White Paper: 10GbE Fiber A Practical Understanding and Functional Approach

Transcription

1 White Paper: 10GbE Fiber A Practical Understanding and Functional Approach Dennis Manes, RCDD Senior Applications Engineer Leviton Network Solutions E/8 #2509

2 Table of Contents Introduction 3 Types of Transmitters 4 Design / Specification 7 Termination Type, Method and Equipment 10 Installation 12 Testing / Certification 13 Future Considerations 13 Conclusion 13 Leviton Network Solutions 2

3 Introduction The capabilities and methods of Multimode Fiber Optic cabling being used in the 10GbE arena has been discussed and lauded since the latter part of the 1990s. It has been tested, specified, and standardized. The engineering of the components has been done by numerous manufacturers. From an applications approach, what is needed is an easily followed discussion of how to design, install, and apply this knowledge in a real-world environment. We all understand that customers are requiring higher bandwidth solutions to the ever-increasing appetite for information. This requires that we, as Information Transport Systems Specialists, be knowledgeable of how fiber 10GbE works, how to deploy it, and especially how to assure it will function to the clients expectations. The best practice to assure your installation will perform at the 10GbE level, is a systems approach. What this means, is that you should assure that all of the materials used, such as cable, connectors, distribution enclosures, adapter plates, and optical patch cords all adhere to the same standards and performance parameters. However, the responsibility to assure that the link will support the 10GbE for fiber rests initially with the designer, and finally with the installer. Leviton Network Solutions 3

4 Types of Transmitters The type of transmitter most often used with multimode fiber for a 10GbE application is the VCSEL (Vertical Cavity Surface Emitting Laser). A VCSEL laser light source emits at the 850 nm wavelength and it is capable of a high data rate. An 850 nm LED and an 850 nm VCSEL emit light differently. In technical terms, the launch conditions between these two light sources are different. An LED emits light that is relatively uniform over the entire face of the multimode fiber core. In contrast, a VCSEL source emits light in a narrow beam, which shines bright in the center of the fiber core and quickly dims as it moves away from the center; it does not illuminate the core near the cladding interface. This difference in launch conditions results in different loss measurements. The loss measured with an LED is typically greater than the loss measured with a VCSEL. LC Connector LED core overfill is >100µm SC Connector VCSEL laser core fill is ~30µm The industry standards for structured cabling emphasize the generic nature of structured cabling. They do not make any assumptions for the deployment of the installed links. As the note above states, the signal loss in multimode fibers is greater (worse) for light sources and launch conditions exhibited by LED light sources. Therefore, unless specific light source requirements or link deployment instructions are mentioned in the statement of work, the structured cabling standards recommend the LED as the preferred light source to certify and measure multimode cabling links to cover the worst case situation. It is in your best interest to recommend and if possible get the client to specify the system capability for use of VCSEL light sources. The use of legacy 62.5/125 µm multimode fiber is not a viable option due to bandwidth and transmission distance restrictions, see table 1. Additionally, the use of single-mode fiber is not recognized by ANSI/TIA/EIA-568-B.1 for use in the horizontal, this along with the prohibitive cost of 1300 nm lasers such as Distributed Feedback (DFB) and Fabry Perot (FP) lasers. Leviton Network Solutions 4

5 To assist you in understanding the requirements, and to be able to discuss the needs to accomplish this, will require a basic knowledge of the theory behind the application. The following is a list of the basic terms, definitions, and an understandable explanation of 10GbE. 10-Gigabit Ethernet (10GbE) - A version of Ethernet with a nominal data rate of 10 Gbit/s, a gigabit per second (Gbit/s or Gbps) = 1,000,000,000 (10^9) bits per second, 10GbE over fiber is specified by IEEE Gigabit Ethernet uses digital signaling. The fiber optic version of this is 1000BASE-SX. Laser-optimized 50/125 µm multimode fiber Multimode fiber cable that is specifically designed for 850 nm operation at 1 Gb/s and higher. TIA/EIA-492-AAAC specifies mechanical, geometric and optical characteristics for laser-optimized 50/125 µm multimode fiber. This fiber has been fully adopted into TIA/EIA-568-B.3 and ISO/IEC Differential Mode Delay (DMD) A light pulse launched from a VCSEL will have different mode groups and will, in general, have different propagation times. This results in bandwidth reduction and limiting of the distance for 10GbE transmission. However, as you can see from the following figures, the effect is minimized by use of laser optimized 50 µm multimode fiber. Light Source Core Light Source Core 10 Gbps 850 nm Laser Cladding Detector 10 Gbps 850 nm Laser Cladding Detector The Effect of DMD using non-laser optimized fiber on Transmission Laser Optimized Fiber Reduces DMD resulting in Reliable Transmission Figure 1 The methodology of measuring DMD uses a single-mode pulse (5 µm spot size) which is scanned across the 50/125 µm laser-optimized multimode fiber core in at most 2 µm increments. Figure 2 is an illustration of this methodology. Leviton Network Solutions 5

