ETSI TR 143 901 V13.0.0 (2016-01)

ETSI TR 1143 901 V13.0.0 (201616-01)

TECHNICAL REPORT
Digital cellular telecocommunications system (Phahase 2+);
Feasibility Study on on generic access to A/Gb intinterface
(3GPP TR 43.9.901 version 13.0.0 Release 13 13)

R
GLOBAL SYSTTEME FOR
MOBILE COMMUUNNICATIONS
3GPP TR 43.901 version 13.0.0 Release 13 1 ETSI TR 143 901 V13.0.0 (2016-01)

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3GPP TR 43.901 version 13.0.0 Release 13 2 ETSI TR 143 901 V13.0.0 (2016-01)
Intellectual Property Rights
IPRs essential or potentially essential to the present document may have been declared to ETSI. The information
pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found
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Foreword
This Technical Report (TR) has been produced by ETSI 3rd Generation Partnership Project (3GPP).
The present document may refer to technical specifications or reports using their 3GPP identities, UMTS identities or
GSM identities. These should be interpreted as being references to the corresponding ETSI deliverables.
The cross reference between GSM, UMTS, 3GPP and ETSI identities can be found under
http://webapp.etsi.org/key/queryform.asp.
Modal verbs terminology
In the present document "shall", "shall not", "should", "should not", "may", "need not", "will", "will not", "can" and
"cannot" are to be interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of
provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.
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Contents
Intellectual Property Rights . 2
Foreword . 2
Modal verbs terminology . 2
Foreword . 5
Introduction . 5
1 Scope . 6
2 References . 6
3 Definitions, symbols and abbreviations . 7
3.1 Definitions . 7
3.2 Symbols . 7
3.3 Abbreviations . 7
4 Assumed high level requirements . 9
5 Study results . 9
5.1 Architecture . 9
5.1.1 Architecture for generic access interface . 9
5.1.1.1 Functional architecture . 9
5.1.1.2 Functional entities . 10
5.1.1.2.1 Mobile Stations (MS) . 10
5.1.1.2.2 Generic Access Network Controller (GANC) . 10
5.1.1.3 Interfaces . 11
5.1.1.3.1 A/Gb interfaces . 11
5.1.1.3.2 Up interface . 11
5.1.1.3.3 Wm interface . 11
5.1.1.4 High level description of generic access . 11
5.1.2 Signalling flows . 11
5.1.2.1 Registration for generic access . 11
5.1.2.2 Deregistration . 13
5.1.3 Mobile originated call flow . 14
5.1.4 Mobile terminated call flow . 16
5.1.5 GPRS procedures . 17
5.1.5.1 GAN-GRR user data transport channel . 17
5.1.5.2 GPRS user data transport procedures . 17
5.1.5.3 GPRS signalling and SMS transport procedures . 17
5.1.5.4 Packet paging for GPRS data service . 17
5.1.5.5 Packet paging for CS domain service. 18
5.1.5.6 GPRS Suspend procedure . 18
5.1.5.7 GPRS Resume procedure . 18
5.2 Protocol aspects . 18
5.2.1 New Generic Layer . 18
5.2.1.1 CS domain signalling plane . 18
5.2.1.2 CS domain user plane. 20
5.2.1.3 PS domain signalling plane . 21
5.2.1.4 PS domain user plane . 22
5.2.2 Existing protocols . 23
5.2.2.1 Standard 3GPP protocols . 23
5.2.2.1.1 Short Message Service . 23
5.2.2.1.2 GSM SMS services. 24
5.2.2.1.3 GPRS SMS services . 24
5.2.2.1.4 Supplementary services . 24
5.2.2.2 Standard IP-based protocols . 24
5.3 Security mechanisms . 24
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5.3.1 Security levels . 24
5.3.2 Up interface security . 25
5.4 Multi-mode terminal operation. 25
5.4.1 Mechanism of mode selection in multi-mode terminals . 26
5.4.2 PLMN selection . 26
5.4.3 Re-selection between GERAN/UTRAN and GAN modes . 27
5.4.3.1 Rove-in (from GERAN/UTRAN to GAN) . 27
5.4.3.2 Rove-out (from GAN to GERAN/UTRAN) . 27
5.4.3.3 Ping-pong avoidance between GAN and GERAN/UTRAN . 27
5.4.4 Handovers between GAN and GERAN . 28
5.4.4.1 Cell identifiers in generic access . 28
5.4.4.1.1 GAN cell id for Location Services & billing . 28
5.4.4.1.2 GAN cell id for handover-to-GAN . 29
5.4.4.2 Handover to GAN . 30
5.4.4.3 Handover to GERAN . 32
5.4.4.4 Other handover considerations . 33
5.4.5 Cell Change Order between GAN and GERAN . 34
5.5 Emergency call support in GAN . 34
5.6 Feasibility for support of services available through GERAN while in GAN mode . 34
5.7 Other considerations . 35
6 Potential impacts on current specifications . 35
7 Summary and conclusion . 35
Annex A: Change history . 36
History . 37

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Foreword
rd
This Technical Report has been produced by the 3 Generation Partnership Project (3GPP).
