Long-Reach Passive Optical Networks Russell P
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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009 1 Long-Reach Passive Optical Networks Russell P. Davey, Daniel B. Grossman, Senior Member, IEEE, Michael Rasztovits-Wiech, David B. Payne, Derek Nesset, Member, IEEE, A. E. Kelly, Albert Rafel, Shamil Appathurai, and Sheng-Hui Yang, Member, IEEE Abstract—This paper is a tutorial reviewing research and devel- opment performed over the last few years to extend the reach of passive optical networks using technology such as optical ampli- fiers. Index Terms—Communication systems, networks, optical am- plifiers, optical fiber communications. I. INTRODUCTION HE rapid growth of Internet access and services such as T IP video delivery and voice-over IP (VoIP) is accelerating Fig. 1. Typical configuration for B-PON, GE-PON, and G-PON. demand for broadband access. While most broadband services around the world are delivered via copper access networks, op- tical access technology has been commercially available for sev- eral years and is being deployed in volume in some countries [1]. Where optical access is deployed, passive optical networks (PONs) are often the technology of choice because the trans- mission fiber and the central office equipment can be shared by a large number of customers. Early PON deployments were based on B-PON systems as standardized in the ITU-T G.983 series. Fig. 2. Mid-span GPON extension. Currently being installed in Asian countries such as Japan are Ethernet PON (GE-PON) with gigabit transmission capability operation is made possible using wavelength division multi- that complies with IEEE 802.3ah. Meanwhile, operators in the plexing (WDM) with upstream wavelengths in the 1310 nm United States and Europe are now focusing on gigabit-capable region (1260–1360 nm) and downstream wavelengths in the G-PON systems as standardized in ITU-T G.984 series, with 1490 nm region (1480–1500 nm). Capacity is shared among typical bit rates of 2.5 Gbit/s downstream and 1.2 Gbit/s up- subscribers on the PON using a time-division multiple access stream [2]. (TDMA) protocol that assigns transmission time slots for each Fig. 1 shows the system configuration typical for B-PON, user. The maximum reach and split of a PON are determined GE-PON and G-PON. An optical line terminal (OLT) in the by both the PON protocol and the physical layer optical reach. central office is connected to several optical network units The G-PON protocol supports a maximum logical reach of 60 (ONU) via an optical distribution network (ODN) consisting of km and a maximum logical split of up to 128. In practice most optical fibers and passive optical splitters. The ODN is totally commercial systems conform to the class specification that passive, which is very attractive to an operator. Single-fiber allows a maximum optical loss budget of 28 dB: often this is used to deploy a split size of 32 and reach of up to 20 km. Manuscript received June 29, 2008; revised September 12, 2008. Current ver- Similarly GE-PON specifies a maximum distance of 10 or 20 sion published nulldate This work was supported in part by the European Com- mission’s FP6 Project MUSE. The work of D. B. Payne was supported in part km, reflecting the use of different laser types, and offers loss by the European Union through the Welsh Assembly Government. budgets of 20 and 24 dB excluding optical path penalty. In R. P. Davey, D. Nesset, and A. Rafel are with BT, Ipswich, IP5 3RE, U.K. calculating the achievable reach, the total loss must be within (e-mail: [email protected]). D. B. Grossman and S.-H. Yang are with Motorola Applied Research and the allowed loss budget, taking account of realistic fiber and Technology Center, Marlborough, MA 01752IEEE USA. Proof splitter losses. M. Rasztovits-Wiech is with Siemens IT Solutions and Services PSE, A-1210 The concept of increasing the reach and/or split of PONs via Vienna, Austria. D. B. Payne was with BT, Ipswich, IP5 3RE, U.K., and is now with intermediate equipment such as optical amplifiers has been of the [AUTHOR: PLEASE PROVIDE A COMPLETE MAILING AD- research interest since the 1990s [3], [4]. Recently research has DRESS.—ED.] Institute of Advanced Telecommunications, Swansea focused on extending the reach of G-PON and GE-PON via University, Swansea, U.K. A. E. Kelly is with Amphotonix Ltd, Glasgow, G20 0SP, U.K. midspan optical amplifiers [5] or transponders [6] as shown in S. Appathurai is with the [AUTHOR: PLEASE PROVIDE A COMPLETE Fig. 2. This concept has recently been standardized in ITU-T MAILING ADDRESS.—ED.] BT Design. Recommendation G.984.6. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. WebThe OLT is connected Version via a length of fiber known as the op- Digital Object Identifier 10.1109/JLT.2008.2006991 tical trunk line (OTL) to the active midspan extender equipment. 