In the early 2000’s many telcos deployed GPON Point-to-Multipoint optical networks as an economical architecture for internet access in low-density rural areas. Popular GPON based systems from companies like Calix and Adtran provided overlays for video and means for providing Plain Old Telephone Service (POTS) as well as internet based voice telephony services.
The key benefit of the GPON architecture was the 1:32 or even 1:64 passive splitting of the optical signal. This allowed the cost of the Optical Line Terminal (OLT) and the GPON optical transceiver to be divided across the number of ONTs served by a given GPON port. Because of the relatively high cost of optical transceivers during that time, this sharing of costs across multiple customers served was a huge advantage.
But as customers continue to demand higher speed internet access, the high split ratios used in GPONs are an impediment. Most GPONs are deployed with a 2.5Gbps downstream signal which is shared by all the subscribers. The table below shows the approximate maximum data rate (net of transport overhead) available to each subscriber at various split ratios.
|Max. Data Rate / ONT
To meet the demand for higher internet access speeds for new builds, rural/low-density carriers are increasingly moving away from GPON in favor of direct point-to-point architectures. This article describes a migration path to pt-to-pt for previously deployed GPONs.
OUT WITH THE OLD…
The network diagram below depicts most of the major elements of a GPON network. The Optical Line Terminal, OLT, manages the bi-directional Passive Optical Network (PON) services, typically including voice and data (i.e., Internet access). The RF Video equipment sources uni-directional overlay video services (aka, broadcast and VOD/PPV). The PON services typically use 1490nm wavelength in the ‘downstream’ direction from the OLT to the ONT and 1310nm in the ‘upstream’ while the RF video is typically transmitted at 1550nm. The “WDM”, wavelength division multiplexer, is a passive device that muxes and demuxes the PON and video services on/off of the single fiber transport backbone. The “Splitter” is a passive device that splits/combines the PON/Video traffic between the one single-fiber backbone and a number of single-fiber links, each linking to a single ONT.
The splitter in the GPON network introduces losses on the order of 3dB * Log2(Split Number). So a 1:8 splitter introduces at least 3dB * Log2(8) or 9dB. The table below shows the loss at various common split ratios.
|Split Ratio 1:N||Loss per Doubling (dB)||
|Min. Total Splitter Loss|
Splitter optical power losses of this order requires the use of relatively expensive optical transceivers in GPON head-end equipment (OLTs and RF Video) to transmit at high power levels (sometimes in excess of 5dBm). Also because of the splitter losses the optical receive power levels at the ONTs are relatively low, requiring the use of more expensive transceivers with very low sensitivity levels.
…IN WITH THE NEW
The optical transceivers used in GPON and other networking equipment are defined in standards called Multiple Source Agreements (MSAs). The primary impetus behind MSA standards was to make optical transceivers a commodity. As customers began to realize they did not have to purchase these transceivers at inflated prices from the OEMs but could purchase 100% compatible/identical parts on the open market, the price of optical transceivers of all types has dropped precipitously. In fact, a pair of 1Gbps bi-directional optical transceivers capable of delivering Internet access services up to 20km can be purchased for less than $50!..and 10Gbps BiDi’s are rapidly becoming extremely affordable.
So, is there a way to take advantage of these low cost optics to meet the demand for higher internet access speeds? The answer is “yes”, with a bit of re-arrangement and a few additions an existing GPON network can be upgraded to a state-of-the-art Gigabit Internet access network. The upgrade may be accomplished on an ONT-by-ONT basis or the entire GPON may be upgraded together. The diagram below shows a single ONT (the subscriber on the bottom right) upgrade. The steps are as follows (no particular order),
- Add a 1Gbps BiDi transceiver to an available port in the OLT
- Equip a new RF Video output (or split the one already there)
- Connect the BiDi and RF Video to a new WDM (1310&1490/1550)
- Disconnect the upgraded home-run link from the Splitter
- Connect a new fiber from the WDM to this home-run link
The second approach is upgrading the entire network as a whole, shown in the diagram below. Notice the existing splitter is removed and relocated at or near the RF Video source so that one copy of the video service is made for each ONT served. The OLT may need to be upgraded or replaced so there is sufficient optical ports for each of the ONTs. Also, additional WDM filters (extremely low-cost items) must be installed, again, one for each ONT served. If unused fiber is not available between the head-end and the previous location of the splitter, a sufficient number of fibers must be added.
Most existing GPON ONTs are not upgrade-able to a 1Gbps WAN interface. However, new generation 1Gbps ONTs are available that are very inexpensive and compact. Soon we expect 10Gbps ONTs (primarily for commercial applications) to become cost-effective.
FOR MORE INFORMATION
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