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Question 1: Explain the major classification of Computer Networks.
A computer network is a group of interconnected computers. In the world of computers,
networking is the practice of linking two or more computing devices together for the purpose
of sharing data. Networks are built with a mix of computer hardware and computer software.
The network allows computers to communicate with each other and share resources and
Following can be the several factors for classifying different computer networks.
In networking, the communication language used by computer devices is called the protocol.
Yet another way to classify computer networks is by the set of protocols they support.
Networks often implement multiple protocols to support specific applications. Popular
protocols include TCP/IP, the most common protocol found on the Internet and in home
Wired Vs Wireless Networking:
Many of the same network protocols, like TCP/IP, work in both wired and wireless networks.
Networks with Ethernet cables predominated in businesses, schools, and homes for several
decades. Recently, however, wireless networking alternatives have emerged as the premier
technology for building new computer networks.
Computer networks also differ in their design. The two types of high-level network design are
called client-server and peer-to-peer. Client-server networks feature centralized server
computers that store email, Web pages, files and or applications. On a peer-to-peer network,
conversely, all computers tend to support the same functions. Client-server networks are
much more common in business and peer-to-peer networks much more common in homes.
Often, it is impractical for two devices to be directly, point-to-point connected. This is so for one (or both) of the following contingencies:
The devices are very far apart. It would be inordinately expensive, for example, to string a dedicated link between two devices thousands of miles apart.
There is a set of devices, each of which may require a link to many of the others at various times. Examples are all of the telephones in the world and all of the terminals
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and computers owned by a single organization. Except for the case of a very few devices, it is impractical to provide a dedicated wire between each pair of devices.
The solution to this problem is to attach each device to a communication network.
This is the most basic approach to classify networks. It defines the type of network according
to the geographic area it spans. Local area networks (LANs), for example, typically reach
across a single home, whereas wide area networks (WANs), reach across cities, states, or even
across the world. The Internet is the world's largest public WAN.
WAN spans a large geographic area, such as a state, province or country. WANs often connect
multiple smaller networks, such as local area networks (LANs) or metro area networks (MANs).
Typically, a WAN consists of a number of interconnected switching nodes. A transmission from any one device is routed through these internal nodes to the specified destination device. These nodes (including the boundary nodes) are not concerned with the content of the data; rather, their purpose is to provide a switching facility that will move the data from node to node until they reach their destination.
The world's most popular WAN is the Internet. Some segments of the Internet, like VPN-based extranets, are also WANs in themselves. Finally, many WANs are corporate or research networks that utilize leased lines.
WANs generally utilize different and much more expensive networking equipment than do LANs. Key technologies often found in WANs include SONET, Frame Relay, and ATM.
In a circuit-switched network, a dedicated communications path is established between two stations through the nodes of the network. That path is a connected sequence of physical links between nodes. On each link, a logical channel is dedicated to the connection. Data generated by the source station are transmitted along the dedicated path as rapidly as possible. At each node, incoming data are routed or switched to the appropriate outgoing channel without delay. The most common example of circuit switching is the telephone network.
A quite different approach is used in a packet-switched network. In this case, it is not necessary to dedicate transmission capacity along a path through the network. Rather, data are sent out in a sequence of small chunks, called packets. Each packet is passed through the network from node to node along some path leading from source to destination. At each node,
Wide Area Network (WAN):
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the entire packet is received, stored briefly, and then transmitted to the next node. Packet- switched networks are commonly used for terminal-to-computer and computer-to-computer communications.
Packet switching was developed at a time when digital long-distance transmission facilities exhibited a relatively high error rate compared to today's facilities. As a result, there is a considerable amount of overhead built into packet-switched schemes to compensate for errors. The overhead includes additional bits added to each packet to introduce redundancy and additional processing at the end stations and the intermediate switching nodes to detect and recover from errors. With modern high-speed telecommunications systems, this overhead is unnecessary and counterproductive. It is unnecessary because the rate of errors has been dramatically lowered and any remaining errors can easily be caught in the end systems by logic that operates above the level of the packet-switching logic; it is counterproductive because the overhead involved soaks up a significant fraction of the high capacity provided by the network. Frame relay was developed to take advantage of these high data rates and low error rates. Whereas the original packet-switching networks were designed with a data rate to the end user of about 64 kbps, frame relay networks are designed to operate efficiently at user data rates of up to 2 Mbps. The key to achieving these high data rates is to strip out most of the overhead involved with error control.
Asynchronous transfer mode (ATM), sometimes referred to as cell relay, is a culmination of all of the developments in circuit switching and packet switching over the past 25 years. ATM can be viewed as an evolution from frame relay. The most obvious difference between frame relay and ATM is that frame relay uses variable-length packets, called frames, and ATM uses fixed-length packets, called cells. As with frame relay, ATM provides little overhead for error control, depending on the inherent reliability of the transmission system and on higher layers of logic in the end systems to catch and correct errors. By using a fixed-packet length, the processing overhead is reduced even further for ATM compared to frame relay. The result is that ATM is designed to work in the range of 10s and 100s of Mbps, compared to the 2-Mbps target of frame relay. ATM can also be viewed as an evolution from circuit switching. With circuit switching, only fixed-data-rate circuits are available to the end system. ATM allows the definition of multiple virtual channels with data rates that are dynamically defined at the time the virtual channel is created. By using full, fixed-size cells, ATM is so efficient that it can offer a constant-data- rate channel even though it is using a packet-switching technique. Thus, ATM extends circuit switching to allow multiple channels with the data rate on each channel dynamically set on demand.
