CUSTOMER_CODE
SMUDE
DIVISION_CODE
SMUDE
EVENT_CODE
SMUAPR15
ASSESSMENT_CODE BCA4010_SMUAPR15
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
4097
QUESTION_TEXT
What is Carrier Sense Multiple Access protocols? Explain three
major protocols of CSMA.
SCHEME OF
EVALUATION
*Protocols in which stations listen for a carrier and act accordingly
are called Carrier Sense Protocols. "Multiple Access" describes the
fact that multiple nodes send and receive on the medium.(1 mark)
Protocol 1. 1-Persistent CSMA:
*When a station has data to send, it first listens to the channel to see
if anyone else is transmitting. If the channel is busy, the station waits
until it becomes idle. When the station detects an idle channel, it
transmits a frame. If a collision occurs, the station waits a random
amount of time and starts retransmission.(2marks)
*The protocol is so called because the station transmits with a
probability of a whenever it finds the channel idle.(1 mark)
Protocol 2. Non persistent CSMA:
*Before sending, a station senses the channel if no one else is
sending, the station begins doing so itself. However, if the channel is
already in use, the station does not continuously sense the channel for
the purpose of seizing it immediately upon detecting the end of
previous transmission. Instead, it waits for a random period of time
and then repeats the algorithm. (2marks)
*Intuitively, this algorithm should lead to better channel utilization
and longer delays than 1-persistent CSMA.(1 mark)
Protocol 3: p - persistent CSMA:
*When a station becomes ready to send, it senses the channel. If it is
idle, it transmits with a probability p. With a probability of q = 1- p, it
defers until the next slot. If that slot is also idle, it either transmits or
defers again, with probabilities p and q.This process is repeated until
either the frame has been transmitted or another station has begun
transmitting.(2marks)
*In the latter case, it acts as if there had been a collision. If the station
initially senses the channel busy, it waits until the next slot and
applies the above algorithm.(1 mark)
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
4098
QUESTION_TEXT
Explain Stop-and Wait automatic Repeat Request and Sliding
Window Protocols for Noisy channels.
SCHEME OF
EVALUATION
Stop-and-Wait Automatic Repeat Request .
*This protocol adds a simple error control mechanism to the stopand-wait protocol. To detect and correct corrupted frames, we need to
add redundancy bits to data frame. When the frame arrives at the
receiver site, it is checked and if it is corrupted, it is silently
discarded. The detection of errors in this protocol is manifested by
the silence of the receiver. Lost frames are more difficult to handle
than corrupted ones. The corrupted and lost frames need to be resent
in this protocol.
*If the receiver does not respond when there is an error, how can the
sender know which frame to resend? To remedy this problem, the
sender keeps a copy of the sent frame. At the same time it starts a
timer. If the timer expires and there is no acknowledgement for the
sent frame, the frame is resent, the copy is held and the timer is
restarted. Since the protocol uses the stop-and-wait mechanism, there
is only one specific frame that needs an ACK even though several
copies of the same frame can be in the network.
*Since an ACK frame can also be corrupted and lost, it too needs
redundancy bits and a sequence number. The ACK frame for this
protocol has a sequence number field. In this protocol, the sender
simply discards a corrupted ACK frame or ignores an out-of-order
one. This protocol specifies the frames to be numbered. This is done
by using sequence numbers.(2+2+1 marks)
Sliding Window Protocol:
*In this protocol, the sliding window is an abstract concept that
defines the range of sequence numbers that is the concern of the
sender and receiver. The sender and the receiver need to deal with
only part of the possible sequence numbers. The range which is the
concern of the sender is called the send sliding window; the range
that is the concern of the receiver is called the receive sliding
window.
*The send window is an imaginary box covering the sequence
numbers of the data frames which can be in transit. In each window
position, some of these sequence numbers define the frames that have
been sent; The maximum size of the window is 2m - 1.
*The window at any time divides the possible sequence numbers into
regions.
- The first region, defines the sequence numbers belonging to frames
that are acknowledged.
- The second region, defines the range sequence numbers for frames
that can be sent and have an unknown status.
- The third range defines the range of sequence numbers for frames
that can be sent.
- The fourth region defines sequence numbers that cannot be used
until the window slides. (2+1+2 marks)
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
72445
QUESTION_TEXT Explain different types and classes of routing algorithms.
SCHEME OF
EVALUATION
There are two types of routing algorithm depending upon the
communication subnet.
* Session routing: if subnet uses virtual circuits.
* Forwarding: if subnet uses datagram
* If the subnet uses the virtual circuits internally, routing decision
is made only when a new virtual circuit is set up. Once a virtual
circuit is set up there after the data packets just follows the
previously established route. This is called session routing because a
route remains in force for an entire user session. (2 marks)
* If the subnet uses the datagram's internally, routing decision are
made a new for every data packet since the best route must have
changed since last time. One can think of two processes inside a
router' One process handles each packet as it arrives, looking up the
outgoing line to use for it in the routing tables. This process is called
forwarding. (2 marks)
* The other process of the router is responsible for filling in and
updating the routing tables. To update the routing tables we need to
use the routing algorithms. (1 mark)
Classes of routing algorithms:
* Static or Non-adaptive Routing algorithms
* Dynamic or Adaptive Routing algorithms
Static or Non-adaptive Routing algorithms do not base their routing
decisions on measurements or estimates of the current traffic and
topology. The choice of the route to use to get from source to
destination or from one node to other is computed in advance. It is
done off-line, and downloaded to the routers when the network is
booted. Most of the public switched telephone network (PSTN) uses
pre-computed routing tables, with routes if the most direct route
becomes blocked. (3 marks)
Dynamic or adaptive Routing algorithms in contrast change their
decisions to reflect changes in topology and usually the traffic as
Dynamic routing attempts to solve this problem by constructing
tables automatically, based on information carried by routing
protocols, allowing the network to act nearly autonomously in
avoiding network fail and blockages. (2 marks)
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
72448
QUESTION_TEXT
Briefly explain
a. Fast Ethernet
b. Gigabit Ethernet
SCHEME OF EVALUATION
5 mark each
a. Fast Ethernet
b.
Gigabit Ethernet
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
72449
QUESTION_TEXT
Explain the following TCP congestion control protocols
a. Slow start
b. Congestion avoidance
c. Fast retransmit
d. Fast recovery
2.5 mark each
a. Slow start
SCHEME OF EVALUATION b. Congestion avoidance
c. Fast retransmit
d. Fast recovery
QUESTION_TYPE
DESCRIPTIVE_QUESTION
QUESTION_ID
116175
QUESTION_TEXT
Explain the layers of OSI model in brief.
The OSI Model is composed of seven ordered layers:
•
Layer 1 – The Physical Layer
•
Layer 2 – The Data Link Layer
•
Layer 3 – The Network Layer
•
Layer 4 – The Transport Layer
•
Layer 5 – The Session Layer
•
Layer 6 – The Presentation Layer
•
Layer 7 – The Application Layer
SCHEME OF EVALUATION
(6 marks)
(2 marks)
(2 marks)