Computer Network Notes For Exam Preparation

1. Computer Network Solution
1. Define Computer Network and Distributed network. Why are computer networks needed? What are the goals and applications of computer networks?
Ans:- Computer Network Definition A computer network or data network is a telecommunications network that allows computers to exchange data. In computer networks, networked computing devices pass data to each other along data connections. Data is transferred in the form of packets. The connections (network links) between nodes are established using either cable media or wireless media. The best-known computer network is the Internet. Network computer devices that originate, route and terminate the data are called network nodes.Nodes can include hosts such as personal computers, phones, servers , as well as networking hardware, Distributed Network Definition A distributed network is a type of computer network that is spread over different networks. This provides a single data communication network which can be managed jointly or separately by each network .Beside shared communication with in a network , a distributed network after also distributes processing. Computer Network Needs
 Sharing files. You can access files on other network computers. This can be very handy, for example, when you're paying bills on a laptop in the living room, and you need a file that’s on the computer in your home office. With a network, you can access that file without having to physically go to the other computer. And you're in control: with a network, you can share what you want to share and keep private what you want to keep private.
 Streaming media. Media streaming refers to the process of sending digital media, such as photos, music, or video, over a network to a device that can play the media. For instance, you can view your photos or movies on some current-generation TVs, or you can play music on some compatible stereo receivers that are connected to your network.
 Sharing an Internet connection. You can share a broadband Internet connection—which means you don’t have to buy a separate Internet account for each computer.
 Playing network games. You can play computer games with other people on the Internet, and you can even connect game consoles (such as Microsoft Xbox, Nintendo Wii, and Sony PlayStation) to your network.
 Sharing a printer. Instead of buying a printer to connect to each computer, you can use one printer and connect it to the network. Then everyone on the network can use it.
NETWORK GOALS:
The main goal of networking is "Resource sharing", and it is to make all programs, data and equipment available to anyone on the network with out the regard to the physical location of the resource and the user.
A second goal is to provide high reliability by having alternative sources of supply. For example, all files could be replicated on two or three machines, so if one of them is unavailable, the other copies could be available.
Another goal is saving Money. Small computers have a much better price/performance ratio than larger ones. Mainframes are roughly a factor of ten times faster than the fastest single chip microprocessors, but they cost thousand times more. This imbalance has caused many system designers to build systems consisting of powerful personal computers, one per user, with data kept on one or more shared file server machines. This goal leads to networks with many computers located in the same building. Such a network is called a LAN(local area network).
Another closely related goal is to increase the Systems Performance as the work load increases by just adding more processors. With central mainframes, when the system is full, it must be replaced by a larger one, usually at great expense and with even greater disruption to the users.
Computer networks provide a powerful communication medium. A file that was updated/modified on a network, can be seen by the other users on the network immediately.
NETWORK APPLICATIONS:
1. Access to remote programs.
2. Access to remote databases.
3. Value-added communication facilities.
4. Resource sharing such as printers and storage devices.

