Topology
Topology provides different configurations that are used to create a network. A network is a group of devices connected by communication paths to exchange information and share resources. While connecting computers together to form a network, the user needs to consider the structure, layout and the cabling requirements. A network can be created by using point-to-point connection or multipoint connection. In a point-to-point connection, only two devices are connected to each other. The user can use a point-to-point connection to transfer data from local network to a remote network through a dedicated line. Examples of point-to-point connection are microwave, satellite and television network. In multipoint connection, many devices share the connection and are called as multi-drop connection.
Topology is a pattern of computer devices and describes the way in which these devices are connected. Topologies can be physical or logical. Physical topology refers to the actual physical structure of the network, while a logical topology determines the way in which the data actually passes through the network from one device to other. Different types of topologies are Single node, Star, Bus, Mesh, Tree and Hybrid.
Single Node Topology
In single node topology, the server or host is having all the information for the network. The terminal sometimes called a dumb terminal does not contain necessary files to function independently is connected to the server. A dumb terminal is the cheapest computer device to use on a network but it relies on the resources of the server. Generally, the host is larger and capable of serving host of terminals.
Basic Structure of Single Node Topology
A single node network includes a single device which is connected to the server. The server is the main device that is used for network data storage. If a node does not contain files and information needed for operation, it is known as a “dumb terminal”. These dumb terminals access the required files from the server, operate on them and return them back to the server after completing the task. Terminals, other than the dumb terminals contain their own files and applications. Such terminals use the server only for backing up files on its hard drive or using its printer.
Advantages and Disadvantages of Single Node Topology
Generally a larger computer is used as a server in single node topology. Such server is capable of being connected to hundreds of terminals.
Advantages of single node topology are:
Ø Easy to install, configure and manage
Ø Least expensive, as a “dumb terminal” is the cheapest computer
Ø Single cable is required
Disadvantages of single node topology are:
Ø Only a single device is connected in the entire network
Ø Dumb terminal is dependent on the server for resources
Bus Topology
In a bus topology, devices share a common backbone cable to send and receive data. A thick co-oxial cable is used to connect all devices. Bus topology uses daisy chain scheme to add more devices in a network. In the daisy chain scheme, device 1 is connected to device 2; device 2 is connected to device 3. The first and last devices are connected to 50 ohm terminator resistor.
Basic structure of Bus Topology
In bus topology, the server is connected at one end of the backbone cable and the other devices are connected at different positions along the backbone cable known as trunk. Terminator is placed at each end of the cable, It is used to remove unsent data from the cable if the destination device is faulty. Data is sent to all the devices along the cable. As the message arrives at each device, it checks the destination address, encode in the message. If the address matches, it accepts the data. If the address does not match, the device ignores the data. At a time, only one device can send data. Due to this number of devices waiting to send data on the bus are more. This results in slowing down the performance of a network.
In bus topology, the devices are not responsible for data transmission from one device to another. They can only listen to the information sent on a network. If one device fails, it does not affect the entire network. In an active topology, devices regenerate signals and take part in transfer of data; hence if one device fails then the entire network is affected.
Every device on a network shares a common cable. When two machines transmit data at the same time the signals collide. Due to this, the information is destroyed. This process is called Collision. Collosion can affect the performance of the network.
Advantages and Disadvantages of Bus Topology
Bus topology is called as a linear bus. It is the simplest and most common method of computer networking. However, bus networks work best within a limited number of devices.
Advantages of bus topology are:
Ø Easy to connect a device to linear bus
Ø Requires less cable
Ø Less expensive
Ø Works better for smaller networks
Disadvantages of bus topology are:
Ø Entire network gets affected if there is a break in the main cable
Ø Terminators are required at both ends of the backbone cable.
