9 Interworking to PSTN

09.073GPPGeneral Requirements on Interworking between the Public Land Mobile Network (PLMN) and the Intergrated Services Digital Network (ISDN) or Public Switched Telephone Network (PSTN)TS

9.1 Speech Calls

9.1.1 Interworking indications to PLMN terminal

An indication to inform the PLMN terminal that:

i) instead of receiving out‑of‑band indications for certain types of failure conditions, a tone or announcement will be received in‑band from the PSTN.

ii) the available compatibility information will be not exhaustive for deducing a GSM Basic Service and there will be a limitation on address ‑ the terminal may be required to accept the call on the basis of indicating its compatibility requirements.

iii) (if a DTE) in‑band handshaking signals should be anticipated.

9.1.2 Transmission aspects

Includes control of Speech Processing and Echo Control Devices, see GSM 03.50.

9.1.3 Generation of In‑band Tones and Announcements (PLMN‑PSTN)

In‑band tones and announcements shall be provided for all speech and 3,1 kHz audio bearer services between a PLMN and a PSTN.

9.2 Data Calls

Low Layer Compatibility Checking on the received GSM bearer capability information element will be carried out by the MSC/IWF to check if the call setup is compatible to the bearer service (3,1 kHz audio) provided by a PSTN and to the IWFs provided by the PLMN.

In case the call setup does not conform to these requirements (e.g. an information transfer capability value "unrestricted digital information" is requested), the call shall fail with an error cause indicating that the network is unable to support the service requested.

As well as compatibility checking subscription checking shall be performed. If the subscription check fails the call setup shall be rejected.

For the case where the MS offers negotiable values in the GSM bearer capability information element (e.g. both transparent and non‑transparent connection element) refer to the definitions specified in GSM 07.01.

For interworking of data calls between a PLMN and a PSTN a modem will be utilized to provide the interworking function.

Figure 1: PLMN PSTN interworking for circuit switched calls

9.2.1 Network interworking mobile originated

9.2.1.1 Selection of interworking function

The interworking function will need to negotiate with the user to establish the appropriate modem selection e.g. data rate, modulation scheme, etc. In addition, it will also be required to convert the signalling format, from a combination of out of band and in band, to that suitable for controlling the modem and the autocalling line procedure function where applicable. In the following modem selection procedures it is assumed that the interworking function and modems will be associated with each MSC. As an alternative, a centralized modem resource is possible as a network provider option. Signalling between the MSC and the centralized modem resource is outside the scope of this specification.

For a data call originated by a circuit mode data terminal on the PLMN, the modem selection is done by using the element "modem type" in the call set‑up message (bearer capability).

In addition, other elements of the call setup will indicate the user rate, etc. to be used via that modem. The use of this information however means that the network is only able to select a modem from the modem pool which conforms to the speed which the terminal is utilizing at the DTE/DCE interface at the MS (e.g. V.22 for 1 200 bps). The exception to this is where the user has selected the non transparent service in which case either an autobauding or multi self selecting speed modem (e.g. V.32) may be used.

In case the GSM-BC(s) received with the set-up message indicated a multislot or 14.4kbit/s-operation (refer to GSM 07.01) and the network does not support this service, the GSM-BC(s) sent with the call proceeding message shall not contain the "fixed network user rate", "other modem type" and "user initiated modification indicator" parameters – the MSC shall discard the multislot or 14.4kbit/s-related parameters and use the fall-back bearer service indicated by the remaining parameters of the GSM-BS(s) on a singleslot configuration (refer to GSM 08.20 and GSM 04.21) on the MSC/IWF-BSS link. The MSC/IWF shall modify the relevant parameters in a possibly present LLC accordingly.

If the MSC supports the multislot or 14.4kbit/s-operation, the GSM-BC(s) shall include the "fixed network user rate", "other modem type" and if applicable the "user initiated modification indicator" parameters. The MSC shall apply a singleslot configuration when the "maximum number of traffic channels" indicates ‘1 TCH’ and the "user initiated modification indicator" indicates either ‘user initiated modification not requested’ or ‘user initiated modification up to 1 TCH/F requested’, otherwise a multislot configuration (refer to GSM 08.20 and GSM 04.21) shall be used on the MSC/IWF-BSS link. In case the MS signals an ACC containing TCH/F4.8 only and the network does not support TCH/F4.8 channel coding, then the MSC may act as if TCH/F9.6 were included in the ACC.

In case the GSM-BC(s) received with the set-up message did not indicate a multislot or 14.4kbit/s-operation, the MSC shall not include the "fixed network user rate", "other modem type" and "user initiated modification indicator" parameters in the GSM-BC(s) of the call proceeding message – the MSC shall use a singleslot configuration on the MSC/IWF-BSS link.

The MSC may negotiate parameters with the MS according to the rules defined in GSM 07.01. For multislot and 14,4 kbit/s-operations the MSC/IWF shall modify the relevant parameters in a possibly present LLC accordingly.

