Topic: Fiber Optics In Premises Networks
The Role of Fiber Optics In Premises Networks
While UTP copper has dominated premises cabling, fiber optics has become increasingly popular as computer network speeds have risen to the gigabit range and above. Most large corporate or industrial networks use fiber optics for the LAN backbone cabling. Some have also adopted fiber to the desktop using a centralized fiber architecture which can be quite cost effective. Even fiber to the home architectures are being used in premises networks.
Fiber offers several advantages for LAN backbones. The biggest advantage of optical fiber is the fact it can transport more information longer distances in less time than any other communications medium. In addition, it is unaffected by the interference of electromagnetic radiation which makes it possible to transmit information and data through areas with too much interference for copper wiring with less noise and less error, for example in industrial networks in factories. Fiber is smaller and lighter than copper wires which makes it easier to fit in tight spaces or conduits. A properly designed centralized fiber optic network may save costs over copper wiring when the total cost of installation, support, regeneration, etc. are included.
Centralized Fiber To The Desktop
Replacing UTP copper cables to the desktop with fiber optics was never cost effective, as each link requires converters to connect to the copper port on the PC to fiber and another on the hub/switch end unless dedicated hubs/switches with fiber ports are used. Some users did pay that cost, as they expected to upgrade to speeds that would not run on UTP and did not want to install upgrades each time the network speed increased.
However, the solution to cost-effective fiber in the LAN is using centralized fiber (see right side of diagram above.) Since fiber supports longer links than copper, it's possible to build networks without telecom rooms for intermediate connections, just passive fiber optics from the main equipment room to the work area. In the standards, this is known as centralized fiber architecture. Since the telecom room is not necessary, the user saves the cost of the floor space for the telecom room, the cost of providing uninterrupted power and data ground to the telecom room and year-round air conditioning to remove the heat generated by high speed networking equipment. This will usually more than offset the additional cost of the fiber link and save maintenance costs.
OLANs - Optical LANs
Recently, fiber to the home (FTTH) using a passive optical network (PON) or point-to-point (P2P) links became cost-effective for broadband connections. In the first 5 years of active FTTH installations, almost 100 million homes, apartments and businesses were directly connected on fiber. Such high volume means prices dropped enough to make the cost of a singlemode fiber link almost as cheap as copper, or even cheaper when fibers higher bandwidth capacity allowed for multiple users to share one fiber link. Once suppliers and users realized that a premises LAN was not different than an apartment building (called a MDU - multi-dwelling unit - in FTTH jargon) the FTTH architecture began being used in large LANs. By "large" we mean they started with LANs covering large geographic areas like a campus or large building as well as "large" numbers of users.
These applications became known as "passive optical LANs" (POLs) when using FTTH PON technology and "fiber to the office" (FTTO) when using P2P links. Collectively they are being called OLANs for "optical LANs). OLANs are based on international standards for FTTH but are being considered to be included in the structured cabling standards in the future.
Both FTTO and POL use multiport mini-switches at the user outlet. POLs are designed for triple play services (voice, data and video) but may only carry the services needed by the user. FTTO outlets are usually multiport Ethernet. Data ports are generally Gigabit Ethernet but upgrades to higher bit rates may be done. User terminals may have POE (power over Ethernet) available using the power for the ONT or switch.
OLANs are ideal solutions for many networks. They are essentially not distance limited, so they are ideal for large buildings (convention centers, airports, libraries, sports facilities, hospitals, etc.) or campuses. They scale easily to large networks, with networks of 16,000 users already installed. They use little space compared to traditional structured cabling networks. Not only are telecom rooms not needed, but the entrance facility electronics are small too. And the networks easily accommodate very high data usage; some can handle 10-20Gb/s connections to the outside world. POLs also have another advantage in security. Since they broadcast through a PON splitter to all users, each signal must be encrypted, adding a layer of security for the network.
More on OLANs.
More on fiber optic network design topics.
Other Premises Uses For Fiber
Premises cabling for LANs is where the fiber/copper/wireless arguments generally focus. A century and a half of experience with copper communications cabling gives most users a familiarity with copper that makes them skeptical about any other medium. And in many cases, copper has proven to be a valid choice. Most building management systems use proprietary copper cabling, for example thermostat wiring, as do paging/audio speaker systems. Security monitoring and entry systems, certainly the lower cost ones, still depend on copper, although high security facilities like government and military installations often pay the additional cost for fiber’s more secure nature.
