running voice data/encryption on compal?

Fabio Pietrosanti (naif) lists at
Fri Jan 14 22:50:04 UTC 2011

On 14/01/11 21.29, Harald Welte wrote:
> Aaron,
> the usual problems with end-to-end encryption in GSM come into play:
> * as the voice path is not transparent but transcoded any number of times
>   in the core network, you cannot simply encrypt the codec frames but gqwill
>   have to initiate a CSD ('modem') connection.
Enciphering directly the GSM/AMR codec payload at GSM it's still a cool
hackish idea!

Regarding transcoding it's also to be considered that within the same
operator or at GSM inter-working national level it's much more difficult
that transcoding it's done in the voice path.
Mobile operators tends to avoid transcoding whether possible to reduce
the infrastructural costs associated with media gateway equipments and
directly interconnected mobile operators probably doesn't do much

I asked you a similar question at ph-neutral a couple of years ago and i
remember that there's also an issue related not being able, due to
hardware design of baseband, to control the path between the
encoder/decoder and the transmitter.

So no "hands on" the audio encoded/decoded payload that will get sent to
the network, right?
> * not many networks support it anymore
> * calling rates are more expensive
> * you immediately leave a visible trail, since nobody else uses CSD
> And no, neither is the ARM7 fast enough for any crypto, nor do we implement
> CSD at all.

We all would really like to see incredibly efficient DoV (Data over
Voice) modem technologies reaching at least stable 800bit/s to be able
to run a 600bit/s audio codec along with some encryption on top of it.

For military stuff there's a UK university that made a patent for a
1200bit/s DoV with error correction and 0.03% BER over GSM-to-GSM calls
with GSM codec (13kbit/s).

Some other DoV has been done to develop emergency services such as
in-band transmission of information between emergergy services and law
enforcement operators for european esafety project
( standardizing in-band modem over GSM and AMR as
3GPP TS 26.267 but that uber-patented technology are not designed to
handle realtime transport bust just signaling of messages.


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