EXFO EXpert IPTV Test Tools (FTB-200v2) - Manual (Page 73)

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The join message asks the network to send the requested program or channel to the STB by
joining a multicast group carrying the desired broadcast channel.
IGMP latency, then, is the time between when the join message is sent and the first video
packet is received by the STB.
This parameter measures network performance, but not the end user's experience with
regard to channel changing time.
The IGMP latency plus the time it takes to fill the decode buffer and to decode and display the
content is the total user experience time. However, the buffer fill time and the decode time
are functions of the network architecture and are not variables. This total time is called Zap
Time.