EXFO EXpert IPTV Test Tools (FTB-1 / 2 Pro) - Manual (Page 72)

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IGMP is the signaling protocol used to access broadcast video services that use a multicast
network design to efficiently manage network bandwidth. In this implementation, a join
message is sent from the STB to the network.
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.