5 Normal Procedures

3GPP42.056GSM Cordless Telephony System (CTS), Phase 1Release 17Service descriptionStage 1TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

5.1 Enrolment of CTS-FP

5.1.1 CTS operator procedure for enrolment of CTS-FP

At enrolment, the CTS-FP shall receive data needed for correct operation of the CTS radio interface, e.g. the following:

the set of carrier frequencies;
the maximum transmit power it is allowed to use;
data related to CTS-roaming.

At enrolment, the CTS-FP may also receive other data necessary for the CTS-FP to operate, e.g. the following:

operation data necessary to contact the CTS operator (e.g. a fixed network destination address where the information shall be sent).

As a minimum requirement one of the following three mechanisms to provide data to the CTS-FP shall be supported:

a) by means of network information via the radio interface (e.g. a SMS message, or a system-information message) from the cellular network to the CTS-MS, which then forwards the information to the CTS-FP. This implies that the MS must recognize that this information is intended for CTS-FP use, i.e. it shall forward the relevant information, without any display to the user. A means of passing the initial parameters to the CTS-FP is required;

directly on a fixed network connection between CTS-FP and the CTS operator;
Directly from the GSM network when the CTS-FP is connected via radio to a GSM PLMN.

Cordless access to the CTS services via this CTS-FP shall not be possible unless enrolment has been successfully completed.

Enrolment shall be based on authentication of the FP-SIM.

5.1.2 User procedure for enrolment of CTS-FP

To prevent misuse of the CTS-FP or unintended start of the enrolment procedure, the following user actions may be required:

the CTS-FP and a CTS-MS may be used for CTS-FP enrolment;
a well defined user action at the CTS-FP and/or the CTS-MS in order to enter a CTS-FP enrolment state (e.g. by pressing a button or by entering a certain menu);
the user is informed when the procedure is finished (e.g. tones or icons).

5.2 Enrolment of CTS-MSs

After enrolment of the CTS-FP it may be possible to initialise several CTS-MSs on that CTS-FP.

To avoid unauthorized enrolment of a CTS-MS onto a CTS-FP, CTS-MS enrolment shall involve mutual authentication of the CTS-MS and the CTS-FP. Some security sensitive information, e.g. the CTS-MS authentication key, shall be securely stored inside the MS-SIM and in CTS-FP. Some other information needs to be exchanged between CTS-FP and CTS-MS to create data records required to recognize and access each other. This shall be done directly via signalling on the GSM-CTS Radio Interface.

Each CTS-MS to be enrolled shall indicate that the CTS service is subscribed to.

In addition, depending on the roaming control level defined by the CTS operator, enrolment may be performed with CTS operator control including CTS-FP authentication by the CTS operator and CTS-MS authentication. To prevent misuse of the CTS-FP by unauthorised users, the following actions shall be required:

the CTS-FP and a CTS-MS shall be used for CTS-MS enrolment;
a well defined user action at the CTS-FP and/or the CTS-MS in order to enter a CTS-MS enrolment state (e.g. by pressing a button or by entering a certain menu);
there shall be a mechanism to prove authorisation of that particular CTS-MS to use the CTS-FP;
the user is informed, via the CTS-MS, when the procedure is finished (e.g. tones or icons).

At CTS-MS enrolment a unique CTS-MS number is optionally assigned to the CTS-MS. This number is intended to be used to identify a specific CTS-MS, e.g. for de-enrolment or for addressing an internal call or call transfer towards this CTS-MS. This CTS-MS number shall be in the range of 0 to 99.

5.3 De-enrolment of a CTS-MS

It shall be possible to de-initialise a specific CTS-MS which is enrolled at a CTS-FP. This can be performed either by a CTS-MS, by the CTS-FP or by the CTS operator.

In the case de-enrolment is performed by a CTS-MS, to prevent misuse by unauthorised users or unintended CTS-MS de-enrolment, the following actions shall be required:

the CTS-FP and a CTS-MS shall be used for CTS-MS de-enrolment. The CTS-MS used may be any CTS-MS which is enrolled at the CTS-FP including the one which is going to be de-enrolled;
a well defined user action at the CTS-FP and/or the CTS-MS in order to enter a CTS-MS de-enrolment state (e.g. pressing a button or entering a certain menu);
there shall be a mechanism to prove authorisation of that particular CTS-MS to use the CTS-FP;
the user is informed, via the CTS-MS, when the procedure is finished (e.g. tones or icons).

When de-enrolment is performed by the CTS-FP, the CTS-FP deletes all the data related to the de-enrolled CTS-MS and notifies the CTS-MS when possible.

When de-enrolment is performed by the CTS operator a specific command is sent to the CTS-FP resquesting de-enrolment of a given CTS-MS. The CTS-FP sends a de-enrolment notification to the CTS-MS when possible.

The de-enrolled CTS-MS shall not be able to attach to the CTS-FP until it is again enrolled with the CTS-FP.

5.4 Attachment of CTS-MS

An MS cannot attach with a CTS-FP without being enrolled to that CTS-FP. An MS cannot make calls via a certain CTS-FP, without being attached with that CTS-FP.

To avoid unauthorized attachment of a CTS-MS at a CTS-FP, mutual authentication between the CTS-FP and the CTS‑MS shall be performed during attachment.