9.2.1.2 Modem Selection

In general terms the indication of the bearer capability parameter "Information Transfer Capability" will be utilized in the call set‑up message to determine when the modem should be selected in the call.

In case of single calls, the modem function shall operate in the calling mode in case of mobile originated calls and in the answering mode in case of mobile terminated calls.

In case of dual data calls (alternate speech/data, speech followed by data) the operation mode of the modem (working in calling or answering mode) depend on the initial call setup direction and on the optional parameter "Reverse Call Setup Direction" information element of the MODIFY message. If this information element is omitted the direction is derived from the initial call setup direction, i.e the mode is the same as in case of single calls.

For the attribute value "3,1 kHz audio Ex PLMN" and "facsimile group 3", the modem will be selected immediately. The line procedure according to V.25 will then be carried out using the appropriate modem functions.

For the Bearer Service 61 "Alternate speech/data" or the Teleservice 61 "Alternate speech/facsimile group 3", (if speech is selected as the first service) and the Bearer Service 81 "Speech followed by data", the modem is made available but not selected until the subscriber indicates the change of service request (see subclause 9.3).

In case of the Bearer Service 61 "Alternate speech/data" and the Bearer Service 81 "Speech followed by data", instead of the line procedures for the autocalling mode according to CCITT Recommendation V.25 (i.e. 1 300 Hz tone sending and 2 100 Hz tone recognition for mobile originated single calls) the manual data calling procedure shall apply. For mobile terminated single calls the modem function shall send the 2 100 Hz answering tone.

For "alternate speech/facsimile group 3" calls refer to GSM 03.45 and 03.46.

9.2.1.3 DTE/Modem interface (Filtering)

The DTEs taken into account for the PLMN at the MS side conform to CCITT’s DTE/Modem interface specifications, which assume basically an error‑free environment, i.e.:

‑ limited distance, point‑to‑point local interconnection of the interface circuits for data and status;

‑ steady state signalling.

The envisaged use of these DTEs in the PLMN environment leads to the exposure of these "interconnections" to the PLMN Radio Channel. To assure proper operation even under these conditions appropriate measures have to be taken. In the "non‑transparent case" the RLP satisfies the requirement for both data and status lines. In the "transparent" case, the

‑ data line aspects have to be dealt with end‑to‑end between the users, while

‑ status line aspects are of concern to the network which are dealt with in the following.

The use of the channel control information for the remote control of the DTE/Modem control interchange‑circuits between the MS and the MSC/IWF (the conveyance of which is supported by the rate adaptation scheme adopted for PLMN application) requires alignment to the particular transmission occurrences in the traffic channel to be taken into account within the PLMN. In principle this can be best achieved by:

‑ relying only on the PLMN outband signalling as far as connection control is concerned;

‑ eliminating the dependence upon the transmission of channel control information via the radio link.

Support for this strategy is given to a certain extent by the confinement of PLMN data connection to:

‑ full duplex operation;

‑ switched service (demand access);

‑ mapping of connection‑control relevant conditions of the DTE/DCE control interchange‑circuits to/from outband PLMN signalling according to GSM 04.08 after successful traffic channel synchronization (refer to subclause 9.2.3.4);

‑ flow control by a network entity supported only in non‑transparent mode;

‑ support of connections with the same user data rate only (no TA end‑to‑end flow control in case of transparent mode).

The only DTE/Modem control interchange‑circuit conditions, which actually are not covered by the above confinements, are the indications of readiness for data transmission, i.e. CT106/109 in case of V.‑series interface and I‑circuit in case of X.‑series interface. As the effect of a conditions change of the aforementioned DTE/Modem interchange‑circuits depends on the:

‑ phase within the course of the connection;

‑ direction of change (ON‑OFF or OFF‑ON).

The required precaution to be applied (Filtering) must be determined individually in view of:

‑ function deduced from the change;

‑ resilience of the connection needed;

‑ error condition possibly invoked due to a delay in performing the condition change of the control interchange circuit;

‑ potential loss of performance in connection usage.

The details of the filtering function are laid down in GSM 07‑series. Filtering of channel control information is only relevant at the MS side in the transparent mode of operation.

9.2.1.4 Mapping of BC‑IE from GSM 04.08 to ISUP (or other)

As it cannot be determined from the called address whether the distant network is a PSTN or an ISDN the same mapping takes place as for ISDN calls (see table 6a), if ISDN signalling is used between different MSCs (e.g. on the link VMSC ‑ GMSC).

9.2.2 Network Interworking Mobile terminated PSTN Originated

This subclause describes the interworking of calls where the calling subscriber cannot generate or communicate Compatibility Information exhaustive for deducing a GSM Basic Service to a PLMN (gateway MSC/interrogating node) because of lack of ISDN signalling capability. Thus the HLR is relieved from any compatibility checking for such calls.