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A local area network (LAN) supplies networking capability to a group of computers in close
proximity to each other such as in an office building, a school, or a home. A LAN is useful for
sharing resources like files, printers, games or other applications. A LAN in turn often
connects to other LANs, and to the Internet or other WAN.
Most local area networks are built with relatively inexpensive hardware such as Ethernet
cables, network adapters, and hubs. Wireless LAN and other more advanced LAN hardware
options also exist.
Traditionally, LANs make use of a broadcast network approach rather than a switching approach. With a broadcast communication network, there are no intermediate switching nodes. At each station, there is a transmitter and a receiver that communicates over a medium shared by other stations. A transmission from any one station is broadcast to and received by all other stations. A simple example of this is a CB radio system, in which all users tuned to the same channel may communicate. We will be concerned with networks used to link computers, workstations, and other digital devices. In the latter case, data are usually transmitted in packets. Because the medium is shared, i.e. only one station at a time can transmit a packet.
More recently, examples of switched LANs have appeared. The two most prominent examples are ATM LANs, which simply use an ATM network in a local area, and Fibre Channel.
The most common type of local area network is an Ethernet LAN. The smallest home LAN can
have exactly two computers; a large LAN can accommodate many thousands of computers.
Many LANs are divided into logical groups called subnets. An Internet Protocol (IP) "Class A"
LAN can in theory accommodate more than 16 million devices organized into subnets.
There are several key distinctions between LANs and WANs:
The scope of the LAN is small, typically a single building or a cluster of buildings. This difference in geographic scope leads to different technical solutions.
It is usually the case that the LAN is owned by the same organization that owns the
attached devices. For WANs, this is less often the case, or at least a significant fraction of the network assets are not owned. This has two implications. First, care must be taken in the choice of LAN, as there may be a substantial capital investment (compared
Key Distinctions between LANs and WANs:
Local Area Network (LAN):
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to dial-up or leased charges for wide area networks) for both purchase and maintenance. Second, the network management responsibility for a local network falls solely on the user.
The internal data rates of LANs are typically much greater than those of wide area networks.
Question 2: Explain structure and Functions of OSI layers.
The Open Systems Interconnection (OSI) reference model has been an essential element of
computer network design since its ratification in 1984. The OSI is an abstract model of how
network protocols and equipment should communicate and work together (interoperate).
The OSI Model Stack:
The OSI model divides the complex task of computer-to-computer communications,
traditionally called internetworking, into a series of stages known as layers. Layers in the OSI
model are ordered from lowest level to highest. Together, these layers comprise the OSI
stack. The stack contains seven layers.
Layer 7 – Application Layer:
The application layer is the OSI layer closest to the end user, which means that both the OSI
application layer and the user interact directly with the software application. This layer
interacts with software applications that implement a communicating component. Application
layer functions typically include identifying communication partners, determining resource
availability, and synchronizing communication.
Some examples of application layer implementations include Telnet, Hypertext Transfer
Protocol (HTTP), File Transfer Protocol (FTP) , and Simple Mail Transfer Protocol (SMTP).
Open Systems Interconnection (OSI):
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Layer 6 – Presentation Layer:
The Presentation Layer establishes a context between Application Layer entities. This layer
provides independence from differences in data representation (e.g., encryption) by
translating from application to network format, and vice versa. The presentation layer works
to transform data into the form that the application layer can accept. This layer formats and
encrypts data to be sent across a network, providing freedom from compatibility problems. It
is sometimes called the syntax layer.
Layer 5 – Session Layer:
The Session Layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes check pointing, adjournment, termination, and restart procedures.
Layer 4 – Transport Layer:
The Transport Layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the Transport Layer can keep track of the segments and retransmit those that fail.
Layer 3 – Network Layer:
The Network Layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks, while maintaining the quality of service requested by the Transport Layer. The Network Layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer—sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is hierarchical.
Layer 2 – Data Link Layer:
At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sublayers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.
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Layer 1 – Physical Layer:
The Physical Layer defines the electrical and physical specifications for devices. In particular,
it defines the relationship between a device and a physical medium. This includes the layout
of pins, voltages, cable specifications, Hubs, repeaters, network adapters, Host Bus Adapters
(HBAs used in Storage Area Networks) and more.
This layer conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.
Question 3: What are Protocols and Standards?
In the field of telecommunications, a communications protocol is the set of standard rules for
data representation, signaling, authentication and error detection required to send
information over a communications channel.
An example of a simple communications protocol adapted to voice communication is the case
of a radio dispatcher talking to mobile stations. Communication protocols for digital computer
network communication have features intended to ensure reliable interchange of data over an
imperfect communication channel. Communication protocol is basically following certain rules
so that the system works properly.
