CSE 101 LECTURE 4

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Slide-3
Simultaneous Access
• In organizations, many people may need to use the same data or programs. A network solves this problem.
• Shared data and programs can be stored on a central network server. A server that stores data files may be called a file server.
• Managers may assign access rights to users. Some users may only be able to read data, others may be able to make changes to existing files.
Shared Peripheral Devices
• Because peripheral (external) devices like printers can be expensive, it is cost-effective to connect a device to a network so users can share it.
• Through a process called spooling, users can send multiple documents (called print jobs) to a networked printer at the same time. The documents are temporarily stored on the server and printed in turn.
Personal Communication
• One of the most common uses of networks is for electronic mail (e-mail).
• An e-mail system enables users to exchange written messages (often with data files attached) across the local network or over the Internet.
• Two other popular network-based communications systems are teleconferencing and videoconferencing.
Easier Backup
• Networks enable managers to easily back up (make backup copies of) important data.
• Administrators commonly back up shared data files stored on the server, but may also use the network to back up files on users' PCs.
Slide-4
Server based networking
Server based networking provides a central location for management, backups, updates, anti-virus management, anti-spam management, and anti-spyware management. Server based networking can also provide you with monitoring of employee activities suchas internet usage, web pages viewed and instant messenger use.
• A network in which all client computers use a dedicated central server computer for network functions such as storage, security and other resources.
• A server has a large hard disk for shared storage. It may provide other services to the nodes, as well.
In a file server network, nodes can access files on the server, but not necessarily on other nodes.
mainframe
Mainframe computers are typically large, metal boxed computers with large processing abilities. The terminals are called "dumb terminals" because they only send and receive data, leaving the processing to the mainframe
Client / Server Network
The client/server refers to the way two computer programs interact with each other.
The client makes a request from the server, who then fulfills the request. Although this idea can be used on one computer it is an efficient way for a network of computers in different locations to interconnect.
Advantages
In most cases, a client-server architecture enables the roles and responsibilities of a computing system to be distributed among several independent computers that are known to each other only through a network. This creates an additional advantage to this architecture: greater ease of maintenance. For example, it is possible to replace, repair, upgrade, or even relocate a server while its clients remain both unaware and unaffected by that change. This independence from change is also referred to as encapsulation.
All the data is stored on the servers, which generally have far greater security controls than most clients. Servers can better control access and resources, to guarantee that only those clients with the appropriate permissions may access and change data.
Since data storage is centralized, updates to that data are far easier to administer than what would be possible under a P2P paradigm. Under a P2P architecture, data updates may need to be distributed and applied to each "peer" in the network, which is both time-consuming and error-prone, as there can be thousands or even millions of peers.
Many mature client-server technologies are already available which were designed to ensure security, 'friendliness' of the user interface, and ease of use.
It functions with multiple different clients of different capabilities.
Disadvantages
Traffic congestion on the network has been an issue since the inception of the client-server paradigm. As the number of simultaneous client requests to a given server increases, the server can become severely overloaded. Contrast that to a P2P network, where its bandwidth actually increases as more nodes are added, since the P2P network's overall bandwidth can be roughly computed as the sum of the bandwidths of every node in that network.
The client-server paradigm lacks the robustness of a good P2P network. Under client-server, should a critical server fail, clients’ requests cannot be fulfilled. In P2P networks, resources are usually distributed among many nodes. Even if one or more nodes depart and abandon a downloading file, for example, the remaining nodes should still have the data needed to complete the download.
Specific types of clients include web browsers, email clients, and online chat clients.
Specific types of servers include web servers, ftp servers, application servers, database servers, mail servers, file servers, print servers, and terminal servers. Most web services are also types of servers.
LANs
• LAN stands for Local Area Network.
• The first LANs were created in the late 1970s.
• LANs are small networks constricted to a small area like a house, office, or city.
• A LAN is a network whose computers are located relatively near one another.
• The nodes may be connected by a cable, infrared link, or small transmitters.
• A network transmits data among computers by breaking it into small pieces, called packets.
• Every LAN uses a protocol – a set of rules that governs how packets are configured and transmitted.
• LANs are used to share resources like storage, internet,etc.
• A 'node' on a LAN is a connected computer or device like a printer.
