18-12-2012, 04:57 PM
Computer Networks Questions & Answers
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Q1. Explain about Coaxial Cable.
Coaxial Cable:
Coaxial cable is a common transmission medium. It has better shielding than twisted
pairs, so it can pan longer distances at higher speeds. Two kinds of coaxial cable are widely
used. One kind, 50-ohm cable, is commonly used when it is intended for digital transmission
from the start. The other kind, 75-ohm cable, is commonly used for analog transmission and
cable television but is becoming more important with the advent of Internet over cable. This
distinction is based on historical, rather than technical, factors (e.g., early dipole antennas had
an impedance of 300 ohms, and it was easy to use existing 4:1 impedance matching
transformers).
A coaxial cable consists of a stiff copper wire as the core, surrounded by an insulating
material. The insulator is encased by a cylindrical conductor, often as a closely-woven
braided mesh. The outer conductor is covered in a protective plastic sheath. A cutaway view
of a coaxial cable is shown in Fig.1.
Q2. Explain about Twisted Pair.
Twisted Pair:
Twisted pair is the oldest and most common transmission media. A twisted pair
consists of two insulated copper wires, typically about 1 mm thick. The wires are twisted
together in a helical form, just like a DNA molecule.
Twisting is done because two parallel wires constitute a fine antenna. When the wires
are twisted, the waves from different twists cancel out, so the wire radiates less effectively.
The most common application of the twisted pair is the telephone system. Nearly all
telephones are connected to the telephone company (Telco) office by a twisted pair.
Q3. Explain about Fiber optic cables.
Fiber optic cables:
Fiber optic cables are similar to coax, except without the braid. Fig.3.1 (a) shows a
single fiber viewed from the side. At the canter is the glass core through which the light
propagates. In multimode fibers, the core is typically 50 microns in diameter, about the
thickness of a human hair. In single-mode fibers, the core is 8 to 10 microns.
Q6. Compare circuit-switched and packet-switched networks.
Circuit switching and packet switching differ in many respects. Circuit switching
requires that a circuit be set up end to end before communication begins. Packet switching
does not require any advance setup. The first packet can just be sent as soon as it is available.
The result of the connection setup with circuit switching is the reservation of bandwidth all
the way from the sender to the receiver. All packets follow this path. Among other properties,
having all packets follow the same path means that they cannot arrive out of order. With
packet switching there is no path, so different packets can follow different paths, depending
on network conditions at the time they are sent. They may arrive out of order.
7. Explain dual cable and single cable systems in broadband coaxial cable.
Dual cable:
Dual cable systems have two identical cables, cablel1 is used to transmit data
(inbound communication) and cable2 is used to receive (outbound communication). A
computer that wants to transmit data, it will sent data to cabel1 which is received by a device
called the head-end at the root of the cable tree. The head-end then send this data down the
tree on to cable2 which is received by the receiver. Fig.7 shows dual cable system.