QVidium QVAVQ Series v4 - Manual (Page 16)

User’s Manual v.4 QVidium ® QVARQ QoS Proxy Server™
16 of 24 - Copyright 2010 QVidium
®
Technologies, Inc.
3.3.2 ARQ Error Correction Configuration
The configuration page for ARQ is shown above in Figure 3. To enable ARQ, you must first select
ARQ transport from the Profile dialog. Also, ARQ transport must be enabled at the encoder.
With ARQ selected and the decoder started, the decoder will watch for gaps in the RTP sequence
numbers from the output of the jitter buffer. When it detects missing packets, it sends
retransmission requests to the encoder so that the encoder can quickly resend any missing
packets. Incoming retransmitted ARQ packets bypass the network 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 output
of the ARQ buffer should no longer contain any missing packets.
The ARQ has five parameters that can be configured in manual mode: Target Latency, Burst
Drop
, Robust Mode, Number Retries, and Round Trip Time. In automatic mode, the
system automatically measures and calculates the Round Trip Time and Number Retries. In
most cases automatic mode using the default parameters should be adequate.
Of the required parameters, the first parameter, Target Latency, specifies the total delay, in
milliseconds, allotted for the request, retransmission, and recovery process. The ARQ mechanism
will attempt as many retries as possible within this target latency time. Thus, larger target latency
times increase the delay before video is output, but allows for more chances of requesting and
recovering any missing packets.
A Burst Drop delay can also be specified to delay any retransmission requests for a time equal
to the maximum expected packet loss time, such as from dynamic router changes of other sources
of burst loss.
A Robust Mode can also be selected. When selected (checked), a minimum of two tries will be
attempted for recovering any missing packets. When not selected (unchecked), ARQ will always
try at least once to recover any missing packets.
To help with firewall configuration and to handle potential port conflicts, the user can change the
default port that ARQ uses to send upstream retransmission requests, ARQ Port. However, you
must make certain that the same ARQ port number is set in both the encoder as in the decoder.
Although the automatic measurement of the round-trip time uses multiple measurements to take
into account random network delays, for some networks you might wish to specify a larger round-
trip value, or to specify a larger number of retries in case the upstream link loses some of the
retransmission request messages. In automatic robust-mode of ARQ operation, the system
automatically measures the round-trip time and divides that number into your specified Target
Latency to determine the number of retries to use. However in Manual Mode, you can force ARQ
to always use a specified number or retransmission requests.
NOTE: Remember to click Save after changing any parameters, or your changes will be lost. You
also have to click Start for any changes to take effect.