1. layer23
      1. Usage
      2. Goals

layer23

The layer23 program is the playground we use for developing the GSM Protocol layers 2 and 3 (GSM TS 04.05, 04.06, 04.08)

Using the layer23 program, you can:

  • Connect to /tmp/osmocom_l2 socket created by osmocon to exchange data from L1A_L23_Interface.
  • Receive GSM Layer1 frames.
    • Dump received GSM Layer1 frames and print some analysis.
    • Send the Layer1 frames as GSMTAP packets to wireshark (see WiresharkIntegration)
    • forward the Layer1 frames to the Layer2 / LAPDm implementation inside layer23
    • forward them from Layer2 into Layer3 code inside layer23.
  • receive outgoing frames from Layer3
    • process them through layer2
    • send them as uplink frames to GSMTAP
    • send them to the Layer1 on the phone

The layer23 program is used in combination with the layer1.bin firmware.

Usage

./layer23 -a 871 -i 127.0.0.1

where -a 871 indicates the GSM ARFCN to which layer1 shall tune, and -i 127.0.0.1 indicates the IP address to which the GSMTAP messages are to be sent.

This will connect to the osmocon application and will receive GSM Layer1 frames. The receiving is done in the src/main.c and the frames will be passed to src/l1ctl.c, from there into src/lapdm.c and finally into src/layer3.c

The output of the layer23 application is currently looking like this: 

Found CCCH burst(s): TDMA: (0003/08/39) tc:3 49 06 1b 3b 80 62 f2 20 01 3a c8 07 0a 15 65 04 b9 00 00 80 00 02 1b  si: 0x1b
        SI3 on wrong TC
Found sync burst: SNR: 16384 TDMA: (0003/15/46) bsic: 49
Found sync burst: SNR: 16384 TDMA: (0003/25/05) bsic: 49
Found sync burst: SNR: 16384 TDMA: (0003/09/15) bsic: 49
Found sync burst: SNR: 16384 TDMA: (0003/19/25) bsic: 49
Found CCCH burst(s): TDMA: (0003/07/39) tc:4 31 06 1c 62 f2 20 01 3a 65 04 b9 00 00 80 00 53 2b 2b 2b 2b 2b 2b 2b  si: 0x1c
        SI4 on wrong TC
Found sync burst: SNR: 16384 TDMA: (0003/14/46) bsic: 49
Found sync burst: SNR: 16384 TDMA: (0003/24/05) bsic: 49
Found sync burst: SNR: 16384 TDMA: (0003/08/15) bsic: 49
Found sync burst: SNR: 16384 TDMA: (0003/18/25) bsic: 49
Found CCCH burst(s): TDMA: (0003/06/39) tc:5 01 06 00 b0 00 58 47 eb 4a 92 53 40 62 84 eb 2b 2b 2b 2b 2b 2b 2b 2b  si: 0x0
        SI13 on wrong TC

Goals

The goal for the layer2+ application is to turn it into a complete GSM Stack for Mobile stations. This will include handling of GSM Layer2 (LAPDm, GSM 04.06), GSM Layer3 (GSM 04.08).