Two methods of allocating MS International ISDN Numbers (MSISDNs) are allowed: Firstly, a separate MSISDN may be allocated for each service, or service option, which a subscriber uses for incoming calls; or, alternatively, a single number, applicable for all incoming calls is used.

It should be noted that it is possible for both schemes to co‑exist within the PLMN and that they are not mutually exclusive.

a) Multiple MSISDNs are used ("The Multi‑numbering Scheme"). See figure 2.

In this scheme, the HPLMN will allocate a number of MSISDNs to a subscriber and associate with each of these numbers some interworking information ("IWI"). According to GSM 03.08 this IWI comprises of either one or two complete GSM Bearer Capability (GSM BC) information elements(s) (Contents according to GSM 07.01 and coded as per GSM 04.08) . In either case, when the HLR receives an interrogation relating to an incoming call (i.e. the MAP "Send Routing Information" procedure), it requests a roaming number (MSRN) from the VLR. This request will contain the GSM BC(s) reflecting the service associated with the called MSISDN, i.e. the GSM BC(s) are passed to the VLR.

If two GSM BC‑IE have to be sent to the VLR they are preceded by a repeat indicator information element according to GSM 04.08. These three information elements shall be included within the MAP parameter "GSM Bearer Capability" of the message "Provide Roaming Number".

At the VMSC, when the incoming call arrives, the GSM BC(s) associated with the MSRN are retrieved from the VLR and sent to the MS at call set‑up.

Where the PLMN specific parameters "connection element" and "radio channel" requirements contained in the retrieved GSM BC‑IE, indicate dual capabilities then the VMSC shall set them according to its capabilities/preferences. Additionally the parameters correlated to those mentioned above may have to be modified in accordance with GSM 07.01.

The same applies to the parameter modem type if "autobauding type 1" is indicated but the IWF does not support this feature. The parameter "data compression" may also be modified according to the capabilities of the IWF.

Where single capabilities are indicated then the VMSC shall use the requested values if it is able to support the service requested. If it is unable to support the requested service then it shall set them according to its capabilities/preferences.

Where the Compatibility Information is provided in a degree exhaustive to deduce a GSM Basic Service (see application rules in subclause 10.2.2), then the VMSC in providing the GSM BC IE in the setup message shall set the PLMN specific parameters to its capabilities/preferences.

On receipt of a Set‑up message containing the compatibility information, the MS will analyse the contents to decide whether the service can be supported (with or without modification, see GSM 07.01) and the call will be accepted or rejected as appropriate.

These negotiable parameters in the GSM BC-IE are: Connection Element (Transparent\non-transparent), Data Compression, number of data bits, number of stop bits and parity as well as the correlated parameters Structure, Intermediate Rate, Modem Type and User Information Layer 2 Protocol. For multislot or 14.4kbit/s–operations additionally the parameters Fixed Network User Rate, Other Modem Type and User Initiated Modification Indicator can be negotiated, see GSM 07.01. This negotiation takes place by means of the MS reflecting back to the MSC a complete bearer capability information element in the call confirm message, with the relevant parameters changed. If this does not take place (i.e. if there is no GSM BC present in the call confirmed message), than the MSC will assume that the values originally transmitted to the MS are accepted.

In case the GSM-BC(s) sent with the set-up message contained the "fixed network user rate", "other modem type" and "user initiated modification parameter" parameters and no multislot or 14.4kbit/s–related parameters (refer to GSM 07.01) are received in the GSM-BC(s) of the call confirmed message or no GSM-BC(s) are received, the MSC shall discard the "fixed network user rate", "other modem type" and "user initiated modification parameter" parameters – the MSC shall use the fall-back bearer service indicated by the remaining parameters of the GSM-BC(s) on a singleslot configuration (refer to GSM 08.20 and GSM 04.21) on the MSC/IWF-BSS link.

On the other hand, if the GSM-BC(s) received with the call confirmed message contain(s) multislot or 14.4kbit/s–related parameters the MSC shall apply a singleslot configuration when the "maximum number of traffic channels" indicates ‘1 TCH’ and the "user initiated modification indicator" indicates either ‘user initiated modification not requested’ or ‘user initiated modification up to 1 TCH/F requested’, otherwise a multislot configuration (refer to GSM 08.20 and GSM 04.21) shall be used on the MSC/IWF-BSS link. In case the MS signals an ACC containing TCH/F4.8 only and the network does not support TCH/F4.8 channel coding, then the MSC may act as if TCH/F9.6 were included in the ACC.

In addition the MS may propose to the network to modify the User Rate as well as the correlated parameters Modem Type and Intermediate Rate in the CALL CONFIRMED message. The network may accept or release the call. For multislot or 14.4kbit/s–operations, the MS may also propose to the network to modify the Fixed Network User Rate and Other Modem Type parameters (see GSM 07.01).

b) A Single MSISDNs is used ("The Single‑numbering Scheme" ). See figure 3.

