10G-EPON vs. XGS-PON: Are They Really All That Different?
The U.S. government is investing billions of dollars in broadband infrastructure. That funding will predominately be used to deploy fiber-to-the-premises (FTTP) networks using passive optical network (PON) technology. Most U.S. broadband providers are focused on deploying a 10G variant of PON technology, and two types are widely available in the market today: 10G-EPON and XGS-PON. The similarities between these two solutions far outnumber the differences. Most importantly, both technologies support the same peak capacities in the upstream and downstream, and both are equally capable of supporting symmetrical, multi-gigabit services to the household or business.
A Little PON Technology History
The history of PON reaches back to British Telecom research in the 1980s. The first PON standard, APON, was published in 1998 by the ITU-T. The specification for Gigabit PON (GPON) was published in 2003 and defines a PON that operates 2.5 Gbps downstream and 1.25 Gbps upstream. Following closely, the IEEE published its first PON standard, 1G-EPON, in 2004. This standard operates at 1 Gbps downstream and 1 Gbps upstream and was the most widely deployed PON standard until the mid-2010s.
In 2010, the IEEE became the first to publish a 10G PON standard, officially named 10G-EPON, that operates at 10 Gbps downstream and 10 Gbps upstream. Network operators began deploying 10G-EPON as early as 2012, and it is still being widely deployed today. In 2016, six years after the 10G-EPON standard was published, the ITU-T published XGS-PON, which is also capable of symmetrical 10 Gbps operation. Operators that aligned with ITU-T began deploying XGS-PON in approximately 2018.
Digging Into the Details
PON, including 10G-EPON and XGS-PON, is a system for transmitting data over a shared fiber-optic point-to-multipoint network. At the root of this network is the Optical Line Terminal (OLT). From the OLT, a single fiber extends to a splitter, which joins this single fiber to multiple fibers that extend toward the end-user. Splitters can be cascaded to create an optical network that ultimately connects to a network terminal at the customer premises. This terminal is the Optical Network Unit (ONU). This device might also be referred to as an Optical Network Terminal (ONT), which is functionally the same as an ONU.
In both 10G-EPON and XGS-PON, a scheme called Time Division Multiple Access (TDMA) is used to share the fiber’s upstream resource. In this scheme, each user (the ONU at the user’s premises) is granted a share of time during which they are allowed to transmit data. At all other times, the user is “silent” while other users transmit data.
The downstream resource has only one transmitter: the OLT. By nature of the optical splitting topology, all users receive all data that is transmitted downstream. Each ONU is expected to ignore data that is not addressed to it, but both 10G-EPON and XGS-PON take the extra step of encrypting traffic to prevent nefarious actors from accessing other users’ data.
Frame Transmission by Traffic Direction
Both 10G-EPON and XGS-PON use error correction schemes to achieve a wide split ratio and 10 Gbps transmission over long distances (approximately 20km). Error correction works by sending additional data, called parity, on top of the user’s data as it is being transmitted. This extra data is one form of overhead that consumes a portion of the total available capacity of the PON. Overhead results in a reduction of the usable capacity for both 10G-EPON and XGS-PON. In the end, the usable capacity for 10G-EPON and XGS-PON is approximately 8.8 Gbps.
Both standards require a minimum split ratio that supports 64 users (1:64). Split ratio can be difficult to understand because it’s a value that is determined by the operator’s network design criteria and the specific optical transceivers chosen by the operator. Some will point out that XGS-PON requires a minimum 1:256 split ratio, but this is only a recommendation and is not a requirement. Neither 10G-EPON nor XGS-PON places a maximum on the split ratio, and both standards define multiple options for the optical transceiver performance. XGS-PON and 10G-EPON are both very flexible and the operator is free to design the network to fit its specific needs including split ratios that exceed 1:64.
The native frame format used by 10G-EPON is Ethernet, which is used to carry the Internet Protocol in the vast majority of networks. It seems obvious that 10G-EPON would be capable of carrying user data in Ethernet frames, and indeed it is. Although XGS-PON’s native framing is not Ethernet, XGS-PON is equally capable of carrying users’ data in Ethernet frames and does so with a trivial amount of additional overhead compared with 10G-EPON.
Considering all these similarities, we might ask, “What’s the difference between 10G-EPON and XGS-PON?” For an engineer, it is easy to dig in and find that there are many differences between these two PON standards. However, in the end, an operator chooses one over the other based on two major factors: the network’s legacy and support for the standard in the backend systems.
PON isn’t a new technology; it has been deployed around the globe for over 20 years. The technology has a legacy of poor interoperability over that time, starting with ITU-T PON. Poor interoperability means that an ONU from one vendor will not function correctly when connected to an OLT from another vendor, so both must be sourced from the same vendor. Similarly, systems based on ITU-T PON typically have vendor-specific interfaces to the operator’s network management and provisioning systems. Industry bodies like the Broadband Forum are taking steps to improve interoperability, but this history of poor interoperability is difficult to shake off. If an operator has an existing PON network based on ITU-T standards, then that operator is most likely to adopt XGS-PON.
The DOCSIS® Provisioning of EPON (DPoE) solution from CableLabs is a series of specifications that define interoperability between OLT and ONU and interoperability with cable operators’ network management and provisioning systems. Further, CableLabs created a DPoE certification program that makes sure ONUs and OLTs conform to the specifications and are interoperable among vendors. These factors are the reason some cable operators still prefer 10G-EPON over XGS-PON.
|Ratified in 2016
|Ratified in 2010
|At least 1:64
|At least 1:64
|Usable Data Rate
|Approx. 8.8 Gbps
|Approx. 8.8 Gbps
|Low but improving
|Transports Ethernet and IP
Comparison of XGS-PON vs. 10G-EPON
Future-Proof PON Technology
The similarities between 10G-EPON and XGS-PON far outnumber the differences. Both technologies support the same usable capacity in the upstream and downstream and therefore are equally capable of supporting symmetrical, multi-gigabit services to the household or business. Market data demonstrates that the cost of 10G-EPON and XGS-PON devices is comparable. This is the case because 10G-EPON and XGS-PON ONUs are built using the same underlying hardware components, including identical silicon, and the software determines the mode of operation.
Today, the majority of PON deployments are of the 10 Gbps variety — either 10G-EPON or XGS-PON. Marketeers are trying to cast one or the other of these as better, but in reality, these two technologies are equal in their ability to meet and exceed broadband service requirements today and well into the future.