When it comes to FTTx deployment, many carriers mistakenly assume that PON is the best or only game in town. This addresses some of the myths surrounding Active Ethernet and PON technologies and explains why Active Ethernet networks are becoming the preferred choice among many leading service providers.
Passive Optical Networks (PON)
A Passive Optical Network (PON) consists of an optical line terminator (OLT) located at the Central Office (CO) and a set of associated optical network terminals (ONT) to terminate the fiber – usually located at the customer’s premise. Both of these devices require power. PON gets its name because instead of using powered electronics in the outside plant, it instead uses passive splitters and couplers to divide up the bandwidth among the end users – typically 32 over a maximum distance of 10-20km. Because this is a shared network, it is sometimes referred to as Point to Multipoint or P2MP.
An Active network looks very similar to a PON, however, there are three main differences. First, instead of having passive, unmanageable splitters in the field, it uses environmentally hardened Ethernet electronics to provide fiber access aggregation. Second, instead of sharing bandwidth among multiple subscribers, each end user is provided a dedicated “pipe” that provides full bi-directional bandwidth. Because of its dedicated nature, this type of architecture is sometimes referred to as Point to Point (P2P). The third architectural difference between PON and Active is the distance limitation. In a PON network, the furthest subscriber must be within 10-20km from the CO, depending on the total number of splits (more splits = less distance). An Active network, on the other hand, has a distance limitation of 80km, regardless of the number of subscribers being served. The number of subscribers is limited only by the switches employed, and not by the infrastructure itself, as in the case of PON.
Fibre Network Myths
With the basics of these topologies understood, we can now explore the five most common myths surrounding PON vs. Active networks:
- PON Networks Make Better Use of Fiber
- Active Electronics In the Field Are a Liability
- PON Systems Don’t Require Set Top Boxes for Video
- PON Systems Provide Plenty of Bandwidth
- PON Has Dominant Market Share Over Active.
Myth 1 – PON Systems Make better Use of Fiber
The root of this misperception is one based in reality. However, it is no longer valid because of two technological developments that have occurred in the past year. The first of these developments is the completion of the IEEE 802.3ah Ethernet in the First Mile (EFM) standard that defines, among other things, a method for delivering Ethernet over a single strand of fiber. Before this standard emerged, Active Ethernet solutions required two strands of fiber to every subscriber (one to send, the other to receive). Years ago, when fiber was very expensive, one can understand what a costly proposition this was.
The second development has been the evolution of environmentally hardened Ethernet devices that can be placed in the outside plant. Prior to the availability of this type of gear, network operators would have to pull the fiber from every subscriber all the way back to their CO or else rely on Controlled Environment Vaults (CEVs) or other types of air conditioned/heated Remote Terminals (RTs).
The two of these developments, along with the fact that fiber costs have dropped to a fraction of what they were just a few years ago, make the question of which network uses less fiber virtually a non-issue.
Myth 2 – Active Electronics in the Field are a Liability
To assess where it makes most sense to place powered devices, one should understand the different theories of outside plant design. These designs, along with their pros and cons, are described below.
The promise of PON has been that you can push a single strand of fiber far out into the field and split it with a passive, unmanageable device close to the customer premises. Unfortunately, this approach has a number of drawbacks. One of the biggest disadvantages is that these splitters have no intelligence, and therefore cannot be managed. You cannot communicate with them remotely, and with hundreds or thousands of splitters scattered around in the field, driving to each one to check for problems when a service outage occurs becomes a very slow and a very expensive proposition.
Another major disadvantage to PON is its inflexibility. If a 1×4 splitter is used to serve four homes, hooking up a fifth customer requires pulling a new strand of fiber all the way from the upstream splitter, or re-designing the network to accommodate a larger splitter near the customer premises without violating the 32 split maximum allowed.
Unfortunately, changing any splitter in the network requires all downstream customers to come offline while the work is done.
A logical alternative to alleviate this problem is to under-utilize the capacity in the outside plant. In other words, instead of maxing out each PON port with 32 splits, only deploy 16 or 32 splits to allow room for growth. Important to remember, however, is that each PON port in the OLT carries a very high price tag since it is intended to be amortized across 32 customers. By not fully loading up that PON port, you increase the per-subscriber costs dramatically. The network operator, in this scenario, is forced to decide which is the lesser of two evils – low flexibility or inflated per-subscriber costs.
Finally, since PONs are shared networks, each subscriber becomes a homogenous member of the PON port they are connected to. Each subscriber gets the same bandwidth, each must receive software updates at the same time, and each must have the same ONT at the customer prem. This introduces significant challenges when businesses looking for higher bandwidth services are mixed in with these residential subscribers, when updating a new load of code, or when migrating to new technology.
Passive Star PON
A Passive Star architecture is designed to alleviate some of the flexibility challenges of a traditional PON topology. Instead of pushing the splitters all the way out to the customer premises location, they are pulled back and aggregated in a more centralized location, typically housed in a cabinet. This design helps drive more efficiency and lightens the burden of troubleshooting since the splitters are now more centralized.
