The job of a LAN is to link many PC's to a mainframe or microcomputer. They can be connected through twisted wire cables, phone lines, fiber optics, and infrared light and radio signals. If a network lacks a dedicated server, it is a peer-to-peer network, where each PC acts like a server to other PC's.
Network Operating Systems
Network operating systems are a combination of programs that give some computers and peripherals the ability to accept requests for service across the network and give other computers the ability to correctly use those services. Servers are computers that share their hard-disk drives, attached peripherals such as printers and CD-ROM drives, and communications circuits. Servers inspect requests for proper authorization, check for conflicts, and then provide the requested service.
File servers store files created by application programs. A file server has a hard disk drive large enough to share. They provide economy because many people can store their data on a single hard drive and access the same file simultaneously. File-server software allows shared access to specific segments of data files under controlled conditions.
Print servers accept print jobs from anyone on the network. Spooling the print jobs (saving them in a disk file until the printer is ready) is an important function of print-server software. The software also shows the status of print jobs waiting, and it recognizes the priorities assigned to specific users.
Client software works with the operating system of the computer so it can route from application programs out to the file and printer servers on the network. An important part of client software is called the redirector. The redirector takes service request it has been programmed to recognize and routes them out of the PC across the network for service. Client software is usually run with file- or printer- software on the same computer. Network communications software takes requests form the client and sends the requests across the network. The software has protocols for addressing, ensuring delivery, and ensuring accuracy. Typical network protocols include Apple's Apple File Protocol, Artisoft's Network Basic Input Output System, and Novell's Sequential Packet Exchange and Internetwork Packet Exchange (SPX and IPX).
All of the nodes, or computers, on Bus (Ethernet) network are attached to the LAN as branches off a common line. Before a node can send data along the network, it listens to make sure there are no signals being transmitted along the network. Then, it sends its message to a transceiver, which broadcasts the message in both directions so it will reach all nodes of the network. The message includes address of its destination and source, the data itself, and error-checking packets. Each node in the LAN checks the who the message is addressed to. Nodes to which the message is not addressed to ignore it. When a node recognizes its address in the message, it reads the data, checks for errors, and sends a message to the sender to let it know it reached safely. If two nodes send information at the same time, the collision of the two messages causes electrical interference that is detected by the senders. The first sender which detects a collision sends a special signal which jams the network and lets all the other nodes that the network is blocked. Transmissions from the nodes stop, and each node waits a random length of time before sending its message again. This process repeats until one of the modes sends a message without colliding with another node's message.
On a token-ring network, all the computers are attached together in a circular network. A token, which is a short all-clear message, always circulates through the network. If a node wants to send a message, it grabs the token, changes the binary code to say it's in use, attaches its message with the address it is intended for, and attaches error-correcting code. Since there is only one token, only one message can be circulated in the network at a time. Since electrical resistance may make the deteriorate the message, each node includes a repeater which re-generates the entire message. Each node inspects the message to see if it is addressed to itself. Once the message reaches the node it is intended for, the node makes a copy of the message and continuous sending it along the ring. Once the message comes back to the sender, the node strips the message and puts back the node's original all-clear signal.
A star network consists nodes on attached to separate lines which lead to the same central station. The central station can connect any line to any other line. A node sends a message to the central station which includes the address of which it is intended for, the data, and error checking code. The central station regularly checks the nodes connected to it, and by taking turns opening and closing switches, it prevents a collision from occurring. In order to keep one node from "hogging" the network, the central station only allows a small amount of data to pass through the switches. The message is put on hold until the station comes around it again.