4 Terminal Adaptation Functions for synchronous transparent services

07.033GPPTerminal Adaptation Functions (TAF) for services using synchronous bearer capabilitiesTS

Specification GSM 03.10 [3] refers to the models for connection types supporting synchronous transparent services.

4.1 Rate Adaptation

Rate adaptation on the MS-BS interface is described in GSM 04.21. The synchronous data services make use of the following rate adaptation functions: RA1, RA2, RA1/RA1′ and RA1′. See also Figure 6 in GSM 03.10. The D-bits of the rate adaptation frames are used to convey user data and 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 multislot 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 ‑ V‑series

This is provided as indicated in specification GSM 04.21 [6]. The functions applied in this case are shown in figure 2 (see model 2b in figure 6 of GSM 03.10 [3]).

Figure 2: Rate adaptation for V‑series terminals

4.1.2 Rate adaptation ‑ X.21

This is provided as indicated in specification GSM 04.21 [6]. The functions applied in this case are shown in figure 3 (see model 2b in figure 6 of GSM 03.10 [3]).

Figure 3: Rate adaptation for X.21 terminals

4.1.3 Rate adaptation ‑ S‑interface

The functions applied in this case are shown in figure 4 (see model 2a in figure 6 of GSM 03.10 [3]).

Figure 4a: Rate adaptation for S‑interface

Figure 4b: Rate adaptation for S‑interface (continued)

There are two cases to be considered for the RA1 function:

a) V‑series interface

For the V‑series type of terminal equipments the rate adaptation functions are as described in GSM 04.21 [6].

b) X.21‑interface

For terminal equipments using the X.21‑interface the rate adaptation functions are identical to those described in GSM 04.21 [6], but the notation used is as described in CCITT recommendation X.30/I.461.

The notation used is as follows:

The conversion of the user rates of 2.4 and 4.8 kbit/s to 8 kbit/s and user rate of 9.6 kbit/s to 16 kbit/s shall be implemented by means of the 40 bit frame structure shown in figure 5.

Figure 5 shows that in addition to the basic frame, a two frame multiframe is employed. In odd frames, octet 0 contains all zeros, whilst in even frames octet 0 consists of a one followed by seven E bits. The order of bit transmission of the 40 bit frame is from left‑to‑right and top‑to‑bottom.

This two frame multiframe corresponds to the 80 bit frame structure presented in GSM 04.21 [6] as shown in figure 6. The 24 information bits P1,..,P8, Q1,..Q8, R1,..,R8 of odd frames correspond with D1,..,D24 and those of even frames correspond with D25,..,D48 respectively. For the status bits there is the following correspondence: odd frame SQ,X,SR,SP = S1,X,S3,S4 and even frame SQ,X,SR,SP = S6,X,S8,S9.

Option for a manufacturer of mobile stations:

In transparent mode support of a packet mode TE1 or TE2/TA, which uses flag stuffing.

Bit number

1

2

3

4

5

6

7

8

Octet 0 Odd frames

0

0

0

0

0

0

0

0

Even frames

1

E1

E2

E3

E4

E5

E6

E7

Octet 1

1

P1

P2

P3

P4

P5

P6

SQ

Octet 2

1

P7

P8

Q1

Q2

Q3

Q4

X

Octet 3

1

Q5

Q6

Q7

Q8

R1

R2

SR

Octet 4

1

R3

R4

R5

R6

R7

R8

SP

NOTE: Bit X, if not used for the optional flow control or for the indication of the far end synchronization, shall be set to 0 (see CCITT Recommendation I.463/V.110).

Figure 5: 40 bit frame structure of CCITT X.30

X.30 Two frame multifr. V.110 80‑bit frame 
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
odd 1 P1 P2 P3 P4 P5 P6 SQ 1 D1 D2 D3 D4 D5 D6 S1
frame 1 P7 P8 Q1 Q2 Q3 Q4 X 1 D7 D8 D9 D10 D11 D12 X
1 Q5 Q6 Q7 Q8 R1 R2 SR 1 D13 D14 D15 D16 D17 D18 S3
1 R3 R4 R5 R6 R7 R8 SP 1 D19 D20 D21 D22 D23 D24 S4

