Archive

Archive for May, 2012

Temporal Networks

May 31st, 2012 2 comments

Pádraig and  I discussed Time Respecting Paths  in the datasets we have based on the paper – Temporal Networks[1].

We want to detect the shortest/quickest paths between pairs of nodes, which respect the the temporal paths, so that we can derive a betweeness centrality measure.

The existing implementation we have, based on the BubbleRAP paper, floods the network, and calculates the betweeness centrality using the number of times a node is used in ANY path  – the BubbleRAP implementation uses only the SHORTEST paths. So there is some discrepancy here. The BubbeRAP paper reads as follows:

To calculate the individual centrality value for each node, we take an numerical approach. First we carry out a large number of emulations of unlimited flooding with different uniformly distributed traffic patterns created using the HaggleSim emulator, which can replay the collected mobility traces and emulate different forwarding strategies for every contact event. Then we count the number of times a node acts as a relay for other nodes on all the shortest delay deliveries. Here the shortest delay delivery refers to the case when the same message is delivered to the destination through different paths, and we only count the de- livery with the shortest delay. We call this number the betweenness centrality of this node in this temporal graph. Of course, we can normalise it to the highest value found. Here we use unlimited flooding since it can explore the largest range of delivery alternatives with the shortest delay. This definition captures the spirit of the Freeman centrality[2]

 

The Freeman centrality paper, which I only skimmed, doesn’t appear to refer to temporal graphs directly –  further inspection may yield a better understanding 🙂

There is a question over what observation period we should use, and how to decide which is really the ‘shortest path’  when paths may start/end at different times. In the Bubble RAP paper, they seem to have chosen the path with the least latency, but not considering the start time of the path. In the temporal networks paper [1], the authors talk about a Saw-tooth pattern which governs latency (as regular connections create a latency lag between connections), and mention a method to normalise over this pattern – but not really for the Betweeness Centrality measure.

e.g. If we start at time 0, and measure the time it takes to get to the destination node, that is fine. however, what if there is an intermediate time, 4, where, if we measure the path from there, we find a new shortest path – in Bubble RAP, this is taken as the shortest path, but is this correct?

I will implement the Time Respecting, shortest path (latency) betweeness centrality, (and maybe the shortest path – hops), as per BubbleRAP, and I will also investigate the normalised version as per the Temporal Networks paper.

[1]Holme P, Saramäki J. Temporal Networks. 2011:1-28. Available at: http://arxiv.org/abs/1108.1780. Accessed May 15, 2012..

[2]Freeman L. A set of measures of centrality based on betweenness. Sociometry. 1977. Available at: http://www.jstor.org/stable/10.2307/3033543. Accessed May 31, 2012.