Ø Difficult to detect the faulty device
Ø limited number of devices can be attached
Ø Heavy traffic and addition of devices degrades the performance
Ø Expensive than single node topology
Star Topology
In star topology, multiple devices are connected to a central connection point known as hub or switch. Devices are attached to the hub by using either copper cables or fiber optic cables. Star networks provide a cost-effective method for sharing information between different users. Star topology can be used in airline reservation counters and small business offices where employees want an access to common applications and files.
Basic Structure of Star Topology
Each device is connected point-to-point to a central device which is either a hub or switch. Hub manages and controls all the functions of the network. It also acts as a multi port repeater for the data flow. Hubs can be passive or active. Passive hubs are used as a connection point which routes all the traffic towards the devices. It does not amplify or regenerate the signals. They simply take all the packets they receive on a single port and broadcast them across all ports. Active hub contains electronic circuits that regenerate and retransmit the information. Active hubs have all the features of passive hubs, with the added feature of actually watching the data being sent out. Active hubs take a larger role in Ethernet communication by implementing a technology called store and forward where the hubs actually look at the data they are transmitting before sending it. It also repairs certain “damaged” packets.
Data is passed through the hub before reaching the destination. For example, device A wants to transmit data to device B. Device A first sends data to the hub. If the hub is passive, it rebroadcasts the data to all the devices in star network, except device A. Each device has to read the address specified in data to determine if the data is intended for that device. If the address is different, then that data is discarded. If the hub is active, it regenerates the data before it is rebroadcasted.
A physical star network that uses a central hub operates as per logical bus topology. The hub broadcasts data to all segments and thus uses a bus topology for transmission of data. On the other hand, a physical star network using a switch functions as per logical star topology. This is because the switch has a capacity to read the address mentioned in data and to direct the data to the intended receiver.
Advantages and Disadvantages of Star Topology
A star network requires more cable wires, but failure in any cable will affect only that particular device and not the entire LAN. Star network is easy to install and configure. You can detect faults easily and remove the faulty device.
Advantages of star topology are:
Ø Easy to install, configure and manage
Ø Easy to detect faulty device
Ø Easier to expand
Ø Connecting or removing device does not affect the network
Ø Hub provides a centralized management
Disadvantages of star topology are:
Ø Increased devices causes traffic which makes the network slow
Ø Requires more cable length than a linear topology
Ø Hub failure leads to network failure
Ø More expensive
Ring Topology
In ring topology, each device is connected to adjacent devices forming a circle. Data flows in only one direction, clockwise or anticlockwise. Each device in the ring topology acts as a repeater. It amplifies the signal transmission and transmits it to the next device.
Basic Structure of Ring Topology
All devices are connected to each other in the form of a closed loop like ring. Each device has only two neighbors for communication purpose one is upstream and another is downstream. Data travels through many points before reaching the destination which makes it inefficient network compared to a star network. For example, consider a ring network having eight devices. If device 1 wants to transmit data to device 3, data must travel from device 1, through device 2 and then to its destination at device 3.
The ring topology is used to implement the token ring network at 4 or 16 mbps. In token ring network, a special signal or a small message called token is continually passed from one device to another. The device which has the token is allowed to transmit data over the network. Data is extracted by the appropriate device and the acknowledgement is sent back to the transmitting device. The receipt is removed and empty token is passed on to another device to transmit data. This topology also sends data on priority basis where message of higher priority are sent before messages with lower priority. The nodes on a token ring are connected electrically or optically in ring structure. But physically they are always connected in a star configuration to a hub which is also called as MAU. It is a transceiver device that provides electrical and optical connection between the devices and the network. As soon as the devices are activated, they are connected to the the ring by MAU. It also removes a non-functioning device from the ring.
In ring topology, single ring is used whereas dual ring network topology consists of two concentric rings, primary and secondary, to connect each device on a network. Primary ring carries data in one direction whereas secondary ring carries data in the opposite direction. Secondary ring is used if primary ring stops working. Thus, the secondary ring is redundant. In dual ring topology, it is difficult to add new devices. For this, each station should have two transmitter ports and two receiver ports.
