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Data Communication Introduction to data communication Data communication is one of the most exciting developments in data processing today. It allows us to work at home, at office or anywhere telephone lines and other communication lines are available. With earlier computer systems, all processing was done in a central room or location. All components of the computer systems are literally reaching out to us. Instead of taking processing jobs to the computer room, we are now bringing the computer system to where the action is. But how are these remote computer devices able to communicate with each other? It’s done with data communication systems. The marriage of computers with data communication is one of the most exciting developments in data processing today. In the near 80 years after the introduction of the telephone, a complex network of telecommunications systems was established to link locations throughout the world. The first linkage of computing and communication devices occurred in 1940  when Dr George Stibtiz used telephone lines to send data from Dart Mouth college in New Hampshire to Bell laboratories in New York city. The purpose of a communication system is to convey information from one point to another. Data communication refers to means and methods where by data are transferred between processing locations. It extends the power of the computer behind the computer room and allows computer facilities at remote sites. The distance involved may range from a few feet to million of miles. Needs of Data Communication Data communication is used to link the system both locally and throughout the networks. It helps your organization to increase efficiency, economically and effectively. It can help to increase effiecency and productivity by providing new ways of working in computer-based information system, where there is often a need to share data and programs and other information system, where there is often a need to share data and programs and other information between the various computers. Basic Elements of a Communication System Communication is the process of transferring data from one point to another as shown in the following figure. The five basic elements of any communications process are: 1.	Message: The message is the information (data) to be communicated. 2.	Sender: A sender (source), which created the message to be transmitted it can be a computer, workstation, Telephone, handset, video camera or etc. 3.	Medium: A medium which carries the message is the physical path by which a message travels from sender to receiver. 4.	Receiver: A receiver (sink), is the device that receives the messages. It also can be computer, workstation, telephone, handset, television or etc. 5.	Protocol: A protocol is a set of rules that govern data communication. It represents an agreement between the communicating devices.
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What is a Network? A network consists of two or more computers that are linked in order to share resources (such as printers and CD-ROMs), exchange files, or allow electronic communications. The computers on a network may be linked through cables, telephone lines, radio waves, satellites, or infrared light beams. The three basic types of networks include: •	Local Area Network (LAN) •	Wide Area Network (WAN) •	Metropolitan Area Network (MAN) Local Area Network A Local Area Network (LAN) is a network that is confined to a relatively small area. It is generally limited to a geographic area such as a writing lab, school, or building. Rarely are LAN computers more than a mile apart. In a typical LAN configuration, one computer is designated as the file server. It stores all of the software that controls the network, as well as the software that can be shared by the computers attached to the network. Computers connected to the file server are called workstations. The workstations can be less powerful than the file server, and they may have additional software on their hard drives. On most LANs, cables are used to connect the network interface cards in each computer. See the Topology, Cabling, and Hardware sections of this tutorial for more information on the configuration of a LAN. Wide Area Network Wide Area Networks (WANs) connect larger geographic areas, such as Florida, the United States, or the world. Dedicated transoceanic cabling or satellite uplinks may be used to connect this type of network. Using a WAN, schools in Florida can communicate with places like Tokyo in a matter of minutes, without paying enormous phone bills. A WAN is complicated. It uses multiplexers to connect local and metropolitan networks to global communications networks like the Internet. To users, however, a WAN will not appear to be much different than a LAN or a MAN. Metropolitan Area Network Metropolitan Area Networks or MANs are large computer networks usually spanning a campus or a city. They typically use wireless infrastructure or optical fiber connections to link their sites. Advantages of Installing a School Network •	Speed. Networks provide a very rapid method for sharing and transferring files. Without a network, files are shared by copying them to floppy disks, then carrying or sending the disks from one computer to another. This method of transferring files (referred to as sneaker-net) is very time-consuming. •	Cost. Networkable versions of many popular software programs are available at considerable savings when compared to buying individually licensed copies. Besides monetary savings, sharing a program on a network allows for easier upgrading of the program. The changes have to be done only once, on the file server, instead of on all the individual workstations. •	Security. Files and programs on a network can be designated as "copy inhibit," so that you do not have to worry about illegal copying of programs. Also, passwords can be established for specific directories to restrict access to authorized users. •	Centralized Software Management. One of the greatest benefits of installing a network at a school is the fact that all of the software can be loaded on one computer (the file server). This eliminates that need to spend time and energy installing updates and tracking files on independent computers throughout the building. •	Resource Sharing. Sharing resources is another area in which a network exceeds stand-alone computers. Most schools cannot afford enough laser printers, fax machines, modems, scanners, and CD-ROM players for each computer. However, if these or similar peripherals are added to a network, they can be shared by many users. •	Electronic Mail. The presence of a network provides the hardware necessary to install an e-mail system. E-mail aids in personal and professional communication for all school personnel, and it facilitates the dissemination of general information to the entire school staff. Electronic mail on a LAN can enable students to communicate with teachers and peers at their own school. If the LAN is connected to the Internet, students can communicate with others throughout the world. •	Flexible Access. School networks allow students to access their files from computers throughout the school. Students can begin an assignment in their classroom, save part of it on a public access area of the network, then go to the media center after school to finish their work. Students can also work cooperatively through the network. •	Workgroup Computing. Workgroup software (such as Microsoft BackOffice) allows many users to work on a document or project concurrently. For example, educators located at various schools within a county could simultaneously contribute their ideas about new curriculum standards to the same document and spreadsheets. Disadvantages of Installing a School Network •	Expensive to Install. Although a network will generally save money over time, the initial costs of installation can be prohibitive. Cables, network cards, and software are expensive, and the installation may require the services of a technician. •	Requires Administrative Time. Proper maintenance of a network requires considerable time and expertise. Many schools have installed a network, only to find that they did not budget for the necessary administrative support. •	File Server May Fail. Although a file server is no more susceptible to failure than any other computer, when the files server "goes down," the entire network may come to a halt. When this happens, the entire school may lose access to necessary programs and files. •	Cables May Break. The Topology chapter presents information about the various configurations of cables. Some of the configurations are designed to minimize the inconvenience of a broken cable; with other configurations, one broken cable can stop the entire network. What is a Protocol? A protocol is a set of rules that governs the communications between computers on a network. These rules include guidelines that regulate the following characteristics of a network: access method, allowed physical topologies, types of cabling, and speed of data transfer. See the Topology and Cabling sections of this tutorial for more information.

