3G Voice Works
How to make a 3G voice call with Open Source software -- an update on Osmocom's Iu capabilities
I am glad to announce that we have succeeded in placing a 3G voice call between
two phones using an hNodeB cell and the Osmocom 3G core network. Find attached
a full network trace including IuCS signalling as well as the RTP voice stream.
This, proudly, is the first publicly available pcap of Iuh, IuCS and IuPS, and
it was created using exclusively free software in the core network stack.
Osmocom has had 3G data connectivity working for some months now3, and the
IuPS code has already been merged to OpenBSC's master branch (though it still
requires libosmo-netif, libosmo-sccp and asn1c to be built from the branches
The 3G voice counterpart is taking somewhat longer, not because it's more
difficult per se, but mostly because it needs profound refactoring of our MSC.
So far our MSC was closely tied to the BSC code, and to include IuCS, we need to
Are we done with 3G now? Not quite. Things need to be made fully configurable,
proper 3G authentication needs to be integrated, and all work needs to be put in
a stable release. We would also like to have a proper 2G A interface as a
companion to the 3G IuCS interface, which would allow us to completely replace
the OsmoNITB with the new OsmoCSCN.
Read this as a humble invitation to join NLnet1 and other sysmocom customers
in funding the open source 3G core network development here at sysmocom2.
The resulting software stack is free for everyone, including you, both in the
sense of free speech as well as the proverbial free beer, and we can still use
all the support we can get to wrap this up. If you would like to see this working
sooner rather than later, do not hesitate to contact us2.
So, we're still working on Osmocom 3G, but if you would like to take a look
ahead, here is how:
We have a 3G authentication implementation ready, but since this is not yet
integrated in our HLR/VLR and MSC libraries, we're still working with hardcoded
2G authentication tokens. So to test, you still need specially provisioned SIM
cards. Firstly, they must be incapable of 3G authentication, so that the phone
decides to fall back to 2G auth. Secondly, they must all be programmed with a
000102030405060708090a0b0c0d0e0f. If you need help here, feel free to
contact us2 -- we're in the meantime working on integrating full 3G
authentication with osmo-cscn and osmo-sgsn.
To set up a 3G core network based on free Osmocom software, this is what you
+--------+ ,-->| MGCPGW |<--RTP--... / | | | | |<--MGCP | +--------+ \ / | +------------+<--RTP +--------+ `->+----------+ UE <-->| hNodeB | | HNB-GW | | OsmoCSCN | UE <-->| |<--Iuh---->| |<--IuCS-->| | | | ...-->| | ...-->| | | | | | +----------+ +------------+<--GTP-U | | \ | | +------+ +------+ | | |<--IuPS-->| SGSN |<--GTP-C-->| GGSN | | +--------+ ...-->| | GTP-U-->| | | +------+ / +------+ \_______________________________/
Instead of a traditional NodeB, we use "smaller" hNodeB 3G cell hardware to take
care of the radio interface. This has the advantage that it already has an RNC
integrated, which we would otherwise need to implement separately. The RNC will
talk Iuh, i.e. HNBAP and RANAP4, to OsmoHNBGW running on your box, let's call it
the core network computer (CN).
Besides the HNB-GW, your CN further comprises of OsmoCSCN for voice signalling
as well as the OsmoMGCPGW to direct RTP streams. For data, there are OsmoSGSN
In short, Iuh is the combined voice (IuCS, Iu circuit switched) and data (IuPS,
Iu packet switched) signalling, which the HNB-GW splits to OsmoCSCN (circuit
switched core network) and OsmoSGSN. When a phone (UE, user equipment) starts
a call, OsmoCSCN takes care of all the signalling, from authentication to RAB
assignment, and instructs the MGCPGW to forward the RTP streams from the hNodeB,
in our case, back to the same hNodeB and to the other UE. In the field, the
MGCPGW would instead forward to a remote media gateway.
To set up your CN, build and install the following projects from
http://git.osmocom.org, using below branches; the current state of which have
also been tagged as '3G_2016_09':
- libosmocore: master 9c0751fc60e6282b5f5ff791d53f6f862f1c9c79
- libosmo-abis: master 15d9b7929d449e4138bcb003c614035bceadc3d1
- libosmo-netif: sysmocom/sctp b830719b392dc96fd7987fb3dfba31a92a6fa38b
- libosmo-sccp: sysmocom/iu c1307ee64d25f4b19397bcf4791ba4c85d1dbe79
- libsmpp34: master 2ccf5304ca465fbc70f6ae3283b4f49aaa9b650f
- asn1c: aper-prefix-onto-upstream b9b7c9e54d079c6093a5e77a79aabed409dc9bfb
- libasn1c: master 20d668cbd3c14ef32fcbd09617fbd3c8e6856ec0
- osmo-iuh: master f41b2fa500c209136c3446f4bc9d9da348539f92
- openggsn: master 03dbafb000c88155309dfd67b3bba73f7b389e69
- openbsc: sysmocom/iu 8a9f12dc2f69bf3a4e861cc9a81b71bdc5f13180
Once the CN stack is built, set up the configuration. Find attached files for an
example of a local test setup. Some details explained:
Tell the osmo-hnbgw which local IP address to use to listen for Iuh connections.
