Passive Optical Neworks Optical Distribution Network. White Paper. Northforge Innovations Inc.

Transcription

1 Passive Optical Neworks Optical Distribution Network White Paper Northforge Innovations Inc. February

2 Introduction Passive Optical Networks (PON) are broadly deployed to provide high-bandwidth Fiber to the Premises (FTTP). This is the second whitepaper in our series on Passive Optical Networks. In the first whitepaper we described what PON is, why it is needed, and some of the history of PON. In this whitepaper we dive deeper into the architecture of PON implementations and the characteristics of the Optical Distribution Network. We cover important design considerations, how cable operators are moving towards PON in order to improve their service, and some of the newer methods for implementing and deploying PON. Broadband Architectures PON follows the same basic architecture as other broadband delivery architectures such as DSL and Cable but as with all things in networking, the names of things are different. This is shown in the diagram. In all three cases there is a device at the service provider that terminates the physical connections and manages the overall transmission system. The three headend devices shown in the diagram (DSLAM, CMTS, ) share a few important attributes, primarily that they all provide access to the and other services to downstream endpoints, but they differ in a few ways both functionally and architecturally. The DSL Access Multiplexer (DSLAM) came first. The DSL line carries both voice (POTS) and data. The DSLAM basically modulates the data signal on top of the analog voice channel. Low-pass filters at both ends are used to split out the voice from the data. The Cable Modem Termination System (CMTS) and the Optical Line Termination () are similar in that they are responsible for multiplexing multiple services DSL Cable PON TV/ Video TV/ Video Transport/ Services CO/Head End Distribution Subscriber PSTN VoIP PSTN VoIP Twisted Pair onto the network but since they are working with a much higher bandwidth infrastructure, they can multiplex more services than DSL. Specifically they combine video and voice services (POTS and/or VoIP) as well as service. DSLAM Filter PSTN CMTS Fiber Fiber Fiber Node POS Coax Fiber Filter Cable Modem ONT DSL Modem 2

3 PON Optical Distribution Network The PON Optical Distribution Network is constructed as shown in the following diagram: 1:2 2 Premises ONU ONU ONU ONU ONU Video 1:4 1:8 8 Premises ONU ONU ONU ONU ONU Other Services The Optical Line Terminal,, is the headend. The is responsible for multiplexing services onto the fiber distribution network. Most s support multiple PON networks (two are shown in the diagram above). Each PON supports a number of PON subscribers. The subscribers can be reached through a single splitter (upper PON network) or through a sequence of splitters (lower PON network). A large head-end looks something like this. At the other end of the network is the Optical Network Unit, ONU. This device is often called the ONT or Optical Network Terminal which is the ITU name for a single tenant ONU. The ONU/ONT terminates a fiber link, extracts the various data streams, and presents one or more service interfaces to the user. A service interface can be Ethernet(s) for traffic, wired or wireless, POTS connection(s) for phone service, or a coaxial cable for video and possibly traffic. In the middle is the Passive Optical (POS). This is the device that connects to the PON link from the and splits it into multiple tributaries. The split is usually a power of 2, 1:2, 1:4, 1:8, 1:16, 1:2, 1:64. The POS does not require any power (hence the word passive ).