The contents of the present document are subject to continuing work within the TSG and may change following formal
TSG approval. Should the TSG modify the contents of the present document, it will be re-released by the TSG with an
identifying change of release date and an increase in version number as follows:
Version x.y.z
where:
x the first digit:
1 presented to TSG for information;
2 presented to TSG for approval;
3 or greater indicates TSG approved document under change control.
y the second digit is incremented for all changes of substance, i.e. technical enhancements, corrections,
updates, etc.
z the third digit is incremented when editorial only changes have been incorporated in the document.
Introduction
This document captures the results of the feasibility study of enabling generic access to A/Gb interface using alternate
access means such as ADSL, Cable, Bluetooth, etc. Mobile stations obtain services from the GSM core network using
such generic access means rather than through the traditional GERAN radio interface. The goal is to ensure no impact
to the current A/Gb interface specifications.
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1 Scope
This document studies the feasibility of generic access to A/Gb interface. Specific areas of study are:
- Architecture to enable generic access.
- Access interface protocols required to provide connectivity to A/Gb interface and GSM/GPRS services.
- Security mechanisms to support generic access architecture.
- Determining feasibility for support of services currently supported through GERAN.
The focus of the study shall be on establishing the feasibility for supporting generic access in the home network case,
while also identifying issues with extending the solution to the roaming scenarios.
2 References
The following documents contain provisions, which, through reference in this text, constitute provisions of the present
document.
• References are either specific (identified by date of publication, edition number, version number, etc.) or
non-specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including
a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same
Release as the present document.
[1] 3GPP TS 22.011: "Service accessibility".
[2] 3GPP TS 23.234: "3GPP system to Wireles Local Area Network (WLAN) interworking; System
description".
[3] 3GPP TS 26.103: "Speech codec list for GSM and UMTS".
[4] ITU-T Recommendation V.110: "Support by an ISDN of data terminal equipments with V-Series
type interfaces".
[5] IETF RFC 791: "Internet Protocol".
[6] IETF RFC 793: "Transmission Control Protocol".
[7] IETF RFC 2406: "IP Encapsulating Security Payload (ESP)".
[8] IKEv2
[9] EAP SIM
[10] IPSec NAT
[11] IETF RFC 768: "User Datagram Protocol".
[12] IETF RFC 3550: "A Transport Protocol for Real-Time Applications".
[13] 3GPP TS 33.234: "3G security; Wireless Local Area Network (WLAN) interworking security".
[14] IETF RFC 2406: " IP Encapsulating Security Payload (ESP)".
[15] 3GPP TS 23.122: "Non-Access-Stratum functions related to Mobile Station (MS) in idle mode".
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3 Definitions, symbols and abbreviations
3.1 Definitions
For the purposes of the present document, the following terms and definitions apply.
Generic Access Network: an access network providing access to A/Gb interfaces using broadband IP network.
Generic Access Network Controller: the network node that connects to the MSC and SGSN via the A-interface and
Gb interface respectively and mimics the functionality of the GERAN BSS.
Roving: the action of re-selection between 3GPP access technology and GAN for a mobile station in idle mode.
Rove in: the mobile station reselects from GERAN/UTRAN to GAN.
Rove out: the mobile station reselects from GAN to GERAN/UTRAN.
Handover: a mobile station engaged in a call moves between GERAN/UTRAN and GAN.
Handover in: the mobile station moves from GERAN/UTRAN to GAN.
