EFFENBERGER LAYOUT 2/20/07 3:28 PM Page 51 TOPICS IN OPTICAL COMMUNICATIONS An Introduction to PON Technologies Frank Effenberger, Huawei Technologies US; David Cleary, Calix, Inc.; Onn Haran, PMC Sierra Glen Kramer, Teknovus, Inc.; Ruo Ding Li, Motorola, Inc.; Moshe Oron, Tellabs, Inc. Thomas Pfeiffer, Alcatel-Lucent Germany ABSTRACT adopted IEEE standard that was designed for fixed and mobile access networks. It has a useful Passive optical networks are the most impor- range of about 5 km at a data rate of 70 Mb/s. tant class of fiber access systems in the world WiFi is more mature than WiMAX, but it has a today. This article first reviews the reasons why range of only 100 m and a bit rate of 10–50 the PON as a general architecture is so impor- Mb/s. In spite of this limitation, WiFi is more tant. We then outline in some depth the tech- widely used for access today than WiMAX due nologies used to implement this architecture, to its maturity. including the G-PON and E-PON systems being Although both WiFi and WiMAX are rela- deployed today, and the advanced PON systems tively low cost to deploy, they lack sufficient that provide the evolution path to ever higher bandwidth to support video applications. These bandwidths. wireless technologies use a point-to-multipoint architecture. This means that bandwidth is INTRODUCTION: shared by multiple users — in some cases hun- dreds of users. Consequently, WiFi and WiMAX THE MOTIVATION FOR PON are useful for Web surfing applications, but impractical for higher-bandwidth and higher-rev- One of the most critical decisions for any busi- enue applications such as IPTV. ness involves the purchase of capital equipment. Another access technology option available to Of the many factors that influence this decision, service providers is copper — more specifically, equipment cost and the resulting revenue poten- digital subscriber line (DSL) over copper. Unlike tial are two of the most important. Service pro- wireless, DSL uses a point-to-point architecture. viders face this decision when upgrading existing So instead of sharing 50 Mb/s over all sub- access networks or expanding into new areas. scribers, DSL can provide 50 Mb/s to each sub- They want to minimize the cost of deploying scriber. Unfortunately, DSL shares a access equipment while maximizing revenue shortcoming with wireless: it is a noise-limited from the service offerings. Of these two parame- access technology. In other words, the effective ters, the cost of deployment is easier to deter- bandwidth DSL provides to a subscriber depends mine than revenue potential because future on the level of noise, which in turn depends on revenue involves considerable speculation. As a the length of the copper loop. DSL is capable of result, the raw bandwidth capabilities of an 50 Mb/s for loop lengths less than 300 ft, but can access technology are often used as a proxy for only provide 10 Mb/s at 10,000 ft. If operators revenue potential. Thus, the most important want to offer a compelling video service with 30 decision a service provider makes when purchas- Mb/s, they need to shorten loop lengths to ing network equipment is how to strike a bal- roughly 3000 ft or less. This is a viable approach, ance between minimizing the equipment cost but the cost is only slightly lower than an all- and maximizing the bandwidth. fiber approach. The passive optical network (PON) is just The final option to consider for access tech- one of several access technologies used by ser- nology is fiber. An access network can be archi- vice providers, but it enjoys a dominant posi- tected using either dedicated or shared fibers. A tion in the access market. Before discussing dedicated fiber plant, often referred to as a the specific details of the PON, it is worth- point-to-point network, provides a dedicated while to survey the alternate access technolo- fiber strand between each subscriber and the gies in order to understand the reasons for the central office (CO). PON’s success. In a shared fiber architecture, a single fiber Access networks fall into three categories: from the CO serves several dozen subscribers. wireless, copper, and fiber. Wireless has the low- This fiber is brought to a neighborhood where est deployment cost because it has the lowest the signals are broken out onto separate fibers outside plant costs. WiFi (802.11) and WiMAX that run to the individual subscribers. (802.16) are the standards for wireless access Point-to-point fiber networks have a low mar- and broadband access. WiMAX is a recently ket penetration mainly due to the additional cost IEEE Communications Magazine • March 2007 0163-6804/07/$20.00 © 2007 IEEE S17 Authorized licensed use limited to: IEEE Xplore. Downloaded on December 3, 2008 at 10:42 from IEEE Xplore. Restrictions