WANs
• WAN stands for Wide Area Network.
• WANs are very large networks that interconnect smaller LAN networks, for a large geographic area like a country(i.e., any network whose communications links cross metropolitan, regional, or national boundaries.) [9]
• WANs are usually for private companies, however, some built by internet service providers connect LANs to the internet.
• WAN can use a combination of satellites, microwave, and link and variety of computers from mainframes to terminals.
• A 'node' on a WAN is a LAN.
Personal Area Network (PAN)
A personal area network (PAN) is a computer network used for communication among computer devices.

PAN Usually, uses short range wireless technology is used to connect devises like a cell phone and a PDA.
Home Area Network (HAN)
HAN is short for Home Area Network. HAN is a recently coined term for a small LAN in a home environment, used for lifestyle purposes.
Home Area Network uses cable, wired, or wireless connections to connect a homes' digital devices. For example, fax machines, computers, DVD's etc.
Garden Area Network (GAN)
A similar system such as HAN would be GAN which stands for Garden Area Network, and allows for one system to control such devices like garden lights, sprinkler systems and alarm systems
Slide-8
In peer-to-peer networking there are no dedicated servers or hierarchy among the computers. All of the computers are equal and therefore known as peers. Normally each computer serves as Client/Server and there is no one assigned to be an administrator responsible for the entire network.

Peer-to-peer networks are good choices for needs of small organizations where the users are allocated in the same general area, security is not an issue and the organization and the network will have limited growth within the foreseeable future.

The term Client/server refers to the concept of sharing the work involved in processing data between the client computer and the most powerful server computer.
Slide-9
Network Interface Cards (NIC)
A network card, network adapter or NIC (network interface card) is a piece of computer hardware designed to allow computers to communicate over a computer network. It provides physical access to a networking medium and often provides a low-level addressing system through the use of MAC addresses. It allows users to connect to each other either by using cables or wirelessly.
Repeaters
A repeater is an electronic device that receives a signal and retransmits it at a higher power level, or to the other side of an obstruction, so that the signal can cover longer distances without degradation. In most twisted pair ethernet configurations, repeaters are required for cable runs longer than 100 meters away from the computer.
Hubs
A hub contains multiple ports. When a packet arrives at one port, it is copied to all the ports of the hub for transmission. When the packets are copied, the destination address in the frame does not change to a broadcast address. It does this in a rudimentary way, it simply copies the data to all of the Nodes connected to the hub.[3]
Bridges
A network bridge connects multiple network segments at the data link layer (layer 2) of the OSI model. Bridges do not promiscuously copy traffic to all ports, as hubs do, but learn which MAC addresses are reachable through specific ports. Once the bridge associates a port and an address, it will send traffic for that address only to that port. Bridges do send broadcasts to all ports except the one on which the broadcast was received.
Bridges learn the association of ports and addresses by examining the source address of frames that it sees on various ports. Once a frame arrives through a port, its source address is stored and the bridge assumes that MAC address is associated with that port. The first time that a previously unknown destination address is seen, the bridge will forward the frame to all ports other than the one on which the frame arrived.
Bridges come in three basic types:
Local bridges: Directly connect local area networks (LANs)
Remote bridges: Can be used to create a wide area network (WAN) link between LANs. Remote bridges, where the connecting link is slower than the end networks, largely have been replaced by routers.
Wireless bridges: Can be used to join LANs or connect remote stations to LANs.
Switches
A switch is a device that performs switching. Specifically, it forwards and filters OSI layer 2 datagrams (chunk of data communication) between ports (connected cables) based on the MAC addresses in the packets. This is distinct from a hub in that it only forwards the datagrams to the ports involved in the communications rather than all ports connected. Strictly speaking, a switch is not capable of routing traffic based on IP address (layer 3) which is necessary for communicating between network segments or within a large or complex LAN. Some switches are capable of routing based on IP addresses but are still called switches as a marketing term. A switch normally has numerous ports, with the intention being that most or all of the network is connected directly to the switch, or another switch that is in turn connected to a switch.
Switch is a marketing term that encompasses routers and bridges, as well as devices that may distribute traffic on load or by application content (e.g., a Web URL identifier). Switches may operate at one or more OSI model layers, including physical, data link, network, or transport (i.e., end-to-end). A device that operates simultaneously at more than one of these layers is called a multilayer switch.
