08.623GPPInband Tandem Free Operation (TFO) of speech codecsService descriptionStage 3TS
The IS_System_Identification_Block is a mandatory IS_Extension_Block for the IS_ACK and IS_REQ messages with the 16-bit Information_Field containing the IS_System_Identification. It identifies the system within which the message is generated. Table 22 shows the defined IS_System_Identification codes.
Table 22: Defined IS_System_Identification Codes
Code (in hex)
either 0x53948, if EX == "0.0"
The only defined code so far is GSM_Identification, see also Figure 26.
Figure 26: IS_System_Identification for GSM
All other codes are reserved. Further IS_System_Identification Codes for other systems shall be defined in a way that the audibility is minimal and the hamming distance to the already defined once is maximal.
The IS_System_Identification is protected by the binary, systematic (16,8) block code with generator polynomial g(x) = x^8 + x^7 + x^6 + x^4 + x^2 + x + 1. The minimum Hamming distance of this code is dmin=5, which allows the correction of up to 2 bit errors within each code word of length 16 bits.
Code word 0x0000 is per definition used for GSM.
The resulting 16 bits are placed into the IS_System_Extension_Block as shown in Figure 26 and then the whole 20 bit word is additionally EXORed with the fixed code word 0x53948 to minimise audible effects.
Annex B (Informative):
In Path Equipment: Generic Rules and Guidelines
Inband Signalling Messages (IS Messages) can be used to construct a specific IS Protocol for the communication between telecommunication entities for various purposes. The original purpose is to establish tandem free operation of mobile-to-mobile calls in GSM networks. The IS Messages provide communication channels inside the speech signal paths between the speech transcoders.
In addition IS Messages allow the control of equipment within the speech signal paths between these telecommunication entities (e.g. speech transcoders). These equipments are termed „In Path Equipments“ (IPEs).
Annex A defines the generic structure of these IS Messages and rules for the IS_Sender.
Annex B defines the generic rules with respect to these IS Messages for the IPEs.
Annex A is mandatory for TFO_TRAU Equipment and informative for IPEs.
Annex B is informative for TFO_TRAU Equipment.
It shall be followed by IPEs, which want to be compatible to IS Messages.