/mcpQuality of Service for Voice over IP VoIP QoS over ATM 39 QoSVoIP.mif VoIP QoS over ATM This section describes how to configure VoIP QoS over ATM and includes the following subsections: • VoIP QoS over ATM Scenario • VoIP QoS over ATM Solution Using Separate Data and Voice ATM PVCs • VoIP QoS over ATM Solution Using Shared Data and Voice ATM PVCs VoIP QoS over ATM Scenario ATM technology has inherent advantages in handling VoIP traffic because of its small, fixed-size cells and class of service (CoS) mechanisms. These advantages do not ensure, however, that VoIP traffic will automatically obtain the QoS it needs from the ATM network carrying it. VoIP traffic will not automatically obtain the QoS it needs because QoS definitions at the IP layer, such as the IP Precedence settings in the packet header, do not automatically match ATM CoS settings, namely traffic class (Constant Bit Rate (CBR), Variable Bit Rate (VBR), Available Bit Rate (ABR), Undefined Bit Rate (UBR)) and traffic parameters such as Sustainable Cell Rate (SCR), Peak Cell Rate (PCR), and burst size. Consequently, after data and voice packets are identified and sorted at the IP layer, the network operator must manually configure the ATM virtual circuits (VCs) to ensure QoS for voice packets across an ATM network. This manual provisioning is time-consuming, labor-intensive, error-prone, and, above all, does not scale as more and more voice traffic is introduced into the network. Figure 11 shows an example of VoIP QoS configured to support ATM. frame-relay mincir 256000 Sets the minimum acceptable CIR rate. The mincir value needs to be greater than total priority and bandwidth allocated. frame-relay fragment 320 Enables FRF.12 fragmentation with a fragment size of 320 bytes. service-policy output llq! Attaches the llq QoS policy to the defined map class. In this example, Frame Relay traffic shaping handles speed mismatches, FRF.12 fragmentation prevents VoIP packets from getting delayed behind large data packets, cRTP reduces VoIP bandwidth requirements, and LLQ provides priority to VoIP traffic and guarantees bandwidth to another class. You need to configure these features on both ends of the Frame Relay link. FRF.12 is needed only for links less than 1.2 Mbps, and cRTP is recommended only on links with a low number of VoIP calls and if the CPU is not running too high. Configuration Example 16: QoS for VoIP over Frame Relay WAN Links (continued)