apply. EFFENBERGER LAYOUT 2/20/07 3:28 PM Page 52 GIGABIT PASSIVE Up to 60 km OPTICAL NETWORKS Up to 20 km ONU STANDARDIZATION HISTORY The gigabit-capable PON (G-PON) is speci- fied by International Telecommunication WDM ONU OLT Union — Telecommunication Standardization ONU Up to Sector (ITU-T) G.984 series [1-4]. G-PON 128 split definition began in the Full Service Access Network (FSAN) consortium in 2001. In Jan- Video ONU uary 2003 the first two standards were Tx approved by the ITU-T, covering the require- Downstream: 1490 nm, 28 db, 2.488 Gb/s ONU ments and basic architecture (G.984.1), and Upstream: 1310 nm, 28 db, 1.244 Gb/s the physical-medium-dependent (PMD) layer Downstream video: 1550 nm (G.984.2). In February 2004 G.984.3 specifying the G-PON transmission convergence (TC) layer was ratified, followed by G.984.4, which I Figure 1. G-PON physical network architecture. standardizes the G-PON management require- ments. Since then, a few amendments have reached consent by the ITU-T on most of the it adds over a shared fiber infrastructure. documents in the series. Depending on the average loop length, the con- struction costs of outside plant based on dedicat- PMD LAYER ed fiber exceed those of outside plant based on The G-PON network architecture supports a shared fiber by anywhere from 20 percent to 100 two-wavelength WDM scheme for downstream percent. and upstream digital services (Fig. 1). Addition- In shared fiber architectures, there are two ally, another downstream wavelength is allocated ways the signals are broken out. One method is for distribution of analog video service. The net- called active Ethernet (AE), and the other is the work supports up to 60 km reach, with 20 km PON. With AE the individual signals are split differential reach between optical network units out using electronic equipment near the sub- (ONUs). The split ratio supported by the stan- scriber. In the PON the signals are replicated dard is up to 128. Practical deployments typically passively by the splitter. would have lower reach and split ratio, limited A shared network based on a PON has sev- by the optical budget. eral advantages over one based on AE. The ITU-T G.984.2 specifies the PMD layer for outside plant of a PON incurs lower capital G-PON, covering the range of G-PON upstream expenditures as it has no electronic compo- and downstream bit rates, and the optical param- nents in the field. The PON also lowers the eters for the various rate combinations. operational expenditures, since there is no As network operators requirements evolved, need for the operators to provide and monitor the preferred G-PON bit rate was selected to be electrical power in the field or maintain back- 2.488 Gb/s downstream, 1.244 Gb/s upstream. up batteries. A PON has a higher reliability This focus has then allowed the definition of than AE because in the PON outside plant best practice optical parameters for G-PON, there are no electronic components, which are which was documented as an amendment to prone to failure. Lastly, perhaps one of the G.984.2. The parameters, known as Class B+, most crucial features of a PON-based access apply to a network with or without a video over- network is its signal rate and format trans- lay and to ONUs based on either APD or PIN parency. Upgrading to higher bit rates is sim- technology. pler for a PON than for AE. Both require upgraded electronics in the CO and customer GTC LAYER premises, but, unlike AE, there is nothing that The G-PON TC (GTC) layer specified by [3] needs upgrading in the outside plant for a performs the adaptation of user data onto the PON, as the passive splitters are agnostic to PMD layer. Additionally, the GTC layer pro- PON speed. For all of the reasons cited above, vides basic management of the G-PON network. the PON is by far the most widely deployed The GTC layer defines two adaptation access technology. The rate and signal format methods for data transport: asynchronous transparency became a sort of insurance poli- transfer mode (ATM) and G-PON-encapsula- cy that eased carriers into deploying PON tion-method (GEM). However, as GEM has outside plants with the understanding that an become the preferred method, ATM is not dis- access network could flexibly be upgraded as cussed hereafter. GTC with GEM allows low new technologies mature or new standards overhead adaptation of various protocols, evolve. including Ethernet and time-division multiplex- Not surprisingly, this article is dedicated to ing (TDM). GTC also provides the medium various flavors of PONs that all use the same or access control (MAC) function, coordinating very similar outside plants, but differ significant- the interleaving of upstream transmissions ly in signaling rates, data formats, or protocols from multiple ONUs.