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A network consist of two computers or more that connected “linked” together in order and they can share resources ( e.g. data, printers or CD-ROMs), exchange files, and allow electronic communication. Networking arose from the need to share data in a timely fashion. The computers on the network may be connect through cables, telephone lines, radio waves, satellites, or infrared light beams. We know that the personal computer is wonderful to produce Data, Spreadsheet, Graphics and other kinds of information. From that we can define network as a group of computers that connected together to sharing data. Computers connect to each other via cable to share resources 1.Data. 2.E-mail, messages,scheduling 3.Graphics. 4.Printers. 5.Fax machines. 6.Modems. 7.Applications. 8.Other hardware resources. The benefits of networks:1-File and programs sharing. 2-Resources sharing such as printer. 3-sharing data messages, printers…etc 1 ***Types of networks*** There are basically two types of network:local-area network "LAN" and wide-area network "WAN" , the main difference between the two is the way in which they are geographically distributed. Local-area networks (LAN): LAN are usually designed to cover a small geographical area (such as a single building or afew adjacent buildings) and are generally used in an office environment. Wide-area network (WAN): WAN ara usually distrbuted over a large geographical area (such as the United States). Classification of networks by Scale : Distance 1m example Multicomputer 10 - 1km Local Area Network (LAN) Wan 100km 2 ***Network design*** Data Networking : Transmission technology:• Broadcast • Point to point Broadcast : Single channel shared by all the machines Messages (packets) sent by one machine is seen by all Broadcasting messages is simple Point-to-point : Also called store and forward or packet switched subnet. Impractical (costly) for all communication devices to be point-to-point connected. Devices are typically connected to a communications network . 3 Example of Point-to-Point Subnet Topologies Transmission Mode:Transmission Mode Defines the direction of the signal between linked devices. • Half duplex - Can transmit or receive, but not at the same time. • Full duplex - Each can transmit or receive simultaneously. 4 *There are two kinds of topology physical and logical :The physical topology of a network refers to the configuration of cables, computers, and other peripherals. The logical topology is the method that used to pass the information. Main types of physical topologies :The following parts are discussing the physical topologies used in network and other related topics:o Star brunch from hub. o Ring or loop. o Bus or row. Linear bus. o Tree . 1-Star networks:A network with radial topology in which a central the point to which all other nodes join. "control" node is And we can say from this topology as the following, cable segments to a centralized component connect computers is called a hub. Signals are transmitted from the sending computer through the hub to all computers on the network. In the previously the computers are connected with mainframe computer as alternative the hub. The star network offers centralized resources and management. However, because each computer is connected to a central point, this topology requires a great deal of cable in a large network installation. Also, if the central point fails, the entire network goes down. 5 Star Topology If one computer, or the cable that connects it to the hub, fails on a star network,only failed computer will not be able to send or receive network data. The rest of the network continues to function normally. We can summarization advantages and disadvantage as following for this topology. Advantages of a star topology:• Easy to install and wire. • No disruptions to the network connecting or removing devices. • Easy to detect faults and to remove parts. Disadvantages of a star topology:• Requires more cable length than a linear topology. • If the hub or concentrator fails, nodes attached are disabled. • More expensive than linear bus topologies because of the cost of the concentrators. 2-Ring networks:Ring architecture connects each node in a closed loop, Signals ravel from one node to the next in a single direction around the 6 loop. The server in a ring is just one of several nodes. Advantages of a ring topology:• It is more scalable because the point-to-point links between nodes dose not limit the number of nodes in the ring. Point-to-point: - A direct link between two objects in a network. Disadvantages of a ring topology:• A failure of any link between a node pair takes the bus down and prevents communication by other nodes. Ring Topolpogy *Linear Bus:Alinear bus topolgy consists of amain run of cable with a terminator at each end. all nodes have(fileserver,workstations,and peripherals)are connected to the linear cable. 