5.5 Call procedures

5.5.1 CTS-MS originated calls

Calls from the CTS-MS are set up via the CTS-FP or the GSM network, depending on the current mode and coverage. In parallel mode it should be possible for the user to choose the preferential call origination network and regardless of the preferential origination, to select on per call basis the call set up network.

The CTS-MS shall indicate to the user if the CTS-FP cannot be used to set-up a call, e.g. due to that the line is busy or the CTS-FP is engaged in a CTS internal call.

In case of network origination, the CTS-MS/CTS-FP pair shall behave as a usual terminal of the network on which the CTS-FP is connected.

If the CTS internal call hold/call retrieve feature is supported, the user can put the call to the network on internal hold, e.g. to set up a CTS internal call, and retrieve it afterwards.

5.5.1.1 Number compatibility between GSM formats and fixed network formats

In order to use the GSM number formats (e.g. using ‘+’ as part of a dialled number) when originating a CTS call via the MMI of the CTS-MS, a number conversion from GSM format to fixed network format is required in the CTS-FP.

The following types of conversions are required:

replacing the GSM international prefix (‘+’) with the appropriate fixed network international prefix, as defined by the user, when the country code in the call set-up does not equal the stored (home) country code;
replacing the GSM international prefix (‘+’) and the country code with an appropriate prefix, as defined by the user (e.g. a ‘0’), when the country code in the call set-up equals the stored (home) country code;
route the Emergency call set-up to the appropriate emergency number in the fixed network;
removing a potential (home) area code, as defined by the user.

In order to support the above-mentioned features, the user shall either be able to define and modify the following data in the CTS-MS MMI and then pass it to the CTS-FP or define the conversions in the CTS-FP:

the fixed network international prefix;
the (home) country code;
the (home) area code;
the fixed network emergency number;
prefix to replace "international part" of a national number (e.g. replacing +468123 with 08123).

5.5.2 CTS-MS terminated calls

External calls to the CTS-MS are directed either via the GSM network (using the MSISDN) or via the CTS-FP (using the CTS-FP network or GSM number). Each network could then direct the call to any destination, e.g. according to registration status or normal "Call forwarding" settings.

In addition, internal calls may be supported, as described in subclause 5.5.3.

Two different types of alerting the CTS-MS from the CTS-FP shall be supported:

group alerting: all CTS-MSs attached with the CTS-FP will be paged until one CTS-MS (or another fixed network terminal) accepts the call. The other CTS- MSs will then not be paged any more and shall stop alerting the user;
individual alerting: if a call can be determined, by the CTS-FP, as intended for a certain CTS-MS, only that CTS‑MS will be paged. To determine an individual CTS-MS, different methods can be used, e.g. the following:

– use of the CTS-MS local number;

– use of the MS-ISDN.

When receiving a call, the user shall be informed where the call comes from, i.e. via GSM network or CTS-FP, and in the case of CTS-FP whether it is a CTS internal call or an external call. As an option in the CTS-MS, an indication on the type of alerting (group or individual) should be given. This could be realised e.g. by different alerting tones.

If an incoming call via the CTS-FP is answered by a fixed terminal (e.g. phone or answering machine), the CTS user shall be able to retrieve the call at any time while the call lasts.

If the CTS internal call hold/call retrieve feature is supported, the user can put the call from the external network on internal hold, e.g. to set up a CTS internal call, and retrieve it afterwards.

If a MS is involved in an internal call and an incoming call for this MS is presented on the CTS-FP, the user shall be informed of the new call, e.g. by a tone. It shall be possible to distinguish internal and external calls. The user will have the possibility to ignore the new call or to release or to hold the current call and to take the new one.

5.5.3 CTS internal calls (optional)

To set up an internal call, a specific action or command shall be used in order to distinguish between external calls (to the fixed network or the GSM network) and CTS internal calls. The CTS-MS number, which is assigned at enrolment shall be used for dialling. It should be possible to find the CTS-MS number corresponding to a MSISDN number by a MMI on the CTS-FP and/or on the CTS-MS.

Only the CTS-MS which is associated with the CTS-MS number will be paged. A specific alerting tone or melody may be defined for internal calls in order to differentiate internal and external calls.

5.5.4 CTS internal call transfer (optional)

If the internal call transfer feature is supported, the user can transfer a call from the fixed network to any CTS-MS which is attached at the same CTS-FP. The destined CTS-MS is addressed by its CTS-MS number which is assigned at enrolment.

An external network call can directly be transferred to the destined CTS-MS or it can be put on hold before (see above).

5.6 Maintenance

The data exchanged during the initialisation may be updated, using the same mechanisms as developed for initialisation. The update of the initialisation parameters intended for correct operation of the CTS radio interface (as described in subclause 5.1.1) after initialisation time shall be under the control of the GSM operator.

Means shall be developed so that a CTS-FP cannot be moved to an area , where the CTS frequencies are not allowed to be used by the CTS subscriber, without having to be re-initialised.

The MS when attached to GSM, but not attached to the CTS-FP, and with a user selection for parallel mode or automatic mode with CTS preferred, shall attach with the CTS-FP within 2 minutes after coming in range of the CTS‑FP.

The MS when being switched on within range of the CTS-FP, and with a user selection for CTS mode, automatic mode with CTS preferred, or parallel mode, shall attach with the CTS-FP within 1 minute.