One-Sentence Summary

Designed and delivered an anti-passback capability for a legacy embedded access control system by modeling secure spaces as a lightweight graph and building on newly added broadcast messaging.

The Context

This project came out of the legacy access control product line at LPS in the 1990s, where new customer requirements had to be met on already-installed hardware with very limited communication and memory resources.

The Challenge

Marketing and sales uncovered an a large opportunity with a government agency that required a feature that we did not have:

Do not allow someone (or their key card) to re-enter an area that they already entered. In other words, prevent someone from passing their key card to the person behind them.

Why It Was Hard

We had a real feature gap for a government customer, but the platform gave us very little room for a heavy solution. The design had to enforce anti-passback behavior with minimal code, minimal data, and no hardware replacement.

The Constraints

It was the 90s, we had limited memory (both for code and data) and bandwidth. Any solution would have to be light weight.

The Approach

Take advantage of topological equivalence and map the secure areas and outside to a connected network graph. The edges are the doors between secure areas. Designate the one unsecured area--outside--as area 0.

This approach requires that we:

• Track the current space that everyone is in. This added one byte per person at each door that they had access to.
• Broadcast a someone's new location when they move from one space to another. Fortunately, we had just added broadcast capability.

My Role

I owned this effort end-to-end from initial design through final testing.

Key Decisions

• Model secure areas and outside as a connected graph so anti-passback could be enforced with a simple location-state design instead of a heavier rule engine.
• Reuse the newly added broadcast capability so location changes could propagate without inventing a second communication mechanism.

The Outcome

The simplicity of the design and prior work to implement broadcasting helped to guarantee a happy customer.

Evidence / Signals of Success

• The design required only one byte of additional state per person at each authorized door.
• Prior broadcast work plus the lightweight design helped us meet the customer's requirement and deliver a satisfactory result.

Resume Bullet Seeds

• Designed and implemented anti-passback capability for a legacy embedded access control system, translating a new government requirement into a lightweight graph-based model that fit severe memory and bandwidth limits.
• Owned delivery end-to-end, building on prior broadcast messaging work to add real-time location enforcement without hardware changes.

Interview Story Angles

• End-to-end ownership on a customer-driven feature request.
• Turning a security requirement into a simpler abstract model.
• Designing for hard memory and bandwidth limits on installed hardware.
• Reusing a recent platform capability to accelerate delivery.

Lessons Learned

• Simple designs tend to be simpler to implement, test, and deliver.