A key component in successful and smoothless operations of IPTV and triple-play services is a very good understanding and handling of IGMP — the signalling protocol used to control how multicast MPEG transport streams are distributed effectively throughout an IPTV network.
Especially for narrowband access connections, such as ADSL, VDSL or DOCSIS, it is critical to ensure that the limited available downstream bandwidth is never exceeded.
One thing that easily causes congestion on the downstream access link is when multiple IPTV streams occur at the same time. If only one viewer or set-top-box (STB) exist on the access port, you might think the situation is not possible. Truth is it’s very likely to happen if your access devices such as switches, DSLAMs, CMTSs are using default configuration settings.
A desired solution is that the access device only forwards IPTV MPEG streams to ports where explicit requests have been received. This is achieved by using IGMP snooping techniques that intercept the “IGMP join” messages from the STBs. Once the STB switches to a new channel, or if the STB shuts down, an “IGMP leave message” is sent from the STB with the result that the stream should be stopped towards that port. However, using default settings on the access device, it will take several seconds before the stream is completely stopped. This is due waiting for timer expirations, inherent in the querying procedure between multicast routers and IGMP-aware access devices.
This behavior could have really devastating consequences. For instance, a user that quickly switches between channels could easily generate bandwidth peaks that exceed the available downstream rates by orders of magnitude. A simple example: Assume that it takes 5 seconds for channels to stop being forwarded on the port and that the user is able to zap 2 channels per second. This will result in a peak of 10 concurrent channels on the port. Assume a mix of HDTV and SDTV channels, this would typically result in 50-60 Mbit/s in total traffic, which easily saturates a downstream ADSL link.
Luckily, there are solutions to this problem. The approach is to configure the access device to immediately stop forwarding traffic on the port on which an “IGMP leave” message is received. However, different vendors use different naming and different configuration commands, which makes it rather confusing. For instance here’s a quick look at how different vendors name their features:
|Cisco and Juniper||IGMP immediate-leave|
Regardless of actual naming, by using these features IPTV MPEG streams will be stopped within a few milliseconds, rather than having to wait 4-5 seconds. The actual maximum time depends on the settings of the IGMP querying procedure.
In case you are not sure whether these functions are enabled correctly in your access devices, you either have to check your configurations, or you have to test how your network actually behaves. In Netrounds, you can easily and remotely perform measurements of channel zapping times from an end-user perspective using the built-in features for IPTV. Sign up to Netrounds to find out the details.
A final caveat though: if more than one STB is attached to a single port, it might be counterproductive to use the immediate leave features. Consider a scenario where two STBs are viewing the same channel and one of them switches to a different one. This would temporarily cause an outage until the access switch recovers forwarding when it discovers the IGMP membership report again.