2G
From 1995 to 1998, mobile communication remains in 2G phase.
R99
In 1999, 3GPP release its first edition 3G specifications, with the version number R99. 3G-R99 bring new entities UTRAN, UE, Node-B and so forth. They are all RAN (Radio Access Network) related.
In 3G-R99 architecture, the specifications concerned include:
For A-interface and Iu-interface, 24.008,48.008,3GPP 25.413 UTRAN Iu interface RANAP signalling;
Between MSCs, ISUP Q.761-Q.764, TUP Q.721-Q.724;
MAP, 29.002;
CAP, 22.078, 23.078, 29.078.
R4
3GPP encounters Y2K problem, too. So, from 2000 3GPP names version of its specifications R4, R5, R6, R7, etc. 3G-R4 is the prevailing applied version currently.
3G-R4 bring Nc(Network Control), Nb(Network Bearer), Mc(Media Control) interfaces. It separate bearer from signaling.
In 3G-R4 architecture, the specifications concerned include:
For Nc-interface, Q.1901-Q.1902;
For Mc-interface, Q.1950, H.248;
For Nb-interface, ATM, IP, TDM are supported;
23.205 describes the general architecture of 3G-R4.
Internal implementation
23.018 describes the procedures within a MSC. Many SDL process and procedure figures for basic call are involved in it.