1. What are the three criteria necessary for an
effective and efficient network?
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.
measured by frequency of failure, the time it takes a link to recover from the
failure and the network’s robustness
in a catastrophe.
include protecting data from unauthorized access and viruses.
2. Group the OSI layers by function?
layers of the OSI model belonging to three subgroups.
data link and network layers are the network
support layers; they deal with the physical aspects of moving data from
one device to another.
presentation and application layers are the user
support layers; they allow interoperability among unrelated software
transport layer ensures end-to-end
reliable data transmission.
3. What are header and trailers and how do they get
added and removed?
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?
It decomposes the problem of building a network
into more manageable components.
It provides a more modular design.
5. Why are protocols needed?
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.
the two interfaces provided by protocols?
interface- defines the operations that local objects can perform on the protocol.
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?
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?
distance between a pair of adjacent maxima or minima of a wave, typically
measured in meters, is called wave’s wavelength.
10. Define 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.
the two types of duplex?
duplex-two bit streams can be simultaneously transmitted over the links at the same time, one going in
duplex-it supports data flowing in only one direction at a time.
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?
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.
the different encoding techniques?
15. How does NRZ-L differ from NRZ-I?
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.
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
responsibilities of data link layer include the following. a) Framing b)
Physical addressing c) Flow control d) Error control e) Access control.
the ways to address the framing problem?
18. Distinguish between peer-to-peer relationship and a
primary-secondary relationship. peer -to- peer relationship?
devices share the link equally.
relationship: One device controls traffic and the others must
transmit through it.
19. Mention the types of errors and define the
2 types of errors
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.
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.
4 types of redundancy checks are used in data communication.
Vertical redundancy checks (VRC).
Longitudinal redundancy checks (LRC).
Cyclic redundancy checks (CRC).
21. Write short notes on VRC.
common and least expensive mechanism for error detection is the vertical
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
bit) becomes even.
22. Write short notes on LRC.
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.
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.
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
accepted. Otherwise, the received stream of bits is discarded and the dates are
25. Define checksum.
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
sender follows these steps a) the units are divided into k sections each of n
sections are added together using 2’s complement to get the sum. c) The sum is
and become the checksum. d) The checksum is sent with the data.
the types of error correcting methods.
2 error-correcting methods.
Single bit error correction
Burst error correction.
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.
the purpose of hamming code?
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?
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?
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?
receiving device has a block of memory called a buffer, reserved for storing
incoming data until they are processed.