2. 5 . Exchange of Information by means of eMails and FTP.
6. Information sharing by using Web or Internet.
7. Interaction with other users using dynamic web pages.
8. IP phones
9. Video Conferences
10. Parallel computing
11. Instant Messaging
2.Define Data Communication. What are its major components? What are the Network criteria that must be fulfilled for data communication? Ans:- Data Communication Data communication refers to the exchange of data between a source and a receiver. Data communication is said to be local if communicating devices are in the same building or a similarly restricted geographical area. The meanings of source and receiver are very simple. The device that transmits the data is known as source and the device that receives the transmitted data is known as receiver. Data communication aims at the transfer of data and maintenance of the data during the process but not the actual generation of the information at the source and receiver. Major Components Of Data Communication A Communication system has following components: 1. Message: It is the information or data to be communicated. It can consist of text, numbers, pictures, sound or video or any combination of these. 2. Sender: It is the device/computer that generates and sends that message. 3. Receiver: It is the device or computer that receives the message. The location of receiver computer is generally different from the sender computer. The distance between sender and receiver depends upon the types of network used in between. 4. Medium: It is the channel or physical path through which the message is carried from sender to the receiver. The medium can be wired like twisted pair wire, coaxial cable, fiber-optic cable or wireless like laser, radio waves, and microwaves. 5. Protocol: It is a set of rules that govern the communication between the devices. Both sender and receiver follow same protocols to communicate with each other.
Criteria for a Data Communication Network
The major criteria that a Data Communication Network must meet are:
1.Performance
Performance is the defined as the rate of transferring error free data. It is measured by the Response Time. Response Time is the elasped time between the end of an inquiry and the beginning of a response. Request a file transfer and start the file transfer. Factors that affect Response Time are:
 Number of Users: More users on a network - slower the network will run
 Transmission Speed: speed that data will be transmitted measured in bits per second (bps)
 Media Type: Type of physical connection used to connect nodes together
 Hardware Type: Slow computers such as XT or fast such as Pentiums
 Software Program: How well is the network operating system (NOS) written
2.Consistency

3. Consistency is the predictability of response time and accuracy of data.
 Users prefer to have consistent response times, they develop a feel for normal operating conditions. For example: if the "normal" response time is 3 sec. for printing to a Network Printer and a response time of over 30 sec happens, we know that there is a problem in the system!
 Accuracy of Data determines if the network is reliable! If a system loses data, then the users will not have confidence in the information and will often not use the system.
3.Reliability
Reliability is the measure of how often a network is useable. MTBF (Mean Time Between Failures) is a measure of the average time a component is expected to operate between failures. Normally provided by the manufacturer. A network failure can be: hardware, data carrying medium and Network Operating System.
4.Recovery
Recovery is the Network's ability to return to a prescribed level of operation after a network failure. This level is where the amount of lost data is nonexistent or at a minimum. Recovery is based on having Back-up Files.
5. Security
Security is the protection of Hardware, Software and Data from unauthorized access. Restricted physical access to computers, password protection, limiting user privileges and data encryption are common security methods. Anti-Virus monitoring programs to defend against computer viruses are a security measure.
3.What do you understand by Topology? Explain the major Network Topologies along with their advantages & disadvantages.
Ans:-Topology In communication networks, a topology is a usually schematic description of the arrangement of a network, including its nodes and connecting lines. There are two ways of defining network geometry: the physical topology and the logical (or signal) topology. The physical topology of a network is the actual geometric layout of workstations. There are several common physical topologies, as described below and as shown in the illustration.
1.Bus topology Bus Topology is the simplest of network topologies. In this type of topology, all the nodes (computers as well as servers) are connected to the single cable (called bus), by the help of interface connectors. This central cable is the backbone of the network and is known as Bus (thus the name). Every workstation communicates with the other device through this Bus. A signal from the source is broadcasted and it travels to all workstations connected to bus cable. Although the message is broadcasted but only the intended recipient, whose MAC address or IP address matches, accepts it. If the MAC /IP address of machine doesn’t match with the intended address, machine discards the signal. A terminator is added at ends of the central cable, to prevent bouncing of signals. A barrel connector can be used to extend it. Below I have given a basic diagram of a bus topology and then have discussed advantages and disadvantages of Bus Network Topology
Bus topology diagram
Advantages (benefits) of Linear Bus Topology

