WEB TECHNOLOGIES ACHYUT GODBOLE PDF
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Web Technology: BSc Information Technology - Semester 2. Share your thoughts with other customers. Write a product review. Top Reviews Most recent Top Reviews. There was a problem filtering reviews right now. Please try again later. Paperback Verified Purchase.
Very good book content wise and the delivery from Amazon was also very good. One person found this helpful. Nice book for beginners. Kindle Edition Verified Purchase. Found this book pretty well-written and the contents are easy to understand. Still this book is worth buying See all 6 reviews. Back to top. After the presentation layer receives and processes this data, it adds its own header to the original data and sends it to the next layer in the hierarchy i.
Let us call this together as L6 data. When the fifth session layer sends this data to the next, i. In the end, the original data L7 and all the headers are sent across the physical medium.
To do this, the source and the destination nodes have to agree on a number of factors such as voltage which constitutes a bit value 0, voltage which constitutes bit value 1, what is the bit interval i. It also deals with the electrical and mechanical specifications of the cables, connectors, and interfaces such as RS C, etc.
Signal encoding How are the bits 0 and 1 to be represented? Medium What is the medium used, and what are its properties? Bit synchronization Is the transmission asynchronous or synchronous? Transmission type Is the transmission serial or parallel? Transmission mode Is the transmission simplex, half-duplex, or full-duplex? Topology What is the topology mesh, star, ring, bus or hybrid used?
Interface How are the two closely linked devices connected? Bandwidth Which of baseband or broadband communication is used? Signal type Are analog signals used, or digital ones? The group of bits is generally called as frame. The network layer passes a data unit to the data link layer. At this stage, the data link layer adds the header and trailer information to this, as shown in Fig. This now becomes a data unit to be passed to the physical layer.
The header and trailer, which is not shown, but is instead assumed to be present contains the addresses and other control information. The addresses at this level refer to the physical addresses of the adjacent nodes in the network, between which the frame is being sent.
Thus, these addresses change as the frame travels from. The addresses of the end nodes, i. Therefore, it is not a part of the header and trailer added and deleted at the data link layer. Hence, they remain unchanged as the frame moves through different nodes from the source to the destination.
Let us refer to Fig. Let us imagine that node A wants to send a packet to node D. Let us imagine that we use the datagram approach. In this case, the logical i. The data unit passed by the network layer to the data link layer will contain them. The data unit will look as it is shown in Fig. Let us call this as DN. When this data unit DN is passed from the network layer at node A to the data link layer at node A, the following happens.
Let us imagine that the next node is F, based on the congestion conditions at that time, i. Here, the packet is passed on from the data link layer to the network layer of node F after performing the error-control function i. Now, this DN needs to. For this, the final destination address, i. The frame now has to be sent from node F to node D. Let us say that the chosen path is FG. Here, the physical addresses of F and G are added to form the data unit at the data link layer at node F, as shown in Fig.
There again, the physical addresses are removed to get the original DN, which is passed on to the network layer at node D. The data link layer also performs the flow control function. If it can be sent, the node is ready to send the data. If the connection is a multipoint type i.
Therefore, in LANs, the data ink layer is split into two sub-layers, as shown in Fig. In this case, LLC takes care of normal data link layer functions, such as error control and flow control, etc.
Thus, the data link layer performs the following functions. Addressing Headers and trailers are added, containing the physical addresses of the adjacent nodes, and removed upon a successful delivery. Flow control This avoids overwriting the receivers buffer by regulating the amount of data that can be sent.
Synchronization Headers have bits, which tell the receiver when a frame is arriving. It also contains bits to synchronize its timing to know the bit interval to recognize the bit correctly.
Trailers mark the end of a frame, apart from containing the error control bits. Error control It checks the CRC to ensure the correctness of the frame. If incorrect, it asks for retransmission. Again, here there are multiple schemes positive acknowledgement, negative acknowledgement, go-back-n, sliding window, etc. Node-to-node delivery Finally, it is responsible for error-free delivery of the entire frame to the next adjacent node node-to-node delivery.
