“I don’t want to have to share a pipe. The problem with ‘cable’ is shared pipes. If my neighbor is doing a bunch of stuff over the network, I get impacted too. With fiber I get speed and no shared pipes.”

--- Entrepreneur in a focus group

The notion that subscribers connected to residential fiber networks do not “share pipes” is often misunderstood. For residential fiber networks, sharing pipes is one of the main reasons fiber to the home (FTTH) is even remotely cost-effective for service providers to deploy. But what is most surprising is the following: deploying shared network solutions has led to a more rapid increase in residential broadband speeds than otherwise would have been the case with non-shared access network solutions. I like to call this the Point-to-Multipoint Effect. In the process, sharing pipes has allowed broadband speed growth to surpass the predicted 50% compounded annual growth rate commonly known as Nielsen’s Law of Internet Bandwidth. Read on to learn more…

First, a couple of definitions:

• A (non-shared) point-to-point (P2P) network topology is one in which there is a single dedicated connection between two endpoints. In the case of access networks, one endpoint is typically located at the hub or central office, or could be located at a remote distribution point. The other endpoint is a digital subscriber line (DSL) modem, for example, or a simple Ethernet switch, located on the customer premise. In P2P networks, the peak capacity of a link is used exclusively by only the two
• A (shared) point-to-multipoint (P2MP) network topology is one in which there is a single downstream transmitter and multiple access termination devices that all selectively listen to the same downstream data stream. A key characteristic with P2MP networks is the peak capacity of the network is shared between all connected endpoints. Two examples of P2MP networks are HFC and passive optical networking (PON), shown in the figure below (showing downstream transmission).

Two examples of (shared) point-to-multipoint networks: HFC and PON

The PON solution represents the most prevalent residential fiber solution in the world, primarily due to lower costs compared to P2P fiber solutions. To illustrate the sharing, referring to the diagram above, if 10G-EPON is the technology choice, each optical network unit (ONU) connected to the network transmits upstream at ~10 Gbps, but they don’t transmit simultaneously. Instead, an ONU must be scheduled by the OLT for upstream transmission to avoid collisions with other ONUs. In essence, the scheduling of ONUs results in the sharing of the 10 Gbps peak capacity.  Consequently, there is a whole lotta pipe sharing going on in PON solutions.

Do shared networks necessarily perform better or worse than non-shared networks? It depends on how performance is measured, but in one key area, residential broadband speeds, shared networks have significantly outperformed non-shared networks by a substantial amount.

A recent blog discussed Nielsen’s Law of Internet Bandwidth and how the cable industry was preparing to meet future broadband speeds with 100G-EPON. When Mr. Nielsen made his initial prediction in 1998, residential broadband access was dominated by dialup and ISDN connections, which are both P2P solutions. Indeed, for approximately the first 14 years since that initial 300 bits per second dialup connection in 1982/1983, the progression of available peak service tier bit rates followed the 50% annual growth rate prediction.

**various sources compiled by CableLabs

The release of the first DOCSIS® specifications by CableLabs in 1996 essentially represented the dawn of P2MP solutions, i.e. shared, for residential Internet connectivity. According to the data in the chart above, the tremendous rate of technology advancements resulting from the shared DOCSIS/HFC network solution, and later with the development of shared PON technologies, coupled with the relative cost-effectiveness of these solutions, has far exceeded other P2P technologies for residential broadband. While the initial growth prediction in 1998 was a 50% annual growth rate, the Point-to-Multipoint Effect increased the growth rate closer to 70% for residential Internet connectivity. The Point-to-Multipoint Effect indicates that sharing pipes for residential connectivity has provided a solution that has actually allowed residential high speed data rates to increase at a faster pace! This “sharing” trend is expected to continue with the development of Full Duplex DOCSIS and 100G-EPON, making the introduction of new services possible.  Thus, just like our parents always told us, it is good to share.

In his role as Vice President Wired Technologies at CableLabs, Curtis Knittle leads the activities which focus on cable operator integration of optical technologies in access networks. Curtis is also Chair of the 100G-EPON (IEEE 802.3ca) Task Force.

The telecommunications industry typically uses Nielsen’s Law of Internet Bandwidth to represent historical broadband Internet speeds and to forecast future broadband Internet speeds. Mr. Nielsen predicted many years ago the high-end user’s downstream connection speed grows by approximately 50% compound annual growth rate (CAGR). In reality, actual peak service tiers offered by service providers over the years may be following something closer to 60% compound annual growth rate, as shown in the figure below.

The point of this blog is not to debate whether the growth rate is 50% or 60%, but rather if the growth rate continues, how do we evolve our networks to keep pace?