4 It s All About Power The design of optical networks, in general, is focused on power and this is true of PON networks also. Communications only happens if the signal reaches the destination with enough optical power that it can be correctly and reliably interpreted. A receive signal that has too little power results a lot of errors (and ironically, this is true if the power is too high, also). Power in an optical signal is reduced by three things: 1. Distance this is the biggest issue. The power drops by half roughly every 6000 meters. You often see this written as 0.5dB 1 per kilometer. 2. Connectors at each connector there is some loss as the signal is transferred from one piece of glass to another. This loss is roughly the same as about 600 meters of fiber (.2dB to.db).. Passive You see this referred to as insertion loss. An n-way splitter results in 1 n of the signal to each tributary and in addition there is a little bit of overhead. For a 1:2 splitter you get roughly 1 40 (-16dB) of the power and for a 1:64 splitter you get roughly 1 80 (-19dB) of the power. With this background we can begin to understand the important design considerations in PON networks. Let s say that the optical splitter is 20km from the. This represents a loss of about 10dB (above we noted 0.5dB per km). If we use a 1:2 splitter, we lose another 16dB so now we are 26dB down. We have to go through at least four connectors or about 1dB which brings us to -27dB. BPON and GPON allow for 28dB loss in the distribution network, so we are within that envelope (but just barely). A 1:64 splitter has db more loss and therefore could not be used with 20km of fiber. However, if the 1:64 splitter is just 10km from the, we get 5dB back, keeping it within the envelope. As a result, the standards 2 provide those two distances. At 10km the signal can be split to 64 premises and at 20km it can be split to 2 premises. This is shown in the following diagram. 10km 4X16=64 1:16 16 Premises ONU ONU ONU ONU ONU 1:4 4X8=2 1:8 8 Premises ONU ONU ONU ONU ONU 20km 1 A refresher: The db is a logarithmic representation of loss in a network component. It is 10xlog10 (powerout / powerin ) logarithm. Since the db is a logarithm, serial losses can just be added together. -db is a loss of 50%. -10dB is a loss of 90%. 2 ITU-T G section 9/Physical Reach: Physical reach is the maximum physical distance between the and the ONU/ONT. In GPON, two options are defined for the physical reach: 10 km and 20 km. The document also defines a maximum logical reach of 60 km. This is the maximum allowed distance if physical limitations (i.e., signal loss) weren t an issue. Also in IEEE 802. for 1G EPON there are two PON PHYs defined, 1000BASE-PX10 and 1000BASE-PX20 (and there are similar definitions for 10G EPON). 4

5 Upstream is much harder All of the ONUs are sharing the optical link from the to the POS. This is ok for downstream traffic since all of the traffic is replicated to all of the ONUs. The can transmit continuously (called Continuous Mode transmission). But upstream is harder. If all of the ONUs (or even some of them) transmit at the same time, their data will collide when it goes through the optical combiner (which is the opposite of the splitter). Therefore, transmission from ONUs cannot be continuous they must transmit in bursts (called Burst Mode transmission). But something must coordinate the bursts this is the job of the. The assigns transmission timeslots to each ONU. But each ONU is a different distance from the (they are all the same up to the splitter but different after the splitter) which means that the delay from each ONU is different. The difference is usually very small, resulting in nanoseconds of difference in delay, but the tolerances are tight. Let s say the tells ONU1 that it can transmit for 100µs starting at time X and it tells ONU2 that it can transmit for 100µs starting at time X+100µs. If ONU1 is a kilometer farther than ONU2 from the combiner, then it takes longer for its data to reach the combiner and it could be that the first few bits of ONU2 s transmission collide with the last bits of ONU1 s. Conversely if ONU1 is closer then there could be a few micro-seconds of dead time on the wire after ONU1 s transmission is complete, resulting in lower line usage and less total bandwidth capacity. In the left picture below, the three ONUs transmit based on their assigned timeslots and the data reaches the splitter in sequence. But with the same transmission parameters, if ONU1 is a little closer to the splitter, User 1 and User data collide as shown in the right picture. Upstream Traffic Upstream Traffic ONU1 User 1 ONU1 User ?x?x 2 2 ONU2 User 2 2 ONU2 User ONU User ONU User 1 1 Therefore, the must dynamically determine the delay to each ONU and assign transmission slots to the ONUs based on their distance so that collisions don t happen and upstream bandwidth is maximized. This process is called ranging. 5

