A typical optic fiber link for the use of telephony connects the OLTE to a primary digital Multiplexing equipment handling 2 Mb/s or higher order streams as 8 Mb/s, 34 Mb/s or at still higher as in Synchronous Digital Hierarchy (SDH) streams of 155 Mb/s or more. However irrespective of the input digital stream to the OLTE input at the lowest level the circuits are derived from mostly 2Mb/s digital primary Multiplexing equipment's which process 2 Mb/s streams to convert to 30 voice or data channels of 64 Kb/S. There are compression technique available to increase the number of voice channels from one to even four for every 64 Kb/S digital channels.
The OLTE generates infrared signals which are modulated by the digital streams and are launched to the optical fiber through connectors. The transmission in the fiber the most commonly used method is the mono mode which means that the fiber allows only one mode of transmission thus giving the highest speeds as there is no dispersion to distort the digital pulse shape. Standard OLTES are available from many telecommunication equipment manufacturers and it is required to specify to them the Data rate for the choice of the correct equipment.
For low cost applications simple optical modems are also available which can directly connect computer to computer , create optical fiber LAN . These are more aptly called optical modems and can be used for setting up simple links for the cost sensitive user and whose requirements are specific and whose main aim is not to provide telecom service but to use his own link to handle data.
Optical fiber cable have made a long journey from the early days where handling of the optical fiber required very special handling and joining and fault finding was not easy. It is difficult to differentiate between a optical fiber cable from an ordinary copper cable specially if the optical fiber cable is armored. The optical fiber cables being of light weight have longer drum lengths and can be about 3 Km per drum .
For joining of optical fiber system an instrument known as splicing machine is used . The splicing machine is an automatic machine which ensures that the optical fiber is aligned perfectly and then is fused by the generation of high heat to fuse the glass forming the optical fiber. Before joining the fibers preparation for the joining is required and it consists of stripping the outer covering of the optical fiber. After the outer cover of the optical fiber is stripped off the optical fiber cores are exposed. These fibers have a coating over each which are then required to be stripped off. For stripping off the outer cover tools similar to those used for stripping of traditional cables are used since the mechanical objectives are similar. The exposed fiber end is then cleaned and the ends of the fibers cut using the splicing machine to ensure that the end of the fibers are made flat and perpendicular to the fiber. The two fiber ends so prepared is then put inside a splicing machine and the machine then brings the two ends together and fuses them perfectly to make proper joint. The quality of the joint is immediately made known by the splicing machine through a viewing screen available with the splicing machines. If the quality shown id unsatisfactory then the splice has to rejected and the fibers cut again and the splicing repeated. Normally vibrations , presence of dust at the time of fusion results in the generation of bad splicing.
The portion where splicing is done is a weak area with the raw fiber exposed and mechanical strength available is that of the fiber core itself. Hence what is required is to protect these cores by providing necessary mechanical methods. To make this available optical fiber jointing kits are available which provide the necessary mechanical support to the splices. There are jointing kits suitable for outdoors as well as indoors. The difference between the two mainly consists of the shape and the strength of the outer cover while internally they are same. The indoor kit his box like , rectangular generally and made to look good since it would be visible . The outdoor jointing kit is normally cylindrical to reduce stresses when kept underground and provides better seal against dust etc. Inside both the types of kit there are a number of thin plates. The exposed fiber ends are provided support by these plates and about 4 fibers are supported by one supporting plate. The edges of the plates are shaped to facilitate support to the fibers which are coiled against these shaped edges of the plates. The optic fiber cable need to be clamped properly at the entry and the exit of the jointing kit to ensure that the strains in the cable outside are not transmitted to the fibers inside the jointing kit as otherwise such strains can break the spliced joints.
The cost of a simple optical fibre link is about US $ 15000 per Km varying on cost of laying , size of cable , data raet to be supported.
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