In the single‑numbering scheme, the HPLMN will allocate one MSISDN to a subscriber, applicable to all services.

In this case, when the HLR receives an interrogation relating to an incoming call without compatibility information exhaustive for deducing a GSM Basic Service (i.e. the MAP "Send Routing Information" procedure), the request to the VLR for a roaming number will not contain compatibility information i.e. a GSM BC.

At the VLR, when the incoming call arrives, there is no GSM BC associated with the MSRN and so the call set‑up to the mobile will not contain the GSM BC element.

In this case, the MS will return a complete single or dual GSM BC in the Call Confirmed message, indicating the service required by the mobile subscriber. The VMSC will analyse this GSM BC(s) and optionally perform subscription checking (see GSM 02.01). If the requested GSM BC can be supported the call is established, otherwise the call will be released.

Mobile terminated, PSTN originated call compatibility information provided not exhaustive for deducing a GSM Bearer Service; HLR uses multiple MSISDN numbers with corresponding BCs.

NOTES: (1) The HLR translates the received MSISDN_ called address (MSISDNk) into the relevant bearer capability information (BCk).

(2) Some parameters of BCk may be provided/modified according to the MSC’s capabilities/preferences. See subclause 9.2.2.

(3) In the "Call Confirm" message, the MS may modify some parameters of the BC. See subclause 9.2.2.

Abbr.: SRI ‑ Send Routing Information

PRN ‑ Provide Roaming Number

MSRN ‑ Mobile Station Roaming Number

IAM ‑ Initial Address Message

SIFICSU ‑ Send Information For Incoming Call Set Up

Figure 2

Mobile terminated, PSTN originated call Compatibility Information provided not exhaustive for deducing a GSM Bearer Service; HLR uses single MSISDN numbers (no corresponding BC stored). Per call MSRN allocation.

NOTE: (1) This BC is derived from information stored in the MS, according to its configuration.

Abbreviations: see figure 2.

Figure 3

9.2.3 Transparent service support

NOTE: See GSM 03.10.

GSM 08.20 identifies the rate adaptation scheme to be utilized on the BS to MSC link. The transcoding function will generate the 64 kbit/s rate adapted format utilizing the 8 and 16 kbit/s intermediate data rates. The MSC to MSC/IWF link (e.g. in the case of handover) will utilize the same 64 kbit/s rate adaptation scheme as that indicated in GSM 08.20.

For the transparent service support the MSC/IWF will select the modem and speed based on the Compatibility information contained in either the call set‑up or call confirmed message reference subclause 9.2.1 and 9.2.2. Where the modem type indicated is one of the multi‑speed versions, e.g. V.32, then the MSC/IWF will restrict the modem to the speed indicated in the call set‑up and call confirmed message, respectively, i.e. will inhibit the modem from changing speed, irrespective of the conditions, error rate, encountered on the PSTN link. This scenario is also applicable for the use of "autobauding" modems, in that only the specifically requested modem type and speed will be selected at the MSC/IWF (however Facsimile Gp 3 can use channel mode modify).

9.2.3.1 Not used

9.2.3.2 Rate adaptation process in MSC/IWF

This process is a reverse of that provided in the Terminal Adaptation function of the MS. The rate adaptation RA1 is based on the CCITT V.110 80 bit frame for TCH/F2.4, TCH/F4.8 and TCH/F9.6 and on A-TRAU frame for TCH/F14.4. GSM 04.21 and 08.20, respectively, refer to the rate adaptation mechanisms to be provided. For multislot configurations refer to GSM 03.10.

Figure 4: Rate adaptation schematic

In case of asynchronous bearer services and the facsimile teleservices in the transparent mode, the IWF shall disregard the value of bits E4, E5, E6 and E7 in the data transmission phase.

9.2.3.3 Mapping of signalling MS/MSC/IWF to modem interface requirements

This process also is a reverse of the function provided in the Terminal Adaption function of the MS for the mapping of DTE/DCE signalling information to Dm channel and in band signalling information. GSM 07.02, and 07.03 refer.

Figure 5: Signalling mapping schematic

In general it is not required for the modem in the MSC/IWF to support a "remote looping" request from a modem in the PSTN. In addition the invocation of a "remote looping" request from the mobile subscriber to a modem in the PSTN need not be supported (see also GSM 07.01). Specific test loops for mobile subscribers to contact may be provided at the network operators discretion.

9.2.3.4 Establishment of end‑to‑end terminal synchronizations

Prior to exposing the traffic channel of a PLMN connection to transmission of user data, the controlling entities of the connection have to assure of the availability of the traffic channel. This is done by a so called synchronizations process:

‑ starting on the indication of "physical connection established" resulting from the PLMN‑inherent outband signalling procedure. This indication is given on sending the message CONNECT in case of MOC, CONNECT ACKNOWLEDGEMENT in case of MTC and MODIFY COMPLETE (which is sent after reception of the ASSIGN COMPLETE message) in case of in‑call modification.

