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Q31. What are the advantages and disadvantages of twisted pair.

Twisted pair cabling is a type of wiring in which two conductors (the forward and return conductors of a single

circuit) are twisted together for the purposes of cancelling out electromagnetic interference (EMI) from external

sources; for instance, electromagnetic radiation from unshielded twisted pair (UTP) cables, and crosstalk

between neighbouring pairs.

Advantages

It is a thin, flexible cable that is easy to string between walls.

More lines can be run through the same wiring ducts.

UTP costs less per meter/foot than any other type of LAN cable.

Electrical noise going into or coming from the cable can be prevented.

Cross-talk is minimized.

Disadvantages

It is susceptible to interference and noise.

There is attenuation problem in twisted pair cable.

For analog, repeaters needed every 5-6km

For digital, repeaters needed every 2-3km

It has relatively low bandwidth (3000Hz)

Q32. Why Ethernet is still popular and being used in networking.

Ethernet is the predominant LAN (Local Area Network) technology for home and business networks. Ethernet is

the most popular data link layer protocol in the world, and there are several reasons why this is so. Because it

has been around for more than 20 years, Ethernet hardware, such as NICs and hubs, is inexpensive and easy to

find. The popularity of the protocol also means that experienced Ethernet technicians are common and their

time is also relatively inexpensive. Standard Ethernet is also tolerant of faults like overlong cable segments andloose connections.

Q33. Why twisted pair wire is having less noise than parallel wire?

Interference from devices such as a motor can create unequal noise over two parallel lines. The line that iscloser to the device receives more interference than the one that is farther.Effect of Noise on Parallel Lines

If however, the wires are twisted around each other at regular intervals, each wire is closer to the noise sourcefor half of the time and farther for the other half. Both receive the same amount of noise. The noise from twowires cancel each other out at the receiver.

http://en.wikipedia.org/wiki/Cross-talkhttp://en.wikipedia.org/wiki/Cross-talk

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Q34. What is the form of the signal in twisted pair cable? How does this differ from the signal in

fibre optics cable?

Q35. Explain with justification why the light passing through is reflective instead of refractive in

fibre optics.

When light passes from a medium with one index of refraction (m1) to another medium with a lower index of

refraction (m2), it bends or refracts away from an imaginary line perpendicular to the surface (normal line). As

the angle of the beam through m1 becomes greater with respect to the normal line, the refracted light through

m2 bends further away from the line.

At one particular angle (critical angle), the refracted light will not go into m2, but instead will travel along the

surface between the two media (sine [critical angle]. If the beam through m1 is greater than the critical angle,

then the refracted beam will be reflected entirely back into m1 (total internal reflection), even though m2 may

be transparent.

In an optical fiber, the light travels through the core by constantly reflecting from the cladding because the

angle of the light is always greater than the critical angle. Light reflects from the cladding no matter what angle

the fiber itself gets bent at, even if it's a full circle.

Q36. Explain cyclic redundancy checking (CRC).

Cyclic redundancy checking is used to verify whether the received data is in correct format as it was sent from

the sending end. In data communication data is sent in packets. A CRC bit is placed somewhere in the packet

and is verified at the receiving end.

A CRC is generated by dividing the total number of bits in the block of data being sent by a predetermine

binary number. The remainder is added to the packet and the packet is transmitted. On the receiving end the

reverse mathematical operation is performed to verified the packet contents. If the computation is successful,

the packet is passed to the next step. If it fails, the issuing node is notified and the entire packet is

retransmitted.

Common CRC patterns are 12-bit(CRC 12), CRC 32 etc.

Q37. Explain the light source in optical cable.

Optical communication is the transmission of photon (or light) energy through a low-loss waveguide whosefunction is to propagate the light signals over long distances. In telecommunications systems, the source of thephoton energy may be a light-emitting or semiconductor laser diode, whose function is to produce light energyat a single wavelength. By turning the light source on and off quickly, streams of ones and zeros can betransmitted to form a digital communications channel. The wavelength of the optical light source has beenselected to best match the transmission properties of recognized optical fiber types. The common opticalcommunications wavelengths of 850 nm to 1550 nm fall between the ultraviolet and microwave frequencies inthe light spectrum.

