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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.

Top Bus Topology

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

Top Physical Implementation Of A Bus Network

Bus Network

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.

Top 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.

Top Physical Bus Cable Limits




10Base2 Network Layout




Top Wiring of the DIX Connector

Pin Ethernet IEEE 802.3
1 Shield Control-in Shield
2 Collision presence+ Control-in A
3 Transmit+ Data-out A
4 Reserved Data-in Shield
5 Receive+ Data-in A
6 Power return Voltage common
7 Reserved Control-out A
8 Reserved Control-out Shield
9 Collision presence- Control-in B
10 Transmit- Data-out B
11 Reserved Data-out Shield
12 Receive- Data-in B
13 Power Voltage  
14 Reserved Voltage Shield
15 Reserved Control-out B
Shell --- Protective Ground

10Base5 Network Layout




10BaseT Network Layout

Top Bus Network Topology Summary

Advantages Disadvantages
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

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