Network Topology Design - Computer Network

Topologies

The physical appearance of a network is called topology. Topology refers to the arrangement, configuration, or shape of the wires, computers, or other components that are physically and logically interconnected in data communication.

Logical topology ­– how data flows via the network.

Physical topology – how the network is physically structured.

There are 5 types of topologies.

1. BUS topology

2. Mesh topology

3. RING topology

4. STAR topology

5. TREE topology

We are now having a question about what is the need for these 5 topologies. Is there any advantage that you can store for taking out a topology? Yes, it has the advantage of choosing a particular type of topology. I’ll talk about it when I teach topologies.

1. BUS topology

1. The main cable is laid out and every device or computer is connected to it.
2. It transfers the data in a single direction.
3. It has a multipoint connection.
4. The mainline is the backbone of this topology. If it fails, then the whole system will fail.
5. It is used in 802.3 ethernets and 802.4 standard networks.
6. When a node wishes to transmit a message across the network, it sends the message to the whole network regardless of whether the message has any information is used by all of the devices.
7. The most popular access mechanism is CSMA (Carrier Sense Multiple Access). 

Advantage

1. Cabling cost is less. The only main cable is laid out. But if we attach N computers to it, N drop lines are also needed.
2. Easy to install.
3. Can be scalable.
4. If a device fails, then the rest of the devices can work.
5. It is used to build smaller networks.
6. Reliable.
7. Support up to 10 Mbps data speed.  

Disadvantage

1. Troubleshooting is difficult. Because if a device fails, then there is very difficult to find that device.
2. If two devices send the message at the same time, then a collision happens. CSMA/CD and CSMA/CA came into the picture.
3. In a large topology, repeaters are used in the main cable to regenerate the signal.
4. If the main cable fails, then the topology fails.
5. Not secure communication, because the message is broadcasted to every device.
6. The main cable can be extended by using BNC connectors. A loose BNC connector fails the network. 

2. Mesh Topology

1. Every device is connected to every other device in the topology, with a dedicated link.
2. Secure because of a dedicated line.
3. Mesh topology is the Internet used in WAN (World Area Network).
4. It usually works as a wireless network; hence it requires no cabling cost but different wireless signals cost too much.
5. If there are N devices connected in a topology, then N(N-1)/2  cables are required.  
6. If there are N devices connected in a topology, then N-1 ports are required in every device and this is a very complex task. These days, partial mesh topology is used. In partial mesh, some devices are not connected by a dedicated link. If the sender wants to send a message to the receiver, then an alternative path is used by trusted devices.

Advantage

1. Dedicated line hence secure and private also.
2. Reliable because if a computer fails, it will have no impact on the rest of the computers.
3. Easy configuration.
4. Troubleshooting is easy.
5. No traffic problem.
6. Full bandwidth usage. 

Disadvantage

1. Installation is difficult.
2. Most costly in every topology.
3. Maintenance cost is high. 

3. Ring Topology

1. This topology is in the form of a ring. Each device is connected to its two neighbors on two sides of the cable. The last device is connected to the first device.
2. If the bus topology design rear part of the cable is connected to the front part of the cable, then ring topology is made. It’s an IEEE 802.5 standard.
3. Transmission is unidirectional and several repeaters are attached because if device ‘A’ wants to send data to device ‘F’, it will pass through a long route. So, repeaters are attached so that signal does not attenuate.
4. Bi-directional transmission is also there, but it will add one more main cable as a ring and its direction of transmission is opposite to the other main cable. It is called a Dual-Ring Topology.
5. Rings are generally used as a high-performance topology where full bandwidth is used to share videos, high-resolution images, and audio.
6. Ring topology does token passing. A token is a short message that is passed around the ring. Suppose, A device wants to transfer a message to the other device. So, it will capture the token and put the message on the token, and move it on a unidirectional side. Every device receives the token, that is in the token’s way. They match the address of self to the address of the token. If it does not match with anyone, the token moves to the sending device, and sending device will demolish the token. If the address matches, with any other device, then that device will receive the message and sends an acknowledgment to the sending device by generating a new token. The sending device on receiving an acknowledgment generates a new token, that is passed in a unidirectional way until another device captures it to send the message. 

Advantage

1. Cheap because we use a twisted pair cable and it is inexpensive.
2. Easy to install.
3. Can be scalable.
4. It is used to build smaller networks.
5. Every device can share in an equal manner.

Disadvantage

1. Slower network.
2. Troubleshooting is difficult.
3. Adding or removing a device can change the routing table.
4. Less secure.  
5. Network crashes by the failure of one wire. Some will tell you that if a device fails, the whole network crashes in a ring topology. They are referring to this diagram. If a device or a wire fails, then the whole network crashes.

4. Star Topology

1. In star topology, a hub is in the center of the topology, and computers are connected to the hub by a wire.
2. If N computers are used, then N cables are used.
3. Each Computer provides a dedicated channel with the hub.
4. Reliable because if one computer fails then the rest of the computers work. But if the hub failed then the whole system crashes.
5. If the hub is an active device and based on a switch then it can send the information to only one computer to which it belongs. But if the hub is a passive device then it broadcast the information to all the computers, which breached the security.
6. Example – Ethernet Base 10
7. The hub can be of three types

• Layer-1 device – Repeater or Hub
• Layer-2 device – Switch or Bridge
• Layer-3 device – Router or Gateway.

Advantage

1. Easy to use.
2. Scalable.
3. Secure with an active hub.
4. Cheap.
5. Troubleshooting is easy. 
6. Supports 100 Mbps speed. 

Disadvantage

1. If the hub fails, the whole system crashes.
2. Not secure with a passive hub.

5. Tree Topology

1. All the star topology is controlled by a Central Hub.
2. The central hub is an active device. The remaining hubs may be passive or active devices.
3. The central hub has a repeater that increases the signal distance.

Advantage

1. Several devices can be connected by a tree topology.
2. An isolated network can be achieved.
3. Support broadcasting. Example – Cable TV network, Wi-Fi network.
4. Easily managed.
5. Dedicated link to every computer.

Disadvantage

1. Central hub fails, topology fails.
2. High cost as broadcasting happens.