4 Description

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

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

4.1 System overview

The GSM Cordless Telephone System (GSM-CTS) described in the present document is a radio communications system based on a GSM-compatible cellular interface between a private radio base station called CTS Fixed Part (CTS-FP) and a CTS mobile station (CTS-MS). The CTS-FP is connected either via a wireline to the PSTN/ISDN network or via a radio connection to a GSM network.

When connected to the fixed network, the CTS-FP will be compliant with the existing fixed network standards (e.g. support the telephony 3,1 kHz teleservice (see [7]), the speech bearer service (see [8]) and the 3,1 kHz audio bearer service (see [9] in case of an ISDN CTS-FP and the relevant national standards in case of PSTN CTS-FP). There is no direct radio communication between different CTS-FPs. However, this does not preclude indirect communication, e.g. via the fixed network or via the MS.

When connected to a GSM network, the CTS-FP is compliant with the existing ETSI standards applicable to a GSM mobile. From the GSM network point of view, the CTS-FP is seen as a standard GSM mobile including a standard GSM SIM card. (i.e. the CTS-FP may include 2 SIM cards: one for the GSM subscription and one for the CTS subscription).

An illustration of the CTS concept is shown in figure 1. Due to the low transmit power of the CTS-FP, the coverage area is restricted and limited. When the CTS-MS comes in range of the CTS-FP, it may attach to the CTS. When connected to the fixed network, from then on, the user can make and accept calls directly via the PSTN without the intervention of the public cellular network. When attached, the MS checks whether it is still in range of the CTS and whether it is paged. As an option, the MS may simultaneously work in both the cellular and the cordless mode, in a so called parallel mode, i.e. be attached both in the GSM PLMN and the CTS. When the MS comes out of range of the CTS it may switch to the GSM mode. This switch shall be indicated to the user. When the CTS-MS is in either CTS mode or GSM mode and is searching for a potential channel to attach with in the other mode, it shall be able to respond to paging messages from the mode in which it is currently attached.

The radio interface between the CTS-FP and the CTS-MS is a modified GSM interface. The carrier frequencies used are the same as assigned for cellular service. These carrier frequencies can be part of a licence exempt band or a licensed band.

When CTS-FP operates in a licenced band:

– these frequencies can be reserved by the operator for GSM-CTS usage, or can be shared with the cellular system. In every case, however, the GSM operator controls, on an area and time basis, on which frequencies the CTS is allowed to operate. In case of co-existence in the same operating area, a procedure shall be deployed to minimise interference between GSM PLMN and CTS users.

NOTE: The frequencies used for CTS could be in any frequency band defined for GSM, i.e. GSM900, DCS1800 or PCS1900. This applies both to the CTS-FP and the CTS-MS.

The quality of service (e.g. service accessibility, call set-up time, bit rate for data services) of the ‘GSM only’ subscribers shall remain unaffected by the introduction of CTS. Means to guarantee that the quality of service of GSM subscribers does not degrade due to CTS introduction have to be defined.

Since the MS uses the same frequency spectrum both for the cellular mode and the cordless mode, the ME hardware can be reused. The intention is that hardware modifications in the radio transceiver of a standard GSM ME should not be necessary. It is also a requirement that the introduction of CTS shall not destabilise any existing GSM mobile equipment or degrade the mobile equipment’s dedicated (e.g. in call) or idle mode performance.

When a licensed band is used:

– the CTS operator can administer all CTS-FPs and the corresponding CTS. When the CTS is initialised, the required operation parameters are downloaded into the CTS. This can either be done via the fixed network or via the GSM radio interface.

Figure 1: Concept for the GSM CTS functionality when connected to the PSTN

4.2 Service aspects

4.2.1 General

In the first phase of CTS an application for residential environment serving one cell is intended. The services and features to be supported relate to this assumption.

