MCS-042 June 2018

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1. (a) Explain the DES algorithm with the help of a suitable example.

Answer : -

1. (b) What is the need of,hierarchical routing ? Explain it with an example.

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1. (c) Explain Leaky Bucket and Token Bucket traffic shaping mechanisms and compare them.

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Leaky Bucket - Suppose we have a bucket in which we are pouring water in a random order but we have to get water in a fixed rate, for this we will make a hole at the bottom of the bucket. It will ensure that water coming out is in a some fixed rate, and also if bucket will full we will stop pouring in it.

The input rate can vary, but the output rate remains constant. Similarly, in networking, a technique called leaky bucket can smooth out bursty traffic. Bursty chunks are stored in the bucket and sent out at an average rate.

The leaky bucket is used to implement traffic policing and traffic shaping in Ethernet and cellular data networks. The algorithm can also be used to control metered-bandwidth Internet connections to prevent going over the allotted bandwidth for a month, thereby avoiding extra charges.

Token Bucket - The token bucket algorithm is based on an analogy of a fixed capacity bucket into which tokens (normally representing a unit of bytes or a single packet of predetermined size) are added at a fixed rate. When a packet is ready to be send, it is first checked whether the bucket contains sufficient tokens or not. If sufficient tokens are present in the bucket, the appropriate numbers of tokens [equivalent to the length of the packet in bytes] are removed and the packet is passed for transmission. Else if there is a deficiency of tokens, then the packet has to wait in a queue.

1. (d) A high quality audio signal is limited to 10 kHz. It is to be transmitted using a 7-bit code. Calculate the required bit rate which needs to be supported by the channel.

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1. (e) Enlist any two core protocols of Internet layer in TCP/IP reference model. Describe the functionality of these two protocols.

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2. (a) Explain the concepts of Local Procedure Call (LPC) and Remote Procedure Call (RPC) using suitable examples and diagrams. Why is RPC useful in programming ?

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2. (b) State the need of broadcast routing. Discuss various broadcast routing procedures. Give an example of multicast routing.

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3. (a) system uses Slotted ALOHA protocol. In an observation interval, the channel is busy for 90% of the time. Calculate the system throughput.

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3. (b) Explain connection establishment and connection termination processes in TCP using three-way handshaking mechanism.

Answer : -

Connection Establishment

Step 1 : Device A (Client) sends a TCP segment with the SYN flag set to 1, ACK flag set to 0 and an Sequence Number 2000 (which marks the beginning of the sequence numbers for data that device A will transmit). SYN flag announces an attempt to open a connection.

Step 2 : Device B (Server) receives Device A's TCP segment and returns a TCP segment with SYN = 1, ACK = 1, Sequence Number = 5000 (Device B's Initial Sequence Number) and Acknowledgment Number = 2001 (2000 + 1, the next sequence number Device B expecting from Device A).

Step 3 : Device A sends a TCP segment to Device B that acknowledges receipt of Device B's TCP segment, With flags set as SYN = 0, ACK = 1, Sequence number = 2001 and Acknowledgment number = 5001 (5000 + 1, the next sequence number Device A expecting from Device B).

4. (a) Why is Stop-and-Wait protocol inefficient for satellite links ? How is this inefficiency overcome in Sliding Window protocol ? Explain.

Answer : -

4. (b) Explain the collision occurrences in Ethernet. How is backoff algorithm implemented in Ethernet ?

Answer : - A collision is the situation that occurs when two or more devices attempt to send a signal along the same transmission channel at the same time. The colliding of the signals can result in garbled, and thus useless, messages. The lost information needs to be resent so that the receiver will get it.

Carrier Sense Multiple Access (CSMA) is used to control the flow of data in a transmission media so that packets do not get lost and data integrity is maintained. There are two modifications to CSMA, the CSMA/CD (CSMA/Collision Detection) is used in wired network and CSMA/CA (CSMA/Collision Avoidance) is used in wireless network to detect whether a collision has occurred.

In wired network once a collision is detected, CSMA/CD immediately terminates the transmission so that the transmitter does not have to waste a lot of time in continuing. In wireless network CSMA/CA first check whether the medium is in use or not. If it is busy, then the transmitter waits until it is idle before it starts transmitting.

Example of Ethernet Operation

Station A wishes to send a frame. First, it checks if the medium is available (Carrier Sense). If it isn't, it waits until the current sender on the medium has finished.

Suppose Station A believes the medium is available and attempts to send a frame. Because the medium is shared (Multiple Access), other senders might also attempt to send at the same time. At this point, Station B tries to send a frame at the same time as Station A.

After a while, Station A and Station B realize that there is another device attempting to send a frame (Collision Detect). Each station waits for a random amount of time before sending again. The time after the collision is divided into time slots; Station A and Station B each pick a random slot for attempting a retransmission.

If Station A and Station B attempt to retransmit in the same slot, they extend the number of slots. Each station then picks a new slot, thereby decreasing the probability of retransmitting in the same slot.

5. (a) Illustrate Count-to-Infinity problem. How can it be resolved ?

Answer : - The main issue with Distance Vector Routing protocols is Routing Loops, since Bellman-Ford Algorithm cannot prevent loops. This routing loop in Distance Vector Routing network causes Count to Infinity Problem. Routing loops usually occur when any interface goes down or two-routers send updates at the same time.

Count-to-Infinity Problem

In this example R2 will know that it can get to R3 at a cost of 1, and R1 will know that it can get to R3 via R2 at a cost of 2.

If the link between R2 and R3 is disconnected, then R2 will know that it can no longer get to R3 via that link and will remove it from it’s table. Before it can send any updates it’s possible that it will receive an update from R1 which will be advertising that it can get to R3 at a cost of 2. R2 can get to R1 at a cost of 1, so it will update a route to R3 via R1 at a cost of 3. R1 will then receive updates from R2 later and update its cost to 4. They will then go on feeding each other bad information toward infinity which is called as Count-to-Infinity Problem.

Solution of Count-to-Infinity Problem for Wired Networks

Solution of Count-to-Infinity Problem for Wireless Networks

5. (b) Explain the functionalities of SSL. Discuss the merits of SSL.

Answer : - SSL is the backbone of our secure Internet and it protects your sensitive information as it travels across the world's computer networks. SSL is essential for protecting your website, even if it doesn't handle sensitive information like credit cards. It provides privacy, critical security and data integrity for both your websites and your users' personal information.

SSL Encrypts Sensitive Information - The primary reason why SSL is used is to keep sensitive information sent across the Internet encrypted so that only the intended recipient can access it. This is important because the information you send on the Internet is passed from computer to computer to get to the destination server. Any computer in between you and the server can see your credit card numbers, usernames and passwords, and other sensitive information if it is not encrypted with an SSL certificate. When an SSL certificate is used, the information becomes unreadable to everyone except for the server you are sending the information to. This protects it from hackers and identity thieves.

SSL Provides Authentication - In addition to encryption, a proper SSL certificate also provides authentication. This means you can be sure that you are sending information to the right server and not to an imposter trying to steal your information. Why is this important? The nature of the Internet means that your customers will often be sending information through several computers. Any of these computers could pretend to be your website and trick your users into sending them personal information. It is only possible to avoid this by getting an SSL Certificate from a trusted SSL provider.

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