Recent blog entries for LaForge

Cellular re-broadcast over satellite

I've recently attended a seminar that (among other topics) also covered RF interference hunting. The speaker was talking about various real-world cases of RF interference and illustrating them in detail.

Of course everyone who has any interest in RF or cellular will know about fundamental issues of radio frequency interference. To the biggest part, you have

  • cells of the same operator interfering with each other due to too frequent frequency re-use, adjacent channel interference, etc.
  • cells of different operators interfering with each other due to intermodulation products and the like
  • cells interfering with cable TV, terrestrial TV
  • DECT interfering with cells
  • cells or microwave links interfering with SAT-TV reception
  • all types of general EMC problems

But what the speaker of this seminar covered was actually a cellular base-station being re-broadcast all over Europe via a commercial satellite (!).

It is a well-known fact that most satellites in the sky are basically just "bent pipes", i.e. they consist of a RF receiver on one frequency, a mixer to shift the frequency, and a power amplifier. So basically whatever is sent up on one frequency to the satellite gets re-transmitted back down to earth on another frequency. This is abused by "satellite hijacking" or "transponder hijacking" and has been covered for decades in various publications.

Ok, but how does cellular relate to this? Well, apparently some people are running VSAT terminals (bi-directional satellite terminals) with improperly shielded or broken cables/connectors. In that case, the RF emitted from a nearby cellular base station leaks into that cable, and will get amplified + up-converted by the block up-converter of that VSAT terminal.

The bent-pipe satellite subsequently picks this signal up and re-transmits it all over its coverage area!

I've tried to find some public documents about this, an there's surprisingly little public information about this phenomenon.

However, I could find a slide set from SES, presented at a Satellite Interference Reduction Group: Identifying Rebroadcast (GSM)

It describes a surprisingly manual and low-tech approach at hunting down the source of the interference by using an old nokia net-monitor phone to display the MCC/MNC/LAC/CID of the cell. Even in 2011 there were already open source projects such as airprobe that could have done the job based on sampled IF data. And I'm not even starting to consider proprietary tools.

It should be relatively simple to have a SDR that you can tune to a given satellite transponder, and which then would look for any GSM/UMTS/LTE carrier within its spectrum and dump their identities in a fully automatic way.

But then, maybe it really doesn't happen all that often after all to rectify such a development...

Syndicated 2017-02-15 23:00:00 from LaForge's home page

Towards a real SIGTRAN/SS7 stack in libosmo-sigtran

In the good old days ever since the late 1980ies - and a surprising amount even still today - telecom signaling traffic is still carried over circuit-switched SS7 with its TDM lines as physical layer, and not an IP/Ethernet based transport.

When Holger first created OsmoBSC, the BSC-only version of OpenBSC some 7-8 years ago, he needed to implement a minimal subset of SCCP wrapped in TCP called SCCP Lite. This was due to the simple fact that the MSC to which it should operate implemented this non-standard protocol stacking that was developed + deployed before the IETF SIGTRAN WG specified M3UA or SUA came around. But even after those were specified in 2004, the 3GPP didn't specify how to carry A over IP in a standard way until the end of 2008, when a first A interface over IP study was released.

As time passese, more modern MSCs of course still implement classic circuit-switched SS7, but appear to have dropped SCCPlite in favor of real AoIP as specified by 3GPP meanwhile. So it's time to add this to the osmocom universe and OsmoBSC.

A couple of years ago (2010-2013) implemented both classic SS7 (MTP2/MTP3/SCCP) as well as SIGTRAN stackings (M2PA/M2UA/M3UA/SUA in Erlang. The result has been used in some production deployments, but only with a relatively limited feature set. Unfortunately, this code has nto received any contributions in the time since, and I have to say that as an open source community project, it has failed. Also, while Erlang might be fine for core network equipment, running it on a BSC really is overkill. Keep in miond that we often run OpenBSC on really small ARM926EJS based embedded systems, much more resource constrained than any single smartphone during the late decade.

