Page 50 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Unit 3: Physical Layer

3.4 Keywords Notes

Bandwidth: Refers to the range of frequencies assigned to a channel.
Bounded Media: Refers to the wired transmission systems that employ physical media, which
are tangible.
Coaxial Cable: It is a very robust shielded copper wire two-conductor cable in which a solid
center conductor runs concentrically (coaxial) inside a solid outer circular conductor.

Frequency Spectrum: Refers to the range of frequencies being supported by a particular
transmission medium.
Gauge: Gauge is a measure of the thickness of the conductor.

Graded Index Multimode Fiber: In the case of a graded index multimode fiber, the index of
refraction across the core is gradually changed from a maximum at the center to a minimum
near the edges, hence the name graded index.
Monomode/Singlemode fiber: This has a thinner inner core. In this case, the core diameter of
about 9 µm is much closer in size to the wavelength of light being propagated, about 1.3 µm.
This limits the light transmission to a single ray or mode of light to propagate down the core of
the fiber.

Multimode Fiber: The core diameter is relatively large compared to a wavelength of light.
Optical Fiber: Optical fiber carries the transmitted information in the form of a fluctuating beam
of light in a glass fiber rather than as an electrical signal on a wire.

Propagation Delay: Refers to the length of time required for a signal to travel from transmitter
to receiver across a transmission system.
Shielded Copper or STP: Shielded twisted pair (STP) differs from UTP in that a metallic shield or
screen surrounds the pairs, which may or may not be twisted.
Step Index Multimode Fiber: Different rays travel different distances, and take different amounts
of time to transit the length of a fiber.
Twisted Pair: A twisted pair is a pair of copper wires twisted together and wrapped with a
plastic coating.
Unbounded Media: Refers to wireless transmission systems do not make use of a physical
conductor, or guide, to bind the signal.

Unshielded Twisted Pair (UTP): A UTP cable contains from 2 to 4200 twisted pairs. The advantages
of UTP are the flexibility, low cost media, and can be used for either voice or data communications.

3.5 Review Questions

1. What are the different transmission mediums over which data communication devices
can provide service?
2. What are the major limitations of twisted pair wire?
3. Describe how satellite communication is different from radio broadcast?

4. A receiver in fiber optic system requires 5 microwatt of power. The length of cable is 5 Km
and offers an attenuation loss of 2 dB/km. There is a loss of 1 dB at both the source and the
receiver. Calculate the required level of optical power at the optical source.

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