Background: The Ring Road Approach

µPCN is intended to run on low-cost LEO micro satellies such as CubeSats for rendering planet-wide communication possible.

This underlying idea has been presented e.g. under the term "Ring Road" in [1] and discussed in further detail in [2].

The core idea of the Ring Road concept is to interconnect physically decoupled subnetworks of an overall network via LEO satellites moving between these sites. The satellites are not cross-linked; rather, each satellite functions as a “data mule”, exchanging data directly with each isolated site during its overflight of that site and retaining in local storage any outbound data destined for nodes that are reachable from sites over which the satellite will fly in future. In effect, the approach builds a world-scale message ferrying architecture. To automate the communication among the participating nodes, including LEO satellites, DTN protocols are applied. Every decoupled site includes at least one so-called “cold spot”, a network node equipped with a radio transceiver that has direct contact with Ring Road satellites but not with the Internet. The nodes located within the decoupled network site have access to the satellites via the cold spot(s). Besides cold spots, “hot spots” also interact with the satellites. Those nodes have connections to the Internet as well as radio transceivers. Thus, a path from a node located within a decoupled site and a node in the Internet is always routed via a cold spot to the satellite network and from there via a hot spot to the Internet and thence to the Internet-connected node.

An example of the overall Ring Road concept is visualized in the following figure.

The figure provides an example scenario of Ring Road operation (a modified version of the scenario discussed in [2]). In this scenario node X requests some data from a database located at node C. Node X has established a short temporary ad-hoc connection to node A and transfers the request to this node. Equipped with a radio transceiver, node A is termed a “cold spot” because it has direct contact with Ring Road satellites but not with the Internet.

Node A forwards the request to LEO satellite 2 which physically transports the request in storage while in transit to node B. In the Ring Road terminology node B is called a “hot spot” due to its access to the Internet as well as to the Ring Road satellites. Satellite 2, flying over node B, radiates the request to that node; node B forwards the request via a continuously available Internet connection to node C which processes the request and sends out a reply. The reply is not transferred on the same route as the original request, because it may be several days before satellite 2 flies over node A again.

C's local knowledge of the contact opportunities between cold and hot spots and LEO satellites results in C forwarding the reply to node D, the next node that will be in contact with satellite 3, which in turn is the next satellite that (a) will be in contact with some hot spot and (b) will subsequently fly over node A. Node D buffers the reply and as soon as LEO satellite 3 is passing D's location, it transfers the reply to this ferry. As soon as satellite 3 passes node A, the reply is delivered to A which holds the data in storage until the temporary connection to node X is reestablished.

[1] C. Krupiarz, C. Belleme, D. Gherardi and E. Birrane, “Using SmallSats and DTN for Communication in Developing Countries”, in Proceedings of the 59th International Astronautical Congress, Glasgow, Scotland, 2008.
[2] S. C. Burleigh and E. J. Birrane, “Toward a Communications Satellite Network for Humanitarian Relief”, Proceedings of the 1st International Conference on Wireless Technologies for Humanitarian Relief, Kerala, India, 2011.