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Networks
A Network Topology describes the way network cabling is laid out. This doesn't mean the physical layout (how it loops through walls and floors), but how the logical layout looks when viewed in a simplified diagram. • A Bus Networks is one of the most widely used network topologies. A bus network uses a cable to which all the network devices are attached, either directly or through a junction box. The method of attachment depends on the type of bus network, the network protocol, and the speed of the network. The main cable that is used to connect all the devices is called the backbone.
Figure 19 shows a schematic of a bus network. In figure 19, the backbone has a number of junction boxes (transceivers) attached. This allows for a high-speed backbone that is usually also immune to problems with any network card within a device. The junction box allows traffic through the backbone whether or not a device is attached to the junction box. Each end of the backbone, called the bus, is terminated with a block of resistors or a similar electrical device. A popular variation of the bus network topology is found in many small LAN’s. This consists of a length of cable that snakes from machine to machine. There are no transceivers along the network. Instead, each device is connected into the bus directly using a T-shaped connector (Bus Network Connector) on the network interface card. The connector connects the machine to the two neighbours through two cables, one to each neighbour. At the ends of the network, a simple resistor is added to one side of the T-connector to terminate the network electrically.
Figure 20 shows a schematic of a machine-to-machine bus network. In figure 20, each network device has a T-connector attached to the network interface card, leading to the two neighbours. The two ends of the bus are terminated with resistors. Some devices on this type of network use a telephone jack connector, called RJ-45, instead of a T-connector and BNC jacks. In this case, a special adapter must be coupled into the network backbone to accept the telephone jacks. This connector acts much like a transceiver in the true bus network. This machine-to-machine network, also called a peer-to-peer network, is not capable of sustaining the high speeds possible with a backbone-based bus network. A machine-to-machine network is usually built using coaxial cable. Until recently, these networks were limited to a throughput of about 10 Mbps. Recent improvements allow 100 Mbps on this type of network. The problem with this type of machine-to-machine network is that if one machine is taken off the network cable or the network interface card malfunctions, the backbone is broken and must be tied together again with a jumper of some sort. • A Ring Network is a closed network structure in the form of a circle, to which all nodes are connected. Despite misconceptions, there is no physical loop made of the network cable, at least not in the case of the most common form of ring network called Token Ring. The ring name comes from the design of the central network device, which has a loop inside it to which are attached cables for all the devices on the network. With a Token Ring network, a central control unit called a Media Access Unit (MAU) has a cable ring inside it to which all devices are attached.
Figure 21 shows a schematic of a Token Ring network. In figure 21, with the MAU at the centre of the network containing the bus ring. Attached to the ring through junction boxes are all the network devices. There are some true ring networks that have a physically closed loop of the network cable. The ring network has some advantages from a design point of view in that network problems with traffic collisions are handled more easily than on a bus network. A problem is that as with the bus-based machine-to-machine network, any problem with one machine's connection to the network cable can crash the entire network.
Figure 22 shows the token access method in a Token Ring network. In figure 22, a Token Frame is transported in only one direction, until it reaches it’s destination. Thereafter it’s back transported by the Token Ring network until the sending node recognise it and remove it from the ring. • A Star Network is arranged in a central structure with branches radiating from it. The central point of the star-structure is called a concentrator, into which plug all the cables from individuals machines. On machine on the network usually acts as the central controller or network server. A star network has one major advantage over the machine-to-machine bus and ring networks: When a machine is disconnected from the concentrator, the rest of the network continues functioning unaffected.
Figure 23 shows a schematic of a star network. In figure 23, each cable from the concentrator to the device comes out of one of a row of slots or connectors, each identified by a number. Network traffic on a star network proceeds from your machine to the concentrator, then out to the target machine. A star network needs a lot of cable because each machine has to have a cable straight to the concentrator. • A Hub Network is similar to the bus network in that it uses a backbone cable that has a set of connectors on it. The cable is called a backplane in a hub network. Each connector leads to the hub device, which leads off to network devices. This allows a very high-speed backplane to be used, which can be as long and complex as needed. Hub networks are commonly found in large organisations that must support many network devices and need high speed. The hubs that lead off the backplane can support many devices, depending on the type of connector. They can support hundreds of PC each, so a hub network can be used for very large networks. The cost of a hub network is usually very high because of the high-speed backbone and the fast hub devices.
A Local Area Networks (LAN) is a number of devices (computers, printers, and other special peripherals) that are connected to eachother by some form of wiring, all of which are treated as a single entity for TCP/IP configuration. This usually means they share a subnet IP address in common. A LAN enables independent devices to communicate directly with each other through peer-to-peer communications. A LAN does not exceed a span of about 10 kilometre’s and is usually limited to a single building or group of close buildings. LAN’s use a moderate data rate, which means they are slower than mainframe-to-mainframe links. A LAN is a physical and logical accumulation of machines, called nodes, and cables or other communications method's between the machines, called links. Usually the links are simple coaxial or twisted-pair cables. In larger LAN’s, there may have to be amplifiers or repeaters positioned along the cables to ensure the signal is not lost due to lack of strength.
• The transmission medium (the type of cabling used as the link). • The transmission technique (the technique used to handle transmission on the medium). • The access control method (which decides how a machine accesses the medium). The medium is straightforward: • It's a choice between one type of cable or another, dependent primarily on the speed of the network and the adapter cards, as well as the type of network topology. The transmission technique is usually one of two: • Circuit-Switched networks, this networks uses dedicated connections between any two machines (or more properly, between any two nodes). As long as the circuit exists, the sending machine can always talk directly to the destination machine. The connection between the two machines is left in place until no longer needed. This doesn't mean that a cable has to be strung between the two devices, the connection may be made inside a switching box of some sort, which can connect and disconnect between any two machines running into it quickly and flexibly. The connection between two machines is exclusively used by those two machines only, and no other transmission is allowed on the connection.
Figure 25 shows fragmentation and reassemble of a message on a circuit switching network. • Packet-Switched networks, this networks divides all messages on the local area network into small chunks called packets and attaches information to the front of the packet that identifies the recipient. The packets from all the machines on the local area network are placed on a high-bandwidth cable running through all the machines on the network. As a packet moves around the network, each machine analyses the header to see if the packet is for it. If not, it is sent further on.
Figure 26 shows fragmentation and reassemble of a message on a packet switching network. While packet switching is a more flexible approach than circuit switching, it does have a few problems. The primary problem is network traffic. As the number of nodes on the network increases, the network traffic increases too, sometimes reaching the network limit's. Another problem with packet switching is that there is no guarantee of packets getting from source to destination, which is one of the strong points of circuit switching. Some examples of common used networks:
Figure 27 shows a schematic of a Backbone Network.
Figure 28 shows a schematic of a Thinnet Network.
A Wide Area Networks (WAN) is a number of local area networks that are connected to form a large, logical entity. The LAN’s are connected through a gateway or bridge, cabled to each other with a high-speed network cable. WAN’s can be close together physically or separated by a large distance. For example, the design of the WAN is such that machines-to-machines connections are simpler than going out over the internet, and usually much faster. WAN’s can share a subnet IP address, or they can have different subnets. The design of the WAN is more a choice of logical configuration and can be tailored to meet traffic, security, and speed considerations. WAN’s are used by most corporations that maintain multiple offices.
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