Handover out: the mobile station moves from GAN to GERAN/UTRAN.
Seamless: free from noticeable transitions (i.e., no end-user action is required; speech interruptions are short; service
interruptions are short; incoming calls are not missed; packet sessions are maintained; services work identically).
3.2 Symbols
For the purposes of the present document, the following symbols apply:
A GSM A Interface : the A interface is located between the MSC (Mobile service Switching Centre)
and the BSS (Base station system or radio Sub-System).
Gb GSM Gb Interface : the Gb interface connects the Base Station System (BSS) to the Serving GPRS
Support Node (SGSN)
Up Interface between MS and GAN
Wm Reference point between a Packet Data Gateway and a 3GPP AAA Server or 3GPP AAA proxy

3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
A-GPS Assisted-GPS
AAA Authentication, Authorisation and Accounting
ADSL Asymmetric Digital Subscriber Line
ARFCN Absolute Radio Frequency Channel Number
BCCH Broadcast Control CHannel
BSIC Base transceiver Station Identity Code
BSS Base Station Subsystem
BSSAP Base Station Subsystem Application Part
BSSGP Base Station Subsystem GPRS Protocol
BSSMAP Base Station Subsystem Management Application Part
CBS Cell Broadcast Service
CC Call Control
CGI Cell Global Identity
CI Cell Identity
CM Connection Management
CN Core Network
CS Circuit Switched
DSL Digital Subscriber Line
DTAP Direct Transfer Application Part
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DTM Dual Transfer Mode
ESP Encapsulating Security Payload
FFS For Further Study
GAN Generic Access Network
GANC Generic Access Network Controller
GERAN GSM EDGE Radio Access Network
GMM GPRS Mobility Management
GMMRR GPRS Mobility Management Radio Resource
GRR GPRS Radio Resource
HLR Home Location Register
HO Handover
HPLMN Home Public Land Mobile Network
HTTP Hyper Text Transfer Protocol
IMS IP Multimedia Subsystem
IMSI International Mobile Subscriber Identity
IP Internet Protocol
L1 Layer 1 (physical layer)
LA Location Area
LA Location Area Identity
LCS Location Services
LLC Logical Link Control
MAC Medium Access Control (protocol layering context)
MBMS Multimedia Broadcast and Multicast Service
MM Man Machine
MMI Man Machine Interface
MMS Multimedia Messaging Service
MO Mobile Originated
MS Mobile Station
MSC Mobile Switching Centre
MVNO Mobile Virtual Network Operator
NACC Network Assisted Cell Change
NAS Non-Access Stratum
NC2 Network Control mode 2
OTDOA Observed Time Difference Of Arrival (positioning method)
PCM Pulse Code Modulation
PDP Packet Data Protocol
PDU Protocol Data Unit
PLMN Public Land Mobile Network
PS Packet Switched
RAN Radio Access Network
RR Radio Resources
RRC Radio Resource Control
RTCP Real-Time Transport Control Protocol
RTP Real Time Protocol
SAP Service Access Point
SAPI Service Access Point Identifier
SCCP Signalling Connection Control Part
SGW Security Gateway
SGSN Serving GPRS Support Node
SID SIlence Descriptor
SIM GSM Subscriber Identity Module
SM Session Management
SMS Short Message Service
SS Supplementary Service
SSL Secure Sockets Layer
TBF Temporary Block Flow
TCP Transmission Control Protocol
TFO Tandem Free Operation
TLLI Temporary Logical Link Identity
TMSI Temporary Mobile Subscriber Identity
TrFO Transcoding Free Operation
UDP User Datagram Protocol
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UNC Universal Naming Convention
UTRAN Universal Terrestrial Radio Access Network
VBS Voice Broadcast Service
VGCS Voice Group Call Service
VPLMN Visited Public Land Mobile Network

4 Assumed high level requirements
This clause summarizes the various assumed requirements for the feasibility study, when providing generic access to
A/Gb interfaces:
- GAN interfaces to the core network shall use existing standard A interface to the MSC and Gb interface to the
SGSN. Non-access stratum (NAS) protocols shall not be impacted.
- GAN shall reuse the existing GERAN identifiers toward the core network.
- GAN should support all telecommunication services supported using the A/Gb interfaces.
