SHORT ANSWER QUESTIONS (FROM All 5 Units)
Q-1. List out
the advantages and drawbacks of bus topology.
Ans:
Advantages:
i) Easy to
implement
ii) It is
very cost effective because only a single segment required
iii) It is
very flexible
iv) Moderate
reliability.
v) Can add
new station or delete any station easily (scalable)
Disadvantages:
i) Required
suitable medium access control technique.
ii) Maximum
cable length restriction imposed due to delay and signal
unbalancing
problem.
Q-2. List out
the advantages and drawbacks of ring topology.
Ans:
Advantages:
i) Data insertion,
data reception and data removal can be provided by repeater
ii) It can
provide multicast addressing.
iii)
Point-to-point links to its adjacent nodes (moderate cost)
Disadvantages:
i) The
repeater introduces a delay
ii) The
topology fails if any link disconnects or a node fails.
iii) Direct
link not provided
iv)
It provides complex management
Q-3. Why star
topology is commonly preferred?
Ans:
It gives high reliability, more flexible and higher bandwidth. Since there is a
central control point, the control of network is easy and priority can be given
to selected nodes.
Q-4. Is there
any relationship between transmission media and topology?
Ans:
Yes, medium should be selected based on the topology. For example, for bus
topology coaxial cable medium is suitable, and for ring/star topology
twisted-pair or optical fiber can be used.
Q-5. In what
situations contention based MAC protocols are suitable?
Ans:
Contention based MAC protocols are suitable for bursty nature of traffic under
light to moderate load. These techniques are always decentralized, simple and
easy to implement.
Q-6. What is
vulnerable period? How it affects the performance in MAC protocols?
Ans: The
total period of time when collision may occur for a packet is called vulnerable
period. Let, all packets have a fixed duration λ. Then vulnerable period is 2λ
in pure ALOHA scheme and λ in slotted ALOHA scheme. If vulnerable period is
long, probability of the occurrence collision increases leading to reduction in
throughput.
Q-7. How
throughput is improved in slotted ALOHA over pure ALOHA?
Ans: In pure
ALOHA vulnerable period is 2λ.
So, S/G =
e-2G or throughput S = G e-2G , where G is the total number of packets.
Maximum value
of G = 0.5 or maximum throughput Smax = 1/2e.
In slotted
ALOHA, vulnerable period is λ and S/G = e-G or throughput S = G e-G . Here,
maximum value
of G is 1 and maximum throughput Smax = 1/e.
Q-8. What is
the parameter ‘a’? How does it affect the performance of the CSMA protocol?
Ans: The
efficiency of CSMA scheme depends on propagation delay, which is represented by
a parameter ‘a’ as defined below.
propagation
delay
a = --------------------------------
packet transmission time
Smaller the value of propagation delay,
lower is the vulnerable period and higher is the efficiency. If propagation
delay is zero, collision cannot occur in CSMA scheme. But in practice, there is
some delay and depending on the value of ‘a’ collision occurs.
Q-9. How
performance is improved in CSMA/CD protocol compared to CSMA protocol?
Ans:
In CSMA scheme, a station monitors the channel before sending a packet.
Whenever a collision is detected, it does not stop transmission leading to some
wastage of time. On the other hand, in CSMA/CD scheme, whenever a station
detects a collision, it sends a jamming signal by which other station comes to
know that a collision occurs. As a result, wastage of time is reduced leading
to improvement in performance.
Q-10 What are
the goals in mind of IEEE 802 committee?
Ans: IEEE 802
committee has few goals in mind, namely
– To promote
compatibility
–
Implementation with minimum efforts
– Accommodate
diverse applications
Q-11. List the
functions performed by the physical layer of 802.3 standard?
Ans.
Functions of physical layer are:
i) Data
encoding/decoding (To facilitate synchronization and efficient transfer of
signal through the medium).
ii) Collision
detection (It detects at the transmit side)
iii) Carrier
sensing (Channel access senses a carrier on the channel at both the transmit
and receive sides)
iv)
Transmit/receive the packets (Frame transmitted to all stations connected to
the channel)
v) Topology
and medium used (Mediums are co-axial cable, twisted pair and fiber optic
cable)
Q-12. Why do
you require a limit on the minimum size of Ethernet frame?
Ans. To
detect collision, it is essential that a sender continue sending a frame and at
the same time receives another frame sent by another station. Considering
maximum delay with five Ethernet segments in cascade, the size of frame has
been found to be 64 bytes such that the above condition is satisfied.
Q-4. What are the different types of cabling supported by Ethernet
standard?
Ans. Types of
cabling are:
i) 10 BASE 5
- Maximum cable length is 500 meters using 4” diameter coaxial cable.
ii) 10 BASE 2
- Maximum cable length is 185 meters using 0.25” diameter CATV cable.
iii) 10 BASE
T - Maximum cable length is 100 meters using twisted-pair cable (CAT-3 UTP).
iv)
10 BASE FL - Maximum cable length is 2 Km using multimode fiber optic cable
(125/62.5 micrometer).
Q-13. What is
the advantage of token passing protocol over CSMA/CD protocol?
Ans.
Advantage of token passing protocol over CSMA/CD protocol:
The CSMA/CD
is not a deterministic protocol. A packet may be delivered after many (up to
15) collisions leading to long variable delay. An unfortunate packet may not
get delivered at all. This feature makes CSMA/CD protocol unsuitable for
real-time applications. On the other hand, token passing protocol is a
deterministic
approach, which allows a packet to be delivered within a known time frame. It
also allows priority to be assigned to packets. These are the two key
advantages of token passing protocol over CSMA/CD protocol.
Q-14. What are
the drawbacks of token ring topology?
Ans. Token
ring protocol cannot work if a link or a station fails. So, it is vulnerable to
link and station failure.
Q-15. How the
reliability of token ring topology can be improved?
Ans.
Reliability of the ring network can be improved by implementing the ring
topology using a wiring concentrator. This allows not only to detect fault, but
also to isolate the faulty link/station with the help of a bypass relay.
Q-16. What
role the active token monitor performs?
Ans.
Token ring is maintained with the help of active token monitor. Any one of the
stations has the capability to act as active token monitor, but at a particular
instant only one acts as active token monitor. It monitors various error
situations such as multiple token, orphan packet, etc, and takes appropriate
action to come out of the error situation.