Fiber Selection for Premises Networks
The following is an excerpt from a white paper recently published by Corning that focuses on different types of fiber and the applications for which they are best suited. To read the paper in its entirety, click here.
There are several main types of optical fiber, and the way each is designed, the characteristics they possess, and the way these cause the fiber to operate determine the application to which a given fiber is most appropriate. Some fiber types or specification differences that seem subtle can lead to large functional differences in the system. This can directly affect the current and long-term performance of the network, as well as the cost. For this reason, it is important that network designers and network operators fully understand the fiber types and technologies available in order to make informed choices and select the correct fiber and interconnect technology for their current and future application needs.
Premises Network Design Considerations: The Need for Optical Speed
System designers and consultants must construct communications networks that meet the current and future workload of their customers at the link lengths required for their facility or campus. Rising demand for sophisticated “smart” electronics devices increases the demands on information storage and accessibility/transport infrastructure. These requirements drive the needs for ultra-high speed data interconnects and structured cabling fabric in premises networks such as data centers (DC), Storage Area Networks (SAN), and clustered or supercomputing applications, serving a wide range of sectors from financial and industrial to social networking and health. The interconnect media choices available for constructing such networks include wireless technology, copper cable, and optical fiber cable. Of these, optical fiber offers the highest bandwidth and lowest latency supporting the fastest data rates over the longest link lengths, reliably and securely.
The standardization of transmission protocols such as IEEE Ethernet, for example, is a key industry indicator that the pace of development in high-speed interconnect devices and network building blocks are facilitated by the capabilities of optical fiber. Optical fiber is an easily installed medium that is immune to electromagnetic interference and is also significantly more efficient in terms of power consumption. The latest trends in optical fiber cable interconnect infrastructure also offer space and cost savings, with higher cabling density and port density over copper cabling and lightweight cables that contribute to better cooling efficiency and lower energy costs.
Optical Fiber Options
There are three main categories of optical fiber suitable for use within premises networks: 50/125 μm multimode fiber; 62.5/125 μm multimode fiber; and single-mode fiber. The numeric notation (e.g. 50/125 μm) signifies the diameter of the glass core where the light travels (50 μm) and the outside glass cladding diameter (125 μm). This cladding diameter is common to all major fiber categories in the industry and provides near identical mechanical properties for the three fiber types. However, the different core diameters of each fiber affect the optical properties significantly and have a direct impact on system performance and system cost when balanced against network application needs. The refractive index profile of each product is precisely designed to channel light down the fiber, enhancing certain attributes such as bandwidth, attenuation, and also fiber bending sensitivity. Before comparing specific fiber types, it is first important to understand the two primary optical fiber attributes that have the biggest impact on system performance and cost.
Attenuation is the reduction of signal power, or loss, as light travels through an optical fiber. Fiber attenuation coefficient is measured in decibels per kilometer (dB/km). A higher attenuation coefficient results in a higher rate of signal loss of a given fiber length. Insertion loss is the total attenuation from all sources plus any reflection losses over a specific length. Single-mode fibers in premises networks generally operate at 1310nm (for short-reach applications), whereas multimode fibers are optimized for either 850nm or 1300nm operation. Attenuation has rarely been a primary limiting factor in short-reach premises applications and where multiple connectors in the signal path are generally commonplace. However, in modern day state-of-the-art high-speed interconnects, such as 100 Gb/s, the insertion loss performance of premises networks has become increasingly relevant.
Bandwidth quantifies the intrinsic information-carrying capacity of multimode fiber, given in units of megahertz-kilometer (MHz•km). Bandwidth behavior of multimode fibers arises from multi-modal dispersion (multi-path signal spreading), which occurs as the result of light traveling along different modes (paths) in the core of the fiber. The bandwidth specification or performance of a multimode fiber is verified through optical measurements during fiber manufacture. Actual system performance and data-rate handling is strongly bandwidth-dependent but also governed by transceiver technology and device characteristics. Modal dispersion is suppressed in single-mode fibers by the smaller core diameter. Waveguide chromatic dispersion is rarely a limiting factor for single-mode fiber when operated at 1310nm (near zero dispersion) in premises networks.
