[Linphone-users] What is the adaptive rate control role in relation to video calls lag?

[A A]

Hello,

I am new to VOIP via SIP world but not new to Linux. I am looking for
an open source replacement for Skype for making video calls with
people all over the world. I chose Linphone because its command line
interface brought my attention. Now, I have 2 computers running
Slackware, both in the same LAN connected to the same router,
connected via cable and not Wifi. I started Linphone on both of them.
I use H264 video codec on both of them. I use Linphone in two ways
that I know will be useful to me - inside LAN and via Linphone SIP
service:

1) I use one computer to call another via its IP: <username>@<IP>. I
pick up a call and a few seconds later a video transmission starts.
With adaptive rate control box checked on both computers and with 0
specified as download/upload limits I experience a horrible lag in
video transmission on both computers. On the computer on which I
answer the call it's a bit smaller than on the second one but in both
cases it's about 2 seconds. It makes video call unusable. However,
when I set download limit to 187 Kbit/sec and upload limit to 200
Kbit/sec on the computer on which I pick up the call and leave rate
control box checked and don't change anything on the second computer,
there is absolutely no lag in video transmission. Video artifacts are,
however, pretty frequent.

2) With the same settings as above, I sign in into two different
Linphone accounts on both computers and call each other. This time,
lag persist only on the computer on which download/upload limits are
left to 0. When I changed them to 187/200 and leave adaptive rate
control box checked just the same as on the other computer, lag is
gone. Bad thing is that connection via Linphone service is pretty
unstable here and a signal strength bar in main call window starts
with good or very good but sometimes drops to poor.

All of this makes me think what does adaptive rate control option
actually do? Why do I need to specify values manually, is it normal
that SIP programs are bad at guessing available bandwidth? I am
planning on connecting to my LAN from outside via OpenVPN and use
Linphone to call peers in the same LAN. Do you think it would work
out? What quality can I expect in such situation?