08.623GPPInband Tandem Free Operation (TFO) of speech codecsService descriptionStage 3TS
B.2.1 Typical IPEs are IS_Passive
General: An IPE shall never actively initiate the exchange of IS Messages. The active initiation is only done by terminals or their "representatives". This avoids uncontrolled and unnecessary fluctuation of IS Messages within the network.
Most IPEs shall never actively respond to IS Messages by sending other IS Messages. They are called IS_Passive.
They need not and do not understand the IS Protocol, but let it just pass unaltered and obey the relevant IS_IPE Messages.
Some IPEs may, however, respond on received IS Messages, modify these and/or respond with own IS Messages, if they understand the IS Protocol and can take or bring advantage to the overall system performance or system quality. These IPEs are called IS_Responsive. Examples are GSM-specific Digital Circuit Multiplication Equipments (GCMEs), which reduce transmission costs without degrading the speech quality. These IPEs may be able to step into the IS Protocol, interpret and respond to it and modify the speech signal in an system_compliant way. Thus they become IS_Active Partners themselves.
B.2.2 IS Message_Transparency
When commanded into a Transparent Mode, the IPEs are fully transparent at least for the LSBs in all PCM samples. Therefore the following rules are needed only and only do apply for the IPEs, when in Normal_Mode:
IPEs shall let the IS Messages bypass, respectively re-insert them, from their input to their respective output.
They shall not alter them, nor do any kind of error correction. Exceptions are the IS_Responsive IPEs.
B.2.2.1 First IS Message
During its Normal_Mode an IS_Compliant IPE shall always monitor the incoming PCM data stream for the occurrence of the IS_Header sequence. If the IS_Header is detected after a period without IS Messages, the IPE shall store the following IS_Command and IS_Extension_Block(s). During reception of this first IS Message, the normal operation of the IPE is maintained with the consequence that the first IS Message may not appear at the output of the IPE.
B.2.2.2 IS Messages within a Sequence
All further IS Messages which follow directly after the first detected IS Message in the same phase position shall be passed unaltered to the output of the IPE with exactly that delay the IPE would later introduce when commanded into Transparent_Mode by one of the IS_TRANS commands, see Figure 27.
Figure 27: Transparency and Delay for first and following IS Messages
The upper row symbolizes the speech signal at the input of the IPE, with the PCM samples drawn vertically and the IS Messages inserted into the LSBs. The lower row symbolizes the speech signal at the output of the IPE. The vertical lines denote the boundaries of the IS Message elements.
Figure 27 shows an example where the first IS Message is detected, but not passed through. The distortion caused by the first IS Message is still "somehow" there (indicated by the empty dashed boxes in the LSB), but the message is destroyed. The second and third IS Messages are passed through unaltered. Note, however, that the delay of the speech signal is (in this example) substantially higher than the delay of the IS Messages. They travel faster than the speech signal through this IPE.
B.2.2.3 Isolated IS Message
In cases where the first detected IS Message is not immediately followed by further IS Messages, the IPE shall insert this first IS Message (which the IPE has stored) into its output PCM bit stream, with exactly the delay and phase position a second IS Message would have, see Figure 28, which shows an example where an isolated IS Message is travelling through an IPE.
Figure 28: Transparency and Delay for an isolated IS Message
Note that the delay of an isolated IS Message is depending on its own length! Longer IS Messages will have more delay, shorter less. It could – in principle – happen that a second, shorter isolated IS Message would "bypass" the first longer IS Message – with the consequence that the first one would be destroyed. This is especially important when there are several IPEs in the path, since the delay effects accumulate. Therefore it is not allowed to send shorter isolated IS Messages too close after longer IS Messages. IS Messages with same length have no restriction.
In summary: the first IS Message in a series of IS Messages is "swallowed" by an IPE, while all the following IS Messages pass unaltered and with minimal delay. If an IS Message occurs isolated, then it is not swallowed, but delayed by exactly its own length. The latter mechanism ensures that isolated IS Messages can pass through an unlimited number of IPEs.
B.2.2.4 Check if IS Message is following
The checking, whether an other IS Message is following or not is done "on the fly", i.e. bit by bit. This is possible due to the fact that all messages begin with exactly the same IS_Header. The decision, whether an IS Message is an isolated message or the first message in a series, can be done latest after the last bit of the (next) IS_Header. See Figure 28.
Consequently: after detection of the first IS Message, the IS_Header is in any case inserted at the output in the correct position, regardless, whether a second message follows or not.