In the meantime (2015/2016) we also implemented some minimal SUA support for interfacing with UMTS femto/small cells via Iuh (see OsmoHNBGW).

So in order to proceed to implement the required SCCP-over-M3UA-over-SCTP stacking, I originally thought well, take Holgers old SCCP code, remove it from the IPA multiplex below, stack it on top of a new M3UA codebase that is copied partially from SUA.

However, this falls short of the goals in several ways:

  • The application shouldn't care whether it runs on top of SUA or SCCP, it should use a unified interface towards the SCCP Provider. OsmoHNBGW and the SUA code already introduce such an interface baed on the SCCP-User-SAP implemented using Osmocom primitives (osmo_prim). However, the old OsmoBSC/SCCPlite code doesn't have such abstraction.
  • The code should be modular and reusable for other SIGTRAN stackings as required in the future

So I found myself sketching out what needs to be done and I ended up pretty much with a re-implementation of large parts. Not quite fun, but definitely worth it.

The strategy is:

And then finally stack all those bits on top of each other, rendering a fairly clean and modern implementation that can be used with the IuCS of the virtually unmodified OsmmoHNBGW, OsmoCSCN and OsmoSGSN for testing.

Next steps in the direction of the AoIP are:

  • Implementation of the MTP-SAP based on the IPA transport
  • Binding the new SCCP code on top of that
  • Converting OsmoBSC code base to use the SCCP-User-SAP for its signaling connection

From that point onwards, OsmoBSC doesn't care anymore whether it transports the BSSAP/BSSMAP messages of the A interface over SCCP/IPA/TCP/IP (SCCPlite) SCCP/M3UA/SCTP/IP (3GPP AoIP), or even something like SUA/SCTP/IP.

However, the 3GPP AoIP specs (unlike SCCPlite) actually modify the BSSAP/BSSMAP payload. Rather than using Circuit Identifier Codes and then mapping the CICs to UDP ports based on some secret conventions, they actually encapsulate the IP address and UDP port information for the RTP streams. This is of course the cleaner and more flexible approach, but it means we'll have to do some further changes inside the actual BSC code to accommodate this.

Syndicated 2017-02-12 23:00:00 from LaForge's home page

Testing (not only) telecom protocols

When implementing any kind of communication protocol, one always dreams of some existing test suite that one can simply run against the implementation to check if it performs correct in at least those use cases that matter to the given application.

Of course in the real world, there rarely are protocols where this is true. If test specifications exist at all, they are often just very abstract texts for human consumption that you as the reader should implement yourself.

For some (by far not all) of the protocols found in cellular networks, every so often I have seen some formal/abstract machine-parseable test specifications. Sometimes it was TTCN-2, and sometimes TTCN-3.

If you haven't heard about TTCN-3, it is basically a way to create functional tests in an abstract description (textual + graphical), and then compile that into an actual executable tests suite that you can run against the implementation under test.

However, when I last did some research into this several years ago, I couldn't find any Free / Open Source tools to actually use those formally specified test suites. This is not a big surprise, as even much more fundamental tools for many telecom protocols are missing, such as good/complete ASN.1 compilers, or even CSN.1 compilers.

To my big surprise I now discovered that Ericsson had released their (formerly internal) TITAN TTCN3 Toolset as Free / Open Source Software under EPL 1.0. The project is even part of the Eclipse Foundation. Now I'm certainly not a friend of Java or Eclipse by all means, but well, for running tests I'd certainly not complain.

The project also doesn't seem like it was a one-time code-drop but seems very active with many repositories on gitub. For example for the core module, titan.core shows plenty of activity on an almost daily basis. Also, binary releases for a variety of distributions are made available. They even have a video showing the installation ;)

If you're curious about TTCN-3 and TITAN, Ericsson also have made available a great 200+ pages slide set about TTCN-3 and TITAN.

I haven't yet had time to play with it, but it definitely is rather high on my TODO list to try.

