An optical packet network based on Arrayed Waveguide Gratings

This work presents PrimeNet, a novel architecture for optical multihop networks and investigates a Medium Access Control (MAC) protocol and a fairness algorithm for it. The network architecture is based on an Arrayed Waveguide Grating, a passive optical device that is used widely as wavelength multiplexer/demultiplexer, today. Relying on a physical star architecture logical rings are set up on each wavelength. The number of nodes N has to be a prime number to lead to (N−1) 2 pairs of counterdirectional rings. It is possible to start the deployment of the network with a single ring, and to add further rings when the demand increases. With the addition of new rings the mean hop distance in the multihop network decreases from N/2 down to 1, which is the full mesh. The capacity of this multihop network is compared to a single-hop network. It shows that a small number of fixed transmitters and receivers per node (2 to 3, mostly) are enough to gain the same capacity as a single-hop network with one tunable transmitter/receiver pair. On the basis of the proposed multi-ring architecture a node structure, a MAC protocol, and a fairness algorithm are developed and evaluated analytically and by simulation. An estimation of some physical parameters shows that the network is suitable for the local and metropolitan area. The MAC protocol is based on a carrier-sensing and a fiber delay line (FDL) large enough to store a full packet in order to allow for an unslotted, immediate access to the medium. An aggregation of smaller packets to so-called ”Jumbo”-frames helps to reduce the overhead for optical switching. Because of the potentially very small mean hop distance, we favor local fairness algorithms over global. Among three candidates, the back-pressure based Spatial Reuse Protocol (SRP) was chosen for the fairness algorithm. It had to be modified to suit the optical node architecture. The simulations of the fairness algorithm were performed using unidirectional traffic with a packet size distribution that is typical for today’s Internet. While a fair access to the medium can obviously be guaranteed for this kind of traffic, a modeling of ”real” TCP revealed interactions between the MAC protocol and TCP that lead to unfairness for certain TCP connections. With the introduction of a head-of-line timer to avoid the blocking of the slow start mechanism of TCP, fairness can be guaranteed. Another problem arising from the optical node architecture are reorderings of ”Jumbo” frames. These lead to spurious retransmissions of TCP segments. A discussion of possibilities to make TCP robust against packet reorderings concludes the chapter. At last we give an outlook on the design of large networks based on PrimeNets. Every AWG-based multihop network can be seen as a permutation or Cayley graph. This family of graphs incorporates many of known regular graphs, such as the ring, the hypercube, or the star graph.