WHITE PAPER Is Your Network Ready for GPON? Is Your Network Ready for GPON? Evolution to GPON GPON is a culmination of the best in BPON and EPON techniques. Most agree that eventually, everything will move to IP (voice over IP, video over IP, robust data applications like gaming, video streaming, MPEG3 downloads), and quadruple play applications (network appliances, security, video surveillance). The advantages of GPON are key in driving commitment by the large volume carriers to the GPON standard. As GPON moves to become the standard of choice for fiber-to-the-premise (FTTP) networks, cost reductions and inter-operability will accelerate. The Full Service Access Network (FSAN) committee has been working on GPON since mid 2001, shortly after BPON ratification. The FSAN group was created by a group of service providers to facilitate the creation of suitable access network equipment standards, hence reducing the price of affordable equipment. GPON is making it easier for PON networks to move to an “all IP format” where the external interfaces to the core are moving to an all gigabit ethernet network creating a movement away from the traditional ATM transport to a “pure” IP transport. GPON is IP-centric; allowing the traditional services of voice and video, yet acknowledging the strengths of the service provider to differentiate themselves on quality of service (QoS). GPON continues to have a long reach that effectively eliminates active components in the access network with little or no significant changes to the physical architecture. Standards Bodies There are two standards bodies for the development of PON protocols. One is the International Telecommunications Union (ITU). The ITU typically develops standards for the telecommunications industry. This body has developed standards for HDSL, ADSL, SS7, etc. All of the large carriers in the world belong to and support the ITU. The Institute of Electrical and Electronics Engineers, Inc. (IEEE) is another standards group focused on the engineering community and has increasingly taken on a standards role in the last five to ten years, primarily in the Enterprise IT marketplace. But as data and telephony continue to converge, they are also involved in some of the telecommunications standards work. Standards are important because they drive the underlying protocols and the basic specifications for specific telecom and data systems. These protocols and specifications are the heart of the various PON’s variants. Ultimately, these protocols define the specifics to make interoperability a reality and to ensure the products will perform. Three standards have been ratified: BPON and GPON by the ITU and EPON by the IEEE. Any other protocol is not a standard, however, WDM PON is in development. Page 3 PON Variants In reviewing the various PON standards, the following is offered: APON came and went and was the first iteration of PON for FTTP solutions. As the world continued to migrate to a mix of data and traditional voice solutions, it was realized that APON lacked the bandwidth needed for more robust applications. BPON was standardized in 2001. It provided for 622 Mb/s downstream and 155 Mb/s upstream and added RF video overlay capabilities. This was still a predominately ATM based protocol. As the FTTP market started to gain more traction, higher bandwidth applications started to be developed that required even more bandwidth. EPON or Ethernet in the First Mile (EFM) is an ongoing standard work that uses Ethernet for Packet data. It was initially a best effort solution. Today’s standards are offering higher layer protocols that can be incorporated with some level of QoS. Standards work continues, but the 1.2 Gb/s symmetrical bandwidth may not be enough for higher bandwidth applications needs. EPON is mentioned in this context for standards comparison purposes only. GPON was standardized in 2003. GPON takes the best of BPON, with its QoS capability, and the best of EPON, which is the ability to transport and interface on an all IP network. GPON also address the higher application bandwidth needs by providing 2.4 Gb/s downstream and 1.2 Gb/s upstream. Transitioning from BPON to GPON In transitioning from BPON to GPON, there are three key architectural components: fiber optic cable, class of optics and split ratios. Fiber Optic Cable The various fiber optic cable manufacturers may have similar loss characteristics or they may be quite different. Spectral attenuation occurs when the loss or attenuation of a signal is a direct correlation between the wavelength and the distance. When increasing the split ratios from 1:32 to 1:64 and higher, spectral attenuation will become important. As fiber optic cable ages, attenuation increases due to the phenomena of hydrogen aging. Here, the molecules of hydrogen atoms in the silica or glass tend to breakdown over time making the fiber “less clear” to transport the light pulses. Class of Optics Within the FSANs and IEEE bodies, various “classes” of the optic’s electronics are defined. EPON traditionally uses PX10 and PX20 optics where the maximum link budget is 26 dB. BPON uses Class B optics where the maximum link budget is 25 dB. GPON utilizes Class B+ and Class C optics where the maximum link budgets are 28 dB and 30 dB respectfully. Split Ratios The third component is split ratios. For split ratios within PON point-to-multipoint architectures, (also defined by the FSANs standards body) BPON’s timing only allows for various splits of 1 to 32 or some combination where there are 32 subscribers. For GPON, the FSANs standards body recognized that there are higher speeds (bandwidths); therefore additional split arrangements are allowed. Again, these higher split ratios are governed by the timing and the PON chip sets that will allow the 1:32, or 1:64 combinations, and a future 1:128 ratio. Spectral Attenuation Spectral attenuation is simply the loss characteristics built into the fiber optic cable giving a particular optical wavelength. The lower the wavelength, the higher the spectral attenuation. This is applied to link loss budget calculations where the worst case numbers are applied to the end-to-end loss budget. Not shown here is the fact that after the 1600 nm wavelength, the intrinsic attenuation actually increases. Therefore, it is not logical to say that we can keep extending the wavelength