The interchange circuit signalling mapping at the interface between the TE2 and the MT shall conform to ITU-T recommendation V.24 
. The signal levels at the interface shall conform either to ITU-T recommendation V.28 
, or to IrDA IrPHY Physical signalling standard specification 
, or to PCMCIA 2.1 
, or to PC Card 3.0 
electrical specifications or to later revisions.
The signals required at this interface are shown in Table 2
Specification TS 44.021 refers to the frame structure and identifies the use of status bits for the carriage of signalling information.
Status bits SA, SB and X are used to convey channel control information associated with the data bits in the data transfer state. Table 1
shows the mapping scheme between the ITU-T V.24 
circuit numbers and the status bits for the transparent mode. It also shows how the unused status bits should be handled. It is derived from the general mapping scheme described in annex C. A binary 0 corresponds to the ON condition, a binary 1 to the OFF condition.
The transport of these status bits by the various channel codings is described in subsequent sections.
Use of Network Independent Clocking (applicable to A/Gb mode PLMN only):
Network Independent Clocking is only applicable to calls using ITC value "3.1 kHz audio ex PLMN".
Within the A/Gb mode PLMN the coding of the values for bits associated with NIC is specified in TS 44.021
and 3GPP TS 08.20 
. In the forward (transmitting) direction the multiframes shall be coded in exact accordance with that specified in those specifications. Bit E6 is set to "1" in alternate modified ITU-T V.110 
frames at the transmitter. However, the use of this bit at the receiver for monitoring frame Synchronization, or any other purpose, is not specified and is left to the discretion of the implementor.
A "perfect linear block Code" is used in C1 C5, whose error correction properties may be utilized in the receiver, in order to ensure reliable operation of NIC.
The NIC sending function has to recognize when the difference between the applicable clock speed of the A/Gb mode PLMN and the interface speed generates a positive or negative whole bit requirement. When this positive or negative condition occurs, the NIC codewords specified in specification TS 44.021
are used to transport this condition to the receiving NIC function. Transmission of the codeword shall clear the positive or negative condition related to that codeword at the sending function. The sending function shall not send more than one positive or negative compensation within a contiguous period of time corresponding to 10 000 user data bits minus the number of user data bits necessary to make up an even number of ITU-T V.110 
frames. Between compensations (NIC compensation is coded in two ITU-T V.110 
frames). This results from the requirements to compensate for maximum clock differences of ±100 parts per million. If the receiving function receives NIC compensations more often than a contiguous period of time corresponding to 10 000 user data bits, there is no guarantee that data will not be lost.
The NIC receiving function has to provide the capability to support the compensation requirements of the sending function. This compensation is managed by manipulating the clock speed of the interface, within the standard constraints of that interface.
Overall, the compensation functions have to be capable of managing clock tolerances of ±100 parts per million.
The NIC function has to recognize and manage the conversion of the NIC information received incoming from an ISDN terminal Interface. The conversion has to be made to the NIC format used within the A/Gb mode PLMN System as defined in specifications TS 44.021
and 3GPP TS 08.20 
). The NIC function has to manage the conversion of the PLMN NIC format into that used within the ISDN in the traffic direction towards the ISDN terminal interface.
Due to the incompatibility between the ISDN and the A/Gb mode PLMN requirements NIC interworking is nor provided between these two formats. as such no NIC function is required in providing interworking to the ISDN for unrestricted digital.
Action on loss of synchronization:
If five consecutive NIC multiframes have incorrect framing bit values in E7, the receiver shall stop applying clocking compensation to the received data. Resynchronization shall be attempted and compensation shall resume when synchronization is achieved.
Signal element timing:
Receiver signal element timing (CT115) is generated by MT2. In the A/Gb mode PLMN transparent case, this shall be synchronized to the output of RA1' function. In the UTRAN Iu mode transparent case, this shall be synchronized to output of the RLC. A transition from ON to OFF condition shall nominally indicate the centre of each signal element on CT104.
Transmitter signal element timing is generated by MT2 (CT114), this may be synchronized to CT115.
In the case of alternate Speech/Group 3 Facsimile, there may be a Channel Mode Modify during the course of the facsimile portion of the call. If this occurs, the user data rate changes and this is reflected to the ITU T V.24 
interface as a change in the clock speed on CT 114 and CT 115.