Telecommunication Essay Research Paper Telecommunication1 IntroductionComputer and — страница 4

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represents a black or white area on the image. The image is sent as a set of zeros and ones and is then reconstructed on the receiving end. 4. LANs 4.1. I Donnwanna File-Share! Network Operating Systems (OS) are actually a group of programs that give computers and peripherals the ability to accept requests for service across a network and give other computers the ability to correctly use those services. Servers share their hard disks, attached peripherals such as printers and optical drives, and communication devices. They inspect requests for proper authorization, check for conflicts and errors and then perform the requested service. There is a multitude of different types of servers. File servers are equipped with large hard drives that are used to share files and information,

as well as whole applications. The file-server software allows shared access to specific segments of the data files under controlled conditions. Print servers accept print jobs sent by anyone on the network. These servers are equipped with spooling software (saving data to disk until the printer is ready to accept it) that is vital in the situations where many requests can pour in at the same time. Network Operating Systems package requests from the keyboard and from applications in a succession of data envelopes for transmission across the network. For example, Novell’s NetWare will package a directory request in an IPX (Internetwork Packet Exchange) packet, and the LAN adapter will then package the IPX request into an Ethernet frame. In each step information about data and

error control data is added to the packet. 4.2. Eight Go In One Comes Out The Network Interface Card or LAN adapter, is an interface between the computer and the network cabling. Within the computer it is responsible for the movement of data between the RAM (Random Access Memory) and the card itself. Externally it is responsible for the control of the flow of data in and out of the network cabling system. Since typically computers are faster than the network, the LAN adapter must also function as a buffer between the two. It is also responsible for the change of the form of data from a wide parallel stream coming in eight bits at a time to a narrow stream moving one bit at a time in and out of the network port. To handle these tasks the LAN adapters are equipped with a

microprocessor and 8-64K of RAM. Some of the cards include sockets for ROM chips called Boot ROM. These chips allow computers without hard drives to boot operating systems from the file server. 4.3. Take Your Turn! Ethernet and Token Ring network adapters use similar systems of electrical signaling over the network cable. These signals are very similar to the Baudot and Morse codes. A technique called Manchester encoding uses voltage pulses ranging from -15v to +15v in order to transmit the zeros and ones. The network cable has only one drawback, it can only carry signals from one network card at a time. So each LAN architecture needs a media-access control (MAC) scheme in order to make the network cards take turns transmitting into the cable. Ethernet cards listen to the traffic

on the cable and transmit only if there is a break in the traffic when the channel is quiet. This technique is called Carrier-Sense Multiple Access With Collision detection (CSMA/CD). With collision detection, if two cards start transmitting at the same time, they see the collision, stop, and resume some time later. Token Ring networks use a much more complex process called token passing. Token Ring cards wait for permission in order to transmit into the cable that forms an electrical loop. The cards use their serial numbers in order to find the master interface card. This card starts a message called a token. When a card with information to send receives the token, it sends the data across the network. After the addressed interface card receives the information and returns it to

the originating card, the token is given back to the master to be passed onto the next card. The ARCnet network uses a very similar system to that of the Token Ring. Instead of using a token, the master card keeps a table of all active cards and polls each one in turn, giving permission to transmit. 4.4. Tied In A Knot Various types of cabling are used to connect the LAN adapters to the servers. Unshielded twisted pair wires offer rather slow speed, are very inexpensive, are small, and can only span very short distances. These cables use the RJ-45 connector. Coaxial cable offers fast speed, is rather expensive, has a medium sized diameter, and can span medium distances. Coaxial cable uses BNC connectors. The shielded twisted pair cable offers fast speed, is more expensive than