Introduction to Ps-cs Interoperation

Introduction to PS-CS Interoperation

Issue 01

Date 2012-11-07

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Introduction to PS-CS Interoperation About This Document

About This Document

Change HistoryDate Version Description Author

2011-12-21 V1.0 Completed the first draft. He Wei

Key wordsNetwork operation mode, paging coordination, mobile-terminated call, dual transfer mode (DTM), class-A


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Introduction to PS-CS Interoperation Contents

Contents

About This Document........................................................................ii1 Introduction..................................................................................1

1.1 Overview............................................................................................................................................................11.2 Network Operation Mode..................................................................................................................................1

1.2.1 Network Operation Mode in A/Gb mode.................................................................................................11.2.2 Network Operation Mode in Iu Mode......................................................................................................2

1.3 Gs Interface........................................................................................................................................................21.4 Paging Coordination..........................................................................................................................................41.5 UE Operation Mode...........................................................................................................................................5

1.5.1 A/Gb Mode...............................................................................................................................................51.5.2 Iu Mode.....................................................................................................................................................5

2 Interoperation Scenario.................................................................62.1 Overview............................................................................................................................................................62.2 Interoperation Scenario......................................................................................................................................62.3 DTM Mode........................................................................................................................................................7

2.3.1 Overview..................................................................................................................................................72.3.2 UE DTM Capability.................................................................................................................................82.3.3 SGSN DTM Capability............................................................................................................................8

A References....................................................................................9


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Introduction to PS-CS Interoperation 2 Interoperation Scenario

1 Introduction

1.1 OverviewIn circuit switched (CS)-packet switched (PS) interoperation scenarios, the concepts of network operation mode, Gs interface, paging coordination, and DTM, as well as their relationship easily lead to confusion. Knowledge about these concepts and mutual relationship is essential for analysis on PS-CS interoperation services.

1.2 Network Operation ModeNetwork operation mode is broadcast to a user equipment (UE) through system information and is important for CS-PS interoperation. The UE determines its behavior based on the network operation mode and its radio access capabilities.

1.2.1 Network Operation Mode in A/Gb modeBased on the paging mode of PS and CS services and their cooperation relationship, the GSM/EDGE radio access network (GERAN)/general packet radio service (GPRS) network uses the following network operation modes (for details, see 3GPP TS 23.060):

Network operation mode I: The Gs interface is deployed to support the paging coordination function of the core network. For GPRS-attached UEs, CS paging messages are delivered over the same paging channel as PS paging messages, either over the packet common control channel (PCCCH) (if the PCCCH is configured) or over the common control channel (CCCH). UEs listen to one of the paging channels for reception of both CS paging and PS paging messages. If a UE is assigned resources on a packet data channel (PDCH), the network delivers a paging message to the UE over the PDCH.

Network operation mode II: No Gs interface is deployed to support the paging coordination function of the core network. All paging messages are delivered over the CCCH. UEs listen to only the CCCH. CS paging message delivery continues on the CCCH even if the UE has been assigned resources on a PDCH. In this case, the UE cannot receive CS paging messages unless the base station subsystem (BSS) is configured with the paging coordination function as described in section 1.4 "Paging Coordination."

Network operation mode III: No Gs interface is deployed to support the paging coordination function of the core network. Different from network operation mode I, PS


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paging messages are delivered over the PCCCH and CS paging messages over the CCCH when the PCCCH is configured in network operation mode III. The UE needs to listen to both the paging channel (PCH) and the packet paging channel (PPCH). If the PCCCH is not configured, paging messages are delivered over the CCCH.

Table 1-1 lists available channels and the paging coordination capability of the core network in different network operation modes.

Table 1-1 Available channels in different network operation modes (in A/Gb mode)

Mode

Circuit Paging Channel

GPRS Paging Channel

CN Paging Coordination

Packet paging channel Packet paging channel -

I CCCH paging channel CCCH paging channel Yes

Packet data channel Not applicable -

II CCCH paging channel CCCH paging channel No

III CCCH paging channel Packet paging channel No

CCCH paging channel CCCH paging channel -

1.2.2 Network Operation Mode in Iu ModeIn Iu mode, the network operation mode is divided into two types based on availability of the Gs interface: network operation mode I and network operation mode II.

