13 The RA2 Function

3GPP48.020Rate adaption on the Base Station System - Mobile-services Switching Centre (BSS - MSC) interfaceRelease 17TS

The RA2 function shall be applied only for single slot operations in A/Gb mode. For multislot operations the A-interface Multiplexing Function applies (see clause 14).

This procedure is based on the RA2 function as specified in ITU-T V.110. It shall be used to rate adapt to/from the intermediate rates of 8, 16 or 32 kbit/s from/to the 64 kbit/s rate used at the A-interface.

Table 6a

Intermediate rate	Rate at the A-interface
8 kbit/s	64 kbit/s
16 kbit/s	64 kbit/s
32 kbit/s	64 kbit/s
64 kbit/s	64 kbit/s

For the intermediate and user data rate of 64 kbit/s, the RA2 transmits the bit stream over the A-interface as it is.

It considers the 64 kbit/s stream over the A interface to consist of octets, bits 1 through 8, with bit 1 being transmitted first.

The procedure requires that:

i) The 8 kbit/s stream occupies bit position 1;

ii) The 16 kbit/s bitstream occupies bit positions (1,2);

iii) The 32 kbit/s bitstream occupies bit positions (1,2,3,4) ;

iv) The order of transmission of the bits of the subrate stream is identical before and after rate adaptation.

v) All unused bits in the 64 kbit/s stream are set to binary "1".