A Strategy for Upgrading GPON

I’ve been asked a lot during 2018 if fiber overbuilders ought to be considering the next generation of PON technology that might replace GPON. They hear about the newer technologies from vendors and the press. For example, Verizon announced a few months ago that they would begin introducing Calix NGPON2 into their fiber network next year. The company did a test using the technology recently in Tampa and achieved 8 Gbps speeds. AT&T has been evaluating the other alternate technology, XGS-PON, and may be introducing it into their network in 2019.

Before anybody invests a lot of money in a GPON network it’s a good idea to always ask if there are better alternatives – as should be done for every technology deployed in the network.

One thing to consider is how Verizon plans on using NGPON2. They view this as the least expensive way to deliver bandwidth to a 5G network that consists of multiple small cells mounted on poles. They like PON technology because it accommodates multiple end-points using a single last-mile fiber, meaning a less fiber-rich network than with other 10-gigabit technologies. Verizon also recently began the huge task of consolidating their numerous networks and PON gives them a way to consolidate multi-gigabit connections of all sorts onto a single platform.

Very few of my clients operate networks that have a huge number of 10-gigabit local end points. Anybody that does should consider Verizon’s decision because NGPON2 is an interesting and elegant solution for handling multiple large customer nodes while also reducing the quantity of lit fibers in the network.

Most clients I work with operate PON networks to serve a mix of residential and business customers. The first question I always ask them is if a new technology will solve an existing problem in their network. Is there anything that a new technology can do that GPON can’t do? Are my clients seeing congestion in neighborhood nodes that are overwhelming their GPON network?

Occasionally I’ve been told that they want to provide faster connections to a handful of customers for which the PON network is not sufficient – they might want to offer dedicated gigabit or larger connections to large businesses, cell sites or schools. We’ve always recommended that clients design networks with the capability of large Ethernet connections external to the PON network. There are numerous affordable technologies for delivering a 10-gigabit pipe directly to a customer with active Ethernet. It seems like overkill to consider upgrading the electronics to all customers to satisfy the need of a few large customers rather than overlaying a second technology into the network. We’ve always recommended that networks have some extra fiber pairs in every neighborhood exactly for this purpose.

I’ve not yet heard an ISP tell me that they are overloading a residential PON network due to customer data volumes. This is not surprising. GPON was introduced just over a decade ago, and at that time the big ISPs offered speeds in the range of 25 Mbps to customers. GPON delivers 2.4 gigabits to up to 32 homes and can easily support residential gigabit service. At the time of introduction GPON was at least a forty-times increase in customer capacity compared to DSL and cable modems – a gigantic leap forward in capability. It takes a long time for consumer household usage to grow to fill that much new capacity. The next biggest leap forward we’ve seen was the leap from dial-up to 1 Mbps DSL – a 17-times increase in capacity.

Even if somebody starts reaching capacity on a GPON there are some inexpensive upgrades that are far less expensive than upgrading to a new technology. A GPON network won’t reach capacity evenly and would see it in some neighborhood nodes first. The capacity in a neighborhood GPON node can easily be doubled by cutting the size of the node in half by splitting it to two PONs. I have one client that did the math and said that as long as they can buy GPON equipment they would upgrade by splitting a few times – from 32 to 16 homes and from 16 homes to 8 homes, and maybe even from 8 to 4 customers before they’d consider tearing out GPON for something new. Each such split doubles capacity and splitting nodes three times would be an 8-fold increase in capacity. If we continue on the path of seeing household bandwidth demand double every three years, then splitting nods twice would easily add more than another decade to the life of a PON network. In doing that math it’s important to understand that splitting a node actually more than doubles capacity because it also decreases the oversubscription factor for each customer on the node.

AT CCG we’ve always prided ourselves on being technology neutral and vendor neutral. We think network providers should use the technology that most affordably fits the needs of their end users. We rarely see a residential fiber network where GPON is not the clear winner from a cost and performance perspective. We have clients using numerous active Ethernet technologies that are aimed at serving large businesses or for long-haul transport. But we are always open-minded and would easily recommend NGPON2 or XGS-PON if it is the best solution. We just have not yet seen a network where the new technology is the clear winner.

A New PON Technology

ONTNow that many fiber competitors are providing gigabit Ethernet to a lot of customers we have started to stress the capability of the existing passive optical network (PON) technology. The most predominant type of PON network in place today is GPON (gigabit PON). This technology shares 2.5 gigabits of download data among up to 64 homes (although most providers put fewer customers on a PON).

My clients today tell me that their gigabit customers still don’t use much more data than other customers. I liken this to the time when the industry provided unlimited long distance to households and found out that, on the whole, those customers didn’t call a lot more than before. As long as you can’t tell a big difference in usage between a gigabit customer and a 100 Mbps customer, introducing gigabit speeds alone is not going to break a network.

But what does matter is that all customers, in aggregate, are demanding more downloads over time. Numerous studies have shown that the amount of total data demanded by an average household doubles about every three years. With that kind of exponential growth it won’t take long until almost any network will show stress. But added to the inexorable growth of data usage is a belief that, over time, customers with gigabit speeds are going to find applications that use that speed. When gigabit customers really start using gigabit capabilities the current PON technology will be quickly overstressed.

Several vendors have come out with a new PON technology that has been referred to as XGPON or NGPON1. This new technology increases the shared data stream to 10 gigabits. The primary trouble with this technology is that it is neither easily forward nor backward compatible. Upgrading to 10 gigabits means an outlay for new electronics for an only 4 times increase in bandwidth. I have a hard time recommending that a customer with GPON make a spendy upgrade for a technology that is only slightly better. It won’t take more than a decade until the exponential growth of customer demand catches up to this upgrade.

But there is another new alternative. Both Alcatel-Lucent and Huawei have come out with next generation PON technology which uses TWDM (time and wave division multiplexing) to insert multiple PONs onto the same fiber. The first generation of this technology creates four different light pathways using four different ‘colors’ of light. This is effectively the same as a 4-way node split in that it creates a separate PON for the customers assigned to a given color. Even if you had 64 customers on a PON this technology can instead provide four separate PONs of 16 customers. But with 32 customers this becomes an extremely friendly 8 customer per PON.

This new technology is being referred to as NGPON2. Probably the biggest benefit of the technology is that it doesn’t require a forced migration and upgrade to existing customers. Those customers can stay on the existing color while you migrate or add new customers to the new colors. But any existing customer that is moved onto a new PON color would need to have an upgraded ONT. The best feature of the new technology is that it provides a huge upgrade in bandwidth and can provide either 40 Gbps or 80 Gbps download per an existing PON.

This seems like a no brainer for any service provider who wants to offer gigabit as their only product. An all-gigabit network is going to create choke points in a traditional PON network, but as long as the backbone bandwidth to nodes is increased along with this upgrade it ought to handle gigabit customers seamlessly (when they actually start using their gigabit).

The big question is when does a current provider need to consider this kind of upgrade? I have numerous clients who provide 100 Mbps service on PON who are experiencing very little network contention. One strategy some of them are considering with GPON is to place gigabit customers on their own PON and limit the number of customers on each gigabit PON to a manageable number. With creative strategies like this it might be possible to keep GPON running comfortably for a long time. It’s interesting to see PON providers starting to seriously consider bandwidth management strategies. It’s something that the owners of HFC cable networks have had to do for a decade, and it seems that we are getting to the point where even fiber networks can feel stress from bandwidth growth.