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Mathieu Gallissot May Where this is not the case, the source of material has been acknowledged. The first one tends to merge different kinds of networks into one consisting of more than a billion users across the world. Wireless also had a huge growth over the passing years, first with cellular phones more than one billon users and then with wifi.
Nowadays, merging Internet and wireless technologies seems ineluctable, for reaching a permanent network where big quantities of information are going to travel whatever kind of terminal is used. With the success of this technology, the usage starts to need more and more requirements, for video and voice for example. This report will cover the adaptation of the mechanism of routing into wireless networks, in particular, Wifi.
I would like to thank Maurice Mitchell, my project supervisor, who allowed me to research this really interesting subject. Ad Hoc mode ORG windows application This mode is just using wireless for the end user loop. The second kind of operating mode for a wireless network is the Ad Hoc mode.
This one relies only on wireless communications. Its role is basically to serve as a gateway between a wired network called distribution system and its wireless zone. Mobility is then limited to a coverage zone, and so, has a limited impact on this implementation. This infrastructure design can be seen as a first way to implement wireless technologies into the settled wired world.
In fact, from the technical point of view, only the first two layers are modified in the OSI model. These changes concern only the physical layer going from a wire to a wireless media and the Logical Link Control LLC layer how to access the media. Other layers of the OSI model are working as in a wired network.
Ad Hoc networks introduce a new way of communication. Since communications. This rule is true in telephony phones vs. In Ad Hoc networks, each station is using the network and creating the network. For example, in a wifi network, a laptop is able to send and receive data for itself, but it can also forward others data acting as a router. This design has the main advantage to be independent of any distribution system, or any hierarchy such as an access point.
If A wants to communicate with B, then, they just have to connect to each other and exchange data. But, this design also extends coverage and mobility possibilities. If A wants to communicate with C, but C is out of range, B, a station in both A and C radio zones, can forward packets.
By this way, a single access point where a few users are connected can provide a network access to out-of-range machines. Figure describes this implementation of Ad Hoc networks:. Without Ad Hoc, only station A could access the internet using the access point. But, if each station is able to forward the packets to the Access Point, then, B can access the internet, as well as C and the final user.
A good example is when an army is deploying into a destroyed place or an empty space. In this case, each station can be configured for forwarding communications to the appropriate destination. This example also shows the mobility benefit of the Ad Hoc model. This case also applies in the ocean, in the air or even in space for satellites. The most famous example of community network is FON. FON provide to every registered user with an internet connection and a wifi access point at low cost.
The user must connect this access point to his Internet connection and share his connection with other FON users. The most commonly used, is to extend coverage as explained above. Access points are deployed into a city most common scale depending of the density of users, and then, Ad Hoc is used to extend the coverage of these access points.
In this scenario, the density of access points is important as too many users connected to the same access point can overload its connection.
Also, there must be enough users to relay the network where no coverage exists. This time, each access point is part of the Ad Hoc network, and can be connected or not to a distribution system. A good illustration of this wireless mesh network is the one made by the One Laptop Per Child project. That means that every child with one of these laptops relays data for others, even if the laptop is switched off with a very low consumption wireless chipset.
Some schools can have the role of access point, but this is not an obligation. In this case, a child will be able to connect to others, and Internet, share information and knowledge with others. This is a major problem for security. If people have the correct equipment for a specific signal, they are able to use it i.
Using a wireless communication is equivalent to shouting information from the top of a roof. One of the most effective ways for securing a wireless signal is to encrypt it encrypting data or even the signal.
This problem is due to the radio media. Many parameters can affect a radio liaison: interferences, obstacles, mobility…etc. For Wifi, there are two main frequency spaces, 2. Obstacles also affect radio waves.
It first reduces the power of the signal, and then, it can also reflect the signal, and destroy it in the same way. In a mobile environment, radio waves are subject to the Doppler Effect, causing a frequency distortion.
In addition, bandwidth on a radio link is shared between every device using it. Access methods must be designed for avoiding collisions and improve communication, but, these access methods also reduce the availability of the bandwidth. When a device wants to communicate with a wire, it concentrates all the energy on this wire.
For wireless communication, antennas are usually omni- directional, as they need much more energy. Also, the absorption in the air is very important compared to wires. There are many causes for that. The radio propagation model is the main cause. In theory, connections are symmetric, signal power reduces proportionally to the distance between the emitter and the receptor.
In practice, the antenna design and the environment can cause the device to be able to receive from another device, but will not be able to send to this device. Some chipsets can restore a low-power signal but will not be able to provide enough power to the antenna for responding to this signal.
This is represented in the OSI model as the third layer called Network. The role of routing a network is similar to the role of a road map for a post office, in both cases; we need to locate the destination, and more importantly, the best way to reach it.
It especially has an important role, as the Internet was first designed for military communications. Americans wanted a communication infrastructure able to handle the fact that some part of a network core may be down. In this case, a mechanism should redirect data to its destination.
In the real world, this layer has a very limited role for computers, but, it is the main role for routers, in a network core. For other kinds of network, there are similar mechanisms.
For mobile phones, a database centralises the base station where each mobile is connected. This database is used for every call to a mobile phone, providing the end destination to the network core.
In this case, routers use routing protocol to logically locate themselves, and draw a network topology. With this mechanism, routers are able to define a routing table. This routing table contains the information for helping the router to make a decision on where to forward received packets. Routing protocols helps to build routing tables, as these protocols exchange data between routers, containing information about the network.
Each protocol acts a different way. The forwarding decision can be taken only depending on. Takes its decisions Protocol on the status of links up or down and the shortest path. Table List of famous routing protocols used on wired networks Routing protocols are often qualified depending on the size of information they have to exchange in order to build a correct table.
A routing protocol should not use by itself the entire bandwidth available on a link. They are also qualified on how often they have to exchange data, and how complex they are just link state or using more information on the link. This scenario is totally new. This group has a mission to create and discuss routing protocols. This task is very important, due to the complexity of routing on Ad Hoc networks.
The work started in January , with the publication of the informational RFC This document presents the 4 main constraints for routing on Ad Hoc networks, such as dynamics topology, bandwidth constraints, energy constraints and low physical security. The group has then to comply with these constraints in order to build an efficient algorithm of route calculation. The three mains approaches are proactive protocols, reactive protocols and hybrids.
That means these protocols are constantly making requests to their neighbours if any in order to draw a network topology, and then, build the routing table. The disadvantage of this principle is to not be reactive to topology changes, as the tables are pre-established. At the time the data has to be sent, it is not certain that the gateway designed by the routing table will still be there to forward the data.
Contrary to the proactive algorithm, they ask their neighbours for a route when they have data to send. If the neighbours do not have any known route, they broadcast.
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Routing on Ad Hoc networks