ETSI provides a couple of test suites in TTCN-3 for protocols like DIAMETER, GTP2-C, DMR, IPv6, S1AP, LTE-NAS, 6LoWPAN, SIP, and others at http://forge.etsi.org/websvn/ (It's also the first time I've seen that ETSI has a SVN server. Everyone else is using git these days, but yes, revision control systems rather than periodic ZIP files is definitely a big progress. They should do that for their reference codecs and ASN.1 files, too.

I'm not sure once I'll get around to it. Sadly, there is no TTCN-3 for SCCP, SUA, M3UA or any SIGTRAN related stuff, otherwise I would want to try it right away. But it definitely seems like a very interesting technology (and tool).

Syndicated 2017-02-11 23:00:00 from LaForge's home page

FOSDEM 2017

Last weekend I had the pleasure of attending FOSDEM 2017. For many years, it is probably the most exciting event exclusively on Free Software to attend every year.

My personal highlights (next to meeting plenty of old and new friends) in terms of the talks were:

I was attending but not so excited by Georg Greve's OpenPOWER talk. It was a great talk, and it is an important topic, but the engineer in me would have hoped for some actual beefy technical stuff. But well, I was just not the right audience. I had heard about OpenPOWER quite some time ago and have been following it from a distance.

The LoRaWAN talk couldn't have been any less technical, despite stating technical, political and cultural in the topic. But then, well, just recently 33C3 had the most exciting LoRa PHY Reverse Engineering Talk by Matt Knight.

Other talks whose recordings I still want to watch one of these days:

Syndicated 2017-02-10 23:00:00 from LaForge's home page

Osmocom Conference 2017 on April 21st

I'm very happy that in 2017, we will have the first ever technical conference on the Osmocom cellular infrastructure projects.

For many years, we have had a small, invitation only event by Osmocom developers for Osmocom developers called OsmoDevCon. This was fine for the early years of Osmocom, but during the last few years it became apparent that we also need a public event for our many users. Those range from commercial cellular operators to community based efforts like Rhizomatica, and of course include the many research/lab type users with whom we started.

So now we'll have the public OsmoCon on April 21st, back-to-back with the invitation-only OsmoDevcon from April 22nd through 23rd.

I'm hoping we can bring together a representative sample of our user base at OsmoCon 2017 in April. Looking forward to meet you all. I hope you're also curious to hear more from other users, and of course the development team.

Regards,
Harald

Syndicated 2017-01-31 23:00:00 from LaForge's home page

Autodesk: How to lose loyal EAGLE customers

A few days ago, Autodesk has announecd that the popular EAGLE electronics design automation (EDA) software is moving to a subscription based model.

When previously you paid once for a license and could use that version/license as long as you wanted, there now is a monthly subscription fee. Once you stop paying, you loose the right to use the software. Welcome to the brave new world.

I have remotely observed this subscription model as a general trend in the proprietary software universe. So far it hasn't affected me at all, as the only two proprietary applications I use on a regular basis during the last decade are IDA and EAGLE.

I already have ethical issues with using non-free software, but those two cases have been the exceptions, in order to get to the productivity required by the job. While I can somehow convince my consciousness in those two cases that it's OK - using software under a subscription model is completely out of the question, period. Not only would I end up paying for the rest of my professional career in order to be able to open and maintain old design files, but I would also have to accept software that "calls home" and has "remote kill" features. This is clearly not something I would ever want to use on any of my computers. Also, I don't want software to be associated with any account, and it's not the bloody business of the software maker to know when and where I use my software.

For me - and I hope for many, many other EAGLE users - this move is utterly unacceptable and certainly marks the end of any business between the EAGLE makers and myself and/or my companies. I will happily use my current "old-style" EAGLE 7.x licenses for the near future, and theS see what kind of improvements I would need to contribute to KiCAD or other FOSS EDA software in order to eventually migrate to those.

As expected, this doesn't only upset me, but many other customers, some of whom have been loyal to using EAGLE for many years if not decades, back to the DOS version. This is reflected by some media reports (like this one at hackaday or user posts at element14.com or eaglecentral.ca who are similarly critical of this move.