1 E1 E2 E3 E4 E5 E6 E7 1 E1 E2 E3 E4 E5 E6 E7
even 1 P1 P2 P3 P4 P5 P6 SQ 1 D25 D26 D27 D28 D29 D30 S6
frame 1 P7 P8 Q1 Q2 Q3 Q4 X 1 D31 D32 D33 D34 D35 D36 X
1 Q5 Q6 Q7 Q8 R1 R2 SR 1 D37 D38 D39 D40 D41 D42 S8
1 R3 R4 R5 R6 R7 R8 SP 1 D43 D44 D45 D46 D47 D48 S9

Figure 6: Correspondence of X.30 and V.110 frames

4.2 Interchange Circuit Signalling Mapping

4.2.1 V‑series interchange circuit mapping

The interchange circuit signalling mapping at the interface between the TE2 and the MT shall conform to CCITT recommendation V.24; while the signal levels at the interface shall conform either to CCITT 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 04.21 refers to the frame structure and identifies the use of status bits for the carriage of signalling information.

Status bits

The bits S and X are used to convey channel status information associated with the data bits in the data transfer stage as shown below. The S‑bits are put into two groups SA and SB to carry the condition of two interchange circuits. The X‑bit is used to control the condition of circuit 106.

The mechanism for proper assignment of the control information from the transmitting signal rate adapter interface via these bits to the receiving signal rate adapter interface is shown below in table 1.

For the S and X bits, a ZERO corresponds to the ON condition, a ONE to the OFF condition.

General mapping scheme

Table 1: General mapping scheme for V‑series interchange circuits

Table 2: Minimum set of V‑series interchange circuits

Circuit Number

Circuit Name

Ground

Data

Control

to

from

to

from

TE2

TE2

TE2

TE2

CT102

Common Return

X

CT103

Transmitted

data

X

CT104

Received data

X

CT105

Request to

send

X

CT106

Ready for

sending

X

CT107

Data set ready

X

CT108.2

Data terminal

ready

X

CT109

Data channel

received line

X

signal detector

CT114

Transmitter

signal element

X

timing

CT115

Receiver

signal element

X

timing

CT125

Calling in‑

dicator (note)

X

NOTE: CT125 is used with the AUTO ANSWER function of the TAF.

Use of Network Independent Clocking:

Network Independent Clocking is only applicable to calls using ITC value "3.1 kHz audio ex PLMN".

Within the GSM network the coding of the values for bits associated with NIC is specified in GSM specifications GSM 04.21 [6]/GSM 08.20 [9]. 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 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 GSM network 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 GSM 04.21 [6] 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 compensations 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 V.110 frames between compensations (NIC compensation is coded in two 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 GSM System as defined in specifications 04.21/08.20. The NIC function has to manage the conversion of the GSM 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 GSM 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 will be attempted and compensation will resume when synchronization is achieved.

Signal element timing:

Receiver signal element timing (CT115) is generated by MT2. In the transparent case, this shall be synchronized to the output of RA1′ function. In the non transparent case it is output from the L2R on the basis of the current user data rate. 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 V.24 interface as a change in the clock speed on CT 114 and CT 115.

4.2.1.1 Multislot configurations (Channel coding TCH/F9.6 or TCH/F4.8 kbit/s)

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 (ref GSM 04.21).

4.2.1.2 Channel coding TCH/F14.4

For information on the mapping of the interchange circuit signalling bits in the 14.5 multiframe structure, refer to GSM 04.21.

4.2.2 X.21 Interchange circuit mapping

The interchange circuit signalling mapping at the interface between the TE2 and the MT shall conform to CCITT recommendations X.21 and X.24; while the signal levels at the interface shall conform either to CCITT recommendation X.26 (v.10), or to X.27 (V.11) ‑ see also paragraph 2.1 of CCITT recommendation X.21, 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 3.

Specification 04.21 refers to the frame structure and identifies the use of status bits for the carriage of signalling information.

Status bits (S1,S3,S4,S6,S8,S9):

For the purpose of alignment with the case where the X.21 TE2 is connected to the MT via a TA conforming to CCITT recommendation X.30 (I.461), the notation for the S‑bits will be SP, SQ and SR as in figure 5/GSM 07.03. For the bits SP, SQ and SR, a ZERO corresponds to the ON condition, a ONE to the OFF condition.

The bits SP, SQ and SR are used to convey channel associated status information. The mapping of the information on circuit C of the X.21 interface to the S bits and from the S bits to the circuit I in the distant interface should be done in such a way that the SP, SQ and SR bits are associated with the bit‑groups P, Q and R. To assure proper and secure operation the mapping scheme has to be consistent with CCITT recommendations X.21 and X.24.