4. 1) It is easy to set-up and extend bus network. 2) Cable length required for this topology is the least compared to other networks. 3) Bus topology costs very less. 4) Linear Bus network is mostly used in small networks. Good for LAN.
Disadvantages (Drawbacks) of Linear Bus Topology 1) There is a limit on central cable length and number of nodes that can be connected. 2) Dependency on central cable in this topology has its disadvantages.If the main cable (i.e. bus ) encounters some problem, whole network breaks down. 3) Proper termination is required to dump signals. Use of terminators is must. 4) It is difficult to detect and troubleshoot fault at individual station. 5) Maintenance costs can get higher with time. 6) Efficiency of Bus network reduces, as the number of devices connected to it increases. 7) It is not suitable for networks with heavy traffic. 8) Security is very low because all the computers receive the sent signal from the source.
2.Ring Topology Advantages ▸ The data being transmitted between two nodes passes through all the intermediate nodes. A central server is not required for the management of this topology. ▸ The traffic is unidirectional and the data transmission is high-speed. ▸ In comparison to a bus, a ring is better at handling load. ▸ The adding or removing of network nodes is easy, as the process requires changing only two connections. ▸ The configuration makes it easy to identify faults in network nodes. ▸ In this topology, each node has the opportunity to transmit data. Thus, it is a very organized network topology. ▸ It is less costly than a star topology. Disadvantages ▸ The failure of a single node in the network can cause the entire network to fail. ▸ The movement or changes made to network nodes affect the entire network's performance. ▸ Data sent from one node to another has to pass through all the intermediate nodes. This makes the transmission slower in comparison to that in a star topology. The transmission speed drops with an increase in the number of nodes. ▸ There is heavy dependency on the wire connecting the network nodes in the ring. Back to Index 3.Mesh Topology Advantages ▸ The arrangement of the network nodes is such that it is possible to transmit data from one node to many other nodes at the same time. ▸ The failure of a single node does not cause the entire network to fail as there are alternate paths for data transmission.

5. ▸ It can handle heavy traffic, as there are dedicated paths between any two network nodes. ▸ Point-to-point contact between every pair of nodes, makes it easy to identify faults. Disadvantages ▸ The arrangement wherein every network node is connected to every other node of the network, many connections serve no major purpose. This leads to redundancy of many network connections. ▸ A lot of cabling is required. Thus, the costs incurred in setup and maintenance are high. ▸ Owing to its complexity, the administration of a mesh network is difficult. 4.Star topology In Star topology, all the components of network are connected to the central device called “hub” which may be a hub, a router or a switch. Unlike Bus topology (discussed earlier), where nodes were connected to central cable, here all the workstations are connected to central device with a point-to-point connection. So it can be said that every computer is indirectly connected to every other node by the help of “hub”. All the data on the star topology passes through the central device before reaching the intended destination. Hub acts as a junction to connect different nodes present in Star Network, and at the same time it manages and controls whole of the network. Depending on which central device is used, “hub” can act as repeater or signal booster. Central device can also communicate with other hubs of different network. Unshielded Twisted Pair (UTP) Ethernet cable is used to connect workstations to central node.
Star Topology Diagram
Advantages of Star Topology
1) As compared to Bus topology it gives far much better performance, signals don’t necessarily get transmitted to all the workstations. A sent signal reaches the intended destination after passing through no more than 3-4 devices and 2-3 links. Performance of the network is dependent on the capacity of central hub. 2) Easy to connect new nodes or devices. In star topology new nodes can be added easily without affecting rest of the network. Similarly components can also be removed easily. 3) Centralized management. It helps in monitoring the network. 4) Failure of one node or link doesn’t affect the rest of network. At the same time its easy to detect the failure and troubleshoot it.
Disadvantages of Star Topology
1) Too much dependency on central device has its own drawbacks. If it fails whole network goes down. 2) The use of hub, a router or a switch as central device increases the overall cost of the network. 3) Performance and as well number of nodes which can be added in such topology is depended on capacity of central device.
5.Tree Topology Imagine a hierarchy of network nodes, with the root node serving client nodes, that in turn serve other