This needs to be on an interface reachable by the hNodeB. The IuCS and IuPS
links towards the osmo-cscn and osmo-sgsn are so far still hardcoded as
127.0.0.1 and 127.0.0.2, respectively, i.e. osmo-cscn and osmo-sgsn should run
on the same machine as the osmo-hnbgw. These will listen on the proper port
without further configuration (still hardcoded).
Also tell the MGCPGW (osmo-bsc_mgcp) which local IP address to bind to, which
has to be reachable both by the hNodeB as well as the osmo-cscn process. The
osmo-cscn.cfg is then told where to reach the MGCPGW.
A notable detail for 3G data is that the GGSN has to be reachable by the hNodeB.
Since the GTP standard defines fixed port numbers which both SGSN and GGSN have
to to use, the SGSN may not bind on the same IP address as the GGSN.
Once you have configured the IP addresses, start up your core network: launch
osmo-cscn, osmo-bsc_mgcp, osmo-sgsn, ggsn and osmo-hnbgw. You should see log
messages indicating established IuCS and IuPS links (HNBGW, CSCN and SGSN).
With your CN up and running, configure the hNodeB to contact osmo-hnbgw. Also
make sure the PLMN ID and LAC are configured correctly, to match the MCC and
MNC in the osmo-cscn.cfg -- otherwise the hNodeB may reject all attach requests.
Finally, do authorize the SIM card's IMSI, e.g. using osmo-cscn's telnet VTY,
and if necessary configure the hNodeB to allow access by this IMSI.
The attached pcap file contains a complete network trace of:
- HNBAP of hNodeB registering at the HNB-GW;
- two UEs registering first at the HNB-GW (HNBAP UE Register) and then on IuCS
and IuPS (MM Location Updating, GMM Attach), coming in via Iuh at the HNB-GW
and forwarded to OsmoCSCN and OsmoSGSN;
- the two UEs browsing the websites nlnet.nl5 and the current xkcd webcomic,
with PDP Context allocation as well as GTP-C and GTP-U6;
- a voice call where the one UE calls the other (i.e. MO with Service Request to
MT with Paging), with the RTP stream directed through our MGCP GW using CRCX
and MDCX instructions;
- each UE sending an SMS to the other.
The IP addresses used in attached network trace are:
- 10.9.1.11: hNodeB 3G femto cell;
- 10.9.1.120: CN computer's interface for Iuh and RTP, as well as the SGSN's
GTP-C side towards the GGSN;
- 10.9.1.13: CN computer's interface for GTP-U towards the hNodeB as well as
GTP-C towards the SGSN7;
- 127.0.0.1: loopback on the CN computer for IuCS;
- 127.0.0.2: loopback on the CN computer for IuPS;
- 10.23.42.*: IP addresses given to UEs within the GTP tunnel;
- all other IP addresses are remote servers contacted by the UEs.
When looking at network traces, note the various protocols: Iuh, IuCS and IuPS
communicate via SCTP (as opposed to TCP or UDP). You will see the same Iu
messages twice8, e.g. once on IuCS between HNB-GW and CSCN, encapsulated in
RANAP/SUA9, and again on Iuh between HNB-GW and hNodeB, encapsulated in
RANAP/RUA. In contrast, the MGCP configuration and RTP streams for voice use
UDP, and so do GTP-U and GTP-C for the data link.
In conclusion, we still need some work to reach our goal of a fully operational
3G core network. The attached trace of a 3G voice call using exclusively free
Osmocom software proves that we are now very close indeed.
We invite you to test and use our 3G core network stack, and if you can,
consider joining NLnet and sysmocom as sponsor of the ground breaking work in
the Osmocom community.
1 NLnet foundation, https://nlnet.nl
4 See also this protocol stack diagram
5 nlnet.nl is browsable by https only, so all you see is TLS encrypted data.
I would have liked to see a DNS query for nlnet.nl, but the UE had already
cached its resolution to 220.127.116.11. There are, however, other DNS queries,
as well as a plain http session for xkcd.com.
6 I have, for privacy reasons, filtered from the pcap some services that the
UEs eagerly contacted but which were not browsed explicitly during the test.
7 It is however possible that few GTP-U packets end up at the SGSN between
activating the PDP context and redirecting GTP-U towards the hNodeB's address,
which can only be known after the IuPS RAB Assignment is complete.
8 Note that the timing differences between the internal loopback and eth0
interfaces may cause their ordering to appear slightly out of sync in the pcap.
9 RANAP/SUA is our non-standard choice of SIGTRAN for IuCS and IuPS, since it
has simpler layering than the standard RANAP/SCCP/M3UA. See also4.