http://www.erg.abdn.ac.uk/users/gorry/course/lan-pages/csma-cd.html

The most common protocols are: •	Ethernet •	LocalTalk •	Token Ring •	FDDI •	ATM Ethernet A local-area network (LAN) architecture developed by Xerox Corporation in cooperation with DEC and Intel in 1976. Ethernet uses a bus or star topology and supports data transfer rates of 10 Mbit/s. The Ethernet specification served as the basis for the IEEE 802.3 standard, which specifies the physical and lower software layers. Ethernet uses the CSMA/CD access method to handle simultaneous demands. It is one of the most widely implemented LAN standards. The Ethernet protocol is by far the most widely used. Ethernet uses an access method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection). This is a system where each computer listens to the cable before sending anything through the network. If the network is clear, the computer will transmit. If some other node is already transmitting on the cable, the computer will wait and try again when the line is clear. Sometimes, two computers attempt to transmit at the same instant. When this happens a collision occurs. Each computer then backs off and waits a random amount of time before attempting to retransmit. With this access method, it is normal to have collisions. However, the delay caused by collisions and retransmitting is very small and does not normally effect the speed of transmission on the network. The Ethernet protocol allows for linear bus, star, or tree topologies. Data can be transmitted over wireless access points, twisted pair, coaxial, or fiber optic cable at a speed of 10 Mbit/s up to 1000 Mbit/s.

Fast Ethernet To allow for an increased speed of transmission, the Ethernet protocol has developed a new standard that supports 100 Mbit/s. This is commonly called Fast Ethernet. Fast Ethernet requires the use of different, more expensive network concentrators/hubs and network interface cards. In addition, category 5 twisted pair or fiber optic cable is necessary. Fast Ethernet is becoming common in schools that have been recently wired. A newer version of Ethernet, called 100Base-T (or Fast Ethernet), supports data transfer rates of 100 Mbit/s. And the newest version, Gigabit Ethernet supports data rates of 1 gigabit (1,000 megabits) per second.

Gigabit Ethernet The most recent development in the Ethernet standard is a protocol that has a transmission speed of 1 Gbit/s. Gigabit Ethernet is primarily used for backbones on a network at this time. In the future, it will probably be used for workstation and server connections also. It can be used with both fiber optic cabling and copper. The 1000BaseTX, the copper cable used for Gigabit Ethernet, is expected to become the formal standard in 1999. Abbreviated GbE, a version of Ethernet, which supports data transfer rates of 1 Gigabit (1,000 megabits) per second. The first Gigabit Ethernet standard (802.3z) was ratified by the IEEE 802.3 Committee in 1998. LocalTalk LocalTalk is a network protocol that was developed by Apple Computer, Inc. for Macintosh computers. The method used by LocalTalk is called CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). It is similar to CSMA/CD except that a computer signals its intent to transmit before it actually does so. LocalTalk adapters and special twisted pair cable can be used to connect a series of computers through the serial port. The Macintosh operating system allows the establishment of a peer-to-peer network without the need for additional software. With the addition of the server version of AppleShare software, a client/server network can be established. The LocalTalk protocol allows for linear bus, star, or tree topologies using twisted pair cable. A primary disadvantage of LocalTalk is speed. Its speed of transmission is only 230 kbit/s. The cabling scheme supported by the AppleTalk network protocol for Macintosh computers. Most local-area networks that use AppleTalk, such as TOPS, also conform to the LocalTalk cable system. Such networks are sometimes called LocalTalk networks. Although LocalTalk networks are relatively slow, they are popular because they are easy and inexpensive to install and maintain. An alternative cabling scheme that is faster is Ethernet.

Token Ring The Token Ring protocol was developed by IBM in the mid-1980s. The access method used involves token-passing. In Token Ring, the computers are connected so that the signal travels around the network from one computer to another in a logical ring. A single electronic token moves around the ring from one computer to the next. If a computer does not have information to transmit, it simply passes the token on to the next workstation. If a computer wishes to transmit and receives an empty token, it attaches data to the token. The token then proceeds around the ring until it comes to the computer for which the data is meant. At this point, the data is captured by the receiving computer. The Token Ring protocol requires a star-wired ring using twisted pair or fiber optic cable. It can operate at transmission speeds of 4 Mbit/s or 16 Mbit/s. Due to the increasing popularity of Ethernet, the use of Token Ring in school environments has decreased. (1) A type of computer network in which all the computers are arranged (schematically) in a circle. A token, which is a special bit pattern, travels around the circle. To send a message, a computer catches the token, attaches a message to it, and then lets it continue to travel around the network. Also see token passing. For network diagrams, see Network Topology Diagrams in the Quick Reference section of Webopedia. (2) When capitalized, Token Ring refers to the PC network architecture developed by IBM. The IBM Token-Ring specification has been standardized by the IEEE as the IEEE 802.5 standard.