For point-to-multipoint networks there is a general rule of thumb for determining the peak service tier given a particular peak network capacity. This capacity-to-peak-tier ratio of 2:1 isn’t necessarily based in scientific fact, but comes from years of experience that a 2:1 ratio allows service providers to have a reasonable level of confidence that speed test measurements will accurately reflect a user’s subscription level. For example, for a particular access network technology, if the network supports 2 Gbps transmission rates to/from the access termination device (i.e., a cable modem) then the peak service tier typically won’t exceed 1 Gbps.

The present state of the art access network technology peaks at 10 Gbps^[1]. The IEEE 802.3 10 Gbps Ethernet Passive Optical Network (10G-EPON) has been deployed in China and the United States. ITU-T has recently consented XGS-PON, another 10 Gbps symmetric PON standard that uses the physical layer of XG-PON (ITU-T G.987.2) and 10G-EPON. Even the ITU-T’s NG-PON2 standard, which uses multiple wavelengths to increase network capacity, only defines a single wavelength per optical network unit (ONU), which puts NG-PON2 on par with 10G-EPON and XGS-PON in terms of meeting peak service tier rates. Finally, CableLabs is now certifying DOCSIS 3.1 devices which are capable of 10 Gbps downstream, and soon will certify 10 Gbps symmetric devices based on Full Duplex DOCSIS technology. What does this mean for accommodating Nielsen’s Law? Assuming the peak service tier trends continue, and given the 10 Gbps peak network capacity of current solutions, the maximum peak service tier will level off at approximately 5 Gbps (see red dashed line in chart above) until technology advances to allow higher rates. The telecommunications industry needs a technology roadmap beyond the current state of the art which allows for peak service tiers to exceed 5 Gbps.

CableLabs and its members, along with other service providers and the IEEE, are determined to stay ahead of the trends displayed in the graph above by contributing to the world’s first 100 Gbps EPON solution as part of the IEEE 802.3ca Task Force. The prevailing sentiment of the 802.3ca Task Force is to create a generational standard that allows for growth of peak network capacity (and corresponding peak service tiers) if and when such growth becomes necessary, without creating a new standard. This growth is expected to be achieved through defining four wavelengths, with each wavelength supporting 25 Gbps. Initial product developments will revolve around a single wavelength to provide a 25 Gbps EPON solution. When market conditions demand it, using two wavelengths along with a channel bonding solution will allow an ONU to transmit and receive at up to 50 Gbps. Similarly, with four wavelengths and channel bonding the ONU will transmit and receive at up to 100 Gbps. Examining the chart above, and assuming historical trends continue, the reader can see the 100G-EPON standard will support peak service tiers out to approximately 2030, give or take a couple years, assuming the 50% CAGR predicted by Nielsen continues.

One of the interesting facets of the 802.3ca Task Force activities relates to the improvement in efficiencies in the media access control (MAC). Previously, the IEEE 802.3 standard did not allow frame fragmentation, but recently with the completion of the IEEE 802.3br Interspersing Express Traffic Task Force, frame fragmentation is now allowed in networks based on the 802.3 standard. The 802.3ca Task Force plans to leverage fragmentation to make transmission more efficient in a multi-wavelength, channel-bonded environment. Additionally, contributions to the 802.3ca Task Force will improve the efficiency of the upstream bandwidth allocation process by allowing multiple service flow queue depth reporting and upstream granting in a single message pair. Considering the ITU-T SG15/Q2 is also investigating 25 Gbps per wavelength, the more promising and exciting aspect of these 802.3ca Task Force decisions is that the next generation of IEEE EPON and ITU-T GPON standards could be more closely aligned than ever before in the very near future! This makes a converged optical access solution closer to reality. (see a previous blog regarding a converged optical access initiative)

In his role as Vice President Wired Technologies at CableLabs, Curtis Knittle leads the activities which focus on cable operator integration of optical technologies in access networks. Curtis is also Chair of the IEEE 802.3ca Task Force.

[1] I’m ignoring point-to-point fiber solutions such as Active Ethernet because these are not typically residential solutions.

Service providers invest billions of dollars in their access networks. Ideally, the deployed technology meets consumer demand for many years, allowing service providers to avoid costly upgrades before fully recovering their investments. In addition to technology longevity, service providers also like to see technology evolution, a next generation, to borrow an overused technology term, to ensure future consumer demands can be met by staying within the same technology family. Nowhere is the next generation moniker more prevalent than in the development of passive optical networking (PON) standards.

