Carriers have two fundamental options for extending the reach of a Gigabit PON (GPON) network. One option is to deploy GPON Optical Line Terminals (OLTs) outside of Central Offices (COs). Another option is G.984.6 mid-span extenders, which increase a GPON network’s reach to as much as 60 km. For carriers that only deploy GPON OLTs from their Central Offices (COs), 20km is sufficient to serve only a fraction (perhaps half) of their subscribers. The following diagram shows a basic GPON access architecture without any reach extension.

Remote GPON OLT

One robust option for extending the reach of GPON networks is to install remote GPON OLTs, but this does involve placing relatively complex electronics outside of protected COs. This option provides the most flexibility because the GPON OLT can be deployed in an OutSide Plant (OSP) cabinet or Controlled Environment Vault (CEV) up to 100km or more from the CO. The distance is limited only by the uplink optics chosen for the OLT. The following diagram shows how remote GPON OLTs are deployed in OSP cabinets.

And the picture below shows an actual OSP cabinet and power pedestal.

Mid-Span Extenders

Mid-span extenders are a carrier’s other option. These devices increase GPON network’s reach to as much as 60km, which is the logical limit of GPON’s transmission convergence layer (related to maximum delay between OLT and ONT). G.984.6 intends to allow compatibility with existing ONTs and OLTs as much as possible. The diagram below shows a mid-span extender in a GPON access network

G.984.6 specifies two technology options for mid-span extenders. They may be based on optical amplifiers, which merely provide gain in optical power. Or they may be Optical-Electrical-Optical (OEO) regenerators, which receive the optical signal, convert it to an electrical signal, retime and reshape the signal, and finally convert it to an optical signal for retransmission. Mid-span extenders are bidirectional devices, and the two basic technologies could be mixed (e.g., OEO in upstream and OA in downstream) in a hybrid device.

Mid-span extenders have some disadvantages. They are electronic devices requiring periodic maintenance, which invalidates the passive in Passive Optical Network. They are likely to be incompatible with existing GPON OLT implementations, though compatibility could be achieve with simple changes in GPON OLT parameter values. Mid-span extenders may limit next generation PON deployments unless these technologies are specifically considered in the design of these devices.

Another issue with mid-span extenders is that they will not allow an OTDR full visibility on a GPON local loop, though this is not unique to mid-span extenders. Optical splitters have such high losses (~17db for 32x splitter), and these losses are experienced twice by the OTDR signal (once in each direction for a total of ~34db loss for 32x splitters), that OTDRs can not measure attenuation beyond a splitter either.

Products

Alphion markets their PON.ext mid-span extender. A variety of vendors offer GPON OLTs, and all should be remotely deployable in an environmentally controlled enclosure such as a CEV. For an OSP cabinet deployment, a key criterion is the operating temperature range of the equipment, which should operate reliably from -40 to +65C.

© 2009, The Product Group LLC. All rights reserved.

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1. Frank says:

   March 11, 2010 at 2:44 pm

   John,

   What is the cost (total or per subscriber) of a mid span extender?, or if it is easier to answer, how much can service providers afford to spend on reach extension(vs. the cost of OSP cabinets?)

   Thank you for an otherwise simple and easy to read tutorial.

   Frank