Topology refers to the way in which the network of computers is connected. Each topology is suited to specific tasks and has its own advantages and disadvantages.
The choice of topology is dependent upon
There are FOUR major competing topologies
Most networking software support all topologies.
The bus cable carries the transmitted message along the cable. As the message arrives at each workstation, the workstation computer checks the destination address contained in the message to see if it matches it's own. If the address does not match, the workstation does nothing more.
If the workstation address matches that contained in the message, the workstation processes the message. The message is transmitted along the cable and is visible to all computers connected to that cable.
There are THREE common wiring implementations for bus networks
Physical Implementation Of A Bus
The above diagram shows a number of computers connected to a Bus cable, in this case, implemented as Thin Ethernet. Each computer has a network card installed, which directly attaches to the network bus cable via a T-Connector.
It is becoming common to use 10BaseT (UTP) for implementing Ethernet LANS. Each workstation is wired in star fashion back to a concentrator wiring centre (hub). The hub is a multi-port device supporting up to about 32 ports. One of these ports is connected to a server, or the output of the hub can be connected to other hubs.
Ethernet 802.3: Carrier Sense
Multiple Access with Collision Detection (CSMA/CD)
This protocol is commonly used in bus (Ethernet) implementations.
Multiple access refers to the fact that in bus systems, each station has access to the common cable.
Carrier sense refers to the fact that each station listens to see if no other station is transmitting before sending data.
Collision detection refers to the principle of listening to see if other stations are transmitting whilst we are transmitting.
In bus systems, all stations have access to the same cable medium. It is therefore possible that a station may already be transmitting when another station wants to transmit. Rule 1 is that a station must listen to determine if another station is transmitting before initiating a transmission. If the network is busy, then the station must back off and wait a random interval before trying again.
Rule 2 is that a station which is transmitting must monitor the network to see if another station has begun transmission. This is a collision, and if this occurs, both stations must back off and retry after a random time interval. As it takes a finite time for signals to travel down the cable, it is possible for more than one station to think that the network is free and both grab it at the same time.
CSMA/CD models what happens in the real world. People involved in group conversation tend to obey much the same behavior.
Physical Bus Cable Limits
10Base2 THIN ETHERNET NETWORK LAYOUT
10Base5 THICK ETHERNET NETWORK
Wiring of the DIX Connector
10BaseT UTP NETWORK LAYOUT
Bus Network Topology Summary
|Easy to implement||Limits on cable length and Workstation numbers|
|Low Cost||Difficult to isolate network faults|
|A cable fault affects all workstations|
|As the number of workstations increase, the speed of the network slows down|