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TS 27.003
Terminal Adaptation Functions (TAF) for Services
using Synchronous Bearer Capabilities

V17.0.0 (PDF)  2022/03  19 p.
V16.0.0  2020/06  19 p.
V15.0.0  2018/06  19 p.
V14.0.0  2017/03  19 p.
V13.0.0  2015/12  19 p.
V12.0.0  2014/10  19 p.
V11.0.0  2012/09  19 p.
V10.0.1  2011/04  19 p.
V9.0.0  2009/12  19 p.
V8.0.0  2008/12  19 p.
V7.0.0  2007/06  19 p.
V6.0.0  2005/01  19 p.
V5.0.0  2002/01  19 p.
V4.1.0  2001/04  19 p.
V3.5.0  2000/10  25 p.
GSM Rel-98 v7.0.0  1999/06  32 p.
GSM Rel-97 v6.0.0  1999/06  32 p.
GSM Rel-96 v5.4.1  1998/01  32 p.
GSM Phase-2 v4.5.1  1994/02  34 p.
GSM Phase-1 v3.4.0  1993/01  51 p.
Mrs. Biondic, Nevenka
Ericsson LM

Content for  TS 27.003  Word version:  17.0.0

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1  ScopeWord‑p. 5

The present document defines Terminal Adaptation Functions (TAF) which are integrated in a Mobile Termination (MT) and which enable the use of synchronous bearer services in the PLMN and the attachment of Synchronous synchronous Terminals terminals to an MT (see TS 24.002). For the case where asynchronous terminals are attached to the TAF when using synchronous bearer services in the PLMN, the reader is referred to TS 27.002 for the asynchronous MT-TAF interface specifics and to the present document for synchronous bearer service specifics on the TAF-IWF interface. The general aspects of Terminal Adaptation Functions are contained in specification TS 27.001. The present document covers support of synchronous data services (see TS 22.002) for the following interfaces and procedures:
The support of these services is still optional. The specification of these services is not within the scope of the present document. For that, the reader is referred to GSM R98 or R99 respectively.

2  ReferencesWord‑p. 5

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.
  • References are either specific (identified by date of publication, edition number, version number, etc.) or non specific.
  • For a specific reference, subsequent revisions do not apply.
  • For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.
TS 43.010: "GSM Public Land Mobile Network (PLMN) connection types".
TS 24.002: "GSM - UMTS Public Land Mobile Natwork (PLMN) access reference configuration "
TS 44.021: "Rate adaption on the Mobile Station - Base Station System (MS - BSS) interface".
GSM TS 08.20: "Rate adaption on the Base Station System - Mobile services Switching Centre (BSS - MSC) interface".
TS 22.002: "Circuit Bearer Services (BS) supported by Public Land Mobile Network (PLMN)".
TS 24.008: "Mobile Radio Interface Layer 3 specification; Core Network Protocols-Stage 3".
[8]  Void.
TS 27.001: "General on Terminal Adaptation Functions (TAF) for Mobile Stations (MS)".
TR 21.905: "Vocabulary for 3GPP Specifications ".
ITU-T Recommendation I.420 (1998): "Basic user network interface".
[12]  Void.
[13]  Void.
[14]  Void.
ITU-T Recommendation V.22 (1988): "1200 bits per second duplex modem standardized for use in the general switched telephone network and on point-to-point 2-wire leased telephone-type circuits".
ITU-T Recommendation V.22 bis (1988): "2400 bits per second duplex modem using the frequency division technique standardized for use on the general switched telephone network and on point-to-point 2-wire leased telephone-type circuits".
ITU-T Recommendation V.24 (1996):"List of definitions for interchange circuits between data terminal equipment (DTE) and data circuit terminating equipment (DCE)".
ITU-T Recommendation V.25 bis (1996): "Synchronous and asynchronous automatic dialling procedures on switched networks".
ITU-T Recommendation V.26 ter (1988): "2400 bits per second duplex modem using the echo cancellation technique standardized for use on the general switched telephone network and on point-to-point 2-wire leased telephone-type circuits".
ITU-T Recommendation V.28 (1993): "Electrical characteristics for unbalanced double current interchange circuits".
[21]  Void.
ITU-T Recommendation V.110 (1996): "Support of data terminal equipments with V Series interfaces by an integrated services digital network".
[23]  Void.
[24]  Void.
[25]  Void.
[26]  Void.
[27]  Void.
[28]  Void.
[29]  Void.
ITU-T Recommendation X.32 (1996): "Interface between Data terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for terminals operating in packet mode and accessing a Packet-Switched Public Data Network through a public switched telephone network or an Integrated Services Digital Network or a Circuit-Switched Public Data Network".
Personal Computer Memory Card Association: "PCMCIA 2.1 or PC Card 3.0 electrical specification or later revisions".
Infrared Data Association IrDA: "IrPHY Physical layer signalling standard".
TS 27.002: "Terminal adaptation functions (TAF) for services using asynchronous bearer capabilities".

