1.4 NETWORK TOPOLOGIES

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.

There are different types of the topologies like bus, ring, tree, mesh etc. However, we will discuss only the first three to introduce you to the concepts.

1.4.1 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 1.7. 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 most widely used with local area network design. 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.

1.4.2 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 8. 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 neighboring 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 peer-to-peer 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.

1.4.3 Star Topology

In star topology, each computer on a network communicates with a central hub (also called as a concentrator) that re-sends the message either to all the computers or only to the destination computer. A hub expands one network connection into many. For example, a four-port 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 does not 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 detected-It 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 requires 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.