Networking: Basic Topologies

Network Topologies

What is a Topology?
- The way in which the connections are made among all the computers is called the topology of the network.
- Network topology specifically refers to the physical layout of the network, specially the location of the computers and how the cable is run between them.

The most common topologies are:
Bus
Star
Ring 
Mesh

Bus:
The bus topology is the simplest and most common.
It is often used when a network installation is small, simple, or temporary.
It is a Passive topology. This means that computers on the bus only listen for data being sent, they are not responsible for moving the data from one computer to the next.

- In an active topology network, the computers regenerate signals and are responsible for moving the data through the network.
- On a  bus network, all the computer are connected to a single cable.
- When one computer sends a signal using the cable, all the computers on the network receive the information, but only one (Addressee) accepts it. The rest disregard the message.

Advantages of Bus:
1. The bus is simple, reliable in very small network, and easy to use.
2. The bus requires the least amount of cable to connect the computers together and is therefore less expensive than other cabling arrangements.
3. Failure of one node does not affect the rest of network.

Disadvantages of Bus:
1. Heavy network traffic can slow a bus considerably.
2. A break in the cable or lake of proper termination can bring the network down.
3. It is difficult to troubleshoot a bus.

Appropriateness of the bus topology:
- The network is small
- The network will not be frequently reconfigured
- The least expensive solution is required
- The network is not expected to grow much

Star Topology:
- In a star topology, each device has a dedicated point to point link only to central controller, usually called a hub/server/host.
- Each computer on a star network communicates with a central hub that resends the message appropriate computer(s)
- The hub can be active or passive.
- An active hub regenerate the electrical signal and sends it to all the computers connected to it.

Advantages of Star Topology:
1. It is easy to modify and add new computers to a star network
2. During adding/deleting a node network can function normally.
3. When the capacity of the central hub is exceeded, it can be replaced with one that has a larger number of ports to plug lines into.
4. Provide for centralised monitoring and management of the network.
5. Single computer failure do not necessarily bring down the whole star network.    

Disadvantages of Star Topology:
1. If the central hub fails, the whole network fails to operate.
2. It cost more to cable a star network.
3. Require dedicated server and NOS

Appropriateness of Star Topology:
- It must be easy to add or remove client computer.
- It must be easy to troubleshoot.
- The network is large.
- The network is expected to grow in the future.

Ring Topology:
- In a ring topology, each computer is connected directly to the next computer in line, forming a circle of cable.
- It uses token to pass the information from one computer to another.
- Every computer is connected to the next compute in the ring, and each retransmit what it receives from the previous computer.
- The message flow around the ring in one direction.
- Ring is an active topology.
- There is no termination because there is no end to the ring

Token Passing method:
Token passing a method of sending data in a ring topology
A small packet, called the token passed around the ring to each computer in tern
If a computer has information to send, it modifies the token, adds address information and the data and sends it down the ring.
The information travels around the ring until it either reaches its destination or returns to the sender.
A token can circle a ring 200 meters in diameter at about 10,000 times a second.

Advantages of Ring Topology:
1. All the computers have equal access to the network.
2. Even with many users, network performance is even
3. Allows error checking, and acknowledgement.

Disadvantages of Ring Topology:
1. Failure of one computer can affect the whole network.
2. It is difficult to troubleshoot the ring network.
3. Adding or removing computers disturbs the network.

Appropriateness of Ring Topology:
- The network must operate reasonably under a heavy load
- A higher-speed network is required.
- The network will not be frequently reconfigured.

Mesh Topology:
- In a mesh topology, every devices has a dedicated point to point  link to every other device.
- A fully connected mesh network therefore has n(n-1)/2 physical channels to link n devices.
- To accommodate that many links, every device on the network must have n-1 input/output ports.

Advantages of Mesh Topology:
1. Because of the dedicated link, no traffic between computers.
2. Failure of one node computer not affect rest of the network.
3. Because of the dedicated link privacy and security are guaranteed
4. Point to point links make fault identification and fault isolation easy.

Disadvantages of Mesh Topology:
1. Due to the amount of cabling and number of input output ports, it is expensive.
2. Large space is require to run the cables.

Variations of the Major Topologies:
1. Hybrid Star:
A star network can be extended by placing another star hub where a computer might otherwise go, allowing several more computers or hubs to be connected to that hub.

2. Star Bus:
The star bus topology combine the bus and the star, linking several star hubs together with bus trunks. If one computer fails, the hub can detect the fault and isolate the computer.
If a hub fails, computers connected to it will not be able to communicate, and the bus network will be broken into two segments that can not reach each other.

3. Hybrid Topologies:
Often a network combines several topologies as subnetworks linked together is a large topology.
For instance one department of business may have decided to use a bus topology while another department has a ring.
The two can be connected to each other a central controller in a star topology
When two or more topologies are connected together it forms a hybrid topology