Rest in Peace, EAGLE. I hope Autodesk gets what they deserve: A new influx of migrations away from EAGLE into the direction of Open Source EDA software like KiCAD.

In fact, the more I think about it, I'm actually very much inclined to work on good FOSS migration tools / converters - not only for my own use, but to help more people move away from EAGLE. It's not that I don't have enough projects at my hand already, but at least I'm motivated to do something about this betrayal by Autodesk. Let's see what (if any) will come out of this.

So let's see it that way: What Autodesk is doing is raising the level off pain of using EAGLE so high that more people will use and contribute FOSS EDA software. And that is actually a good thing!

Syndicated 2017-01-22 23:00:00 from LaForge's home page

33C3 talk on dissecting cellular modems

Yesterday, together with Holger 'zecke' Freyther, I co-presented at 33C3 about Dissectiong modern (3G/4G) cellular modems.

This presentation covers some of our recent explorations into a specific type of 3G/4G cellular modems, which next to the regular modem/baseband processor also contain a Cortex-A5 core that (unexpectedly) runs Linux.

We want to use such modems for building self-contained M2M devices that run the entire application inside the modem itself, without any external needs except electrical power, SIM card and antenna.

Next to that, they also pose an ideal platform for testing the Osmocom network-side projects for running GSM, GPRS, EDGE, UMTS and HSPA cellular networks.

You can find the Slides and the Video recordings in case you're interested in more details about our work.

The results of our reverse engineering can be found in the wiki at http://osmocom.org/projects/quectel-modems/wiki together with links to the various git repositories containing related tools.

As with all the many projects that I happen to end up doing, it would be great to get more people contributing to them. If you're interested in cellular technology and want to help out, feel free to register at the osmocom.org site and start adding/updating/correcting information to the wiki.

You can e.g. help by

  • playing with the modem and documenting your findings
  • reviewing the source code released by Qualcomm + Quectel and documenting your findings
  • help us to create a working OE build with our own kernel and rootfs images as well as opkg package feeds for the modems
  • help reverse engineering DIAG and QMI protocols as well as the open source programs to interact with them

Syndicated 2016-12-30 00:00:00 from LaForge's home page

Contribute to Osmocom 3.5G and receive a free femtocell

In 2016, Osmocom gained initial 3.5G support with osmo-iuh and the Iu interface extensions of our libmsc and OsmoSGSN coede. This means you can run your own small open source 3.5G cellular network for SMS, Voice and Data services.

However, the project needs more contributors: Become an active member in the Osmocom development community and get your nano3G femtocell for free.

I'm happy to announce that my company sysmocom hereby issues a call for proposals to the general public. Please describe in a short proposal how you would help us improving the Osmocom project if you were to receive one of those free femtocells.

Details of this proposal can be found at https://sysmocom.de/downloads/accelerate_3g5_cfp.pdf

Please contact mailto:accelerate3g5@sysmocom.de in case of any questions.

Syndicated 2016-12-29 00:00:00 from LaForge's home page

Accessing 3GPP specs in PDF format

When you work with GSM/cellular systems, the definite resource are the specifications. They were originally released by ETSI, later by 3GPP.

The problem start with the fact that there are separate numbering schemes. Everyone in the cellular industry I know always uses the GSM/3GPP TS numbering scheme, i.e. something like 3GPP TS 44.008. However, ETSI assigns its own numbers to the specs, like ETSI TS 144008. Now in most cases, it is as simple s removing the '.' and prefixing the '1' in the beginning. However, that's not always true and there are exceptions such as 3GPP TS 01.01 mapping to ETSI TS 101855. To make things harder, there doesn't seem to be a machine-readable translation table betwen the spec numbers, but there's a website for spec number conversion at http://webapp.etsi.org/key/queryform.asp

When I started to work on GSM related topics somewhere between my work at Openmoko and the start of the OpenBSC project, I manually downloaded the PDF files of GSM specifications from the ETSI website. This was a cumbersome process, as you had to enter the spec number (e.g. TS 04.08) in a search window, look for the latest version in the search results, click on that and then click again for accessing the PDF file (rather than a proprietary Microsoft Word file).

