The idea of mesh networks is quite easy. You have a group of mobile phones and only few of them are directly connected to an access point. The other phones use their connected neighbours as routers to get an access to the global network too. Thus this technology makes possible free calls over internet without using the GSM service of mobile phone providers. TerraNet made already some tests in Tansania and Equador.

TerraNet’s chief Carlius says that the mobile phone providers are not that very much ready to invest money in development of Mesh networks. It’s obviously that it would mean the end of GSM era. But I’m more than sure that in several years when this technology gets ripe enough and mesh network components will be the standard equipment at mobile phones like Bluetooth or Wlan, it will displace current expensive phone services.

Taking into consideration all these facts, it’s obvious that the standard approaches applicable to the usual networks with pre-existing infrastructure aren’t suitable in general for the ad hoc networks. In following I’d like to tell a couple of words to ISO/OSI layers and explain briefly how they are realized in ad hoc networks.

Data Link Layer.

In Ethernet the Carrier Sense Multiple Access Collision Detection (CSMA/CD) is mostly used as the Medium Access Control Protocol. This protocol does not work well in ad hoc network. So imagine there is a node A that wants to send a packet to the other node B. A has to be sure that no other neighbour is sending at the moment to avoid collisions. If medium is already in use, A has to wait. The main lack of this approach is so called Hidden Terminal Problem. The picture depicts it. The node C is hidden from the node A. If C is already sending and A wants to send, it can’t find any sending nodes nearby, so it starts sending too. It leads to a collisions at the node B.

That’s why IEEE 802.11 offers the alternative MAC protocol that partly solves this problem. If the node A wants to send some packet to the node B and the medium is free, A sends first of all a short RTS (ready to send) signal. All the neighbours of A know then that A is going to send. When B recieves RTS it sends CTS (clear to send) signal, and thus all the neihbours of B know that B is going to recieve. After the transmition B sends an ACK and it releases the medium for the neighbours.

Network Layer.

The most important and interesting question is how one node finds a multi-hop path to the other node. Routing protocols for mobile ad hoc networks are devided into three large classes proactive, reactive and hybrid routing protocols. Reactive protocols are designed for the ad hoc networks with high dynamics, where the network topology changes frequently. Each time when one node sends a packet to the other node, the routing path is built up again. Proactive protocols use a kind of routing tables. It is assumed that the routes change not so often and they can be saved for a while in a table. The tables are then changed by the nodes. And at last Hybrid protocols use both of these technics. The list of most popular algorithms can be found for example in Wikipedia.

Transport Layer.

In most ad hoc networks various realizations of TCP are used as the transport protocol. All the realizations like TCP Reno, TCP New-Reno differ from each other by algorithms that set the congestion window. The “normal” TCP doesn’t work well in ad hoc networks because the packets are frequently lost being propagating over the air. “Normal” TCP has to be adopted to these conditions.

Application Layer.

Unlike the Application Layer in common networks, application layer in mobile ad hoc networks can’t be completely isolated from the network layer. The developing applications have to be aware of the dynamics of the network and depending on the frequency of the topology changes apply for example the appropriate routing protocol.

• MANets – Mobile Ad-hoc Networks. It’s the classical ad hoc network that came from the military sector. These networks were developed to connect planes, tanks, troops at the battlefield. They are completely self-organizing. The other possible areas where they could be applied are disaster recovery, car-to-car communication, home networking. This class of ad hoc networks can be seen as the forefather of the other classes.
• WMN – Wireless Mesh Networks. Like MANets, but there is a set of nodes, stationary mesh routers which form a wireless multi-hop ad hoc backbone. The mesh routers can be connected to the Internet and thus all the participants of the backbone have an access to the Internet. Imagine you are at the airpot where there are only few access points (mesh routers). You have no direct contact to a one, but still you have an Internet access due to the other guests of the airport. The mesh routers make the routing task more simple and the protocols running on them allow the backbone to be easy to set up and self healing.
• WSN – Wireless Sensor Networks. This special class of ad hoc networks is used to monitor some phenomenon in a certain area. The nodes have often only the simple task to deliver the monitoring information such as measured temperature, air polution, median speed of the cars on a highway etc. to the minitoring system. The devices partitipating in a sensor network are just special sensors that differ greatly in their purpose from the user devices. Thus the solutions designed for the general multi-hop ad hoc networks where mobile devices execute such applications as Skype, instant messaging, streaming applications etc., just don’t suite for sensor networks.

The idea of ad hoc networks is not that very innovative. It’s almost as old as the Internet. In the last years the interest to the ad hoc networks has grown very much. They should be the networks of the 4-th generation (4G). But I should say that there are still no services or software solutions for the ad hoc networks so far. All the efforts of the science are concentrated on the research of the routing that is a great challenge at the moment. The other problem is that there are no suitabe divices on the markt. So all the research results that were made so far are mostly of the theoretic nature and base on simulation. So I guess that it will take two or three years till the first serious software solutions for the ad hoc networks will be ready to appear on the markt.

Another point is the politics of the markt. Do you still know how expensive were the licenses for UMTS? It will take several years yet till the mobile providers get their money back and be able to look for new technologies. In the near futre there will appear a set of online services for mobile phones based on geo informations. Maybe there appears something else, but the fact is that the place for the ad hoc network will appear on the markt not that soon. In any case I’m persuade that mobile ad hoc networks are our future and much interesting things will happen in this sector.

Next time I’m going to tell a little bit about routing, classification of routing protocols and the main problems connecting with routing.

In this new Blog category, “Ad Hoc Networks”, I’m going to write about the progress of my diploma thesis and to deal with my knowledge and research results on MANets. Because of the contract conditions I can’t tell about all the details. But most of the research work I’ll have to do will be of my own and it will find a reflection in the postings of my IT-Blog.