QVidium QVMP2C-1011 - Manual (Page 12)

User’s Manual v.28 QVidium™ MPEG2+4 Codec
12 of 45 - Copyright 2007-2008 QVidium™ Technologies, Inc.
2.4.3 ARQ: Automatic Retransmission Request
To enable Automatic Retransmission Request (ARQ), you must first select ARQ transport from the
Profile
dialog. ARQ transport must also be enabled at the decoder. With ARQ selected and the
encoder started, the encoder will begin to save outgoing packets for later retransmission, when
necessary. You must also be certain to configure any firewalls to allow the ARQ retransmission
request packets through. The default port for these upstream ARQ request packets is UDP port
7020, although you can configure this to any other valid, non-conflicting UDP port. However, if you
choose to change the ARQ request port, you must make certain the settings at the encoder and
decoder match.
At the receiver, the ARQ error correction mechanism first buffers incoming packets in a
synchronized network de-jitter buffer that maintains a constant, configurable delay for incoming
packets. The de-jitter buffer processes RTP sequence numbers and reorders packets as necessary
to restore proper packet sequence and to buffer against erratic network delays. Its goal is to output
a smooth, consistent flow of packets with increasing sequence numbers. This output feeds the ARQ
error correction mechanism.
The ARQ unit at the receiver watches for gaps in RTP sequence number from the de-jitter buffer
output, and immediately sends out retransmission requests when necessary. Incoming
retransmitted ARQ packets bypass the de-jitter queue and find their correct place in the ARQ buffer,
so that by the time packet are output by the ARQ buffer, the ARQ error correction mechanism
should no longer contain any missing packets.
When the encoder and decoder are both started, the decoder automatically measures the round-trip
delay and packet jitter to the encoder and automatically configures ARQ for optimal operation
according to a few basic user-specified parameters, as described in Section 3.4.3.
Note: For testing ARQ error correction, select ARQ transport at both the encoder and decoder and
use the default ARQ parameters at the decoder (receiver).