How FASTag works: why your car doesn't stop at the toll (System Design Notes)
The full FASTag system design: what's actually on the sticker, how the barrier opens without the bank, the NPCI and bank flow, failure cases, and how to explain it in an interview.
By InternHack Team · · 11 min read · System Design
The toll barrier opens, your car drives through, and your bank hasn't deducted a single rupee yet. The SMS sometimes arrives 20 minutes later, sometimes 2 hours later. That's not a bug. It's the design.
In these notes we break FASTag down like a system design problem: who the actors are, where the data lives, what the barrier opens on, when and how the money is actually debited, and what happens when something fails. At the end there's a ready answer you can give in an interview.
All facts come from the official NPCI/IHMCL toll interface spec (ICD 2.5) and NHAI/news sources, listed below. Keep in mind: this version of ICD 2.5 is from 2020, so some timers may have changed since.
There's no money on the sticker: what a FASTag actually holds
The biggest misconception: "the balance is stored on the sticker". It isn't. The FASTag on your windshield is a passive UHF RFID tag. It has no battery. The lane reader sends out a radio wave, the tag wakes up using that energy, and reflects back (backscatters) whatever is stored on it.
The tag holds three things that matter:
| Memory bank | What it is | Why it matters |
|---|---|---|
| TID | 96-bit chip ID, written and locked by the chip maker, unique to every chip | The chip's true identity |
| EPC / Tag ID | 96 bits (24 hex characters), the unique serial number that identifies the vehicle in the NETC system | The rest of the system looks the vehicle up by this ID |
| User memory | A 512-bit digital signature (P-256 ECC + SHA-256) | Proof that nobody tampered with the tag's data |
That's it. No balance, no bank account number, no KYC. The balance lives in the systems of your issuer bank (whoever gave you the FASTag, e.g. ICICI, HDFC, SBI).
This is also why the viral "someone scanned a FASTag with a smartwatch and stole money" video was false. NPCI clarified that NETC debits can only be initiated by registered merchants (toll plazas, through their acquirer bank). There's simply nothing on the tag that can move money.
System design lesson: Keep the client device (the tag) dumb and cheap. It only holds identity plus a tamper-proof signature. State (the balance) stays server-side.
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Aage ke sections free InternHack account ke saath khulte hain:
- The actors in the system
- What happens in the lane: how the barrier opens without the bank
- After the car leaves: how the money is debited
- When something fails
- Double debits and "my car was at home, but I still got charged"
- Numbers worth remembering
- The design trade-off: availability over consistency
- How to answer this in an interview
Sources
- NPCI/IHMCL, ICD Manual v2.5 (Interface Control Document, revision 28 Feb 2020): https://ihmcl.co.in/wp-content/uploads/2021/07/ICD-2-5-Document.pdf
- NPCI circular on ICD 2.5 mandate (Jan 2023): https://www.npci.org.in/PDF/netc/circular/2023/Circular-Mandate-of-ICD-2.5-Implementation-and-discontinuation-of-ICD-2.4-for-all-NETC-Plazas.pdf
- NHAI clarification on real-time tag status and the Feb 2025 rules: https://www.nbmcw.com/news/nhai-clarifies-no-impact-on-fastag-transactions.html
- NPCI circular NETC 004A (Jan 2025), via BOOM: https://boomlive.in/explainers/blacklisting-chargebacks-penalties-what-are-the-new-fastag-rules-27786
- Wait time 734s to 47s, FASTag share, NH plazas (Rajya Sabha reply): https://swarajyamag.com/amp/story/news-brief/national-highways-toll-fee-collection-more-than-doubles-in-five-years-surpasses-rs-55000-crore
- NETC monthly volume (NPCI data via Dataful): https://dataful.in/datasets/337/
- Active vs issued tags: https://www.outlookmoney.com/news/59-crore-fastags-active-against-1186-crore-issued-gadkari
- Wrong deductions, camera images and complaint channels: https://www.onmanorama.com/news/kerala/2026/07/19/fastag-fraud-recovery.html
- Barrier-less MLFF at Choryasi, NH-48: https://www.tribuneindia.com/news/business/no-congestion-no-waiting-india-launches-first-barrier-less-tolling-system-on-nh-48-in-gujarat/
- MLFF rollout status (Jul 2026): https://www.nbmcw.com/news/roads-highways/over-77-lakh-fastag-annual-passes-issued-nhai-expands-barrier-free-tolling.html
ICD 2.5 is a 2020 document. Some timers may have changed in later circulars.
