tree 819721bf3ee366ff6a333da64cb646519e1e0a2c
parent 6bfa7445fca074fdf94707681d93e92ec0993bbd
author Andreas Eversberg <jolly@eversberg.eu> 1375958333 +0200
committer Holger Hans Peter Freyther <holger@moiji-mobile.com> 1375964833 +0200

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>