FDDI Fiber Distributed Data Interface (FDDI) is a network protocol that is used primarily to interconnect two or more local area networks, often over large distances. The access method used by FDDI involves token-passing. FDDI uses a dual ring physical topology. Transmission normally occurs on one of the rings; however, if a break occurs, the system keeps information moving by automatically using portions of the second ring to create a new complete ring. A major advantage of FDDI is speed. It operates over fiber optic cable at 100 Mbit/s. Abbreviation of Fiber Distributed Data Interface, a set of ANSI protocols for sending digital data over fiber optic cable. FDDI networks are token-passing networks, and support data rates of up to 100 Mbit/s (100 million bits) per second. FDDI networks are typically used as backbones for wide-area networks. An extension to FDDI, called FDDI-2, supports the transmission of voice and video information as well as data. Another variation of FDDI, called FDDI Full Duplex Technology (FFDT) uses the same network infrastructure but can potentially support data rates up to 200 Mbit/s. http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/fddi.htm ATM Asynchronous Transfer Mode (ATM) is a network protocol that transmits data at a speed of 155 Mbit/s and higher. ATM works by transmitting all data in small packets of a fixed size; whereas, other protocols transfer variable length packets. ATM supports a variety of media such as video, CD-quality audio, and imaging. ATM employs a star topology, which can work with fiber optic as well as twisted pair cable. ATM is most often used to interconnect two or more local area networks. It is also frequently used by Internet Service Providers to utilize high-speed access to the Internet for their clients. As ATM technology becomes more cost-effective, it will provide another solution for constructing faster local area networks. Short for Asynchronous Transfer Mode, a network technology based on transferring data in cells or packets of a fixed size. The cell used with ATM is relatively small compared to units used with older technologies. The small, constant cell size allows ATM equipment to transmit video, audio, and computer data over the same network, and assure that no single type of data hogs the line. Some people think that ATM holds the answer to the Internet bandwidth problem, but others are skeptical. ATM creates a fixed channel, or route, between two points whenever data transfer begins. This differs from TCP/IP, in which messages are divided into packets and each packet can take a different route from source to destination. This difference makes it easier to track and bill data usage across an ATM network, but it makes it less adaptable to sudden surges in network traffic. When purchasing ATM service, you generally have a choice of four different types of service: • constant bit rate (CBR): specifies a fixed bit rate so that data is sent in a steady stream. This is analogous to a leased line. • variable bit rate (VBR): provides a specified throughput capacity but data is not sent evenly. This is a popular choice for voice and videoconferencing data. • available bit rate (ABR): provides a guaranteed minimum capacity but allows data to be bursted at higher capacities when the network is free. • unspecified bit rate (UBR): does not guarantee any throughput levels. This is used for applications, such as file transfer, that can tolerate delays.

Protocol Summary Protocol	Cable	Speed	Topology Ethernet	Twisted Pair, Coaxial, Fiber	10 Mbit/s	Linear Bus, Star, Tree Fast Ethernet	Twisted Pair, Fiber	100 Mbit/s	Star LocalTalk	Twisted Pair	.23 Mbit/s	Linear Bus or Star Token Ring	Twisted Pair	4 Mbit/s - 16 Mbit/s	Star-Wired Ring FDDI	Fiber	100 Mbit/s	Dual ring ATM	Twisted Pair, Fiber	155-2488 Mbit/s	Linear Bus, Star, Tree

What is Networking Hardware? Networking hardware includes all computers, peripherals, interface cards and other equipment needed to perform data-processing and communications within the network. CLICK on the terms below to learn more about those pieces of networking hardware. This section provides information on the following components: •	File Servers •	Workstations •	Network Interface Cards •	Switches •	Repeaters •	Bridges •	Routers File Servers A file server stands at the heart of most networks. It is a very fast computer with a large amount of RAM and storage space, along with a fast network interface card. The network operating system software resides on this computer, along with any software applications and data files that need to be shared. The f e server controls the communication of information between the nodes on a network. For example, it may be asked to send a word processor program to one workstation, receive a database file from another workstation, and store an e-mail message during the same time period. This requires a computer that can store a lot of information and share it very quickly. File servers should have at least the following characteristics: •	800 megahertz or faster microprocessor (Pentium 3 or 4, G4 or G5) •	A fast hard drive with at least 120 gigabytes of storage •	A RAID (Redundant Array of Inexpensive Disks) to preserve data after a disk casualty •	A tape back-up unit (i.e. DAT, JAZ, Zip, or CD-RW drive) •	Numerous expansion slots •	Fast network interface card •	At least of 512 MB of RAM Workstations All of the user computers connected to a network are called workstations. A typical workstation is a computer that is configured with a network interface card, networking software, and the appropriate cables. Workstations do not necessarily need floppy disk drives because files can be saved on the file server. Almost any computer can serve as a network workstation. Network Interface Cards The network interface card (NIC) provides the physical connection between the network and the computer workstation. Most NICs are internal, with the card fitting into an expansion slot inside the computer. Some computers, such as Mac Classics, use external boxes which are attached to a serial port or a SCSI port. Laptop computers can now be purchased with a network interface card built-in or with network cards that slip into a PCMCIA slot. PCMCIA slot Network interface cards are a major factor in determining the speed and performance of a network. It is a good idea to use the fastest network card available for the type of workstation you are using. The three most common network interface connections are Ethernet cards, LocalTalk connectors, and Token Ring cards. According to an International Data Corporation study, Ethernet is the most popular, followed by Token Ring and LocalTalk (Sant'Angelo, R. (1995). NetWare Unleashed, Indianapolis, IN: Sams Publishing). Before data can be sent over the network, the NIC must change it from a form the computer can understand to a form that can travel over a network cable Data moves through a computer along paths called buses. These are actually several data paths placed side by side. Because the paths are side by side (parallel), data can move along them in lateral groups instead of in a single (serial) data stream. Older buses were known as 8-bit buses because they could move data 8 bits at a time, The IBM PC/AT computer used a 16-bit bus, which means it could move data 16 bits at a time. Computers manufactured today use 32-bit buses. When data travels on a computer's bus, it is said to be traveling in parallel because the 32 bits are moving alongside by side. Think of a 32-bit bus as a 32-lane highway with 32 cars moving side by side (moving in parallel), each carrying one bit of data On the network cable, however, data must travel in a single stream of bits. When data travels on a network cable it is said to be traveling as a serial transmission because one bit follows another. In other words, the cable is a one-lane highway, and the data always travels in one direction The NIC takes data that is traveling in parallel as a group and restructures it so that it will flow through the 1-bit-serial path of the network cable, The component responsible for this is the transceiver (transmitter/receiver) Network Address In addition to transforming data, the NIC also has to advertise its own location, or address, to the rest of the network to distinguish it from all the other cards on the network. (MAC address), The NIC also participates in several other functions in sequence as it takes data from the computer and gets it ready for the network cable: 1.	The NIC signals the computer, requesting the computer's data. 2.	The computer's bus moves the data from the computer's memory to the NIC. Because data can often move faster on the bus or the cable than the NIC can handle, the data is sent to the card's buffer, a reserved portion of RAM. Here it is held temporarily during both the transmission and reception of data. Network Cabling and Connectors A thinnet network connection uses a coaxial BNC connector as shown in Figure A thicknet network connection uses a 15-pin attachment unit interface (AUI) cable to connect the 15-pin (DB-15) connector on the back of the NIC An unshielded twisted-pair connection uses a RJ-45 connector, The RJ-45 connector is similar to a RJ-11 telephone connector but is larger in size and has eight conductors; a RJ-11 only has 4 conductors

Ethernet Cards Ethernet cards are usually purchased separately from a computer, although many computers (such as the Macintosh) now include an option for a pre-installed Ethernet card. Ethernet cards contain connections for either coaxial or twisted pair cables (or both) (See fig. 1). If it is designed for coaxial cable, the connection will be BNC. If it is designed for twisted pair, it will have a RJ-45 connection. Some Ethernet cards also contain an AUI connector. This can be used to attach coaxial, twisted pair, or fiber optics cable to an Ethernet card. When this method is used there is always an external transceiver attached to the workstation. (See the Cabling section for more information on connectors.)