The packet at network layer is usually referred to as a datagram. This layer ensures the successful delivery of a packet to the destination node. To perform this, it has to choose a route. As discussed before, a route could be chosen before sending all the packets belonging to the same message virtual circuit or it could be chosen for each packet at each node datagram.
This layer is also responsible for tackling the congestion problem at a node, when there are too many packets stored at a node to be forwarded to the next node. Whenever there is only one small network based on broadcast philosophy e. There are many private or public subnet operators who provide the hardware links and the software consisting of physical, data link and network layers e.
They guarantee an error-free delivery of a packet to the destination at a charge. This layer has to carry out the accounting function to facilitate this billing based on how many packets are routed, when and, etc. When packets are sent across national boundaries, the rates may change, thus making this accounting function complex. A router can connect two networks with different protocols, packet lengths and formats. The network layer is responsible for the creation of a homogeneous network by helping to overcome these problems.
At this layer, a header is added to a packet, which includes the source and destination addresses logical addresses.
These are not the same as the physical addresses between each pair of adjacent nodes at the data link layer, as seen before. If we refer to Fig. These addresses and, in fact, the whole of DN remains unchanged throughout the journey of the packet from A to F to G to D. Only physical addresses of the adjacent nodes keep getting added and removed, as the packet travels from A to F to G to D. Finally, at node D, after verifying the addresses,.
Routing As discussed before. Congestion control As discussed before. Logical addressing Source and destination logical addresses e. Address transformations Interpreting logical addresses to get their physical equivalent e. We shall discuss this in detail later in the book. Accounting and billing As discussed before.
Source to Destination error-free delivery of a packet. Therefore, a header at the transport layer contains information that helps to send the message to the corresponding layer at the destination node, although the message broken into packets may travel through a number of intermediate nodes.
As we know, each end node may be running several processes may be for several users through several terminals. The transport layer ensures that the complete message arrives at the destination, and in the proper order and is passed on to the proper application. The transport layer takes care of error control and flow control, both at the source and at the destination for the entire message, rather than only for a packet. Incidentally, a packet is either termed as a segment or as a datagram at the transport layer.
As we know, these days, a computer can run many applications at the same time. All these applications could need communication with the same or different remote computers at the same time.
For example, suppose we have two computers A and B. Let us say A hosts a file server, in which B is interested. Similarly, suppose another messaging application on A wants to send a message to B. Since the two different applications want to communicate with their counterparts on remote computers at the same time, it is very essential that a. This is the job of the transport layer.
It enables communication between two applications residing on different computers. The transport layer receives data from the session layer on the source computer, which needs to be sent across to the other computer.
For this, the transport layer on the source computer breaks the data into smaller packets and gives them to the lower layer network layer , from which it goes to still lower layers and finally gets transmitted to the destination computer. If the original data is to be re-created at the session layer of the destination computer, we would need some mechanism for identifying the sequence in which the data was fragmented into packets by the transport layer at the source computer.
For this purpose, when it breaks the session layer data into segments, the transport layer of the source computer adds sequence numbers to the segments. Now, the transport layer at the destination can reassemble them to create the original data and present it to the session layer.
A connection is a logical path that is associated with all the packets of a message, between the source and the destination. A connection consists of three phases which are, establishment, data transfer and connection release. By using connections, the transport layer can perform the sequencing, error detection and correction in a better way. To summarize, the responsibilities of the transport layer are as follows.
Host-to-host message delivery Ensuring that all the segments of a message sent by a source node arrive at the intended destination. Application-to-application communication The transport layer enables communication between two applications running on different computers.
Segmentation and reassembly The transport layer breaks a message into segments, numbers them by adding sequence numbers at the source, and uses the sequence numbers at the destination to reassemble the original message.
