In video services, if the VOD service is normal but the live service encounters a...

Live TV uses multicast, while VOD uses unicast. If VOD works but live TV is a black screen, typical causes are multicast service-path/config mismatches or priority starvation for multicast control/traffic:

C: If the ONU uses different VLANs for VOD and live but the OLT has only one video service flow, multicast frames will not match the correct service-port, causing black screen.

D: A low-priority traffic profile for multicast join/report or stream on the OLT uplink may drop IGMP joins or multicast packets under load, resulting in black screen while unicast VOD still passes.

Queue scheduling choices such as PQ or DWRR (A/B) are generic congestion schemes; by themselves they are not direct root causes of a live-only black screen unless mis-classified, whereas C and D are recognized multicast-specific fault points.