6 Core of Fiber Light Source 5µm 50µm Multimode Fiber Front view Figure 2 Effective Modal Bandwidth (EMB) The Fiber bandwidth measurement technique to ensure 50/125-µm laser-optimized multimode fiber will reliably support 10GbE transmission. Calculated Effective Modal Bandwidth (EMBc) The main purpose of the EMBc calculation is to ensure that a fiber s effective modal bandwidth will meet the 10Gbits/sec requirement of 2000 MHz km with any conforming laser. Minimum Calculated Effective Modal Bandwidth (minembc) - The second method of predicting EMB from DMD. DMD Mask - Is the translating of DMD data into an EMB prediction. Return Loss - Is the power of the optical signal that returns towards the optical source against the direction of signal propagation. Contributors to return loss are Fresnel reflections (back reflected light due to interfaces at mated connectors and mechanical splices) and Rayleigh back scattering (scattered light due to intrinsic fiber properties). Bit Error Rate (BER) - Is the number of bit errors per unit time compared to the total number of bits transmitted per unit time. Error free propagation of bits of data through the fiber link is the ultimate design goal of the MMF and connectors in a 10GbE optical link. Leviton Network Solutions 6

7 Now that we have gone over the preliminaries, it is time to get to the functional areas. There are three principle areas to be considered: Design / Specification Installation Testing / Certification This table illustrates the different multimode fiber distance capabilities as specified in the 10GbE standard. Fiber Type Modal 850 nm (min) (MHz.km) Operating Range (meters) 62.5 µm Multimode to to µm Multimode to to to 300 Table 1 Design / Specification Design of a 10GbE fiber solution should start with the fiber. The selection of the optical properties of fiber should also be carried over to the fiber patch cords and also to the connector if you will be using a pre-polished connector or manufactured terminated cabling. The use of 50/125 µm laser optimized fiber for building backbone, campus backbone, horizontal cabling, centralized cabling and data centers is recommended, so long as it does not exceed the operating range shown on Table 1. This provides the user with the ability to operate at slower data-rate speeds initially, while additionally offering the ability to migrate to higher data-rate laser-based systems as demands for bandwidth increase. Selection of fiber grade must offer an upgrade path to 1 Gb/s and to 10 Gb/s. When trying to decide the cable specifications for your installation, TIA/EIA-492-AAAC specifies mechanical, geometric and optical characteristics for laser-optimized 50/125 µm multimode fiber. This fiber has been fully adopted into TIA/EIA- 568-B.3 and ISO/IEC When specifying multimode fiber intended for use at data rates or 1 Gb/s or greater, you should use minembc values rather than pass/fail performance indicated by the DMD mask method, as DMD mask method only provides a prediction of true EMB. Leviton Network Solutions 7

8 Next would be the selection of connector and termination methodology. The connector type selected will affect the termination methodology. Here is a list of primary connector types with polish possibilities and the common method/s of termination. Connector Fiber Polish Termination Method/s MTP MM/SM Factory SC MM/SM PC/UPC/APC PC Field, other Factory / No-Epoxy-No-Polish LC MM/SM PC/UPC/APC PC Field, other Factory / No-Epoxy-No-Polish MU MM/SM PC/UPC/APC PC Field, other Factory / No-Epoxy-No-Polish PC physical contact, the standard polish with a normal / typical back reflection of < -25dB UPC Ultra PC, normal / typical back reflection of < -55dB APC Angled PC, normal / typical back reflection of < -65dB Table 2 LC Connector SC Connector SC-APC Connector MU Connector MTP Connector Leviton Network Solutions 8

9 Appropriate selection of connector and termination method is necessary in order to meet system requirements for insertion loss and return loss. Basically, loss is the reduction of a signal that passes through any medium (i.e., optical fiber cabling, copper cabling, or air) for a distance. The determination of your link budget is a function of transmitter power, channel insertion loss, power penalties, budget margin, and receiver minimum sensitivity. It is a basic formula of; Power - channel insertion loss - power penalties - budget margin > receiver minimum sensitivity. Figure 3 is a visual of this. Channel Insertion Loss Power Penalties Power Budget Margin Transmitter Minimum Power Link Power Budget = Minimum Transmit Power - Minimum Receiver Sensitivity Figure 3 Receiver Minimum Sensitivity Loss is a function of distance through the medium as well as through interfaces (e.g., connectors, splices or couplers) that impede the propagation of the signal. The quality of optical fiber link terminations directly affects channel insertion loss. Poor quality terminations cause an increase in loss while high-performance terminations produce less loss. Generally, components like connectors and mechanical splices are specified as reflectance, and system (cumulative effect of Fresnel reflections and Rayleigh scattering) sensitivity is specified as return loss. See figure 4. Fiber Optic Cabling Channel Equipment Connection Equipment Connection Equipment Cable Building Cable Figure 4 Equipment Cable Leviton Network Solutions 9

