text: 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.
  @cite_22: Feature Issue on Optical Access Networks (OAN) We describe two schemes developed to carry very-high-data-rate DSL (VDSL) signals over a fiber to the cabinet architecture for the upgrade of legacy copper networks. The first technique exploits the subcarrier bandwidth of uncooled semiconductor lasers for the transmission of multiple VDSL signals by using inexpensive interfacing hardware in the optical network unit (ONU). With a hybrid fiber-copper link comprising 100 m of twisted-pair copper cable and as much as 45 km of single-mode fiber, data transmission comparable with that for fiber to the home is achieved. In an extension to this design we demonstrate a system using a reflective semiconductor optical amplifier in a carrierless remote ONU configuration. Such a design might provide wavelength agility in the fiber access network for signal routing or active bandwidth allocation.
summary: Two physical-layer systems to bridge VDSL signals over a fiber access network were proposed in @cite_22 . Individual VDSL signals are converted to be spectrally stacked into a composite signal that modulates an optical carrier. In the first system the optical carrier is supplied by a laser at the ONU and in the second system the optical carrier is supplied by a laser in the OLT that is reflected and modulated by a Reflective Semiconductor Optical Amplifier (RSOA) at the ONU. The optical carrier provides 1 GHz of spectral width accommodating 40 VDSL lines without guard bands and 25 VDSL lines with guard bands. Although, this approach to a hybrid PON xDSL allows the drop-point device to avoid buffering as well as contain simple logic by pulling the DSLAM functionality into the OLT, the design requires the PON to carry the full bandwidth of each VDSL line even when idle. Designs that operate at the link layer rather than physical layer can avoid transmission of idle data on the PON thereby increasing the number of subscribers that can be supported by capitalizing on statistical multiplexing gains.