Table 1-1 Network configuration in different network operation modes (in Iu mode)

Mode Network Configuration Combined Procedure by Mobile Terminal (MT)

I Gs interface is present. Yes

II Gs interface is not present. No

1.3 Gs InterfaceThe Gs interface is an optional interface that connects the mobile switching center (MSC) and the SGSN. The Gs interface is used in network operation mode I to support the paging coordination function of the core network. Through the Gs interface, the SGSN and the MSC/VLR are associated and the association establishment is initiated by the SGSN. The Gs interface is established through the combined GPRS/International mobile subscriber identity (IMSI) attach procedure and combined routing area update (RAU)/location update procedure.


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Figure 1-1 Combined GPRS/IMSI attach procedure

7c. Cancel Location Ack

7b. Cancel Location

3. Identity Response

2. Identification Response

2. Identification Request 1. Attach Request

5. IMEI Check

3. Identity Request

4. Authentication

7a. Update Location

MS RAN new SGSN old SGSN GGSN HLR EIR old

MSC/VLR new

MSC/VLR

8d. Cancel Location Ack

8c. Cancel Location 8b. Update Location

8g. Update Location Ack

8e. Insert Subscriber Data

8f. Insert Subscriber Data Ack

7f. Insert Subscriber Data

8a. Location Update Request

8h. Location Update Accept

7h. Update Location Ack

7g. Insert Subscriber Data Ack

10. Attach Complete

9. Attach Accept

11. TMSI Reallocation Complete

C1

7d. Delete PDP Context Request 7e. Delete PDP Context Response

6. Delete PDP Context Request

6. Delete PDP Context Response


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Figure 1-2 Combined RAU/location update procedure

4b. Cancel Location

4c. Cancel Location Ack

4e. Insert Subscriber Data Ack

4d. Insert Subscriber Data

HLRMS BSSnew

MSC/VLRSGSNold

MSC/VLR

3. Location Update Request

1. Routeing Area Update Request

2. Security Functions

4a. Update Location

4f. Update Location Ack

8. TMSI Reallocation Complete

6. Routeing Area Update Accept

5. Location Update Accept

7. Routeing Area Update Complete

C1

A Gs-based network has the following advantages:

Radio resources are saved. Theoretically, a routing area (RA) is smaller than a location area (LA). Paging messages sending to a PS RA involves a smaller number of cells. In this case, paging resources are substantially saved since cell broadcast and paging messages are the major load in a RA/LA. On the other hand, CS attach is performed through PS attach and therefore signaling overheads are reduced by half. In this case, the entire-network bottleneck is optimized since the end-to-end (E2E) bottleneck is located on the wireless network.

Signaling overheads in UE attach are reduced by half, which facilitates UE power consumption.

1.4 Paging CoordinationPaging coordination allows a UE to simultaneously receive CS paging and PS paging messages when the ME listens to only one paging channel. In network operation mode I, the Gs interface is deployed to support the paging coordination function of the core network. In network operation modes II and III, CS paging and PS paging are independently performed in the core network and the core network does not perform paging coordination.

However, in network operation modes II and III, paging coordination can also be performed by the BSS (3GPP TS 23.060). To perform paging coordination, the BSC needs to store one context, which records the service type of the UE, CS or PS. The context is indexed based on the IMSI because the IMSI is the common identifier of the PS domain and the CS domain. If the BSS detects that the UE is performing PS services, CS paging messages are delivered over


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the PS service channel. If the BSS detects that the UE is performing CS services, PS paging messages are delivered over the dedicated CS service channel.

If the context has not been set up, CS paging messages are delivered over the CCCH and PS paging messages are delivered over the CCCH or the PCCCH (if the PCCCH is configured).

If BSS paging coordination is activated, the BSS notifies the UE of the paging coordination information through the system information.

1.5 UE Operation Mode1.5.1 A/Gb Mode

In A/Gb mode, GPRS UEs are classified into class A, class B, and class C (for details, see 3GPP TS 22.060). A/Gb-mode UEs of class B are commonly used.

Class-A mode of operation: The UE is attached to both PS and CS domain, and the UE supports simultaneous running of PS and CS domain services.

Class-B mode of operation: The UE is attached to both PS and CS domain, but the UE can only run one type of services, PS or CS services, at a time.

Class-C mode of operation: The UE is attached to only one domain, PS or CS, at a time.

1.5.2 Iu ModeAn Iu-mode UE operates in one of the following three modes (3GPP TS 23.060):

CS/PS operation mode: The UE is attached to both the PS domain and CS domain, and the UE is capable of simultaneously signaling with the PS and CS core network domains.