Both the ITU-T and IEEE are creating next generation PON standards. The ITU has approved two documents from their G.989 series defining Next Generation PON 2 (NG-PON2), and a third document is currently undergoing final comments. The NG-PON2 architecture settled on a time and wavelength division multiplexing (TWDM) method which stacks four wavelengths in a coordinated manner onto a single fiber, with each wavelength delivering 10 Gb/s. Total NG-PON2 TWDM-PON capacity is therefore 40 Gb/s. Since the ONU only receives one wavelength, the bit rate received by a single ONU is capped at 10 Gb/s. In such a solution, the dynamic bandwidth allocation (DBA) would likely be independent for each wavelength. Multiple vendors have reported demonstrating the TWDM-PON solution. A four-wavelength TWDM-PON is illustrated in the figure below.

40 Gb/s Time and Wavelength Division Multiplexing (TWDM) PON.

Is a 100 Gb/s Solution on the Horizon?

The IEEE 802.3 Working Group has recently formed a Study Group to develop objectives for the next generation of Ethernet Passive Optical Networking (NG-EPON). Several key technology decisions await the NG-EPON Study Group: (1) number of wavelengths, (2) bit rate per wavelength, (3) transceiver tunability, and (4) channel bonding. Faced with the same consumer demands and industry competition as other access network technologies, the NG-EPON Study Group could relatively easily define a four-wavelength, 10 Gb/s-per-wavelength PON to put it on par with NG-PON2, shown above. However, alternative solutions are being investigated by NG-EPON participants that would take PON technology multiple steps beyond, and presumably allow consumer demand to be met for many years into the future. For example, using advanced modulation techniques to provide 25 Gb/s per wavelength, combined with four multiplexed wavelengths on a single fiber, could yield the first 100 Gb/s PON solution. By incorporating channel bonding, a concept popularized by CableLabs and the cable industry in the DOCSIS® 3.0 specifications, an NG-EPON ONU would be capable of receiving one or more wavelengths, potentially receiving 50 Gb/s or more. In a channel-bonded solution the DBA will closely coordinate upstream transmissions on one or more wavelengths simultaneously. Are the optical transceivers tunable? That is another of the many important technology decisions yet to be made. A channel-bonded, time-wavelength division multiplexed PON is shown in the diagram below.

Channel-bonded, time and wavelength division multiplexed 100 Gb/s PON.

With a keen eye on vendor implementation schedules and interoperability, the increased capacity of NG-EPON ONUs and OLTs could be staged according to a timeline that aligns with consumer demand and service provider requirements, alleviating the need to develop yet another next generation standard (next-next generation?).

These technology decisions will begin to take shape within the NG-EPON Study Group (and subsequent Task Force) starting at the September 2015 802.3 Interim meeting. Anyone with the desire to contribute is welcome to attend. Perhaps the ITU-T will also investigate these same approaches for NG-PON2, ultimately resulting in another step toward a converged optical access solution (See the OnePON blog regarding a converged optical access initiative).

In his role as Vice President Wired Technologies at CableLabs, Curtis Knittle leads the activities which focus on cable operator integration of optical technologies in access networks. Curtis is also Chair of the 100G-EPON (IEEE 802.3ca) Task Force.

APON, BPON, EPON, GEPON, GPON, G.epon, NGPON1, XGPON1, XGPON2, NGEPON, NGPON2, TWDM-PON, WDMPON – did I leave any out? I’m sure I did.

The alphabetical possibilities to represent different versions of passive optical networking (PON) technologies will soon be exhausted. I’m being overly dramatic, of course, but trying to make the point that all these different versions of PON, driven by membership in two somewhat competing standards development organizations, not only causes the eyes of network engineers to glaze over, but has contributed to confusion and indecision among service providers globally.

What is a PON?

In the strictest sense, a PON is an all-fiber network which consists of only passive optical components (specifically splitters and combiners), except at the endpoints of each fiber, where there is an electrically-powered termination device – either an optical line terminal (OLT) or optical network unit (ONU). The aforementioned PON technologies also assume a point-to-multipoint topology in which downstream transmission from an OLT is received by all ONUs, but upstream transmission by an ONU is received only by the OLT.

Admittedly, not all of the acronyms above represent actual PON standards, so here is an unofficial, very short description of the meaning of each acronym:

APON (ATM PON) – The first PON technology standardized by the ITU. APON used Asynchronous Transfer Mode (ATM) cells to transport data across the PON. APONs typically provided up to 622 Mbps downstream and up to 155 Mbps upstream.

