Constant Bit Rate (CBR) guarantees that the entire network, including each intervening switch from origin to destination, will provide an agreed-upon quantity of bandwidth at all times. From the standpoint of a carrier, CBR is expensive to provide, because the guaranteed bandwidth must be reserved for that customer even if the customer isn't using it. The carrier can't let another customer use that bandwidth in the switch, because the purpose of CBR is to provide an absolute guarantee, which also assures that no cells will be lost.
A quality-of-service contract, like any other contract, has two sides. An application using CBR will ask for a certain quantity of bandwidth, such as two megabits per second. The adapter, in turn, has to guarantee that it won't send more than the agreed upon amount of data. If it exceeds that limit, the switch may dump the extra bits. Making sure that no cells are lost depends on the adapter fulfilling its side of the contract. Lossless transmission is possible, but only if the adapter is careful. If not, the switch can simply dump packets that exceed the terms of the contract. For this reason, the adapter on the PC must do traffic shaping, or flow control, so that nothing is lost.
A good example of CBR in action would be a situation with 200 incoming voice lines, with 200 simultaneous vital calls, which are being mapped to 200 hardware voice lines. In the middle is an ATM network. Because there is no control over the incoming data, it must be handled in real time, which would require CBR.