08.623GPPInband Tandem Free Operation (TFO) of speech codecsService descriptionStage 3TS
A.2.1 IS_REQ Message
With the IS_REQ Message an IS_Sender can test, if there is an IS Partner and indicates that it is willing to negotiate.
IS_REQ is used to initiate the IS Protocol or to indicate changes in the configuration, etc.
IS_REQ has at least one IS_Extension_Block, containing the IS_System_Identification. (see A.5).
Other IS_Extension_Blocks may follow, see Figure 17.
Figure 17: General Construction of an IS_REQ Message
In general an IS_REQ Message shall be as short as possible. Special care must be taken in the design of the IS_Extension_Blocks to avoid audible effects, since sometimes an IS_REQ Message may be transmitted for quite some time (several seconds).
A.2.2 IS_ACK Message
With the IS_ACK Message an IS Partner typically answers an IS_REQ Message or an IS_ACK Message. It can also be used to submit further information to the other IS Partner. IS_REQ and IS_ACK are the main message types between IS Partners.
The IS_ACK has at least an IS_Extension_Block containing the IS_System_Identification (see A.5).
Other IS_Extension_Blocks may follow, see Figure 18.
Figure 18: General Construction of an IS_ACK Message
No specific design constraints with respect to audibility exist, since IS_ACK is typically not sent very often.
A.2.3 IS_IPE, IS_TRANS and IS_NORMAL Messages
The IPE command denotes IS_IPE Messages. An IPE shall always look for this type of message and follow the instruction. An IS_Sender shall use this IS_IPE Message to command all IPEs into a specific mode of "Bit Transparency".
This Message has one IS_Extension_Block, indicating the requested IPE_Mode. See Figure 19.
Figure 19: General Construction of an IS_IPE Message
No specific design constraints with respect to audibility exist, since IS_IPE is typically not sent very often.
Table 21 defines 16 out of 32 possible IPE_Commands. The other codes are reserved for future extensions.
Table 21: Defined IPE_Modes
MEANING / ACTION
pass 1 LSB; 7 upper Bits are used
pass 2 LSBs; 6 upper Bits are used
pass 3 LSBs; 5 upper Bits are used
pass 4 LSBs; 4 upper Bits are used
pass 5 LSBs; 3 upper Bits are used
pass 6 LSBs; 2 upper Bits are used
pass 7 LSBs; 1 upper Bit is used
Full Transparent Mode for all eight bits
pass 1 LSB; 7 upper Bits are free and unused
pass 2 LSBs; 6 upper Bits are free and unused
pass 3 LSBs; 5 upper Bits are free and unused
pass 4 LSBs; 4 upper Bits are free and unused
pass 5 LSBs; 3 upper Bits are free and unused
pass 6 LSBs; 2 upper Bits are free and unused
pass 7 LSBs; 1 upper Bit is free and unused
The IPE_Mode is protected by the binary, systematic (16,5) block code with generator polynomial g(x) = x^11 + x^7 + x^5 + x^4 + x^2 + x + 1. The minimum Hamming distance of this code is dmin=7, which allows the correction of up to 3 bit errors within each code word of length 16 bits.
Bits 1 (MSB) and 11 are the synchronisation bits and set to "0", see Figure 20. The EX field is set to "0.0" in all currently defined IPE_Modes, i.e. no further IS_Extension_Block is following.
Table 21 defines the coding in hexadecimal notation for the complete IPE_Mode_Extension_Block, with EX := 00.
Figure 20: IPE_Mode_Extension_Block for the IS_IPE Message
An IS_ IPE Message containing the NORMAL command is termed IS_NORMAL Message.
An IS_ IPE Message containing a TRANS_x command is termed IS_TRANS_x Message.
An IS_ IPE Message containing a TRANS_x_u command is termed IS_TRANS_x_u Message.
The latter two are sometimes also termed IS_TRANS Message, if the details are not important.
The behaviour of IPEs, when receiving such commands, is described in Annex B.
The first IS Message in a series is often "swallowed" by IPEs (see Annex B). An IS_IPE Message must therefor never be the first message of a series of IS Messages, i.e. it shall be sent as an isolated IS Message or after a (sufficiently long) uninterrupted IS Protocol.
A.2.4 IS_FILL Message
The IS_FILL Message has no IS_Extension_Block and no specific meaning. An IS_ Sender can use the IS_FILL Message to fill a temporary gap in the protocol flow. This may be important to keep all IPEs in synchronization and open for further IS Messages. See Figure 23. An IS_FILL Message shall also be used by the IS_Sender to resynchronize all IPEs in case of a phase shift of the Keep_Open_Indication.
Figure 21: Construction of the IS_FILL Message
IS_FILL is designed in a way that multiple repetitions cause minimal audible effects.
A.2.5 IS_DUP Message
The IS_DUP Message may be used between IS Partners to indicate an half duplex mode. It may be especially important in Handover situations. The IS_DUP Message has no IS_Extension_Block, see Figure 22.
Figure 22: Construction of the IS_DUP Message
A.2.6 IS_SYL Message
The IS_SYL Message may be used between IS Partners to indicate the loss of synchronisation. It may be especially important in Handover situations. The IS_SYL Message has no IS_Extension_Block, see Figure 22.
Figure 23: Construction of the IS_SYL Message