Available Bit Rate (ABR) is the newest and least expensive of the three already covered. ABR is the most complex QoS, but is the cheapest service to buy from a carrier. ABR employs a feedback loop between the adapter and switch. ABR lets an adapter say that it would like, for example, two megabits, but will take whatever it can get. The switch says that it can provide two megabits initially, but the elaborate protocol in ABR allows the switch to lower that bandwidth. Packets from the switch tell the card that its bandwidth is being reduced, and the card can agree to source a reduced amount of data. Later on, the switch might reduce bandwidth again and later increase it. A constant feedback loop guarantees that each side knows the status of the other. Because the adapter always knows the available bandwidth, it doesn't overrun the limit.
ABR is very popular because it resembles a LAN: although in theory it has a certain amount of bandwidth available, in practice data is transferred at a much lower rate. Sometimes it will be fast, and sometimes it will be slow. ABR makes sense because QoS applications are rare today. Customers like the idea of QoS, but might initially still be using Ethernet, which acts like ABR. Therefore, when customers use ATM cards, their applications will often use ABR.
A major advantage of ABR is that it is inexpensive, because the switch is providing whatever bandwidth is available. Net managers can migrate to ATM, and everything they have today will still work because of ABR. And the presence of ATM means that when an application needs the enhanced capabilities of ATM, the speed of ATM will be available, providing migrating users with the best of both worlds.