6 What about Cable Companies? Compared to running fiber to the premises, cable companies are at a disadvantage because Hybrid Fiber-Coax (HFC) uses cable for the last mile. Cable can t deliver as much bandwidth as fiber, cable can t run as far as fiber (without amplifiers), and the conversion from fiber to cable is active, which means that the Optical Node is more expensive, requires more maintenance, and is powered (unlike a passive optical splitter). Cable systems use a technique called DOCSIS (Data Over Cable Service Interface Specification) developed by CableLabs to deliver access over cable. DOCSIS has been around for many years and has evolved over several versions to increase both functionality and bandwidth. DOCSIS defines how the carrier channels are allocated for downstream traffic, how the data is encoded, how the upstream traffic is multiplexed, etc. The cable companies are not sitting still in the face of the fiber threat. They have two approaches to providing an all-fiber solution, RF over Glass (RFoG) and DOCSIS Provisioning over EPON (DPoE). RFoG is the simpler approach in terms of maintaining existing infrastucture. It just transmits the RF signals (the standard CATV spectrum from about 50MHz up to 1GHz in 6MHz channels) in an optical wavelength using PON. The CMTS and backend devices don t have to change at all since they are designed to transmit on fiber in HFC. The distribution network has to change since the HFC Optical Node gets replaced with a passive optical splitter and the Cable Modem (coax) is replaced by an ONU. The ONU has a coaxial output for video so in-house wiring doesn t have to change. RFoG isn t an ideal solution, however. First, it is not designed to grow it captures the current state of the DOCSIS infrastructure. In addition, there are some technical issues which cause upstream bandwidth to be a bit less than the comparable HFC approach. Since it uses PON for optical distribution, RFoG does benefit from the operational benefits of passive distribution. The preferred alternative is DPoE. With DPoE, the CMTS is replaced with an, but the entire DOCSIS management infrastructure that sits behind the CMTS stays the same. The is special. It includes a virtual Cable Modem (vcm in the diagram), so the DOCSIS domain actually terminates in the. The virtual cable modem then, in effect, transmits its subscriber side through the EPON network to a special DPoE ONU which then provides the same subscriber interfaces (e.g., Ethernet, CATV, POTS) as the real cable modem. In effect, the cable modem functionality which is in a single device in the HFC network is spread across the network from the vcm to the ONU. DPoE is a more forward looking solution and allows the cable companies to evolve their distribution network based on the evolution of standards-based Ethernet PON. The following diagram shows a standard HFC-based DOCSIS network and an analogous DPoEbased network. CMTS vcmts vcm DOCSIS IP HFC Network DOCSIS EPON Network Ethernet Cable Modem ONU Ethernet/RF IP Gateway Set Top Box IP Gateway Set Top Box 6

7 PON Provides A Way To Get Fiber Everywhere Clearly the goal is to continue to drive fiber to the premises. Bandwidth requirements continue to increase and subscribers want more bandwidth with good service quality. PON provides a cost-effective way to achieve this. But as both the bandwidth and the number of subscribers goes up, even PON has to improve and grow into new and more flexible architectures. In this whitepaper we reviewed the basic architecture of PON, described the important design constraints, and discussed some of the ways that these constraints are being addressed. We also described some of the ways that cable providers are addressing the technology threat posed by PON. In the final whitepaper in this series we will describe some of the next generation approaches to implementing PON networks in order to improve performance and reduce infrastructure cost. This includes both evolutionary approaches such as PON Extenders and Remote s and are practical extensions of the current architecture and revolutionary, new solutions that are re-architecting the Central Office such as VHA (virtual Hardware Abstraction) that are based on virtualized network functions and software control. Northforge Innovation has been working with a number of equipment vendors to develop software for various aspects of PON implementations, and Northforge s engineering team has expertise developing software for both FPGA and ASIC-based PON implementations using many of the common ASICs used in today s PON and ONU implementations. Northforge is also taking the lead in developing many of the new software-defined capabilities such as VHA, for a new generation of PON products. 7

8 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 NORTHFORGE INNOVATIONS INC. GATINEAU DEVELOPMENT CENTER (Development Center) 72 Laval Street, rd Level Gatineau (QC) J8X H MONTREAL DEVELOPMENT CENTER 40 Saint-Nicolas Street, Suite 026 Montreal, QC H2Y 2P5 USA OFFICE (Sales Office) One Boston Place, Suite 2600 Boston, MA General Inquiries Consulting Inquiries info@northforgeinc.com Copyright Northforge Innovations Inc. 8