- GAN shall be able to operate over existing generic IP access networks (e.g. Cable, DSL, etc.). GAN-specific
functionality shall not be required in the generic IP access network.
- Multi-mode terminals shall be able to perform automatic roving between GERAN/UTRAN and GAN, subject to
the policies of the operator.
- Multi-mode terminals shall be able to perform seamless handover between GERAN/UTRAN and GAN, subject
to the policies of the operator.
- PLMN selection and mechanisms for the avoidance of ping-pong between GERAN/UTRAN and GAN modes
shall follow the principles enunciated in 3GPP TS 22.011.
- The home operator providing GAN service shall control access to Generic Access in all scenarios, including
roaming.
- GAN shall provide security at least as good as GERAN for all traffic between mobile station and GANC. This
includes support of bilateral authentication and encryption of all signalling and user plane traffic between mobile
station and GANC.
- GAN should not require any change to existing standards e.g. the behaviour of MS in GERAN. Non-GAN
capable MSs shall not be impacted due to GAN deployment.
- GAN shall be easily scaled with increasing users and traffic. It should efficiently use the resources of the generic
IP access network.
- Existing charging mechanisms should be used for GAN.
5 Study results
5.1 Architecture
5.1.1 Architecture for generic access interface
5.1.1.1 Functional architecture
An option for the Generic Access Network architecture is illustrated below.
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Figure 5.1.1.1.1: GAN Functional Architecture
5.1.1.2 Functional entities
5.1.1.2.1 Mobile Stations (MS)
The MS contains a new functional block to access a generic access network (GAN).
5.1.1.2.2 Generic Access Network Controller (GANC)
The Generic Access Network Controller (GANC) appears to the core network as a GERAN base station subsystem
(BSS). This entity mimics the role of the BSC in the GERAN architecure as seen from the perspective of the A/Gb
interface. Thus the CN to which the GANC is connected to, is unaware of the different access mechanism being
supported by the GANC compared to the BSC. A generic IP access network provides connectivity between the MS and
the GANC.
The functionality provided by the GANC includes the following:
- User plane circuit switched services:
Inter-working circuit switched bearers over Up interface to circuit switched bearers over A-interface, including
transcoding voice to/from the MS to PCM voice from/to the MSC (when TFO/TrFO features are not being
utilized).
- User plane packet switched services:
Inter-working data transport channels over Up interface to packet flows over Gb interface
- Control plane functionality
- Security Gateway (SGW) for the set-up of secure tunnel with MS for mutual authentication, encryption and
data integrity
- Registration for GAN service access and providing system information
- Set-up of GAN bearer paths for CS and PS services. This includes establishment, management, and teardown
of signalling and user plane bearers between the MS and the GANC.
- GAN functions equivalent to GSM RR and GPRS RLC such as for paging and handovers.
- Transparent transfer of L3 messages between the MS and core network.
NOTE: The AAA server is out of scope of the current study. It is used to authenticate the MS when it first sets up
a secure tunnel to the GAN, specifically to the SGW.
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5.1.1.3 Interfaces
5.1.1.3.1 A/Gb interfaces
The GAN co-exists with the GERAN and interconnects to the core network via the same interfaces used by a standard
GERAN BSS network element:
- GSM A-interface for circuit switched services
No changes are seen necessary to the A interface protocols.
- GPRS Gb-interface for packet services
No changes are seen necessary to the Gb interface protocols.
5.1.1.3.2 Up interface
A single new interface, the Up interface, is defined between the GANC and the MS.
5.1.1.3.3 Wm interface
The Wm interface is used between the GANC-SGW and AAA server, as defined by 3GPP TS 23.234. The Wm
interface is out of scope of the current study.
5.1.1.4 High level description of generic access
The following provides a general outline for how an MS accesses a generic access network which in turn is connected
to the core network through the A/Gb interfaces, using the proposed GAN functional architecture:
1 The MS shall first setup a secure tunnel over the new access interface Up, with the GANC-SGW. The identity of
the GANC and its SGW, in terms of the IP address or equivalent shall be made known to the MS through any
one of various mechanisms, such as provisioning in the terminal, etc. The MS shall be authenticated and
authorized using SIM credentials, via the AAA server.
