running voice data/encryption on compal?

David Wilson dw at botanicus.net
Sat Jan 15 23:23:31 UTC 2011


Hi Fabio,

On 15 January 2011 20:56, Fabio Pietrosanti (naif)
<lists at infosecurity.ch> wrote:
>
> Maybe there are an area of conditions where transcoding doesn't happen,
> like within the same operator or within national gsm operators
> interconnections.
>
> If that could be confirmed, it would be possible to have a low latency
> full-duplex communication data path at 13.3kbit/s (GSM codec) or
> 12.2kbit (GSM EFR/AMR codec) to be managed such like a serial connection.
>
> On top of a protocol alike HDLC could provide a serial connection over a
> GSM voice raw transport
>
> Someone willing to make a try?

Note the L1 channel coding is intimately linked to characteristics of
the codec/connection type, and it's just not as simple as treating the
bits that usually carry codec frames as a serial link. For example
block size and error correction varies by codec and even between
individual frame types within a codec. I believe L2 has no automatic
retransmission on frame error for voice channels, so that must also be
handled.

I'm not suggesting a solution, just pointing out these are not simple
"boxes and arrows" to be freely substituted without detailed knowledge
of the surrounding system.


David




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