Q-1 Calculate Cyclic Redundancy Check (CRC), where Message bits M=1010101110 and Generator bits G=10001.
ans)
b )Write an algorithm to compute the shortest path using Dijkstra’s algorithm.
c)Describe the configuration of BUS, STAR, and RING network topologies, with their
one advantage and one disadvantage.
ans)Bus Topology: --In a bus topology, all stations are attached to the same cable. In the Bus Network, messages are sent in both directions from a single point and are read by the node (computer or peripheral on the network) identified by the code with the message. Most Local Area Networks (LANs) are Bus Networks because the network will continue to function even if one computer is down. The purpose of the
terminators (resistors) at either end of the network is to stop the signal being reflected
back. If a bus network is not terminated, or if the terminator has the wrong level of
resistance, each signal may travel across the bus several times instead of just once.
This problem increases the number of signal collisions, degrading network
performance.
advantage-:
i) Bus topologies are relatively easy to install and don't require much cabling
compared to other topologies.
disadvantage
i) Entire network shuts down if there is a break in the main cable.
STAR TOPOLOGY-:
In a Star Network, all the nodes (PCs, printers and other shared
peripherals) are connected to the central server. It has a central connection point - like a switch. A star topology is designed with each node (file server, workstations, and
peripherals) connected directly to a central network hub or concentrator.All traffic emanates from the switch of the star. Data on a star network passes through
the switch or concentrator before continuing to its destination. The switch or
concentrator manages and controls all functions of the network. It also acts as a repeater for the data flow. This configuration is common with twisted pair cable; however, it can also be used with coaxial cable or fiber optic cable. The switch offers a common connection for all stations on the network. Each station has its own direct cable connection to the switch.
ADVANTAGE-:
i) Easy to add new stations as each station has its own direct cable connection to
the switch. If a cable is cut, it only affects the computer that was attached to it.
DISADVANTAGE-:
i) Depending on where the switches are located, star networks can require more
cable length than a linear topology
RING TOPOLOGY-:
All the nodes in a ring network are connected in a closed circle of
cable as shown in figure 3. Messages that are transmitted travel around the ring until
they reach the computer that they are addressed to. The signal being transmitted is
refreshed by each node in the ring between the sender and receiver. In a ring network,
every device has exactly two neighbors for communication purposes All messages travel through a ring in the same direction. There are no terminated ends to
the cable; the signal travels around the circle and terminated by the source.
ADVANTAGE-:
i) Growth of system has minimal impact on performance. The ring networks can
be larger than bus or star because each node regenerates the signal.
DISADVANTAGE-:
i) Expensive topology.
(d) Describe the different techniques used to multiplex signals?
There are four basic multiplexing techniques:
• Frequency division multiplexing (FDM)
• Time division Multiplexing (TDM)
• Code division Multiplexing (CDM)
• Space-division Multiplexing (SDM
FREQUENCY DIVISION MULTIPLEXING-:
Frequency division multiplexing (FDM) is the technique used to divide the available
bandwidth into a number of smaller independent logical channels with each channel
having a small bandwidth. The method of using a number of carrier frequencies each
of which is modulated by an independent speech signal is in fact frequency division
multiplexing.
TIME DIVISION MULTIPLEXING-:
Time Division Multiplexing (TDM) is another popular method of utilizing the
capacity of a physical channel effectively. Each user of the channel is allotted a small
time interval during which it may transmit a message. Thus the total time available in
the channel is divided and each user is allocated a time slot. Data from each user is
multiplexed into a frame which is transmitted over the channel. In TDM, user’s
messages are buffered as they received and read from the buffer during its time slot to
make a frame. Therefore each user can use the full channel bandwidth. The channel
capacity is fully utilized in TDM by interleaving a number of messages belonging to
different users into one long message. This message sent through the physical channel must be separated at the receiving end. Individual chunks of message sent by each user should be reassembled into a full message.
CODE DIVISION MULTIPLEXING-:
CDMA uses spread-spectrum technology and a special coding scheme (where each
transmitter is assigned a code generally pseudorandom code) to allow multiple users
to be multiplexed over the same physical channel. By contrast, time division multiple
access (TDMA) divides access by time, while frequency-division multiple access
(FDMA) divides it by frequency. CDMA is a form of spread-spectrum signalling,
since the modulated coded signal has a much higher data bandwidth than the data
being communicated. This allows more users to communicate on the same network at
one time than if each user was allotted a specific frequency range. Remember that
CDMA is a digital technology, so analog signals must be digitized before being
transmitted on the network.