Fig. 1. Ethernet card. From top to bottom: RJ-45, AUI, and BNC connectors

LocalTalk Connectors LocalTalk is Apple's built-in solution for networking Macintosh computers. It utilizes a special adapter box and a cable that plugs into the printer port of a Macintosh (See fig. 2). A major disadvantage of LocalTalk is that it is slow in comparison to Ethernet. Most Ethernet connections operate at 10 Mbit/s (Megabits per second). In contrast, LocalTalk operates at only 230 kbit/s (or .23 Mbit/s).

Fig.2. LocalTalk connectors

Ethernet Cards vs. LocalTalk Connections Ethernet 	LocalTalk Fast data transfer (10 to 100 Mbit/s) 	Slow data transfer (.23 Mbit/s) Expensive - purchased separately 	Built into Macintosh computers Requires computer slot 	No computer slot necessary Available for most computers 	Works only on Macintosh computers

Token Ring Cards Token Ring network cards look similar to Ethernet cards. One visible difference is the type of connector on the back end of the card. Token Ring cards generally have a nine pin DIN type connector to attach the card to the network cable. Switch A concentratorمتمرکزکننده is a device that provides a central connection point for cables from workstations, servers, and peripherals. In a star topology, twisted-pair wire is run from each workstation to a central switch/hub. Most switches are active, that is they electrically amplifyتقويت کردن the signal as it moves from one device to another. Switches no longer broadcast network packets as hubs did in the past, they memorize addressing of computers and send the information to the correct location directly. Switches are: •	Usually configured with 8, 12, or 24 RJ-45 ports •	Often used in a star or star-wired ring topology •	Sold with specialized software for port management •	Also called hubs •	Usually installed in a standardized metal rack that also may store netmodems, bridges, or routers.

switch SOURCE: Whatis.com DATE: 03 Nov, 2005

Also see repeater, hub, bridge, gateway, and router. In a telecommunications network, a switch is a device that channels incoming data from any of multiple input ports to the specific output port that will take the data toward its intended destination. In the traditional circuit-switched telephone network, one or more switches are used to set up a dedicated though temporary connection or circuit for an exchange between two or more parties. On an Ethernet local area network (LAN), a switch determines from the physical device (Media Access Control or MAC) address in each incoming message frame which output port to forward it to and out of. In a wide area packet-switched network such as the Internet, a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination. In the Open Systems Interconnection (OSI) communications model, a switch performs the layer 2 or Data-Link layer function. That is, it simply looks at each packet or data unit and determines from a physical address (the "MAC address") which device a data unit is intended for and switches it out toward that device. However, in wide area networks such as the Internet, the destination address requires a look-up in a routing table by a device known as a router. Some newer switches also perform routing functions (layer 3 or the Network layer functions in OSI) and are sometimes called IP switches. On larger networks, the trip from one switch point to another in the network is called a hop. The time a switch takes to figure out where to forward a data unit is called its latency. The price paid for having the flexibility that switches provide in a network is this latency. Switches are found at the backbone and gateway levels of a network where one network connects with another and at the subnetwork level where data is being forwarded close to its destination or origin. The former are often known as core switches and the latter as desktop switches. In the simplest networks, a switch is not required for messages that are sent and received within the network. For example, a local area network may be organized in a Token Ring or bus arrangement in which each possible destination inspects each message and reads any message with its address. Circuit-Switching version Packet-Switching A network's paths can be used exclusively for a certain duration by two or more parties and then switched for use to another set of parties. This type of "switching" is known as circuit-switching and is really a dedicated and continuously connected path for its duration. Today, an ordinary voice phone call generally uses circuit-switching. Most data today is sent, using digital signals, over networks that use packet-switching. Using packet-switching, all network users can share the same paths at the same time and the particular route a data unit travels can be varied as conditions change. In packet-switching, a message is divided into packets, which are units of a certain number of bytes. The network addresses of the sender and of the destination are added to the packet. Each network point looks at the packet to see where to send it next. Packets in the same message may travel different routes and may not arrive in the same order that they were sent. At the destination, the packets in a message are collected and reassembled into the original message.

Repeaters Since a signal loses strength as it passes along a cable, it is often necessary to boost the signal with a device called a repeater. The repeater electrically amplifies the signal it receives and rebroadcasts it. Repeaters can be separate devices or they can be incorporated into a concentrator. They are used when the total length of your network cable exceeds the standards set for the type of cable being used. A good example of the use of repeaters would be in a local area network using a star topology with unshielded twisted-pair cabling. The length limit for unshielded twisted-pair cable is 100 meters. The most common configuration is for each workstation to be connected by twisted-pair cable to a multi-port active concentrator. The concentrator amplifies all the signals that pass through it allowing for the total length of cable on the network to exceed the 100 meter limit. Bridges A bridge is a device that allows you to segment a large network into two smaller, more efficient networks. If you are adding to an older wiring scheme and want the new network to be up-to-date, a bridge can connect the two. A bridge monitors the information traffic on both sides of the network so that it can pass packets of information to the correct location. Most bridges can "listen" to the network and automatically figure out the address of each computer on both sides of the bridge. The bridge can inspect each message and, if necessary, broadcast it on the other side of the network. The bridge manages the traffic to maintain optimum performance on both sides of the network. You might say that the bridge is like a traffic cop at a busy intersection during rush hour. It keeps information flowing on both sides of the network, but it does not allow unnecessary traffic through. Bridges can be used to connect different types of cabling, or physical topologies. They must, however, be used between networks with the same protocol.

