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Gateway and Gateway 4 Manual v1.2

© Tieline Research Pty. Ltd. 2021

connections

for

STLs

with

connections  that
aren’t  shared  &
have  quality

service (QoS).

Important Notes:

The

FEC Delay

configured should take into account the packet arrival (jitter buffer)

strategy at the remote codec. For example, if the maximum jitter buffer at the remote
codec is 1000 ms, the FEC Delay setting should be lower, to ensure there is enough
time for FEC packets to arrive and replace lost packets prior to audio playout.

By default, the codec will use the audio stream IP address as the remote FEC IP
address as well. This can be adjusted in the

Program Manager panel

in the HTML5

Toolbox web-GUI.

The default local and remote UDP audio FEC ports are 9002.

Any of the available algorithms can be selected when configuring RFC 2733 FEC in the
codec.

How does FEC work?

If you enter a FEC setting of 20% and you are losing one packet in every five sent, the lost packet
will be replaced by FEC to maintain the quality of the connection. If you are losing more packets
than this, say one in three, it will be necessary to increase the FEC setting to 33% to compensate.

Note

: There is an inverse relationship between FEC settings and the jitter-buffer millisecond setting

that you use for IP connections.

So  why  not  use  100%  FEC  every  time?  The  answer  is  because  you  need  twice  the  bit  rate  to
achieve  full  redundancy  and  depending  on  the  link  conditions,  this  could  potentially  cause  more
dropouts  because  of  network  congestion  than  it  fixes.  Here  is  a  simple  rule  to  remember: Your
maximum uplink speed is the maximum bandwidth you have to play with. As a rule of thumb, try  not
to  exceed  more  than  80%  of  your  maximum  bandwidth.  If  your  link  is  shared,  be  even  more
conservative.

You  should  also consider  the  maximum  upload  speed  at  the  remote  end  too.  Is  the  connection
shared  at  either  end?  Your  bit  rates,  FEC  settings  and  buffer  rates  must  be  pre-configured  at  both
ends before you connect, so it's always better to  set  your  connection  speed  and  balance  your  FEC
according to the available uplink bandwidth at each end for best performance.

As an example, if you want 15 kHz mono  (using  the  Tieline  Music  Algorithm)  you  will  need  at  least
a  24kbps  connection  for  audio.  Adding  100%  FEC  will  add  another  24kbps  making  your  bit  rate
48kbps plus some  overhead  of  around  10kbps  is  required.  If  you're  on  a  64kbps  uplink,  you  should
consider reducing your FEC to minimize the likelihood of exceeding your bandwidth capacity.

Here is another example, if you want  15  kHz  stereo,  you  need  at  least  56kbps  for  the  audio.  100%
FEC  requires  at  least  112kbps  and  50%  FEC  requires  at  least  84kbps.  If  your  uplink  speed  is
256kbps  and  you're  on  a  shared  connection,  then  choosing  a  lower  FEC  setting  of  20%-33%  may
give you better results.

Conserving Bandwidth with FEC

There is a trade-off between the quality and the reliability of an IP connection – particularly when
FEC is activated on your codecs. However, it is possible in certain situations to configure different