6.1 Computer Network Summary
Computer network can be used for numerous services， both for companies and for individuals. For companies， networks of personal computers using shared servers often provide flexibility and a good price/performance ratio. For individuals， networks offer access to a variety of information and entertainment resources.
Roughly speaking， networks can be divided up into LANs， MANs， WANs， and internet works， each with their own characteristics， technologies， speeds， and niches. LANs cover a building， MANs cover a city， and WANs cover a country or continent. LANs and MANs are un-switched （i.e.， do not have routers）; WANs are switched.
Network software consists of protocols， or rules by which processes can communicate. Protocols can be either connectionless or connection-oriented. Most networks support protocol hierarchies， with each layer providing services to the lower layers. Protocol stacks are typically based either on the OSI model or the TCP/IP model. Both of these have network， transport， and application layers， but they differ on the other layers.
Well-known networks have included: LAN， Ethernet， BUS Network， STAR Network， Ring Network， Novell Netware and Novell Inc.
1. LAN
LAN， acronym for Local Area Network is a group of computers and other devices dispersed over a relatively limited area and connected by a communication link that enables any device to interact with any other on the network. LANs commonly include microcomputers and shared （after expensive） resources such as laser printers and large hard disks. Most （modem） LANs can support a wide variety of computers and other devices. Each device must use the proper physical and data-link protocols for the particular LAN， and all devices that want to communicate with each other on the LAN must use the same upper-level communication protocol. Although single LAN is geographically limited， （to a department or an office building， for example） separate LAN can be connected to form larger networks. Similar LANs are linked by bridges， which act as transfer points between networks;dissimilar LANs are linked by gateways， which both transfer data and convert it according to the protocols used by the receiving network.
The devices on a LAN are known as nodes （NODE）， and the nodes are connected by cables through which messages are transmitted. Types of cables include twisted-pair wiring， coaxial cable， or fiber-optic （light-transmitting） cable. Nodes on a LAN can be wired together in any of three basic topologies， known as bus， ring， and star. As implied by their names， a bus network is more or less linear， a ring network forms a loop， and a star network radiates from a central hub.
To avoid potential collisions when two or more nodes attempt to transmit at the same time， LANs use either contention and collision detection or token passing to regular traffic.
2. Ethernet
Ethernet is a local area network developed by Xerox in 1976， originally for linking minicomputers at the Palo Alto Research Center. A widely implemented network from which the IEEE802.3 standard for contention networks was developed. Ethernet uses a bus topology （configuration） and relies on the form of access known as CSMA/CD to regulate traffic on the main communication line. Network nodes are connected by coaxial cable （in either of two varieties， known as thin and thick） or by twisted-pair wiring. Thin Ethernet cable is 5 millimeters （about 0.2 inch） in diameter and can connect network stations over a distance of 300 meters （about 1000 feet）;thick Ethernet cable is 1 centimeter （about 0.4 inch） in diameter and can connect stations up to 1000 meters （about 3300 feet） apart. Information on an Ethernet network is sent in variable-length frames containing delivery and control information plus up to 1500 bytes of data. The original Ethernet standard provides for base band transmission at 10 megabits （10 million bits） per second.
3. BUS Network
Bus Network， in computer science， is a topology （configuration） for a local area network in which all nodes are connected to a main communications line （bus）. On a bus network， each node monitors activity on the line. Messages are detected by all nodes but are accepted only by the node（s） to which they are addressed. Because a bus network relies on a common data “highway”， a malfunctioning node simply ceases to communicate. To avoid collisions that occur when two or more nodes try to use the line at the same time， bus network commonly relies on collision detection or Token Passing to regulate traffic.
4. STAR Network
Star Network is a local area network in which each device （node） is connected to a central computer in a star-shaped configuration （topology）; commonly， a network consisting of a central computer （the hub） surrounded by terminals. In a star network， messages pass directly from a node to the central computer， which handles any further routing （as to another node） that might be necessary. A star network is reliable in the sense that a node can fail without affecting any other node on the network. Its weakness， however， is that failure of the central computer results in a shutdown of the entire network. And because each node is individually wired to the hub， cable costs can be high.
5. Ring Network