08.623GPPInband Tandem Free Operation (TFO) of speech codecsService descriptionStage 3TS
Concerning the IS Protocol, an IPE can be described with five major States in two main Modes, where the States describe the IPE with respect to the IS Protocol and the Modes describe the IPE with respect to the operation on PCM data. Figure 29 shows a graphical representation of the State diagram of an IPE.
Figure 29: Principle of a State Diagram of an IPE
An IS Message shall be recognized as "error-free", if no error can be detected, neither within the IS_Header, nor in the IS_Command nor in any IS_Extension_Block.
An IS Message shall be recognized as "single-error", if no more than one bit position differs in the IS_Header or the IS_Command_Block or the IPE_Mode_Block or one EX-field or one Sync bit.
An IS Message shall be recognized as "correctable", if the phase position is as in preceeding IS Messages and:
– no more than 2 bit position differs in the IS_Header; and
– no more than 1 error is detected within the IS_Command_Block; and
– no more than 3 errors are detected within the IPE_Mode_Block; and
– no more than 0 error is detected within the EX-field(s); and
– no more than 1 error is detected within the Sync-Bit(s); and
– the total number of detected errors is not higher than 3.
IS Messages, which are error-free, single-error or correctable are also called "valid" IS Messages.
An IS Message shall be recognized as "present", if the phase position is as in preceeding IS Messages and:
no more than 4 bit position differs in the IS_Header and
– no more than 2 errors are detected within the IS_Command_Block; and
– no more than 3 errors are detected within the IPE_Mode_Block; and
– no more than 1 error is detected within the EX-field(s); and
– no more than 2 errors are detected within the Sync-Bit(s); and
– the total number of detected errors is not higher than 4.
Sequences, which differ in more than "present" are not recognized as IS Messages at all ("not_present").
Note that the insertion of T_Bits may change the phase position of an IS Message. The IS Message shall in that case be classified after the removal of the T_Bits.
An octet slip may also change the phase position of an IS Message. If an error-free or a single-error IS Message can be found after considering a hypotetical octet slip (1 sample), then it may be regarded as error-free or single-error and the new phase position shall be regarded as valid, if no valid or present IS Message can be found at the old phase position.
B.3.1 IPE in Sync_Not_Found
After start-up or after a long interruption of the IS Protocol an IPE is in Normal_Mode, performing its normal operation. IS Messages have not been found and consequently no bypassing of IS Messages is performed.
The algorithm for initial synchronization shall be able to detect each single IS Message, especially the first or an isolated one. An IPE shall always, during Normal_Mode and during Transparent_Mode, search for the IS_Header and consequently for complete IS Messages. When found, it can be assumed that with high probability the following IS Messages and the Keep_Open _Indication will stay within the found "grid"or "phase" of every 16th PCM sample, the 16_PCM_Sample_Grid.
An IPE transits from Sync_Not_Found into Sync_Found, if and only if an error_free IS Message is detected. Then the IPE lets the following IS Messages bypass, as described above.
If the first IS Message is an error_free IS_TRANS Message, then the IPE transits directly into the Transparent_Mode.
B.3.2 IPE in Sync_Found
The IPE continues its normal operation, but opens an "IS_Door" every 16th LSB for the bypassing IS Messages.
An IPE shall regard sync as continued, i.e. stay in Sync_Found, if after each IS Message another valid IS Message follows within the same phase position, i.e. within the 16_PCM_Sample_Grid.
For any deviations from a valid IS Message, the IPE transits to Sync_Lost.
If an error_free or correctable IS_TRANS is received in Sync_Found, then the IPE transits into the Transparent_Mode.
B.3.3 IPE in Sync_Lost
In Sync_Lost, an IPE shall search for IS Messages on all positions as for initial synchronisation. In parallel, an IPE shall bypass not_valid, but present IS Messages at the found phase position for a maximum of one second. An IPE shall close the IS_Door after that, if no valid IS Message is following, i.e. transit into Sync_Not_Found.
A single valid IS Message brings the IPE back into Sync_Found.
As soon as the IPE detects in Sync_Found or in Sync_Lost a single or more deviations from an error_free IS Message, then the IPE may optionally open the IS_Door also at positions 1 around the present (0) phase position for a maximum of one second to allow other IPEs in the path for parallel re-synchronization. See Figure 30. The IPE may try to find a continuation of the disturbed IS Message at these 3 positions. If the IPE can detect an error-free or a single-error IS Message in this way, then it shall accept the new phase position, if no IS Message can be found at the old phase position anymore.
Figure 30: Handling of octet slip for fast and parallel re-synchronization of all IPEs (optional)
B.3.4 IPE in Keep_Open_Sync
The IPE enters this State by receiving a valid IS_TRANS Message. This is the main State of the Transparent_Mode.
It depends on the specific IPE, if this Transparent_Mode is active only for the commanded direction (that is the default assumption) or in both directions (because for a specific IPE it might be useless or impossible to maintain Normal_Mode in one direction and Transparent_Mode in the other one).
The IPE shall bypass the commanded LSBs and handle the upper bits accordingly (IPE specific).
The IPE shall search in parallel for IS_IPE Messages (IS_TRANS, IS_NORMAL) and
transit – if necessary – to Normal_Mode or an other Transparent_Modes (other number of transparent LSBs).
The IPE shall monitor the bypassing bit stream for the Keep_Open_Indication and accept the Keep_Open_Indication only at the phase position defined by the preceding IS Message.
If the Keep_Open_Indication is not seen anymore then the IPE transits into Keep_Open_Lost.
B.3.5 IPE in Keep_Open_Lost
The IPE shall continue its operation in Transparent_Mode and Keep_Open_Lost for a maximum of one second before it shall return to Normal_Mode.During that time the IPE shall try to resynchronize either by finding an IS Message or by finding the Keep_Open_Indication at positions 1 and 0 around the present phase position (handle of Octet Slip).
The IPE may take advantage of the fact that T_Bits are inserted or deleted by the IS_Sender in case of an intentional phase adjustment.
An IS Message at any arbitrary phase position followed by a valid Keep_Open_Indication is accepted as re-defining the Keep_Open phase position, if and only if the Keep_Open_Indication is no longer present at the old phase position.
A Keep_Open_Indication at a phase position 1 PCM sample interval around the old phase position is accepted as re-defining the Keep_Open phase position, if and only if the Keep_Open_Indication is no longer present at the old phase position.
The Keep_Open_Indication is valid, as long as at least 40 "0"-Bits are seen at the correct positions within a sliding window of length of one second. At least one "1"-Bit must be seen in between each pair of the expected "0"-Bits.