2 The MS shall then setup a signalling connection over the Up with the GANC and register with the GAN. During
GAN registration, the MS provides information regarding its identity, location and capabilities. If the GAN
accepts the registration, it provides system information to the MS, which is required for obtaining all the
supported services and potentially handover functionality. The stored MS identity is used by the GAN, to
support unicast paging using the Up signalling connection to the MS and other functions.
3 At this point, the MS can switch from 3GPP mode to GAN mode. Layer 3 messages (MM, CC, CM, SS) are
transparently transferred to the core network through the signalling connection over the Up interface. When
needed, the GANC and MS set up CS-domain and PS-domain user plane bearers over the GAN. GANC also
provides additional functions to support handovers, etc.
NOTE: 3GPP mode refers to the mode wherein the physical layer specification for that mode is part of 3GPP
specifications, e.g. GERAN L1 and UTRAN L1.
Identification by the MS of availability of generic access is out of scope of the current study.
5.1.2 Signalling flows
This clause explains how typical functionality such as MO, MT calls, paging, etc may be accomplished for the
identified architecture options.
5.1.2.1 Registration for generic access
The GAN Registration procedure is performed between the MS and the GANC. It serves the following functions:
- It informs the GANC that the MS is now connected through a generic IP access network and is available at a
particular IP address. The GANC maintains the registration context for the purposes of (for example) mobile-
terminated calling.
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NOTE: The feasibility study is limited to cases wherein the IP address of the MS remains the same throughout the
lifetime of the current registration to the GAN.
- It provides the MS with the operating parameters associated with the GAN service. The "GSM System
Information" message content that is applicable to the GAN cell is delivered to the MS during the GAN
registration process. This enables the MS to switch to GAN mode, and following the Registration procedure
trigger NAS procedures with the core network (such as Location/Routing Area Update, mobile originated calls,
mobile terminated calls, etc.).
This procedure is applicable only if the MS is operating in GAN-only, GAN-preferred or GERAN/UTRAN-preferred
modes.
In all other cases, the MS will not execute this procedure, and since it will not have successfully registered with a
GANC, all subsequent GAN procedures will also not be executed by the MS.
The Registration procedures consist of the following steps:
- Attaching to a generic IP access network
- Registration with a GANC
During the registration process, the MS can be redirected to another GANC, for reasons such as:
- MS provided GERAN cell information. For example, the "appropriate" GANC is the one that is a neighbour of
the GERAN cell the MS is currently located in, where " neighbour cells" are defined in clause 5.2.1.
- Load balancing, operator policy, roaming agreements, etc.
The GAN may also reject the MS's request for registration.
If no GSM coverage is available when an MS connects to the GANC, then the GANC may not reliably be able to
determine the location of the MS for the purposes of assigning the MS to the appropriate GANC. The GANC
functionality should permit the operator to determine the service policy in this case; e.g., the operator could provide
GAN service to the user with certain limitations or deny GAN service.
The MS sets up a secure connection to the GANC, and executes the GAN-RR Registration procedures:
- The MS sends a GAN-RR Register Request message that carries the MS identity (IMSI), GERAN cell
information (either current camping GSM CGI, or LAI of last cell where the MS successfully registered) and
generic IP access network point of attachment.
- The MS additionally provides information about whether there is on-going CS connection in GERAN at the time
of registration. This permits the GANC to redirect the MS to an appropriate GANC for enabling seamless
handovers.
- If this GANC is the appropriate GANC, it returns a GAN-RR Register Accept message, which contains "System
Information" for the cell represented by the GANC, including cell description, LAI and CI. The GANC may also
omit information providing the identity of the GAN cell, in order to prohit handovers being triggered from
GERAN.
- If this GANC is not the appropriate GANC, it returns a GAN-RR Register Redirect message, using the GERAN
cell information provided by the MS to identify the appropriate GANC (FQDN or IP address) and its associated
SGW. In this case, the MS will repeat this Registration procedure with the redirected GANC.
- Alternately, the GANC may return a GAN-RR Register Reject message thereby denying GAN service to the
MS.
The Registration procedure can support the Access Class functionality by providing the necessary system information in
the Registration message to the MS.
The GAN Registration Update procedure allows the MS to update information in the GANC regarding changes in the
identity of the overlapping GERAN cell or changes in the generic IP access network point of attachment, by sending a
GAN-RR Register Update message to the GANC carrying the updated information. This may result in the MS being
redirected to another serving GANC, or being denied service e.g. due to operator policy.