Overemphasizing the ill-defined term "switch" often leads to confusion when first trying to understand networking. Many experienced network designers and operators recommend starting with the logic of devices dealing with only one protocol level, not all of which are covered by OSI. Multilayer device selection is an advanced topic that may lead to selecting particular implementations, but multilayer switching is simply not a real-world design concept.
Routers
Routers are networking devices that forward data packets between networks using headers and forwarding tables to determine the best path to forward the packets. Routers work at the network layer of the TCP/IP model or layer 3 of the OSI model. Routers also provide interconnectivity between like and unlike media (RFC 1812). This is accomplished by examining the Header of a data packet, and making a decision on the next hop to which it should be sent (RFC 1812) They use preconfigured static routes, status of their hardware interfaces, and routing protocols to select the best route between any two subnets. A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP's network. Some DSL and cable modems, for home (and even office) use, have been integrated with routers to allow multiple home/office computers to access the Internet through the same connection. Many of these new devices also consist of wireless access points (waps) or wireless routers to allow for IEEE 802.11b/g wireless enabled devices to connect to the network without the need for cabled connections.
Slide – 12
Intranet
An intranet is a set of networks, using the Internet Protocol and IP-based tools such as web browsers and file transfer applications, that is under the control of a single administrative entity. That administrative entity closes the intranet to all but specific, authorized users. Most commonly, an intranet is the internal network of an organization. A large intranet will typically have at least one web server to provide users with organizational information.
Extranet
An extranet is a network or internetwork that is limited in scope to a single organization or entity but which also has limited connections to the networks of one or more other usually, but not necessarily, trusted organizations or entities (e.g. a company's customers may be given access to some part of its intranet creating in this way an extranet, while at the same time the customers may not be considered 'trusted' from a security standpoint). Technically, an extranet may also be categorized as a CAN, MAN, WAN, or other type of network, although, by definition, an extranet cannot consist of a single LAN; it must have at least one connection with an external network.
INTERNET
• The Internet was created by the Advanced Research Projects Agency (ARPA) and the U.S. Department of Defense for scientific and military communications.
• The Internet is a network of interconnected networks. Even if part of its infrastructure was destroyed, data could flow through the remaining networks.
• The Internet uses high-speed data lines, called backbones, to carry data. Smaller networks connect to the backbone, enabling any user on any network to exchange data with any other user.
The Internet is a worldwide, publicly accessible series of interconnected computer networks that transmit data by packet switching using the standard Internet Protocol. It is a "network of networks" that consists of millions of smaller networks, which together carry various information and services, such as electronic mail, online chat, file transfer, and the interlinked web pages and other resources of the World Wide Web (WWW). So the Internet is a collection of interconnected computer networks, linked by copper wires, fiber-optic cables, wireless connections, etc. In contrast, the Web is a collection of interconnected documents and other resources, linked by hyperlinks and URLs. The World Wide Web is one of the services accessible via the Internet, along with various others including e-mail, file sharing, online gaming and others.
Common uses
E-mail
The concept of sending electronic text messages between parties in a way analogous to mailing letters or memos predates the creation of the Internet. Even today it can be important to distinguish between Internet and internal e-mail systems. Internet e-mail may travel and be stored unencrypted on many other networks and machines out of both the sender's and the recipient's control. During this time it is quite possible for the content to be read and even tampered with by third parties, if anyone considers it important enough. Purely internal or intranet mail systems, where the information never leaves the corporate or organization's network, are much more secure, although in any organization there will be IT and other personnel whose job may involve monitoring, and occasionally accessing, the e-mail of other employees not addressed to them.
The World Wide Web
Many people use the terms Internet and World Wide Web (or just the Web) interchangeably, but, as discussed above, the two terms are not synonymous.
The World Wide Web is a huge set of interlinked documents, images and other resources, linked by hyperlinks and URLs. These hyperlinks and URLs allow the web servers and other machines that store originals, and cached copies, of these resources to deliver them as required using HTTP (Hypertext Transfer Protocol). HTTP is only one of the communication protocols used on the Internet.
Web services also use HTTP to allow software systems to communicate in order to share and exchange business logic and data.
