Physical Topologies

A topology is basically a map of a network. The physical topology describes how the devices (workstations, printers . . .etc) are interconnected with each other. The cables or connections in a physical topology are often referred to as network media (or physical media). Choosing how computers will be connected in a company’s network is important. A wrong decision in the physical topology makes the media difficult to correct because it is costly and disruptive to change an entire installation once it is in place. The typical organization changes the physical layout and physical media of a network only once about every 10 years, so it is important to choose a configuration that you can live with and that allows for growth. In the following sections, we will look at the fourmost common topologies:

·         Bus

·         Star

·         Ring

·         Mesh

Bus Topology

In bus topology, all computers are attached to a single continuous cable that is terminated at both ends, which is the simplest way to create a physical network. Originally computers were attached to the cable with wire taps. Physical media used to connect all computers is coaxial cable. To connect all PC’s,T (T connecter) is used.

When communicating on a network that uses a bus topology, all computers are able to see the data on the wire. This does not create chaos, though, because the only computer that actually accepts the data is the one to which it is addressed. You can think of a bus network as a small party. Amit is already there, along with 10 other people. Amit would like to tell Abhay something. Amit yells out, “Abhay! Will you grab me a cup of coffee, please?” Everyone in the party can hear Amit, but only Abhay will respond.

Workflow

Suppose computer having IP address 192.16.20.2 wants to send data to computer 192.16.20.6, the process will be as follow:

1.      First, PC having 192.16.20.2 generates the jamming signal and sends to all PC’s.

2.      After receiving the signal issued by PC 192.16.20.2, all remaining computers will get an idea that PC 192.16.20.2 wants to send data.

3.      After completing the task of sending the data, again PC 192.16.20.2 will generate release signal so remaining computers can start to send their data.

Suppose two PC’s sends jamming signals simultaneously, then at that time, PC having higher priority will be selected to send its data. But there is one important point which is essential to discuss is; when PC 192.16.20.2 will send data (packets), it will be seen by all and packets will constantly move to every PC. When packet will reach to last PC, again that same packet will traverse in reverse position and reaches to first PC. At this particular time there are chances of collisions (accidents) of packets. So to eliminate these problems we need to keep two terminators at two ends. By keeping these terminators, packets will be stoppedto move further to any PC.

Advantages

·         It is simple to install

·         It is relatively inexpensive

·         Uses less cables than other topologies

·         If any one computer fails to perform then the network is not disturbed.

·         Additional computers can be easily added and removed.

Disadvantages

·         Multiple computers can’t transfer data at a time

·         Low speed: 1 – 10 mbps

·         It is difficult to troubleshoot

Star Topology

Unlike those in a bus topology, each computer in a star topology is connected to a central point by a separate cable or wireless connection. The central point is a device known by such names as hub, switch, and access point, depending on the underlying technology. It is also known as concentrator. To connect nodes, twisted pair cables are used.

Compared to the bus technology, a star network generally requires more cable, but a failure in any star network cable will only take down one computer’s network access and not the entire LAN. (If the hub fails, however, the entire network also fails.)

Advantages

·         New stations can be added easily and quickly

·         Multiple computers can transfer data simultaneously

·         A single cable failure won’t bring down the entire network

·         It is relatively easy to troubleshoot

Disadvantages

·         Total installation cost can be higher because of the larger number of cables, but prices are constantly becoming more and more competitive.

·         If a concentrator fails, whole network fails.

Ring Topology

In the ring topology, each computer is connected directly to two other computers in the network. Data moves down a one-way path from one computer to another. The good news about laying out cable in a ring is that the cable design is simple. The bad news is that, adding or removing a computer, disrupts the entire network. Also, because you have to break the ring in order to add another station, it is very difficult to reconfigure without bringing down the whole network. For this reason, the physical ring topology is seldom used.

Workflow

Suppose PC having 172.18.20.1 IP addresswants to send data to PC172.18.20.4, it will generate token(packet). This token will be passed to every computer until it reaches to its final destination. When PC 172.18.20.4 receives token, it will extract the data from the token and sends the empty token back to the sender. After receiving this empty token by sender, it will destroy that token. In this topology, data is passed in two ways:

1.      Clock-wise

2.      Counter clock-wise

In clock-wise, computer1 will send data to computer 4 and in counter clock-wise,data will be send from computer4 to computer1. We can move data in any direction unless we have cables in both directions.

Note: The PC which generates the token can only destroy that particular token. Until the token is destroyed by sender, no other PC can send data.

Advantages

·         The ring topology is relatively easy to troubleshoot

·         If a station fails to receive the data from its above neigbour, it will get to know that there is some problem in the network.

Disadvantages

·        Expensive, because multiple cables are needed for each workstation.

·        Difficult to reconfigure

·        No fault tolerant. A single cable fault can bring down the entire network down

·        If there are two directions like counter-wise and counter clock-wise, then cable’s cost would get increase.

·        Speed : 1 – 10 mbps

Mesh Topology

In a mesh topology, a path exists from each station to every station in the network, resulting in the most physical connections per node of any topology. While not usually seen in LANs, a variation on this type of topology – the hybrid mesh – is used on the Internet and other WANs in a limited fashion. In addition, it’s called a hybrid because other types of topologies might be mixed in as well.

A mesh topology can become quite complex as wiring and connections increase exponentially. For every n stations, you will have n(n-1)/2 connections. For example, in a network of 4 computers, you will have 4(4-1)/2 connections, or 6 connections. If your network grows to only 10 computers, you will have 45 connections to manage! Given this impossible overhead, only small systems can be connected this way. The payoff for all this work is a more fail-safe, or fault tolerant, network, at least as far as cabling is concerned.

Advantages

·         The use of dedicated links guarantees that each connection can carry its own data load, thus eliminating the traffic problems that can occur when links must be shared by multiple devices.

·         It is robust. If one of link becomes unusable, it does not incapacitate the entire system.

·         There is advantage of piracy or security. When any message travels along a dedicated line, only the intended recipient sees it. Physical boundaries prevent other users from gaining other access to messages.

Disadvantages

·         First, every device must be connected to every other device, installation and reconnection is difficult.

·         Second, the sheer bulk of the wiring can be greater than the available space (in walls, ceilings, or floors) can accommodate.

·         Finally, the hardware requires to connect each link (I/O ports and cable) can be prohibitively expensive.

Today, the mesh topology is rarely used, and then only in a WAN environment and only because the mesh topology is fault tolerant. Computers or network devices can switch between these multiple, redundant connections if the need arises.

One practical example of a mesh technology is the connection of telephone regional offices in which each regional office needs to be connected to every other regional office.

 Posted by: Chaudhary Amit V.