Suggestive questions of Data Communication and Networking

Multiple choice question:

1. What is the size of MAC Address?
(a) 48 bits
2. _________ defines the variations in the packet delay.
(a) Jitter
3. UDP stands for_____.
(a) User Datagram Protocol
4. Which multiple access technique is used by IEEE 802.11 standard for wireless LAN?
(a) CSMA/CA
5. CSMA stands for
(a) carrier sense multiple access
6. In virtual circuit network each packet contains ___________.
(a) Virtual Circuit Identifier
7. Automatic repeat request error management mechanism is provided by ____
(a) media access control sublayer
8. The data link layer takes the packets from ___ and encapsulates them into frames for
transmission.
(a) physical layer
9. Write one difference between UDP and TCP.
(a) UDP is connectionless protocol and TCP is connection-oriented protocol.
10. Which topology requires a central controller or hub?
(a) Star topology

Short answer type question:

1. Explain the functions of Data Link Layer.

The data link layer is responsible for moving frames from one hop (node) to the next. Other
responsibilities of the data link layer include the following:
• Framing. The data link layer divides the stream of bits received from the network layer
into manageable data units called frames.
• Physical addressing. If frames are to be distributed to different systems on the network,
the data link layer adds a header to the frame to define the sender and/or receiver of the
frame. If the frame is intended for a system outside the sender's network, the receiver address
is the address of the device that connects the network to the next one.
• Flow control. If the rate at which the data are absorbed by the receiver is less than the rate
at which data are produced in the sender, the data link layer imposes a flow control
mechanism to avoid overwhelming the receiver.
• Error control. The data link layer adds reliability to the physical layer by adding
mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to
recognize duplicate frames. Error control is normally achieved through a trailer added to the
end of the frame.
• Access control. When two or more devices are connected to the same link, data link layer
protocols are necessary to determine which device has control over the link at any given time.
 2. A 4 Mbps token ring has a token holding timer value 15 ms. What is
the longest frame that can be sent on this ring?

If token holding time is 15 ms then a user can transmit only for 15 ms, maximum time till it
can hold token.

In 15 ms a user can transmit 4Mbps*15ms = 4 * 106 bits per second * 15 * 10-3 seconds = 60
*103 bits

So, max frame size = 60 Kbits.

3. Explain Token Passing controlled access method.

In the token-passing method, the stations in a network are organized in a logical ring. In other
words, for each station, there is a predecessor and a successor. The predecessor is the station
which is logically before the station in the ring; the successor is the station which is after the
station in the ring. The current station is the one that is accessing the channel now. The right
to this access has been passed from the predecessor to the current station. The right will be
passed to the successor when the current station has no more data to send. But how is the
right to access the channel passed from one station to another? In this method, a special packet
called a token circulates through the ring. The possession of the token gives the station the
right to access the channel and send its data. When a station has some data to send, it waits
until it receives the token from its predecessor. It then holds the token and sends its data.
When the station has no more data to send, it releases the token, passing it to the next logical
station in the ring. The station cannot send data until it receives the token again in the next
round. In this process, when a station receives the token and has no data to send, it just passes
the data to the next station. Token management is needed for this access method. Stations
must be limited in the time they can have possession of the token. The token must be
monitored to ensure it has not been lost or destroyed. For example, if a station that is holding
the token fails, the token will disappear from the network. Another function of token
management is to assign priorities to the stations and to the types of data being transmitted.
And finally, token management is needed to make low-priority stations release the token to
high priority stations.
4. What is WDM?

Wavelength-division multiplexing (WDM) is designed to use the high-data-rate capability of

fiber-optic cable. The optical fiber data rate is higher than the data rate of metallic transmission
cable. Using a fiber-optic cable for one single line wastes the available bandwidth.
Multiplexing allows us to combine several lines into one. WDM is conceptually the same as
FDM, except that the multiplexing and demultiplexing involve optical signals transmitted
through fiber-optic channels. The idea is the same: We are combining different signals of
different frequencies. The difference is that the frequencies are very high. Very narrow bands
of light from different sources are combined to make a wider band of light. At the receiver,
the signals are separated by the demultiplexer. WDM is an analog multiplexing technique to
combine optical signals. Although WDM technology is very complex, the basic idea is very
simple. We want to combine multiple light sources into one single light at the multiplexer and
do the reverse at the demultiplexer. The combining and splitting of light sources are easily
handled by a prism. Recall from basic physics that a prism bends a beam of light based on the
angle of incidence and the frequency. Using this technique, a multiplexer can be made to
combine several input beams of light, each containing a narrow band of frequencies, into one
output beam of a wider band of frequencies. A demultiplexer can also be made to reverse the
process. One application of WDM is the SONET network in which multiple optical fiber lines
are multiplexed and demultiplexed.