Routers A router translates information from one network to another; it is similar to a superintelligent bridge. Routers select the best path to route a message, based on the destination address and origin. The router can direct traffic to prevent head-on collisions, and is smart enough to know when to direct traffic along back roads and shortcuts. While bridges know the addresses of all computers on each side of the network, routers know the addresses of computers, bridges, and other routers on the network. Routers can even "listen" to the entire network to determine which sections are busiest—they can then redirect data around those sections until they clear up. If you have a school LAN that you want to connect to the Internet, you will need to purchase a router. In this case, the router serves as the translator between the information on your LAN and the Internet. It also determines the best route to send the data over the Internet. Routers can: •	Direct signal traffic efficiently •	Route messages between any two protocols •	Route messages between linear bus, star, and star-wired ring topologies •	Route messages across fiber optic, coaxial, and twisted-pair cabling What is Network Cabling? Cable is the medium through which information usually moves from one network device to another. There are several types of cable which are commonly used with LANs. In some cases, a network will utilize only one type of cable, other networks will use a variety of cable types. The type of cable chosen for a network is related to the network's topology, protocol, and size. Understanding the characteristics of different types of cable and how they relate to other aspects of a network is necessary for the development of a successful network. The following sections discuss the types of cables used in networks and other related topics. •	Unshielded Twisted Pair (UTP) Cable •	Shielded Twisted Pair (STP) Cable •	Coaxial Cable •	Fiber Optic Cable •	Wireless LANs •	Cable Installation Guides Unshielded Twisted Pair (UTP) Cable Twisted pair cabling comes in two varieties: shielded and unshielded. Unshielded twisted pair (UTP) is the most popular and is generally the best option for school networks (See fig. 1). Fig.1. Unshielded twisted pair The quality of UTP may vary from telephone-grade wire to extremely high-speed cable. The cable has four pairs of wires inside the jacket. Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs and other electrical devices. The tighter the twisting, the higher the supported transmission rate and the greater the cost per foot. The EIA/TIA (Electronic Industry Association/Telecommunication Industry Association) has established standards of UTP and rated five categories of wire. Categories of Unshielded Twisted Pair Type	Use Category 1	Voice Only (Telephone Wire) Category 2	Data to 4 Mbit/s (LocalTalk) Category 3	Data to 10 Mbit/s (Ethernet) Category 4	Data to 20 Mbit/s (16 Mbit/s Token Ring) Category 5	Data to 100 Mbit/s (Fast Ethernet)

Buy the best cable you can afford; most schools purchase Category 3 or Category 5. If you are designing a 10 Mbit/s Ethernet network and are considering the cost savings of buying Category 3 wire instead of Category 5, remember that the Category 5 cable will provide more "room to grow" as transmission technologies increase. Both Category 3 and Category 5 UTP have a maximum segment length of 100 meters. In Florida, Category 5 cable is required for retrofit grants. 10BaseT refers to the specifications for unshielded twisted pair cable (Category 3, 4, or 5) carrying Ethernet signals. Category 6 is relatively new and is used for gigabit connections. Unshielded Twisted Pair Connector The standard connector for unshielded twisted pair cabling is an RJ-45 connector. This is a plastic connector that looks like a large telephone-style connector (See fig. 2). A slot allows the RJ-45 to be inserted only one way. RJ stands for Registered Jack, implying that the connector follows a standard borrowed from the telephone industry. This standard designates which wire goes with each pin inside the connector. Fig. 2. RJ-45 connector Shielded Twisted Pair (STP) Cable A disadvantage of UTP is that it may be susceptible to radio and electrical frequency interference. Shielded twisted pair (STP) is suitable for environments with electrical interference; however, the extra shielding can make the cables quite bulky. Shielded twisted pair is often used on networks using Token Ring topology. Coaxial Cable Coaxial cabling has a single copper conductor at its center. A plastic layer provides insulation between the center conductor and a braided metal shield (See fig. 3). The metal shield helps to block any outside interference from fluorescent lights, motors, and other computers.

Fig. 3. Coaxial cable Although coaxial cabling is difficult to install, it is highly resistant to signal interference. In addition, it can support greater cable lengths between network devices than twisted pair cable. The two types of coaxial cabling are thick coaxial and thin coaxial. Thin coaxial cable is also referred to as thinnet. 10Base2 refers to the specifications for thin coaxial cable carrying Ethernet signals. The 2 refers to the approximate maximum segment length being 200 meters. In actual fact the maximum segment length is 185 meters. Thin coaxial cable is popular in school networks, especially linear bus networks. Thick coaxial cable is also referred to as thicknet. 10Base5 refers to the specifications for thick coaxial cable carrying Ethernet signals. The 5 refers to the maximum segment length being 500 meters. Thick coaxial cable has an extra protective plastic cover that helps keep moisture away from the center conductor. This makes thick coaxial a great choice when running longer lengths in a linear bus network. One disadvantage of thick coaxial is that it does not bend easily and is difficult to install. Coaxial Cable Connectors The most common type of connector used with coaxial cables is the Bayone-Neill-Concelman (BNC) connector (See fig. 4). Different types of adapters are available for BNC connectors, including a T-connector, barrel connector, and terminator. Connectors on the cable are the weakest points in any network. To help avoid problems with your network, always use the BNC connectors that crimp, rather than screw, onto the cable.

Fig. 4. BNC connector Fiber Optic Cable Fiber optic cabling consists of a center glass core surrounded by several layers of protective materials (See fig. 5). It transmits light rather than electronic signals eliminating the problem of electrical interference. This makes it ideal for certain environments that contain a large amount of electrical interference. It has also made it the standard for connecting networks between buildings, due to its immunity to the effects of moisture and lighting. Fiber optic cable has the ability to transmit signals over much longer distances than coaxial and twisted pair. It also has the capability to carry information at vastly greater speeds. This capacity broadens communication possibilities to include services such as video conferencing and interactive services. The cost of fiber optic cabling is comparable to copper cabling; however, it is more difficult to install and modify. 10BaseF refers to the specifications for fiber optic cable carrying Ethernet signals.