The mechanism for mapping is as follows:

‑ In all cases where X.21‑byte timing interchange circuit B is not provided, the status bits SP, SQ and SR of the bit groups P, Q and R are evaluated by sampling the circuit C in the middle of the 8th bit of the respective preceding bit group. On the other hand, the conditions of the status bits SP, SQ and SR are adopted by the circuit I beginning with transition of the respective 8th bit of a bit‑group P, Q and R to the first bit of the consecutive bit group on the circuit R.

‑ In the case where X.21‑byte timing interchange circuit B is provided for character alignment, the circuit C is sampled together with the bit 8 of the preceding octet and the circuit I is changing its state at the boundaries between the old and new octets at the circuit R. This operation is defined in CCITT recommendation X.24.

Table 3: X.21 interchange circuits

Interchange circuit

Interchange circuit name

Data

Control

Timing toTE2

to TE2

from TE2

to TE2

from TE2

G

Common return

Ga

TE2 common return

T

Transmit

X

X

R

Receive

X

C

Control

X

I

Indication

X

S

Signal element timing

X

B

Byte timing (note)

X

NOTE: According to CCITT recommendation X.21 the provision of the 8 bit timing interchange circuit B is not mandatory.

4.2.3 Case of S‑interface

At the S‑interface an X.30 rate adapted bit stream is provided by the TE1 or TE2‑TA combination (see figure 4). The terminal adaptation function within the MT does not have any interchange circuit signalling mapping function to perform.

4.3 Call establishment signalling mapping at TE/MT interface

4.3.1 V‑series interfaces

4.3.1.1 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 Mobile Station and the IWF. The mapping for this is shown in section 6.

4.3.1.2 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.

4.3.1.3 V.25 bis auto call/auto answer

The mapping of the V.25 bis procedures to the messages of the PLMN Dm‑channel signalling (GSM 04.08 [5]) is defined in section 4.

Auto Call:

This procedure is provided according to V.25 bis using only circuit 108/2. A subset of V.25 bis 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 V.25 bis shall be generated in the IWF, where appropriate.

Auto Answer:

This procedure is provided according to V.25 bis.

Table 4: Minimum set of V.25 bis Call Set‑up Commands and Indications

Description

IA5Characters

Commands

Call Request with Number

CRN

from TE2

provided 0,1..9,*,#,A,B,C,D

Disregard Incoming Call

DIC

Connect Incoming Call

CIC

Indications

Call Failure Indication

CFI XX

to TE2

XX = CB,AB,NT,FC (Note)

INcoming Call

INC

VALid

VAL

INValid

INV

NOTE to table 4: CB = Local MT busy AB = Abort call NT = No answer FC = Forbidden call *

* Forbidden call indication results from contravention of rules for repeat call attempts as defined by the appropriate national approvals administration. It is recommended that this is the responsibility of the MT, not the TE2.

4.3.2 X‑series interfaces

4.3.2.1 X.21 bis 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.

4.3.2.2 X.21 bis/V.25 bis call establishment signalling mapping

The mapping of the V.25 bis procedures to the messages of the PLMN Dm‑channel signalling (GSM 04.08 [5]) is defined in section 6.

Auto Call:

This procedure is provided according to V.25 bis using only circuit 108/2. A subset of V.25 bis is shown in table 4. This subset gives minimum level of control and indication.

Auto Answer:

This procedure is provided according to V.25 bis.

4.3.2.3 X.21 call establishment signalling mapping

The mapping of the X.21 procedures to the messages of the PLMN Dm‑channel signalling (GSM 04.08 [5]) is defined in section 7.

4.3.3 S‑interface (I.420) signalling mapping

The mapping of Q.931 signalling to 04.08 signalling requires the inclusion, by the MT, of PLMN specific elements (eg. transparent or not, half or full rate channel). The required Bearer Capability Elements are shown in GSM 07.01 [8] Annex 2.

4.3.4 X.25 procedures mapping

User terminals are connected to mobile termination either at S reference point (TE1 or TE2/TA) or at R reference point (TE2). For the physical interface of TE2s all different possibilities are shown in table 9 in section 8.

For more details, see CCITT X.25 and the appropriate interface recommendations.

The mapping is described in section 8.