SPACE DIVISION MULTIPLEXING-:
When we want to transmit multiple messages through any of the communication
media, the ultimate goal is to maximize the use of the given resources (e.g. time and
frequency in general). It involves grouping many separate wires into a common cable
enclosure. A cable that has, for example, 50 twisted pairs inside it can support 50
channels. SDM has the unique advantage of not requiring any multiplexing
equipment. It is usually combined with other multiplexing techniques to better utilize
the individual physical channels. For example, if there are six persons in the office
and all of them want to talk at the same time, this will give rise to interference
between the conversations. To reduce the interference they may divide themselves
into three groups of two, such that the conversation is between each pair of people. If
the pairs continue talking whilst sitting next to each other, the interference would still
be present. The best way for each pair to converse with minimal interference would
be to sit a few feet away from the other pairs (within the same room) and converse.
They would still be sharing the same medium for their conversations but the physical
space in the room would be divided for each conversation. This is the simplest
example of Space Division Multiplexing.
2 (a) What is Pulse Code Modulation (PCM)? Assume the maximum signal bandwidth of speech data is 4,000Hz. What is the sampling rate for converting speech into digital data using Pulse Code Modulation (PCM)? If each sample is encoded by 8 bits, what is the data rate of the encoded signal
ans)
Pulse code modulation (PCM) is a digital scheme for transmitting analog data. The signals in PCM are binary; that is, there are only two possible states, represented by logic 1 (high) and logic0 (low). This is true no matter how complex the analog waveform happens to be. Using PCM, it is possible to digitize all forms of analog data, including full-motion video, voices, music, telemetry, and virtual reality (VR).
b)Write RSA algorithm and explain it using an example
the algorithm involves multiplying two large prime numbers (a prime number is a number divisible only by that number and 1) and through additional operations deriving a set of two numbers that constitutes the public key and another set that is the private key. Once the keys have been developed, the original prime numbers are no longer important and can be discarded. Both the public and the private keys are needed for encryption /decryption but only the owner of a private key ever needs to know it. Using the RSA system, the private key never needs to be sent across the Internet.The private key is used to decrypt text that has been encrypted with the public key. Thus, if I send you a message, I can find out your public key (but not your private key) from a central administrator and encrypt a message to you using your public key. When you receive it, you decrypt it with your private key. In addition to encrypting messages (which ensures privacy), you can authenticate yourself to me (so I know that it is really you who sent the message) by using your private key to encrypt a digital certificate. When I receive it, I can use your public key to decrypt it. A table might help us remember this.
(c) Show the signals that will be generated when the sequence 01010001 is encoded using NRZI and Differential Manchester encoding schemes.
SOL)
(d) Why are the wires twisted in a twisted-pair copper wire?
Twisted pair is most widely used media for local data distribution. Twisted-pair cable
is a type of cabling that is used for telephone communications and most modern
Ethernet networks. A pair of wires forms a circuit that can transmit data. The pairs are
twisted to provide protection against crosstalk, and noise generated by adjacent pairs.When electrical current flows through a wire, it creates a small, circular magnetic
field around the wire. When two wires in an electrical circuit are placed close
together, their magnetic fields are the exact opposite of each other. Thus, the two
magnetic fields cancel each other out. They also cancel out any outside magnetic
fields. Twisting the wires can enhance this cancellation effect. Using cancellation
together with twisting the wires, cable designers can effectively provide self shielding
for wire pairs within the network media.
3 (a) What are the major differences between wired LANs and wireless LANs? What are the two main problems with wireless LANs. Briefly, describe the physical layer standards of IEEE 802.11.
ans)LAN stands for Local Area Network, which is a collection of computers and other network devices in a certain location that are connected together by switches and the routers that facilitate the communication of the network elements. Each computer or network element is connected to the switches/routers via a UTP cable. The added letter in WLAN stands for wireless. This is a type of network where the data is not transmitted via cables but over the air through the use of wireless transmitters and receivers.