Surveillance systems are becoming more prevalent in buildings, especially airports, government offices, banks, casinos or other buildings that are considered possible security risks. While coax connections are common in short links and structured cabling can run cameras limited distances on Cat 5E or Cat 6 UPT like computer networks, fiber has become a much more common choice. Besides offering greater flexibility in camera placement because of its distance capability, fiber optic cabling is much smaller and lightweight, allowing easier installation, especially in older facilities like airports or large buildings that may have available spaces already filled with many generations of copper cabling.
Industrial networks have used fiber for many years. In a factory environment, immunity from the electrical noise generated by machinery is often the primary reason for using fiber instead of copper cables. The long distances in large buildings and the need to have small cables that can easily be pulled in conduit also argue for fiber's use.
A Quick Fiber Primer
Optical fiber is comprised of a light carrying core surrounded by another optical layer called the cladding that traps light into the core. Fiber is characterized by the size and composition of the core which determines how the light is carried in the core.
Step index multimode fiber has a core comprised of one single type of optical material, either glass or plastic. It has higher attenuation and is too slow for many uses, due to the dispersion caused by the different path lengths of the various modes travelling in the core. Step index fiber is not widely used - only POF (plastic optical fiber) and PCS/HCS (plastic or hard clad silica, plastic cladding on a glass core) use a step index design today.
Graded index multimode fiber uses variations in the composition of the glass in the core to compensate for the different path lengths of the modes. It offers hundreds of times more bandwidth than step index fiber - up to about 2 gigahertz. Two types are in use, 50/125 and 62.5/125, where the numbers represent the core/cladding diameter in microns.
Singlemode fiber shrinks the core down so small that the light can only travel in one ray. This increases the bandwidth to almost infinity - but it's practically limited to about 100,000 gigahertz - that's still a lot! Singlemode fiber has a core diameter of 8-10 microns, specified as "mode field diameter," the effective size of the core, and a cladding diameter of 125 microns.
Most premises cabling is multimode fiber but some backbones OLANs, telecom and CATV signals use singlemode. Multimode fiber comes in several sizes, defined by its core size and bandwidth specs. For many years premises applications primarily used 62.5/125 multimode fiber, originally called "FDDI fiber" because the first fiber-only LAN used it, but now internationally standardized as OM1 fiber. With the advent of Gigabit Ethernet and Fibre Channel at gigabit speeds, the low bandwidth capability of OM1 fiber with 850 nm VCSEL laser sources used for gigabit transmitters limited it's link lengths, so many users switched to 50/125 fiber which had been optimized for 850 nm lasers in the earliest days of fiber optics for telephone links. OM2 fiber had good bandwidth, but manufacturers developed OM3 fiber with even higher bandwidth and longer link capability. OM3 is generally the fiber of choice for premises LAN use today, but even higher bandwidth 50/125 fiber, called OM4, is in development.
It's important to remember that premises cabling standards may not be the same as OSP cabling standards or manufacturer's specifications which tend to be better than the lower specs of the standards. Here, for example, are the premises cabling standards.
Structured Cabling Fiber Optic Cable Performance Standards
Note that these specs are quite conservative, compared to what is routinely available in the marketplace. The spec notes also that the cable manufacturer can use the fiber manufacturer's data on bandwidth, so they do not have to test it.
More on fibers.
Fiber Optic Data Links
Fiber Optic Datalink
Fiber optic transmission systems all use data links that work similar to the diagram shown above. Each fiber link consists of a transmitter on one end of a fiber and a receiver on the other end. Most systems operate by transmitting in one direction on one fiber and in the reverse direction on another fiber for full duplex operation. It's possible to transmit both directions on one fiber but it requires couplers to do so and fiber is less expensive than couplers. A FTTH passive optical network (PON) is one of the only systems using bidirectional transmission over a single fiber because its network architecture is based around couplers already.
Fiber Optic Transceiver
Most systems use a "transceiver" which includes both transmission and receiver in a single module. The transmitter takes an electrical input and converts it to an optical output from a laser diode or LED. The light from the transmitter is coupled into the fiber with a connector and is transmitted through the fiber optic cable plant. The light from the end of the fiber is coupled to a receiver where a detector converts the light into an electrical signal which is then conditioned properly for use by the receiving equipment.