Software products that can access the resources of the Web are correctly termed user agents. In normal use, web browsers, such as Internet Explorer and Firefox, access web pages and allow users to navigate from one to another via hyperlinks. Web documents may contain almost any combination of computer data including graphics, sounds, text, video, multimedia and interactive content including games, office applications and scientific demonstrations.
Using the Web, it is also easier than ever before for individuals and organisations to publish ideas and information to an extremely large audience. Anyone can find ways to publish a web page, a blog or build a website for very little initial cost. Publishing and maintaining large, professional websites full of attractive, diverse and up-to-date information is still a difficult and expensive proposition, however.
Many individuals and some companies and groups use "web logs" or blogs, which are largely used as easily updatable online diaries. Some commercial organisations encourage staff to fill them with advice on their areas of specialization in the hope that visitors will be impressed by the expert knowledge and free information, and be attracted to the corporation as a result. One example of this practice is Microsoft, whose product developers publish their personal blogs in order to pique the public's interest in their work.

Remote access
The Internet allows computer users to connect to other computers and information stores easily, wherever they may be across the world. They may do this with or without the use of security, authentication and encryption technologies, depending on the requirements.
This is encouraging new ways of working from home, collaboration and information sharing in many industries. An accountant sitting at home can audit the books of a company based in another country, on a server situated in a third country that is remotely maintained by IT specialists in a fourth. These accounts could have been created by home-working bookkeepers, in other remote locations, based on information e-mailed to them from offices all over the world. Some of these things were possible before the widespread use of the Internet, but the cost of private leased lines would have made many of them infeasible in practice.
An office worker away from his desk, perhaps on the other side of the world on a business trip or a holiday, can open a remote desktop session into his normal office PC using a secure Virtual Private Network (VPN) connection via the Internet. This gives the worker complete access to all of his or her normal files and data, including e-mail and other applications, while away from the office.
This concept is also referred to by some network security people as the Virtual Private Nightmare, because it extends the secure perimeter of a corporate network into its employees' homes; this has been the source of some notable security breaches, but also provides security for the workers.
Collaboration
The low cost and nearly instantaneous sharing of ideas, knowledge, and skills has made collaborative work dramatically easier. Not only can a group cheaply communicate and test, but the wide reach of the Internet allows such groups to easily form in the first place, even among niche interests. An example of this is the free software movement in software development, which produced GNU and Linux from scratch and has taken over development of Mozilla and OpenOffice.org (formerly known as Netscape Communicator and StarOffice).
Internet "chat", whether in the form of IRC "chat rooms" or channels, or via instant messaging systems, allow colleagues to stay in touch in a very convenient way when working at their computers during the day. Messages can be sent and viewed even more quickly and conveniently than via e-mail. Extension to these systems may allow files to be exchanged, "whiteboard" drawings to be shared as well as voice and video contact between team members.
Version control systems allow collaborating teams to work on shared sets of documents without either accidentally overwriting each other's work or having members wait until they get "sent" documents to be able to add their thoughts and changes.
File sharing
A computer file can be e-mailed to customers, colleagues and friends as an attachment. It can be uploaded to a website or FTP server for easy download by others. It can be put into a "shared location" or onto a file server for instant use by colleagues. The load of bulk downloads to many users can be eased by the use of "mirror" servers or peer-to-peer networks.
In any of these cases, access to the file may be controlled by user authentication; the transit of the file over the Internet may be obscured by encryption, and money may change hands before or after access to the file is given. The price can be paid by the remote charging of funds from, for example, a credit card whose details are also passed—hopefully fully encrypted—across the Internet. The origin and authenticity of the file received may be checked by digital signatures or by MD5 or other message digests.
These simple features of the Internet, over a worldwide basis, are changing the basis for the production, sale, and distribution of anything that can be reduced to a computer file for transmission. This includes all manner of print publications, software products, news, music, film, video, photography, graphics and the other arts. This in turn has caused seismic shifts in each of the existing industries that previously controlled the production and distribution of these products.
Internet collaboration technology enables business and project teams to share documents, calendars and other information. Such collaboration occurs in a wide variety of areas including scientific research, software development, conference planning, political activism and creative writing.
