Multi-gig PON 101: Q&A with CableLabs

optical network
This week's interview features Curtis Knittle, VP of wired technologies at CableLabs. (sbelov/Getty Images)

Fierce’s Multi-gig PON 101 Q&A series digs deep into the high-speed future, aiming to help readers build in-depth knowledge about key technologies and the overall market landscape through progressive interviews with industry experts. This collection will delve into the forces driving operators toward multi-gig PON technology; how XGS-PON and other 10G technologies are helping deliver a new breed of ultra-fast broadband; and what’s coming down the pipe with 25G and beyond.

This week's interview features Curtis Knittle, VP of wired technologies at CableLabs. The interview transcript has been lightly edited.

Fierce Telecom (FT): First things first: what PON standards offer multi-gigabit capabilities? How widespread are these today?

Curtis Knittle, CableLabs

Curtis Knittle (CK): There are basically four PON standards today which are capable of supporting symmetric multi-gigabit services. The set includes the two 10G PON standards (IEEE 10G-EPON and ITU-T XGS-PON), the IEEE 25G/50G-EPON standard and the ITU-T NG-PON2 standard. There is also a Multi-Source Agreement (MSA) that just wrapped up for 25GS-PON. An MSA is more like a specification, rather than a standard, but for all practical purposes it is nearly the same thing, bringing the total to five multi-gigabit PON options.

If we’re focusing on multi-gigabit services over PON, the 10G PON technologies are leading the way. 10G-EPON was first out of the gate, both with the standard as well as initial deployments. If XGS-PON hasn’t already overtaken 10G-EPON in terms of total number of ports deployed, it soon will. The 25G PON options are relatively new – IEEE ratified the 25G/50G standard in July 2020 and the 25GS-PON MSA literally wrapped up this month, so we have yet to see any 25G PON deployments.

FT: What is the one thing people misunderstand the most about PON technology or the market more generally?

CK: I’d say the most misunderstood thing about PON is this – PON is PON. What do I mean? Whether the standard comes from IEEE or the ITU, they both accomplish the same thing in roughly the same manner. The role of time division multiplexing (TDM) PON in access networks is to transport Ethernet frames from one side of the optical distribution network to the other, using TDM in the downstream and time division multiple access (TDMA) in the upstream.

FT: Can you help us better understand the different PON standards? What is G-PON vs XGS-PON vs 25G PON? Do these just offer progressively faster speeds or are there other differences?

CK: That is correct, successive standards from a particular standards organization is mainly increasing the peak capacity. For the ITU flavors of PON, GPON provided ~2.5Gbps downstream, XGS-PON delivers symmetric 10 Gbps, and 25GS-PON (the MSA specification) delivers, you guessed it, symmetric 25 Gbps. For IEEE flavors, it is nearly the same thing – 1G-EPON delivered symmetric 1 Gbps, 10G-EPON delivers symmetric 10 Gbps and 25G/50G-EPON will deliver 25 Gbps or 50 Gbps downstream and 10 Gbps, 25 Gbps, or 50 Gbps upstream.

FT: What’s the difference between the PON standards from the ITU vs those put out by IEEE?

CK: The main difference between the multitude of PON flavors is which Standards Development Organization (SDO) defined the technology. There are differences in the media access control, but many similarities in the physical components. In the final analysis, they are very similar in terms of capabilities, the primary role for PON being “Ethernet In – Ethernet Out”, whether the standard comes from IEEE or ITU. In fact, XGS-PON more or less borrowed the physical layer from 10G-EPON, and the same thing is happening with 25GS-PON, which is borrowing the physical layer from 25G-EPON. In both cases – XGS-PON and 25GS-PON – the ITU flavor of media access control is layered on top of the physical layer. Plus, ASICs today are capable of supporting both flavors of PON, so essentially even the chipset in the customer premise equipment is the same. This “sharing” of physical layer components and ASICs really helps every operator deploying PON because of the increased volume.

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FT: It seems 10G is starting to proliferate and we’ve already heard buzz about 25G. What’s the next step in PON’s evolution?

CK: 10G PON deployments are definitely ramping up, with 25G PON on the horizon. There are two activities addressing the next step in PON’s evolution. The first is from the ITU which has undertaken a standardization effort for single-wavelength 50 Gbps PON. The second is from CableLabs which has recently started a working group to develop specifications for a single-wavelength 100 Gbps PON.

FT: Can you tell us a bit more about coherent PON? Why is it important and how far away is it/what needs to happen to make it a reality?

CK: Coherent PON (CPON) uses coherent modulation and reception, which is capable of supporting significantly higher peak capacities on a single wavelength relative to the traditional intensity modulation with direct detect (IM-DD). With each successive increase in capacity for IM-DD technology, the solution necessarily becomes more complex, and therefore the devices will be more complex and likely cost more. We believe we should use that increase in device complexity to make the jump to coherent modulation because it has a much longer runway, for example to 200 Gbps or 400 Gbps.

The important characteristic of CPON isn’t necessarily that impressive peak capacity and the technology roadmap. The important characteristic for CPON is it will reach four times farther than traditional PON for the same split ratio, or for a 20 km distance the split ratio can be 16 times higher than IM-DD PON. This means that instead of only supporting 32 or 64 homes on a single IM-DD OLT port, a CPON OLT port will be capable of supporting in the neighborhood of 512 homes (for 20 km nominal distance). That’s a significant reduction in the number of OLT ports. And remember, with this increased reach and split ratio, you’re also getting 10 times higher capacity compared to 10G PON, or four times higher capacity compared to 25G PON.

Coherent optical technology, whether in a point-to-point configuration or point-to-multipoint (aka PON) configuration is the future of optical transmission in access networks.