7 Computer on the bus topology network communication:by addresing data to aparticular computer and putting that data on the cable in the form of electronic signal To understand how computer communication on bus you need to be familiar with three concepts: Sending the signal Signal bounce The terminator Sending the signal: Network data in the form of electronic signals is sent to all of the computer on the network;however,the information is accepted only by the computer whose address matches the address encoded in the original signal.only on computer at atime can send messages . Because only one computer at atime can send data on abus network,network performance is affected by the number of computers attached to the bus,the more computer there will be waiting to put data on the bus,and the slower the network. There is no standard measure for the imact of numbers of computers on any given network. The amount the network slows down is not solely related to the number of computer on the network. It depends on numerous factors including: Hardware capabilities of computer on the network. Number of times computer on the network transmit data. 8 Type of applications begin run on the network. Types of cable used on the network. Distance between computers on the network. The bus is apassive topology .computer on abus only listen for data being sent on the network.there are not responsible for moving data form one computer to the next. If one computer fails,it does not affect the rest of network.in active toplogy. Computer regenerate signals and move data along the network. signal bounce:-because the data,or electronic signal,is sent to the entire network,it will travel from one end of the cable to the other. noise of the network communication. Abreak in the cable will occur if the cable is physically cut into tow pieces or if one end of the cable becomes disconnected.in either case,one or more ends of the cable will not have aterminateor,the signal will bounce,and all network activity will stop. this is referred to as the network being”down”. Advantage of alinear bus topology: Easy to connect acomputer or peripheral to alinear bus. Requires less cable length than astar topology. The bus is simple,reiable in very small networks,easy to use,and easy to understand. 9 The bus requires the least amount of cable to connect the computers together and is therefore less expensive than oyher cabling arrangment. It is easy to extend abus. Disadvantages of alinear bus topology: Entire network shuts down if there is abreak in the main cable. It difficult to troubleshoot the bus. Tree topology:Atree topology combines characteristics of linear bus and star topologies. It consists of groups of star- configured workstation connected to a linear but backbone cable. Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs. Advantages of a tree toplogy: point-to point wiring for individual segments. Supported by several hardare and software venders. disavantages of a tree topology: Overall length of each segment is limited by the type of cabling used. If the backbone line breaks, the entire segment goes down. More difficult to configure and wire than other topolgies. 10 Parts of a Network: Servers: Provide shared resources to users Clients: Use the shared resources from servers Media: Device for connection Shared peripherals: Resources provided by servers such as printers, modems Wiring Concentrators, Hubs, and Switches :Wiring concentrators, hubs, and switches provide a common physical connection point for computing devices. Most hubs and all wiring concentrators and switches have built-in signal repeating capability and thus perform signal repair and retransmission. In most cases, hubs, wiring concentrators, and switches are proprietary, standalone hardware. Occasionally, hub technology consists of hub cards and software that work together in a standard computer. Next figure shows one of common hardware-based connection devices: a token-ring switch 11 Hubs:One network component that is becoming standard equipment in more and more networks is yhe hub. Hub is the central component in astar topology. Hub consideration:Hubs are versatile and offer several advantages over system , In astandard linear bus topology ,abreak in the cable will take the network down .with hubs,however,abreak in any of the cables attached to the hub affect only the segment . The rest of the network keeps functioning. The benfits of hub-based topologyies include: Changing or expanding wiring as needed.simply plug in another computer or another hub. Using different ports to accommodate avariety of cabling types. Modem : Modems convert digital (computer) signals to analog (audio) signals, and vice versa, by modulating and demodulating a carrier frequency. The most common modems transmit and receive data across ordinary voicegrade telephone lines. A transmitting modem converts (modulates) the encoded data signal to an audible signal and transmits it. A modem connected at the other end of the line listens to the audible signal and converts it back into a digital signal (demodulates it) for the computer on the receiving end of 12 the communication link. Modems are commonly used for inexpensive, intermittent communications between geographically isolated computers and a main network. Microwave Transmitters : - Microwave transmitters and receivers, especially satellite systems, are commonly used to transmit network signals over great distances. A microwave transmitter uses the atmosphere or outer space as the transmission medium to send the signal to a microwave receiver. The microwave receiver either relays the signal to another microwave transmitter, which sends it to another microwave receiver, or the receiving station translates the signal to some other form, such as digital impulses, and sends it along on some other suitable medium. Figure 4 shows a satellite microwave link. 13 Internetworking Devices: Bridges and Routers Bridges and routers are the devices used to interconnect subnetworks. They can be either hardware or software based. Software-based routers and bridges can be part of a server's operating system or can at least run in the server with the operating system. Hardware-based bridges and routers can also be installed on standard computers to create dedicated, standalone devices. To understand internetworking, it is not essential that you understand all the technical