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The GAN Registration Update procedure also allows the GANC to update the GAN system information in the MS, if
needed, by sending a GAN-RR Register Update message to the MS carrying the updated information.
5.1.2.2 Deregistration
The GAN-RR Deregistration procedure allows the MS to explicitly inform the GANC that it is leaving GAN mode (e.g.
when it detaches from the generic IP access network), by sending a GAN-RR Deregister message to the GANC,
allowing the GANC to free resources that it assigned to the MS. The GANC also supports "implicit GAN
deregistration", when the secure connection to the MS is abruptly lost.
The GANC can also autonomously release the MS registration context, and send a GAN-RR Deregister message to the
MS. Alternately, the GANC can implicitly deregister the MS by closing the secure connection with the MS.
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5.1.3 Mobile originated call flow

MS GANC
1. GAN-RR UL DIRECT TRANSFER (CM Service Request)
2. Complete Layer 3 Info
3. Authentication
4. Cipher-Mode Command
5. GAN-RR CIPHERING MODE COMMAND
6. GAN-RR CIPHERING MODE COMPLETE
7. Cipher-Mode Complete Command
8. GAN-RR DL DIRECT TRANSFER (CM Service Accept)
9. GAN-RR UL DIRECT TRANSFER (Setup)
10. GAN-RR DL DIRECT TRANSFER (Call Proceeding)
11. Assignment Request
12. GAN-RR ACTIVATE CHANNEL
13. Uplink user plane RTP Stream
14. GAN-RR ACTIVATE CHANNEL ACK
15. Downlink user plane RTP Stream
16. Assignment Complete
17. GAN-RR ACTIVATE CHANNEL COMPLETE
18. GAN-RR DL DIRECT TRANSFER (Alerting)
19. GAN-RR DL DIRECT TRANSFER (Connect)
20. GAN-RR UL DIRECT TRANSFER (Connect Ack)
21. VOICE TRAFFIC
Figure 5.1.3.1: Mobile originated speech call
The description of the procedure in this sub-clause assumes the MS is in GAN mode i.e. it has successfully registered
with the GANC and GAN-RR is the serving RR entity in the MS.
1) Upon request from the user to originate a call, the MS sends the CM Service Request to the GANC in the GAN-
RR UL DIRECT TRANSFER.
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2) The GANC establishes an SCCP connection to the CN. The GANC then forwards the CM Service Request to the
CN using the Complete Layer 3 Information. Subsequent layer-3 messages between mobile station and core
network will be sent between GANC and CN via DTAP.
3) The CN may optionally authenticate the MS using standard GERAN authentication procedures.
4) The CN may optionally update the ciphering parameters in the GANC using the Cipher-Mode Command.
5) The GANC signals the permitted ciphering algorithms to the MS using the GAN-RR CIPHERING-MODE
COMMAND. These algorithms do not apply for GAN. The MS stores this information for possible future use
after a handover to GERAN.
6) The MS signals the selected ciphering algorithm in the GAN-RR CIPHERING-MODE COMPLETE message.
7) The GANC signals the selected ciphering algorithm to the CN using Cipher-Mode Complete.
8) The CN sends the CM service accept message in the GAN-RR DL DIRECT TRANSFER to the MS.
9) The MS sends the Setup message providing details on the call to the CN and its bearer capability and supported
codecs. This message is contained within the GAN-RR UL DIRECT TRANSFER.
10)The CN indicates it has received the call setup and it will accept no additional call-establishment information
using the GAN-RR DL DIRECT TRANSFER to convey the Call Proceeding indication to the MS.
11) The CN requests the GANC to assign call resources using Assignment Request.
12) The GANC sends the GAN-RR ACTIVATE CHANNEL to the MS including bearer path setup information such
as:
- channel mode
- Multi-rate codec configuration
- the UDP port & the IP address for the uplink stream
- the voice sample size
- the cipher mode (for use in case of subsequent handover to GERAN)
13) The MS establishes the RTP path to the GANC. MS optionally sends idle RTP/UDP packets to the GANC but
has not connected the user to the audio path.
14) The MS sends the GAN-RR ACTIVATE CHANNEL ACK to the GANC indicating the UDP port and IP
address for the downlink stream.
