QoS Traffic Requirements Voice

• Requires guaranteed priority bandwidth per call

• 150 bps (+ Layer 2 overhead) guaranteed bandwidth for voice-control traffic per call

*one-way requirements

• Requires guaranteed priority bandwidth per call

• 150 bps (+ Layer 2 overhead) guaranteed bandwidth for voice-control traffic per call

*one-way requirements

Voice quality is directly affected by these QoS quality factors:

■ Loss: Loss causes voice clipping and skips. The industry standard coded algorithms that are used in Cisco digital signal processors (DSPs) can correct for up to 30 ms of lost voice. For example, if a 20 ms sample of voice payload is used per Voice over IP (VoIP) packet, only a single voice packet can be lost during any given time. If two successive voice packets are lost, the 30-ms correctable window is exceeded and voice quality begins to degrade.

■ Delay: Delay can cause voice quality degradation if the delay is above 200 ms. The International Telecommunication Union (ITU) standard for VoIP (G. 114) states that a 150-ms one-way delay budget is acceptable for high voice quality.

■ Jitter: There are dynamic and adaptive jitter buffers within Cisco IP telephony devices. However, these can usually only compensate for 20 to 50 ms of jitter, or delay variations.

To determine the bandwidth requirement per each VoIP call, several factors must be considered. These factors include the codec used, the sampling rate, and the Layer 2 protocol.

Each VoIP call also requires 150 bps of bandwidth for the voice control (call-signaling) traffic.

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