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A protocol is used for communication between entities in different systems. The terms "entity" and "system" are used in a very general sense. Examples of entities are user application programs, file transfer packages, data-base management systems, electronic mail facilities, and terminals. Examples of systems are computers, terminals, and remote sensors. In general, an entity is anything capable of sending or receiving information, and a system is a physically distinct object that contains one or more entities. For two entities to communicate successfully, they must "speak the same language". What is communicated, how it is communicated, and when it is communicated must conform to some mutually acceptable conventions between the entities involved. The conventions are referred to as a protocol, which may be defined as a set of rules governing the exchange of data between two entities. The key elements of a protocol are:
Syntax. Includes such things as data format and signal levels. Semantics. Includes control information for coordination and error handling. Timing. Includes speed matching and sequencing.
It has long been accepted in the communications industry that standards are required to govern the physical, electrical, and procedural characteristics of communication equipment. communication-equipment vendors recognize that their equipment will generally interface to and communicate with other vendors' equipment, computer vendors have traditionally attempted to lock their customers into proprietary equipment.
A standard assures that there will be a large market for a particular piece of equipment or software. This encourages mass production and, in some cases, the use of large- scale-integration (LSI) or very-large-scale-integration (VLSI) techniques, resulting in lower costs.
A standard allows products from multiple vendors to communicate, giving the purchaser
more flexibility in equipment selection and use.
A standard tends to freeze the technology. By the time a standard is developed, subjected to review and compromise, and promulgated, more efficient techniques are possible.
There are multiple standards for the same thing. This is not a disadvantage of standards
per se, but of the current way things are done. Fortunately, in recent years the various standards-making organizations have begun to cooperate more closely. Nevertheless, there are still areas where multiple conflicting standards exist.
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Question 4: Define Channel Capacity and Explain Shannon Capacity Formula
Channel capacity is the tightest upper bound on the amount of information that can be
reliably transmitted over a communications channel. It is the limiting information rate (in
units of information per unit time) that can be achieved with arbitrarily small error
One can intuitively reason that, for a given communication system, as the information rate increases the number of errors per second will also increase. Surprisingly, however, this is not the case.z
A given communication system has a maximum rate of information C known as the channel capacity.
If the information rate R is less than C, then one can approach arbitrarily small error
probabilities by using intelligent coding techniques.
Thus, if R ≤ C then transmission may be accomplished without error in the presence of noise.
Consider a band limited Gaussian channel operating in the presence of additive Gaussian noise:
Then according to the Shannon’s Capacity Formula, the channel capacity is given by:
C = B log2 (1 + S/N)
Where; C is the capacity in bits per second, Bis the bandwidth of the channel in Hertz, and S/Nis the signal-to-noise ratio.
Shannon’s Capacity Formula:
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Question 5: A digital Signaling is required at 11200 bps
A. If a signal element encodes 8 bit word, what is the minimum required bandwidth of the channel?
B. In case of 32-bit word? Since,
C = 2B log2 M
C is the capacity in bits per second, Bis the bandwidth of the channel in Hertz, and M is the number of levels
C = 11200 bps log2M = 8 (i.e. number of levels for 8 bits is 256) 11200 = 2 x B x 8 B = 700 Hz
C = 11200 bps log2M = 32 (i.e. M = 4294967296) 11200 = 2 x B x 32 B = 175 Hz
Question 6: Given a channel with intended capacity of 32 Mbps, the bandwidth of the channel is 3 MHz. Assuming white thermal noise what signal to noise ratio is required to achieve this capacity.
According to the Shannon’s Capacity Formula:
C = B log2 (1 + S/N)
Where; C is the capacity in bits per second, Bis the bandwidth of the channel in Hertz, and S/Nis the signal-to-noise ratio. C = 32 Mbps B = 3 MHz
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32 x 106 = 3 x 106 log2 (1 + S/N) 32/3 = log2 (1 + S/N) 232/3 – 1 = S/N S/N = 1624.4986 SNRdb = 10 log10 (1624.4986) SNRdb = 32.107 db
Question 7: A periodic signal has been decomposed using Fourier Analysis to yield four sine waves of frequencies 100, 400, 600 and 800 Hz. What is the bandwidth of the resulting periodic signal. Resulting Bandwidth = Frequency max – Frequency min = 800 Hz – 100 Hz = 100 Hz
Question 8: Suppose that a digitized TV picture is to be transmitted from a source that uses a matrix of 800 X 600 picture elements (pixels), where each pixel can take on one of 256 intensity values. Assume that 30 frames are sent per second. Find the source rate R (bps)? Intensity values or levels = M = 256 Therefore the number of bits for each level = n = log2 M = 8 Frames sent per second = 30 Total pixels = 800 x 600 = 480000 The source rate is given as: R = (Total pixels) x (Frames per second) x n R = 480000 x 30 x 8 R = 115.2 Mbps
Question 9: Encode the following digital data using following encoding schemes. 1 0 1 0 0 1 1 0 1 1 0 0 0 0 0 1 1 1 1 1 0 0 0 0 0 0 0 0 0 1 0 0 1 1