QoS DSCP for Call-Signaling
Call-Signaling Traffic The following are key QoS requirements and recommendations for Call-Signaling traffic: • Call-Signaling traffic should be marked as DSCP CS3 per the QoS Baseline (during migration, it may also be marked the legacy value of DSCP AF31). • 150 bps (plus Layer 2 overhead) per phone of guaranteed bandwidth is required for voice control traffic; more may be required, depending on the call signaling protocol(s) in use. Call-Signaling traffic was originally marked by Cisco IP Telephony equipment to DSCP AF31. However, the Assured Forwarding classes, as defined in RFC 2597, were intended for flows that could be subject to markdown and - subsequently - the aggressive dropping of marked-down values. Marking down and aggressively dropping Call-Signaling could result in noticeable delay-to-dial-tone (DDT) and lengthy call setup times, both of which generally translate to poor user experiences. The QoS Baseline changed the ma...