6. lower-level nodes. The top-level node is mostly a mainframe computer while other nodes in the hierarchy are mini or microcomputers. In this arrangement, the node at each level could be forming a star network with the nodes it serves. In this case, the structure combines star and bus topologies and inherits their advantages and disadvantages. Advantages ▸ The tree topology is useful in cases where a star or bus cannot be implemented individually. It is most- suited in networking multiple departments of a university or corporation, where each unit (star segment) functions separately, and is also connected with the main node (root node). ▸ The advantages of centralization that are achieved in a star topology are inherited by the individual star segments in a tree network. ▸ Each star segment gets a dedicated link from the central bus. Thus, failing of one segment does not affect the rest of the network. ▸ Fault identification is easy. ▸ The network can be expanded by the addition of secondary nodes. Thus, scalability is achieved. Disadvantages ▸ As multiple segments are connected to a central bus, the network depends heavily on the bus. Its failure affects the entire network. ▸ Owing to its size and complexity, maintenance is not easy and costs are high. Also, configuration is difficult in comparison to that in other topologies. ▸ Though it is scalable, the number of nodes that can be added depends on the capacity of the central bus and on the cable type.
4.Explain the OSI Reference model mentioning the functions of each layer in detail.
Ans:- Open Systems Interconnection (OSI) is a set of internationally recognized, non-proprietary standards for networking and for operating system involved in networking functions.
OSI layer is divided into 7 layers:-
Functions of the Layers
1. Physical Layer
The physical layer coordinates the functions required to transmit a bit stream over a physical medium.

7. III
The physical layer is concerned with the following:
Physical characteristics of interfaces and media - The physical layer defines the characteristics of the interface between the devices and the transmission medium.
Representation of bits - To transmit the stream of bits, it must be encoded to signals. The physical layer defines the type of encoding.
Data Rate or Transmission rate - The number of bits sent each second is also defined by the physical layer.
Synchronization of bits - The sender and receiver must be synchronized at the bit level. Their clocks must be synchronized.
Line Configuration - In a point-to-point configuration, two devices are connected together through a dedicated link. In a multipoint configuration, a link is shared between several devices.
Physical Topology - The physical topology defines how devices are connected to make a network. Devices can be connected using a mesh, bus, star or ring topology.
Transmission Mode - The physical layer also defines the direction of transmission between two devices: simplex, half-duplex or full-duplex
2. Data Link Layer
It is responsible for transmitting frames from one node to next node.
The other responsibilities of this layer are:
Framing - Divides the stream of bits received into data units called frames.
Physical addressing – If frames are to be distributed to different systems on the n/w , data link layer adds a header to the frame to define the sender and receiver.
Flow control- If the rate at which the data are absorbed by the receiver is less than the rate produced in the sender ,the Data link layer imposes a flow ctrl mechanism.
Error control- Used for detecting and retransmitting damaged or lost frames and to prevent duplication of frames. This is achieved through a trailer added at the end of the frame.
Access control -Used to determine which device has control over the link at any given time.
3. NETWORK LAYER
This layer is responsible for the delivery of packets from source to destination.
It is mainly required, when it is necessary to send information from one network to another.

8. The other responsibilities of this layer are
Logical addressing - If a packet passes the n/w boundary, we need another addressing system for source and destination called logical address.
Routing – The devices which connects various networks called routers are responsible for delivering packets to final destination.
4. TRANSPORT LAYER
 It is responsible for Process to Process delivery.
 It also ensures whether the message arrives in order or not.
The other responsibilities of this layer are
Port addressing - The header in this must therefore include a address called port address. This layer gets the entire message to the correct process on that computer.
Segmentation and reassembly - The message is divided into segments and each segment is assigned a sequence number. These numbers are arranged correctly on the arrival side by this layer.
Connection control - This can either be connectionless or connection-oriented. The connectionless treats each segment as a individual packet and delivers to the destination. The connection-oriented makes connection on the destination side before the delivery. After the delivery the termination will be terminated.
Flow and error control - Similar to data link layer, but process to process take place.
5.SESSION LAYER
This layer establishes, manages and terminates connections between applications.
The other responsibilities of this layer are
Dialog control - This session allows two systems to enter into a dialog either in half duplex or full duplex.
Synchronization-This allows to add checkpoints into a stream of data.
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6.PRESENTATION LAYER

9. It is concerned with the syntax and semantics of information exchanged between two systems.
The other responsibilities of this layer are

Translation – Different computers use different encoding system, this
layer is responsible for interoperability between these different encoding
methods. It will change the message into some common format.

