Dear Osmocom community,
Those of you who had occasion to work with CSD probably noticed that
the standard RTP payload format prescribed for AoIP by 3GPP Rel8+
(64 kbit/s CLEARMODE) is extremely wasteful: 160 bytes of payload
every 20 ms (compared to a maximum of 33 bytes every 20 ms for voice
calls), and most of that 160-byte CLEARMODE payload is constant
filler. The actual information content for CSD channels traveling
across A interface (data bits plus all metadata preserved, strictly
lossless conversion) fits into 16 bytes for CSD modes up to 4.8 kbit/s,
32 bytes for 9.6 kbit/s or 37 bytes for 14.4 kbit/s - much more compact
than the 160-byte standard!
Because my non-profit org in USA (American 2G Cooperative) will soon
be operating a public GSM/2G network as a membership co-op, we have a
legal and ethical obligation to provide all services strictly at cost.
Radio resource usage is exactly the same between voice and data calls,
while CPU load on tw-border-mgw machines in the CN will often be even
less with CSD than with EFR or AMR speech transcoding - hence there is
no legitimate reason for CSD calls to be more expensive than voice.
Yet the bloated RTP format prescribed by 3GPP Rel8+ artificially
increases the cost of IP transport within the network.
My solution to the problem: yet another TW-TS, specifying a modified
version of AoIP interface with compressed CSD. Here is the spec:
https://www.freecalypso.org/specs/tw-ts-007-v010001.txt
An updated version of TW-TS-003 (BSSMAP extension spec) spells out how
the MSC requests the use of compressed CSD instead of standard TS 48.103
on AoIP by adding RTPext IE to ASSIGNMENT REQUEST or HANDOVER REQUEST,
with the right bit set to request TW-TS-007 RTP format.
In terms of implementation of this newly defined feature in Osmocom,
my plan is as follows:
* Within the past couple of years I extended osmo_trau2rtp() and
osmo_rtp2trau() to support CSD on E1 BTS, using standard CLEARMODE
format on RTP side. I plan to extend these functions to support
TW-TS-007 as well.
* Combination of mainline OsmoBSC plus tw-e1abis-mgw has fully working
CSD with E1 BTS, already tested with Nokia InSite. If my idea for
representing TW-TS-007 compressed CSD format in MGCP/SDP is acceptable
(please see chapter 9 and Annex A in the spec linked above), I would
like to extend OsmoBSC to be able to command tw-e1abis-mgw via
MGCP/SDP to emit either standard CLEARMODE or TW-TS-007 per request
from MSC.
The above points are for E1 BTS. With IP-native OsmoBTS, we have two
options:
1) We could add more logic to OsmoBTS to emit either standard CLEARMODE
or TW-TS-007, just like we already do for TW-TS-001/002 carrying
TRAU-UL metadata bits for FR/HR/EFR.
2) We could leave OsmoBTS completely unchanged, speaking only standard
CLEARMODE format for CSD, and have OsmoBSC-associated OsmoMGW
convert on the fly (we are dealing with strictly lossless conversion
here, not bringing back extra metadata as with other TW-TS) if the
MSC asked for TW-TS-007 compressed RTP format.
Option 2 seems easier to implement, and I am leaning toward this
option. Of course this option only makes sense when osmo-bsc,
osmo-mgw and osmo-bts-trx run on the same SBC next to or inside
SDR-based BTS, or when the link between the BSC and the BTS is a local
Ethernet cable, while A interface travels across public Internet or
other networks where bandwidth matters.
What does the community think? For my own operation E1 BTS is a higher
priority (I still plan on using surplus Ericsson RBS6k gear), hence
there is plenty of time for Osmocom community to provide feedback on
how y'all would like to see this mechanism working with IP-native
OsmoBTS.
In terms of upcoming Gerrit patches related to TW-TS-007, I plan to
work on libosmo-abis next, implementing lossless conversion functions
and extending osmo_trau2rtp() and osmo_rtp2trau() for efficient,
native support of compressed CSD.
GSM/2G Forever,
Mother Mychaela
P.S. If anyone has constructive criticism regarding chapter 9 or
Annex A in the TW-TS-007 spec linked above (AoIP and SDP aspects), it
is not too late to revise those parts.
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Hello,
I am sorry, this might not be the right place to ask. I am kind of a
newbie to the osmocom stack as well, so please be gentle with me.
I recently got an FCC license in the 900MHz range and I am running
sysmobts 1002 at home. I have updated it from NITB to the modern world
of course.
Everything seems to work as it should but there is one issue:
A Pixel 9 Pro Android phone with a sysmocom SIM card (999/70) connected
to the sysmobts disassociates -in certain cases- with the BTS/BSC after
a call and displays "No network". It will never reassociate again,
unless I put the phone in airplane mode and back on. Then it
reassociates immediately again.
I cannot see anything particular in the osmo-bsc/osmo-bts-sysmo logs but
as I said, I am a newbie. I will be happy to post the logs if this is
helpful.
The issue does not arise for each call, only certain calls. I use
osmo-sip-connector to an asterisk.
Is this a known issue? Is it an issue with the phone? Did I configure
something in a wrong way?
I will do some experiments with another GSM phone.
Thank you!
Christoph Lauter
Hi,
a couple month ago I proposed to make the VLR code of the osmo-msc independent of the osmo-msc
and create a new library called libosmo-vlr. Later use the libosmo-vlr code to integrate it into
osmo-sgsn.
But after take more time of this approach, I'm not sure if it is a very good idea to guarantee
high stability of libosmo-vlr as we do for other osmocom libraries.
Because the code is heavily integrated with both daemon and some code path are quite complex and partly
not fully understood. I would rather guarantee runtime stability within a library version, but not at build time.
The strategy would be increasing the library major version more often.
Any thoughts?
Best,
lynxis
PS. The current concept (because of time pressure) is to keep 2 copies of libosmo-vlr in the code base of osmo-msc and
osmo-sgsn and to move it later out of it into an own library.
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