Streaming media
Many existing radio and television broadcasters provide Internet "feeds" of their live audio and video streams (for example, the BBC). They may also allow time-shift viewing or listening such as Preview, Classic Clips and Listen Again features. These providers have been joined by a range of pure Internet "broadcasters" who never had on-air licenses. This means that an Internet-connected device, such as a computer or something more specific, can be used to access on-line media in much the same way as was previously possible only with a television or radio receiver. The range of material is much wider, from pornography to highly specialized, technical webcasts. Podcasting is a variation on this theme, where—usually audio—material is first downloaded in full and then may be played back on a computer or shifted to a digital audio player to be listened to on the move. These techniques using simple equipment allow anybody, with little censorship or licensing control, to broadcast audio-visual material on a worldwide basis.
Webcams can be seen as an even lower-budget extension of this phenomenon. While some webcams can give full-frame-rate video, the picture is usually either small or updates slowly. Internet users can watch animals around an African waterhole, ships in the Panama Canal, the traffic at a local roundabout or their own premises, live and in real time. Video chat rooms, video conferencing, and remote controllable webcams are also popular. Many uses can be found for personal webcams in and around the home, with and without two-way sound.
YouTube, sometimes described as an Internet phenomenon because of the vast amount of users and how rapidly the site's popularity has grown, was founded on February 15, 2005. It is now the leading website for free streaming video. It uses a flash-based web player which streams video files in the format FLV. Users are able to watch videos without signing up; however, if users do sign up they are able to upload an unlimited amount of videos and they are given their own personal profile. It is currently estimated that there are 64,000,000 videos on YouTube, and it is also currently estimated that 825,000 new videos are uploaded every day.
Voice telephony (VoIP)
VoIP stands for Voice over IP, where IP refers to the Internet Protocol that underlies all Internet communication. This phenomenon began as an optional two-way voice extension to some of the instant messaging systems that took off around the year 2000. In recent years many VoIP systems have become as easy to use and as convenient as a normal telephone. The benefit is that, as the Internet carries the actual voice traffic, VoIP can be free or cost much less than a normal telephone call, especially over long distances and especially for those with always-on Internet connections such as cable or ADSL.
Thus, VoIP is maturing into a viable alternative to traditional telephones. Interoperability between different providers has improved and the ability to call or receive a call from a traditional telephone is available. Simple, inexpensive VoIP modems are now available that eliminate the need for a PC.
Voice quality can still vary from call to call but is often equal to and can even exceed that of traditional calls.
Remaining problems for VoIP include emergency telephone number dialing and reliability. Currently, a few VoIP providers provide an emergency service, but it is not universally available. Traditional phones are line-powered and operate during a power failure; VoIP does not do so without a backup power source for the electronics.
Most VoIP providers offer unlimited national calling, but the direction in VoIP is clearly toward global coverage with unlimited minutes for a low monthly fee.
VoIP has also become increasingly popular within the gaming world, as a form of communication between players. Popular gaming VoIP clients include Ventrilo and Teamspeak, and there are others available also. The PlayStation 3 and Xbox 360 also offer VoIP chat features.
Slide -13
Network Topology Network topology are the physical layout of the network that the locations of the computers and how the cable is run between them. It is important to use the right topology. Each topology has its own strengths and weakness
Mesh Topology
• It is also called a point-to-point topology.
• Each device is connected directly to all other network devices.
• In a mesh topology, every device in the network is physically connected to every other device in the network. A message can be sent on different possible paths from source to destination. Mesh topology provides improved performance and reliability. Mesh networks are not used much in local area networks. It is mostly used in wide area networks.
Advantages
The use of dedicated link guarantees that each connection can carry its own data load. It eliminates traffic problem.
If one link becomes unusable, it does not harm the entire system.
It is easy to troubleshoot.
Disadvantages
A full mesh network can be very expensive.
It is difficult to install and reconfigure.
Bus Topology
• It is a multipoint topology.
• Each device shares the connection.
• Only one device at a time can send.
• Bus topology is the cheapest way of connecting computers to form a workgroup or departmental LAN, but it has the disadvantage that a single loose connection or cable break can bring down the entire LAN
Termination is important issue in bus networks. The electrical signal from a transmitting computer is free to travel the entire length of the cable. Without the termination, when the signal reaches the end of the wire, it bounces back and travels back up the wire. When a signal echoes back and forth along an unterminated bus, it is called ringing. The terminators absorb the electrical energy and stop the reflections.
