On mitigating packet reordering in FiWi networks

In an integrated fiber and wireless (FiWi) access network, multi-path routing may be applied in the wireless subnetwork to improve throughput. Due to different delays along multiple paths, packets may arrive out of order, which may cause TCP performance degradation. Although the effect of packet reordering due to multi-path routing has been well studied, remedy solutions are either to schedule packets at the source node to proactively reduce the chance of packet reordering, or to modify TCP protocol. Resequencing packets arrived out-of-order has only been considered at the end systems which can cause long delay as packets must be buffered until there is no sequence gap. As all traffic in a FiWi network is sent to the Optical Line Terminal (OLT), the OLT serves as a convergence node which naturally makes it possible to resequence packets at the OLT before they are sent to the Internet. However, the challenge is that OLT must re-sequence packets effectively with a very small delay to avoid a performance hit. In this paper, we propose a scheduling algorithm at the OLT to resequence packets while providing fairness. Simulation results validate that our packet scheduling algorithm is effective in improving the performance of TCP flows. Since resequencing is conducted in the access network which has a much fewer number of flows compared with those at routers, our proposed work provides a scalable solution to mitigate the side-effect of packet reordering caused by multi-path routing.