networking in the Developing World

Before packets can be forwarded and routed to the Internet, layers one (the physical) and two (the data link) need to be connected. Without link local connectivity, network nodes cannot talk to each other and route packets.

To provide physical connectivity, wireless network devices must operate in the same part of the radio spectrum. As we saw in chapter two, this means that 802.11a radios will talk to 802.11a radios at around 5GHz, and 802.11b/g radios will talk to other 802.11b/g radios at around 2.4GHz. But an 802.11a device cannot interoperate with an 802.11b/g device, since they use completely different parts of the electromagnetic spectrum.

More specifically, wireless cards must agree on a common channel. If one 802.11b radio card is set to channel 2 while another is set to channel 11, then the radios cannot communicate with each other.

When two wireless cards are configured to use the same protocol on the same radio channel, then they are ready to negotiate data link layer connectivity. Each 802.11a/b/g device can operate in one of four possible modes:

1. Master mode (also called AP or infrastructure mode) is used to create a service that looks like a traditional access point. The wireless card creates a network with a specified name (called the SSID) and channel, and offers network services on it. While in master mode, wireless cards manage all communications related to the network (authenticating wireless clients, handling channel contention, repeating packets, etc.) Wireless cards in master mode can only communicate with cards that are associated with it in managed mode.

2. Managed mode is sometimes also referred to as client mode. Wireless cards in managed mode will join a network created by a master, and will automatically change their channel to match it. They then present any necessary credentials to the master, and if those credentials are accepted, they are said to be associated with the master. Managed mode cards do not communicate with each other directly, and will only communicate with an associated master.

3. Ad-hoc mode creates a multipoint-to-multipoint network where there is no single master node or AP. In ad-hoc mode, each wireless card communicates directly with its neighbors. Nodes must be in range of each other to communicate, and must agree on a network name and channel.

4. Monitor mode is used by some tools (such as Kismet, chapter six) to passively listen to all radio traffic on a given channel. When in monitor mode, wireless cards transmit no data. This is useful for analyzing problems on a wireless link or observing spectrum usage in the local area. Monitor mode is not used for normal communications.

When implementing a point-to-point or point-to-multipoint link, one radio will typically operate in master mode, while the other(s) operate in managed mode. In a multipoint-to-multipoint mesh, the radios all operate in ad-hoc mode so that they can communicate with each other directly.

It is important to keep these modes in mind when designing your network layout. Remember that managed mode clients cannot communicate with each other directly, so it is likely that you will want to run a high repeater site in master or ad-hoc mode. As we will see later in this chapter, ad-hoc is more flexible but has a number of performance issues as compared to using the master / managed modes.

Now that your wireless cards are providing physical and data link connectivity, they are ready to start passing around packets on layer 3: the internetworking layer.

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Data networks are often described as being built on many layers. Each layer depends on the operation of all of the underlying layers before communication can take place, but only needs to exchange data with the layer above or beneath it. The TCP/IP model of networking describes five layers, as shown in this diagram:

The previous section on network layouts described layer one: the physical layer. This is the physical medium over which communications take place. This can be a copper CAT5 cable, a fiber optic bundle, radio waves, or just about any other medium.

The next layer up is referred to as the data link layer. Whenever two or more nodes share the same physical medium (for example, several computers plugged into a hub, or a room full of laptops all using the same radio channel) they use the data link layer to determine whose turn it is to transmit on the medium. Common examples of data link protocols are Ethernet, Token Ring, ATM, and the wireless networking protocols (802.11a/b/g). Communication on this layer is said to be link local, since all nodes connected at this layer can communicate with each other directly. On networks modeled after Ethernet, nodes are referred to by their MAC address, which is a unique 48 bit number assigned to every networking device when it is manufactured.

Just above the data link layer is the Internet layer. For TCP/IP, this is the Internet Protocol (IP). At the Internet layer, packets can leave the link local network and be retransmitted on other networks. Routers perform this function on a network by having at least two network interfaces, one on each of the networks to be interconnected. Nodes on the Internet are reached by their globally unique IP address.