Fig.5. Fiber optic cable Facts about fiber optic cables: •	Outer insulating jacket is made of Teflon or PVC. •	Kevlar fiber helps to strengthen the cable and prevent breakage. •	A plastic coating is used to cushion the fiber center. •	Center (core) is made of glass or plastic fibers. Fiber Optic Connector The most common connector used with fiber optic cable is an ST connector. It is barrel shaped, similar to a BNC connector. A newer connector, the SC, is becoming more popular. It has a squared face and is easier to connect in a confined space. Ethernet Cable Summary Specification	Cable Type	Maximum length 10BaseT	Unshielded Twisted Pair	100 meters 10Base2	Thin Coaxial	185 meters 10Base5	Thick Coaxial	500 meters 10BaseF	Fiber Optic	2000 meters 100BaseT	Unshielded Twisted Pair	100 meters 100BaseTX	Unshielded Twisted Pair	220 meters

Wireless LANs Not all networks are connected with cabling; some networks are wireless. Wireless LANs use high frequency radio signals, infrared light beams, or lasers to communicate between the workstations and the file server or hubs. Each workstation and file server on a wireless network has some sort of transceiver/antenna to send and receive the data. Information is relayed between transceivers as if they were physically connected. For longer distance, wireless communications can also take place through cellular telephone technology, microwave transmission, or by satellite. Wireless networks are great for allowing laptop computers or remote computers to connect to the LAN. Wireless networks are also beneficial in older buildings where it may be difficult or impossible to install cables. The two most common types of infrared communications used in schools are line-of-sight and scattered broadcast. Line-of-sight communication means that there must be an unblocked direct line between the workstation and the transceiver. If a person walks within the line-of-sight while there is a transmission, the information would need to be sent again. This kind of obstruction can slow down the wireless network. Scattered infrared communication is a broadcast of infrared transmissions sent out in multiple directions that bounces off walls and ceilings until it eventually hits the receiver. Networking communications with laser are virtually the same as line-of-sight infrared networks. Wireless LANs have several disadvantages. They provide poor security, and are susceptible to interference from lights and electronic devices. They are also slower than LANs using cabling. Installing Cable - Some Guidelines When running cable, it is best to follow a few simple rules: •	Always use more cable than you need. Leave plenty of slack. •	Test every part of a network as you install it. Even if it is brand new, it may have problems that will be difficult to isolate later. •	Stay at least 3 feet away from fluorescent light boxes and other sources of electrical interference. •	If it is necessary to run cable across the floor, cover the cable with cable protectors. •	Label both ends of each cable. •	Use cable ties (not tape) to keep cables in the same location together. What is a Topology? The physical topology of a network refers to the configuration of cables, computers, and other peripherals. Physical topology should not be confused with logical topology which is the method used to pass information between workstations. Logical topology was discussed in the Protocol chapter. Main Types of Physical Topologies The following sections discuss the physical topologies used in networks and other related topics. •	Linear Bus •	Star •	Star-Wired Ring •	Tree •	Considerations When Choosing a Topology •	Summary Chart Linear Bus A linear bus topology consists of a main run of cable with a terminator at each end (See fig. 1). All nodes (file server, workstations, and peripherals) are connected to the linear cable. Ethernet and LocalTalk networks use a linear bus topology. Fig. 1. Linear Bus topology Advantages of a Linear Bus Topology •	Easy to connect a computer or peripheral to a linear bus. •	Requires less cable length than a star topology. Disadvantages of a Linear Bus Topology •	Entire network shuts down if there is a break in the main cable. •	Terminators are required at both ends of the backbone cable. •	Difficult to identify the problem if the entire network shuts down. •	Not meant to be used as a stand-alone solution in a large building. Star A star topology is designed with each node (file server, workstations, and peripherals) connected directly to a central network hub or concentrator (See fig. 2). Data on a star network passes through the hub or concentrator before continuing to its destination. The hub or concentrator manages and controls all functions of the network. It also acts as a repeater for the data flow. This configuration is common with twisted pair cable; however, it can also be used with coaxial cable or fiber optic cable. Fig. 2. Star topology Advantages of a Star Topology •	Easy to install and wire. •	No disruptions to the network then 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. The protocols used with star configurations are usually Ethernet or LocalTalk. Token Ring uses a similar topology, called the star-wired ring. Star-Wired Ring A star-wired ring topology may appear (externally) to be the same as a star topology. Internally, the MAU (multistation access unit) of a star-wired ring contains wiring that allows information to pass from one device to another in a circle or ring (See fig. 3). The Token Ring protocol uses a star-wired ring topology. Tree A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable (See fig. 4). Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs. Fig. 4. Tree topology Advantages of a Tree Topology •	Point-to-point wiring for individual segments. •	Supported by several hardware and software venders. Disadvantages 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 topologies. 5-4-3 Rule A consideration in setting up a tree topology using Ethernet protocol is the 5-4-3 rule. One aspect of the Ethernet protocol requires that a signal sent out on the network cable reach every part of the network within a specified length of time. Each concentrator or repeater that a signal goes through adds a small amount of time. This leads to the rule that between any two nodes on the network there can only be a maximum of 5 segments, connected through 4 repeaters/concentrators. In addition, only 3 of the segments may be populated (trunk) segments if they are made of coaxial cable. A populated segment is one which has one or more nodes attached to it. In Figure 4, the 5-4-3 rule is adhered to. The furthest two nodes on the network have 4 segments and 3 repeaters/concentrators between them. This rule does not apply to other network protocols or Ethernet networks where all fiber optic cabling or a combination of a fiber backbone with UTP cabling is used. If there is a combination of fiber optic backbone and UTP cabling, the rule is simply translated to 7-6-5 rule. Considerations When Choosing a Topology: •	Money. A linear bus network may be the least expensive way to install a 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, expanding a network is easily done by adding another concentrator. •	Cable type. The most common cable in schools is unshielded twisted pair, which is most often used with star topologies. Summary Chart: Physical Topology	Common Cable	Common Protocol Linear Bus	Twisted Pair Coaxial Fiber	Ethernet LocalTalk Star	Twisted Pair Fiber	Ethernet LocalTalk Star-Wired Ring	Twisted Pair	Token Ring Tree	Twisted Pair Coaxial Fiber	Ethernet

