3 Terminal Adaptation Functions for transparent services

07.023GPPTerminal Adaptation Functions (TAF) for services using asynchronous bearer capabilitiesTS

GSM 03.10 [3] refers to the connection types supporting the transparent services.

3.1 Rate Adaptation

GSM 04.21 [6] describes the rate adaptation scheme to be utilized over the Base Station (BS) to Mobile Station (MS) link. GSM 03.10 [3] refers to the rate adaptation elements to be provided in the MS.

3.1.1 Rate Adaptation ‑ R interface

This is provided as indicated in GSM 04.21 [6].

3.1.2 Rate Adaptation ‑ S Interface (I.420)

The ISDN rate adapted frame format is modified to the PLMN rate adapted format as indicated in GSM 04.21 [6].

3.2 Interchange Circuit Signalling Mapping – V-series interface

The interchange circuit signalling at the interface between the TE2 and the MT shall conform to CCITT Recommendation V.24 [14]. The signals required at this interface are shown in table 3.

The mapping of these signals to the pins of a 25 pin D-type connector is given in ISO 2110. The mapping for a commonly used 9 pin connector is given in Annex B.

3.2.1 Mapping of V.24 circuits to status bits

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 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.

Table 1: Mapping scheme at the MT for the transparent mode

Signal at TE2/MT interface or condition within the MT

Mapping
direction: MT to IWF

Mapping
direction: IWF to MT

CT 105

not mapped (note 1)

CT 106

from status bit X (note 7)

CT 107

not mapped (note 5)

CT 108/2

not mapped (note 6)

CT 109

from status bit SB (note 7)

CT 133

not mapped (note 2)

always ON

to status bit SA (note 3)

always ON

to status bit SB (note 1)

always ON

to status bit X (note 4)

ignored by MT

from status bit SA (note 3)

NOTE 1. The SB bit towards the IWF, according to the General Mapping (annex C), could be used to carry CT 105. However, CT 105 should always be ON in the data transfer state since only duplex operation is supported. Also, many DTEs use the connector pin assigned to CT 105 for CT 133. No interchange circuit shall be mapped to the SB bit which shall always be set to ON in the data transfer state.

NOTE 2. CT 133 is not mapped since there is no flow control in transparent mode.

NOTE 3. The SA bits in both directions are available only with certain channel codings. Therefore, for maximum compatibility, they should not be mapped.

NOTE 4. The X bit towards the IWF is not mapped and shall always be set to ON in the data transfer state since there is no flow control in transparent mode.

NOTE 5. CT 107 is controlled by the channel synchronisation process (07.01).

NOTE 6. CT 108/2 may be used in the call setup and answering processes.

NOTE 7. The status bits are filtered before being mapped to the V.24 circuits (07.01).

3.2.2 Single slot configurations (TCH/F9.6 or TCH/F4.8)

GSM 04.21 [6] refers to the frame structure and identifies the use of the status bits for the carriage of signalling information in transparent mode. The S bits are put into two groups. SA is carried by bits S1,S3,S6,S8 and SB by bits S4,S9 in the ITU‑T V.110 80-bit intermediate rate frame.

3.2.3 Multislot configurations (TCH/F9.6 or TCH/F4.8)

In transparent multislot configurations, status bits S1, S3 and the X-bit between the D12 and D13 – in the ITU‑T V.110 80-bit intermediate rate frame – are used for transferring substream numbering information. The S4-bit is used for frame synchronization between the parallel substreams (reference GSM 04.21). The remaining S bits are put into two groups. SA is carried by bits S6,S8 and SB by bit S9. The remaining X bits can be used as described in section 3.2.1.

3.2.4 Channel coding TCH/F14.4

For information on the mapping of the interchange circuit signalling bits in the 14,5 kbit/s multiframe structure, refer to GSM 04.21. There is no SA bit in this channel coding. Only the SB and X bits are carried.

3.3 Interface Signal Levels – R interface

The signal levels at the interface between the TE2 and the MT shall conform to CCITT V.28, or to IrDA IrPHY physical signalling standard specification, or to PCMCIA 2.1, or to PC‑Card 3.0 electrical specification or to later revisions.

3.4 Call Establishment and Clearing Signalling Mapping

3.4.1 V-series interface Autocalling/answering

The mapping of the V.25 bis [11] procedures to the messages of the PLMN signalling in GSM 04.08 [5] is defined in section 5.

a) Auto Calling

This procedure is provided according to V.25 bis [11] using only 108/2.

A subset of V.25 bis is shown in table 3. This subset gives minimum level of control and indication.

During the call establishment phase, i.e. after signalling, calling tone according to V.25 [32] shall be generated in the IWF (GSM 09.07 [31]).

An alternative to CCITT V.25bis [11] is to use the ITU‑T V.25 ter [12] dial command as specified in GSM 07.07 [8].

b) Auto Answer

This procedure is provided according to V.25bis [11] or to V.25 ter [12].

During the call establishment phase:

– the states of the V.24 interchange circuits shall be according to GSM 07.01[7],

– the data and status bits from the IWF shall not be mapped,

– the data and status bits towards the IWF shall be according to GSM 07.01[7].

3.4.2 S Interface (I.420) Signalling Mapping

The mapping of Q.931 signalling to GSM 04.08 [5] signalling requires the inclusion, by the MT, of PLMN specific elements (e.g. transparent or not, half/full rate channel). For asynchronous Bearer services, requests for bearer capabilities not listed in table 4 (or where the "Users information layer 1 protocol" element does not indicate V.110) will result in call rejection.

3.4.3 Call Establishment Manual Operation ‑ Utilizing Alternate Speech/Data or Speech Followed By Data Capabilities

During manual call establishment, the mobile user shall be able to hear network supervisory tones and answer tone.

On hearing answer tone, the user invokes the transition from speech to data in both the MS and the IWF. The mapping for this is shown in section 5.

3.4.4 Call Establishment Manual Operation ‑ Utilizing the Unrestricted Digital Capability

In this case the user will not hear network supervisory tones or answer tone. The data transfer phase will be entered automatically.

3.4.5 V-series interface Call Clearing

This procedure is provided according to V.25 bis [11] using CT 108/2. An alternative to CCITT V.25bis [11] is to use the V.25 ter [12] hook control command or the hangup commands specified in GSM 07.07 [8]. The mapping of the V.25 bis [11] procedures to the messages of the PLMN signalling in GSM 04.08 [5] is defined in section 5.

During the call clearing phase:

– the states of the V.24 interchange circuits shall be according to CCITT V.24 [14],

– the data and status bits from the IWF shall not be mapped or used by the MT in any way,

– the data and status bits towards the IWF have no significance and may be set to 1 and OFF respectively.