3.4 TOPOLOGIES

 In computer networking, topology refers to the layout of connected devices. You can think of a topology as a structure of a network. This shape does not necessarily correspond to the actual physical layout of the devices on the network.

Network Topology is the study of the arrangement or mapping of the elements (links, nodes, etc.) of a network interconnection between the nodes. It also determines the strategy for physically expanding the network, in future Topologies can be physical or logical. Physical Topology means the physical design of a network including the devices, location and cable installation. Logical Topology refers to the fact that how data actually transfers in a network as opposed to its design.

Following are the considerations for choosing a Topology:

• · Costing: Some kind of topology may be less expensive compared to others in terms of installation (for example Bus topology).
• · Length of cable: It is needed to connect machines in a network.
• · Network scalability: The topology should support future growth of network without much expenditure.
• · Cable types: There are difference types of cable media: unshielded twisted pair, coaxial cable, Fiber optics. The most common cable is unshielded twisted pair, which is most often used with star topology .

Now we shall learn about different types of the topologies like bus, ring, tree, mesh in detail.

3.4.1 Bus Topology

Bus topology is a single common communication to which all the computers are connected. It has a single length of cable with a terminator at each end as shown in the Figure 1. It is a passive topology which means only one computer at a time can send a message. Hence, the number of computers attached to a bus network can significantly affect the speed of the network. A computer must wait until the bus is free before it can transmit. Each node is connected to others nodes. The network operating system keeps track of a unique address of each node and manages the flow of data between machines.

The bus topology is the simplest and mostly widely used with local area network design. A bus topology is considered to be a passive topology. The computers on the bus keep on listening. When they hear data that belongs to them, they receive.

When one device on the network wants to send a broadcast message to another device on the network, it first makes sure no one else on the bus is transmitting, and then it sends information out on the media. All other devices on the network see it, but only the intended recipient accepts and processes it. This is accomplished by using data frames which contain source and destination addresses.

Advantages

a) It is simple, reliable, and easy to be used in a small sized local area network.

b) It requires least amount of cable to connect computers together and is therefore less expensive than other cabling arrangements.

c) It is easy to implement and extend using connectors.

d) If one computer on the bus fails, it does not affect the rest of the traffic on the bus.

Disadvantages

a) In this topology, no two computers can transmit data at the same time.

b) It does not cope well with heavy load which can slow down a bus considerably.

c) Performance degrades as additional computers are added.

d) Terminators are required at both ends of the cable.

e) It is hard to reconfigure once it's set up.

3.4.2 Ring Topology

Ring topology is also known as circular topology. This layout is similar to the linear bus, except that the nodes are connected in a circle as shown in Figure 2. In this topology, each node is connected to two and only two neighboring nodes. The ring does not have an end. It is made of short segments that connect one PC to the next PC and so on Data is accepted from one of the neighbouring nodes and is transmitted onwards to another node .Therefore data travels in only direction from node to node around the rings. Since, each computer retransmits what it receives, a ring is an active network and is not subject to the signal loss problems. There is no termination because there is no end to the ring.

This type of topology can be found in peertopeer networks, in which each machine manages both information processing and the distribution of data files. Examples of such topology:

1) IBM Token Ring

2) Fiber Distributed Data Interface (FDDI)

 

Advantages

a) It is an orderly network where every device has access to the token (control signal) and the opportunity to transmit – because every computer is given equal access to the token, no computer can monopolize the network.

b) It performs better than a star topology under heavy network load.

c) It can create much larger network using Token Ring.

d) It does not require network server to manage the connectivity between the computers.

Disadvantages

a) Network adapter cards and Multi Access Units used in this topology are much more expensive than Ethernet cards and hubs used in bus topology.

b) It is much slower than an Ethernet network under normal load.

c) It is difficult to troubleshoot.

d) One malfunctioning node or bad port in the Multi Access Units can create problems for the entire network

3.4.3 Star Topology

In star topology each computer on a network communicates with a central hub (also called as a concentrator) that resends the message either to all the computers or only to the destination computer. A hub expands one network connection into many. For example a fourport hub connects up to four machines. A single hub is sufficient for a small network, however large networks require multiple hubs. But, it increases hardware and cabling costs. 

Advantages

a) It is more reliable (if one connection fails, it doesn't affect others) –The centre of a star network is a good place to diagnose network faults and if one computer fails whole network is not disturbed. Hub detects the fault and isolates the faulty computer.

b) It is easy to replace, install or remove hosts or other devices, problem can be easily detectedIt is easier to modify or add a new computer without disturbing the rest of the network by simply running a new line from the computer to the central location and plugging it to the hub.

c) Use of multiple cables types in a same network with a hub.

d) It has good performance

Disadvantages

a) It is expensive to install as it require more cable, it costs more to cable a star network because all network cables must be pulled to one central point, requiring more cable length than other networking topologies.

b) Central node dependency, if central hub fails, the whole network fails to operate.

c) Many star networks require a device at the central point to rebroadcast or switch the network traffic.

3.4.4 Tree Topology

It is one of the most common types of network setups that is similar to a bus topology and a star topology. Tree topology integrates multiple star topologies together onto a bus. In its simplest form, only hub devices connect directly to the tree bus and each hub functions as the "root" of a tree of devices as shown in Figure 4.

Advantages

a) It required pointtopoint wiring for individual segments.

b) It is easily supported by several hardware and software vendors.

Disadvantages

a) In this topology, the overall length of each segment is limited by the type of cabling used.

b) If the backbone line breaks, the entire segment goes down.

c) It is more difficult to configure and wire than other topologies

3.4.5 Mesh Topology

A type of network setup where each of the computers and network devices are interconnected with one another as shown in Figure 5 allowing for most transmissions to be distributed, even if one of the connections goes down, the network remains intact.

While the data is traveling on the Mesh Network it is automatically configured to reach the destination by taking the shortest route which means the least number of hops.

Advantages

a) It provides redundant paths between devices and hence it is more reliable and stable.

b) The network can be expanded without disruption to current users.

Disadvantages

a) It requires more cabling than the other LAN topologies.

b) The implementation of this topology is quite complicated.

3.4.6 Hybrid Topology

With the hybrid topology, two or more topologies are combined to form a complete network. For example, a hybrid topology could be the combination of a star and bus topology. These are also the most common in use.

In a starbus topology, several star topology networks are linked to a bus connection. In this topology, if a computer fails, it will not affect the rest of the network. However, if the central component, or hub, that attaches all computers in a star, fails, then you have big problems since no computer will be able to communicate.

Advantages

a) Network expansion is very simple in case of hybrid technology

b) If one client fails, the entire network does not fail.

Disadvantages

a) If one hub fails, all connections to that hub fail, although other hubs continue to function.