WLANs are deployed in areas where a wide number of computers may connect to the network but not at the same time. Places like cofee shops often add WLAN to their shops to entice more customers who do not stay for extended periods. Even at home where you have a somewhat fixed number of computers that connect to the network, WLAN is also preferred as it gives users the freedom to move around the house and carry their laptops with them without needing to fuss with cables. For areas where the computers are pretty much fixed, a wired LAN is very desirable due to the advantages that it offers.
- the main problem in wlan is security issues the radio waves used in wireless networking typically penetrate outside the building, creating a real risk that the network can be hacked from the parking lot or the street..
- wlan is slower than lan network.Lan network is much faster than WLAN
The Institute of Electronic and Electrical Engineers (IEEE) has released IEEE 802.11 in June 1997. The standard defined physical and MAC layers of wireless local area networks (WLANs).
The physical layer of the original 802.11 standardized three wireless data exchange techniques:
- Infrared (IR);
- Frequency hopping spread spectrum (FHSS);
- Direct sequence spread spectrum (DSSS).
The physical layer in 802.11 is split into Physical Layer Convergence Protocol (PLCP) and the Physical Medium Dependent (PMD) sub layers. The PLCP prepares/parses data units transmitted/received using various 802.11 media access techniques. The PMD performs the data transmission/reception and modulation/demodulation directly accessing air under the guidance of the PLCP. The 802.11 MAC layer to the great extend is affected by the nature of the media. For example, it implements a relatively complex for the second layer fragmentation of PDUs.
(b) Make a comparative analysis between IPV4 and IPV6.
ans)The number of IPv4 unique addresses is not that large in relation to the current rate of expansion of the Internet. Consequently, a new addressing system has been devised which is a part of Internet Protocol version 6 (IPv6), which uses 128-bit addresses, means total addresses will be 2128 .. IPv4 uses 32-bit addresses, means total addresseswill be 232 around 4,294,967,296 unique addresses. IPv6 has almost 7.9x1028 times moreaddresses than IPv4. It is possible that IPv6 would not be used or implemented completely in the coming couple of years. This IPv6 (Internet Protocol version 6) is a revision of the earlier Internet Protocol (IP) version 4. As you know IPv4 address is 32 bit and divided intofour octets separated by dot for example 192.186.12.10, on the other hand IPv6 addresses are consist of eight groups of four hexadecimal digits separated by colons, for example 2001:0db8:85a3:0042:0000:8a2e:0370:7334. IPv6 is designed to swap the existing IPv4, which is the main communications protocol for most Internet traffic as of today. IPv6 was developed to deal with the long-anticipated problem of IPv4 running out of addresses, some of the reasons and need for implementing IPv6 are following:
• The short term solutions like sub-netting, classless addressing cannot fulfill the
massive future demand of address space.
• The internet must accommodate the real-time audio and video transmission with
best quality of services.
• Internet protocol must provide the necessary security implementation for some
applications.
• There is a need of multicasting in current IPv4, where the transmission of a packet
to multiple destinations can be sent in a single send operation.
• IPv4 need a major revision in various issues like privacy, mobility, routing, QoS
(quality of services), extendibility and addressing
(c) What is congestion? What are the main reasons for occurring congestion in the network? Explain the techniques to solve the problem of congestion.
ans)Congestion: When too many packets are in a subnet or a part of subnet, performance
degrades as depicted in figure 9. This situation is called congestion.
Factors Causing the Congestion
1. Many input lines demanding the same output lines.
2. Slow receiver fast sender.
3. Low bandwidth lines can also cause congestion.
4. Congestion itself (duplicacy).
5. Traffic is bursty
a) Open loop: In open loop solution the good designs are being developed to solve
the problem so that congestion does not occur at first place once the system is
setup and running, no mid pores connection is made. In open loop control, tools
are included to decide when to accept new traffic, when to discard packets and
which ones. And making scheduling decisions at various points in the network.
The decisions are offline decision is not based on current state of network close
loop solution.
The concept of feedback loop is used in closed loop solution. This approach has
three parts, when apply to the congestion control.
i) Monitor the system to detect when and where congestion occurs.
ii) Pass the information to places where the action can be taken.
iii) Adjust system operation to correct the problem
Traffic Shaping (Congestion Control Policy in ATM)
One of the main causes of congestion is that traffic is often bursty. If hosts could be
made to transmit a uniform rate, congestion would be less common. Another open
loop method to help manage congestion is forcing the packet to be transmitted at a
more predictable rate. This approach to congestion management is widely used in
ATM networks and is called traffic shaping.