More on fiber optic transceivers and their components
Premises networks are quite different from long-haul outside plant systems. Long haul systems use singlemode fiber which has the lowest attenuation and virtually unlimited bandwidth. Premises cabling distances are short so attenuation of the fiber is of less concern, although bandwidth can be a major issue with gigabit networks and faster. Most premises networks use multimode fiber since it uses inexpensive sources like LEDs and VCSELs. Early, and slower, premises systems used LED sources with 62.5/125 micron (called OM1) fibers (and 100/140 in the earliest LANs), but LEDs are not useable above about 250 Mb/s. With the advent of Gigabit Ethernet and the faster versions of Fibre Channel, premises networks switched to transmitters using 850 nm VCSELs, vertical cavity surface-emitting lasers, that offered adequate speed at a very low cost.
Above 1 Gb/s, fiber bandwidth became an issue, as the distance limitation was fiber bandwidth not attenuation, especially with OM1 fiber. With the advent of Gigabit Ethernet, fiber manufacturers brought back an older fiber design, 50/125 micron (now called OM2 fiber), that had higher bandwidth since it was originally designed for use with lasers around 1980. Further recent developments of 50/125 fiber has provided extremely high bandwidth capability (OM3 fiber.) Most current networks use OM3 fiber for new installations as it provides adequate bandwidth for future 10 gigabit networks. Future networks at 40-100 Gb/s have spurred development of OM4 fiber with even higher bandwidth. More on fibers.
Many networks not only use the highest bandwidth multimode fiber, but also install hybrid cables which contain both multimode and singlemode fibers in the backbone. Some current applications already use singlemode, like CATV video or some telephone or cellular antenna systems.
Since many premises networks already have 62.5/125 fiber systems, adding 50/125 for new systems requires not mixing them, as connecting 62.5/125 fiber to 50/125 fiber will cause large mismatch fiber diameter losses. Color coding the OM3 fiber in aqua per the standards is one good way to distinguish them. Another solution is to use LC connectors on OM2/OM3 systems which are not intermateable with ST or SC connectors commonly used on OM1 fiber cables. More on specifying OM3 cable systems.
Fiber Optic Cable Choice
Cable Types: (L>R): Zipcord, Distribution, Loose Tube, Breakout
Unlike UTP copper cables which are all 4-pair cables, fiber optic cables can be chosen with different fiber counts and even different cable types to allow choosing the optimum cable for the application.
Patchcords are made from simplex tight buffer cables, equipment connection cords or even some short links will use zipcord which is just two simplex cables molded together.
Most premises cables, especially backbone cables, are of the distribution type, which has the highest fiber count for the smallest cable diameter. Distribution cables have buffered fibers that can be directly terminated and placed in patch panels.
Breakout cables are bundles of simplex cables in a common jacket. Breakout cable is the most rugged premises cable, easily terminated directly on each subcable which is protected and needs no patch panels or boxes for protection. It is ideal for industrial applications or in equipment rooms.
Loose tube cables are designed for outside plant environments where high pulling tension and moisture protection is needed. They are difficult to terminate because of the bare fibers inside the tubes and are rarely used indoors. Since their primary use is outdoors where they need jackets that are resistant to moisture, sun, etc. they are not generally rated for flame retardance and cannot be used indoors.
All premises cables must be rated for fire retardance per NEC Article 770. Cables are rated for general purpose use, riser rated (more fire retardant) or plenum rated (low emissions for use in air-handling areas.)
Cables without UL or other fire retardance markings should never be installed indoors as they will not pass building inspections! Outdoor cables are not fire-rated and can only be used up to 50 feet indoors. If you need to bring an outdoor cable indoors, consider a double-jacketed cable with PE jacket over a PVC UL-rated indoor jacket. Simply remove the outdoor jacket when you come indoors and you will not have to terminate at the entry point.
More on cables.
Most, but not all, premises fiber optic cables have jackets color-coded to indicate the fibers in the cable. Multimode cables are traditionally orange and singlemode are yellow. With the addition of OM2 and OM3 fibers to the mix, OM3 cable jackets is color-coded aqua. The table below shows the color codes specifed in TIA-598.
Colored outer jackets or print may be used on Premises Distribution Cable, Premises Interconnect Cable or Interconnect Cord, or Premises Breakout Cable to identify the classification and fiber sizes of the fiber. When colored jackets are used to identify the type of fiber in cable containing only one fiber type, the colors shall be as indicated in Table. Other colors may be used providing that the print on the outer jacket identifies fiber classifications in accordance with subclause 4.3.3. Such colors should be as agreed upon between manufacturer and user.
Unless otherwise specified, the outer jacket of premises cable containing more than one fiber type shall use a printed legend to identify the quantities and types of fibers within the cable. Table 3 shows the preferred nomenclature for the various fiber types, for example "12 Fiber 8 x 50/125, 4 x 62.5/125." When the print on the outer jacket of premises cable is used to identify the types and classifications of the fiber, the nomenclature of the table is preferred for the various fiber types.
