Zapping is switching from one channel to another and the zap latency or zap time is the time interval from which the viewer changes a particular channel to the time when the channel is displayed.These delays while switching from one channel to another occurs in general television and more prominently in IPTV’s.
Delay -How long does it take to switch from one channel to another.
If the customer changes channels in rapid successions is the delay worse.
Impact when there are more number of channels .
Multicast is one of the key technologies that enables the IPTV services.
So each group of the IGMP is considered as a channel.It is not necessary that the network is flooded with stream.So each time the viewer changes the channel a leave message is sent and each time the viewer switches to a channel a join message is sent to that group.Hence this delay in sending and receiving joins and leaves is the zap latency.
How to test zapping in spirent test center,
Zap testing using Sequencer: STC –> Wizards –> Triple Play –> IP TV
For Type’, select “Channel Zapping Test”. Set the time duration for the Zap test. Set the total number of channels to be Zapped and the starting multicast group IP address. Set the location for storing the test results.
The following is the configuration for Multicase Downstream traffic during channel zapping.
Select the STC port that is connected to the NT side.
Create a traffic stream for DS multicast traffic at the NT side.
IGMP Client creation at the ONT side to send join:
Set Zap Behavior to ‘Zap only’, Zap Direction = Up, Zap Interval type = Leave to leave.
Set Zap interval = 1000 / Number of Zaps per second,
Ex: If the number of zaps required per second is 2, then zap interval = 1000 / 2 = 500 msec.
Set Join Latency = Leave Latency = Zap interval
To Run Zap Test: