What is a contention ratio? How broadband is shared, explained simply
You've seen "contention ratio 50:1" or "uncontended" and want to know what it means for your speed.
A worked example
Imagine 50 homes, each on a 100 Mb/s plan, sharing a 1,000 Mb/s (1 Gb/s) link. If everyone used their full speed at once they'd need 5,000 Mb/s, so the link is 5 times smaller than the total of the plans: a 5:1 ratio.
| Homes busy at once | Share each (fairly split) | What it feels like on a 100 Mb/s plan |
|---|---|---|
| 5 | 200 Mb/s, more than the plan | Full speed |
| 10 | 100 Mb/s | Full speed |
| 20 | 50 Mb/s | 4K still fine, big downloads take twice as long |
| 40 | 25 Mb/s | One 4K stream just about, downloads slow |
| 50 | 20 Mb/s | Noticeably slower; films may step down to 1080p |
In practice homes aren't all doing the same thing: one is on a 4 Mb/s video call, one is watching 15 Mb/s 4K, one is downloading as fast as it can. The link is shared so that small users get what they need and the biggest ones share what's left (called max-min fairness). Try it in the interactive Wirebook lesson with a street of houses, taps and pipes.
The old numbers: 50:1 and 20:1
In the ADSL era providers often quoted 50:1 for home plans and 20:1 for business plans. Those numbers were never exact promises, and the real bottleneck could be anywhere: the exchange, the provider's links between cities, or its connections to the rest of the internet. Today most home providers don't quote a ratio at all. They watch how full each link gets at the busiest time and upgrade it when it gets close. A provider that upgrades in time feels uncontended; one that doesn't feels slow every evening, whatever its ratio was on paper.
Is full fibre shared?
Yes, but at much larger sizes. Most full-fibre (FTTP) networks use PON: one fibre from the exchange is split with glass splitters to up to about 32 homes (sometimes more).
- GPON: about 2.5 Gb/s down and 1.25 Gb/s up shared by the split, so 32 homes on 500 Mb/s plans are about 6:1 at that point.
- XGS-PON: about 10 Gb/s each way shared, which is why multi-gigabit plans are possible.
Behind the split, the provider's own links are shared again. Cable broadband (DOCSIS) shares capacity between the homes on each node in a similar way. Mobile and home 5G share each mast's capacity between everyone connected to it, which is why they vary so much by time and place.
Does a lower ratio mean faster?
Only at busy times. During the day, a 50:1 link and a 5:1 link both feel fast because few people are using them. In the evening the lower ratio (or simply a provider who keeps links upgraded) keeps speeds closer to the plan. Two things to look at instead of a ratio:
- Peak-time speed: many providers publish an average speed at 8 to 10 pm, and some regulators report it too.
- Minimum guaranteed speed: a figure some providers give at sign-up, below which they must try to fix it.
Uncontended and leased lines
An uncontended service (often a business leased line or "dedicated internet access") reserves its full speed for you all the way to the provider's core network, usually with the same speed up and down and a service-level agreement. It costs far more because the provider can't share that capacity with anyone else.
Related
Broadband speed calculator: what your household needs · Internet slow in the evening · How much broadband speed do I need? · Lag when someone downloads (bufferbloat) · Video keeps buffering · Mbps vs MB/s · Interactive: turn on the taps
Questions
What does a 50:1 contention ratio mean?
Up to 50 customers share capacity that could only give them all their full plan speed if they took turns. It matters only when many of them are busy at the same time, usually in the evening.
What is a good contention ratio?
Lower is better for evening speeds, but home providers rarely publish one now. A published peak-time average speed or a minimum guaranteed speed is a more useful thing to compare.
Is fibre broadband contended?
Yes. Full fibre is usually shared by up to about 32 homes on one split (2.5 Gb/s for GPON, 10 Gb/s for XGS-PON), and the provider's links behind that are shared too. The amounts are much larger than with copper, so it's less often the bottleneck.
What is uncontended broadband?
A service whose full speed is reserved just for you up to the provider's core network, typically a business leased line. It's much more expensive than home broadband.
Why don't providers give everyone dedicated capacity?
Most people use their full speed only for short bursts. Sharing lets providers sell fast plans at home-broadband prices; dedicated capacity costs several times more.
Published .