Passive Optical Nework An Introduction

White Paper Northforge Innovations Inc.

December 2017

1 Introduction

Internet users are like race drivers, they always want to go faster and drive faster machines. Faster performance and speed requires higher bandwidth. Once you taste that high bandwidth at the office, you naturally want to have that performance wherever you are, especially when you’re connecting to the web at home. Northforge Innovations sees Passive Optical Networks (PON) as a key way to help deliver high bandwidth and to advance the network communications of its customers. PON requires a lower building cost relative to active optical net- works along with lower maintenance costs. Because there are fewer moving or electrical parts, there’s simply less that can go wrong in a PON.

Fiber For Everyone

There was a time when high bandwidth network connections were limited to businesses and primarily to big businesses. Today, however, that’s not the case. Today, everybody needs high bandwidth.

Businesses still need high bandwidth to transfer files, databases, and virtual machines, between locations, and especially for newer applications such as video conferencing. Most personal/residential applications are still low bandwidth—mail, web, messaging, etc. But over the past several years, streaming video and gaming have become huge drivers of bandwidth—as much as 70% of all Internet bandwidth during primetime!1. And these new services not only require higher bandwidth, but they need more consistent bandwidth, a better quality of service.

Delivering that much bandwidth to residences is a major challenge for service providers. In an area with hundreds or thousands of businesses, there are tens of thousands of residences, or more. Also, residences are far more cost-sensitive than businesses, increasing the challenge. Fiber optics has been the solution for high bandwidth business connections for quite a while. Office parks and buildings have had direct fiber access for years, but running fiber to that many residences in a cost-effective way is a challenge.

Ten years ago most homes got their TV service either over the air, from satellite, or through . Vid- eo-on-demand, if it was available at all, was usually delivered in the same way. Phone service was delivered on from the central office. Internet service came over coaxial cable or via telephone lines (DSL).

Fiber can deliver all of these services and has many benefits over copper. For a given amount of bandwidth, fiber can carry a signal much farther than copper—copper can transfer one gigabit per second a few kilometers and

1 https://www.recode.net/2015/12/7/11621218/streaming-video-now-accounts-for-70-percent-of--usage

Fiber vs. Copper, the difference is bigger than you think! The difference between copper and fiber in terms of the distance that a signal can be carried is profound. As a signal progresses down a channel from its source, it gets weaker and weaker. There is a minimum signal strength that a receiver can detect and correctly interpret—if the signal reaching the receiver is too weak then there is no reliable communication.

We can compare the distance that weakens the signal by half. Note that the rate that the signal degrades goes up with the frequency of the signal. On RG6 (modern TV coaxial cable), for a 216MHz signal (US channel 13), the signal degrades by half every 22 meters. At 950MHz (common frequency for satellite dish interfaces), the signal degrades by half every 10 meters. Contrast these distances with fiber. For standard single-mode fiber at 1310nm (standard SONET/SDH), the signal degrades by half every 6000 meters!

2 fiber can transfer one gigabit per second over 100km (see inset). Also, fiber can carry much more bandwidth than Optical Fibers Metallic Cables copper (with multiple wavelengths, a single fiber pair can carry a terabit per second these days). Fiber is more secure than copper—it is harder to FTTN surreptitiously tap into fiber than copper. Also, signals 1000ft. (300m) on copper can be corrupted by electro-magnetic interference—e.g., motors and lightening—but fiber is immune.

As a result there has been a steady movement towards fiber over the past 10 to 15 years. The goal has been to FTTC push fiber closer and closer to the end user. Fiber to the 1000ft. (300m) premises/home (FTTP/FTTH) is the goal. This has clearly been achieved with many offerings deployed around the world (Verizon FiOS being the most widely deployed example of FTTP in the United States), but there are a variety of technologies that have been deployed that FTT have been moving fiber closer to the end user. This diagram2 shows this evolution.

With FTTNode, fiber is run to a node in a neighborhood or campus and then the data is transferred to copper that is run to commercial buildings and homes. This approach FTTP is most common in cable TV deployments where it is called Hybrid Fiber Coax (HFC), which is discussed below. (Also, AT&T’s U-Verse is commonly implemented as FTTN.) FTTCurb is a variant of FTTN where the node is pushed even closer to the end user. The benefit is that the copper run is shorter and therefore more bandwidth can be delivered. Fiber to the Building (FTTB) is an approach taken with some multi-unit dwellings such as apartment buildings. A fiber node is placed in the basement and copper “risers” are run to each unit.

But the ultimate goal is FTTP, running the fiber right to the end user.

Making Fiber Cost Effective

Using fiber is more expensive than copper. The fiber cable is more expensive than copper cable, the fiber transmitters and receivers are more expensive than copper transmitters and receivers, etc. There are long term savings to be had by combining all services on a single channel and by having a channel that can provide increased bandwidth as needed (e.g., by adding wavelengths), but given the cost sensitivity, an architecture is needed that minimizes the initial investment as well as minimizing long-term maintenance.