5. What is meant by random access method? Give example of random

access protocols.

In random access or contention methods, no station is superior to another station and none is
assigned the control over another. No station permits, or does not permit, another station to
send. At each instance, a station that has data to send uses a procedure defined by the protocol
to make a decision on whether or not to send. This decision depends on the state of the
medium (idle or busy). In other words, each station can transmit when it desires on the
condition that it follows the predefined procedure, including the testing of the state of the
medium.
The random-access protocol ALOHA is a very simple procedure called multiple access (MA).
It was designed for a radio (wireless) LAN, but it can be used on any shared medium. It is
obvious that there are potential collisions in this arrangement. The medium is shared between
the stations. When a station sends data, another station may attempt to do so at the same time.
The data from the two stations collide and become garbled. The original ALOHA protocol is
called pure ALOHA. This is a simple, but elegant protocol. The idea is that each station sends
a frame whenever it has a frame to send. However, since there is only one channel to share,
there is the possibility of collision between frames from different stations. It is obvious that
we need to resend the frames that have been destroyed during transmission. The pure
ALOHA protocol relies on acknowledgments from the receiver. When a station sends a frame,
it expects the receiver to send an acknowledgment. If the acknowledgment does not arrive
after a time-out period; the station assumes that the frame (or the acknowledgment) has been
destroyed and resends the frame. A collision involves two or more stations. If all these stations
try to resend their frames after the time-out, the frames will collide again. Pure ALOHA
dictates that when the time-out period passes, each station waits a random amount of time
before resending its frame. The randomness will help avoid more collisions. We call this time
the back-off time TB.

6. Describe Domain Name System.

To identify an entity, TCP/IP protocols use the IP address, which uniquely identifies the
connection of a host to the Internet. However, people prefer to use names instead of numeric
addresses. Therefore, we need a system that can map a name to an address or an address to a
name. This huge amount of information is divided into smaller parts and store each part on a
different computer. In this method, the host that needs mapping can contact the closest
computer holding the needed information. This method is used by the Domain Name System
(DNS). To have a hierarchical name space, a domain name space was designed. In this
design the names are defined in an inverted-tree structure with the root at the top. The tree
can have only 128 levels: level 0 (root) to level 127 Label. Each node in the tree has a label,
which is a string with a maximum of 63 characters. The root label is a null string (empty
string). DNS requires that children of a node (nodes that branch from the same node) have
different labels, which guarantees the uniqueness of the domain names. Each node in the
tree has a domain name. A full domain name is a sequence of labels separated by dots (.).
The domain names are always read from the node up to the root. The last label is the label
of the root (null). This means that a full domain name always ends in a null label, which
means the last character is a dot because the null string is nothing.

7. Why TCP is called connection-oriented reliable protocol?

TCP, unlike UDP, is a connection-oriented protocol. When a process at site A wants to send
and receive data from another process at site B, the following occurs:
1. The two TCPs establish a connection between them.
2. Data are exchanged in both directions.
3. The connection is terminated.
Note that this is a virtual connection, not a physical connection. The TCP segment is
encapsulated in an IP datagram and can be sent out of order, or lost, or corrupted, and then
resent. Each may use a different path to reach the destination. There is no physical connection.
TCP creates a stream-oriented environment in which it accepts the responsibility of delivering
the bytes in order to the other site. The situation is similar to creating a bridge that spans
multiple islands and passing all the bytes from one island to another in one single connection.
TCP is a reliable transport layer protocol. This means that an application program that delivers
a stream of data to TCP relies on TCP to deliver the entire stream to the application program
on the other end in order, without error, and without any part lost or duplicated. TCP
provides reliability using error control. Error control includes mechanisms for detecting
corrupted segments, lost segments, out-of-order segments, and duplicated segments. Error
control also includes a mechanism for correcting errors after they are detected. Error detection
and correction in TCP is achieved through the use of three simple tools: checksum,
acknowledgment, and time-out.

8. Explain why the send window size of Go-Back-N Sliding Window

protocol must be less than 2m where m is the size of the sequence
number field in bits.

We can now show why the size of the send window must be less than 2m. As an example, we
choose m =2, which means the size of the window can be 2m - 1, or 3. Figure compares a
window size of 3 against a window size of 4. If the size of the window is 3 (less than 22) and
all three acknowledgments are lost, the frame timer expires and all three frames are resent.
The receiver is now expecting frame 3, not frame 0, so the duplicate frame is correctly
discarded. On the other hand, if the size of the window is 4 (equal to 22) and all
acknowledgments are lost, the sender will send a duplicate of frame 0. However, this time the
window of the receiver expects to receive frame 0, so it accepts frame 0, not as a duplicate, but
as the first frame in the next cycle. This is an error.