Cellular Network Infrastructure » Radio Access Network » OsmoBSC

In gsm48_send_rr_ciph_mode() I ran into a magic formula:

        if (lchan->encr.alg_id <= RSL_ENC_ALG_A5(0))
                ciph_mod_set = 0;
        else
                ciph_mod_set = (lchan->encr.alg_id-2)<<1 | 1;

To clarify:

ciph_mode_set should match 3GPP TS 44.018 10.5.2.9:

bits
4 3 2
0 0 0   A5/1
0 0 1   A5/2
0 1 0   A5/3
0 1 1   A5/4
1 0 0   A5/5
1 0 1   A5/6
1 1 0   A5/7
1 1 1   reserved

which as in a5/x means ciph_mod_set = x - 1

In bssmap_handle_cipher_mode(), we call gsm0808_cipher_mode(cipher) with cipher either 0 or 1. We want cipher x as in a5/x (e.g. a5/3 means cipher 3).

We then set

conn->lchan->encr.alg_id = RSL_ENC_ALG_A5(cipher) == cipher + 1;

This is the encoding used in the RSL Channel Activation message.

Finally we convert back to x-1, and shift one bit to the left:

 ciph_mod_set = (lchan->encr.alg_id-2)<<1 | 1;

So it is correct, but it would be much easier to read if we stored the a5_n value and converted in the appropriate places, instead of storing one encoding and converting back to the other by a magic formula.

Otherwise at least place comments to clarify.