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New Member

lip synchronization problem with IP/TV multicast


I am trialling IP/TV 3.2 demo version on our University campus network, which has a Catalyst 5500 and RSM as core router/switch feeding other buildings via Gbps Ethernet and is configured to support multicast and Cisco CGMP.

I am multicasting MPEG1 and MPEG2 video clips contained in the demo CD. The qualtiy is really good except for the lip sync problem mentioned below.

1.With on-demand video, there is no problem with lip sync, no matter where the viewing pc is.

2.With scheduled multicast program,

a.There is no problem with lip sync if the viewing pc and the IP/TV server are on the same switch(Catalyst 2950T) and on the same vlan.

b.Audio has a delay of 600-800 ms if the viewing pc is 5 switches away from the server(2x2950T and 3x3550T, pure switching) and on the same vlan as the server.

c.Audio has a delay of 600-800 ms if the viewing pc is on a different vlan from that of the server. That means the multicast traffic is going over the core router/switch 5500.

What puzzles me is the cause of the lip sync. Is it caused by the way the switch/router handles the multicast or the way IP/TV handles the multicast? We haven't implemented QOS on campus at present. Well, you can't do much QOS with 5500. Also, 2.b seems to be quite strange. I would think 2950 and 3550 are powerful enough.

Any suggestion?




Dennis Su, BSc,MSc,CCNP,CCDP

Team Leader, Network Admin & Development

ITS, University


Re: lip synchronization problem with IP/TV multicast

HI Dennis:

Was the same PC used for all of the tests?

What are the specs (CPU/RAM/etc) for the computers you used?

Were the switches (and router) set / optimized to handle the Multicast traffic (i.e., QOS, IGMP, CGMP, ...)?

If you have a packet/traffic analyzer available, did you verify that the multicast streams are arriving out-of-sync?

It certainly sounds like a multicast vs. unicast issue: that can be verified by concurrently "smallcasting" (unicast) the same video to a peer of the PC that is receiving a multicast ("Scheduled") program. Both PCs should be comparable (processor, OS, etc).

I've seen and heard about the sync problem in the past (we have a full stack of old and new IPTV equipment in the Lab). Some tweaking has usually resolved the infrastructure-related issues. In some cases, older PCs (PII450 / 256M) has sync issues relating to the bandwidth of the audio/video stream that was sent to it.

Good Luck


New Member

Re: lip synchronization problem with IP/TV multicast

Three PCs have been used for all the tests with the demo version of IPTV3.2. The same result has happened to them.

Their specs are

PIII 667/320M

PIII 667/128M

PII 500/128M

I have used packet analyzer to capture some traffic but couldn't tell what is wrong. How do you verify that the multicast streams are arriving out-of-sync?

Yes, we are using IGMP and CGMP and IGMP snooping too in some latest-model switches. But we do not have QOS.