differences between a bridge and router. In fact, without some study, this can be a confusing area. For example, if you read about multiprotocol Routers, you will find that these routers also perform what is called source-route bridging. However, without a basic understanding of bridging and routing technology, you will find it difficult to understand the capabilities of some products and the reasons such capabilities are useful or important. Keep in mind that bridges and routers have one important thing in common: They both allow the transfer of data packets (frames) between subnetworks with different network addresses. Bridges:A bridge operates at the data-link layer (layer two) of the OSI model. A bridge acts as an address filter; it relays data between subnetworks (with different addresses) based on information contained at the media access control level. Simple bridges are used to connect networks that use the same physical-layer protocol and the 14 same MAC and logical link protocols (OSI layers one and two). Simple bridges are not capable of translating between different protocols. Other types of bridges, such as translational bridges, can connect networks that use different layer-one and MAClevel protocols; they are capable of translating, then relaying, frames. After a physical connection is made (at OSI layer one), a bridge receives all frames from each of the subnetworks it connects and checks the network address of each received frame. The network address is contained in the MAC header. When a bridge receives a frame from one subnetwork that is addressed to a workstation on another subnetwork, it passes the frame to the intended subnetwork. Routers Routers function at the network layer of the OSI model (one layer above bridges). To communicate, routers must use the same network-layer protocol. And, of course, the sending and receiving workstations on different networks must either share identical protocols at all OSI layers above layer three, or there must be necessary protocol translation at these layers. 15 Like some bridges, routers can allow the transfer of data between networks that use different protocols at OSI layers one and two (the physical layer and the data-link layer, which includes sublayers for media access control and logical link control). Routers can receive, reformat, and retransmit data packets assembled by different layer-one and layer-two protocols. Different routers are built to manage different protocol sets. Figure 11 illustrates how a router transfers data packets. Types of local area network(lAN) Peer-to-Peer Networks; Also Called Workgroups 1. Each computer acts as both a client and server and is located in small area. 2. No dedicated or central server: no hierarchy 3. Each user determines the data to be shared and is also responsible for the security of the data. 4. Involves small groups, usually fewer than ten computers 5. Peer-to-peer networking is built into operating systems such as Microsoft Windows 3.11 (WFW), Microsoft Windows 95, and Microsoft Windows NT Workstation; no additional software is needed. When to Use Peer-to-Peer Networks Ten or fewer users Limited growth expected for network Users clustered in same area 16 Security is not important Users can act as administrators and plan their own security Potential Limitations of Peer-to-Peer Networks:1. Each user acts as his or her own administrator. 2. Users share their resources (printers, fax cards, and so on) as they want. 3. Users must be knowledgeable in network management (managing users and security, making resources available, maintaining applications and data, installing and upgrading application software). 4. Requires CPU time to maintain the server function 5. Limited to resource password security 6. Each resource has its own password. 7. Very little control over peer-to-peer networks. 1. 2. 3. 4. Server-Based Networks Dedicated server(s) More servers can be added when necessary. Users are grouped logically. Servers can be specialized. Why server-based networking? 1.Centrally administered and controlled. 17 2.Provides access to network resources without compromising performace. 3.Security:Access to network resources can be limited to specific users. 4.Backup and redundancy:All data backed up to one location. 5.Data can be duplicated(fault tolerant)more users,larger network. Comparing peer-to-peer and server-based networks:1. Multiple operating systems can used 2.Peer-to-peer processor: Should be at least a 486 33 or supported RISC processor. 3.Hard disk requirements: Vary with system; one GB or more is recommended. 4.Multiple hard drives for super servers. Network Adapters : A network adapter is the hardware installed in computers that enables them to communicate on a network. Network adapters are manufactured in a variety of forms. The most common form is the printed circuit board, which is designed to be installed directly into a standard expansion slot inside a microcomputer. Other network adapters are designed for mobile computing. They are small and lightweight and can be connected to standard connectors on the back of notebook computers so that the computer and network adapter can be easily 18 transported from network to network. Network adapters are now being built into many computers, especially notebook computers. Network adapters are manufactured for connection to virtually any type of guided medium, including twisted-pair wire, coaxial cable, and fiber-optic cable. They are also manufactured for connection to devices that transmit and receive visible light, infrared light, and radio microwaves, to enable wireless networking across the unguided media of Earth's atmosphere and outer space. The hardware used to make connections between network adapters and different transmission media depends on the type of medium used. For example, twist-on BNC connectors are commonly used for connection to coaxial cable, while snap-in telephone-type jacks are ordinarily used for connection to twisted-pair wiring. The figure below shows two different types of network adapters connected to different computers and media, using different types of connectors. 