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Dense Wavelength Division Multiplexing 2.2 Challenges of Today’s Telecommunications Network:- To understand the importance of DWDM technology and optical networking, these capabilities must be discussed in the context of the challenges faced by the telecommunications industry, and in particular, service providers. Forecasts of the amount of bandwidth capacity needed for networks were calculated on the presumptions that a given individual would only use network bandwidth six minutes of each hour. These formulas did not factor in the amount of traffic generated by Internet access (300 percent growth per year) ,faxes, multiple phone lines,modems,teleconferencing , data and video transmission had these factors been included, a far different estimate would have emerged. In fact, today many people use the bandwidth equivalent of 180 minutes or more each hour. Therefore, an enormous amount of bandwidth capacity is required to provide the services demanded by consumers. In addition to this explosion in consumer demand for bandwidth, many service providers are coping with fiber exhaust in their networks. An industry survey indicated that in 1995, the amount of embedded fiber already in use in the average network was between 70 percent and 80 percent. Today, many carriers are nearing one hundred percent capacity utilization across significant portions of their networks. Another problem for Carriers are the challenge of deploying and integrating diverse technology in one physical infrastructure. Consumer demands and competitive pressures mandate that carriers offer diverse services Economically and deploy them over the embedded network. DWDM provides service providers an answer to that demand. 2.3 Resolving the Capacity Crisis:- Faced with the multifaceted challenges of increased service needs, fiber exhaust & layered bandwidth management, service providers need options to provide an economical solution. One way to alleviate fiber exhaust is to lay more fiber, and for those networks where the cost of laying new fiber is minimal, these will prove the most economical solution. However, laying new fiber will not necessarily enable the service provider to provide new services or utilize the bandwidth management capability of a unifying optical layer. A second choice is to increase the bit rate using time division multiplexing (TDM) ,where TDM increases the capacity of a fiber by slicing time into smaller intervals so that more bits (data) can Division Of Computer Engineering, SOE 7
Dense Wavelength Division Multiplexing be transmitted per second. Traditionally, this has been the industry method of choice (DS-1, DS- 2, DS-3 etc.). However, when service providers use this approach exclusively, they must make the leap to the higher bit rate in one jump, having purchased more capacity than they initially need. Based on the SONET hierarchy, the next incremental step from 10 Gbps TDM is 40 Gbps – a quantum leap that many believe will not be possible for TDM technology in the near future. This method has also been used with transport networks that are based on either the synchronous optical network (SONET) standard for North America or the synchronous digital network (SDH) standard for international networks. Options for Increasing Carrier Bandwidth Faced with the challenge of dramatically increasing capacity while constraining costs, carriers have two options: Install new fiber or increase the effective bandwidth of existing fiber. Laying new fiber is the traditional means used by carriers to expand their networks. Deploying new fiber, however, is a costly proposition. It is estimated at about $70,000 per mile, most of which is the cost of permits and construction rather than the fiber itself. Laying new fiber may make sense only when it is desirable to expand the embedded base. Increasing the effective capacity of existing fiber can be accomplished in two ways: • Increase the bit rate of existing systems. • Increase the number of wavelengths on a fiber. Increase the Bit Rate Using TDM, data is now routinely transmitted at 2.5 Gbps (OC-48) and, increasingly, at 10 Gbps (OC-192); recent advances have resulted in speeds of 40 Gbps (OC-768). The electronic circuitry that makes this possible, however, is complex and costly, both to purchase and to maintain. In addition, there are significant technical issues that may restrict the applicability of this approach. Transmission at OC-192 over single-mode (SM) fiber, for example, is 16 times more affected by chromatic dispersion than the next lower aggregate speed, OC-48. The greater transmission power required by the higher bit rates also introduces nonlinear effects that can affect waveform quality. Finally, polarization mode dispersion, another effect that limits the distance a light pulse can travel without degradation, is also an issue. Division Of Computer Engineering, SOE 8
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