Chapter 3: Mesh networking with OLSR
Posted by Mungo under Chapter 3 on March 7th, 2007.Most WiFi networks operate in infrastructure mode - they consist of an access point somewhere (with a radio operating in master mode), attached to a DSL line or other large scale wired network. In such a hotspot the access point usually acts as a master station that is distributing Internet access to its clients, which operate in managed mode. This topology is similar to a mobile phone (GSM) service. Mobile phones connect to a base station - without the presence of such a base station mobiles can’t communicate with each other. If you make a joke call to a friend that is sitting on the other side of the table, your phone sends data to the base station of your provider that may be a mile away - the base station then sends data back to the phone of your friend.
WiFi cards in managed mode can’t communicate directly, either. Clients - for example, two laptops on the same table - have to use the access point as a relay. Any traffic between clients connected to an access point has to be sent twice. If client A and C communicate, client A sends data to the access point B, and then the access point will retransmit the data to client C. A single transmission may have a speed of 600 kByte/sec (thats about the maximum speed you could achieve with 802.11b) in our example - thus, because the data has to be repeated by the access point before it reaches its target, the effective speed between both clients will be only 300 kByte/sec.
In ad-hoc mode there is no hierarchical master-client relationship. Nodes can communicate directly as long as they are within the range of their wireless interfaces. Thus, in our example both computers could achieve full speed when operating ad-hoc, under ideal circumstances.
The disadvantage to ad-hoc mode is that clients do not repeat traffic destined for other clients. In the access point example, if two clients A and C can’t directly “see” each other with their wireless interfaces, they still can communicate as long as the AP is in the wireless range of both clients.
Ad-hoc nodes do not repeat by default, but they can effectively do the same if routing is applied. Mesh networks are based on the strategy that every mesh-enabled node acts as a relay to extend coverage of the wireless network. The more nodes, the better the radio coverage and range of the mesh cloud.
There is one big tradeoff that must be mentioned at this point. If the device only uses one radio interface, the available bandwidth is significantly reduced every time traffic is repeated by intermediate nodes on the way from A to B. Also, there will be interference in transmission due to nodes sharing the same channel. Thus, cheap ad-hoc mesh networks can provide good radio coverage on the last mile(s) of a community wireless network at the cost of speed– especially if the density of nodes and transmit power is high.
If an ad-hoc network consists of only a few nodes that are up and running at all time, don’t move and always have stable radio links - a long list of ifs - it is possible to write individual routing tables for all nodes by hand.
Unfortunately, those conditions are rarely met in the real world. Nodes can fail, WiFi enabled devices roam around, and interference can make radio links unusable at any time. And no one wants to update several routing tables by hand if one node is added to the network. By using routing protocols that automatically maintain individual routing tables in all nodes involved, we can avoid these issues. Popular routing protocols from the wired world (such as OSPF) do not work well in such an environment because they are not designed to deal with lossy links or rapidly changing topology.