Multimode fiber uses a graded index profile (i.e. parabolic in shape) to minimize modal dispersion. This design maximizes bandwidth while maintaining larger core diameters (as compared to single-mode) for simplified system assembly, connectivity, and lower network costs. The bandwidth specification of multimode fibers is a major performance factor in network design. This has led the industry to exploit the higher intrinsic bandwidth potential of 50/125μm fiber when developing higher-performance multimode fiber systems. Since the advent of the Gigabit Ethernet standard, low-cost laser-based transceivers have been developed for multimode fibers operating at 850nm and utilizing the greater bandwidth advantage of 50/125μm fiber. This has led to the near obsolescence in applications and deployment of 62.5μm multimode fiber.
Fiber-Transceiver: Technology Evolution and Parallel Optics
An optical transceiver is a package, usually a pluggable module comprising of a) the optical light source(s) (typically a laser diode or in legacy applications, LED – light emitting diode) and b) optical receiver(s) (photo detector). Over the last decade, multimode bandwidth specifications and measurement methods have evolved alongside compatible transceiver technology, developed to keep pace with delivery of higher transmission speeds. The combination of transceiver and fiber is a major factor that determines a fiber’s practical link length capability depending on the bit rate or application protocol speed. More importantly for high bit-rate applications at or greater than Gigabit speeds, the component costs of transceiver devices dominate over the cost of passive interconnect hardware (cable and connectors). For 10G-based systems, it is estimated that the transceivers account for up to ~25% of the total cost of the physical interconnect and switch electronics, while the passive optical cable and fiber technology is less than 5%. For short-range interconnects supporting high speeds above 10G, parallel optics solutions for multimode offer modular 10G “lanes” for transmission speeds of 40G and 100G Ethernet. Similar multi-lane transmission schemes are used by other interconnect protocols such as Infiniband.
System Costs: Single-mode vs. Multimode Fibers
Within premises networking applications, the most significant differences between single-mode and multimode optical fiber types are the core diameter size and primary operating wavelengths. These key factors therefore impact associated transceiver technology, ease of connectivity, and cost. To utilize the fundamental attributes of single-mode fibers, which are generally geared towards longer distance applications (multi-kilometer reach), requires transceivers with lasers that operate at longer wavelengths with smaller spot-size and generally narrower spectral width. These transceiver characteristics, combined with the need for higher-precision alignment and tighter connector tolerances to smaller core diameters, result in significantly higher transceiver costs and overall higher interconnect costs for single-mode fiber interconnects. Fabrication methods for VCSEL (vertical-cavity surface emitting laser)-based transceivers that are optimized for use with multimode fibers are more easily manufactured into array devices and are lower cost than equivalent single-mode transceivers. Despite the use of multiple fiber lanes and multi-transceivers arrays, there are significant cost savings over single-mode technology employing single or multichannel operation over simplex-duplex connectivity.
Transceivers, utilizing low-cost VCSEL technology developed for 50/125μm multimode fibers, take advantage of the larger core diameter enabling high coupling efficiency and accommodation of wider geometrical tolerances. Data rates from 10 Mb/s to 100 Gb/s are supported by the higher modal bandwidths offered by OM3 and OM4 50/125μm multimode fibers, which generally offer the lowest system cost and upgrade path to 100G for standards-based premises applications using parallel-optic-based interconnects.
Bend Insensitive Multimode Fiber (BIMMF)
Corning developed bend-insensitive ClearCurve multimode fiber to withstand tighter cable bends commonly found in the indoor “patching-interconnect” architecture that is prominent in applications such as data centers, enterprise networks, and LANs. Excessive signal loss or increased attenuation due to tight bending can degrade signal quality and if undetected, can impose severe restrictions on upgrade paths or lead to transmission reliability issues. The improved bend performance allows for greater positive spare margin (the difference between the power budget and the total losses induced in a link), which provides “network system protection” against unexpected or inadvertent cable bends that may occur during routine structured cabling operations, e.g. neighboring rack maintenance or MACs (moves, adds, and changes).
Multimode fibers are graded according to specification and chiefly by their bandwidth performance. These are commonly classified by the TIA/ISO OM (optical multimode) designation. This indicates the class of fiber in terms of bandwidth as specified in the ISO/IEC 11801 structured cabling standard. Various standards organizations have defined the relationship between transmission data rates, link length, and bandwidth for specific protocols, applications, and transceiver types.
Choosing the right fiber for your network application is a critical decision. Understanding your system requirements in order to select the appropriate fiber will maximize the value and performance of your cabling system, which the capability of the network depends on, now and in the future.
By Carl Roberts and Dr. Russell Ellis, Corning