Advantages and Disadvantages of Ring Topology
In ring topology, collisions are avoided as only one device is allowed to transmit at a time. Bad network cards or improper hub units can still affect the network.
Advantages of ring topology are:
Ø Easier to manage as compared to Bus network
Ø Handles large volume of traffic
Ø Very reliable and offers great speed
Ø Provides a good communication over a long distance
Ø No terminators are required
Disadvantages of ring topology are:
Ø More cable wires compared to bus topology are required
Ø only faulty device affects the entire network
Ø Addition and changes of devices can affect the network
Mesh Topology
In Mesh topology, each device is connected to every other device. A device can send data to all the devices in the network. Data sent by the device can take any possible paths to reach the destination.
Mesh topology is not commonly used because of its complex structure. But it performs well when the devices are spread in the network and do not lie in the same region. WAN is an example of mesh topology. It is an interconnection of multiple LANs. Thus, in WAN, multiple paths are available to reach the destination.
Advantages of Mesh topology are:
Ø Provides redundant paths which help in sending data using an alternate path if a specific path fails to send the data.
Ø Entire network does not get affected by one faulty device
Ø Centralized management is not required as in star topology
Ø Easy to detect the faulty device
Disadvantages of mesh topology are:
Ø More cable is required as compared to bus, star and ring topologies
Ø Each link from one device to another requires an individual NIC
Ø Very expensive as compared to bus, star and ring topologies
Ø Data carrying capacity is underutilized unless all devices are transmitting almost constantly
Ø Difficult to reconfigure
Tree Topology
A tree topology combines the characteristics of linear bus and star topology. In tree topology, group of star networks are connected to the linear bus backbone. Tree topology enables the user to expand and configure the existing network depending on the requirements. Twisted pair cable is commonly used by tree topology. Tree topology is also called as hierarchical structure.
Basic Structure of Tree Topology
A tree topology connects multiple star networks. According to the structure of star topology, all devices are connected to a central hub. There are multiple such hub devices which are connected directly to a tree bus. Thus, the hub is used as the root of the tree of number of devices. Tree networks thus may have branches that contain multiple devices connected point-to-point.
Signal from transmitting device first reaches the hub to which it is connected. Hub then directs this signal to tree bus which then travels along the entire network.
Advantages of tree topology are:
Ø Point-to-point wiring for each device
Ø Easy to detect a faulty device
Ø Easy to expand
Disadvantages of tree topology are:
Ø More difficult to configure as compared to other topologies
Ø fault in the backbone affects the entire network
Ø More expensive than other topologies
Ø Overall length of each segment is limited by the type of cabling used
Hybrid Topology
Hybrid topology is a combination of different network topologies. It is also known as a Special Topology. This topology is useful for corporate offices to link their internal LANs together while adding external networks through Wide Area Networks (WANs). The commonly used combination of topologies is Star-Bus or Star-Ring. A star-bus network consists of two or more star topologies connected using a bus trunk. Whereas a tree topology, can be viewed as a collection of star networks arranged in a hierarchy. Here the function of the central node may be distributed unlike the star topology.
Basic Structure of Hybrid Topology
Hybrid topology is a network where two or more topologies are connected in such a way that the resulting network does not have one of the standard forms. This network is a combination of two or more topologies.
In hybrid topology, multiple physical topologies are combined to form one large network. Each topology has its own strengths and weakness. When different types of topologies are combined, their total strength improves the network performance.
Advantages of Hybrid Topology are:
Ø Useful for implementing larger networks
Ø handles larger volume of traffic
Ø Easy to detect and remove faulty devices
Disadvantages of Hybrid Topology are:
Ø More cabling is required as compared to all other topologies
Ø More expensive than all other topologies
Ø Complex structure
Ø MAU is required
Ø Installation and reconfiguration is difficult