What is a Network Operating System? Unlike operating systems, such as DOS and Windows, that are designed for single users to control one computer, network operating systems (NOS) coordinate the activities of multiple computers across a network. The network operating system acts as a director to keep the network running smoothly. The two major types of network operating systems are: •	Peer-to-Peer •	Client/Server Peer-to-Peer Peer-to-peer network operating systems allow users to share resources and files located on their computers and to access shared resources found on other computers. However, they do not have a file server or a centralized management source (See fig. 1). In a peer-to-peer network, all computers are considered equal; they all have the same abilities to use the resources available on the network. Peer-to-peer networks are designed primarily for small to medium local area networks. AppleShare and Windows for Workgroups are examples of programs that can function as peer-to-peer network operating systems. Fig. 1. Peer-to-peer network Advantages of a peer-to-peer network: •	Less initial expense - No need for a dedicated server. •	Setup - An operating system (such as Windows XP) already in place may only need to be reconfigured for peer-to-peer operations. Disadvantages of a peer-to-peer network: •	Decentralized - No central repository for files and applications. •	Security - Does not provide the security available on a client/server network. Client/Server Client/server network operating systems allow the network to centralize functions and applications in one or more dedicated file servers (See fig. 2). The file servers become the heart of the system, providing access to resources and providing security. Individual workstations (clients) have access to the resources available on the file servers. The network operating system provides the mechanism to integrate all the components of the network and allow multiple users to simultaneously share the same resources irrespective of physical location. Novell Netware and Windows 2000 Server are examples of client/server network operating systems. Fig. 2. Client/server network Advantages of a client/server network: •	Centralized - Resources and data security are controlled through the server. •	Scalability - Any or all elements can be replaced individually as needs increase. •	Flexibility - New technology can be easily integrated into system. •	Interoperability - All components (client/network/server) work together. •	Accessibility - Server can be accessed remotely and across multiple platforms. Disadvantages of a client/server network: •	Expense - Requires initial investment in dedicated server. •	Maintenance - Large networks will require a staff to ensure efficient operation. •	Dependence - When server goes down, operations will cease across the network. Examples of network operating systems The following list includes some of the more popular peer-to-peer and client/server network operating systems. •	AppleShare •	Microsoft Windows Server •	Novell Netware 10Base2 - Ethernet specification for thin coaxial cable, transmits signals at 10 Mbit/s (megabits per second) with a distance limit of 185 meters per segment. 10Base5 - Ethernet specification for thick coaxial cable, transmits signals at 10 Mbit/s (megabits per second) with a distance limit of 500 meters per segment. 10BaseF - Ethernet specification for fiber optic cable, transmits signals at 10 Mbit/s (megabits per second) with a distance limit of 2000 meters per segment. 10BaseT - Ethernet specification for unshielded twisted pair cable (category 3, 4, or 5), transmits signals at 10 Mbit/s (megabits per second) with a distance limit of 100 meters per segment. 100BaseT - Ethernet specification for unshielded twisted pair cabling that is used to transmit data at 100 Mbit/s (megabits per second) with a distance limit of 100 meters per segment. 1000BaseTX -Ethernet specification for unshielded twisted pair cabling that is used to trasmit data at 1 Gbit/s (gigabits per second) with a distance limitation of 220 meters per segment. Asynchronous Transfer Mode (ATM) - A network protocol that transmits data at a speed of 155 Mbit/s and higher. It is most often used to interconnect two or more local area networks. AppleTalk - Apple Computer's network protocol originally designed to run over LocalTalk networks, but can also run on Ethernet and Token Ring. AUI Connector (Attachment Unit Interface) - A 15 pin connector found on Ethernet cards that can be used for attaching coaxial, fiber optic, or twisted pair cable. Backbone - A cable to which multiple nodes or workstations are attached. Bit - Binary digit in the binary numbering system. Its value can be 0 or 1. In an 8-bit character scheme, it takes 8 bits to make a byte (character) of data. BNC Connector (Bayone-Neill-Concelman) - Standard connector used to connect 10Base2 coaxial cable. Bridge - Devices that connect and pass packets between two network segments that use the same communications protocol. Cable - Transmission medium of copper wire or optical fiber wrapped in a protective cover. Client/Server - A networking system in which one or more file servers (Server) provide services; such as network management, application and centralized data storage for workstations (Clients). CSMA/CA - Carrier Sense Multiple Access Collision Avoidance is a network access method in which each device signals its intent to transmit before it actually does so. This prevents other devices from sending information, thus preventing collisions from occurring between signals from two or more devices. This is the access method used by LocalTalk. CSMA/CD - Carrier Sense Multiple Access Collision Detection is a network access method in which devices that are ready to transmit data first check the channel for a carrier. If no carrier is sensed, a device can transmit. If two devices transmit at once, a collision occurs and each computer backs off and waits a random amount of time before attempting to retransmit. This is the access method used by Ethernet. Coaxial Cable - Cable consisting of a single copper conductor in the center surrounded by a plastic layer for insulation and a braided metal outer shield. Concentrator - A device that provides a central connection point for cables from workstations, servers, and peripherals. Most concentrators contain the ability to amplify the electrical signal they receive. DIN - A plug and socket connector consisting of a circular pattern of pins in a metal sleeve. This type of connector is commonly seen on keyboards. Dumb Terminal - Refers to devices that are designed to communicate exclusively with a host (main frame) computer. It receives all screen layouts from the host computer and sends all keyboard entry to the host. It cannot function without the host computer. E-mail - An electronic mail message sent from a host computer to a remote computer. End User - Refers to the human executing applications on the workstation. Ethernet - A network protocol invented by Xerox Corporation and developed jointly by Xerox, Intel and Digital Equipment Corporation. Ethernet networks use CSMA/CD and run over a variety of cable types at 10 Mbit/s (megabits per second). Expansion Slot - Area in a computer that accepts additional input/output boards to increase the capability of the computer. Fast Ethernet - A new Ethernet standard that supports 100 Mbit/s using category 5 twisted pair or fiber optic cable. Fiber Distributed Data Interface (FDDI) - A network protocol that is used primarily to interconnect two or more local area networks, often over large distances. Fiber Optic Cable - A cable, consisting of a center glass core surrounded by layers of plastic, that transmits data using light