Useful Q & A on Data Communication & Networks - Part 2

Hello Friends,

Based on the types of questions being asked in UGC NET, I have compiled some important questions on the topic of Data Communication & Networks. Please not just read this post, but also understand the concept behind the given answers. I have given explanation of these questions to help this process simple for you. Since Data Communication & Networks is one of the subjects on which lots of questions are asked, do make a note of preparing this subject well.

Please share your ideas and doubts also after reading the post.......

Q 1. What is the bucket size, when the overlapping and collision occur at same time?

Ans.: If there is only one entry possible in the bucket, when the collision occurs, there is no way to accommodate the colliding value. This results in the overlapping of values.

Q 2. Using RSA algorithm, what is the value of cipher text C, if the plain text M = 5 and p=3, q=11 & d=7 ?

A. 33            B. 5              C. 25            D. 26

Here p and q will be used to calculate public keys. Steps for transmission:

1. calculate n = p * q = 3 * 11 = 33

2. calculate z = ( p - 1 ) * ( q - 1 ) = ( 3 - 1 ) * ( 11 - 1 ) = 20

3. choose a prime number k, such that k is co-prime to z, i.e, z is not divisible by k. We have several choices for k: 7, 11, 13, 17, 19 (we cannot use 5, because 20 is divisible by 5).

Let's pick k=7 (smaller k, "less math").

4. the numbers n = 33 and k = 7 become the Server's public key.

5. the Server has to calculate it's secret key. Here is how.

6. k * j = 1 ( mod z )

7.)  7 * j = 1 ( mod 20 )

8.) ( 7 * j ) / 20 = ? with the remainder of 1 (the "?" here means: "something, but don't wory about it"; we are only interested in the remainder).

Since we selected (on purpose) to work with small numbers, we can easily conclude that 21 / 20 gives "something" with the remainder of 1. So, 7 * j = 21, and j = 3. This is our secret key. We MUST NOT give this key to anyone. Now, server sends n=33 and k=7 back to the Browser

Q 3. If a packet arrive with an M-bit value is ‘1’ and a fragmentation offset value ‘0’, then it is ______ fragment.
(A) First                                 (B) Middle                         (C) Last         (D) All of the above

Answer is A. If the M bit is 0, it means that there are no more fragments. If the M bit is 1, it means that there is at least one more fragment. This fragment can be the first one or a middle one, but not the last one. Offset value '0' always means this is the first fragment. To determine the position of the first byte in a particular fragment, we would multiply given offset value with 8. For example, offset is 200, then the position of first byte in that fragment is 1600.

Fragment Offset field solves the problem of sequencing fragments by indicating to the recipient device where in the overall message each particular fragment should be placed. The field is 13 bits wide, so the offset can be from 0 to 8191. Fragments are specified in units of 8 bytes, which is why fragment length must be a multiple of 8. Uncoincidentally, 8191 * 8 is 65,528, just about the maximum size allowed for an IP datagram.

IMP: When an MTU requirement forces a datagram to be fragmented, it is split into several smaller IP datagrams, each containing part of the original. The header of the original datagram is changed into the header of the first fragment, and new headers are created for the other fragments. Each is set to the same Identification value to mark them as part of the same original datagram. The Fragment Offset of each is set to the location where the fragment belongs in the original. The More Fragments field is set to 1 for all fragments but the last, to let the recipient know when it has received all the fragments.

Q 4. In a network of LANs connected by bridges, packets are sent from one LAN to another through intermediate bridges. Since more than one path may exist between two LANs, packets may have to be routed through multiple bridges. Why is the spanning tree algorithm used for bridge-routing? (GATE 2005)

(a) For shortest path routing between LANs         (b) For avoiding loops in the routing paths

(c) For fault tolerance                                      (d) For minimizing collisions

SOLUTION: Spanning tree is a protocol that allows the bridges to exchange information so that only one of them will handle a given message that is being sent between two computers within the network. This  protocol prevents the condition known as a BRIDGE LOOP. It is typical to add a second bridge between two network segments as a backup in case the primary bridge fails (both bridges need to have some way to understand which bridge is the primary one). To do this, they have a separate path connection just between the bridges in which they exchange information, using bridge protocol data units (BPDUs).

The program in each bridge that allows it to determine how to use the protocol is known as the spanning tree algorithm. The algorithm is specifically constructed to avoid bridge loops (for a bridge using only the most efficient path when faced with multiple paths). If the best path fails, the algorithm recalculates the network and finds the next best route.

Q 5. How many 8-bit characters can be transmitted per second over a 9600 baud serial communication link using asynchronous mode of transmission with one start bit, eight data bits, two stop bits and one parity bit?
(1) 600        (2) 800        (3) 876        (4) 1200

For 9600 baud, 1 bit=1/9600=0.104mS. Each char would require 11 bits. That means, to transmit one char, it would take 1.144 mS. So, applying the formula 1000/1.144 (mS in one Sec/transmission time for one char)=approx 875. Nearest answer is (3)

Q 6. The single stage network is also called

A) one sided network                             B) two sided network

C) recirculating network                      D) pipeline network

Single-Stage Network is a single stage of switching elements (SEs) existing between the inputs and the outputs of the network. Data is circulated a number of times around the network.

Q 7. If a class B network on the Internet has a subnet mask of 255.255.248.0, what is the maximum number of hosts per subnet?
(a) 1022 (b) 1023 (c) 2046 (d) 2047

Explanation: Convert the subnet mask into binary format.

255.255.248.0 = 11111111.11111111.11111000.00000000

Number of 1's in the subnet mask indicates the Network-ID and the Subnet-ID part. Number of 0's in the subnet mask indicates the Host-ID part. Maximum number of Hosts per subnet = 2¹¹ = 2048, where 11 = Number of 0's in the Subnet Mask. Out of 2048 values, 2 addresses are reserved, hence we remove them (2048-2 = 2046). Note: In the host part of the address:- all bits as 1 is reserved as broadcast address and all bits as 0 is used as network address of subnet.

Q 8. Which of the following system calls results in the sending of SYN packets?(a) socket (b) bind (c) listen (d) connect

The connect system call is normally called by the client process to connect to the server process. The socket system call creates a new socket and assigns the protocol and resources to the created socket descriptor. The bind system call associates a local network transport address with a socket. For a client process, it is not mandatory to issue a bind call. The kernel takes care of doing an implicit binding when the client process issues the connect system call. 

It is often necessary for a server process to issue an explicit bind request before it can accept connections or start communication with clients. The listen call indicates to the protocol that the server process is ready to accept any new incoming connections on the socket. There is a limit on the number of connections that can be queued up, after which any further connection requests are ignored.

What is the maximum size of data that the application layer can pass on to the TCP layer below?
Q 9. (a) Any size (b) 2¹⁶ bytes-size of TCP header     (c) 2¹⁶ bytes   (d) 1500 bytes

Which of the following transport layer protocols is used to support electronic mail?

(a) SMTP (b) IP  (c) TCP  (d) UDP

Q 10. When a host on network A sends a message to a host on network B, which address does the router look at ?
(a) Port                  (b) IP           (c) Physical              (d) Subnet mask

Q 11. The ....................... field in the SNMP PDU, reports an error in a response message.

(A) Error index         (B) Error status       (C) Set request                  (D) Agent index

Error status: An integer value that is used in a Response-PDU to tell the requesting SNMP entity the result of its request. A value of zero indicates that no error occurred; the other values indicate what sort of error happened.

Error Index: When Error Status is non-zero, this field contains a pointer that specifies which object generated the error. Always zero in a request.