4.2.2 User aspects

The user of the CTS shall be required to perform a simple installation process in order to initialise the CTS operation. This process shall include:

connection of the CTS-FP to the power supply and to either the fixed network or the GSM network interface;
enrolment of the CTS-FP (CTS-FP enrolment).
As the CTS-FP shall not be GSM operator specific, it has to be enrolled with operation specific data, e.g. a list of allowed frequencies (Generic Frequency List GFL);
enrolment of one or several CTS-MSs with the CTS-FP (CTS-MS enrolment).

Once initialised, CTS operation shall not require any specific action of the user to maintain the operation as long as the CTS-FP is not de-activated (see clause 6).

To have service granted by the CTS-FP, a CTS-MS shall attach at the CTS-FP. In order to attach, a CTS‑MS has to be enrolled with the CTS-FP, the user has to bring the CTS-MS in range of the CTS-FP and the appropriate operation mode has to be selected (see subclause 4.2.5.1).

4.2.3 Subscriptions

The CTS-FP owner shall have a valid CTS subscription from a CTS operator. This subscription is materialised by an FP-SIM. It gives the right to the CTS-FP owner to initialise the CTS-FP and enroll CTS-MSs under conditions defined by the CTS operator. In exchange of a subscription to a CTS operator, the CTS-FP owner will receive a CTS-GFL in the CTS-FP.

The user of the GSM CTS service shall have a subscription with a GSM operator, which allows CTS service. This CTS authorisation may be given to all the CTS-MSs of a PLMN or may be linked to a specific service subscription given by the GSM operator. By default the CTS-MS are not allowed to do CTS operations. The use of a GSM subscription for ‘CTS only’ service is not precluded. Corresponding information may be stored in the SIM.

In order to be able to use the CTS service on the fixed network, a subscription to the fixed network is also required. When connected to the GSM network, the fixed network subscription is replaced by a GSM subscription and the CTS-FP shall include a GSM SIM card corresponding to this GSM subscription.

The fixed network, GSM and CTS subscriptions may be independent of each other. However at CTS-FP enrolment (see subclause 5.1) the CTS-FP may obtain operation data specific to the CTS operator, the user of the CTS service has subscribed to, thus the CTS-FP operation may be tied to a specific GSM operator.

The CTS-FP is normally not subscribed to, it is the users’ property. After the CTS service subscription to one operator is cancelled, it shall be possible to use the same CTS-FP for a CTS service subscription to another operator.

4.2.3.1 CTS-roaming aspects

A CTS-FP owner may only have one CTS subscription to a single CTS operator. This operator defines which CTS-MSs are allowed to enroll onto a particular CTS-FP. Several control levels are offered by the CTS:

– uncontrolled roaming: all CTS-MS from any GSM operator are allowed to enrol on the CTS-FP without CTS-operator control;

– no roaming: the CTS-operator and the GSM operator are the same and only the CTS-MSs from the CTS operator are allowed to attach on the CTS-FP;

– control roaming: on a case by case basis, the CTS-operator controls CTS-MS enrolment using CTS-MS IMSI.

In any cases, the CTS-FP owner authorisation is mandatory for CTS-MS enrolment.

NOTE: CTS-roaming has no impact on CTS-FP operational mode defined by the CTS-operator (e.g. frequency management).

4.2.3.2 Withdrawal of CTS subscription

When used within a licensed band:

– if the CTS subscription is withdrawn, the user looses the right to use a CTS- operator’s frequency spectrum for CTS usage and CTS operation shall stop.

As the CTS service is not obtained directly from the GSM network like other GSM services (e.g. SMS), means of subscription control have to be defined.

4.2.4 GSM coverage

The concept should allow CTS operation during all GSM coverage scenarios, e.g. the following:

area with no GSM coverage;
area with GSM coverage of the HPLMN operator;
area with no HPLMN coverage, but coverage of other PLMN operator(s).

Depending on the enrolment concept being used (see subclause 5.1), a limitation may be that the MS need to be in HPLMN (or at least PLMN) coverage once, before the CTS-FP can be correctly initialised (if the GSM radio interface is used for enrolment data download).

In the case of a CTS-FP connected to a PLMN, a GSM coverage is necessary for CTS operations.