2.1  AbbreviationsWord‑p. 7

In addition to the abbreviations listed below, the present document also uses termslisted in TR 21.905.
Infrared Data Association
InfraredPHYsical layer
ITU-Telecommunication Standardization Sector

2.2  DefinitionsWord‑p. 7

The term 'mobile station' (MS) in the present document is synonymous with the term 'user equipment' (UE) as defined in TR 21.905.
The term 'TE2' in the present document is synonymous with the term 'TE' as defined in TR 21.905.
The term 'MT2' in the present document is synonymous with the term 'MT' as defined in TR 21.905.
For the purposes of the present document the following internal definitions apply in addition to those given in TS 21.905.
A/Gb mode:
indicates that the text applies only to a system or sub-system which operate in A/Gb mode of operation, i.e. with a functional division that is in accordance with the use of an A or a Gb interface between the radio access network and the core network
Iu mode:
indicates that the text applies only to a system or a sub-system which operates in Iu mode of operation, i.e. with a functional division that is in accordance with the use of an Iu-CS or Iu-PS interface between the radio access network and the core network

3  GeneralWord‑p. 7

3.1  Customer access configurationWord‑p. 7

The PLMN access reference configuration is described in Figure 1 of TS 24.002 and Figure 1 of TS 27.001. The present document specifically refers to the MTs which support terminal equipments (TE1 or TE2) that use synchronous bearer capabilities.

3.2  Terminal Adaptation FunctionWord‑p. 8

The TAF is functionally part of an MT0, MT1 or MT2 (see TS 24.002). The terminal adaptation provides facilities to allow manual or automatic call control functions associated with circuit switched data services, in case of ITU-T V series interfaces. The following functions are included:
  • conversion of electrical, mechanical, functional and procedural characteristics of the ITU-T V series, type interfaces to those required by a PLMN;
  • bit rate adaptation of ITU-T V series and ITU-T X series data signalling rates and the ISDN 64 kbit/s to that provided in an A/Gb mode PLMN;
  • the mapping of ITU-T V.25 bis [18] AUTO CALL/AUTO ANSWER procedures to the PLMN Layer 3 signalling;
  • the mapping functions necessary to convert ITU-T S interface signalling to PLMN Layer 3 signalling;
  • synchronization procedure, which means the task of synchronizing the entry to and the exit from the data transfer phase between two subscriber terminals. This is described in the specification TS 27.001;
  • filtering of channel control information. This is described in the specification TS 27.001;
  • compatibility checking (see TS 27.001);
  • flow control;
  • in Call Modification function (see clause 4);
  • splitting and combining of the data flow in case of multi substream data configurations.

3.3  TAF Interfacing to other MT functionsWord‑p. 8

TAF interfacing is shown in Figure 1.
Reproduction of 3GPP TS 27.003, Fig. 1: TAF interfacing to other MT functions

4  Terminal Adaptation Functions for synchronous transparent servicesWord‑p. 8

Specification TS 43.010 refers to the models for connection types supporting synchronous transparent services.

4.1  Rate Adaptation in A/Gb modeWord‑p. 9

Rate adaptation on the MS-BS interface is described in TS 44.021. The synchronous data services make use of the following rate adaptation functions: RA1, RA2, RA1/RA1', RA1' and in case of TCH/F28.8 usage, EDGE-MUX. See also Figures 6, 7 and 8 in TS 43.010. The D-bits of the rate adaptation frames are used to convey user data. The S- and X-bits are used to convey channel status information associated with the data bits in the data transfer state, or to carry substream numbering between the Split/Combine functions in case of mult substream operation. For the S- and X bits, a ZERO corresponds to the ON condition, a ONE to the OFF condition.

4.1.1  Rate adaptation - ITU-T V-seriesWord‑p. 9

This is provided as indicated in specification TS 44.021. The functions applied in this case are shown in Figure 2 (see model 2b in Figures 6, 7 and 8 of TS 43.010).
Reproduction of 3GPP TS 27.003, Fig. 2: Rate adaptation for V-series terminals

4.1.2  Rate adaptation - ITU-T X.21Word‑p. 9


4.1.3  Rate adaptation - ITU-T S-interfaceWord‑p. 9


4.2  Interchange Circuit Signalling MappingWord‑p. 9

4.2.1  ITU-T V-series interchange circuit mappingWord‑p. 9

The interchange circuit signalling mapping at the interface between the TE2 and the MT shall conform to ITU-T recommendation V.24 [17]. The signal levels at the interface shall conform either to ITU-T recommendation V.28 [20], or to IrDA IrPHY Physical signalling standard specification [34], or to PCMCIA 2.1 [33], or to PC Card 3.0 [33] 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 [17] 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.
Signal at TE2/MT interface or condition within the MT Mapping direction: MT to IWF Mapping direction: IWF to MT
CT 105not mapped (note 1)
CT 106from status bit X (note 7)
CT 107not mapped (note 5)
CT 108/2not mapped (note 6)
CT 109from status bit SB (note 7)
CT 133not mapped (note 2)
always ONto status bit SA (note 3)
always ONto status bit SB (note 1)
always ONto status bit X (note 4)
ignored by MTfrom status bit SA (note 3)
Circuit Number Circuit Name Ground Data Control
to TE2 from TE2 to TE2 from TE2
CT102Common ReturnX
CT103Transmitted dataX
CT104Received dataX
CT105Request to sendX
CT106Ready for sendingX
CT107Data set readyX
CT108.2Data terminal readyX
CT109Data channel received line signal detectorX
CT114Transmitter signal element timingX
CT115Receiver signal element timingX
CT125Calling indicator (note)X
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 [5]. 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 [22] 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 [22] frames. Between compensations (NIC compensation is coded in two ITU-T V.110 [22] 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 [5]). 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 [17] interface as a change in the clock speed on CT 114 and CT 115.
Up  Multislot configurations (Channel coding TCH/F9.6 or TCH/F4.8 kbit/s)Word‑p. 11

