## The Network

Basically the network is a flow diagram showing the sequence of operations of a process. Each individual operation is known as an activity and each meeting point or transfer stage between one activity and another is an event or node. If the activities are represented by straight lines and the events by circles, it is very simple to draw their relationships graphically, and the resulting diagram is known as the network. In order to show which activity has to be performed

Figure 20.1

before its neighbour, arrow heads are placed on the straight lines, but it must be explained that the length or orientation of these lines is quite arbitrary.

It can be seen, therefore, that each activity has two nodes or events, one at the beginning and one at the end (Figure 20.1). Thus events 1 and 2 in the figure show the start and finish of activity A. The arrow head indicates that 1 comes before 2, i.e. the operation flows towards 2. We can now describe the activity in two ways:

1 By its activity title (in this case, A)

2 By its starting and finishing event nodes 1-2.

For analysis purposes, the second method must be used. Basic rules

Before proceeding further it may be prudent at this stage to list some very simple but basic rules for network presentation, which must be adhered to rigidly:

1 Where the starting node of an activity is also the finishing node of one or more other activities, it means that all the activities with this finishing node must be completed before the activity starting from that node can be commenced. For example, in Figure 20.2, 1-3(A) and 2-3(B) must be completed before 3-4(C) can be started.

2 Each activity must have a different set of starting and finishing node numbers. This poses a problem when two activities start and finish at the same event node, and means that the example shown in Figure 20.3 is incorrect. In order to apply this rule, therefore, an artificial or 'dummy' activity is introduced into the network (Figure 20.4). This 'dummy' has a duration

Figure 20.2

Wrong

Wrong

Figure 20.5

of zero time and thus does not affect the logic or overall time of the project. It can be seen that activity A still starts at 1 and takes 7 units of time before being completed at event 3. Activity B also still takes 7 units of time before being completed at 3 but it starts at node 2. The activity between 1 and 2 is a timeless dummy.

3 When two chains of activities are inter-related, this can be shown by joining the two chains either by a linking activity or a 'dummy' (Figure 20.5). The dummy's function is to show that all the activities preceding it, i.e. 1-2(A) and 2-3(B) shown in Figure 20.5, must be completed before activity 7-8(F) can be started. Needless to say, activities 5-6(D), 6-7(E) as well as 2-6(G) must also be completed before 7-8(F) can be started.

4 Each activity (except the last) must run into another activity. Failure to do so creates a loose end or 'dangle' (Figure 20.6). Dangles create premature 'ends' of a part of a project, so that the relationship between this end and the actual final completion node cannot be seen. Hence the loose ends must be joined to the final node (in this case, node 6 in Figure 20.7) to enable the analysis to be completed.

5 No chain of activities must be permitted to form a loop, i.e. such a sequence that the last activity in the chain has an influence on the first. Clearly, such a loop makes nonsense of any logic since, if one considers activities 2-3(B), 3-4(C), 4-5(E) and 5-2(F) in Figure 20.8, one finds that B, C and E must precede F, yet F must be completed before B can start. Such a situation cannot occur in nature and defies analysis.

Apart from strictly following the basic rules 1 to 5 set out above, the following points are worth remembering to obtain the maximum benefit from network techniques.

1 Maximize the number of activities which can be carried out in parallel. This obviously (resources permitting) cuts down the overall programme time.

2 Beware of imposing unnecessary restraints on any activity. If a restraint is convenient rather than imperative, it should best be omitted. The use of resource restraints is a trap to be particularly avoided since additional resources can often be mustered - even if at additional cost.

3 Start activities as early as possible and connect them to the rest of the network as late as possible (Figures 20.9 and 20.10). This avoids unnecessary restraints and gives maximum float.

4 Resist the temptation to use a conveniently close node point as a 'staging post' for a dummy activity used as a restraint. Such a break in a restraint could impose an additional unnecessary restraint on the succeeding activity. In Figure 20.11 the intent is to restrain activity E by B and D and activity G by D. However, because the dummy from B uses node 6 as a staging post, activity G is also restrained by B. The correct network is shown in Figure 20.12. It must be remembered that the restraint on G may have to be added at a later stage, so that the effect of B in Figure 20.11 may well be overlooked.

5 When drawing ladder networks (see page 99) beware of the danger of trying to economize on dummy activities as described later (Figures 20.24 and 20.25).

Figure 20.12

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