Physical Structures of Network

In this article, you’ll learn about the Physical Structures of networks, Types of Connection, and Physical typologies.

In physical structures, we need to define some network attributes.

1. Type of Connection
2. Physical Topology

Type of Connection (Line configuration)

There are two possible types of connections:

1.Point-to-Point:- A point-to-point connection provides a dedicated link between two devices. The entire capacity of the link is reserved for transmission between those two devices. Most point-to-point connections use an actual length of wire or cable to connect the two ends, but other options, such as microwave or satellite links, are also possible.

When you change television channels by infrared remote control, you are establishing a point-to-point connection between the remote control and the television’s control system. 

Example

1. Communication between Television and remote control for changing the channels through infrared ray.
2. A computer connected by a telephone line.

2.Multipoint :- A multipoint (multi drop) connection is one in which more than two specific devices share a single link. In a multipoint environment, the capacity of the channel is shared, either spatially or temporally.

Example

ATM is an example of Multipoint connection

Two types of Multipoint Connections

1. Spatial Sharing: If several computers can share the link simultaneously, its called Spatially shared line configuration.
2. Temporal (Time) Sharing: If users must take turns, it is a timeshared connection.

Physical Topology

The term physical topology refers to the way in which a network is laid out physically two or more devices connect  two or more links form a topology. The topology of a network is the geometric representation of the relationship of all the links and linking devices (usually called nodes) to one another.

There are four basic topologies possible:

1. Mesh Topology
2. Star Topology
3. Bus Topology and
4. Ring Topology

Categories of Topology

1.Mesh Topology

In a mesh topology, every device has a dedicated point-to-point link to every other device. The term dedicated means that the link carries traffic only between the two devices it connects.

To find the number of physical links in a fully connected mesh network with n nodes, we first consider that each node must be connected to every other node. Node 1 must be connected to n – 1 nodes, node 2 must be connected to n – 1 nodes, and finally node n must be connected to n – 1 nodes. We need n(n – 1) physical links. However, if each physical link allows communication in both directions (duplex mode), we can divide the number of links by 2.

In other words, we can say that in a mesh topology, we need n(n -1) /2 duplex-mode links. 

2.Star Topology

In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub. The devices are not directly linked to one another. Unlike a mesh topology, a star topology does not allow direct traffic between devices. The controller acts as an exchange: If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device. 

Advantages of Star Topology

• A star topology is less expensive than a mesh topology.
• In a star, each device needs only one link and one I/O port to connect it to any number of others. This factor also makes it easy to install and reconfigure.
• Far less cabling needs to be housed, and additions, moves, and deletions involve only one connection: between that device and the hub.
•  It includes robustness. If one link fails, only that link is affected. All other links remain active. This factor also lends itself to easy fault identification and fault isolation. As long as the hub is working, it can be used to monitor link problems and bypass defective links. 

Disadvantages of Star Topology

• A star topology is a dependency of the whole topology on one single point, the hub. If the hub goes down, the whole system is dead. 

Although a star requires far less cable than a mesh, each node must be linked to a central hub. For this reason, often more cabling is required in a star than in some other topologies (such as ring or bus). 

3.Bus Topology

The preceding examples all describe point-to-point connections. A bus topology, on the other hand, is multipoint. One long cable acts as a backbone to link all the devices in a network. 

Nodes are connected to the bus cable by drop lines and taps. A drop line is a connection running between the device and the main cable. A tap is a connector that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core. As a signal travels along the backbone, some of its energy is transformed into heat. Therefore, it becomes weaker and weaker as it travels farther and farther. For this reason there is a limit on the number of taps a bus can support and on the distance between those taps. 

Advantages of Bus Topology

• Advantages of bus topology include ease of installation. Backbone cable can be laid along the most efficient path, then connected to the nodes by drop lines of various lengths.
• In this way, a bus uses less cabling than mesh or star topologies. In a star, for example, four network devices in the same room require four lengths of cable reaching all the way to the hub.
• In a bus, this redundancy is eliminated. Only the backbone cable stretches through the entire facility. Each drop line has to reach only as far as the nearest point on the backbone. 

Disadvantages of Bus Topology

Disadvantages include difficult reconnection and fault isolation.

• A bus is usually designed to be optimally efficient at installation. It can therefore be difficult to add new devices.
• Signal reflection at the taps can cause degradation in quality. This degradation can be controlled by limiting the number and spacing of devices connected to a given length of cable. Adding new devices may therefore require modification or replacement of the backbone. 
• In addition, a fault or break in the bus cable stops all transmission, even between devices on the same side of the problem. The damaged area reflects signals back in the direction of origin, creating noise in both directions. 

4.Ring Topology

In a ring topology, each device has a dedicated point-to-point connection with only the two devices on either side of it. A signal is passed along the ring in one direction, from device to device, until it reaches its destination. Each device in the ring incorporates a repeater. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.

Advantages of Ring Topology

• A ring is relatively easy to install and reconfigure. Each device is linked to only its immediate neighbors (either physically or logically). To add or delete a device requires changing only two connections. The only constraints are media and traffic considerations (maximum ring length and number of devices).

• In addition, fault isolation is simplified. Generally in a ring, a signal is circulating at all times. If one device does not receive a signal within a specified period, it can issue an alarm. The alarm alerts the network operator to the problem and its location. 

Disadvantages of Ring Topology

• In Uni-directional Ring, a knowledge packet (token) must undergo all the nodes.
• In a simple ring, a break in the ring (such as a disabled station) can disable the entire network. This weakness can be solved by using a dual ring or a switch capable of closing off the break. 

Ring topology was prevalent when IBM introduced its local-area network Token Ring. Today, the need for higher-speed LANs has made this topology less popular. 

5.Hybrid Topology

It may be a combination of quite two topologies. computer networking, a network structure that contains quite two topologies is understood as hybrid topology. It inherits the benefits and drawbacks of included topologies.

The hybrid topology includes a mixture of bus, mesh, ring topology, star, and tree topology. The mix of topologies depends on the necessity of a corporation.

Types of Hybrid Network Topologies

There are two types of hybrid network topologies counting on the essential requirement of a corporation. Still, the foremost commonly used one is Star-Ring and Star-Bus topologies that structure the hybrid.

Advantages of Hybrid Topology

• The hybrid network combines the advantages of various sorts of topologies
• Can be modified as per requirement
• It is very reliable
• It is extremely flexible.
• It is expensive
• It is easily scalable
• design is complex.

Disadvantages of Hybrid Topology

• Hardware changes are required to attach topology to a different topology.

Thank you for reading, If you have reached so far, please like the article, It will encourage me to write more such articles. Do share your valuable suggestions, I appreciate your honest feedback!