[bbs-revival] Plans for #36C3 / Status Report

Christian Berger christian at clarke-3.de
Mon Oct 21 05:34:01 UTC 2019


On 20.10.19 22:46, Harald Welte wrote:
> There is currently no plan for that.  As the POC is all-IP these days, I don't
> have particularly high hopes regarding how well it can carry Modem communication,
> with all the jitter that's going on.  A few years ago, the POC was still entirely
> ISDN based, but those days are over :/

Well jitter isn't the potential problem, that's what the jitter buffer
is for. The potential problem is clock skew. Once the clock of the
sender and the receiver differ by more than one packet (e.g. 20ms) audio
data either gets lost or has to be added. Since 20 ms is a lot of
damaged signal, modems will probably disconnect here.

The solution is to make sure you only have a single clock domain, which
is hard when you have 2 sides using TDM of different protocols (ISDN and
DECT). It is much simpler, BTW to do this with IP-based softmodems,
since there you don't actually have any clock. You just get RTP packets
in, and for every incoming packet you send one out. Hypothetically there
is something called a "lost packet", but that only occurs on faulty
networks (e.g. overloaded networks, wireless networks).

BTW any Codec is lossy and G.711 also has its losses. The actual issue
is if those losses are something the modem can handle. Surely you cannot
send 38k over a 32k DECT channel, but lower rates should work.

Servus
  Casandro


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