Connection The transport layer might create a logical connection between the source and the destination for the duration of the complete message transfer for better control over the message transfer. It makes sure that a session once established is closed gracefully, and not abruptly. For example, suppose that a user wants to send a very big document consisting of pages to another user on a different computer.
Suppose that after the first pages have been sent, the connection between the two hosts is broken for some reason. The question now is, when the connection between the two hosts is restored after some time, must the transmission start all over again, i. Or can the user start with the th page? These issues are the concerns of the session layer. The session layer checks and establishes connections between the hosts of two different users.
For this, the users might need to enter identification information such as login and password. Besides this, the session layer also decides things such as whether both users can send as well as receive data at the same time, or whether only one host can send and the other can receive, and so on i. Let us reiterate our earlier example of the transmission of a very big document between two hosts.
To avoid a complete retransmission from the first page, the session layer between the two hosts could create subsessions. After each sub-session is over, a checkpoint can be taken.
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For instance, the session layers at the two hosts could decide that after a successful transmission of a set of every 10 pages, they would take a checkpoint.
This means that if the connection breaks after the first pages have been transmitted, after the connection is restored, the transmission would start at the st page. This is because the last checkpoint would have been taken after the th page was transmitted. The session layer is shown in Fig. Regardless of whether it is required or not, when the session layer receives data from the presentation layer, it adds a header to it, which among other things also contains information as to whether there is any checkpointing, and if there is, at what point.
Sessions and sub-sessions The session layer divides a session into sub-sessions for avoiding retransmission of entire messages by adding the checkpointing feature. Synchronization The session layer decides the order in which data needs to be passed to the transport layer. Session closure The session layer ensures that the session between the hosts is closed gracefully. The presentation layer is responsible for taking care of such differences.
It is also responsible for a data encryption and decryption for security and b data compression and decompression for more efficiency in data transmission. Translation The translation between the senders and the receivers message formats is done by the presentation layer if the two formats are different.
Encryption The presentation layer performs data encryption and decryption for security. Compression For efficient transmission, the presentation layer performs data compression before sending and decompression at the destination. The application programs using the network services also reside at this layer. The users and application programs interact with a physical network at this layer.
This should not be confused with the application system like accounting or purchasing, etc. If an accounting application requires an access to a remote database, or wants a file to be transferred, it will invoke the appropriate application layer protocol e.
Thus, the application layer provides an abstracted view of the layers underneath, and allows the users and applications to concentrate on their tasks, rather than worrying about lower level network protocols.
The conceptual position of the application layer is shown in Fig. Network abstraction The application layer provides an abstraction of the underlying network to an end user and an application. Mail services It allows the users to use the mail services. Remote login It allows logging in a host, which is remote.
Protocol means convention. When computers need to communicate with each other either to exchange information or for sharing common resources, they use a common protocol. There are a number of requirements for data communication, such as data transmission, flow control, error control, routing, data compression, encryption, etc. These features are logically sub-grouped and then the sub-groups are further grouped into groups called as layers.
The model of communication protocols defines seven such layers, i. Each layer has an interface with its adjacent layers, and performs specific functions. The physical layer is concerned with sending raw bits between the adjacent nodes, across the communication medium. The data link layer is responsible for transmitting a group of bits between the adjacent nodes. The network layer is responsible for routing a packet within the subnet, i.
The transport layer is responsible for host-to-host message delivery, application-to-application communication, segmentation and reassembly, and logical connection management between the source and the destination.
The main functions of the session layer are to establish, maintain and synchronize the interaction between two communicating hosts. When two hosts are communicating with each other, they might be using different encoding standards and character sets for representing data internally. The presentation layer is responsible to take care of such differences. The application layer, the topmost layer in the OSI model, enables a user to access the network.
NAK is a acknowledgement. The speed mismatch between the sender and the receiver is called as. In order that a bigger transmission does not overhaul a smaller one, the data is sent in the form of. The layer is the lowest layer in the OSI model. The layer is the topmost layer in the OSI model. The intermediate nodes are concerned with the layers only. The layer is responsible for node to node delivery of packets. The layer is responsible for routing packets within or across networks.
