Topology

The network we are building and operating is targeted to a coverage area of $25 Km^2$, which is the complete city of Leiden (The Netherlands) and its surroundings. There are no hills in the target area. Other natural obstacles such as large forests are also absent. It is a small old town center with some moderately high buildings and some 10-15 story appartment buildings in the suburbs. These appartment flats are surrounded by small houses (max. height 3 floors).

The total number of people living in this area is approximately 160,000. In this area we want to provide outdoor coverage. For using the network indoors, a small antenna connected to the client computer has to be sufficient. The antenna should preferably have a line of sight to the nearest network access point. This is required as the maximum distance between the client and the network access point (a network node) is limited due to the national legislation implementing European (EU / ERC) regulations (restricting maximum radio frequency output power and restricting antenna gain). Combining this knowledge with the anticipated traffic and bandwidth needs, it is evident that we need multiple nodes distributed over the coverage area. The nodes themselves have to be interconnected. A plot of the radio coverage of the current set-up (the historical center of Leiden ($10 Km^2$)) is shown in figure 1. To provide full outdoor coverage of the target area an estimate of 25 network nodes will be needed.

Figure 1: Current Wireless Leiden radio coverage plot. The total area shown is approx. $20 Km^2$. Overlayed in black are contours of equal field strength of the areas in which the Wireless Leiden network is available when using a standard wireless network card connected to a $7 dBi$ gain antenna located outdoors. (data courtesy of www.wirelessdesign.nl)

The interconnection of the nodes also uses wireless links, making the network completely independent of the local (wired) infrastructure and thus very cost-effective and without significant monthly or other regularly repeating costs.

A mesh between the nodes will be formed, as each node is connected to the other nodes by at least 2 different (wireless) connections. With this approach, adding extra nodes to the network will add redundant paths and will therefore also increase the total available bandwidth. The topology as seen from the user is comparable to cellular telephony. Cells for users are created. In these cells the users share the total available bandwidth of an access point. The cells themselves are interconnected by point-to-point wireless connections. These connections form the backbone of the network.