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If you're familiar with frame relay's CIR (Committed Information Rate), you'll recognize ATM SCR (Sustainable Cell Rate), which is the amount of traffic the end point is allowed to burst into the network. ATM's PCR (Peak Cell Rate) defines the maximum amount of traffic the network will accept under any conditions, with the bottom-line effect of frame relay port speed. For bursty traffic, the MBS (Maximum Burst Size) defines the maximum number of cells accepted over a period of time.

CBR (Constant Bit Rate) traffic looks like a nailed-up connection similar to a leased line, at a rate defined by the PCR parameter. It's good for circuit-emulation services but wastes bandwidth.

VBR (Variable Bit Rate) traffic guarantees that traffic under the SCR will be delivered, but traffic sent above SCR may be discarded if it exceeds the MBS. As with any traffic class, PCR can never be exceeded.

VBR-rt (VBR-real time) is simply VBR traffic with a low-latency (and latency-variation) guarantee, tighter connection-admission control and more congestion control, often designed with the goal of carrying real-time traffic, including compressed voice and video.

ABR (Available Bit Rate) traffic specifies a bandwidth setting but no maximum delay. The network may buffer cells and signal the sender to throttle back. UBR (Unspecified Bit Rate) traffic doesn't offer any guaranteed bandwidth or latency, effectively similar to a frame relay 0-CIR service without delay bounds.