But Passive Star is still subject to the inherent drawbacks of a PON network. One of these is the lack of diverse paths through the network. PONs, by their nature, subscribe
to tree-based topologies. Even if the splitters are pulled back to an aggregation cabinet, there is still only one physical path upstream, and that introduces a dangerous dependency on that link. Because these splitters have no intelligence, there is no ability to provide emergency fail over to a diverse path in the event of a link failure.
Another drawback is high first subscriber costs. As mentioned earlier, each PON port carries a high price because it is expected to be divided by 32 subscribers. Therefore, to activate that first subscriber, a significant Capex investment must be made to provide them service.
An Active Star architecture has one arguable drawback from a deployment perspective, and that’s the requirement for power in the outside plant. However, Active electronics in the field are nothing new. Telcos have been deploying DLC networks for decades that have powered electronics in the field. As the types of services evolve to include advanced, bandwidth intensive content like video, having intelligent devices at the edge of a network actually becomes an extremely significant advantage for the following reasons.
For video applications, intelligence at the edge of a network allows multicast streams to be replicated for downstream delivery using IGMP. This means regardless of how many people downstream of the switch are watching the same channel, only one stream is pulled down from the head end. That multicast stream is then replicated in the Access switch and sent to the subscribers. This not only speeds up channel change times, but it also makes more efficient use of your network backbone.
Another benefit to having Active electronics in the field is resiliency. By ringing these nodes together and choosing a vendor that supports Ethernet Protection Switching Rings (EPSR), true carrier-class resiliency is introduced which provides sub-50ms failover in the event of a link failure. For a video customer, it means a split second of picture tiling in the worst case, and for a voice customer, it means the call is not dropped.
Other advantages, of course, include full management and troubleshooting capabilities, high flexibility for deploying different services to residential and business customers, and low first subscriber costs. When striving for “Five Nines” reliability and maximum flexibility in a FTTP network, one can quickly see how venerable a PON network is to link failure, and how deploying Active electronics in the field actually becomes an asset instead of a liability.
Myth 3 – PON Systems Don’t require set To Box for Video
PON systems are able to support a somewhat complex method for deploying video services that most closely resembles what MSO’s (ie. cable TV providers) do today. This method utilizes EDFA’s (Erbium-Doped Fiber Amplifiers) to send an RF signal over a separate wavelength on the fiber to deliver a signal to the customer prem. This is sometimes referred to as delivering video “out of band” since it is outside of the IP data stream.
If this signal contains analogue video when it reaches the customer premise, it can be delivered straight to the television without the need for a set top box – just like the basic service many people receive from cable TV providers today. The experience is exactly the same – the same video quality and the same mono sound.
If the signal contains digital video when it reaches the customer premises, however, a digital set top box must be introduced – again just as in a digital cable service offered today by the MSO’s. This set top box descrambles the digital video signal and delivers it to the TV.
Where it starts to get more complex is when advanced, interactive services are introduced like video on demand (VoD). Since an RF signal is a one-way communication, the IP stream must be utilized to send commands up to the head end. In order to do this, an RF adaptor must be added at the customer premises to demodulate the upstream set top box communications and translate them into IP packets. Once converted to IP, they are sent up through the IP path (or “in-band”) to the head end to make the specified request. The requested content is then sent back down over the RF path (out of band) using DWDM to that specific subscriber location.
Obviously, this requires a good bit of effort to provide a service that many would consider to be the same as what cable TV operators are already doing today. As with any “me too” service, this approach leaves very little opportunity to differentiate based on anything other than price.
The alternative, and what many consider to be the best way of delivering advanced video services, is using IP to deliver the content. This is sometimes called IP Video or Switched Digital Video. In this solution, instead of having a traditional digital set top box, an IP set top box is used to receive the IP packets, decode them, and provide audio and video output to the TV. The experience is a powerful, all-digital experience with full Dolby 5.1 sound, crisp, high quality video, and interactivity unmatched by any other service available today. Think of the interactive power of the Internet combined with high quality broadcast TV. The result is strong differentiation that allows service providers to compete on things other than just price. While IP Set Top Boxes carry a slightly higher price tag today than traditional digital set top boxes, when you add in the cost of the RF adaptor required at each customer premises, the Capex costs for both solutions are quite comparable.
From the network perspective, the most compelling advantage for deploying IP video is having a fully converged IP network to manage and maintain. IP has long been the protocol-of-choice for delivering data services, and in recent years we have seen the rapid emergence of Voice over IP (VoIP) as the preferred method for delivering voice services as well. Using IP for video as well allows a network operator to use the same infrastructure for all three services and realize significant Capex and Pox savings by standardizing on one network infrastructure instead of two as is required in an RF video deployment.
