Transmission Media

1 January 2017

Optical Signal is carried by photon pulses through thin (8 to 10 microns) glass strands (optical fibers), light waves are produced either by Light emitting diodes (LEDs) or injection laser diode (ILD). at transmitting and receiving end, signal is converted from and reconverted to electrical form by optical modems such as an avalanche photo diode. Greater capacity: Data rates of hundreds of Gbps Smaller size & weight Lower attenuation Electromagnetic isolation Greater repeater spacing: 10s of km at least

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Immunity to corrosive materials More immune to tapping * Long-haul trunks: about 1500 km in length & 20,000-60,000 voice channels. undersea optical fiber * Metropolitan trunks: -about 12 km in length & 100,000 voice channels -underground conduits joining telephone exchanges. * Rural exchange trunks: – about 40 – 160 km in length & less than 5000 voice channels. * Subscriber loops:- handling voice, data, image and video. * LANs: Capacity of 100 Mbps to 1 Gbps. 2- Distinguish the characteristics of optical fiber from twisted pair or coaxial cable.

The potential bandwidth, and hence data rate, of optical fiber is immense; data rates of 2 Gbps over tens of kilometers have been demonstrated. Compare this capability to the practical maximum of hundreds of Mbps over about 1 km for coaxial cable and just a few Mbps over 1 km or up to 100 Mbps over a few tens of meters for twisted pair. 2) Smaller size and lighter weight: Optical fibers are considerably thinner than coaxial cable or bundled twisted-pair cable-at least an order of magnitude thinner for comparable information-transmission capacity.

For cramped conduits in buildings and underground along public rights-of-way, the advantage of small size is considerable. The corresponding reduction in weight reduces structural support requirements. 3) Lower attenuation: Attenuation is significantly lower for optical fiber than for coaxial cable or twisted pair and is constant over a wide range. 4) Electromagnetic isolation: Optical fiber systems are not affected by external electromagnetic fields. Thus, the system is not vulnerable to interference, impulse noise, or crosstalk.

By the same token, fibers do not radiate energy, thereby causing little nterference with other equipment and thus providing a high degree of security from eavesdropping. In addition, fiber is inherently difficult to tap. 5) Greater repeater spacing: Fewer repeaters means lower cost and fewer sources of error. The performance of optical fiber systems from this point of view has been steadily improving. For example, AT&T has developed a fiber transmission system that achieves a data rate of 3. 5 Gbps over a distance of 318 km [PARK921 without repeaters. Coaxial and twisted-pair systems generally have repeaters every few kilometers.

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