‑ ending by indicating the successful execution of this process to the controlling entity, which then takes care of the further use of the inband information (data, status).

Network interworking within an MSC/IWF is concerned with the terminating side (to the MS) and the transit side (to the fixed network) of a connection. Both sides have to be treated individually related to the synchronizations process.

With respect to the terminating side the procedure for all traffic channel types except TCH/F14.4 is as follows:

‑ sending of synchronizations pattern 1/OFF (all data bits"1"/all status bits "OFF") to the MS using the RA1/RA2 rate adaptation function. In multislot transparent operation, the synchronisation pattern sent is 1/OFF with the exception of the bit positions S1, first X, S3, and S4 which contain the substream number and multiframe alignment pattern (Ref. GSM TS 04.21).

‑ searching for detection of the synchronizations pattern from the MS within valid V.110 frames, and in multislot operation, also searching for the multiframe alignment pattern "0000 1001 0110 0111 1100 0110 1110 101" (Ref. to GSM 04.21) in bit position S4 and substream numbers in bit positions S1, first X, and S3. This implies that the E1, E2 and E3 bit of the V.110 frame shall be checked for the appropriate user rate in order to distinguish the synchronization pattern from the BSS idle data frame.

With respect to the terminating side the procedure for TCH/F14.4 is as follows:

‑ Sending A-TRAU frames with the data rate set in the bits C1-C4 (TS 08.20) and data bits set to one, sending the multiframe structure with the alignment pattern (bit M1) and with the status bits OFF (bit M2) and, in a multislot case, sending substream numbers (bit M2).

– Searching for the detection of the multiframe alignment pattern „0000 1001 0110 0111 1100 0110 1110 101" (TS 04.21) in the bit M1 and, in a multislot case, searching for substream numbers in the bit M2. (Any 5 bit sequence in the multiframe alignment pattern is unique, i.e. the multiframe alignment can take place by recognition of five successive M1 bits.)

With respect to the transit side the procedure is as follows:

‑ holding the modem interchange circuits (with the exception of CT108) in the OFF condition until timer T (see below) expires, when they are switched to ON.

When the frame alignment pattern and, in case of multislot operation or TCH/F14,4, the multiframe alignment pattern have been recognized as a steady state, the MSC/IWF continues sending the synchronizations patterns to the MS unless a timer T (= 500 ms) expires. From this time, after the expiration of the timer T of each allocated traffic channel, the information on CT106 and CT109 from the Local Modem are directly mapped to the Sb and X bits toward the MS, for TCH/F14.4 the Sb and X bits are mapped to the M2 multiframe bits according. to GSM 04.21. The IWF is allowed to map CT 104 to the data bits sent towards the MS and to map data bits received from the MS to CT 103.

Mobile Originated

At the start of timer T for each of the allocated traffic channel(s) of the call, i.e. on receipt of the synchronizations pattern from the MS, circuit 108 to the selected modem associated with the connection will be switched from the "OFF" to "ON" condition, thus initiating the auto calling sequence.

Mobile Terminated

At the start of timer T for each of the allocated traffic channel(s) of the call, i.e. on receipt of the synchronizations pattern from the MS, circuit 108 to the selected modem associated with the connection will be switched from the "OFF" to "ON" condition, thus initiating the establishment of the modem connection.

It should be noted that in a GSM‑PLMN V.‑series and X.‑series interfaces are only supported in full duplex mode. Thus the call control phase can be mapped almost completely to the signalling procedure (the S‑bits during the call control phase are irrelevant). However, the "ready for data" condition (i.e. CT106/109, in case of V.‑series interface, and I‑circuit, in case of X.‑series interface) is mapped directly to the applicable status bits of a V.110 frame towards the MS (see also filtering of channel control information).

9.2.3.5 Network Independent Clocking (NIC)

The network independent clocking function is invoked by the VMSC/IWF when the service requested (MO or MT) is 3,1 kHz Ex PLMN and synchronous. The above rule applies irrespective of the information contained in the GSM 04.08 setup message regarding NIC. For all other services NIC is not used.

Within the GSM network the coding of the values for bits associated with NIC is specified in GSM 04.21/08.20. In the forward (transmitting) direction the multiframes shall be coded in exact accordance with that specified in those GSM 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 implementer.

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 GSM 04.21 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 maximum NIC code framing delay (e.g. in the case of TCH/F2.4, TCH/F4.8 or TCH/F9.6, the number of user data bits necessary to make up an even number of V.110 frames between compensation). NIC compensation is coded in two V.110 frames in the case of TCH/F2.4, TCH/F4.8 or TCH/F9.6 and in one multiframe in the case of TCH/F14.4. This results from the requirements to compensate for maximum clock differences of ±100 parts per million. If the receiving function receives NIC compensations in the average more often than a contiguous period of time corresponding to 10000 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.