At some point a poor girlfriend of mine was kind enough to do this manual process for each and every 3GPP spec, and then create a corresponding symbolic link so that you could type something like evince /spae/openmoko/gsm-specs/by_chapter/44.008.pdf into your command line and get instant access to the respective spec.

However, of course, this gets out of date over time, and by now almost a decade has passed without a systematic update of that archive.

To the rescue, 3GPP started at some long time ago to not only provide the obnoxious M$ Word DOC files, but have deep links to ETSI. So you could go to http://www.3gpp.org/DynaReport/44-series.htm and then click on 44.008, and one further click you had the desired PDF, served by ETSI (3GPP apparently never provided PDF files).

However, in their infinite wisdom, at some point in 2016 the 3GPP webmaster decided to remove those deep links. Rather than a nice long list of released versions of a given spec, http://www.3gpp.org/DynaReport/44008.htm now points to some crappy JavaScript tabbed page, where you can click on the version number and then get a ZIP file with a single Word DOC file inside. You can hardly male it any more inconvenient and cumbersome. The PDF links would open immediately in modern browsers built-in JavaScript PDF viewer or your favorite PDF viewer. Single click to the information you want. But no, the PDF links had to go and replaced with ZIP file downloads that you first need to extract, and then open in something like LibreOffice, taking ages to load the document, rendering it improperly in a word processor. I don't want to edit the spec, I want to read it, sigh.

So since the usability of this 3GPP specification resource had been artificially crippled, I was annoyed sufficiently well to come up with a solution:

  • first create a complete mirror of all ETSI TS (technical specifications) by using a recursive wget on http://www.etsi.org/deliver/etsi_ts/
  • then use a shell script that utilizes pdfgrep and awk to determine the 3GPP specification number (it is written in the title on the first page of the document) and creating a sym-link. Now I have something like 44.008-4.0.0.pdf -> ts_144008v040000p.pdf

It's such a waste of resources to have to download all those files and then write a script using pdfgrep+awk to re-gain the same usability that the 3GPP chose to remove from their website. Now we can wait for ETSI to disable indexing/recursion on their server, and easy and quick spec access would be gone forever :/

Why does nobody care about efficiency these days?

If you're also an avid 3GPP spec reader, I'm publishing the rather trivial scripts used at http://git.osmocom.org/3gpp-etsi-pdf-links

If you have contacts to the 3GPP webmaster, please try to motivate them to reinstate the direct PDF links.

Syndicated 2016-12-16 00:00:00 from LaForge's home page

Open Hardware IEEE 802.15.4 adapter "ATUSB" available again

Many years ago, in the aftermath of Openmoko shutting down, fellow former Linux kernel hacker Werner Almesberger was working on an IEEE 802.15.4 (WPAN) adapter for the Ben Nanonote.

As a spin-off to that, the ATUSB device was designed: A general-purpose open hardware (and FOSS firmware + driver) IEEE 802.15.4 adapter that can be plugged into any USB port.

/images/atusb.jpg

This adapter has received a mainline linux kernel driver written by Werner Almesberger and Stefan Schmidt, which was eventually merged into mainline Linux in May 2015 (kernel v4.2 and later).

Earlier in 2016, Stefan Schmidt (the current ATUSB Linux driver maintainer) approached me about the situation that ATUSB hardware was frequently asked for, but currently unavailable in its physical/manufactured form. As we run a shop with smaller electronics items for the wider Osmocom community at sysmocom, and we also frequently deal with contract manufacturers for low-volume electronics like the SIMtrace device anyway, it was easy to say "yes, we'll do it".

As a result, ready-built, programmed and tested ATUSB devices are now finally available from the sysmocom webshop

Note: I was never involved with the development of the ATUSB hardware, firmware or driver software at any point in time. All credits go to Werner, Stefan and other contributors around ATUSB.

Syndicated 2016-12-07 00:00:00 from LaForge's home page

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