5 Exceptions and clarifications for basic call

09.123GPPApplication of ISDN User Part (ISUP) Version 2 for the Integrated Services Digital Network (ISDN) - Public Land Mobile Network (PLMN) signalling interfacePart 1: Protocol specificationTS

5.1 Formats and codes

The formats and codes specified in ETS 300 356‑1 [1] with the exceptions and clarifications described in the tables in annex D shall be applicable.

5.2 Basic call signalling procedures

The basic call procedures of ETS 300 356‑1 [1] apply with the following exceptions and clarifications.

5.2.1 Signalling procedures for connection type allowing fallback

The gateway in the preceeding network (PLMN or ISDN) should always perform fallback according to the procedures defined in ETS 300 356‑1 [1] when the succeeding network does not support fallback.

5.2.2 Echo control procedure

The dynamic echo control procedure according to ETS 300 356‑1 [1] is not supported. Instead the echo control procedure described in ETS 300 121 [19] is applicable.

According to ETS 300 540 [21], for speech and 3,1 kHz audio connection types an acoustic echo control shall be provided in the MS. The PLMN may provide electric echo control.

Echo control procedures are not applicable to 64 kbit/s, 2 ´ 64 kbit/s, 384 kbit/s, 1 536 kbit/s or 1 920 kbit/s unrestricted connection types.

Examples of echo control procedures are shown in annex E.

5.2.3 Calls from the PLMN to the fixed network Gateway in the PLMN

The Initial Address Message (IAM) sent by the gateway in the PLMN is coded taking into account the following exceptions and clarifications. Initial address message

Called party number

The called party number parameter may contain a Mobile Station Roaming Number (MSRN).

– Internal network number indicator:

0 if the MSRN is included;

1 otherwise.

The use of the Internal network number indicator is an option.

User service information

The IAM shall always include a user service information parameter for calls to the fixed network when the ISDN access indicator is set to ISDN.

5.2.4 Calls from the fixed network to the PLMN Gateway in the fixed network Initial address message

Called party number

The called party number parameter contains the MSISDN or MSRN.

– Internal network number indicator:

0 if MSRN is included;

1 if MSISDN is included.The use of the Internal network number indicator is an option.

NOTE: The value of the internal network number indicator is set in the exchange having the relevant knowledge, and is passed transparently through the fixed network. Due to interworking with old signalling systems, an MSRN may be received with the internal network number indicator set to "0" or "1". Gateway in the PLMN Address complete message

Significant delays may be incurred while interrogating location registers and while paging the terminating MS.

Due to the possible delays involved in setting up mobile calls, the PLMN needs to take special action to ensure that timer T7 (awaiting address complete timer) does not expire.

For this reason, whenever a call enters the PLMN, the PLMN should run a timer, the PLMN/ISDN early Address Complete Message (ACM) timer (value 5 to 20 seconds). The timer is started after receiving the complete number. If the timer expires, an address complete message is sent. The PLMN should stop the timer on sending a first backward message (address complete message or connect message).

The ACM shall contain the backward call indicators set to "0" except for:

Backward call indicators

– Charge indicator:

00 (no indication),

01 (no charge) or

10 (charge);

NOTE: The setting of the charge indicator is network specific.

– ISDN access indicator:

1 (ISDN) (preferred value, see table D.3);

– Echo control device indicator:

set according to echo control procedure described in annex E;

– ISDN user part indicator:

1 (ISDN user part used all the way).

Once the PLMN gateway has sent backwards one ACM for a call (either because it has internally generated the ACM or because it has transited an ACM received from another node) it shall not send further ACMs. Information in a subsequent ACM received in the PLMN gateway is mapped to a call progress message. The Event indicator in the CPG message is set according to the mapping rules defined in ETS 300 356‑15 [14].