Action on loss of synchronization

If five consecutive NIC multiframes in the V.110 frame have incorrect framing bit values in E7 or if the A-TRAU multiframe synchronisation is lost, the receiver shall stop applying clocking compensation to the received data. Resynchronization will be attempted and compensation will resume when synchronization is achieved.

9.2.4 Non‑transparent service support

NOTE: See GSM 03.10.

GSM 08.20 identifies the corresponding necessary support concerning the rate adaptation scheme to be utilized on the BS‑MSC link.

For the non-transparent service support the MSC/IWF will select the modem and speed based on the Compatibility information contained in either the call set‑up or call confirmed message, reference subclause 9.2.1 and 9.2.2. Where the Modem Type indicated is autobauding type 1, the MSC/IWF may select any speed and modem type according to what it can negotiate with the remote modem. In this case User Rate and Fixed Network User Rate, if present, has no meaning.

9.2.4.1 MSC‑IWF Rate adaptation scheme

This will be the same as for the transparent case.

9.2.4.2 Protocol layer structure in the MSC/IWF

GSM 03.10 identifies the protocol layer structures for the non‑transparent case, the physical layer to the PSTN is provided by means of a modem.

9.2.4.3 Re‑constitution of user data

GSM 04.22 refers to the frame of user data in the radio link protocol. The layer 2 relay functions in the MS and the MSC/IWF (identified in GSM 03.10) contain the mechanism for packing and unpacking the user data into the L2R protocol data units.

9.2.4.4 Layer 2 relay functionality

Specific functionality is required of the L2R dependant upon the service which is being requested to be supported. The selection of the appropriate L2R function will be determined by the MSC/IWF on the basis of the bearer capability information signalled in either the call set‑up request, or call confirmation messages. The prime information element being transparent or non transparent service indication. In addition the particular L2R function will be selected on the basis of the users layer 2 indication ‑ type of protocol to be terminated and mode of flow control to be applied (see appropriate clauses of the 07 series).

The specific interaction between the L2R function and the RLP function and the L2R frame structure will be the same as that detailed in the annex to the appropriate GSM 07 series.

9.2.4.5 In band signalling mapping flow control

This entails the L2R function providing the means of controlling and responding to flow control functions of the modem plus any synchronization requirements related to flow control. For synchronous services flow control is covered by the protocol indicated, whereas for asynchronous services a specific rule applies for flow control (see GSM 07.01).

The flow control function chosen will be dependent upon the information contained or not contained in the "user information layer 2" information element of the GSM BC received from the MS.

If flow control is provided, irrespective of the type used the L2R function must:

(a) provide immediate indication of flow control to the fixed network on receipt of flow control request from the MS.

and/or

(b) provide immediate indication of flow control to the MS on receipt of flow control request from the fixed network i.e. in the next available L2R status octet to be transmitted.

Where in‑band (X‑on/X‑off) flow control is in use, then the X‑on/X‑off characters will not be passed across the radio interface.

For outband flow control refer to subclause 9.2.4.9.

If no flow control is provided, the involved end systems are responsible for performing in‑band flow control on their own by taking into account the buffer capacity of the MSC/IWF stated below.

9.2.4.5.1 Conditions requiring flow control towards the fixed network

The L2R function will initiate flow control ‑ if flow control is present ‑ in the following circumstances:

1) The transmit buffer reaches a preset threshold (BACK PRESSURE).

2) The L2R function receives an explicit "flow control active" indication.

No flow control initiation/removal will take place at the L2R function and loss of data may occur if no flow control is provided.

On removal of buffer congestion or receipt of L2R "flow control inactive" the flow control will be removed.

9.2.4.5.2 Conditions requiring flow control towards the MS

The L2R function will transmit to the MS an explicit "flow control active indication" if flow control is provided in the following circumstances:

1) If the receive buffer from the radio side reaches a preset threshold (BACK PRESSURE).

2) If a flow control indication is received from the fixed network customer. On receipt of this flow control indication, transmission of data from the receive buffers towards the fixed network terminal is halted.

On removal of the buffer congestion or fixed network flow control indication, the L2R function will send a "flow control inactive" indication towards the MS. In addition, for the fixed network indication, transmission of data from the receive buffers will be restarted.

If no flow control is provided at the L2R function, no flow control initiation/removal will take place by the MSC/IWF. Data might be lost without any indication by the MSC/IWF to the end systems involved.

9.2.4.6 Data buffers

9.2.4.6.1 Transmit buffers (towards MS)

Incoming data from the fixed network customer shall be buffered such that if the MSC/IWF is unable to transfer data over the radio path the data is not lost.

The buffer shall be capable of holding the data. Its size is up to the implementers. When the buffer is half full flow control towards the fixed network shall be initiated if flow control is provided as per subclause 9.2.4.5.1.

