What is a Network Diagram?
A network diagram is a graphical representation of activities with their relationships. A network diagram is used to develop a schedule and to do schedule calculations to determine critical and near critical paths.
Types of Network Diagrams
There are basically two types of network diagrams that are used in developing a schedule.
- Arrow Network or AoA (Activity on Arrow) or ADM (Arrow Diagramming Method)
- Node Network or AoN (Activity or Node) or PDM (Precedence Diagramming Method)
1) Arrow Network or ADM (Arrow Diagramming Method)
ADM (Arrow diagramming method) is type of network diagram in which activities are represented by arrows, which are connected to each other through nodes. Tail of the arrow represents start of the activity and head of the arrow represents finish of the activity. Length of the arrow is often proportioned to the duration of the activity. Arrow network only shows finish to start or ‘FS’ relationship between activities.
An activity originates and ends at a node. Nodes are used to represent start or end of an activity in a sequence. Starting node of an activity is also referred as activity’s i-node. And ending node of an activity is also referred as activity’s j-node. This method is also known as I-J method of diagramming.
Relationship between activities often needs a dummy activity to correctly represent them. A dummy activity has no duration. It is only introduced to establish complex relationships between activities which cannot be established otherwise. A dummy activity is represented by dotted lines in a network diagram. In below example, activity C will start when both activity B and activity E will finish. And activity F will start when activity E will Finnish. So there is need of a dummy activity to establish relationship between C and E which cannot be established from activities.
Method to create an Arrow Network Diagram
Lets see how to make an arrow network diagram. Consider the following data in which there are 6 activities from A to F. Predecessors of each activity are written next to them.
To create an arrow diagram draw a node to represent project start. In this case, node 1 is representing the project start (see below solution). Remember that you can number the nodes as you like. You can number the nodes as 1, 2, 3 or 10, 20, 30 etc. Now see that activity A has no predecessor. So draw an arrow originating from node 1 and ending at node 2. As B and C both has predecessor A so they must originate from node 2 (end of activity A) and end at their respective nodes (3 and 4 in this case). Similarly draw activities D, E. As activity F has 2 predecessors so the arrow must originate when both D and E will finish, or from node 5. Node 6 is representing end of activity F and as well as end of the project.
Lags and leads can also be shown in ADM network, however a new activity is to be drawn in order to show a lag or lead. In the example below, concrete is placed which requires 7 days cure time till forms can be stripped off. This relationship can be shown in ADM as below.
ADM method has a large contribution in network diagrams in past. However tis method is no longer used. Now the network diagrams are created by precedence diagramming method or PDM. Scheduling softwares like Primavera P6 also used PDM method for network diagramming.
2) Node Network or PDM (Precedence Diagramming Method)
PDM (Precedence diagramming method) is type of network diagram in which activities are represented by nodes, which are connected to each other through arrows. These arrows show relationships between activities. In precedence diagramming method, all four types of relationships can be represented between activities. These relationships are Finish to Start ‘FS’, Start to Start ‘SS’, Finish to Finish ‘FF’ and Start to Finish ‘SF’.
Method to create an Precedence Network Diagram
Lets see how to make a precedence network diagram. Consider the following data in which there are 6 activities from A to F. Predecessors of each activity are written next to them.
To create precedence diagram, first draw a box to represent activity A. You can also start by drawing a box of Project start and connecting it with activity A. As B and C both have predecessor A so now draw two boxes for B and C and connect them with activity A. Similarly draw boxed for D and E and connect them with activity B and C respectively. Now draw a box for activity F and connect it to activities D and E.
Lags and leads can also be shown in PDM network. No separate activity need to be added in PDM to show lags or leads. These can be simply added on the relationship line connecting the activities. In the example below, concrete is placed which requires 7 days cure time till forms can be stripped off. This relationship can be shown in PDM as below.
Comparison of ADM and PDM Diagramming Methods
From above discussion, we can list down some differences of ADM and PDM methods of diagramming. For example:
- PDM diagramming method is easy and is widely used. ADM method is not used now.
- There is no need to include dummy activities to establish complex relationships in PDM. You can establish any relationship through relationship lines. In ADM, dummy activities are needed to establish complex relationships.
- All four type of relationships can be drawn in PDM. In ADM only Finish to Start (FS) relationship can be drawn.
- In PDM, lags and leads can be shown on relationship lines. However in ADM, separate activities need to be added to show lag and leads.
There are two basic types of network diagramming techniques. ADM or Arrow Network Diagram and PDM or Precedence Network Diagram. In ADM, activities are shown as arrows while in PDM, activities are shown as nodes or boxes. PDM is easy to use and can accommodate any complexity level of a schedule. PDM is widely used now a days and it is also used in different scheduling softwares like Primavera P6.