Encryption and decryption-It means that sender transforms the
original information to another form and sends the resulting message
over the n/w. and vice versa.
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Compression and expansion-Compression reduces the number of bits
contained in the information particularly in text, audio and video.
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APPLICATION LAYER
This layer enables the user to access the n/w. This allows the user to log
on to remote user.
The other responsibilities of this layer are:-
FTAM (file transfer,access,mgmt) - Allows user to access files in a remote host.
Mail services - Provides email forwarding and storage.
Directory services - Provides database sources to access information about various sources and objects.
5. Compare & Contrast the OSI model and TCP/IP model for data communication. Ans:-The Internet Protocol Suite also known as TCP/IP is the set of communications protocols used for the Internet and other similar networks. It is named from two of the most important protocols in it: the Transmission Control Protocol (TCP) and the Internet Protocol (IP), which were the first two networking protocols defined in this standard. IP networking represents a synthesis of several developments that began to evolve in the 1960s and 1970s, namely the Internet and LANs (Local Area Networks), which emerged in the mid- to late-1980s, together with the advent of the World Wide Web in early 1990s.The Internet Protocol Suite, like many protocol suites, may be viewed as a set of layers. Each layer solves a set of problems involving the transmission of data, and provides a well-defined service to the upper layer protocols based on using services from some lower layers. Upper layers are logically closer to the user and deal with more abstract data, relying on lower layer protocols to translate data into forms that can eventually be physically transmitted. The main differences between the two models are as follows:

10. 1.OSI is a reference model and TCP/IP is an implementation of OSI model. 2.TCP/IP Protocols are considered to be standards around which the internet has developed. The OSI model however is a "generic, protocol-independent standard." 3. TCP/IP combines the presentation and session layer issues into its application layer. 4. TCP/IP combines the OSI data link and physical layers into the network access layer. 5. TCP/IP appears to be a simpler model and this is mainly due to the fact that it has fewer layers. 6 .TCP/IP is considered to be a more credible model- This is mainly due to the fact because TCP/IP protocols are the standards around which the internet was developed therefore it mainly gains creditability due to this reason. Where as in contrast networks are not usually built around the OSI model as it is merely used as a guidance tool. 7. The OSI model consists of 7 architectural layers whereas the TCP/IP only has 4 layers. 8. In the TCP/IP model of the Internet, protocols are deliberately not as rigidly designed into strict layers as the OSI model.[6] RFC 3439 contains a section entitled "Layering considered harmful." However, TCP/IP does recognize four broad layers of functionality which are derived from the operating scope of their contained protocols, namely the scope of the software application, the end-to-end transport connection, the internetworking range, and lastly the scope of the direct links to other nodes on the local network. 9. The presumably strict consumer/producer layering of OSI as it is usually described does not present contradictions in TCP/IP, as it is permissible that protocol usage does not follow the hierarchy implied in a layered model. Such examples exist in some routing protocols (e.g., OSPF), or in the description of tunneling protocols, which provide a Link Layer for an application, although the tunnel host protocol may well be a Transport or even an Application Layer protocol in its own right. 10. The TCP/IP design generally favors decisions based on simplicity, efficiency and ease of implementation