9.2.4.6.2 Receive buffers (from MS)

Incoming data from the MS is buffered such that if the fixed network terminal is unable to accept the data then it is not lost.

The buffer shall be capable of holding the data. Its size is up to the implementers. When the buffer becomes half full, the L2R function will send a "flow control active" indication towards the MS if flow control is provided at the L2R function, as per subclause 9.2.4.5.2.

9.2.4.7 Transportation of the Break condition

The "BREAK" condition must be recognized by the L2R function and passed immediately to the MS. The L2R will generate a "BREAK" condition towards the fixed network on receipt of a break indication from the MS. The action of the "BREAK" on the L2R transmit and receive and the length of the "BREAK" signal to be generated towards the fixed network is described in GSM 07.02.

9.2.4.8 In band signalling mapping modem status information

Status information from the modem will be carried by the L2R function to the L2R function in the terminal adaptation function. Status information carried from the L2R function in the terminal adaptation function to the L2R function in the MSC/IWF may be discarded in which case CT 108 and CT 105 are clamped to ON. The MSC/IWF is not intended to utilize status information for any purpose. The use of "Data carrier detect" or "clear to send" by the terminal adaptation function to determine PSTN link establishment or failure is not utilized by the MSC/IWF; e.g. call clearing, in event of line failure, will be generated normally by the MS and not by the MSC/IWF.

9.2.4.9 Support of out‑band flow control

Out‑band flow control in case of PSTN requires V.42 functionality of the modem (refer to GSM 07.01).

If this functionality is requested by the MS but cannot be provided by the MSC/IWF or the remote (fixed network) modem for any reason, the call shall be supported without V.42 functionality (fall back to the non‑error correction mode according to V.42).

This implies that no flow control initiation/removal (refer to subclause 9.2.4.5.1) is possible towards the fixed network. In this case the L2R transmit buffers in the IWF (towards the MS, refer to subclause 9.2.4.6.1) shall overbridge temporary throughput problems on the radio interface and the case where the MS initiates flow control. The IWF however shall release the connection if an overflow of these buffers occurs in order to prevent loss of data.

9.2.4.10 Establishment of end‑to‑end terminal synchronizations

Prior to exposing the traffic channel of a PLMN connection to transmission of user data, the controlling entities of the connection have to assure of the availability of the traffic channel. This is done by a so called synchronization process

‑ starting on the indication of "physical connection established" resulting from the PLMN‑inherent outband signalling procedure. This indication is given on sending the message CONNECT in case of MOC, CONNECT ACKNOWLEDGEMENT in case of MTC and MODIFY COMPLETE (which is sent after reception of the ASSIGN COMPLETE message) in case of in‑call modification.

‑ ending by indicating the successful execution of this process to the controlling entity, which then takes care of the further use of the in‑band information (data, status).

Network interworking within an MSC/IWF is concerned with the terminating side (to the MS) and the transit side (to the fixed network) of a connection. Both sides have to be treated individually related to the synchronization process.

With respect to the terminating side the procedure is as follows:

– reception of V.110 or A-TRAU frames on all allocated traffic channels for the call is required before the MSC/IWF shall reply with an RLP-UA frame to the MT’s RLP link establishment request (if the MSC/IWF initiates the RLP link establishment, reception of V.110 frames or A-TRAU on all allocated traffic channels for the call must be detected first).

‑ waiting for the RLP link establishment by the MT (in addition the MSC/IWF may initiate the RLP establishment).

Depending upon implementation ‑ CT108 will be turned ON to enable the autocalling/autoanswering function of the selected modem either when the RLP has been established or in parallel to RLP establishment. If CT 108 is turned ON in parallel to the RLP establishment, the modem connection may be established before the RLP is established. In this case, data received from the transit side during RLP establishment shall be stored within the L2R buffers until the RLP establishment at the terminating side has been finished. When the RLP has been established, the information from/to the RLP including status changes will be mapped by the L2R entity applicable to the particular bearer capability.

It should be noted that in a GSM‑PLMN V.‑series and X.‑series interfaces are only supported in full duplex mode. Thus the call control phase can be mapped almost completely to the signalling procedure (the S‑bits during the call control phase are irrelevant). However, the "ready for data" condition (i.e. CT106/109, in case of V.‑series interface, and I‑circuit, in case of X.‑series interface) is derived directly from the traffic channel (see also filtering of channel control information).

9.2.4.11 Data compression

When data compression is invoked within a non‑transparent bearer service, interworking to the fixed network is realized as follows:

The GSM BC is used to indicate the interworking modem type and user rate. The modems must try to negotiate data compression and flow control. If negotiation of data compression fails in the fixed network, the call continues with data compression between MS and IWF only.

9.3 Interworking Alternate Speech / Data Calls

Data in this context is defined here to mean 3,1kHz Ex PLMN , or Facsimile Group 3.