19 Network cabling Cable can be recovered in plastic rubber, be careful to get the correct type of cable for the type of network card used. 1. Primary cable types:Coaxial Twisted pair Shielded Unsheilded Fiber optic Twisted pair The least_expansive and most widely_used guided transmition medium is twisted pair. cable show ing vario us layer s 20 Physical description A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern. wire pair acts as a single communication link. Typically,anumber of these pairs are bundled together into a cable by wrapping them in a tought protictive sheath. Cable may contain hundred of pairs . The twisting tends to decrease the crosstalk interfrence between adjacent pairs in acable .that links in the long _distance,the twist length typically varies from two to six inches in apair have thicknesses of from 0.016 to 0.036 inches. Applications That used in the telephone network as well as being the workhorse for communication within building,for local area network supporting personal computer. Tisted pair is much less expensive than the other commonly used guided transmission media(coaxial cable,optical fiber)and is easier to work .it is more limited in terms of data rate and distance. Transmittion characteristics Twisted pair may be used to transmit both analog and digital signals,for analog signal,amplifiers are required about every 5 to 6 km,for digtal signal ,repeter are required every 2 or 3 km.twisted pair is limited in distance. The attenution for twisted is a very strong function of fequency . 21 Unshilded and shildeed twisted pair Twisted pair comes in two varities:unshilded and shilded. Unshilded twisted pair(utp)is ordinary telephone wire.office buildings,by universal practice,this occur noise and attenution also,this is the least expensive of all the transmitiom media commonly used for local area networks and asimple to insulate. UTP Unshielded twistedpair and shielded twisted-pair cables STP Shielding Recognises three categories of utp cabling: Category 3.utp cables and associated connecting hardware whose transmition characteristics are specified up to 16 MHZ. Category 4.utp cables and associated connecting hardware whose transmition chracteristics are specified up to 20MHZ. Category 5.utp cables ans associated connecting hardware whose transmition charactrisics are specified up to 100 MHZ. The difference between category 3 and category 5:The difference between category 3 and category 5 cable is the number of twists in the cable per unit distance,category 5 is much more tightly twisted typically 3 to 4 twists per inch,compared to 3 to 4 twists per foot for category 3, 22 The tighter twisting is more expensive but provides much better performance than category 3. Coaxial cable Physical description Coaxial cable,like twisted pair ,consists of two conductors,it consists of ahellow outer cylinderical conductor that surrounds asingle inner wire conductor. The inner conductor is held in place by either regularly spaced insulating rings or a solid dielectric material. The outer conductor is coverd with ajacet or shiled. Coaxial cable has adiamter of from 0.4 to 1 inch. Applications:Coaxial cable is perhaps the most veratile transmition medium and is enjoying widespred use in a wide varietly of applications; Television distrbution Long_distance telephone transmition Short_run computer system links Local area networks 23 Transmition chractristics Coaxial cable is used to transmit both analog and digital signals. For long_distance transmition of analog signals,amplifirs are needed every kelometers. Outshield Insulation (PVC,Teflon) Coaxial cable showing various layersxial Copper wire mesh or aluminum sleeve Conducting core Optical fiber *Physical description An optical fiber is a thin(2 to 125 Mm). Greater capacity Smaller size and higter weight. Lower attenution. Electomagnatic isolation Greater repeter spacing. *Transmitiom chractrisics Optical fiber system operate in the range of about 10^14 to 10^15 HZ. 24 **Considerations when choosing a topology** • Money. A linear bus network may be the least expensive way to install network you do not have to purchase concentrators • Length of cable needed. The linear bus network uses shorter lengths of cable. • Future growth. With a star topology, adding another concentrator easily dose expanding a network. • Cable type. The most common cable in schools is unshielded twisted pair, which is most often used with star topologies. SELECTING A TOPLOGY:There are many factors to consider when determining what toplogy best suits the needs of an organization. The following table provies some guidelines for selecting a topology. Toplogy Advantages Bus Economical use of cable. Media is inexpensive and easy to work with simple, reliable. Easy to extend. Ring Equal access for all Computers. Even Performance despite many users. Star Easy to modify and add New compters. Disadvantages Netwok can slow Down in heavy traffic. Problems are difficult to isolate. Cable break can affect many users. Failure one one comupter can impact the rest of the network. Problems bard to isolate. Network Reconfiguration Disrupts operation. If the centralized point fails, the network fails . 25 26