Network Topology
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Network Topology and Architecture
Networks come in a few standard forms, and each form is a compete system of compatible hardware, protocols, transmission media, and topologies.
What is a topology?
The way computers on a LAN are connected is called a topology. A topology is a map of the network. It is a plan for how the nodes will be interconnected and how they will function. The three most widely used topologies are bus, star bus, and ring. When discussing topologies, two basic categories are normally mentioned, physical topology, which defines the way the network looks, and logical topology, which defines the way the data passes among the nodes.
Bus Topology A bus topology connects each computer to a single cable. At each end of the cable is a terminating resistor, or a terminator. A signal is passed back and forth along the cable past the workstations and between the two terminators.The "bus" carries a message from one end of the network to the other. As the bus passes each workstation, the workstation checks the destination address on the message. If the address in the message matches the workstation's address, the workstation receives the message. If the address doesn't match, the bus carries the message to the next workstation, and so on.
One type of bus network, called a local bus, is shown in Figure 1.7. A local bus uses a T-connector to connect the cable to the workstation's network adapter card. A terminator is connected to the last T-connector at each end of the network.
A local bus network connects the network cable directly to
each computer using T-connectors.
A bus network, as shown in Figure 1.8, uses drop cables to connect each workstation to the main "backbone" cable.
A regular bus network uses drop cables
to connect each
computer to the network cable.
Bus topology is a passive network, meaning that workstations only listen for data being sent on the network and are not responsible for moving data from one workstation to the next. If one workstation fails, it doesn't affect the entire LAN. On the other hand, if the connection to one workstation comes loose, or if a cable breaks, the entire cable segment (the length between two terminators) loses its connectivity, causing the entire segment to be nonfunctional until it can be repaired.
The following table lists some advantages and disadvantages of bus networks.
| Advantages | Disadvantages |
| Failure of a single workstation doesn't affect the entire LAN | Cable break can affect large number of users |
| Easy cable connections; flexible | Limited cable length and number of workstations |
| Inexpensive cable and connectors | Difficult to isolate network cabling errors |
| Performance degradation is not graceful |
Star Bus Topology
In a network using a star configuration, each workstation is connected to a special unit called a hub. The hub provides a common connection so that all of the computers can communicate with one another.
Star bus topology uses signal splitters in the hub to send out signals in different directions on the cable connections. Both active and passive hubs are allowed. Active hubs can transmit a stronger signal to feed a longer cable and/or more signal splitters.
Each computer on a star network is connected to a hub.
Hubs however, may be connected together to extend the network.
The following table lists some advantages and disadvantages of star bus topology.
| Advantages | Disadvantages |
| Easy to add new workstations | Failure of hub cripples the workstations connected to the hub |
| Central monitoring and network management |
Token-Ring Topology
On a token-ring network, workstations are situated on a continuous network loop on which a "token" is passed from one workstation to the next. Although the name token ring implies a ring, the token ring is physically implemented as a star and electrically a ring. Workstations are centrally connected to a hub called a Media Access Unit (MAU) and are wired in a star configuration. Workstations use a token to transmit data and must wait for a free token in order to transfer messages.
A computer communicates on a token-ring
network by "grabbing" the
token and sending it around an electric ring.
The token contains the address of the sender and the address of the recipient. When the recipient has copied the information from the received message, it returns the token to the originating workstation to verify that the message was received. The original sender then passes the token to the next workstation in the ring so that workstation can send information over the network.
The following table lists some advantages and disadvantages of a token-ring topology.
| Advantages | Disadvantages |
| Cable failures affect a small number of users | Costly wiring and connections |
| Equal access for all workstations | |
| Graceful performance degradation as the size of the network grows |
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Copyrighted 1997 Lim Pei Mun J
Last updated : 8 June 1998
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