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Options for FTTH Optical Fiber Cables


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FTTH (fiber to the home) provides an excellent platform for high or ultra-high speed access technologies. And FTTH solutions bring great benefits to fixed access networks and advanced wireless networks. FTTH network constitutes a fiber-based access network, connecting a large number of end users to a central point known as an access node or point of presence (POP). What kind of optical fiber cables can be used in FTTH deployment? This paper will tell the answer.

Factors to Consider

There are several types of fiber optic cables that can be used in FTTH network. Usually, the future proofed FTTH schemes are based on single-mode fiber (SMF) cables. But in some specific situations, the multimode fiber (MMF) cables can also be used. A number of factors should be taken into consideration when making final decision.

Network Architecture

The choice of network architecture has an impact on the data rate that must be delivered by the fiber and the available optical power budget of the network. And both factors affect the choice of optical fiber.

Network Size

Network size refers to the number of premises served by the network. However, in this context it refers to the physical distance across the network. The available power budget will determine how far the POP can be located from the subscriber. Power budgets are influenced by all the components in the optical path including the fiber.

Fiber Type of Existing Network

If an existing network is expanded, the optical fiber in the new network segments must be compatible with the fiber in the existing network.

Expected Lifetime

FTTH networks are designed with a lifespan of at least 30 years. So it is imperative that investments to the FTTH infrastructure are suitable for future needs. Changes to the choice of fiber during the expected lifespan of the FTTH network are not a realistic option.

Optical Fiber Basis

Optical fiber is effectively a light pipe carrying pulses of light generated by lasers or other optical sources to a receiving sensor (detector). Transmission of light in an optical fiber can be achieved over considerable distances, supporting high-speed applications unsustainable by today’s copper-based networks. Fiber is made from high purity silica. Initially formed into glass-like rods, they are drawn into fine hair-like strands and covered with a thin protective plastic coating. It consists of a core, cladding and outer coating. Light pulses are launched into the core region. The surrounding cladding keeps the light traveling down the core and prevents it from leaking out and an outer coating, usually made of a polymer, is applied during the drawing process.


 


 

The fiber core can be designed in various geometrical sizes which depending how the light pulse travels, produces different optical performances. Two main parameters determine how efficiently light pulses are transmitted down the fiber, which are attenuation and dispersion.

Attenuation refers to the reduction of optical power over distance. Even with the extremely pure materials used to manufacture the fiber core and cladding, power is lost over distance by scattering and absorption within the fiber. Fiber attenuation limits the distance light pulses can travel and still remain detectable. Dispersion can broadly be described as the amount of distortion or spreading of a pulse during transmission. If pulses spread too far, the detector at the other end of the fiber is not able to distinguish one pulse from the next, causing loss of information. Chromatic dispersion occurs in all fibers and is caused by the various colors of light (components of a light pulse) traveling at slightly different speeds along the fiber. Dispersion is inversely related to bandwidth, which is the information carrying capacity.

Options for FTTH Optical Fiber Cables

Three types of optical fiber cables are introduced in this tutorial for FTTH deployment.

Single-mode Fiber Cable

Single-mode fiber cable has a small core size (<10um) which supports only one mode of light. It provides the lowest optical attenuation loss and the highest bandwidth transmission carrying capacity of all the fiber types. But it incurs higher equipment cost than multimode fiber systems. For FTTH applications, the ITUT G.652 recommendations for single-mode fiber are sufficient to cover the needs of most networks. And wide scale FTTH deployments may reduce single-mode equipment costs through increased volumes.

Multimode Fiber Cable

Multimode fiber cable has a larger core size (50 or 62.5um) which supports many modes (different light paths through the core). It can operate with cheaper light sources and connectors. However, the fiber itself is more expensive than single-mode. Multimode fiber is used extensively in data centers and sometimes used in campus networks. It has lower bandwidth capability and restricted transmission distance. A consideration for using multimode fiber is the connection to the existing fiber base, which is most likely to single-mode fiber. This will require some form of intermediate active equipment. Multimode fibers can successfully be installed in flats or multi-tenant buildings using a local active node in the basement.

 


 

Bend Insensitive Fiber Cable

When cabling inside buildings, many areas prove difficult for conventional fibers resulting in possible poor optical performance. To avoid this, very careful and skilled installation practices are required or special fiber protection is needed with ducts and cable designs. However, for some time fiber types with the ITU-T G.657 standard have been widely available allowing fiber optic cables to be installed as easily as conventional copper cables. The fibers inside these cables, which are termed “bend-insensitive”, are capable of operating at a bend radius down to 7.5mm, with some fibers fully compliant down to 5mm.

 

 

 

The recommended G567 describes two categories of single-mode fibers which are suitable for use in access networks. Both categories A and B contain sub-categories which differ in macro-bending loss thus the difference between these fibers is in the permissible bending radius.

Category A contains the recommended attributes and values needed to support optimized access network installation with respect to macro-bending loss. However, the recommended values for the other attributes still remain within the range recommended in G.652.D and emphasizes backward compatibility with G.652.D fibers. This category has three sub-categories with different macro-bending requirements: G.657.A1, G.657.A2 and G.657.A3 fiber. Category B contains the recommended attributes and values needed to support very low bending radii particularly applicable to in-building installations. For the mode-field diameter and chromatic dispersion coefficients, the recommended range of value might be outside the range of values recommended in ITU-T G.652 and thus not necessarily backward compatible. This category has two sub-categories with different macro-bending requirements: G.657.B2 fiber and G.657.B3 fiber.

Summary

This tutorial has introduced three types of optical fiber cables for FTTH deployment and the factors that should be considered when selecting one over the other. And other factors may need to be considered on a case-by-case basis. So which kind of cable to use may need to depend on the actual network requirements.You may find one you need from this paper or contact us for more information: sales@gelink.com.cn