Network active parts
|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). 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 Connectors section for more information on connectors.)||
A concentrator 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 amplify 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:
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.
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.
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
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
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.
Schematic of a linear bus topology
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 electrical
Schematic of a machine-to-machine bus network.
In figure 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.
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.
Schematic of a star network
In figure 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 Tree Network
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. Tree topologies allow for the expansion of an existing network, and enable administrators to configure a network to meet their needs.
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