to achieve lower and lower losses. For PON, the equipment operates in the ranges between 1310 nm and 1550 nm. The fact that we are using CWDM transceivers, the total bandwidth variants can be between +/- 10 nm to a +/- 50 nm. In telecommunications, the spectral bandwidth for single peak devices is the difference between wavelengths at which the radiant intensity is 50%, or 3 dB down from the maximum value. When applying to distances and calculating the link loss budget for a given architecture, fiber optic cable manufacturers must specify the spectral attenuation for their products. In designing the FTTP network, the design engineer, will initially design for the wavelength with the highest loss characteristics. For BPON and GPON, this will always be 1310 nm. The loss will vary depending on manufacturer. Some of the ranges are from 0.31 db/km for premium SMF-28e (Corning), to standard SMF-28e 0.34 db/km to a high of 0.40 dB/km. When we start increasing the split ratios in a given network, the spectral attenuation will be closely monitored. PON Optics Optical link budgets are determined by individual vendor’s active components: PON chips within the electronics, lasers, and receivers. The PON optics “classes” have been defined as Class A, B, & C. Traditional BPON equipment has always used Class B optics. When looking at the link budget for the Class B optics, one will find that the maximum link is set at 25 dB. It was determined that some of the PON networks of 20 km were actually Is Your Network Ready for GPON? WHITE PAPER Website: www.adc.com From North America, Call Toll Free: 1-800-366-3891 • Outside of North America: +1-952-938-8080 Fax: +1-952-917-3237 • For a listing of ADC’s global sales office locations, please refer to our website. ADC Telecommunications, Inc., P.O. Box 1101, Minneapolis, Minnesota USA 55440-1101 Specifications published here are current as of the date of publication of this document. Because we are continuously improving our products, ADC reserves the right to change specifications without prior notice. At any time, you may verify product specifications by contacting our headquarters office in Minneapolis. ADC Telecommunications, Inc. views its patent portfolio as an important corporate asset and vigorously enforces its patents. Products or features contained herein may be covered by one or more U.S. or foreign patents. An Equal Opportunity Employer 105525AE 11/07 Original © 2007 ADC Telecommunications, Inc. All Rights Reserved stretching these budgets to the limit, and the active equipment manufacturers were forced to increase these budgets to 26.5 dB. With the advent of these increased budgets and the possible need to increase the split ratios of GPON, the Class B optics were given an increase in the receiver photo detectors to allow for a 28 dB loss budget. While still not in the Class C optics range, these components were given the distinction of Class B+ optics. The key differentiation is that the cost of these Class B+ optics has not increased to the Class C pricing while maintaining better PON loss characteristics. In the future, the need to transport to longer reaches (30 km or 40 km) and even higher split ratios (1:128) may force the equipment manufacturers to the Class C optics. Splitter Loss Splitters are passive devices because they require no external energy source other than the incident light beam. They are broadband and add only loss, mostly due to the fact that they divide up the input (downstream) power. This loss, called splitter loss or splitting ratio, is usually expressed in dB and depends mainly on the number of output ports. It should be noted that, contrary to what one might expect, the splitter adds approximately the same loss even for light traveling in the upstream direction. Regardless of the splitting architectures or PON technologies used, when calculating the link loss budget, one should account for the following splitter loss configurations: 1x2: 3.70 dB, 1x3: 5.10 dB, 1x4: 7.25 dB, 1x8: 10.38 dB, 1x16: 14.10 dB, and 1x32: 17.45 dB. These losses include SC APC connectors and are the maximum losses defined by the ITU G.671 and Telcordia GR-1209. When GPON required the 1x64 split ratio, it originally forced use of a single 1x2 splitter interfacing two 1x32 splitters to make up the 1x64 configuration. With today’s packaging, this is allowable, however, the design engineer is taking the 1x2 maximum of 3.7 dB and the 1x32 maximum of 17.45 dB and adding them, to equal 21.15 dB. When this is done using Class B optics (at 26.5 dB loss), 5.35 dB of what‘s called “head room” is left. Even with the best fiber manufactured, where spectral attenuation is 0.31 dB p/km, we are only able to design for a 17.25 km PON network. All this, and connectors have still not been included within the Central Office or Head-End or the splices in the OSP. The FSANS standards body and splitter manufacturers using newer planer techniques are pushing the loss of the 1x64 splitter to around 20.4 dB. However, the design engineer does have some options. In designing the network, premium splitters and low loss connectors can be in the network to ensure fusion splices are well below 0.05 dB/splice. There are other techniques that will be used until the standards have caught up to the technology for 1x64 and higher split ratios. PON Standards Summary FSAN will be the dominant PON protocol. GPON will be the dominant protocol for mass deployment. GPON (G.984) has made improvements over BPON (G.983) in terms of efficiency and scalability. There is dual mode support of ATM as well as Ethernet frames. In terms of scalability, GPON is a more economical means of achieving higher speeds, and will realize lower costs due to relaxed timing requirements and its support of both TDM and Ethernet interfaces at the Optical Line Termination (OLT). In terms of value, GPON supports TDM voice today, has a true migration platform to an all IP optical network, and most importantly, affords existing architectures to migrate without forklift upgrades. WHITE PAPER . www .adc. com From North America, Call Toll Free: 1-800-366-3891 • Outside of North America: +1-952-938-8080 Fax: +1-952-917-3237 • For a listing of ADC s. listing of ADC s global sales office locations, please refer to our website. ADC Telecommunications, Inc., P.O. Box 1101, Minneapolis, Minnesota USA 55440-1101