The Network Diagram

The Network Diagram

This is the first in a series of Primavera P6 scheduling articles on the Network Diagram and Relationship Types.

While most people recognize a P6 schedule by its Gantt Chart, the chronological roadmap of a project, the Network Diagram is the true engine behind the dates. It is the primary tool for visualizing project logic, focusing purely on how tasks interconnect rather than where they sit on a calendar.

For further details on the network diagram in P6 Professional, we recommend this Ten Six insight:

The Primavera P6 Professional Network Diagram

The Network Diagram uses the same data found in the Gantt chart (tasks and Work Breakdown Structure (WBS) elements, i.e., deliverables), but it strips away the calendar timescale to highlight the project’s “web” of dependencies. It consists of two fundamental elements:

  • Nodes (Activity Boxes) – These represent individual tasks. They display key data, such as Activity ID and Start/Finish dates, which the system uses to calculate the project’s total duration.
  • Arrows (Logical Relationships) – These lines connect the nodes, defining the specific dependencies that dictate the sequence of work.

Nodes are connected by four types of Arrows or Relationship lines that help the scheduler describe the true scheduling situation.

This article covers the essential elements of a network diagram, providing the practical foundation for building an effective project schedule.

Dynamic Scheduling

A proper Network Diagram is necessary for dynamic scheduling. A scheduler should not include manually assigning the start and finish dates of every task in the schedule. Manually assigning dates negates the purpose of the scheduling software, turning a dynamic tool into a static document.

Instead, the goal is to build a dynamic schedule in which P6 automatically calculates dates from core, high-quality inputs. A dynamic P6 schedule calculates each task’s start and finish time based on its duration and relationships with predecessor and successor activities. Robust schedules leverage these logical relationships across the network to ensure that every task is dynamically tied to the project as a whole.

Elements of a Network Diagram

The network diagram consists of Activity Boxes connected by Arrows. Like the Gantt chart, it displays critical date information; however, these dates are not determined by the box’s physical position on a timeline. Instead, the dates are populated directly inside each Activity Box as the software’s output data fields. This allows the scheduler to focus on the project’s logical flow rather than its placement on a calendar grid.

Activity Box’s Pertinent Information

The minimum information a scheduler provides for an activity is a descriptive task name and its duration. Once these are entered – along with the relationships between them (see the arrows section below) – and the schedule is calculated, P6 populates the remaining fields. Figure 1 illustrates the standard layout of an Activity Box:

Activity Box
Figure 1

Breakdown of the Activity Box

  1. Task (Center Middle) – Descriptive name of the task effort.
  2. Duration (Center Top) – The duration in days to complete the task.
  3. Early Start (ES, Upper Left) – The earliest a task can begin.
  4. Early Finish (EF, Upper Right) – The earliest a task can conclude, calculated as ES + Duration.
  5. Late Start (LS, Lower Left) – The latest date the task can start, calculated as LF – Duration.
  6. Late Finish (LF, Lower Right) – The latest date the task can finish without delaying the project’s completion date.
  7. Total Float – The time in days a task can delay without delaying the project’s finish date.

Arrows

The arrows graphically show how one Activity Box relates to another, always pointing in the direction of workflow (left to right). These connections serve three critical roles:

  • Defining Dependency: They establish the sequence of events by designating predecessors and successors.
  • Driving the Dates: They act as the “sinew” of the schedule, transmitting timing data from one task to the next.
  • Path Generation: Linking these nodes establishes the logical network necessary for P6 to identify the Critical Path.

Requirements for a Practical Working Network

Good logic is the backbone of a healthy schedule. The following requirements are essential for maintaining a network with mathematical and operational integrity.

  1. Eliminate Dangling Activities: Every activity must have at least one predecessor and one successor. Without these links, a task becomes unresponsive to network changes, masking the ripple effect of delays and preventing an accurate completion forecast.
  2. Prioritize Dynamic Logic: Avoid Hard Constraints (like Mandatory Finish) which override network relationships. These constraints “freeze” activities, making them unresponsive to changes in predecessors and preventing automatic date recalculations.
  3. Confirm Driving Logic: Every task must belong to a network where driving predecessors are identifiable. This ensures the software accurately calculates and communicates which specific activities are controlling the project timeline.

Summary

The network diagram is the engine of the schedule; it determines the start and finish dates of tasks. The network diagram is composed of Activity Boxes and connecting Arrows. The minimum required information to calculate start/finish dates is the task duration and logic connections.

But the Activity Box also includes ES, EF, LS, LF and Total Float data. An arrow between boxes displays how one task relates to another. A working, practical network requires eliminating dangling tasks, prioritizing dynamic logic and confirming the driving logic.

In P6 Professional, the network diagram includes WBS swim lanes, which organize the network by deliverables and offer deep visibility into how tasks across different WBS elements interconnect.