15) The GANC establishes the downlink RTP path between itself and the MS. The GANC may start sending idle
RTP/UDP packets to the MS.
16) The GANC signals to the CN that the call resources have been allocated by sending an Assignment Complete
message.
17) The GANC signals the completion of the bearer path to the MS with the GAN-RR ACTIVATE CHANNEL
COMPLETE message. An end-to-end audio path now exists between the MS and the CN. The MS can now
connect the user to the audio path.
18) The CN signals that the called subscriber's phone is ringing, via the Alerting message. If the MS has not
connected the audio path to the user, it shall generate ring back to the calling party. Otherwise, the network-
generated ring back will be returned to the calling party.
19) The CN signals that the called party has answered, via the Connect message. It connects the user to the audio
path. If the mobile station is generating ring back, it stops and connects the user to the audio path.
20) The MS sends the Connect ACK in response, and the two parties are connected for the voice call.
21) Bi-directional voice traffic flows between the MS and CN through the GANC.
ETSI
3GPP TR 43.901 version 13.0.0 Release 13 16 ETSI TR 143 901 V13.0.0 (2016-01)
5.1.4 Mobile terminated call flow

MS  GANC    CN
1. Paging Request
2. GAN-RR PAGING REQUEST
3. GAN-RR PAGING RESPONSE
4. Complete Layer 3 Info
5. Authentication
6. Ciphering Configuration
7. GAN-RR DL DIRECT TRANSFER (Setup)
8. GAN-RR UL DIRECT TRANSFER (Call Confirmed)
9. RTP stream setup Assignment Procedure
10. GAN-RR UL DIRECT TRANSFER (Alerting)
11. GAN-RR UL DIRECT TRANSFER (Connect)
12. GAN-RR DL DIRECT TRANSFER (Connect Ack)
13. V OICE TRAFFIC
Figure 5.1.4.1: Mobile terminated speech call
The description of the procedure in this sub-clause assumes the MS is in GAN mode i.e. it has successfully registered
with the GANC and GAN-RR is the serving RR entity in the MS.
1) A mobile-terminated call arrives at the CN. The CN sends a Paging Request to the GANC identified through the
last Location Update received by it and includes the TMSI if available. The IMSI of the mobile being paged is
always included in the request.
2) GANC identifies the MS registration context using the IMSI provided by the CN. It then pages the MS using the
GAN-RR PAGING REQUEST message. The message includes the TMSI if available in the request from the
CN, else it includes only the IMSI of the mobile.
3) The MS responds with a GAN-RR PAGING RESPONSE including the MS Classmark and ciphering key
sequence number.
4) The GANC establishes an SCCP connection to the CN. The GANC then forwards the paging response to the CN
using the Complete Layer 3 Information message.
5) The CN may optionally authenticate the MS using standard GERAN authentication procedures.
6) The CN may optionally update the ciphering configuration in the MS, via the GANC, as described in steps 4-7 of
the Mobile Originated calling scenario.
7) The CN initiates call setup using the Setup message sent to the MS using the GAN-RR DL DIRECT
TRANSFER.
ETSI
3GPP TR 43.901 version 13.0.0 Release 13 17 ETSI TR 143 901 V13.0.0 (2016-01)
8) The MS responds with Call Confirmed using the GAN-RR UL DIRECT TRANSFER after checking it's
compatibility with the bearer service requested in the Setup and modifying the bearer service as needed. If the
Setup included the signal information element, the MS alerts the user using the indicated signal, else the MS
alerts the user after the successful configuration of the user plane.
9) The CN initiates the assignment procedure with the GANC, which triggers the setup of the RTP stream (voice
bearer channel) between the GANC and MS, same as steps 11-17 in MO call scenario.
10) The MS signals that it is alerting the user, via the Alerting message. The CN sends a corresponding alerting
message to the calling party.
11) The MS signals that the called party has answered, via the Connect message. The CN sends a corresponding
Connect message to the calling party and through connects the audio. The MS connects the user to the audio
path.
12) The CN acknowledges via the Connect Ack message. The two parties on the call are connected on the audio path.
13) Bi-directional voice traffic flows between the MS and CN through the GANC.
5.1.5 GPRS procedures
5.1.5.1 GAN-GRR user data transport channel
GPRS user data transfer
...