The term physical topology refers to the way in which a network is laid out physically input/output or more devices connect to a link; two or more links form a topology.
Definition of Network Topology
The term topology in computer networking refers to the way in which a network is laid out physically. Two or more devices connect to a link; two or more links form a topology.
The topology of a network is the geometric representation of the relationship of all links and linking devices (usually called nodes) to one another.
The cost and flexibility of a network installation are partly affected by as is system reliability. Many network topologies are commonly used, but they all have certain similarities.
Information is carried either through space (wireless) or cable. The cable must control the movement of information on the network so that data can be transmitted in a reliable manner.
Network Topology is the schematic description of a network arrangement, connecting various nodes(sender and receiver) through lines of connection.
There are four basic topologies possible: mesh, star, bus, and ring.
The Different types of Network Topologies
- Mesh Topology
- Star Topology
- Bus Topology
- Ring Topology
- Hybrid Topology
1. Mesh Topology
In a mesh topology, every device has a dedicated point-topoint link to every other device.
The term dedicated means that the link carries traffic only between the two devices it connects.
In a mesh topology, Node1 must be connected to n 1 nodes, node2 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. In other words, we can say that in a mesh topology, we need 2 /)1( nn .
Advantages of Mesh Topology
- 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.
- A mesh topology is robust. If one link becomes unusable, it does not incapacitate the entire system.
- There is the advantage of privacy or security. When every message travels along a dedicated line, only the intended recipient sees it.
- Point-to-point links make fault identification and fault isolation easy.
- Data can be transmitted from different devices simultaneously. This topology can withstand high traffic.
- Even if one of the components fails there is always an alternative present. So data transfer doesn‟t get affected.
- Expansion and modification in topology can be done without disrupting other nodes.
Disadvantages of Mesh Topology
- Amount of cabling and the number of I/O ports required.
- The sheer bulk of the wiring can be greater than the available space (in walls, ceilings, or floors) can accommodate.
- The hardware required to connect each link (I/O ports and cable) can be prohibitively expensive.
- There are high chances of redundancy in many of the network connections.
- Overall cost of this network is way too high as compared to other network topologies.
- Set-up and maintenance of this topology is very difficult. Even administration of the network is tough.
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.
Star topology, computers are connected by cable segments to centralized component, called a hub or switch.
Signals are transmitted from the sending computer through the hub or switch to all computers on the network. This topology originated in the early days of computing with computers connected to a centralized mainframe computer.
It is now a common topology in microcomputer networking.
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.
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.
- Other advantages include robustness. If one link fails, only that link is affected. All other links remain active.
- Easy to connect new nodes or devices. In star topology new nodes can be added easily without affecting rest of the network. Similarly components can also be removed easily.
- Centralized management. It helps in monitoring the network.
- Failure of one node or link doesn‟t affect the rest of network. At the same time it is easy to detect the failure and troubleshoot it.
Disadvantages of Star Topology
- The dependency of the whole topology bon 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.
- Too much dependency on central device has its own drawbacks. If it fails whole network goes down.
- The use of hub, a router or a switch as central device increases the overall cost of the network.
- Performance and as well number of nodes which can be added in such topology is depended on capacity of central device.
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.
The bus topology is also known as linear bus. In other words, all the nodes (computers and servers) are connected to the single cable (called bus), by the help of interface connectors.
This central cable is the back bone of the network and every workstation communicates with the other device through this bus. It depends on numerous factors including:
- Hardware capacities of computers on the network
- Number of times computers on the network transmit data
- Type of applications being run on the network
- Types of cable used on the network
- Distance between computers on the network
Advantages Bus Topology
- Bus topology includes ease of installation.
- A bus uses less cabling than mesh or star topologies.
- It is easy to set-up and extend bus network.
- Cable length required for this topology is the least compared to other networks.
- Bus topology very cheap.
- Linear Bus network is mostly used in small networks.
Disadvantages of Bus Topology
- There is a limit on central cable length and number of nodes that can be connected.
- Dependency on central cable in this topology has its disadvantages. If the main cable (i.e. bus) encounters some problem, whole network breaks down.