9.3.1 Alternate Speech/Data Interworking

9.3.1.1 General

The procedure for the alternate speech/data bearer services is invoked at MS‑MSC link during the call set‑up phase. This service is invoked by indication of repeated bearer capability information elements in the setup message and/or call confirmed message respectively (preceded by a repeat indicator "circular"), one indicating speech and the other indicating the specific data service, as for normal data calls. The data service requested will be indicated by the information transfer capability, either "3,1kHz audio Ex PLMN" and the specific user data rate etc. or "facsimile group 3", as for normal data calls. The bearer capability first indicated i.e. speech or 3,1 kHz Audio Ex PLMN, etc. determines the first selection required of the network by the subscriber. Depending on the type of service requested and direction of call establishment (M0/MT, see relevant clauses of GSM 07 series) low layer and high layer capabilities may also be included. The MSC/IWF will perform both compatibility checking and subscription checking on both sets of capabilities as for normal data calls. If either the subscription check or the compatibility check fails then the call will be rejected. The only exception to this is when TS61/TS62 negotiation takes place, see GSM 07.01.

The applicable rules for provision of supplementary services are laid down in GSM 02.04.

The "speech" phase of the call, when invoked is handled by the transcoder and will utilize normal telephony teleservice interworking requirements and mobile network capabilities. This includes any requirements for echo cancellers etc. as indicated in subclause 9.1. The "3,1kHz Audio Ex PLMN" and "facsimile group 3" phase of the call, when invoked, will utilize the appropriate data interworking capability (IWF including modem) and may use either the transparent or non‑transparent mobile network capability.

The network shall provide, for service and operational reasons, a rapid and reliable changeover of capability upon request from the mobile user. This changeover may involve the disabling, by‑passing or introduction of particular network functions (e.g. speech coder, modem etc.) and change of the channel configuration on the radio interface. This changeover is initiated on the receipt of the "MODIFY" message (see GSM 04.08) from the MS. The network itself will not initiate a changeover.

9.3.1.2 Mobile originated PSTN terminated calls

The call is set up in the normal manner (but with repeated bearer capability information elements as described in subclause 9.3.1.1 and handled by the MSC/IWF as indicated in the general clause.

9.3.1.3 PSTN originated mobile terminated calls

The call set up request for this particular service is performed in a similar manner to that indicated in subclause 9.2 for normal PSTN originated calls.

When multiple MSISDNs are used by the HLR ("Multi‑numbering scheme"), two GSM BC‑IE (contents in principle according to GSM 07.01, facsimile group 3 constitutes an exception, coded as per GSM 04.08) preceded by a repeat indicator "circular" are passed to the VLR in the MAP operation "provide roaming number" exceptionally for facsimile group 3 where only one GSM BC‑IE with the ITC value set to "alternate speech/facsimile group 3, starting with speech" is needed. The VLR stores this information against the MSRN.

When the call arrives at the visited MSC this information is retrieved from the VLR and sent to the MS in the setup message as defined in GSM 07.01.

If the ITC of the GSM BC‑IE retrieved from the VLR has the value "alternate speech/facsimile group 3, starting with speech" this GSM BC‑IE has to be mapped to two GSM BC‑IEs (preceded by a repeat indicator "circular"), one representing speech, the other representing facsimile group 3. The order in which these two GSM BC‑IEs are sent towards the MS, in the setup message, is a network option.

In order to allow auto answering mode for the data phase (i.e. the call starts automatically with the data phase), the MS can reflect back to MSC the dual Bearer Capability in the Call Confirm message with the BC elements interchanged to those in the original Call Set‑up message (i.e. data element first or negotiate to facsimile only, see subclause 9.2.2 and GSM 07.01). In all other aspects it is handled as indicated for mobile originated.

NOTE: However, the PLMN specific parameters "connection element" and "radio channel requirements" of the retrieved GSM BC‑IE may be modified, or added in line with the principles identified in subclause 9.2.2.

When a single MSISDN is allocated to the subscriber ("single numbering scheme"), the call is handled as described in case b) of subclause 9.2.2. In the "call confirmed" message, however, two GSM BC‑IEs are preceded by a repeat indicator "circular", with the first GSM BC‑IE indicating the initial phase of the connection.

9.3.2 Speech followed by data interworking

9.3.2.1 General

The set up and selection of interworking function for this service is the same as that indicated for the alternate speech/data service. The service will be indicated by repeated GSM BC‑IEs, (the first indicating speech and the second indicating the data service) with the information transfer capability "3,1kHz audio Ex PLMN" and the specific data service parameters as for normal data calls. The GSM BC‑IEs are preceded by a repeat indicator "sequential". The only difference in this service is that speech will always be the first bearer capability selection and once the changeover command "MODIFY" message is received from the MS then all network resources associated with the handling of the speech call may be released for reallocation to other calls, i.e. they will not be required again in the handling of this call. Both mobile originated and terminated are dealt with as detailed in subclauses 9.3.1.2 and 9.3.1.3.