The layer ensures a correct delivery of a complete message. Encryption is handled by the layer. Detailed Questions Explain the term protocol in general. Explain the different layers and their roles in protocols of computer communications. Explain the different layers in the OSI model. Explain the physical layer in OSI model. How does the data link layer in OSI model work? Discuss the role of network layer in OSI model. How does the transport layer ensure that the complete message arrives at the destination, and in the proper order?
Explain how a session layer establishes, maintains and synchronizes the interaction between two communicating hosts. Explain the role played by the presentation layer in handling different data formats. Explain the topmost layer in the OSI model, the application layer. Exercises 1. How similar or different are they from the OSI model?
Study the background and need for the OSI model. Investigate which of the OSI layers are considered to be very useful and which ones are not quite in use. Consider an analogy wherein a person who knows only French wants to send a fax message to a person who knows only Urdu. Describe this process with reference to the appropriate OSI model layers. Investigate the reasons behind this. In the previous chapter, we have studied the basic principles of protocols. Let us now study another extremely important concept of connecting many such computer networks together.
This is called as internetworking. A network of computer networks is called as an internetwork or simply, internet note the lowercase i. The worldwide Internet note the uppercase I is an example of the internetworking technology.
The Internet, as we have seen, is a huge network of computer networks. The following sections describe the motivations behind such a technology, as well as how it actually works. When two or more devices have to be connected for sharing data or resources or exchanging messages, we call it as networking.
When two networks need to be connected for the same purpose, we call it internetworking.
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The main difference between networking and internetworking is that whereas in case of networking all the devices are compatible with each other e. When we want to connect two or more networks to form an internetwork, it is quite possible that the networks are incompatible with each other in many respects.
All the three types of networks are quite different from each other. They differ in terms of their topologies, signaling, transmission mechanism, as well as wiring, etc. Therefore, the challenge in internetworking is more in terms of handling these incompatibilities and bringing all the incompatible networks to a common platform.
In this chapter, we shall discuss various connecting devices that are required to facilitate networking and internetworking. These devices form the backbones of any network or internetwork abbreviated as internet, which is different from the worldwide network of networks, i. The Internet has been acknowledged as one of the greatest things to happen during the 20th century. In fact, people talk about the Internet in the same way as the revolutionary inventions such as electricity and the printing press, among others.
The Internet is here to stay even if the dotcoms have perished. In this chapter, we shall look at the fundamentals of the Internet technology. More specifically, we shall study how the Internet is organized and how it works. We shall also take a look at the historical perspective of the Internet. We shall first study the basic concepts behind the Internet. We shall then see how the different components of the Internet work. The Internet is basically the worlds largest network of computer networks.
Many different. We shall discuss those in detail. We shall see how it works. The main reason for having an internet is that each computer network is designed with a specific task in mind. For example, a LAN is typically used to connect computers in a smaller area such as an office and it provides fast communication between these computers. On the other hand, WAN technologies are used for communication over longer distances.
As a result, networks become specialized entities. Moreover, a large organization having diversifying needs has multiple networks. In many cases, these networks do not use the same technology in terms of the hardware as well as communication protocols. Consequently, a computer can only communicate with other computers attached to the same network.
As more and more organizations had multiple computer networks in the s, this became a major issue. Computer networks became small islands! In many cases, an employee had to physically move for using computers connected to different networks. For example, to print a document, the employee would need to use a computer that is connected to a print server.
Similarly, for accessing a file on another network, the employee had to use a computer on that network, and so on. Clearly, this was a nuisance. This affected productivity, as people did not like to move around for performing trivial tasks. As a result, the concept of universal service came into being. In simple terms, it means that there was no dependence on the underlying physical technology, or on the fact that there were many separate physical networks. Like a telephone network, people wanted a single computer network in their organization.
