Add special 7-bit encoding and decoding functions for USSD coding
Handling 7-bit coding is a little different for USSD, as TS 03.38
states:
To avoid the situation where the receiving entity confuses 7 binary
zero pad bits as the @ character, the carriage return or <CR>
character shall be used for padding in this situation [...].
If <CR> is intended to be the last character and the message
(including the wanted <CR>) ends on an octet boundary, then another
<CR> must be added together with a padding bit 0. The receiving entity
will perform the carriage return function twice, but this will not
result in misoperation as the definition of <CR> [...] is identical to
the definition of <CR><CR>.
The receiving entity shall remove the final <CR> character where the
message ends on an octet boundary with <CR> as the last character.
Jacob has verified the fix with fakeBTS and the wireshark dissector.
Fixes: OW#947
Reviewed-by: Jacob Erlbeck <jerlbeck@sysmocom.de>
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 8a020db..1b985e1 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -132,8 +132,10 @@
gsm48_rr_att_tlvdef;
gsm_7bit_decode;
+gsm_7bit_decode_ussd;
gsm_7bit_decode_hdr;
gsm_7bit_encode;
+gsm_7bit_encode_ussd;
gsm_7bit_encode_oct;
gsm_arfcn2band;