Network and Network topologies

A network is a set of devices interconnected by a communication medium. Each device is referred to as a node. A node can be a computer, printer or any other computing device.

Network criteria

A network must satisfy following criteria.

• Performance – Performance can be measured by transit time (propagation delay) and response time (speed of operation). Performance is decided by many factors such as number of users, type of transmission medium, hardware and software.
• Reliability – A network reliability is measured by accuracy, failure rate, establishment time and robustness.
• Security – Network security concerned with protection of data from unauthorized access.

Physical structure

Physical structure includes some network attributes such as type of connections and topologies.

Types of connections

The nodes in computer network are interconnected by some link. The link can be of two types:

1. Point-to-point
2. Multipoint/Multidrop

1. Point-to-point

• In case of Point-to-point link, there is a dedicated link between two devices/nodes.
• The link capacity is shared between the two nodes only. The link can be a cable or microwave link. Typical example is TV and its remote control.

2. Multipoint

• When two or more devices/nodes share a common link, it is called as multipoint connection. It is also called multidrop.

Network topologies

• The physical topology of a local area network refers to the way in which the stations are physically interconnected.
• Topology is also referred as, the manner in which nodes are geometrically arranged and connected is known as the topology of the network.
• Physical topology of a local area network should have the following desirable features.

1. The topology should be flexible to accommodate changes in physical locations of the stations, increase in the number of stations and increase in the LAN geographic coverage.
2. The cost of physical media and installation should be minimum.
3. The network should not have any single point of complete failures.

• Network topology refers to the physical layout of the network. Each topology has its own strengths and weakness.
• Four types of topologies are commonly used in the network. They are bus, star, ring and mesh topology.

Bus topology

• Bus topology also called horizontal topology.
• In bus topology, multiple devices are connected one by one, by means of connectors or drop cables.
• When one computer sends a signal up (and down) the wire, all the computers on the network receive the information, but only one accepts the information (using addresses matching). The rest discard the message.
• Bus is passive topology because it requires termination. Cable cannot be left unterminated in a bus network.

Advantages of bus

1. Easy to use and easy to install.
2. Needs fewer physical connectivity devices.
3. A repeater can also be used to extend a bus topology network.
4. Low cost.

Disadvantages of bus

1. Heavy network traffic can slow a bus considerably.
2. It is difficult to troubleshoot a bus.
3. Failure of cable affects all devices on the network.
4. Difficult to add new node.

Star topology

• A star topology consists of a number of number of devices connected by point-to-point links to a central hub.
• Easy to control and traffic flow is simple.
• Data travels from the sender to central hub and then to the receiver.

Advantages of star

1. It is easy to modify and add new nodes to a star network without disturbing the rest of the network.
2. Troubleshooting techniques are easy.
3. Failures of any nodes do not bring down the whole star network.

Disadvantages of star

1. If the central hub fails, the whole network fails to operate.
2. Each device requires its own cable segment.
3. Installation can be moderately difficult, especially in the hierarchical network.

Ring topology

• In the ring topology, each computer is connected to the next computer, with the last one connected to the first. The signal travel on the cable in only one direction. Since each computer retransmits what it receives.
• Ring is an active network. Termination is not required.

Advantages of ring

1. Cable failures are easily found.
2. Because every node is given equal access to the token, no one node can monopolize the network.

Disadvantages of ring

1. Adding or removing nodes disrupts the network.
2. It is difficult to troubleshoot a ring network.
3. Failure of one node on the ring can affect the whole network.
4. Cos of cable is more in ring network.

Mesh topology

• The mesh topology has a link between each device in the network. It is more difficult to install as the number of devices increases.
• Mesh networks are easy to troubleshoot.
• Much of the bandwidth available in mesh configuration is wasted.
• Most mesh topology networks are not true mesh networks. Rather, they are true mesh networks, which contain some most important sites with multiple links.

Advantages of mesh

1. Troubleshooting is easy.
2. Isolation of network failures is easy.

Disadvantages of mesh

1. Difficulty of installation.
2. Costly because of maintaining redundant links.
3. Difficult of recognition.

Hybrid topology

A hybrid topology is a combination of two or more topologies. For example, bus topology connected in each brand of star network.