PS operation mode: The UE is attached to the PS domain only and may only run services of the PS domain.

CS operation mode: The UE is attached to the CS domain only and may only run services of the CS domain.

All combinations of different operation modes described in A/Gb mode and Iu mode can be used by UEs which support both GERAN and UTRAN. For example, CS/PS-mode class-B UEs are commonly used today.


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2 Interoperation Scenario

2.1 OverviewPS-CS interoperation depends on user subscription, network capabilities, and UE capabilities. In the GERAN, most UEs (class-B) cannot be attached to both PS and CS domain at the same time. In the UTRAN, only class-A UEs are supported, indicating that both PS and CS services can be performed at the same time.

2.2 Interoperation ScenarioAccording to the preceding description of network operation mode, in the GERAN, class-A UEs in network operation mode I can receive CS paging messages while performing packet services; class-B UEs in network operation mode I can also receive CS paging messages while performing packet services but must suspend packet services until the CS service is complete.

In network operation mode II, class-B UEs cannot receive CS paging messages while performing packet services. An exception is that, if the BSS supports paging coordination, the BSC can deliver paging messages received on the A interface over the PCH and Packet Associated Control Channel (PACCH) simultaneously even if there is no Gs interface deployed in network operation mode II.

In the GERAN, class-B UEs do not support concurrent services. If CS services are to be initiated when PS services are performed, PS services must be immediately suspended through the Suspend procedure and then be recovered through the Resume procedure after the CS services are complete. This is because the dedicate mode (DM) (for CS services) has a higher priority.

In the UTRAN, class-B UEs support concurrent services. When a UE is handed over from the UTRAN to the GERAN, PS services are suspended because a class-B UE do not support concurrent services in the GERAN (in non-DTM mode).


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Figure 2-1 Suspend and Resume procedure

2. Suspend

6. Routeing Area Update Request

1. Dedicated Mode

MS BSS SGSN MSC/VLR

3. Suspend

4. Resume

5. Channel Release

3. Suspend Ack

4. Resume Ack

2.3 DTM Mode2.3.1 Overview

DTM is a technology allowing simultaneous operation of voice services and data services in the GERAN. According to the initial definition of the GPRS protocol, only class-A UEs can be attached to both the CS domain and PS domain. With the DTM technology, DTM-capable UEs can simultaneously perform PS services and CS services under the coverage of a DTM-capable BSC. Therefore, DTM-capable UEs are also known as simple class-A UEs. With the DTM function, service continuity (for both PS and CS services) in WCDMA-GSM interoperation becomes possible.

With the BSC DTM function, class-B UEs in network operation mode II can perform mobile-terminated calls while performing PS services. The BSS paging coordination function must be activated in DTM mode.

NOTEFor details, see the description in 3GPP TS 43.055. When the establishment of the CS connection is initiated by the network, the CS paging message may come directly from the MSC or via the SGSN if the Gs interface is present. The BSS shall be able to verify in both cases if the paged mobile station is in packet transfer mode and shall send the CS page on the PACCH. This paging co-ordination can be reused for GPRS mobile stations in mode of operation B, so that

the mobile station does not need to listen to the PCH. This feature breaks the link between the presence of the Gs interface and the network capability to

perform paging co-ordination. Alignment of 3G TS 23.060 is needed.

If a cell is DTM-capable, the DTM capability is broadcast over the broadcast control channel (BCCH) or packet broadcast control channel (PBCCH). Huawei BSCs support paging coordination.


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Figure 2-1 CS paging procedure during PS data transmission

2.3.2 UE DTM CapabilityThe DTM capability of a UE is specified in the two information elements (IEs): MS Classmark 3 and MS Radio Access Capability. DTM-capable UEs, known as simple class A UEs, also supports DTM in addition to the DM, idle/packet idle mode, and packet transfer mode (PTM) that are supported by traditional class-B UEs.

2.3.3 SGSN DTM CapabilityTo support DTM, an SGSN needs to satisfy the following requirements, which are all supported by Huawei SGSNs.

Stores information about the UE DTM capability. Delivers IMSIs to a BSC. Delivers the updated UE radio access capabilities to a BSC. Supports the Suspend and Resume procedure.


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Introduction to PS-CS Interoperation A References

A References

[1] 3GPP TS 23.060 "Service description"

[2] 3GPP TS 22.060 "Service description, Stage 1"

[3] 3GPP TS 43.055 "Dual Transfer Mode (DTM)"


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Introduction to Ps-cs Interoperation

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