Myth 4 – PON Systems provide Plenty of Bandwidth
To determine how much bandwidth is enough, a service provider must evaluate what services they intend to offer over the life of the network. Since the capacity and longevity of a fiber infrastructure is virtually limitless, one must look out as far as possible to ensure the equipment that is deployed will handle the bandwidth needs for the foreseeable future.
Different technologies will offer different levels of bandwidth in the upstream and downstream directions. APON and BPON, for example, deliver 622Mbps in the downstream direction, so assuming it is split among 32 subscribers, it provides 19Mbps to each customer. In the upstream direction, it provides 155Mbps split 32 ways, resulting in under 5Mbps. EPON is a technology that provides 1Gbps in both the downstream and upstream directions, providing 30Mbps of bi-directional bandwidth to each subscriber. Finally, GPON, the newest standard for PON technology, provides 1.2Gbps in the downstream direction resulting in 38Mbps per subscriber assuming 32 splits, and 622Mbps in the upstream direction allowing 19Mbps per subscriber.
Active Ethernet, in comparison, provides dedicated bandwidth to each subscriber, which means there is no sharing of network traffic. Speeds most commonly found in the marketplace today are 100Mbps bi-directional to residential customers and even 1Gbps to business customers.
When you consider that Active Ethernet solutions, on average, cost less to deploy and maintain than comparable PON systems, the opportunity to secure more bandwidth for less investment is certainly a compelling proposition. But how much bandwidth is enough? All of these speeds are compelling by today’s DSL and Cable broadband standards, but deploying advanced IP video and other next generation services takes things to a completely new level.
Over their Active Ethernet FTTP network, they offer residential customers a service bundle of 200 digital channels with interactive TV services such as video on demand, voice service with a full package of features such as call waiting and various messaging options, and 10Mbps of high-speed internet for one low monthly price. To deliver these types of services, a network would require at least 22Mbps of bandwidth assuming 3 TV’s per house at 4Mbps per video stream plus the 10Mbps for high speed Internet.
Based on the bandwidth capabilities of PON-based architectures, it is clear this network would already be beyond the capabilities of BPON and pushing up against the capabilities of even the newest PON standard, GPON.
Now if we look at what is just over the horizon – HDTV – we can quickly see how a PON network will not scale to meet the requirements. This is before we even begin to consider how other applications will take off such as distance learning, video conferencing, smart home services, personal video recorders (eg. TiVo), etc. Each of these will carry their own bandwidth requirements that must be planned for.
Myth 5 – PON has Dominant market Share vs. Active
According to Render, Vanderslice & Associates, a noted research firm focused exclusively on the FTTP market, the number of actual deployments of FTTP networks shows a nearly equal split between PON and Active technologies at 48% and 46% respectively (Figure 9). When the RBOCs proclaimed they would deploy BPON in the “Super RFP,” many believed that PON would become the dominant technology choice for all FTTP deployments. When you stop to consider that their legacy infrastructure is ATM-based and that video is not a priority for them, however, it becomes clear that while it makes perfect sense for their needs, it most definitely will not be the best choice for all deployments.
For network operators interested in offering video as a key differentiator and are not locked into a legacy ATM infrastructure, on the other hand, most are realizing the value of deploying a fully converged IP network that is based on the most ubiquitous technology in the world – Ethernet.
Making the Active vs. PON Call
In the end, each network operator will make their decision of which technology to deploy based on their own unique circumstances. The purpose of this paper is not to suggest that one technology is the best for every situation. On the contrary, the intention of this paper is simply to make network operators aware that there is an attractive alternative to PON-based FTTP architectures that leverages the benefits of IP and Ethernet to deliver services that will provide compelling differentiation based on the unique experience it delivers to customers and not just on price.
(Wavelength Division Multiplexing-Passive Optical Network) is an innovative concept for access and backhaul networks. It uses multiple different wavelengths (WDM) over a physical point-to-multipoint fiber infrastructure that contains no active components (PON).
The use of different wavelengths allows for traffic separation within the same physical fiber. The result is a network that provides logical point-to-point connections over a physical point-to-multipoint network topology. WDM-PON allows operators to deliver high bandwidth to multiple endpoints over long distances. Most of the Fiber-to-the-Home deployments in recent years have been based on industry standard technologies such as Gigabit Ethernet Passive Optical Networks (GEPON) and Gigabit PON (GPON).
The success of these deployments has led to significant innovation in both system architecture and the components that are used to build these systems, and the next generation of passive optical networks will inevitably be far more advanced than what is typically deployed today. At the forefront of PON development there have been two separate approaches that appear to compete for next-generation systems: 10 Gbps PON (be it 10G EPON or 10G GPON), and WDM-PON. Each approach has its own advantages and its own issues, but the progress with both new technologies has accelerated in recent years.
While WDM-PON has already had early success in Korea, its adoption in other parts of the world has been slowed by relatively high costs compared to GEPON and GPON technologies. That seems to be changing as WDM-PON competes head-to-head with 10G PON and Point-to-Point systems for next-generation FTTH deployments.