Project Network Diagrams
A project network illustrates the relationships between activities / tasks in the project. Showing the activities as nodes or on arrows between event nodes are two main ways to draw those relationships.
Two types of Network Logic:-
v Activity on Arrow (AOA)
v Activity on Node (AON)
Activities on Arrow diagrams are used to show only the start and end of the activties.
Figure 1 : Activity on Arrow (AOA)
• Arcs = only indicate activities
• Nodes = events (activity start or end)
Activity on Node (AON) diagrams place the activity on the node with interconnection arrows showing the dependency relationships within the activities. Since the activity is on a node, the emphasis (and more data) usually can be placed on the activity.
Figure 2 : Activity on Node (AON)
• Arcs = dependencies between activities. (Events not directly represented)
• Nodes = activities
The main difference between AOA & AON is AOA diagrams emphasize the milestones (events); AON networks emphasize the tasks.
Activity on Arrow Advantages:
· An arrow denotes passage of time and therefore is better suited (than a node) to represent a task.
· Scheduling (manually) on an AOA diagram is easier than on an AON diagram.
· In general, AOA needs fewer arrows than AON and therefore the network will be more compact and clear.
Activity on Arrow Disadvantages
· The AOA network can only show finish-to-start relationships. It simply is not possible to show lead and lag except by adding or subtracting times, which makes project tracking difficult if not impossible.
· There are instances when “dummy” activities can occur in an AOA network. Dummy activities are activities that show the dependency of one task on other tasks but for other than technical reasons. For example, a task may be dependent on another because it would be more cost effective to use the same resources for the two; otherwise, the two tasks could be accomplished in parallel. Dummy activities do not have durations associated with them; they simply show that a task has some kind of dependence on another task.
· AOA diagrams are not as widely used as AON simply because the latter are somewhat simpler to use and all project management software programs can accommodate AON networks, where as not all can accommodate AOA networks.
Activity on Node Advantages:
· AON does not have dummy activities since the arrows only resent dependencies.
· AON can accommodate any kind of task relationship, including:
- Finish to start
- Finish to finish
- Start to start
- Start to finish
- Lead and lag
· AON is the most widely used network - diagramming method today and all project management software supports it.
Activity on Node Disadvantages:
· AON diagram do not clearly show time line for a project.
· Large networks require dedicated maintenance and analysis.· Network diagram do not lead themselves to easy reproduction /distribution.
In general, because of the serious constraint, Activity-on-Arrow diagrams are now rarely used.