Once Internet routing is possible, a method is needed to reach a particular service at a given IP address. This function is filled by the next layer, the transport layer. TCP and UDP are common examples of transport layer protocols. Some protocols at the transport layer (such as TCP) ensure that all of the data has arrived at the destination, and is reassembled and delivered to the next layer in the proper order.

Finally, at the top of the pile we have the application layer. This is the layer that most network users are exposed to, and is the level at which human communication happens. HTTP, FTP, and SMTP are all application layer protocols. The human sits at the top of all of the layers, and needs little or no knowledge of the layers beneath to effectively use the network.

One way to look at the TCP/IP model is to think of a person delivering a letter to an office building downtown. They first need to interact with the road itself (the physical layer), pay attention to other traffic on the road (the data link layer), turn at the proper place to connect to other roads and arrive at the correct address (the Internet layer), go to the proper floor and room number (the transport layer), and finally find the recipient or a receptionist who can take the letter from there (the application layer). The five layers can be easily remembered by using the mnemonic “Please Don’t Look In The Attic,” which of course stands for “Physical / Data Link / Internet / Transport / Application.”

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Communication is only possible when the participants speak a common language. But once the communication becomes more complex than a simple ongoing broadcast, protocol becomes just as important as language. All of the people in an auditorium may speak English, but without a set of rules in place to establish who has the right to use the microphone, the communication of an individual’s ideas to the entire room is nearly impossible. Now imagine an auditorium as big as the world, full of all of the computers that exist. Without a common set of communication protocols to regulate when and how each computer can speak, the Internet would be a chaotic mess where every machine tries to speak at once.

TCP/IP refers to the suite of protocols that permit conversations to happen on the global Internet. By understanding TCP/IP, you can build networks that will scale to virtually any size, and will ultimately become part of the global Internet.

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All of these network designs can be used to complement each other in a large network, and can obviously make use of traditional wired networking techniques whenever possible. It is a common practice, for example, to use a long distance wireless link to provide Internet access to a remote location, and then set up an access point on the remote side to provide local access. One of the clients to this access point may also act as a mesh node, allowing the network to spread organically between laptop users who all ultimately use the original point-to-point link to access the Internet.

Now that we have a clear idea of the way that wireless networks are typically arranged, we can begin to understand how communication is possible over such networks.

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The third type of network layout is multipoint-to-multipoint, which is also referred to as an ad-hoc or mesh network. In a multipoint-to-multipoint network, there is no central authority. Every node on the network carries the traffic of every other as needed, and all nodes communicate with each other directly.

The benefit of this network layout is that even if none of the nodes are in range of a central access point, they can still communicate with each other. Good mesh network implementations are self-healing, in that they automatically detect routing problems and fix them as needed. Extending a mesh network is as simple as adding more nodes. If one of the nodes in the “cloud” happens to be an Internet gateway, then that connection can be shared among all of the clients.

Two big disadvantages to this topology are increased complexity and lower performance. Security in such a network is also a concern, since every participant potentially carries the traffic of every other. Multipoint-to-multipoint networks tend to be complicated to troubleshoot, due to the large number of changing variables as nodes move around. Multipoint-to-multipoint clouds typically do not have the same capacity as point-to-point or point-to-multipoint networks, due to the additional overhead of managing the network routing and increased contention in the radio spectrum.

Nevertheless, mesh networks are useful in many circumstances. We will see an example of how to build a multipoint-to-multipoint mesh network using a routing protocol called OLSR at the end of this chapter.

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The next most commonly encountered network layout is point-to-multipoint. Whenever several nodes are talking to a central point of access, this is a point-to-multipoint application. The typical example of a point-to-multipoint layout is the use of a wireless access point that provides a connection to several laptops. The laptops do not communicate with each other directly, but must be in range of the access point in order to use the network.