- Proper termination is required to dump signals. Use of terminators is must.
- It is difficult to detect and troubleshoot fault at individual station.
- Maintenance costs can get higher with time.
- Efficiency of Bus network reduces, as the number of devices connected to it increases.
- It is not suitable for networks with heavy traffic.
- Security is very low because all the computers receive the sent signal from the source.
- Difficult re-connection and fault isolation.
- A fault or break in the bus cable stops all transmission, even between devices on the same side of the problem.
4. Ring Topology
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.
The ring topology connects computers on a single circle of cable. There are no terminated ends. A ring topology connects one host to the next and the last host to the first.
The signal travels around the loop in one direction and pass through each computer.
Unlike the passive bus topology, each computer acts like a repeater to boost the signal and send it on to the next computer.
Because the signal passes through each computer, the failure of one computer can impact the entire network.
A signal is passed along the ring in one direction, from device to device, until it reaches its destination.
Advantages of Ring Topology
- A ring is relatively easy to install and reconfigure. Fault isolation is simplified.
- Cheap to install and expand
- Even when the load on the network increases, its performance is better than that of Bus topology.
- There is no need for network server to control the connectivity between workstations.
- Additional components do not affect the performance of network.
- Each computer has equal access to resources.
Disadvantages of Ring Topology
- Unidirectional traffic can be a disadvantage.
- In a simple ring, a break in the ring (such as a disabled station) can disable the entire network.
- Each packet of data must pass through all the computers between source and destination. This makes it slower than Star topology.
- If one workstation or port goes down, the entire network gets affected.
- Network is highly dependent on the wire which connects different components.
- MAU‟s and network cards are expensive as compared to Ethernet cards and hubs.
5. Hybrid Topology
Before starting about Hybrid topology, we saw that a network topology is a connection of various links and nodes, communicating with each other for transfer of data.
We also saw various advantages and disadvantages of Star, Bus, Ring, Mesh. Hybrid, as the name suggests, is mixture of two different things.
Similarly in this type of topology we integrate two or more different topologies to form a resultant topology which has good points (as well as weaknesses) of all the constituent basic topologies rather than having characteristics of one specific topology.
This combination of topologies is done according to the requirements of the organization.
It is two different types of topologies which is a mixture of two or more topologies.
For example, if there is an existing ring topology in one office department while a bus topology in another department, connecting these two will result in Hybrid topology.
Remember connecting two similar topologies cannot be termed as Hybrid topology. Star-Ring and Star-Bus networks are most common examples of hybrid network.
For example if in an office in one department ring topology is used and in another star topology is used, connecting these topologies will result in Hybrid Topology (ring topology and star topology).
Advantage of Hybrid Topology
- Reliable: Unlike other networks, fault detection and troubleshooting is easy in this type of topology. The part in which fault is detected can be isolated from the rest of network and required corrective measures can be taken, WITHOUT affecting the functioning of rest of the network.
- Scalable: It‟s easy to increase the size of network by adding new components, without disturbing existing architecture.
- Flexible: Hybrid Network can be designed according to the requirements of the organization and by optimizing the available resources. Special care can be given to nodes where traffic is high as well as where chances of fault are high.
- Effective: Hybrid topology is the combination of two or more topologies, so we can design it in such a way that strengths of constituent topologies are maximized while there weaknesses are neutralized. For example we saw Ring Topology has good data reliability (achieved by use of tokens) and Star topology has high tolerance capability (as each node is not directly connected to other but through central device), so these two can be used effectively in hybrid star-ring topology.
Disadvantage of Hybrid Topology
- Complexity of Design: One of the biggest drawbacks of hybrid topology is its design. It is not easy to design this type of architecture and it‟s a tough job for designers. Configuration and installation process needs to be very efficient.
- Costly Hub: The hubs used to connect two distinct networks, are very expensive. These hubs are different from usual hubs as they need to be intelligent enough to work with different architectures and should be function even if a part of network is down.
- Costly Infrastructure: As hybrid architectures are usually larger in scale, they require a lot of cables; cooling systems, sophisticate network devices, etc