Stream selection is a simple selection algorithm that is applicable to any number of input streams. It is a prerequisite for stream selection that RTPv2 encapsulation be used in UDP.
Each service is identified by multicast source, group/destination address and current synchronization source (SSRC). After the service has been identified, the ISA monitors its ingress for:
traffic with a DA of the multicast group
traffic with a DA of the ISA (unicast)
Traffic is further checked as having RTP-in-UDP payload, RTP version 2.
The SSRC of each incoming RTP packet is learned as a unique source. Only one SSRC is supported for each stream; as SSRC may change during abnormal situations (such as encoder failover), it can be updated.
An SSRC can only be updated when a Loss of Transport (LoT) occurs, as other perfect streams (with the original SSRC) may still be operational. When an LoT occurs, the SSRC is deleted, the buffers are purged, and the RTP sequence counters are reset. The SSRC is extracted from the next valid RTP packet and the sequence starts over.
One RTP packet from the perfect stream is selected for insertion into the video ISA buffer. After a packet is selected, the RTP sequence counter is incremented and any further RTP packets received by the ISA with the previous sequence number are discarded.
In summary, perfect stream selection is a FIFO algorithm for RTP packet selection; this is considered optimal because:
All stream sources are identical. Therefore, for any sequence number, the payload should also be identical.
Most bit errors should be detected by the CRC-32 algorithm applied to Ethernet, SDH, and so on.
These devices typically discard frames where bit errors occur, with the net result being that the video ISA receives a bit error-free stream (though packet loss can occur).
The UDP checksum is verified by the video ISA (after input VQM) and any failures result in a silent discard of the packet.
VQM can be used in conjunction with perfect stream protection.
VQM can be used to monitor the quality of two duplicate ingress multicast streams and the egress multicast stream (after perfect stream selection). This is particularly useful to compare between the ingress and egress multicast. Monitoring the egress multicast after perfect stream selection can provide an insight to the customer viewing experience.