Point-to-multipoint networking can also apply to our earlier example at the university. Suppose the remote building on top of the hill is connected to the central campus with a point-to-point link. Rather than setting up several point-to-point links to distribute the Internet connection, a single antenna could be used that is visible from several remote buildings. This is a classic example of a wide area point (remote site on the hill) to multipoint (many buildings in the valley below) connection.

Note that there are a number of performance issues with using point-to-multipoint over very long distance, which will be addressed later in this chapter. Such links are possible and useful in many circumstances, but don’t make the classic mistake of installing a single high powered radio tower in the middle of town and expecting to be able to serve thousands of clients, as you would with an FM radio station. As we will see, data networks behave very differently than broadcast radio.

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Point-to-point links typically provide an Internet connection where such access isn’t otherwise available. One side of a point-to-point link will have an Internet connection, while the other uses the link to reach the Internet. For example, a university may have a fast frame relay or VSAT connection in the middle of campus, but cannot afford such a connection for an important building just off campus. If the main building has an unobstructed view of the remote site, a point-to-point connection can be used to link the two together. This can augment or even replace existing dial-up links. With proper antennas and clear line of sight, reliable point-to-point links in excess of thirty kilometers are possible.

Of course, once a single point-to-point connection has been made, more can be used to extend the network even further. If the remote building in our example is at the top of a tall hill, it may be able to see other important locations that can’t be seen directly from the central campus. By installing another point-to-point link at the remote site, another node can join the network and make use of the central Internet connection.

Point-to-point links don’t necessarily have to involve Internet access. Suppose you have to physically drive to a remote weather monitoring station, high in the hills, in order to collect the data which it records over time. You could connect the site with a point-to-point link, allowing data collection and monitoring to happen in realtime, without the need to actually travel to the site. Wireless networks can provide enough bandwidth to carry large amounts of data (including audio and video) between any two points that have a connection to each other, even if there is no direct connection to the Internet.

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It may seem odd to talk about the “physical” network when building wireless networks. After all, where is the physical part of the network? In wireless networks, the physical medium we use for communication is obviously electromagnetic energy. But in the context of this chapter, the physical network refers to the mundane topic of where to put things. How do you arrange the equipment so that you can reach your wireless clients? Whether they fill an office building or stretch across many miles, wireless networks are naturally arranged in these three logical configurations:

• Point-to-point links
• Point-to-multipoint links
• Multipoint-to-multipoint clouds

The physical network layout you choose will depend on the nature of the problem you are trying to solve. While different parts of your network can take advantage of all three of these configurations, any individual link will fall into one of the above topologies. The application of each of these topologies is best described by example.

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Before purchasing equipment or deciding on a hardware platform, you should have a clear idea of the nature of your communications problem. Most likely, you are reading this book because you need to connect computer networks together in order to share resources and ultimately reach the larger global Internet. The network design you choose to implement should fit the communications problem you are trying to solve. Do you need to connect a remote site to an Internet connection in the center of your campus? Will your network likely grow to include several remote sites? Will most of your network components be installed in fixed locations, or will your network expand to include hundreds of roaming laptops and other devices?

When solving a complex problem, it is often useful to draw a picture of your resources and problems. In this chapter, we will look at how other people have built wireless networks to solve their communication problems, including diagrams of the essential network structure. We will then cover the networking concepts that define TCP/IP, the primary networking language currently spoken on the Internet. We will then demonstrate several common methods for getting your information to flow efficiently through your network and on to the rest of the world.

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Don’t worry if the concepts in this chapter seem challenging. Understanding how radio waves propagate and interact with the environment is a complex field of study in itself. Most people find it difficult to understand phenomenon that they can’t even see with their own eyes. By now you should understand that radio waves don’t travel in a straight, predictable path. To make reliable communication networks, you will need to be able to calculate how much power is needed to cross a given distance, and predict how the waves will travel along the way.

There is much more to learn about radio physics than we have room for here. For more information about this evolving field, see the resources list in Appendix A. Now that you have an idea of how to predict how radio waves will interact in the real world, you are ready to start using them for communications.

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