Multisite QoS: Addressing QoS Concerns
-Lack of bandwidth
50 packets per second
Less than 150ms of delay
Less than 30ms of jitter
Less than 1 percent of packet loss
PLCA hides up to 35% packet loss
Depends on codec complexity
Replay similar packets
Dejitter buffer – Monitors arrival time of RTP packets. Timestamps are used and dejitter buffer can compensate delay.
Timestamps are not in UDP header therefore RTP assistance is required for Sequence numbers and Timestamps
RTP ports are even and RTCP are odd
LLQ – How?
Class Map – Match
Policy Map – What to do?
Lower vlan numbers for voice due to STP failover! Not in documentation!
Skinny (SCCP) Protocol
The Skinny protocol runs over TCP port 2000, and its DSCP/ToS value is derived from the configurable setting located within the Service Parameter IpTosCm2Dvce mentioned earlier in this document. The default DSCP value is 26 (AF31 or a ToS value of 3, which equals “flash” traffic).
Voice Packets/RTP Traffic
This includes any audio-bearing packet that uses the IP/UDP/RTP protocol stack. All UDP packets are unacknowledged. Therefore, the implementation of QoS mechanisms for this type of traffic is critical in order to ensure voice quality from end-to-end. By default, Cisco CallManager always instructs controlled end devices (IP phones, some MGCP gateways, and so forth) to use a DSCP value of 46 (EF or IP precedence 5). For IOS-based gateways (using MGCP or H.323 for signaling), this is the default value but can be changed on the CLI. There is also an option with Cisco CallManager to change this value. However, Cisco very strongly recommends that it not be changed from the default service.