CS2302 – COMPUTER NETWORKS
TWO MARKS
UNIT1- Network architecture – layers – Physical
links – Channel access on links – Hybrid
multiple access techniques -
Issues in the data link layer - Framing – Error correction
and detection – Link-level Flow
Control
1. What are the three criteria necessary for an effective and efficient
network?
The most important
criteria are performance, reliability and security.
Performance of the network depends on number of users, type
of transmission medium, and the capabilities of the connected h/w and the
efficiency of the s/w.
Reliability is measured by frequency of failure, the time it
takes a link to recover from the failure and the network’s robustness in a
catastrophe.
Security issues include protecting data from unauthorized
access and viruses.
2. Group the OSI layers by function?
The seven layers
of the OSI model belonging to three subgroups.
Physical, data link and network
layers are the network support layers;
they deal with the physical aspects of moving data from one device to another.
Session, presentation and
application layers are the user
support layers; they allow interoperability among unrelated software
systems.
The transport layer ensures end-to-end reliable data transmission.
3. What are header and trailers and how do they get added and removed?
Each layer in
the sending machine adds its own information to the message it receives from
the layer just above it and passes the whole package to the layer just below
it. This information is added in the form of headers or trailers. Headers are
added to the message at the layers 6,5,4,3, and 2. A trailer is added at
layer2. At the receiving machine, the headers or trailers attached to the data
unit at the corresponding sending layers are removed, and actions appropriate
to that layer are taken.
4. What are the features provided by layering?
Two nice features:
·
It decomposes the problem of building a network
into more manageable components.
·
It provides a more modular design.
5. Why are protocols needed?
In networks,
communication occurs between the entities in different systems. Two entities
cannot just send bit streams to each other and expect to be understood. For
communication, the entities must agree on a protocol. A protocol is a set of
rules that govern data communication.
6. What are the two interfaces
provided by protocols?
·
Service interface
·
Peer interface
Service interface- defines
the operations that local objects can perform on the protocol.
Peer interface- defines
the form and meaning of messages exchanged between protocol peers to implement
the communication service.
7. Mention the different physical media?
·
Twisted pair(the wire that your phone connects
to)
·
Coaxial cable(the wire that your TV
connects to)
·
Optical fiber(the medium most commonly used for
high-bandwidth, long-distance links)
·
Space(the stuff that radio waves, microwaves and
infra red beams propagate through)
8. Define Signals?
Signals are actually electromagnetic waves traveling at the speed of
light. The speed of light is, however, medium dependent-electromagnetic waves
traveling through copper and fiber do so at about two-thirds the speed of light
in vacuum.
9. What is wave’s wavelength?
The distance between a pair of
adjacent maxima or minima of a wave, typically measured in meters, is called
wave’s wavelength.
10. Define Modulation?
Modulation -varying the
frequency, amplitude or phase of the signal to effect the transmission of
information. A simple example of modulation is to vary the power (amplitude) of
a single wavelength.
11. Explain the two types of duplex?
·
Full
duplex-two bit streams can be simultaneously transmitted over the links at
the same time, one going in each direction.
·
Half
duplex-it supports data flowing in only one direction at a time.
12. What is CODEC?
A device that encodes analog voice
into a digital ISDN link is called a CODEC, for coder/decoder.
13. What is spread spectrum and explain the
two types of spread spectrum?
Spread spectrum is to spread the
signal over a wider frequency band than normal in such a way as to minimize the
impact of interference from other devices.
·
Frequency Hopping
·
Direct sequence
14. What are the different encoding techniques?
·
NRZ
·
NRZI
·
Manchester
·
4B/5B
15. How does NRZ-L differ from
NRZ-I?
In the NRZ-L
sequence, positive and negative voltages have specific meanings: positive for 0
and negative for 1. in the NRZ-I sequence, the voltages are meaningless.
Instead, the receiver looks for
changes from one level to another as its basis for recognition of 1s.
16. What are the responsibilities of data link layer?
Specific
responsibilities of data link layer include the following. a) Framing b)
Physical addressing c) Flow control d) Error control e) Access control.
17. What are the ways to address the framing problem?
·
Byte-Oriented Protocols(PPP)
·
Bit-Oriented Protocols(HDLC)
·
Clock-Based Framing(SONET)
18. Distinguish between peer-to-peer relationship and a primary-secondary
relationship. peer -to- peer relationship?
All
the devices share the link equally.
Primary-secondary relationship:
One device controls traffic and the others must transmit through it.
19. Mention the types of errors and define the terms?
There are 2 types of errors
·
Single-bit error.
·
Burst-bit error.
Single bit
error: The term single bit error means that only one bit of a given data unit
(such as byte character/data unit or packet) is changed from 1 to 0 or from 0
to 1.
Burst
error: Means that 2 or more bits in the
data unit have changed from 1 to 0 from 0 to 1.
20. List out the available
detection methods.
There are 4 types of redundancy
checks are used in data communication.
·
Vertical redundancy checks (VRC).
·
Longitudinal redundancy checks (LRC).
·
Cyclic redundancy checks (CRC).
·
Checksum.