4.2.5 Mobility

4.2.5.1 Mode selection between CTS and PLMN

The CTS-MS may operate in one of six different operation modes. It is not required that the CTS-MS support all the modes. It shall be possible for the user to select between the supported modes at any time, except during an active call.

a) Manual mode: GSM only, i.e. the normal GSM operation.

b) Manual mode: CTS only, i.e. calls can only be originated or received via the CTS-FP. The MS is not attached to the GSM PLMN.

c) Automatic mode with CTS preferred: The MS attaches to CTS, when in range, and then automatically tries to attach with GSM PLMN, when out of CTS range. An MS attached with a GSM PLMN and coming in range of the CTS shall not attach to CTS if a connection in the GSM PLMN is active. In that case the MS shall wait until the connection is finished.

d) Automatic mode with GSM preferred: the MS attaches to GSM PLMN, when in range, and then automatically tries to attach with CTS, when out of GSM PLMN range. A MS attached with CTS and coming in range of a GSM PLMN shall not attach to the GSM PLMN, if a connection in the CTS is active. In that case the MS shall wait until the connection is finished.

e) Parallel mode – single active mode: the MS tries to attach with both GSM PLMN and CTS. If a CTS-MS is attached to both CTS-FP and GSM, it shall offer double service in idle mode (i.e. full service on both systems in idle mode), and at least single service in active mode (i.e. when involved in an active call on one system, no service is required on the other system). Techniques shall avoid excessive signalling load on the GSM PLMN.

f) Parallel mode – double active mode: the MS tries to attach with both GSM PLMN and CTS. If a CTS-MS is attached to both CTS-FP and GSM, it shall offer full service on both types of networks, e.g. the CTS-MS shall be able to successfully handle all applicable terminating calls that are addressed to its MSISDN and to the CTS-FP number.

When switched on, the CTS-MS enters the last selected operation mode.

NOTE: The user may, optionally, define a default mode setting, in which case the default mode is always entered when the CTS-MS is switched on.

The selected mode and the current coverage (CTS and/or GSM) shall be indicated to the user.

The user of the CTS-MS might have subscribed to the CTS service only. Access of the CTS-MS to the GSM network may however be required in order to establish a signalling link, e.g. to exchange operation control data.

4.2.5.2 Attachment of the same CTS-MS with multiple CTS-FPs

A CTS-MS shall be able to attach to all CTS-FPs it is initialised with however it may only be attached with only one CTS-FP at a time.

A list of all CTS-FPs, the CTS-MS is enrolled to, shall be stored in the CTS-MS, preferably on the MS-SIM. This list shall for each CTS-FP/CTS-MS pair contain all data which defines their association.

This list may also define the preference of attachment in the case that the CTS-MS is in range of several CTS-FPs when entering the CTS mode of operation. The user may be able to change the preference order of CTS-FPs in the list.

A manual attachment mode may be implemented which enables the user to scan for CTS-FPs the CTS-MS is enrolled to in order to select a specific CTS-FP, of those in range, for attachment.

4.2.5.3 Attachment of multiple CTS-MSs with the same CTS-FP

Enrolment and parallel attachment of multiple MS on a single CTS-FP shall be possible. The CTS-FP control however what MSs are allowed to be enrolled and attached.

A minimum number of 8 CTS-MSs shall be possible to attach with the same CTS-FP.

4.2.5.4 Attachment status of a CTS-MS

A mechanism to provide information about the attachment status of a CTS-MS shall be supported by the CTS-FP. This mechanism can be used for network information or routing purposes. This mechanism shall provide a notification in the following situations:

a) when attaching with the CTS;

b) when de-attaching with the CTS.

This mechanism shall be transparent to the user.

In addition, when the mobile is leaving the GSM network for CTS attachment (i.e. not in the parallel mode), it shall behave towards the GSM network as if it was being switched off.

4.2.6 CTS applications

4.2.6.1 Tele-services

The first phase of CTS shall support the speech teleservice on the CTS radio interface.

Emergency calls shall be supported for attached MSs only. The call origination of an emergency call shall be according to the selected mode. The quality of the speech teleservice shall be at least equal to existing GSM speech qualities. The existing GSM phase 2 requirements on voice codecs according to ref. [xx] shall be applicable to CTS.