Traffic shaping is about regulating the average rate (and burstiness) of data
transmission.
LEAKY BUCKET ALGORITHM-:Leaky bucket algorithm can be understood as “The leaky bucket consists of finite
queue when a packet arrives, if there is room on the queue it is appended to the queue,
otherwise it is discarded. At every clock tick, one packet is transmitted
TOKEN BUCKET ALGORITHM-:In many applications, it is better to allow the output to speed up somewhat when large
bursts arrive so a more flexible algorithm is needed, preferably one that never losses
data one such algorithm is token bus algorithm.
In token bucket algorithm, the leaky bucket holds ‘tokens’ generated by a clock at the
rate of one token every DT sec.
The token bucket algorithm provides a different kind of traffic shaping than the leaky
bucket algorithm. Bust of up to n. packets can be sent at once, allowing some
burstiness in the output stream and giving faster response to sodden bursts of input.
A token bucket algorithm throws away tokens when the bucket fills up but never
discards packets.
4(a) Explain the features of HDLC and describe in detail the Frame Format
ans)The High Level Data Link Control protocol was developed by the International Organization for Standardization (ISO). It is used for switched and non-switched networks and is a bit oriented architecture. The High Level Data Link Control has been accepted and used widely because it supports almost all kinds of functionalities like half duplex, full duplex, point to point and multi-point networks. The bit patterns are standard in HDLC and therefore the information exchanged follows the same pattern. This minimizes the chances of any errors.
The main features of HDLC are divided into various aspects
• The modes for operation
• Stations
• Configuration
• Frames and Structures
• The subsets of HDLC
• The modes for operation
• Stations
• Configuration
• Frames and Structures
• The subsets of HDLC
HDLC frames can be transmitted over synchronous or asynchronous links. Those links have no mechanism to mark the beginning or end of a frame, so the beginning and end of each frame has to be identified. This is done by using a frame delimiter, or flag, which is a unique sequence of bits that is guaranteed not to be seen inside a frame. This sequence is '01111110', or, in hexadecimal notation, 0x7E. Each frame begins and ends with a frame delimiter. A frame delimiter at the end of a frame may also mark the start of the next frame. A sequence of 7 or more consecutive 1-bits within a frame will cause the frame to be aborted.
When no frames are being transmitted on a simplex or full-duplex synchronous link, a frame delimiter is continuously transmitted on the link. Using the standard NRZI encoding from bits to line levels (0 bit = transition, 1 bit = no transition), this generates one of two continuous waveforms, depending on the initial state
This is used by modems to train and synchronize their clocks via phase-locked loops. Some protocols allow the 0-bit at the end of a frame delimiter to be shared with the start of the next frame delimiter, i.e. '011111101111110'.
For half-duplex or multi-drop communication, where several transmitters share a line, a receiver on the line will see continuous idling 1-bits in the inter-frame period when no transmitter is active.
Since the flag sequence could appear in user data, such sequences must be modified during transmission to keep the receiver from detecting a false frame delimiter. The receiver must also detect when this has occurred so that the original data stream can be restored before it is passed to higher layer protocols. This can be done using bit stuffing, in which a "0" is added after the occurrence of every "11111" in the data. When the receiver detects these "11111" in the data, it removes the "0" added by the transmitter.
Make a comparison of channel utilization versus load for various random access
protocols.
(ans)
(c) Discuss why repeaters are needed in a communication channel. What is a key difference between an analog repeater and a digital repeater?
A repeater is an electronic device that receives a signal and retransmits it at a higher
level or higher power, or onto the other side of an obstruction, so that the signal can
cover longer distances without degradation.Because repeaters work with the actual physical signal, and do not attempt to interpret
the data being transmitted, they operate on the Physical layer, the first layer of the
OSI model.In telecommunication, the term repeater has the following standardized meanings:
An analog device that amplifies an input signal regardless of its nature (analog or
digital).
A digital device that amplifies, reshapes, retimes, or performs a combination of any of
these functions on a digital input signal for retransmission.
Author Name
Author Description!
Get Free Email Updates to your Inbox!
