[1] A contended service is a service which offers the users of the network a minimum statistically guaranteed contention ratio, while typically offering peaks of usage of up to the maximum bandwidth supplied to the user.
In the United Kingdom, a Rate Adaptive Digital Subscriber Line (RADSL) connection used to be marketed with a contention ratio between 20:1 and 50:1 within the British Telecom network, meaning that 20 to 50 subscribers, each assigned or sold a bandwidth of up to 8 Mbit/s for instance, may be sharing 8 Mbit/s of downlink bandwidth.
[n 1] With the advent of ADSL2+ (up to 20 Mbit/s service, though in theory, ADSL2+ provides up to 24 Mbit/s), FTTC (Fibre to the Cabinet) offering 40 Mbit/s services and even FTTP (Fibre to the Premises) offering 100 Mbit/s, BT no longer work on "contention ratio" as a planning rule.
In the United States and on satellite internet connections, the contention ratio is often higher, and other formulas are used, such as counting only those users who are actually online at a particular time.
In telephony, 20 users each likely to make a call 10% of the time need 8 lines to ensure that there is less than 0.1% chance of being blocked.