Decoding the Gb Interface with Wireshark

Posted March 31st 2010

The Gb interface is part of the packet radio data network (GPRS) in GSM, it sits between the BSC and the SGSN and carries subscriber data headed to and from the internet.

Wireshark understands how to decode the Gb interface, so you can use wireshark to look through data sniffed from a Gb interface by a Corelatus GTH. Here's what it looks like:

How to tell the GTH to capture Gb

To look at a GPRS network like this, you need to do a few things:

1. Connect one of the GTH's E1 interfaces to the E1 (or T1) interface carrying the Gb interfaces. You typically do that at a cross connect panel, using a G.772 monitor point.
2. Enable the E1 interface you connected.
3. Tell the GTH to start decoding frame relay on that interface
4. Convert the captured data to the file format which wireshark understands, libpcap.
5. Open the captured file in wireshark. (On unix-like operating systems, including OSX, you can also pipe into wireshark to get a live view of the capture.)

Taking those steps one at a time, starting with #2:

Enable the E1 interface

  <set name='pcm3A'><attribute name='monitoring' value='true'/></set>

Tell the GTH to start decoding frame relay

The Gb interface uses frame relay on E1. Different sites use different configurations of timeslots. One common setup is to use timeslots 1--15. Another common setup is to 1--15 + 17--31. The GTH can handle any setup.

  <new>
    <fr_monitor ip_addr='172.16.2.1' ip_port='1234'>
      <pcm_source span='3A' timeslot='1'/>
      <pcm_source span='3A' timeslot='2'/>
      <pcm_source span='3A' timeslot='3'/>
      ..
      <pcm_source span='3A' timeslot='15'/>
    </fr_monitor>
  </new>

Convert the captured data

GTH sends out data in a format described in the API manual. Wireshark wants the data to be in libpcap format. save_to_pcap.erl, in the sample Erlang code for GTH can do the conversion, like this:

  save_to_pcap:from_file("/tmp/captured.raw", "/tmp/captured.pcap").

A lazier approach is to let save_to_pcap.erl configure the GTH and start the capture:

  save_to_pcap:frame_relay("172.16.2.7", "3A", lists:seq(1,15), "gprs.pcap").

The C version of save_to_pcap can currently only convert MTP-2, not frame relay. If you want it extended, send mail.

Start up wireshark

By default, wireshark decodes frame relay as 'FRF 3.2/CISCO HDLC'. That's not quite what we want. Go to Edit/Preferences/Protocols/FR and change the encapsulation to 'GPRS Network Service'. Now you get full decoding.