Large diameter optical fiber (50- and 62.5-m) is required to adequately support transmission of light sourceswith larger apertures such as LEDs and vertical-cavity surface-emitting lasers (VCSELs) by minimizing signal

loss and maximizing transmit distances. Small diameter optical fiber (e.g. 9-m) is required to adequatelysupport transmission of laser light sources.

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Q38. What are the advantages of coaxial cable?

The advantages are:

- they are cheap to make

- cheap to install

- easy to modify

- good bandwith

- great channel capacity

- noise immunity due to low error rate

The advantages of using coaxial cable include the following:

One advantage of coaxial cable over other types of radio transmission line is that in an ideal coaxial cable theelectromagnetic field carrying the signal exists only in the space between the inner and outer conductors. Thisallows coaxial cable runs to be installed next to metal objects such as gutters without the power losses thatoccur in other types of transmission lines. Coaxial cable also provides protection of the signal from externalelectromagnetic interference.

The advantage of coaxial design is that the electric and magnetic fields are confined to the dielectric with little

leakage outside the shield. On the converse, electric and magnetic fields outside the cable are largely kept

from causing interference to signals inside the cable. This property makes coaxial cable a good choice for

carrying weak signals that cannot tolerate interference from the environment or for higher electrical signals

that must not be allowed to radiate or couple into adjacent structures or circuits.[2]

Broadband system:

Coax has a sufficient frequency range to support multiple channels, which allows for much greater throughput.

Greater channel capacity

Each of the multiple channels offers substantial capacity. The capacity depends on where you are in the world.In the North American system, each channel in the cable TV system is 6MHz wide, according to the NationalTelevision Systems Committee (NTSC) standard. In Europe, with the Phase Alternate Line (PAL) standard, thechannels are 8MHz wide. Within one of these channels, you can provision high-speed Internet access-that's howcable modems operate. But that one channel is now being shared by everyone using that coax from thatneighborhood node, which can range from 200 to 2,000 homes.

Greater bandwidth

Compared to twisted-pair, coax provides greater bandwidth systemwide, and it also offers greater bandwidth

for each channel. Because it has greater bandwidth per channel, it supports a mixed range of services. Voice,data, and even video and multimedia can benefit from the enhanced capacity.

Lower error rates

Because the inner conductor is in a Faraday shield, noise immunity is improved, and coax has lower error ratesand therefore slightly better performance than twisted-pair. The error rate is generally 10-9 (i.e., 1 in 1 billion)bps.

Greater spacing between amplifiers

Coax's cable shielding reduces noise and crosstalk, which means amplifiers can be spaced farther apart thanwith twisted-pair.

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Q39. On a transmission channel, 600 character message using ASCii7 bit code is used. For

synchronous data stream, there are two SYN characters and a single error detection character is

added.

In case of asynchronous data transmission, there is one start bit and one stop bit and a

single error detection character is added.

Calculate the efficiency of transmission in the above two types of transmission mode.

For synchronous mode of transmission

Total number of characters transmitted=600+2(SYN characters)+1(error detection character).

=603 characters

=603X7=4221 bits

But number of bits carrying actual message=600 characters or 600X7=4200 bits

Hence, efficiency X100=99.5%

For asynchronous mode of transmission

For every character in this mode, there is a start and stop bit. So for each character, we need to send 9 bits i.e.

7+1(start bit)+1(stop bit)

Total number of characters to be sent=600+1(error character) or 601X9=5409 bits

Total number of message bits=600X7 or 4200 bits

Hence, efficiency X100=77.65%

Q40. A message of 2000 characters has to be transmitted from station A to station B. Calculate the

time taken for transmission by synchronous communication scheme in which a 16 bit sync is sent

after every block of 10 characters. Assume each character is represented by a 7 bit ASCII and also

assume the speed to be 1200bps.

Number of character per block is 10 characters. So there will be 200 blocks in the total message. Since 16 bit

sync is added after every 10 characters, number of bits per 10 characters is 10X7+16 =86 bits. For 200 blocks,

number of bits is 86X200=17200 bits.

So time taken to send 17200 bits is =14.33sec