Traditional phone service, POTS3, is one of the only technologies that runs separate communications cables from the service provider to each endpoint. With POTS, each drop (run to the endpoint) is just two pair of rela- tively inexpensive 22 or 24AWG copper, packaged in large bundles of 25 or 50 pairs. This can be done because the bandwidth requirements are low (equivalent to kbps) for analog phone service. Adding DSL which transfers

2 https://en.wikipedia.org/wiki/Fiber_to_the_x 3 POTS is the common name for Plain Old Telephone Service.

3 Coaxial Cable Mux 500-2000 Homes

Fiber Fiber Mux Ring Fiber Node Fiber

Cable Head Mux End

Fiber Fiber Node

Mbps on the voice-grade line is more challenging, but the best DSL performance can only be achieved over a few kilometers. With cable TV, it is not realistic (economically or physically) to run a separate cable (coax or fiber) from the service provider’s office to each residence. And this is true of other high performance broadband solutions. Cable providers addressed this problem by (initially) running coax from the headend (video central office) to a hub in each neighborhood and then running “drops” to individual users. As bandwidth requirements increased (due to Internet, video on demand, and additional channels), the run from the headend to the neighborhood was changed to fiber, which resulted in the current hybrid fiber/coax (HFC) distribution architecture used by cable providers. Fiber runs from the headend to a Fiber Node in each neighborhood where the optical signals are converted to electric signals and then distributed to several hundred to a few thousand premises. This could be FTTN, FTTC, or even FTTB. The Fiber Node can be reached either by directly-connected fiber or via a fiber ring that covers a regional or metropolitan area and connects potentially many Fiber Nodes4.

The Fiber Node has electronics to receive and convert the optical signal, to drive the coaxial cable, and to transmit optical signals back to the headend for Internet traffic and other control traffic. This means that it must be powered and it also means that it must be maintained—there are components that can fail over time. Also, since the signal degrades quickly over coaxial cable (see inset above), amplifiers may be required for longer runs from the fiber node.

In theory, HFC could be modified to provide FTTP. The Fiber Node could distribute the data from the head end (uplink) to a number of fiber optic downlinks/drops at layer 1 (repeater/transponder) or it could also be done at layer 2 ( bridge) if everything were being delivered as Ethernet. This would be a more expensive solution. Not only does it need electronics, but instead of a single fiber (back to the headend) it would also need fiber for each of the drops. In addition to the actual capital cost, this “active” fiber node would require a substantial amount of power and long term maintenance.

Passive Optical Networks (PON)

The solution is to employ smoke and mirrors—well, just the mirrors. Rather than using “active” electronics to transfer information from the central office to each of the active drops, it is possible to split the optical signal using a passive (i.e., unpowered), device very much like a coaxial cable splitter is used to fan out a coaxial cable. The passive optical splitter (POS) splits the incoming signal into 2, 4, 8, 16, 32 or even more separate downlinks, a point-to-multipoint solution. Each downlink is an exact copy of the uplink but has lower power (see inset).

4 The “Mux” element on the ring in the diagram could be a SONET/SDH Add-Drop Multiplexer (ADM) if the ring were a SONET/SDH ring, it could be an Ethernet Bridge if the traffic consisted entirely of packet data, or it could be a ROADM (reconfigurable optical ADM) if wavelengths were being split off for each subtending device.

4 Split 2. 4. 8. 16. 32 C A D C B A ONU Frames D C B A POS Central Office B D C B A ONU O LT

S P N L S P N L POS S P N L S P N L POS L S P N L ONU

Since each downlink is a copy of the uplink (this would also be true of the active solution at layer 1 using a multi- port repeater), all of the traffic is delivered to every premises. Steps must be taken to ensure that users don’t eavesdrop on other users. This could include encryption or just filtering in the termination device (called the ONU or ONT) at the user location.

In the diagram, the device at the central office (called Optical Line Termination or OTL) has two fiber runs going to two Passive Optical Splitters (POS). The top splitter might split the signal into 32 drops. All streams see the same traffic and each ONU selects the traffic destined for it. The bottom splitter might only split the signal into 16 streams, but each of those could go through another 1:2 splitter resulting, also, in 32 drops.

Upstream traffic is a bit trickier (this is also true of HFC). Since the downstream network is point-to-multipoint, the upstream is multipoint-to-point. Multiple users are all trying to send traffic upstream and they are all multiplexed onto the same transmission channel. It is necessary to ensure that they don’t step on each other. This is done by having the OLT assign transmission opportunities to each of the ONUs. This chopping up of the upstream bandwidth is why many PON implementations have lower upstream bandwidth than downstream, a limitation that has been overcome in some implementations by allocating more wavelengths to upstream bandwidth.

Passive optical distribution is much less expensive than an active fiber solution. The Optical Distribution Nodes (splitters) have no electronics and require no power so they are less expensive to operate and maintain, and they don’t require optical transceivers since they are just physically transferring some of the optical signal from the uplink to the downlink. 1:32 splitters are common and depending on distance, a single fiber could drive more than 32 end points either by using a bigger split (e.g. 1:64) or possibly by splitting each downlink again (e.g., 1:2).