BPON (Broadband PON) – ATM is known to be an inefficient transport technology, so APON was renamed to BPON to distance the technology from ATM, even though it still used ATM for transport. Supported bit rate also increased to 1.2 Mbps downstream and 622 Mbps upstream, but most (all?) were deployed at 622 Mbps downstream and 155 Mbps upstream.

EPON (Ethernet PON) – The first PON standard developed by IEEE which uses Ethernet frames to transport data across the PON.  1G EPON provides symmetric 1 Gbps links, while 10G EPON provides symmetric 10 Gbps links. There is also a variant that provides 10 Gbps downstream and 1 Gbps upstream.

GEPON (Gigabit Ethernet PON) – Many people use GEPON to refer to EPON. Others use GEPON to refer to an early, unstandardized version of 100 Mbps PON used in Japan.

GPON (Gigabit PON) – A non-ATM based PON standard from the ITU which uses GPON Encapsulation Method (GEM) to transport Ethernet frames, ATM cells, and native TDM data across the PON. GPON provides 2.5 Gbps downstream and 1.25 Gbps upstream. ATM and TDM are de-facto obsolete, and no GPON equipment supports it, resulting in a network solution which also transports only Ethernet frames – like EPON.

G.epon (EPON with a GPON flavor) – Not to be confused with GEPON, which existed first, G.epon is an ITU term used for EPON devices which can be configured using the GPON mode of ONU configuration called ONU Management Control Interface (OMCI).

NGPON1 (Next Generation GPON) – ITU name for a family of 10 Gbps PON standards (see XGPON1 and XGPON2) – this was the first “next generation” activity in the ITU.

XGPON1 / XGPON2 (10G GPON) – XGPON1 represents an ITU standardized versions of GPON capable of 10 Gbps downstream and 2.5 Gbps upstream. XGPON2 was supposed to represent symmetric 10G bps GPON, but it was never completed.

NGEPON – This is the name for the current IEEE activity to define an EPON standard beyond 10 Gbps – the next generation of EPON.

NGPON2 – This is the name for the current ITU activity to define a family of post-10 Gbps GPON solution(s) - the second “next generation” PON effort in the ITU. The technology of choice for this new standard uses Time and Wavelength Division multiplexing – the origin of the TWDM-PON acronym.

I’m sure I missed a few hyphens in all those acronyms, but I give up! It’s definitely clear as mud, right? Actually, it is very challenging to explain all the different PON standards and terminology. The bottom line is, service providers today are forced to choose between two PON solutions that do exactly the same thing – transport Ethernet frames from one side of an optical distribution network to the other side. Judging from the NGEPON (IEEE) and NGPON2 (ITU) activities, it looks like the madness will continue and the industry will have two next generation PON standards to choose from, much to the disadvantage of both service providers and vendors.

Learning from the Wireless Industry

Once upon a time in the wireless industry there were several competing standards simultaneously operating: GSM, North American TDMA, and Qualcomm’s CDMA. In addition, around the year 2000, WiMax made an entrance. These competing standards operated for anywhere between 8 and 12 years. Around 2006 the GSM/TDMA/CDMA trifecta finally converged to become what is today the LTE wireless standard.   The interesting aspect of this technology convergence is the affect on CPE dongle pricing, as shown in the graph below.

Clearly, a single technical solution drives prices downward much quicker by achieving truly global scale. This has been a huge benefit for wireless service providers. Also beneficial for wireless device vendors, they get to focus their precious and limited development dollars on a single technical solution for the entire wireless market.

OnePON™ Initiative

Clearly the global telecommunications industry does not need two different technologies to transport Ethernet frames across a PON. Today, the IEEE and the ITU are both working on yet another generation of PON standards which will continue the alphabet soup trend and continue to bifurcate the PON market. If the wireless industry may serve as an example, this will force service providers to unnecessarily pay higher prices, and device manufacturers to spend nearly twice the amount for product development if they wish to target the entire market.

The CableLabs OnePON™ Initiative is building toward creating a single PON solution that satisfies the requirements of the global service provider market. OnePON™ is not EPON, and it is not GPON. A third standardized PON solution is not a goal of the OnePON™ Initiative. On the contrary, the goal is to combine the best aspects of both IEEE and ITU solutions to create a single next generation solution.  Service providers will reap the benefits of lower capital expenses for network deployments. Vendors will be able to focus on a single technical solution for PON which addresses the global PON market, resulting in saving a tremendous amount of development costs (compared to two PON solutions), which will help translate directly to lower costs for service providers.

As Director of Optical Technologies at CableLabs, Curtis Knittle leads the activities which focus on standardizing EPON and GPON technologies for use in cable operator networks. On a daily basis he witnesses firsthand the challenges and increased costs associated with two similar PON technologies.