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Network active parts & Topology

Active parts are main stuffs for running network. 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.

Network active parts

Ethernet card

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.)


Ethernet card.
From top to bottom:
RJ-45, AUI, and BNC 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:

  • Usually configured with 8, 12, or 24 RJ-45 ports
  • Often used in a star or star-wired ring topology
  • Sold with specialized software for port management
  • Also called hubs
  • Usually installed in a standardized metal rack that also may store netmodems, bridges, or routers


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:

Network Topology

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 Network

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.

  • 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.


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.

Tree topology

Diagnostic functions

Several diagnostic functions were added to the TP circuitry. One of them is local loopback that returns transmitted data from AUI(TX) to the receiver output AUI(RX). The schematics above assume use of this function in diagnostic mode only; however, quite a number of devices turn this function on by default. Then, during transmission, transmitted data are being received at the same time. However, if a collision occurs, the loopback is turned off, and true received data are on the output. So, the entire circuit behaves as if coax cable was used instead of TP.

Link test

Link test is the primary diagnostic function. The following figure shows its block schematic:


This circuit checks line connection and polarity. The method is simple - whenever there's nothing to transmit for a long time, usually about 16 ms, the following link test pulse is transmitted.

Picture ideal waveform

corrected waveform
after filtering

Receiving circuits use a time window specifying the minimum (about 4 ms) and maximum (about 60 ms) times from the previous activity (transmission of data or the previous link pulse) of the other side, where link test pulses may be received. If none are detected within about 64 ms, "connection lost" is signalled. Usually, a "Link" LED on the device turns off. Since the polarity and shape of the link test pulse is known, it can be used to detect whether the wire pair has been reversed, therefore reversing signal polarity. This kind of error is detected by polarity detector that can also correct for it. Receiver does not pass link test pulses to its output. Link test is a standard diagnostic tool and should be turned ON whenever possible. Modified versions are used to detect device capabilities; most important, ability to communicate at 100 Mbps and full-duplex. For example, full-duplex capability is signalled by 16 link test pulses in standard timing followed by one "fast" pulse that is about 5 ms apart from the previous one.

SQE (Heartbeat) Test

Signal Quality Test, also called Heartbeat test is a supplemental diagnostic function. It is used to test collision signalization at the AUI. If turned on, collision signal (CD) also signals end of packet transmission, besides collisions. Specifically, the device waits 800 ns (8 bits) after transmission ends, and then sends 8 bits to the collision detection (CD) output. The situation is detailed in the following figure:

Picture TX (AUI)
transmitted signal

CD +/- (AUI)
collision signal

SQE is a supplemental test, and its default setting is OFF, unless required otherwise. Using a MAU with SQE test turned on together with a dumb repeater usually leads to problems.

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