Passive Splitters When a passive (unpowered) device is used to split an electrical or optical signal, the resulting split signal is an exact copy of the incoming signal, but the amount of power in each of “n” tributaries is a little bit less than 1/n of the incoming power. This makes sense. If the incoming signal has 1W of power and the splitter doesn’t add power, then the sum of the outgoing signals can’t be more than 1W and is actually a bit less since there is always some loss in the splitter. This is true of cable TV splitters and of Passive Optical Splitters. Note that there are electrical and optical splitters that allow uneven split of the power, but the sum is still a little less than the incoming power.

This explains the evolution of pushing the splitter closer and closer to the end user. The farther you push the fiber, the lower the attenuation (compared to copper), and the more power that is available to be split into the downlinks which means that there can be more downlinks.

5 The Flavors of PON

Deployment of Passive Optical Networks started in the late 1990s. The original PON implementations were based on ATM (cell) connections and were called APON (ATM PON). This was standardized by ITU-T and called BPON (Broadband PON). Since then, the ITU-T has developed (and continues to develop) standards pushing PON technology forward. The Full Service Access Network5 (FSAN) forum has been instrumental in driving adoption of new ITU PON technology.

In 2004, the IEEE Ethernet standards group developed an enhancement to the IEEE 802.3 Ethernet standard, called Ethernet in the First Mile or EFM (802.3ah). EFM included the definition of two PON Ethernet interfaces ().

The result is that there are two parallel tracks for PON development, one in the ITU-T and the other in IEEE. The following table summarizes the major steps in these develop tracks.

Name Year Bandwidth Bandwidth Standard Notes (approx.) Down Up ITU-T based PON APON late 1990s 622M 155M pre- Based on ATM standard BPON 2001 622M 155M G.983.3 Standardized version of APON GPON 2009 2.488G 1.244G G.984 Gigabit version of BPON. Kept ATM encapsulation and added new packet encapsulation called GEM (GPON Encapsulation Method). 10G-PON / XG-PON1 2010 10G 2.5G G.987 10G version of GPON XGS-PON / XG-PON2 2016 10G 10G G.9807 Symmetric version of XG- PON1 NG-PON2 2015 40G (≤ 10 10G G.989 Uses 4 asymmetric per sub) wavelengths (10G/2.5G) NGPON2 and XGS-PON can co-exist with GPON (i.e., operate in the same Optical Distribution Network). They use different wavelengths. By installing a wavelength splitter in front of the Passive Optical Splitter, both the old and new formats can be used. This is important because it allows the network to upgraded and newer endpoints to be installed without replacing the installed base. IEEE based PON EPON / GEPON 2004 1G 1G 802.3 (ah) Based on Ethernet Framing. Part of the Ethernet in the First Mile (EFM) initiative. 10G-EPON 2009 10G 10G 802.3 (av) 10G version of EPON

5 http://www.fsan.org

6 Ethernet PON solutions are less expensive than the older B/GPON solutions but since they require completely different equipment, they are being used in new PON deployments but old ones are not usually retrofitted.

ITU-based PON (GPON and later) is widely deployed in the US because it is the technology that Verizon’s FiOS service is built on. However, worldwide, IEEE-based PON has prevailed. In 2014, there were over 40 million installed EPON ports, making it the most widely deployed PON technology globally6. This includes 10G-EPON which has been deployed in several venues7.

First Whitepaper in a series on PON

In this first of three white papers on PON, we’ve reviewed the basic framework of how passive optical networks can be a solution for service providers needing to deliver higher bandwidth. Our second white paper will go deeper into the components and the architecture of traditional PON networks, and outline recent improvements. In the final white paper on this topic, we’ll look ahead into the near future and discuss how next generation PON architectures will further advance network communications.

6 http://en.wikipedia.org/wiki/Passive_optical_network 7 Example deployment: https://www.fibre-systems.com/news/starman-and-nokia-deploy-first-nationwide-10g-epon-europe

7 About Northforge Innovations Inc.

Northforge Innovations is an expert software consulting and development company focused on advancing network communications. We target network security, network infrastructure, and media services, with the mission and passion to meet the industry’s demands in the evolving cloud infrastructure, virtualization and software-defined networking.

With an average of 15 years of experience, our consultants comprise a worldwide resource pool that’s based in North America. Northforge employs top technical and project management talent to give customers the “intellectual capital” they need for their network communications software development. Our developers have extensive technical and domain expertise across a breadth of technologies. With expertise extending beyond software development services, our team tackles our customer’s most demanding challenges and delivers innovative solutions. Our culture stresses innovation at every step, from our ability to understand and address our customers’ needs, our constant exchange of innovative ideas to the continuous value that we create for our customers.

For more information about Northforge Innovations Inc., please visit www.gonorthforge.com.

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