0733-8724/$25.00 © 2009 IEEE 2 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009 This in turn is connected to the ODN and ONU. Note the in- deployed in an outdoor environment, there is the potential tention is for the OLT and ONU equipment to be essentially opportunity to close the smaller central offices altogether, unchanged compared to the traditional deployment configura- once all legacy (e.g., copper transmission) equipment has tion shown in Fig. 1. Placement of remote OLTs in the field is a been retired. Section IV will describe how network simpli- possible alternative to reach extension. In this scenario, an OLT fication of this kind is important to reduce end-to-end costs with a small number of PON ports and one or two backhaul ports in an environment where revenues do not increase in pro- is environmentally hardened and deployed in the same fashion portion to bandwidth. as a PON extender. The great advantage of reach extension in The next two sections will discuss development of two PON these applications is in the relative simplicity of the hardware extender technologies. Section II discusses a semiconductor deployed in the field. optical amplifier approach to extending the reach of a stan- Some deployments overlay cable television signals at dard G-PON. Section III describes PON extension applied 1550–1560 nm over the same fibers as G-PON (or B-PON or to a 10-Gbit/s, 100-km reach PON. Section IV discusses the GE-PON) using the enhancement band approach described in drivers for fiber access and simplifying the network architecture ITU-T Recommendation G.984.5. In these situations the cable through PON reach extension. television signals can be conveniently amplified using separate erbium doped fiber amplifiers. II. SEMICONDUCTOR OPTICALLY AMPLIFIED GPON Operators value greatly the passive nature of the access net- Semiconductor optical amplifiers (SOAs) are attractive can- work enabled by the PON architecture, and it is not the intention didates for GPON reach extension. They can provide high gain, of PON reach extension to move away from this. Nevertheless, low noise figure (NF), low polarization dependent loss, and fast having the option of an active midspan reach extender can pro- gain dynamics that are suitable for midspan PON signal ampli- vide several benefits, given as follows. fication. They can be designed to provide gain in the 1310-nm 1) Installing fiber cables represents a significant capital in- (O-band) and 1490-nm (S-band) windows used by GPON. The vestment and so PONs are often deployed in greenfield de- only alternative optical amplifier technology capable of oper- ployments, where cable installation costs (whether copper ating in the O- and S- bands are, respectively, praseodymium or fiber) are an inescapable fact. Often greenfield deploy- and thulium doped fluoride fiber amplifiers. These fibers are dif- ments can be located a long way from existing central of- ficult to work with [1], and the technology did not appear to fice buildings—potentially beyond the reach of G-PON or be ready for commercialization; however, recent work appears GE-PON. In this solution, one could build a new central promising [8]. Other advantages of SOAs relative to fiber ampli- office building or house PON OLT equipment in a street fiers in this application include their small size, high reliability, cabinet. Clearly, there are significant capital and opera- and low power consumption. tional costs associated with building a central office, nor is An important consideration for optical amplification of PONs it especially attractive to deploy a full OLT in a street cab- is burst mode operation in the upstream. In GPON, upstream inet. An attractive alternative could therefore be a simple bursts from different ONUs can have a dynamic range of up midspan PON extender box deployed in a street cabinet to 10 dB, due to differential fiber loss between the nearest and (or underground footway box). To offer benefits over the furthest ONU on the PON, and variation in input signal level. street-based OLT approach, the PON extender should be Fiber amplifiers exhibit a slow relaxation response to variations compact, low-power, and cost-effective and require min- in input signal level, which results in output signal distortion. imal configuration and management. As a result, the amplifier’s average output power varies slowly 2) A remote PON extender could give operators more flex- over many bit times at the beginning of each burst. The deci- ibility in deployments. When deploying PONs on long sion threshold circuit in the OLT cannot track receive power loops, the loss budget may not allow the operator to deploy variations on this scale, resulting in excessive BER.