A user should be able to print a document or send a message to any other user from his computer, without needing to use a separate computer on another network for each such task. For this to be possible, all computer networks should be connected together. This means that there should be a network of physically separate networks.
This forms the basis of internetworking. However, one must also remember that organizations invest so much when they build computer networks in terms of cost as well as infrastructure cabling, providing space in the building for it, etc. Therefore, they would want to reuse their existing infrastructure rather than creating everything from scratch. However, there are problems in this.
Electrical as well as software incompatibility makes it impossible to form a network merely by interconnecting wires from two networks. Similarly, one network could use a packet size of say bytes, whereas another could use byte packets. There could be many more such differences like routing algorithms, etc. Thus, any two networks cannot directly communicate with each other by just connecting a wire between them.
Since there are many incompatible networking technologies, the problem becomes more acute. An organization could have many networks of different types. Therefore, the concept of universal service through internetworking is not simple to achieve, although it is highly desirable. The incompatibility issues are addressed in two respects. This hardware component is most commonly a router.
A router is a special-purpose computer that is used specifically for internetworking purposes. A router has a processor CPU and memory like any other computer. From a networks point of view, connecting to a router is not extraordinary in any way. A network connects to a router in the same way as it connects to any other computer.
A router connects two or more computer networks, as shown in Fig.
A network has many computers or nodes attached to it. If a router is treated as yet another computer by the network, it means that the router basically has two addressesone for each network, at points X and Y, as shown in the figure. These two NICs correspond to the two physical addresses of the router. The most important point in this discussion is that a router can connect incompatible networks. A router has the capability to connect them together. How is this possible?
For this, a router has the necessary hardware NIC for each type of network as well as software protocols that make it possible. The router handles all these incompatibilities as well. Again, this is possible because of the hardware and software contained by a router. The point is that A and B in the figure could be arbitrary networks. However, the router would still be able to interconnect them.
Interestingly, the Internet note the uppercase I looks as shown in Fig. A router connects two networks through two NICs that are contained by each such router. If computer X on network A wants to send a message to computer Y on network D, the message can be sent in different routes or paths given below.
The router is responsible for routing the packets to the destination. To do this, the software computes the routing algorithm, and based on this, each router stores the routing table, which states for each destination, the next hop, to which the packet is to be sent. It is for this reason that the router is supposed to act at the network layer of the OSI model. It neither examines the contents of the packet, nor tries to interpret them.
Figure 2. At the software level, routers must agree about the way in which information from the source computer on one network would be transmitted to destination computer on a different network. Since this information is likely to travel via one or more routers, there must be a pre-specified standard to which all routers must conform.
This task is not easy. Packet formats and addressing mechanisms used by the underlying networks may not be the same. Does the router actually perform the conversion and re-conversion of the packets corresponding to the different network formats? Though not impossible, this approach is very difficult and cumbersome.
This is done by defining a standard packet format in which the sender breaks down the original message. We will study this later. Therefore, some networking protocols are required that can standardize communication between incompatible networks. Only then, the concept of universal service can be truly realized. Each packet has the source and destination addresses of X and Y. Ethernet frame can be carried only on the Ethernet network in this case, Net A.
After this, the CRC is computed and appended to the Ethernet frame. It contains the final source and destination addresses of X and Y. Thus, the packet reaches R10, etc. The destination address is verified and the packet is stored. The internal details of many real, actual networks connecting together to form it are hidden, and instead, it appears to be a single, large network.
Every computer on the Internet has an address assigned to it. This is like the postal address assigned to a home. Using this address, any user can send packets to any other computer on the Internet. The users of the Internet do not have to be bothered about the internal structure of the physical networks, their interconnection, routing decisions, or the presence of routers themselves.
Thus, an illusion of a virtual network is created. This is an abstracted view presented to a common user, who is not interested in knowing the internal organization of the communication system.
