after recent discussions with some colleagues RedLibre
, I decided to discuss some technical aspects of wireless networks, aimed primarily at people outside the little world of telecommunications. So this is a text "superficial" and try to explain technical concepts more understandable possible. This text is mainly focused on clarifying issues related to performance / speed wireless networks, especially interesting for those who are interested in participating in community networking projects, such as the already appointed RedLibre 
or
guifi. net. or necessary technical concepts of computer networks (both wireless and wired) Duplex: is the ability of a computer network to send and receive information in both directions, using the same means of information transport. The opposite of this is called simplex. An example is the telephone duplex as using the same apparatus and the same line we can talk in both directions. An example of simplex is television, where there is a sender and a receiver, but it is not possible to exchange the roles of both parties.
Full Duplex:
is the ability of a computer network to send and receive information
in both directions simultaneously. An example is a road where vehicles can pass (information) in both directions, without interference between them.
Half Duplex: is the ability of a computer network to send and receive information in both directions, but not at the same time. First transmits a party, then the other party. The example Most obviously, a walkie-talkie, where you can talk to another device, but need to press a button to talk and release it to listen. Middle share: when there is a shared medium, all parties involved in the communication using the same channel, and therefore, have to wait their turn to communicate. An example of shared media is a phone line, where only you can make a call each time, and if they call both at the same time, one gets the busy signal. An example of unshared
half would be a telephone, where you can receive multiple calls simultaneously, and each has its own channel of communication independent.
Channel: is a concept borrowed from the radio. Each channel is the frequency emitted by a wireless access point. For two computers to communicate by wireless, it is necessary inter alia, that the two tune in the same channel. Spain has approved thirteen channels, slightly overlapping each other. If two nearby wifi stations broadcast in the same channel will interfere with each other, lowering the yield. Symmetry: the relationship between the speed of dispatch and receipt of a computer network. If speed is the same, the network is symmetric, if different, asymmetrical. An example is not the same down stairs to upload, so the stairs are asymmetric, it will always cost more time to upload them down.
Access Point: simplified, is the device that "translates" the wireless signal to the signal cable.
Node: in the context of community wireless networks, is the computer system that controls network access and interconnection to the rest of the network.
With these concepts clarified, we can begin to explain why the performance of different cable networks with and without it. A typical local cable network meets the following requirements: full duplex, half not shared, and the channel does not apply, because it works cable, and each computer has its own cable exclusively. Symmetrical.
A typical wireless network meets the following requirements: half duplex shared medium, and 13 partially overlapping channels, which leads to interference. Symmetrical. An old coaxial network, to an intermediate example: half duplex shared medium, without channel, with a single cable shared by all computers, but without outside interference. Symmetrical.
RONJA
(wireless optical): full duplex, half not shared, not channels, but susceptible to weather interference. Symmetrical.
The typical ADSL: full duplex, half not shared, without channels. Asymmetric
. Only
already said it is clear that a wireless network will always have worse performance than a cable. Obviously it compensates with other qualities, but when plants are part of a wireless community must take into account the network performance can be lower than a modern Internet connection (then I will clarify this). Suppose
a wireless network from 11 megabits, which although already surpassed by 54, still widely used. Moreover, in case of interference, a network of 54 low performance, and is not very surprising that operate at 11. Also in case of very long distance links rarely move from the 11 megabits. If only one client connected to the access point, has all the available bandwidth, which in the case of a network of 11 tend to be about 700 kilobytes per second. But to be shared medium are 700 KB / s total, so if the only client is sending and receiving at the same time, then send 350 and receive a 350 (slightly less in reality). If two clients are receiving at the same time, then you will have 350 KB / s for each, but if for example one of them sends and receives at the same time, the situation will be more like 230 KB / s reception for everyone, and 230 KB / s of transmission for the sending client. If customers share information between them, will not be much better, because clients do not communicate directly with each other, but send the information to the access point, and this repeated for B to receive it, which is divided half speed. If for example A and B are downloading information, and also switching between them simultaneously in both directions, the speed is about 100 KB / s for each discharge and for each exchange. This number comes from: 2 downloads + 2 exchanges, due to be forwarded by the access point, consuming twice the bandwidth. By now it is very obvious that the more customers are added the situation gets worse very quickly.
is important to name the concept of simultaneity. Although there are a large number of clients connected to the same access point, in most cases none will make intensive use of the network. So if a customer downloads a web page really full of photos, for a few seconds will be occupying the available bandwidth, but after a few minutes will not take anything, as you read the above page. If in these minutes, another customer transfers information, will again all the available bandwidth. Therefore, it usually is not too noticeable loss of performance. In communities
wireless use two systems to counter these problems. On one side are the proxy
, the QoS
and blocking P2P. On the other hand is the way it made the links between nodes. On a link of this type there are only 2 parties, and not allowed to connect to customers. Thus, if a client connects to a node, and communicates with a client on another node, the equipment actually acts as an intermediary node, and "channeled" information between two nodes. Thus, if multiple clients from one node communicate with some of the other node, control equipment on both ends to deal with "bundled" multiple streams of information into one. Thus, although no loss of performance, not as high as if the network was out of control.
Mirror. This is one of the strengths of community wireless networks. Although downloading at 700 KB / s does not sound like much given the current speed Internet connections. However send at that speed is impossible with the current connections, which rarely move from megabit (that is 128 kilobytes / s). And all this taking into account networks of 11 megabits. Today, in many cases these networks are 54 megabits (about 2300 kilobytes / s real helpful), so that although the inherent problems still exist, are mitigated by higher network speed.
I hope I have clarified some concepts. My main idea by my past experiences, no one takes esque disappointment when you collaborate with a community network. Performance is what it is, and should not be the main factor to help. If you want to work it is for many other potential factors, such as learning about networking, building communities, communicate freely, etc etc
