Cloud Radio Access Network (C-RAN) architecture claims to reduce capital costs and facilitate the implementation of multi-site coordination mechanisms. This paper studies the delay constraints imposed by the Common Public Radio Interface (CPRI) protocol in ring-star topologies used by mobile operators. Simulations demonstrate that centralised implementations are feasible via functional split in the baseband processing chain. We derive theoretical expressions for propagation and queueing delay, assuming a G/G/1 queueing model. Then, we examine the properties of the fronthaul traffic flows and their behaviour when they are mixed. We show that the theoretical queueing delay estimations are an upper bound on the simulation output and accurate under certain conditions. Based on our results, we further propose a packetisation strategy of the fronthaul traffic which helps reduce the worst case aggregated queueing delay by 30%. Also, the benefits of a bidirectional ring topology are shown, achieving a worst average queueing delay 10 times lower than that of unidirectional topologies.
