Designing the physical network
It may seem odd to talk about the “physical” network when building wireless
networks. In wireless networks, the physical medium we use for communication is obviously electromagnetic energy. But the physical network refers to the mundane topic of where to put things.
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.
Point-to-point
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. 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.
Point-to-multipoint
The next most commonly encountered network layout is point-tomultipoint.
Whenever several nodes are talking to a central point of access, this is a point-to-multipoint application. The typical example of a pointto- 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
*A node is any device capable of sending and receiving data on a network. Access points, routers, computers, and laptops are all examples of nodes.*
could be used that is visible from several remote buildings.
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.
Multipoint-to-multipoint
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 participantpotentially 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-tomultipoint 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.
networks. In wireless networks, the physical medium we use for communication is obviously electromagnetic energy. But the physical network refers to the mundane topic of where to put things.
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.
Point-to-point
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. 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.
Point-to-multipoint
The next most commonly encountered network layout is point-tomultipoint.
Whenever several nodes are talking to a central point of access, this is a point-to-multipoint application. The typical example of a pointto- 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
*A node is any device capable of sending and receiving data on a network. Access points, routers, computers, and laptops are all examples of nodes.*
could be used that is visible from several remote buildings.
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.
Multipoint-to-multipoint
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 participantpotentially 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-tomultipoint 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.
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