User’s Manual v.15
QVidium
®
H.264 HD Video Codecs
™
Copyright 2011-2014 QVidium
®
Technologies, Inc.
Page 41 of 71
5.5.2 ARQ: Automatic Retransmission Request
A
utomatic
R
etransmission Re
q
uest (ARQ) tries to recover any packets lost during transport to the
decoder by adding a small amount of delay at the decoder during which time the decoder would
have time to detect and request any missing packets. The size of this delay should also include
adequate time for the missing packet to be received and inserted into the play out queue so that
the video stream can continue to flow smoothly and unimpeded to the MPEG decoder.
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.
ARQ has three parameters that can be configured in
manual
mode:
Round-Trip Time,
Number Retries
, and
Burst-Drop
time. In
automatic
mode, the system automatically
measures and calculates the
Round Trip Time
and
Number Retries
. Automatic mode only
uses three parameters:
Target Latency
and
Burst-Drop
time and
Robust mode
. In most
cases automatic mode and 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. However, automatic mode also sets an internal maximum limit
based on its measurements that may be less than the target latency time.
The
Burst Drop
delay can 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.
Robust Mode
can also be selected for
Auto
mode of operation. 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:
For testing ARQ error correction, select ARQ transport at both the encoder and decoder and
use the default ARQ parameters at the decoder (receiver). Please also be certain that ARQ has
also been selected at the encoder.