Advantages:
Bus is easy to use and understand and inexpensive simple network
It is easy to extend a network by adding cable with a repeater that boosts the signal and allows it to travel a longer distance.
Disadvantage:
A bus topology becomes slow by heavy network traffic with a lot of computer because networks do not coordinate with each other to reserve times to transmit.
It is difficult to troubleshoot a bus because a cable break or loose connector will cause reflections and bring down the whole network.
Ring Topology
• It is a circle with no ends.
• Packets are sent from one device to the next
Advantages :
One computer cannot monopolize the network.
It continue to function after capacity is exceeded but the speed will be slow.
Disadvantages :
Failure of one computer can affect the whole network.
It is difficult to troubleshoot.
Adding and removing computers disrupts the network.
Star topology
• All devices are connected to a central device.
• The hub receives and forwards packets.
Advantages :
The failure of a single computer or cable doesn't bring down the entire network.
The centralized networking equipment can reduce costs in the long run by making network management much easier.
It allows several cable types in same network with a hub that can accommodate multiple cable types.
Disadvantages :
Failure of the central hub causes the whole network failure.
It is slightly more expensive than using bus topology.
Hybrid Topology
• Variations of two or more topologies.
• Star and Bus
• Star and Ring
Slide-15
Collection of interrelated data
Set of programs to access the data
DMBS contains information about a particular enterprise
DBMS provides an environment that it both convenient and efficient to use
Physical level: describes how a record (e.g. customer) is stored.
Logical level: describes data stored in database, and the relationships among the data.
type customer = record
name: string;
street: string;
city: integer;
end;
View level: application programs hide details of data types. Views can also hide information (e.g. salary) for security purposes.
Slide-16
A schema is the overall design of the database. It describes the data contents, structure and some other aspects of the database also called the intension of the database
The instance is the collection of data stored in the database at a particular time, also called the extension of the database.
Slide-17
A transaction is a collection of operations that performs a single logical function in a database application.
Transaction-management component ensures that the database remains in a consistent (correct) state despite system failures (e.g. power failures and operating system crashes) and transaction failures.
Concurrency-control manager controls the interaction among the concurrent transactions, to ensure the consistency of the database.
A storage manager is a program module that provides the interface between the low-level data stored in the database and the application programs and queries submitted to the system.
The storage manager is responsible for the following tasks:
Interaction with the file manager
Efficient storing, retrieving, and updating of data
Slide- 18
Users are differentiated by the way they expect to interact with
the system
• Application programmers – interact with system through DML calls
• Sophisticated users – form requests in a database query language
• Specialized users – write specialized database applications that do not fit into the traditional data processing framework
• Naïve users – invoke one of the permanent application programs that have been written previously
– Examples, people accessing database over the web, bank tellers, clerical staff
Slide-20
File System Data Management
Requires extensive programming in third-generation language (3GL)
Time consuming
Makes ad hoc queries impossible
Leads to islands of information
Data Dependence
Change in file’s data characteristics requires modification of data access programs
Must tell program what to do and how
Makes file systems cumbersome from programming and data management views
Structural Dependence
Change in file structure requires modification of related programs
Field Definitions and Naming Conventions
Flexible record definition anticipates reporting requirements
Selection of proper field names important
Attention to length of field names
Use of unique record identifiers
Data Redundancy
Different and conflicting versions of same data
Results of uncontrolled data redundancy
Data anomalies
Modification
Insertion
Deletion
Data inconsistency
Lack of data integrity
A good DBMS performs the following functions
maintain data dictionary
support multiple views of data
enforce integrity constraints
enforce access constraints
support concurrency control
support backup and recovery procedures
support logical transactions
Purpose of Database System
Built on top of file systems
Drawbacks of using file systems:
Atomicity of updates
Concurrent access by multiple users
Security problems
Database systems offer solutions to all the above problems
Slide -21
E-R model of real world
Entities (objects)
E.g. customers, accounts, bank branch
Relationships between entities
E.g. Account A-101 is held by customer Johnson
Relationship set depositor associates customers with accounts
Widely used for database design
Database design in E-R model usually converted to design in the relational model (coming up next) which is used for storage and processing