11. 6.What do you understand by Switching? Explain the three types of switching methods in detail?
Ans:-In large networks there might be multiple paths linking sender and receiver. Information may be switched as it travels through various communication channels. There are three typical switching techniques available for digital traffic.
• Circuit Switching
• Message Switching
• Packet Switching
Circuit Switching
A circuit-switched network is made of a set of switches connected by physical links, in which each link is divided into n channels.
• Circuit switching is a technique that directly connects the sender and the receiver in an unbroken path.
• Telephone switching equipment, for example, establishes a path that connects the caller's telephone to the receiver's telephone by making a physical connection.
• With this type of switching technique, once a connection is established, a dedicated path exists between both ends until the connection is terminated.
• Routing decisions must be made when the circuit is first established, but there are no decisions made after that time.
• Circuit switching in a network operates almost the same way as the telephone system works.
• A complete end-to-end path must exist before communication can take place.
• The computer initiating the data transfer must ask for a connection to the destination.
• Once the connection has been initiated and completed to the destination device, the destination device must acknowledge that it is ready and willing to carry on a transfer.
Advantages:
The communication channel (once established) is dedicated.
Disadvantages:
1.Possible long wait to establish a connection, (10 seconds, more on long- distance or international calls.) during which no data can be transmitted.
2. More expensive than any other switching techniques, because a dedicated path is required for each connection.
3.Inefficient use of the communication channel, because the channel is not used when the connected systems are not using it.
Message Switching
• With message switching there is no need to establish a dedicated path between two stations.
• When a station sends a message, the destination address is appended to the message.
• The message is then transmitted through the network, in its entirety, from node to node.
• Each node receives the entire message, stores it in its entirety on disk, and then transmits the message to the next node.
• This type of network is called a store-and-forward network.

12. A message-switching node is typically a general-purpose computer. The device needs sufficient secondary-storage capacity to store the incoming messages, which could be long. A time delay is introduced using this type of scheme due to store- and-forward time, plus the time required to find the next node in the transmission path.
Advantages:
• Channel efficiency can be greater compared to circuit-switched systems, because more devices are sharing the channel.
• Traffic congestion can be reduced, because messages may be temporarily stored in route.
• Message priorities can be established due to store-and-forward technique.
• Message broadcasting can be achieved with the use of broadcast address appended in the message.
Disadvantages
• Message switching is not compatible with interactive applications.
• Store-and-forward devices are expensive, because they must have large disks to hold potentially long messages.
Packet Switching
• Packet switching can be seen as a solution that tries to combine the advantages of message and circuit switching and to minimize the disadvantages of both.
• There are two methods of packet switching: Datagram and virtual circuit.
• In both packet switching methods, a message is broken into small parts, called packets.
• Each packet is tagged with appropriate source and destination addresses.
• Since packets have a strictly defined maximum length, they can be stored in main memory instead of disk, therefore access delay and cost are minimized.
• Also the transmission speeds, between nodes, are optimized.
• With current technology, packets are generally accepted onto the network on a first come, first-served basis. If the network becomes overloaded, packets are delayed or discarded (``dropped'').
• In packet switching, the analog signal from your phone is converted into a digital data stream. That series of digital bits is then divided into relatively tiny clusters of bits, called packets. Each packet has at its beginning the digital address -- a long number -- to which it is being sent. The system blasts out all those tiny packets, as fast as it can, and they travel across the nation's digital backbone systems to their destination: the telephone, or rather the telephone system, of the person you're calling.
• They do not necessarily travel together; they do not travel sequentially. They don't even all travel via the same route. But eventually they arrive at the right point -- that digital address added to the front of each string of digital data -- and at their destination are reassembled into the correct order, then converted to analog form, so your friend can understand what you're saying.

13. Advantages:
• Packet switching is cost effective, because switching devices do not need massive amount of secondary storage.
• Packet switching offers improved delay characteristics because there are no long messages in the queue (maximum packet size is fixed).
• Packet can be rerouted if there is any problem, such as, busy or disabled links.
• The advantage of packet switching is that many network users can share the same channel at the same time. Packet switching can maximize link efficiency by making optimal use of link bandwidth.
Disadvantages:
• Protocols for packet switching are typically more complex.
• It can add some initial costs in implementation.
• If packet is lost, sender needs to retransmit the data.
• Another disadvantage is that packet-switched systems still can’t deliver the same quality as dedicated circuits in applications requiring very little delay - like voice conversations or moving images.

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