Upstream Polling Protocols for Flow Control in PON/xDSL Hybrid Access Networks

In a hybrid PON/xDSL access network, multiple customer premise equipment (CPE) nodes connect over individual digital subscriber lines (DSLs) to a drop-point device. The drop-point device, which is typically reverse powered from the customer, is co-located with an optical network unit (ONU) of the passive optical network (PON). We demonstrate that the drop-point experiences very high buffer occupancies when no flow control or standard Ethernet PAUSE frame flow control is employed. In order to reduce the buffer occupancies in the drop-point, we introduce two gated flow control protocols that extend the polling-based PON medium access control to the DSL segments between the CPEs and the ONUs. We analyze the timing of the gated flow control mechanisms to specify the latest possible time instant when CPEs can start the DSL upstream transmissions so that the ONU can forward the upstream transmissions at the full PON upstream transmission bit rate. Through extensive simulations for a wide range of bursty traffic models, we find that the gated flow control mechanisms, specifically, the ONU and CPE grant sizing policies, enable effective control of the maximum drop-point buffer occupancies.

I. Introduction

Access networks are communication networks that interconnect private local area networks, such as the networks in the homes of individuals, with public metropolitan and core networks, such as those constructed by service providers to connect paying subscribers to the Internet. Private local area networks often employ high speed wired and wireless communications technologies, such as IEEE 802.3 Ethernet (up to 1 Gbit/sec) and IEEE 802.11 WiFi (up to 600 Mbit/sec). These high-speed communications technologies are cost effective in private local area networks due to the short distances involved and subsequent low installation costs. Public metropolitan and core networks employ a variety of communication technologies that include dense wavelength division multiplexed technologies over fiber optic transmission channels (up to 1 Tbit/sec). These high-speed communication technologies are cost effective due to the cost sharing over many paying subscribers. Access networks require significantly higher installation costs compared to private local area networks due to longer distances that must be covered. At the same time, access networks have significantly smaller degrees of cost sharing compared to public metropolitan and core networks; thereby increasing cost per paying subscriber. As a result, access network technologies must keep installation costs low, which can be achieved by utilizing existing bandwidth-limited copper wire or shared optical fiber [1].