If the user attempts to set up calls other than speech or emergency calls, this request shall be sent to the CTS-FP. The CTS-FP may issue a rejection cause.

4.2.6.2 Supplementary services

Supplementary services invoked from the CTS-MS are directed towards 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 network and regardless of the preferential origination, to select on a per transaction basis the used network.

As an option (both in the CTS-MS and CTS-FP) the user with CTS should be capable of supporting all the standardized supplementary services that are provided in the GSM. From the point of view of the CTS-MS and the radio interface, the operation of the services shall be the same in GSM and in cordless mode. The CTS radio interface protocol shall therefore support the GSM supplementary services and correspond to the GSM radio interface with respect to the supplementary services. Additional optional procedures may be provided in the CTS-FP, i.e. an interworking function between the CTS radio interface protocol and the fixed network access protocol for the support of supplementary services. Services not supported in the CTS-FP shall be rejected in the same way as in the GSM.

The CTS-MS shall indicate to the user if the CTS-FP cannot be used to handle the requested service, e.g. due to that the line is busy or the CTS-FP do not support the above-mentioned interworking.

4.2.6.3 Additional support for fixed network supplementary services

In order to enable support of supplementary services in the fixed network, DTMF handling on the CTS Radio Interface, during CTS calls, shall be handled in the same way as in GSM. The CTS-FP shall generate the corresponding tones and messages for the fixed network.

Since some fixed networks make use of hook-flash signalling, it shall be possible for the user to send a hook-flash request to the CTS-FP, which shall then perform the hook-flash signalling on the line interface.

4.2.6.4 Man Machine Interface (MMI)

If the CTS mode is selected as the preferential mode for mobile originated activities (see subclauses 4.2.6.2 and 5.5.1 for supplementary service operations and call set-up, respectively), the handset should not change its behaviour regarding input MMI string. Normal rules according to GSM 02.30 should be used by the MS and the CTS-FP, which would then perform the proper interworking towards the fixed network or the GSM network on which the CTS-FP is connected.

4.2.6.5 CTS internal calls

In the case when more than one CTS-MS are attached to a CTS-FP, internal calls (without billing) between the MSs at this CTS-FP should be possible, as an option (both in the CTS-MS and CTS-FP).

4.2.6.6 CTS internal call transfer

As an option (both in the CTS-MS and CTS-FP) the user should be able to internally transfer an external network call from one CTS-MS to another if both CTS-MSs are attached to the same CTS-FP.

4.2.6.7 CTS internal call hold

As an option (both in the CTS-MS and CTS-FP) the user should be able to put a external network call on internal hold, i.e. to release the call on the CTS Radio Interface but keep it active on the external network interface. If this option is supported, it shall also be possible to retrieve the call.

4.2.6.8 Mobile hunting

As an option, it should be possible to trigger a general procedure which makes all the attached MSs ring on a special way and therefore allows the user to locate them. The procedure should be triggered by the CTS-FP or CTS-MS.

4.2.6.9 SMS-MT

As an option the user should be able to have a Mobile Terminated Short Message presented, provided this message is made available to the CTS-FP. The support of SMS-MT on the CTS radio interface is mandatory for the CTS-MS.

4.2.6.10 SMS-MO

As an option the user should be able to generate a Mobile Originated Short Message and send it towards the CTS-FP. The CTS-FP may issue a rejection cause, if the feature is not supported.

4.2.6.11 Other services

Other services are not required for the first phase.

4.3 Radio interface and GSM co-existence aspects

4.3.1 General principles

It should be possible to use the GSM-Cordless Telephony System (CTS) on the same carrier frequencies as assigned for the cellular service. These carrier frequencies can be part of a licence exempt band or a licensed band. In the second case these frequencies can be reserved for GSM-CTS usage, or can be shared with the cellular system. In any case a set of frequencies (Generic Frequency List) needs to be defined by a CTS operator or a regulator to enable CTS operations.

