Packet Loss
Lost abstracts packets are recoverable if the endpoints can appeal retransmission. Absent voice
packets, as depicted in Figure 2-3, are not recoverable, because the audio charge be played
out in real-time and retransmission is not an option.
60 Authorized Self-Study Guide: Cisco Articulation over IP (CVOICE)
Lost Audio
Packet 1 Absent Packet 2 Packet 3
Figure 2-3 Aftereffect of Packet Loss
Voice packets ability be alone beneath the afterward conditions:
■ The arrangement is ambiguous (flapping links).
■ The arrangement is congested.
■ Too abundant capricious adjournment exists in the network, because packets ability access too late
to be accepted into an interface’s dejitter buffer.
Packet accident causes articulation abridgement and skips. As a result, the adviser hears gaps in the conversation,
as apparent in Figure 2-3. The industry accepted codec algorithms that are used
in Cisco DSPs actual for 20 ms to 50 ms of absent articulation through the use of Packet Loss
Concealment (PLC) algorithms. PLC intelligently analyzes missing packets and generates
a reasonable backup packet to advance the articulation quality. Cisco VoIP technology
uses 20 ms samples of articulation burden per VoIP packet by default. Effective codec correction
algorithms crave that alone a distinct packet can be absent at any accustomed time. If more
packets are lost, the adviser adventures gaps.
If a chat adventures packet loss, the aftereffect is anon heard. If the talker
says, “Watson, appear here. I appetite you,” the adviser ability hear, “Wat——, appear here,
———you.”