In transparent multislot configurations status bits S1, S3 and the X-bit between the D12 and D13 in the ITU-T V.110 [22] 80-bit intermediate rate frame - are used for transferring substream numbering information. The S4 bit is used for frame synchronization between the parallel substreams (ref TS 44.021).  Channel coding TCH/F14.4 and TCH/F28.8Word‑p. 12

For information on the mapping of the interchange circuit signalling bits in the 14,5 multiframe structure, refer to TS 44.021.

4.2.2  ITU-T X.21 [23] Interchange circuit mappingWord‑p. 12


4.2.3  Case of ITU-T S-interfaceWord‑p. 12


4.3  Call establishment signalling mapping at TE/MT interfaceWord‑p. 12

4.3.1  ITU-T V-series interfacesWord‑p. 12  Call establishment manual operation - utilizing Alternate Speech/Data or Speech followed by Data CapabilitiesWord‑p. 12

Void.  Call establishment manual operation - utilizing the Unrestricted Digital CapabilityWord‑p. 12

In this case the user shall not hear network supervisory tones or answer tone. The data transfer phase shall be entered automatically.  ITU-T V.25bis [18] auto call/auto answerWord‑p. 12

The mapping of the ITU-T V.25bis [18] procedures to the messages of the PLMN Layer 3 signalling (TS 24.008) is defined in clause 4.
Auto Call:
This procedure is provided according to ITU-T V.25bis [18] using only circuit 108/2. A subset of ITU-T V.25bis [18] is shown in Table 4. This subset gives minimum level of control and indication.
During the call establishment phase, i.e. after signalling, call tone according to ITU-T V.25bis [18] shall be generated in the IWF, where appropriate.
Auto Answer:
This procedure is provided according to ITU-T V.25bis [18].
Description IA5Characters
Commands from TE2Call Request with Number
provided 0,1..9,*,#,A,B,C,D
Disregard Incoming CallDIC
Connect Incoming CallCIC
Indications to TE2Call Failure Indication
XX = CB,AB,NT,FC (Note)
INcoming CallINC
Local MT busy
Abort call
No answer
Forbidden call (*)
(*) Forbidden call indication results from contravention of rules for repeat call attempts as defined by the appropriate national approval administration. It is recommended that this is the responsibility of the MT, not the TE2.

4.3.2  ITU-T X-series interfacesWord‑p. 13


4.3.3  ITU-T S-interface (ITU-T I.420 [11]) signalling mappingWord‑p. 13


4.3.4  X.25 Procedures MappingWord‑p. 13


5  Terminal Adaptation Functions for synchronous non-transparent services.Word‑p. 13


6  V-series interface procedures to 3GPP TS 24.008 [7] mappingWord‑p. 14

Interface procedures not directly mappable to TS 24.008 (ie. ITU-T V.25 bis [18] VAL/INV) are not considered. Mobile management procedures of TS 24.008 are not considered applicable.
Mapping of other call establishment or clearing messages to the S interface e.g. "Call proceeding", etc. have not been included. It is assumed that these may be mapped directly and thus are of no relevance to the ITU-T V.25 bis [18] or manual interface.

6.1  Mobile Originated callsWord‑p. 14

Element Derived from
MMI ITU-T V.25 bis [18] message
Called AddressKeypadCRN/CRI/CRS
Called Sub AddressKeypadCRI
Derived from internal settings or MMI information.
Same as HLC
Same as HSC
TS 27.001 gives allowed values
Element Derived from
MMI ITU-T V.25 bis [18] message
CauseDisplay (optional)CFI

6.2  Mobile Terminated callsWord‑p. 14

Call establishment is initiated by receipt of Setup at the MS:
Element Derived from
MMI ITU-T V.25 bis [18] message
Called AddressDisplay (optional)INC
Called Sub AddressDisplay (optional)Not applicable
HLCDisplay (optional)Not applicable
LLCDisplay (optional)Not applicable
BCDisplay (optional)Not applicable
Information for the BC element in the call confirm is derived from e.g. MMI or by internal settings.
Connect is sent in response, CIC from ITU-T V.25bis [18] or in response from MMI.

7  ITU-T X.21 [23] interface procedures to 3GPP TS 24.008 [7] mappingWord‑p. 15


8  Support for packet serviceWord‑p. 15


A (Normative)  L2R FunctionalityWord‑p. 16


$  Change historyWord‑p. 17

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