A network shows the sequence and interdependence of the activities in a project in a diagrammatic form using standard symbols. For example, CPM network of a Pumping Station construction project is drawn in fig.




Network Elements

The two basic elements used in a network are activity and event. In addition, where necessary, dummy activities are introduced to maintain logic.

Activity

A project can be broken down into various operations and processes necessary for its completion. Each of these operations or processes, which consume time and possibly, resources is called in activity. Acquiring land, fixing steel, collecting materials, building a wall, constructing a roof and curing the concrete are examples of some of the common activities in a building construction project.
The activities are represented by arrows pointing in the forward direction. The tail of the arrow depicts the commencement of an activity and the arrow head, its termination. The various ways by which the arrow of an activity 'A' can be drawn are:



Note: 
1.     The description of an activity is written above the arrow and its duration in the middle underneath. An activity involving excavation, and needing two units of time for its completion, would be shown as:

      
2.     The length of the arrow is not drawn to scale. As far as possible, the arrow should be sufficiently long so that the description of the activity could be written over it.

3.     Arrows are neither curved nor are they drawn in reverse direction.

Event

It is the state between the completion of a preceding activity and the beginning of the succeeding one. It has no duration; it represents only a point in time. Symbolically, an event is shown by a circle or an ellipse, as:



An event thus refers to a state in the progressing of a project. This state can be named in two ways: either it can describe the completion of the preceding activity, such as 'design approved' for the designing activity; or, it can depict the start of the succeeding activity, such as 'foundation commences'. The states are ordinarily described in terms of the completion of the preceding activity.
The events are labelled numerically to identify them and describe the connecting activities. The procedure for labelling events is covered in the subsequent paragraphs. An activity (i-j) would be shown as:



Note:
1.     The first event of a project is called the start event and the last, the end event.

2.     An event at which an activity starts is termed as the preceding event. The event by which it terminates is called the succeeding event.

3.     An event of significance, such as 'electrification complete', 'buildings ready for occupation' etc., is called a key event. The occurrence of a key event is termed as the milestone.
4.   A key event common to two or more sub networks is called the interface event.

Dummy activity

It is a superimposed activity, which does not represent any specific operation or process. It has zero duration and consumes no resources. Its purpose is twofold:

1.     To provide a logical link to maintain the correct relationship of activities.

2.     To simplify the description of concurrent activities in terms of event numbers.

The dummy activity is drawn like any other activity, but with dotted lines, as:



Example: Consider a simple network consisting of six activities: A, B, C, D, E, and F. The durations of these activities are 2, 3, 2, 1, 3, and 2 days, respectively. The network is shown in fig.




The network logic shows that: activities A, B and C start at the same time; D follows the completion of A and B: E starts after C and D are completed; and, F follows C. The project is over when E and F are completed. The points to note are:

1.     The activities A and B are concurrent (see fig 1 and 2). To enable their description by event numbers, the dummy activity 'G' has been used.


2. To depict relationship among C, D, E, and F, the dummy activity 'H' is introduced in fig.