Session: SpaceWire Networks and Protocols Long Paper

SpaceWire networks may include units, such as memory or processor units, which are bandwidth limited and in high demand by several other units. Such modules are termed Hot Modules. SpaceWire uses wormhole routing to deliver packets comprising multiple flits (N-chars). Wormhole routing helps minimize the number of buffers and the transmission latency. On the other hand, under high load, the network can become congested due to long packets that occupy resources at multiple network nodes, and block the paths of many other packets. This situation may be exacerbated at the presence of Hot Modules. In this paper we demonstrate that a single Hot Module can both dramatically reduce the network efficiency and cause an unfair allocation of system resources. First, the network efficiency is reduced because many packets can wait in different routers for the Hot Module to be available, while blocking other packets that are not destined to the Hot Module. Second, the allocation of system resources is unfair because in order to reach the Hot Module, packets from farther nodes need to win more arbitration cases than packets from nodes that are closer to the Hot Module. We explore several solutions for the Hot Module problem in SpaceWire networks. Possible solutions include provisions for priority-based routing and end-to-end access regulation mechanisms. We employ network simulations to investigate how the solutions address the network-efficiency and access-fairness problems.