  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.