The CTS licence exempt mode is only applicable in countries where the national regulator allocates a suitable band.

A mechanism shall be employed to insure that the CTS-FP will only operate in Licence exempt mode in an area or country where it is authorized to do so.

When used within a licensed band:

– it is necessary for the CTS operator to have complete control, on an area and time basis, over the frequencies the CTS can use. These frequencies shall be part of the frequency set under its responsibility.

Interference to the cellular system or other CTS systems shall be minimised.

The requirements in respect to the receiver and transmitter characteristics of the CTS FP shall allow communication with a GSM MS.

The CTS-FP shall also have a self-controlling feature to minimise interference in the case where no attached MS is in range of the CTS-FP.

In the case of the CTS-Fixed Part connected to PLMN, means should be implemented in the CTS-FP in order to minimize unnecessary radio transmissions to the PLMN.

4.3.2 Broadcast channel functionality

For CTS the BCCH-like functionality shall include the following:

reveal the presence of the CTS;
page the terminals attached to the CTS;
accept access requests from the terminals attached at the CTS;
support intracell handover;
synchronisation.

4.4 Security requirements

4.4.1 General

1. The use of CTS shall not compromise the security of any PLMN or fixed network.

2. There shall be a mechanism that ensures that security options and parameters for the CTS-FP provided by the CTS operator or by the regulator are followed by the CTS-FP.

3. There shall be a mechanism that ensures that the CTS-FP only obeys security options and parameter received from the PLMN operator to which it is enrolled.

4.4.2 Enrolment

Enrolment of the MS to the FP shall include authentication of the MS-SIM by the PLMN in accordance with the requirements for subscriber identity authentication defined in GSM 02.09, clause 3.

It shall be possible to conduct all CTS-FP and CTS-MS security mechanisms across the fixed network.

Enrolment of the MS to the FP shall include mutual authentication between the CTS-FP and the CTS-MS.

The mutual authentication of MS and FP shall use a CTS specific secret key.

4.4.3 Attachment

Attachment of the MS to the FP shall include mutual authentication between the CTS-FP and the CTS-MS.

The mutual authentication of MS and FP shall use a CTS specific secret key. This key shall not be a key input to the MS via the MS MMI, but shall be a key installed/derived during the enrolment of the MS to the FP.

It shall not be possible for an MS that has not been enrolled to an FP to attach to that FP.

4.4.4 Protection of communications

Local CTS communication (both signalling and user data) shall be protected against unauthorized eavesdropping.

This protection need not be as secure as for the GSM air interface, but it shall employ cryptographic methods and a CTS specific encryption key.

A CTS mobile subscriber shall be identified on the CTS radio path by a local temporary, number, similar to, but not, the TMSI.

Communication across the fixed network of sensitive data required for CTS operation shall be protected against unauthorized eavesdropping.

4.4.5 Security Parameter Storage Requirements

It shall be possible for the GSM operator to ascertain the IMEI of the MS when the CTS-MS is attached with a CTS-FP. Additionally, in order to detect changes in the ME used for CTS, the IMEI shall be passed to the FP when requested.

The CTS-FP shall have a unique identity (IFPEI) stored in a secure way, in accordance with the requirements for storage of the IMEI, described in GSM 02.09.

It shall be possible for the GSM operator to ascertain the IFPEI of the CTS-FP.

The common keys used by the MS and FP for mutual authentication and encryption of local CTS communication shall be securely stored within the MS and FP. The secret keys used for enrolment and attachment shall be stored in the MS-SIM.

The FP shall not contain any algorithms used for GSM security (e.g. A3, A8) unless the storage of these algorithms within the FP meets the requirements for storage of authentication functionality within the MS as defined in 02.09, subclause 3.2.3.

Key management shall be under the control of the PLMN operator.

4.4.6 De-activation

It shall be possible within the operator defined time for the GSM operator to de-activate the CTS service of its subscribers (i.e. de-activate the MS).

It shall be possible, within the operator defined time, for the CTS operator to de-activate a CTS-FP with respect to the use of the operator’s frequencies by that CTS-FP (see also subclause 6.3).