Traffic shaping is important in technologies that implement virtual circuits (VCs), such as Frame Relay or ATM, where the Committed Information Rate (CIR) might be less than the physical speed of the interface, the port speed. In such scenarios, it is possible for traffic to burst above the CIR. Depending on the Service Level Agreement (SLA), a carrier might mark excess traffic as Discard Eligible (DE). The carrier might delay or discard excess traffic if congestion is detected within the network of the carrier. This behavior is unacceptable for voice traffic, which must minimize delay and jitter to achieve optimal voice quality. To solve this issue, Frame Relay traffic shaping gives an administrator tools to limit the transmission rate on a Frame Relay virtual circuit to the CIR.

A popular misconception is that voice traffic can be confined to the CIR while data traffic can be allowed to burst. But this does not know how Frame Relay works. No QoS mechanism for Frame Relay is negotiated between service providers and customers. Service providers view all traffic equally and mark any packet that exceeds the CIR as DE, even if the packet is high-priority voice. Therefore, the only way to guarantee optimal performance for voice traffic is to restrict the traffic rate to the CIR.