PON Technology, Part 1: The Present
Fiber-to-the-premises (FTTx) service via passive optical networking (PON) has proven to be an efficient method of delivering maximum bandwidth to the doorsteps of homes, businesses, and other customers. In the first of a two-part series, we’ll review the PON technologies currently deployed in telco and cable/MSO networks, and discuss some of the challenges driving new developments.
All PON technologies in place today use single fiber technology to connect the OLT (optical line terminal) at the central office with the ONU (optical network unit) at the customer premises. It does so over a point-to-multipoint architecture enabled by a passive splitter. The splitting ratio can vary widely, but is most commonly 1:32, or 32 customers per OLT port. splitter.
A simplified standard PON architecture is shown below:
Some of technologies currently deployed include:
• GPON and EPON/GE-PON. The first PON technologies developed and deployed, GPON was favored by telcos, while EPON found more adopters in the cable/MSO space. GPON offers 2.5G service downstream (to the customer/ONU) and 1.25G upstream (to the central office/OLT), and EPON offers symmetrical 1.25G service down and upstream. Both protocols can support splitting ratios of up to 1:64, although 1:32 remains more common.
• XGS-PON and 10G EPON. As with GPON and EPON, these technologies are favored by telcos and cable MSOs, respectively. Both provide symmetrical 10G Ethernet transmission. The advantage of these protocols is that they operate on different wavelengths from their 1G counterparts, and can therefore coexist on the same fiber infrastructure, as shown in the graph below.
• G.FAST. This technology uses existing copper infrastructure in situations in which the cost and inconvenience of fiber installation is deemed excessive. While the physical limitations of the copper wiring place a strict limit on distances, this remains an effective solution for high-density urban and semi-urban environments (e.g., high-rise buildings, rowhouses, shopping centers, etc).
• RFoG. A niche application for cable providers seeking to use their legacy video head-end to transmit over the same fiber network used for 1G and 10G EPON. RFoG is generally viewed as a stopgap measure with a finite lifespan that merely extends the useful life of equipment, but its deployment may continue well into the next decade.
As revolutionary as FTTx once was, there is clear need for advancement. In some markets, GPON/EPON deployments are over a decade old, and are beginning to struggle with current demand due to the same forces affecting all networks (e.g., IoT connectivity, competition from 5G, and so on). G.FAST was never meant to be a permanent solution. Similarly, the utility of RFoG will eventually diminish as streaming video continues to supplant traditional video delivery. Check back early next week for an exploration of what’s next.
Check out the datasheets below to learn more about GPON transceivers and FTTx passives.