21. Write short notes on VRC.
The most common
and least expensive mechanism for error detection is the vertical redundancy
check (VRC) often called a parity check. In this technique a redundant bit
called a parity bit, is appended to every data unit so, that the total number
of 0’s in the unit (including the parity bit) becomes even.
22. Write short notes on LRC.
In longitudinal
redundancy check (LRC), a block of bits is divided into rows and a redundant
row of bits is added to the whole block.
23. Write short notes on
CRC.
The third and
most powerful of the redundancy checking techniques is the cyclic redundancy
checks (CRC) CRC is based on binary division. Here a sequence of redundant
bits, called the CRC remainder is appended to the end of data unit.
24. Write short notes on CRC checker.
A CRC checker
functions exactly like a generator. After receiving the data appended with the
CRC it does the same modulo-2 division. If the remainder is all 0’s the CRC is
dropped and the data accepted. Otherwise, the received stream of bits is
discarded and the dates are resent.
25. Define checksum.
The error
detection method used by the higher layer protocol is called checksum. Checksum
is based on the concept of redundancy.
26. What are the steps followed in checksum generator?
The sender
follows these steps a) the units are divided into k sections each of n bits. b)
All sections are added together using 2’s complement to get the sum. c) The sum
is complemented and become the checksum. d) The checksum is sent with the data.
27. Mention the types of error correcting methods.
There are 2
error-correcting methods.
·
Single bit error correction
·
Burst error correction.
28. Write short notes on
error correction?
It
is the mechanism to correct the errors and it can be handled in 2 ways.
·
When an error is discovered, the receiver can
have the sender retransmit the entire data unit.
·
A receiver can use an error correcting coder,
which automatically corrects certain errors.
29. What is the purpose of hamming code?
A hamming code
can be designed to correct burst errors of certain lengths. So the simple
strategy used by the hamming code to correct single bit errors must be
redesigned to be applicable for multiple bit correction.
30. What is redundancy?
It is the error
detecting mechanism, which means a shorter group of bits or extra bits may be
appended at the destination of each unit.
31. Define flow control?
Flow control
refers to a set of procedures used to restrict the amount of data. The sender
can send before waiting for acknowledgment.
32. Mention the categories of flow control?
There are 2
methods have been developed to control flow of data across communication links.
a) Stop and wait- send one from at a time. b) Sliding window- send several
frames at a time.
33. What is a buffer?
Each receiving
device has a block of memory called a buffer, reserved for storing incoming
data until they are processed.
34.What is the difference between a passive
and an active hub?
An active hub contains a repeater that
regenerates the received bit patterns before sending them out. A passive hub
provides a simple physical connection between the attached devices.
35. For
n devices in a network, what is the number of cable links required for a
mesh and ring topology?
·
Mesh topology – n (n-1)/2
·
Ring topology – n
36. Group the OSI layers by function.
(MAY/JUNE2007)
The seven layers of the OSI model belonging
to three subgroups. Physical, data link and network layers are the network
support layers; they deal with the physical aspects of moving data from one
device to another. Session, presentation and application layers are the user
support layers; they allow interoperability among unrelated software systems.
The transport layer ensures end-to-end reliable data transmission.
37.We have a channel
with a 1 MHz bandwidth. The SNR for this channel is 63; what is the appropriate
bit rate and signal level?
First, we use the Shannon formula
to find our upper limit.
C = B log2 (1 + SNR)
= 106 log2 (1 + 63) = 106 log2 (64)
= 6 Mbps
Then we use the Nyquist formula to
find the
number of signal levels.
4 Mbps = 2 ´ 1
MHz ´
log2 L è L = 4
= B log2 (1) = B ´ 0 = 0
38.List the
Channelization Protocols
n
Frequency Division Multiple Access (FDMA)
n
The total bandwidth is divided into channels.
n
Time Division Multiple Access (TDMA)
n
The band is divided into one channel that is
time shared
n
Code Division Multiple Access (CDMA)
n
One channel carries all transmission
simultaneously
39.What
is protocol?What are its key elements?(NOV/DEC 2007)
Set
of rules that govern the data communication is protocol. The key elements are
i)Syntax ii)Semantics iii)Timing
40.We have a channel
with a 1 MHz bandwidth. The SNR for this channel is 63; what is the appropriate
bit rate and signal level?
First, we use the Shannon formula
to find our upper limit.
C = B log2 (1 + SNR)
= 106 log2 (1 + 63) = 106 log2 (64)
= 6 Mbps
Then we use the Nyquist formula to
find the
number of signal levels.
4 Mbps = 2 ´ 1
MHz ´
log2 L è L = 4
= B log2 (1) = B ´ 0 = 0
41.List the
Channelization Protocols
n
Frequency Division Multiple Access (FDMA)
n
The total bandwidth is divided into channels.
n
Time Division Multiple Access (TDMA)
n
The band is divided into one channel that is
time shared
n
Code Division Multiple Access (CDMA)
n
One channel carries all transmission
simultaneously
42.What
is protocol?What are its key elements?(NOV/DEC 2007)
Set
of rules that govern the data communication is protocol.The key elements are
i)Syntax ii)Semantics iii)Timing
ALSO READ ..!
UNIT I NOTES
UNIT II NOTES
UNIT III NOTES
UNIT IV NOTES
UNIT V NOTES
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