Page 48 - DCAP406_DCAP_207_Computer Networks

Unit 3: Physical Layer

it, converts it back into an optical signal, and places it onto a fiber, and so on. There may be many Notes
such optical repeaters in a long haul transmission system, although typically far fewer than
would be required using other transmission media.

Figure 3.10: Fiber Optic Systems

Bandwidth: Fiber offers by far the greatest bandwidth of any transmission system, often
in excess of 2 Gbps in long haul carrier networks. Systems with 40 Gbps have been tested
successfully on numerous occasions. The theoretical capacity of fiber is in the terabit
(Tbps) range, with current monomode fiber capacity being expandable to that level.
Error Performance: Fibre being a dielectric (a nonconductor of direct electric current), it is
not susceptible to Electro Magnetic Interference/Radio Frequency Interference (EMI/RFI).
This also does not emit EMI/RFI. The light signal will suffer from attenuation, although
less so than other media. Scattering of the optical signal, bending in the fiber cable,
translation of light energy to heat, and splices in the cable system can cause such optical
attenuation.
Distance: Monomode fiber optic systems routinely are capable of transmitting signals
over distances in excess of 325 km. Hence relatively few optical repeaters are required in
a long-haul system. This will reduce costs, and eliminate points of potential failure.
Security: Fiber is intrinsically secure, as it is virtually impossible to place a physical tap
without detection because no light is radiated outside the cable. Therefore, interception of
signal is almost impossible. Additionally, the fiber system supports such a high volume
of traffic that it is difficult to intercept and distinguish a single transmission from the tens
of thousands of other transmissions that might ride the same cable system. The digital
nature of most fibers coupled with encryption techniques frequently used to protect from
interception make fibers highly secure.
Cost: While the acquisition, deployment, and rearrangement costs of fiber are relatively
high, the immense bandwidth can outweigh that cost in bandwidth-intensive applications.
At Gbps speeds, a single set of fibers can carry huge volumes of digital transmissions over
longer distances than alternative systems, thereby lowering the transport cost per bit and
cost per conversation to fractions of a penny per minute.

Applications: Applications for fiber optic transmission systems are bandwidth intensive.
Such applications include backbone carrier networks, international submarine cables,
backbone LANs (FDDI), interoffice trunking, computer-to-computer distribution networks
(CATV and Information Superhighway) and fiber to the desktop (Computer Aided Design).

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