Oversubscription and "Unlimited" Marketing — The Hidden Math of Broadband

Every fibre broadband plan in Singapore claims to be “unlimited.” No data caps, no throttling. But there’s a dirty secret: the physical pipe coming into your neighbourhood is finite. The math only works because ISPs bet you won’t all use it at the same time.

The Contradiction

An ISP buys a 10 Gbps connection to serve a neighbourhood. They sell “unlimited 10 Gbps” to 100 households in that neighbourhood for $30 each.

That’s $3,000/month revenue from 100 customers, but the pipe is only 10 Gbps. If all 100 customers simultaneously ran a speed test, each would get 100 Mbps — not 10 Gbps. The plan is mathematically impossible to deliver to everyone at once.

The Statistical Multiplexing Bet

ISPs rely on statistical multiplexing — the observation that not everyone uses their full bandwidth simultaneously.

At 8 PM in a typical housing block: - 20 households are watching Netflix (~25 Mbps each) - 5 are gaming (~20 Mbps each) - 10 are browsing social media (~5 Mbps each) - 10 are on Zoom calls (~10 Mbps each) - 55 are doing nothing significant (~1 Mbps each)

Total simultaneous usage: 20×25 + 5×20 + 10×5 + 10×10 + 55×1 = ~955 Mbps — well within the 10 Gbps pipe.

At 8% utilization, the pipe is nearly empty. The ISP could sell 10x more “unlimited” plans and still be fine.

The Contention Ratio

The oversubscription ratio (or contention ratio) is how many customers share a port:

ISP	Typical Contention	Peak Headroom
Singtel GPON	~8-12:1	Comfortable
Simba GPON	24:1 (per their own docs)	Tight
Typical residential	20-50:1	Aggressive

Simba’s documentation admits: “a single 10Gbps port is shared by up to 24 users… the theoretical lowest speed at any time could be as low as ~400 Mbps.”

Why Unlimited Works Despite the Math

Three reasons “unlimited” doesn’t collapse the network:

1. Peak Hours Are Short

The 8 PM crunch lasts 2-3 hours. The rest of the day, utilization is under 5%. ISPs buy enough transit to handle the peaks, but they don’t overprovision for worst-case scenarios.

2. Applications Don’t Actually Need 10 Gbps

Activity	Bandwidth Needed
4K Netflix	25 Mbps
Zoom 4K	15 Mbps
Web browsing	1-5 Mbps
Steam download	Burst then idle
YouTube 1080p	5 Mbps

Even heavy users rarely sustain more than 100 Mbps for more than a few minutes.

3. You Can’t Physically Consume It

Your Wi-Fi caps at ~270 Mbps (as I showed in my connection audit). Even wired Gigabit Ethernet caps at ~940 Mbps. To use even 1 Gbps, you need a 2.5 GbE NIC and a willing server on the other end — most people don’t have this.

The Truth

“Unlimited” doesn’t mean unlimited. It means “unlimited at the oversubscribed ratio we’ve calculated you won’t exceed, based on decades of usage statistics.”

The plans work because: - You don’t use your full speed most of the time - Even when you try, your hardware caps you - ISPs have decades of data on peak usage patterns

The only time this breaks is when there’s a massive shift in behavior (like everyone switching to 4K streaming overnight) or when an ISP oversubscribes too aggressively (hello, Simba 24:1).