To protect against network interruption or reconvergence, it is often more efficient to protect the stream using an alternate transmission path. This can be a separate physical interface, transmission link, system, or even technology.
Perfect stream is also known as duplicate stream in some markets. Perfect stream protection allows an operator to split a single multicast stream (single S,G and common SSRC) into two different transmission paths that may have different transmission characteristics, such as latency or jitter. Instead of selecting one stream for retransmission to the client, the perfect stream protection feature evaluates each stream packet-by-packet, selecting the first, valid packet for retransmission.
A circular buffer is used for perfect stream protection which incorporates both packet-by-packet selection (based on RTP sequence number/timestamp and SSRC) and a re-ordering function whereby any out-of-sequence packets are placed into the buffer in order, therefore creating a corrected, in-order stream.
Playout rate is a function in ingest rate, however because the two streams may be delayed between one-another a few assumptions are made:
The first arriving packet is always put into the buffer, allowing for the backup medium to wander in terms of latency and jitter.
Because the source is the same, the rate at which a packet is put into the buffer (from either stream) can be assumed to be the normal bitrate.
The output RTP stream is always maintained in-sequence and the playout speed is always controlled. A moving window calculated against the time stamp smooths jitter that may occur between packets or the two contributing streams. The playout stream is described in Perfect stream selection.