rather than electricity. It has the ability to carry more information over much longer distances. File Server - A computer connected to the network that contains primary files/applications and shares them as requested with the other computers on the network. If the file server is dedicated for that purpose only, it is connected to a client/server network. An example of a client/server network is Novell Netware. All the computers connected to a peer-to-peer network are capable of being the file server. Two examples of peer-to-peer networks are LANtastic and Windows for Workgroups. Gigabit Ethernet - An Ethernet protocol that raises the transmission rates to 1 Gbit/s (gigabits per second). It is primarily used for a high speed backbone of a network. Gigabyte (GB) - One billion bytes of information. One thousand megabytes. Hub - A hardware device that contains multiple independent but connected modules of network and internetwork equipment. Hubs can be active (where they repeat signals sent through them) or passive (where they do not repeat but merely split signals sent through them). Infrared - Electromagnetic waves whose frequency range is above that of microwaves, but below that of the visible spectrum. Intranet - Network internal to an organization that uses Internet protocols. Internet - A global network of networks used to exchange information using the TCP/IP protocol. It allows for electronic mail and the accessing ad retrieval of information from remote sources. LAN (Local Area Network) - A network connecting computers in a relatively small area such as a building. Linear Bus - A network topology in which each node attaches directly to a common cable. LocalTalk - Apple Corporation proprietary protocol that uses CSMA/CA media access scheme and supports transmissions at speeds of 230 kbit/s (Kilobits per second). MAN (Metropolitan Area Network) - A network connecting computers over a large geographical area, such as a city or school district. MAU (Multistation Access Unit) - A Token Ring wiring hub. Modem (Modulator/Demodulator) - Devices that convert digital and analog signals. Modems allow computer data (digital) to be transmitted over voice-grade telephone lines (analog). Multiplexer - A device that allows multiple logical signals to be transmitted simultaneously across a single physical channel. Network Modem - A modem connected to a Local Area Network (LAN) that is accessible from any workstation on the network. Network Interface Card (NIC) - A board that provides network communication capabilities to and from a computer. Network Operating System (NOS) - Operating system designed to pass information and communicate between more than one computer. Examples include AppleShare, Novell NetWare, and Windows NT Server. Node - End point of a network connection. Nodes include any device attached to a network such as file servers, printers, or workstations. Node Devices - Any computer or peripheral that is connected to the network. PCMCIA - An expansion slot found in many laptop computers. Peer-to-Peer Network - A network in which resources and files are shared without a centralized management source. Physical Topology - The physical layout of the network; how the cables are arranged; and how the computers are connected. Point-to-Point - A direct link between two objects in a network. Ports - A connection point for a cable. Protocol -A formal description of a set of rules and conventions that govern how devices on a network exchange information. RAID (Redundant Array of Inexpensive Disks) - A configuration of multiple disks designed to preserve data after a disk casualty. RAM (Random Access Memory) - The working memory of a computer where data and programs are temporarily stored. RAM only holds information when the computer is on. Repeater - A device used in a network to strengthen a signal as it is passed along the network cable. RJ-45 - Standard connectors used for unshielded twisted-pair cable. Router -A device that routes information between interconnected networks. It can select the best path to route a message, as well as translate information from one network to another. It is similar to a superintelligent bridge. SCSI (Small Computer Serial Interface) - An interface controller that allows several peripherals to be connected to the same port on a computer. Segment - Refers to a section of cable on a network. In Ethernet networks, two types of segments are defined. A populated or trunk segment is a network cable that has one or more nodes attached to it. A link segment is a cable that connects a computer to an interconnecting device, such as a repeater or concentrator, or connects an interconnecting device to another interconnecting device. Sneaker-Net - Refers to a manual method of sharing files in which a file is copied from a computer to a floppy disk, transported to a second computer by a person physically walking (apparently wearing sneakers) to the second computer, and manually transferring the file from floppy disk to the second computer. Speed of Data Transfer - The rate at which information travels through a network, usually measured in megabits per second. Star Topology - LAN topology in which each node on a network is connected directly to a central network hub or concentrator. Star-Wired Ring - Network topology that connects network devices (such as computers and printers) in a complete circle. Tape Back-Up - Copying all the data and programs of a computer system on magnetic tape. On tape, data is stored sequentially. When retrieving data, the tape is searched from the beginning of tape until the data is found. Terminator - A device that provides electrical resistance at the end of a transmission line. Its function is to absorb signals on the line, thereby keeping them from bouncing back and being received again by the network. Thicknet - A thick coaxial cable that is used with a 10Base5 Ethernet LAN. Thinnet - A thin coaxial cable that is used with a 10Base2 Ethernet LAN. Token - A special packet that contains data and acts as a messenger or carrier between each computer and device on a ring topology. Each computer must wait for the messenger to stop at its node before it can send data over the network. Token Ring - A network protocol developed by IBM in which computers access the network through token-passing. Usually uses a star-wired ring topology. Topology - There are two types of topology: physical and logical. The physical topology of a network refers to the configuration of cables, computers, and other peripherals. Logical topology is the method used to pass the information between workstations. Issues involving logical topologies are discussed on the Protocol chapter Transceiver (Transmitter/Receiver) - A Device that receives and sends signals over a medium. In networks, it is generally used to allow for the connection between two different types of cable connectors, such as AUI and RJ-45. Tree Topology - LAN topology similar to linear bus topology, except that tree networks can contain branches with multiple nodes. Twisted Pair - Network cabling that consists of four pairs of wires that are manufactured with the wires twisted to certain specifications. Available in shielded and unshielded versions. USB (Universal Serial Bus) Port - A hardware interface for low-speed peripherals such as the keyboard, mouse, joystick, scanner, printer, and telephony devices. WAN (Wide Area Network) - A network connecting computers within very large areas, such as states, countries, and the world. Workgroup - A collection of workstations and servers on a LAN that are designated to communicate and exchange data with one another. Workstation - A computer connected to a network at which users interact with software stored on the network.