Transparent data transmission means that there is no GSM error correction supplied over the air interface.
Non-Transparent data only applies to data transfers and not faxing since the Group 3 standard for fax transmission is a transparent protocol.
Most GSM networks support both non transparent and transparent data communications.
A transparent data call can be started on V.110 (link a ISDN data call) or on V.32.( like in a PSTN modem). The benefit of a V.110 call is that the two modems ore synchonise faster then in a use of V.32.
The transmit by RLP has every time a small delay included. If you have to come to real time then you have to use the not save transparent tranmission.