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Named vs Floating CAD Licensing: The Queueing Theory Model for IT Admins

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Named vs Floating CAD Licensing: The Queueing Theory Model for IT Admins

Over the last five years, major CAD vendors have aggressively eliminated perpetual network licenses in favor of single-user named subscriptions. For enterprise design teams with fluctuating workloads, this shift from shared floating pools to fixed 1:1 seat ratios has inflated annual software expenditure by 200% to 350%.

To determine whether an engineering organization should push back during enterprise renewal negotiations, CAD administrators can apply Operations Research (Queueing Theory) rather than arbitrary guesswork.

Here is the mathematical framework for modeling concurrent CAD seat utilization and finding the optimal licensing mix.

1. The Mathematical Trap of 1:1 Named Subscriptions

In a multidisciplinary firm of 120 engineers, drafters, and project managers, not every team member uses high-end CAD simultaneously:

  • Core Drafters (30%): Active 7–8 hours/day. High concurrency.
  • Project Engineers (45%): Active 1.5–3 hours/day (reviewing markups, section cuts, bill of materials).
  • Managers & Field Supervisors (25%): Active <45 minutes/day (occasional viewing and printing).

When forced into 1:1 Named Subscriptions at $2,000+/year per seat, the effective hourly cost for a casual user skyrockets to over $25/hour of software idle time.

2. Modeling Concurrency with the Erlang C Equation

In network license servers (such as FLEXlm, FlexNet, or Reprise RLM), concurrent checkouts represent an M/M/c/c Queueing System (Erlang loss model):

Given:

  • $\lambda$: Average arrival rate of users requesting a CAD license per hour.
  • $\mu$: Average session service rate ($1 / \mu$ = mean session duration).
  • $A = \lambda / \mu$: Total traffic intensity (in Erlangs).
  • $C$: Number of floating network seats configured on the license server.

The probability of a license denial (the Block Rate $P_b$) is governed by:

$$P_b = \frac{\frac{A^C}{C!}}{\sum_{k=0}^{C} \frac{A^k}{k!}}$$

For a team of 100 designers with an aggregate traffic intensity of $A = 22$ Erlangs, maintaining an enterprise-grade denial rate of $P_b < 0.01$ (99% license availability) requires only 31 floating seats.

Procuring 100 named subscriptions instead of 31 floating seats represents 69 redundant seats, wasting over $138,000 annually.

3. Procurement Action Plan: The Tiered Licensing Strategy

To protect engineering budgets, IT administrators should implement a hybrid three-tier licensing architecture:

  1. Tier 1 (Heavy Production): High-end specialized packages (SolidWorks, Revit, NX) for dedicated full-time modelers.
  2. Tier 2 (General Drafting): High-performance perpetual DWG platforms (such as GstarCAD or ZWCAD) with network dongle or floating server pools for mid-tier engineers.
  3. Tier 3 (Zero-Cloud Client Verification): Lightweight browser-based verification utilities that eliminate license checkouts entirely for casual inspections.

4. Engineering Verification Tools

Before negotiating your next enterprise renewal or purchasing seat packs:

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