Termination: Connectors and Splices
All fiber must have connectors which allow patching cables into links and connecting transmission equipment. Sometimes cables are permanently connected using splices, either fusion splices which are made by welding fibers together in an electrical arc or mechanical slices which have simple alignment fixtures that clamp fibers together. Connectors, not splices, are used in most premises cable plants, as their easy connection/disconnection/reconnection offer the ability to reconfigure cable runs, test individual links and connect hardware where needed.
Premises fiber optic connectors: SC, ST, LC
Early structured cabling standards called for SC connectors as the standard, but users balked, as many had systems already installed with other types, primarily the ST. The standards committees then created the FOCIS documents, Fiber Optic Connector Intermateability Standard, and allowed the use of any connector with FOCIS documentation. Over time, many systems migrated toward the SC, but now the LC is gaining in popularity.
Most transceivers at 1 Gb/s or above use LC connectors for their smaller size and precision, making them a logical choice for cabling being used at high speeds. In addition, since newer cable plants are using 50/125 OM2 or OM3 fiber instead of the older 62.5/125 OM1 fiber for the higher bandwidth capability, using LC connectors on the OM2 and OM2 cable plants prevents mating 62.5/125 fiber to 50/125 fiber which can cause high excess loss from mismatched fibers when connecting the larger fiber to the smaller.
Multimode connectors are usually installed in the field on the cables after pulling, while singlemode connectors are usually installed by splicing a factory-made "pigtail" onto the fiber. Field terminations on multimode may be made using adhesive/polish or prepolished/splice terminations. One can also design and install prefabricated systems, cables already teminated to proper lengths that only need installation and plugging into breakout modules.
Fiber optic cable is, for the most part, installed in buildings the same way as copper wiring. Most cables are installed bare, without connectors, which are then installed in the field. Many installers feel that termination of fiber is no more difficult than Cat 6 copper, as installation techniques are not as likely to affect performance specs as termination of copper cables. These installers generally use traditional adhesive/polish termination procedures. The other choices for fiber termination are to use prepolished/splice connectors that use a simple prepare the cable and fiber, then crimp on the connector. Another option now considered both technically and economically viable is to install preterminated systems. These use factory-manufactured systems with miniature multifiber connectors that can be installed as easily as unterminated cables, but then only require plugging into breakout modules in a patch panel.
Most premises cables are short enough that the primary cause of loss is the loss is the connectors, and, since they are generally field-terminated, connectors will be the focus of testing. Each connector should have three tests: 1) Visual inspection with a microscope to verify polishing if field polished and to ensure no dust or other contamination is present. 2) Loss, called insertion loss, measured by a light source and power meter. 3) Polarization, that is the fibers are arranged so one end of each fiber link is connected to a transmitter and the other end to a receiver.
Fiber testing is much simpler than copper or wireless testing, since the installation needs only testing for end-to-end loss with a simple fiber test set. Unlike copper, installation of connections is unlikely to affect the bandwidth of the fiber, only the loss, and fiber cables have no crosstalk problems, so simple loss testing is all that is required. Most premises networks are too short to be tested by OTDRs so OTDR testing is not required by any premises standard. Testing, therefore is simple, fast and inexpensive. Standards call for insertion loss testing using a light source and power meter with reference launch and receive cables which match the fiber size and connector type of the cable plant being tested as shown in the diagram below and described in TIA OFSTP-14.
Since most premises cables use multimode fiber, one must be careful to control the test source launch conditions in order to get trustworthy test results.
When the networking or transmission equipment is installed, an optical power meter can be used to test the transmitter and receive power in the link to determine if the system is withing the manufacturer's specifications. More on fiber optic testing.
With the small size of glass optical fibers, dirt is a major concern. Dust particles are large compared to the core of fibers and may scratch connectors if not removed by cleaning. Patch panels have mating adapters that can become contaminated by dust if left open to the air. Test equipment has fiber-bulkhead outputs that need periodic cleaning, since they may have hundreds of insertions of test cables. Always keep dust caps on connectors, bulkhead splices, patch panels or anything else that is going to have a connection made with it. Not only will it prevent additional dust buildup, but it will prevent contamination from being touched or damaged from dropping. Always clean connectors before insertion, whether testing or connecting patchcords and equipment. More on cleaning.
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To learn more about fiber optics, see the FOA Fiber Optic Reference Guide sections on fiber.
Table of Contents: The FOA Reference Guide To Fiber Optics