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The three main topologies are the bus, ring, and star.

Bus Topology - consists of a segment of cable running from one PC to the next. Each terminator must be connected to the end of each segment.

Advantages of the Bus:
The bus is simple, reliable in very small networks, easy to use, and easy to understand.
The bus requires the least amount of cable to connect the computers together and is therefore less expensive than other cabling arrangements.
It is easy to extend a bus. Two cables can be joined into one longer cable with a BNC barrel connector, making a longer cable and allowing more computers to join the network.

Disadvantages of the Bus:

Heavy network traffic can slow a bus considerably. 
Each barrel connector weakens the electrical signal, and too many may prevent the signal from being correctly received all along the bus.
It is difficult to troubleshoot the bus.

Ring Topology - A continuous path for data with no logical beginning or ending point. When data is transmitted onto the ring, it goes from node to node until the destination node is reached. Originally, data went in one direction only, but new high-speed ring technologies consist of two loops for redundant data transmission in opposite directions. The Ring is a high-performance network.

Advantages of a Ring Network:
Because every computer is given equal access to the token, no one computer can monopolize the network.
The fair sharing of the network allows the network to degrade gracefully (continue to function in a useful, if slower, manner rather than fail once capacity is exceeded) as more users are added.

Disadvantages of the Ring:

Failure of one computer on the ring can affect the entire network
It is difficult to troubleshoot a ring network
Adding or removing computers disrupts the network

 

Star Topology - looks as if it is the shape of a physical star. This is the oldest communications method coming from telephone switching systems. New designs and equipment make this a viable approach today. All the cables run form the computers to a central location where they are all connected by a device called a hub.. Malfunctioning nodes can be located quickly and taken off the system without effecting the operation of the network.

Stars are used:

In concentrated networks, where the endpoints are directly reachable from a central location
When a network expansion is expected.  Expansion can be done very easily by placing another star hub where a computer might otherwise go, allowing several more computers   or hubs to be connected to that hub.
When the greater reliabilty of a star topology is needed.

Advantages of the Star:

Easy to modify and add new computers to a star without disturbing the rest of the network.
The center of a star network is a good place to diagnose network faults.   Intelligent hubs can be used to provide additional monitoring and management ability.
Single computer failures do not necessary bring down the the whole network.
Different cable types can be used in the same network

Disadvantages of the Star:

If the central hub fails, the entire network is down
Many star networks require a device at the central point to rebroadcast or switch network traffic

There are also combinations of basic topologies.

Star Bus  - each ray radiating from the hub is a separate bus segment. It is still terminated at the end of the segment. Multiple hubs can be connected to enlarge the network. The line that connects the hubs is called the backbone.  If one computer fails, the hub can detect the fault and isolate the computer.  If a hub fails, computers connected ot it will not be able to communicate, and the bus network will be broken into two segments that cannot reach each other.

Star Ring - The network cables are laid out like a star, but a ring is implemented in the central hub.   Outlying hubs can be connected to the inner gun, effectively extending a loop of the inner ring.  Token ring is considered a star ring.  Although its toplogy is physically a star, it functions logically in a ring.

Token Ring—As defined in IEEE 802.5, a communications method that uses a token to control access to the LAN. The difference between a token bus and a token ring is that with a token ring, the LAN does not use a master controller to control the token. Instead, each computer knows the address of the computer that should receive the token next. When a computer with the token has nothing to transmit, it passes the token to the next computer in line.

Mesh - uses redundant links between devices.  A true mesh has a link between each device in the netwok.  Most are hybrid mesh networks which have some redundant linkss but not all.  Installation of a mesh is more difficult because of the quantity of links.  A true mesh with six devices would require 15 connections (5+4+3+2+1).  A mesh network is relatively easy to troubleshoot and are very fault tolerant.  Media failure has less impact on a mesh topology than on any other topology.  The redundant links enable data to be sent over several different paths.  Reconfiguration gets progressively more difficult as the number of devices increases.

Advantages of a Mesh:

Major advantage in fault tolerance
Guaranteed communications channel capapcity
Relatively easy to troubleshoot.

Disadvantages of a Mesh:

Difficulaty of installation and reconfiguration
Cost of maintaining redundant link

Study Questions:

1. Suppose the following situation exists: Your company has decided to network some of its existing computers. The manager would like to get the operations staff networked together first, then, if all goes well, move on to the rest of the company. There are 22 computers in operations, and 45 computers in the rest of the company.

Required result: You must have a highly reliable topology that tolerates problems with individual computers
Optional desired results: The lowest setup costs of any topology. The network should be relatively easy to expand.
Proposed solution: Implement a ring topology network.

Does the proposed solution produce the required result and/or the optional desired result?

2. The Internet is a collection of redundant high-speed wide area links. Which network topology does the Internet use?

3. A series of star networks connected together (hubs linked, but not in a loop) is what?

4. What network type uses a MAU?

5. You need to implement a network among 9 computers in one large office.  The computers will be moved in 6 weeks, and there is no cabling installed in the building.  Which network topology should be used? (answer)

6. You have a 15 user, thinnet coaxial bus network that has been   in use for about two years.  Five new client computers have just been added to the network.  Since the clients have been added, no one has been able to access the server.  What two conditions can cause this problem? (answer)

Answers

1. Answer: The proposed solution does not produce the desired result.  A ring network will not tolerate problems with individual computers well. A ring network is more expensive to implement and would require additional down time to expand.  The problem would indicate a star network would be better suited for this application.

2. Answer: Mesh

3. Answer: A star bus

4. Answer: Token Ring

5. Bus

6. The bus network might be missing a terminator and you exceeded the maximum cable length for the network by adding new cable.

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Last updated: April 08, 1999.