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Which Optical Fiber Should You Use for Your Fiber to The Home (FTTH) Network?

29 March, 2014

Source: ICT Africa

 
Jabulani Dhliwayo

While we take pride in Africa for leapfrogging some technologies and embracing the most advanced of mobile technologies, such as 4G LTE, it is inevitable that advanced wireline technologies are developed alongside mobile technologies. There will always be applications requiring reliable, secure, cost efficient and very high speed broadband applications that even the most advanced mobile technologies will be inadequate. Interestingly, Global leaders in the deployment of advanced mobile networks, like NTT (Japan) and Verizon (USA) are also leaders in the deployment of fiber to the home (FTTH) networks. This is out of the realization that mobile networks alone, however advanced, will not meet all the broadband requirements of an increasingly bandwidth greedy world. A number of African operators and service providers are also cognizant of this fact and are either deploying or planning to deploy FTTH or other high broadband wireline networks such as fiber to the cabinet (FTTC) or hybrid fiber co-axial (HFC).

ICT Africa routinely addresses a number of questions from some operators and service providers contemplating the deployment of FTTH networks. Most of the questions relate to International FTTH standards, architecture and fiber types to consider when building such networks. We plan to have a comprehensive discussion on these topics during our fiber optic training workshops in selected African cities but in this article, we summarize some of the factors to consider when selecting fiber for FTTH networks.

Not all FTTH networks are the same and as a result there are variations in the fiber requirements for each unique network. It is impossible to offer a blanket recommendation for fibers to use in all FTTH networks. Factors such as the density of the population to be served, whether fiber is deployed inside a building, whether fiber is deployed in protective ducts or whether fiber is deployed over power lines, will have a profound impact on the fiber type to be used. In most cases, different fiber types are used in different parts of the FTTH network. We review important fiber attributes and the different types of fiber available for different network sections.

Operating wavelength and Attenuation
FTTH standards require the use of the 1310nm, 1490nm and 1550nm wavelength windows for upstream, downstream and RF video overlay transmission. This means that the fiber you deploy from the central office to the customer premises equipment (CPE) should be capable of transmitting at these wavelength windows. The fiber attenuation at these wavelengths windows must be sufficiently low to meet the power budget requirements of the network.

The Gigabit Passive Optical Network standard, ITU-T G.984.1, for example recommends a power budget of 28dB in the access network. This budget caters for all losses incurred in the network including cable attenuation, splice losses, macro-bend induced losses, connector losses and repair budget. By using fiber with low attenuation at the wavelength windows in question, the network designer will not only be able to meet the power budget requirement, but may also be able to get additional margin to cater for rampant fiber cuts that are common in Africa.

It is therefore recommended that ITU-G.652.D compliant fiber with the lowest attenuation in the 1310nm window and reduced water peak at 1383nm be used at least in the feeder and distribution parts of the FTTH network. For operators and service providers who have used non-zero dispersion shifted fiber (ITU-T G.655 or G.656) in their long haul networks and are contemplating using the same fiber for their FTTH networks for uniformity and backward compatibility, we strongly discourage them.

The typical cutoff wavelengths of G.655 and G.656 fiber are 1480nm and 1450nm, respectively. This means that single mode operation of these NZDS fibers is not guaranteed below these wavelengths and the attenuation, which is never specified for these fibers at 1310nm, may be significantly high.

Macro-Bend Loss
One short coming of optical fiber is that light escapes from the core when the fiber is bent. As fiber is deployed deeper into the access network and inside premises such as homes, apartments and other multiple dwelling units, it is subjected to very tight bends. Such bends can lead to complete failure of service when standard fiber is used.

In response to this challenge, a relatively new ITU-T standard, G.657 was ratified. The standard describes tighter bend requirements beyond the G.652 fibers whose macro-bend specification requires that if it is wound around a 30mm radius mandrel 100 times, the induced loss at 1550nm should be no more than 0.1dB. This specification was no longer adequate for access networks in which bend radii as low as 5mm are now a reality.

The G.657 standard describes two categories of single mode fiber that can be used in access networks. Category A fibers are optimized for transmission in the entire optical fiber spectrum, from 1260nm to 1625nm and they comply with the G.652 recommendations. Category B fibers do not have to comply with G.652 recommendations but they are suitable for transmission at wavelength windows required for access networks, albeit for restricted distances. The two tables are further divided into sub-tables A1, A2, B2 and B3, each with a different level of macro-bend performance. Sub tables A1, A2/B2 and B3 specify maximum acceptable bend induced loss for bend radii as low as 10mm, 7.5mm and 5mm respectively. For the detailed specification, please review the G.657 recommendation document.

Mode Field Diameter
Network designers are encouraged to pay attention to the mode field diameter of the fiber. Splicing fibers with different mode field diameter leads to large splice losses which should be accounted for in the network budget calculation. Some of the G.657 B compliant fibers achieve bend performance through very low MFD which can result in large splice losses when the fiber is spliced to G.672.D or G.657 A fiber.

So which fiber should you use in an FTTH network?
Including G.652.D fibers, you now have six different fiber types to choose from for any portion of your FTTH network. This presents a complexity in the decision process especially when vendors are giving you different suggestions on which fiber should be used where. It is up to the network designer to predict the level of bends the fiber is likely going to be subjected to and select the relevant fiber. We recommend that you consider using G.652.D or G.657 A1 for your feeder and distribution networks, G.657 A1 or G.657 A2 for the drop network and G.657 B3 compliant fiber for inside wiring, depending on the levels of bend in your network.

Many service providers insist on using one fiber for the entire access network. Unfortunately, higher bend performance fiber, such as G.657 B3 compliant fiber, only makes business sense when used in short distance applications such as in-building wiring. G.657 B3 fibers are typically significantly more expensive than fibers with lower bend performance.

The MFD of your access network fiber should be large enough to closely match the MFD of your installed base or other sections of the access network.


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