10 Termination Type, Method and Equipment The application in which the system is to be used will also dictate density of the fiber distribution enclosure. For example, in a data center where space is at a premium, you would typically use an MT cable for core connection and high density LC or MU panels for discreet access interconnect. These cables are factory terminated and tested. The most common and recommended methodology within the Data Center is to use a Plug-n-Play System comprised of MTP Cable Assemblies and MTP Brackets. The MTP cable assemblies are used to provide connection at the Main Distribution point and access to the active equipment, while the MTP brackets provide a coupling capability in the equipment zone. This design provides the best practices approach to assure optical power head room for the 10G network. Previous older style Plug-n-Play Systems used a cassette system which resulted in additional attenuation in the topology. This additional loss often resulted in the system loss being in excess of the 10GbE budget. Typical loss should not exceed 2.6dB for a 10GbE circuit. Typical older Plug-n-Play systems resulted in loss totaling 4.8dB, which is 2.2dB over budget. The recommended methodology comprised of MTP Cable Assemblies and MTP Brackets results in a total loss of 2.0dB, which is.6db under budget. The following illustrates the recommended MTP cable assembly and MTP bracket solution. Leviton Network Solutions 10

11 Server to Core Crossconnect LC - MTP Trunk LC - MTP Trunk In Equipment Zone In Equipment Zone LC - LC Jumper MTP - LC Harness 2.0 db Loss Total.6 db Under Budget MTP - LC Harness Leviton Network Solutions 11

12 While in a normal Telecommunications Room (TR) with limited fiber need, the use of a 6 strand 50/125 µm laser optimized backbone feeder cable terminated in SC duplex is a viable solution. From a standard rack mounted enclosure in the Equipment Room, to a wall mounted enclosure in the TR. Installation Follow the industry standards for the passive components, specifically TIA/EIA-568-B, Commercial Building Telecommunications Cabling Standard, TIA/EIA 569-A, Commercial Building Standards for Telecommunications Pathways and Spaces, and TIA-942, Telecommunications Infrastructure Standard for Data Centers, this offers guidance, recommendations and a template for the solution. A final note on the installation of a 10GbE fiber solution: Fiber connector and coupler adapter cleanliness and fiber connector end face polish are the most often over looked cause of system failure to support a 10Gbe solution. Therefore, it is good practice to thoroughly clean and visually inspect all connectors, couplers, and patch cords during the installation process. Additionally, bend radius and tension placed on the fiber cable will have a significant impact on the capacity of a fiber network to support a 10-GbE solution. Careful attention to installation methodology and support hardware is a must. Leviton Network Solutions 12

13 Testing / Certification Deployment of high data-rate 10-GbE systems requires attention to detail as cabling length and attenuation requirements are more stringent. TIA TR-42.8 outlines in TSB-140 the field testing of optical fiber cabling. This document describes field-testing of length, optical loss and polarity in optical fiber cabling using an optical loss test set (OLTS), optical time domain reflectometer (OTDR) and a visual fault locator (VFL). The purpose of this TSB is to clarify, not replace ANSI/TIA/ EIA and ANSI/TIA/EIA A. Standards, such as the IEEE802.3ae, typically specify the maximum channel link loss as a primary link parameter. Although measured installed link loss is an important parameter, the link loss can not guarantee whether or not a fiber is capable of supporting a 10Gb/s error free transmission. The most precise measure of channel performance is bit error rate (BER) testing. At this time an economical field BER testing device is not available, so you must rely on conventional link loss testing. Installers of fiber optic systems can be confident that the installation will handle 10GbE and provide a robust and effective means of transmission if the total channel link loss is less than 2.6dB. Future Considerations Currently the TSB-172 Committee is being asked to review a new draft specification (492AAAD) for a new 50μm multi-mode fiber. This new fiber commonly know as OM4 is being considered to handle up to 1000-GbE via 850-nm laser-optimized, 50μm multi-mode fiber. The IEEE Higher Speed Study Group (HSSG) has voted to approve the next standard speed for Ethernet to be 100Gb/s. HSSG is currently working on developing the next generations of Ethernet. It is obvious that optical fiber will be a significant portion of that future. Conclusion A 10-GbE fiber optic network is not a feat of magic; it is the result of a well planned design that takes into account the specific current and future needs of the client. It follows basic guidelines for Cabling, Pathways and Spaces, and Infrastructure for Data Centers. It assures that all of the components are compatible; follow the same specifications for interoperability and use quality products and materials. It also assures the testing and documentation is completed and follows standards and recommendations. Leviton Network Solutions 13

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