Network Topologies
Network Topologies
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A computer network is a group of computers connected through a transmission medium to share software and hardware resources in a workplace, in a city or among different countries in the world. There are three main types of networks: the local area network (LAN), the metropolitan area network (MAN), and the wide area network (WAN), which are categorized according to the area covered. As mentioned earlier, a network consists of computers and a medium that connects them using different networking technologies. The physical layout of a network depends on various factors such as speed, reliability, accuracy, distance etc. This physical arrangement of computers in a network is known as network topology. The following are the major topologies associated with local area networking:
It is a type of physical network design where all computers in the network are connected through a single coaxial cable. When any computer sends a signal through the network, it passes through all the computers, but only the computer to which it is addressed, receives the signal, and the others discard it. Only one computer can send signal at a time, so all the other computers have to wait for their turn, making the performance slow. In this topology, 50-ohm terminators are used at both ends of the network to terminate the signals. Otherwise, when the signal reaches on one end, it bounces back, and the network will not become free.
This topology uses minimum cabling, and is, therefore, the simplest and least expensive topology for small networks. Its expansion is easy, and if a repeater is used, signals can be sent over a long distance. The disadvantage of this topology is that if a lot of computers are added in the network, the resultant heavy traffic can slow down the performance. Bus network is difficult to troubleshoot, as a break at any point along the cable can cause the entire network to be divided into two different networks, which cannot communicate with each other. The result is that the entire network goes down.
Note: Bus topology is used by two Ethernet Coaxial Cable standards namely 10 Base 5 (Thick Ethernet) and 10 Base 2 (Thin Ethernet).
It is a type of physical network design where each computer in the network is connected to a central device, called a hub, through an unshielded twisted-pair (UTP) wire. Signals from the sending computer go to the hub, which amplifies and transmits them to all the computers in the network. This is known as a broadcast star network. In another type of star topology, only the destination computer gets the signal. It is known as a switched star network. In this type of topology, more than one computer can send signals at a time to the central device.
Star topology is the most reliable and widely used topology in the present day concentrated network environments. The advantage of a star topology is that its expansion or contraction is easy. When needed, a new computer can be attached to the central hub, and when a computer needs to be removed, it can be simply detached from the hub without affecting the entire network. Since each workstation has a separate connection to the hub, it is easy to troubleshoot. The disadvantage of this topology is that it is fully dependent on the central hub, whose failure can bring down the entire network. It also requires more cable because all the computers should be connected to a central device, i.e. the hub
It is a type of physical network design where all computers in the network are connected in a closed loop. Each computer or device in a Ring topology network acts as a repeater. It transmits data by passing a token around the network in order to prevent the collision of data between two computers that want to send messages at the same time. If a token is free, the computer waiting to send data takes it, attaches the data and destination address to the token, and sends it. When the token reaches its destination computer, the data is received. Then, the token gets back to the originator. If the originator finds that the message has been received, it removes the message from the token. Now, the token is free, and can be used by the other computers in the network to send data. In practice, only logical ring networks are used where all the computers are attached to a central device known as a multistation access unit (MAU).
The advantage of this topology is that every computer in the ring gets equal opportunity to send its data, and no computer can dominate the network. This also leads to the slow performance of the network, when more computers are added. In this topology, if one computer fails, the entire network goes down. Its troubleshooting is difficult, and addition or removal of any computer can disrupt the entire network.
Note: Ring topology is used by several data-link layer protocols like Ethernet, Token Ring and Fiber Distributed Data Interface (FDDI).
Mesh network topology is a type of physical network design where all the computers in the network are connected to one another with many redundant connections. It provides multiple paths for the data traveling on the network to reach its destination. Mesh topology also provides redundancy in the network. It employs the full mesh and the partial mesh methods to connect the devices. In a full mesh topology network, each computer is connected to all the other computers. In a partial mesh topology network, some of the computers are connected to all the computers, whereas some are connected to only those computers with which they frequently exchange data.
The main advantage of mesh topology is easy troubleshooting. It is easy to find and correct faults in this topology. The other advantage is that communication within the network is guaranteed, as it provides many communication channels. But these networks are difficult to install and configure, and require more cabling.
Note: Mesh topology is not practicable in present day LANs because it requires the computers to have multiple network interface cards to enable them to connect to every computer in the network.
Wireless topology is the latest type of physical network design, which eliminates the need for cables. The most common wireless media are infrared light and radio waves. A wireless LAN consists of a transceiver unit (which is a communication device capable of both transmitting and receiving signals) called a wireless access point (WAP). This unit is connected to servers or directly to the network and other devices using a standard cabled network protocol. In wireless networks, client computers communicate with the transceivers connected to the network, using their own transceivers or wireless NICs.
The main advantages of wireless topology are that it is easy to install and reconfigure. Its installation is inexpensive, as it does not require expensive cabling. It provides mobility to the once fixed networks. The disadvantage of this topology is that the signals sent over the network are not secure, and can be received by any one. A fault in the WAP can bring the whole network down. Other electrical devices using the same spectrum of radio signals can cause interference to a wireless network.
There are also other less used topologies besides the above-mentioned topologies, such as Star Bus topology, Star Ring topology, and Hybrid Mesh topologies, which have limited use. The choice of opting for a topology depends entirely upon the flexibility, pricing, area to be covered, the expected speed, ease of use, and such other factors, which may vary from person to person and from organization to organization.
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