For example, a telephone user simply wants to dial someones number and talk with that person instead of knowing how the signaling system works or how many telephone exchanges exist in the system and how they function. Similarly, an Internet user is merely interested in communicating with another user of the Internet, using the computer address of the other user, or he is interested in using the services on that computer.
The concept of a virtual network is very important. It ensures that different computer networks can not only be connected together, but also be looked upon and used as a single network. This forms the basis of the biggest network of networks, the Internet. This concept is illustrated in Fig. The figure shows the illusion of a single, large virtual network corresponding to the real network shown in Fig.
Each of them has another level of classification, as shown in Fig. We have discussed routers in brief in the previous chapters. Note that in each of the last three cases, the device is present in the layer mentioned in the table, as well as one level below it.
That is, a bridge is present in the data link layer as well as the physical layer. A repeater is already at the lowest OSI layer i. A repeater, also called as a regenerator, is an electronic device, which simply regenerates a signal. It works at the physical layer of the OSI protocol, as shown in Fig.
This means that the integrity of the data, carried by the signal, is in danger. A repeater receives such a signal, which is likely to become weak or corrupted, and regenerates it.
For instance, let us assume that a computer works on a convention that 5 volts represent 1, and 0 volts represent 0. That is, the repeater simply recreates the bit pattern of the signal, and puts this regenerated signal back on to the transmission medium.
In effect, the original signal is created once again. Note that a repeater does not anyway change the data that is being transmitted, or the characteristics of a network. The only responsibility of a repeater is to take a stream of bits, in the form of a signal, regenerate it so that the signal is accurate now, and send it forward.
It does not perform any intelligent function. By the time the signal sent by host A can reach host D, it becomes very weak. Therefore, host D may not be able to get it in the form of the original signal. Instead, the bits could change to say before the signal reaches host D. Of course, at a higher level, the error control functions would detect and correct such an anomaly. However, even before this can happen, at the lowest level, the repeater simply prevents it from occurring by taking the input signal corresponding to bits sent by host A, simply regenerating it to create a signal with the same bit format and the original signal strength, and sending it forward.
However, they are different. An amplifier is used for analog signals. In analog signals, it is impossible to separate the original signal and the noise.
An amplifier, therefore, amplifies an original signal as well as the noise in the signal, as it cannot differentiate between the two. On the other hand, a repeater knows that the signal has to be identified as either 0 or 1 only. Therefore, it does not amplify the incoming signalit regenerates it in the original bit pattern. Since a signal must reach a repeater before it becomes too weak to be unidentifiable, the placement of repeaters is an important concern.
A signal must reach a repeater before too much noise is introduced in the signal. Otherwise, the noise can change the bits in the signal i. After corruption, if a repeater regenerates it, incorrect data would be forwarded by the repeater.
A bridge does not run application programs, and instead, facilitates host-to-host communication within a network. It operates at the physical as well as data link layers of the OSI protocol hierarchy.
This is shown in Fig. The main idea of using a bridge is to divide a big network into smaller sub-networks, called as segments. Here, the bridge splits the entire network into two segments, shown with dotted lines.
Due to the bridge, the two segments act as a part of the single network. So adhering to these principles will consummate our team efforts to the promising start of the student's academics. The assignment framing policy need to address the average students and inclusive of an element to attract and promote the intelligent students. The instructor may set multiple sets of assignments and distribute among batches of students. Encourage students for appropriate use of Hungarian notation, proper indentation and comments.
Use of open source software is to be encouraged. In addition to these, instructor may assign one real life application in the form of a mini-project based on the concepts learned. Instructor may also set one assignment or mini-project that is suitable to respective branch beyond the scope of syllabus. Suggested List of Laboratory Assignments 1.We study static Web pages here. Explain the different layers in the OSI model. Which contains photos and videos of celebrities, big booty girls cute teen?
The latter is called time out. Guidelines for Student Journal The laboratory assignments are to be submitted by student in the form of journal.
A user should be able to print a document or send a message to any other user from his computer, without needing to use a separate computer on another network for each such task.
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