Gigabit-capable Passive Optical Network (GPON) extends fiber all the way to the home or premises, and uses an entirely passive outside plant (with the exception of Optical Network Terminals, ONTs, that are sometimes located outside of homes). While the point-to-point links of Vectored VDSL, G.now or G.fast are entirely separate until they are aggregated by Ethernet switching at a DSLAM or DPU, GPON shares the fiber medium among multiple subscribers. This sharing is performed with Time-Division Multiple Access (TDMA) under the scheduling control of the Optical Line Terminal (OLT). In this way, multiple (typically 32) subscriber lines are combined into a single fiber running into an exchange and terminating on an OLT. Thus, there are relatively few ports on the network-end active equipment.
GPON is standardized by the ITU-T G.984 series of Recommendations and typically supports aggregate line rates of 2,488 Mbps in the downstream direction and 1,244 Mbps in the upstream direction on two separate wavelengths. While the speeds of GPON may seem ample, they are shared across many users and may at some times be exhausted, particularly for high-bandwidth services that are not amenable to concentration such as unicast video streaming during prime-time. GPON can be upgraded by splitting nodes, e.g., serving 32 users per OLT port instead of 64. Or, GPON can be upgraded to new, higher-speed systems with no change to the fiber components of the outside plant. XG-PON supports 10 Gbps down and 2.5 Gbps up using TDMA similar to GPON, but at a faster line speed. XG-PON has been standardized, and equipment is now becoming available. Beyond that, NG-PON2 runs each of multiple pairs of wavelengths as a single TDMA PON, each carrying up to 10 Gbps down and 2.5 Gbps up; this is called Time and Wavelength Division Multiplexed (TWDM) PON. NG-PON2 can use up to 8 wavelengths for each direction, increasing overall speed by up to a factor of 8 compared to XG-PON. NG-PON2 standards are nearing completion in the ITU-T.
These next-generation PON systems should be able to work with existing PON outside plant, and are compatible with GPON and each other on the same glass plant due to their wavelength assignments. An upgrade should require no change in the outside plant; however, the OLTs and ONTs would have to be replaced and upgraded.
The main cost of GPON is installation of fiber all the way to the home or apartment; the fiber-drop and ONT installation are particularly costly. Greenfield deployments are clear winners for GPON, however they represent roughly only 1% of subscribers per year. In brownfield deployments, aerial plant is easier to upgrade to fiber than buried plant. Generally speaking, only a subset of locations is cost-effective for GPON installation based purely on return-on-investment criteria. An alternative model for deployment is to rely on “pull” demand from subscribers, whereby GPON installation takes place only after a certain number of customers have committed to purchasing the service.
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