PCAP and protocol analysis

pcap is a data format for captured packets of communication protocols. It is used by a library called libpcap, which in turn is
used by popular network protocol analyzer projects such as tcpdump and wireshark.

In the Ethernet/Internet world, you typically capture packets from your ethernet card using RAW sockets and promiscuous mode.

With GSM protocols such as A-bis, it is obviously not that simple - since they are at least traditionally not transported over IP.

Recording and viewing A-bis communication


Method 1: osmo-nitb PCAP option (obsolete)

The osmo-nitb application inside openbsc provides a command line option to automatically create a PCAP file. The resulting dump is only a subset of what is actually transmitted over the wire. Currently only Link Access Protol D-Channel (LAPD) messages are logged, the actual LAPD header is spoofed and only the TEI and SAPI information is invalid. This is mostly due mISDN not providing us with a LAPD header/frame and the encapsulation we use for wiretap/pcap. In the future there might be a dedicated encapsulation type for the complete mISDN traffic.

To write the protocol dump simply invoke osmo-nitb:

  ./osmo-nitb -p networking.pcap

Method 2: Using misdn_log

This is the preferred method in case you are using the mISDN input driver for OpenBSC, e.g. with a BS-11 BTS.

In order to obtain a A-bis capture and save it in a pcap file, please use the misdn_log tool (part of mISDNuser)
the following way:

  misdn_log -c0 -w networking.pcap

Please make sure to first start osmo-nitb and only then start misdn_log

Method 3: Using tcpdump

If you're using an A-bis over IP based BTS such as the [nanoBTS], then you can use a regular tool like
tcpdump to create a pcap file

  tcpdump -ni eth0 -s 0 -w networking.pcap

where eth0 is the name of the network device connected to the same network as the nanoBTS.


Wireshark already provides dissectors for the various protocols we use (LAPD, RSL, GSM-A, GSM-SMS...). The LAPD protocol dissector needs some minor configuration though. Go to Edit -> Preferences -> Protocols -> LAPD and check the checkbox saying "Use GSM Sapi Values". Afterwards wireshark will be able to display a lot of the A-bis protocol. There are some glitches in the protocol analysis, some missing features and dissection of OML is completely missing.

Also, only the most recent wireshark development versions contain a dissector for the ip.access A-bis over IP protocol.
We recommend you to build wireshark from the latest source code, or alternatively apply the patch that is found in the wireshark
directory of our git repository.

A-bis OML dissector

To add a dissector for the GSM 12.21 A-bis Organization and Maintenance Layer (OML), you can use the abis_oml.patch file
from the wireshark directory of our git repository. This will be submitted for inclusion into wireshark soon.

Dumps for you

Here are some dumps that might be useful. Make sure that you only provide data from your own network and equipment (no IMSI/IMEI you do not know...)

accept-network.pcap - Setup + Location Updating Request + Accept + SMS (4.8 KB) anonymous, 01/03/2009 03:48 PM

reject-network.pcap - Setup + Location Updating Request + Reject (4.39 KB) anonymous, 01/03/2009 04:08 PM

bs11-call.pcap - pcap file containing a MO and MT call leg on a BS-11 (7.92 KB) laforge, 05/09/2011 03:16 PM