3G Roaming Restriction

1 Overview

When being powered on or roaming, a UE's first task is to find and connect to a network to

obtain the services provided by the network. In this case, the UE in idle mode may

perform the following operations: selection or reselection of a PLMN, selection or

reselection of a cell, and location registration.

After a UE is powered on, it first selects a PLMN, then selects a cell, and finally completes

location update.

1.1 Selection or Reselection of a PLMN After a UE is powered on, it first selects a PLMN, and then selects a cell of this PLMN.

After finding a cell of the PLMN, the UE learns of the information about the neighboring

cells from broadcast system messages, and then selects to reside in a cell providing the

best-quality signal among these cells. Next, the UE initiates a location registration

procedure. After successful location registration, the UE resides in the cell providing the

best-quality signal successfully. The benefits of cell residence are as follows:

Enables a UE to receive system messages broadcast by the PLMN.

Enables a UE to initiate a random access within the cell.

Enables a UE to receive paging messages within the network.

Enables a UE to receive broadcast messages within the cell.

The selection or reselection of a PLMN aims to select an available PLMN. To achieve this

goal, a UE generates a list where the available PLMNs are arranged in the sequence of

priority, and then searches the list for the PLMN with the highest priority. The selection or

reselection of a PLMN can be completed in two modes: automatic and manual. Automatic

selection of a PLMN means that a UE automatically selects a PLMN based on PLMN

priorities. Manual selection of a PLMN means that a subscriber selects a PLMN among all

the displayed available PLMNs. In the PLMN list, the registered PLMN (RPLMN) enjoys

the highest priority. There are two files in a USIM, namely EFLOCI and EFPSLOCI,

which record the information about the RPLMN. The content of EFLOCI and

EFPSLOCI is as follows:

Table 1-1 Content of EFLOCI

Bytes Description M/O Length

1 to 4 TMSI M 4 bytes

5 to 9 LAI M 5 bytes

10 RFU M 1 byte

11 Location update status M 1 byte

Table 1-2 Content of EFPSLOCI

Bytes Description M/O Length

1 to 4 P-TMSI M 4 bytes

5 to 7 P-TMSI signature value M 3 bytes

8 to13 RAI M 6 bytes

14 Routing Area update status M 1 byte

An LAI or RAI is saved in one of the two files. The LAI (= MCC + MNC + LAC) or RAI

(= LAI + RAC) contains the MCC and MNC, which indicate the registered RPLMN.

In either automatic or manual selection of a PLMN, after a UE is powered on, it first tries

to select the RPLMN. If the selection of the RPLMN is successful, the PLMN selection

procedure ends. If the selection of the RPLMN fails, the UE generates a PLMN list in

which the PLMNs are sequenced by priority:

1. Home PLMN (HPLMN)

2. PLMNs indicated in the file named User Controlled PLMN Selector with Access

Technology in the USIM, that is, a list of subscriber controlled PLMNs

3. PLMNs indicated in the file named Operator Controlled PLMN Selector with

Access Technology in the USIM, that is, a list of carrier controlled PLMNs

4. PLMNs with better signal quality (sequenced randomly)

5. Other PLMNs (sequenced by signal quality from best to worst)

The IMSI (MCC + MNC + MSIN) is recorded in the USIM, and the UE can obtain the

HPLMN (MCC + MNC) from the IMSI.

The lists of subscriber controlled PLMNs and carrier controlled PLMNs are recorded in

the EFPLMNwAcT and EFOPLMNwACT files of the USIM, in which ACT (access

technology) specifies whether the GSM or UMTS network in the same PLMN is

preferentially selected.

The PLMNs with better signal quality and other PLMNs are found by the UE frequency by

frequency.

The UE searches for the PLMNs one by one based on the preceding PLMN priority

sequence and tries to register its location on a PLMN.

The UMTS network evolves from the GSM network, but the access technologies used by

them are different (GERAN for GSM and UTRAN for UMTS). Therefore, the preferential

access technology must be specified for each PLMN. The priority of the access

technologies is indicated in the file named User Controlled PLMN Selector with Access

Technology or Operator Controlled PLMN Selector with Access Technology. If this

priority is not indicated, usually the UE selects the GERAN preferentially.

The reselection of a PLMN is required in the following scenarios:

PLMN reselection requested by a subscriber: A subscriber can manually initiate a

PLMN reselection procedure in any case.

PLMN reselection after a UE registers with the visited PLMN (VPLMN): After a UE

registers with the VPLMN due to handover or roaming, the UE is aware of this

registration because the VPLMN and the HPLMN have the same MCC but different

MNCs. In this case, the UE tries its best to reselect the HPLMN by periodically

searching for the HPLMN. The search period is written in a file of the USIM, ranging

from six minutes to eight hours. The carriers can prohibit this search function. In this

case, the period in the USIM file is set to 0.

The following figure shows the process of PLMN selection and reselection.

Figure 1-1 Automatic selection and reselection of a PLMN

Null Switch on, no SIM

Switch on with SIM

Yes

SIM inserted

Selectregistered

PLMN

Is

there a

RPLMN

?

No SIM

No

TryingRPLMN

C

Registrationsuccess

Registrationfailure

A B

FIndicateselectedPLMN

LU response"Roamingnot allowed"

E A TryingPLMN

On VPLMN ofhome country

and timeoutoccurs

D

AYes

AnyallowablePLMNs

available?

On PLMN

C

Loss of radiocoverage of

selected PLMN **

Select first *PLMN in list

Select next *PLMN in list

Userre-selection

Registrationsuccessful

Registrationfailure, more in list

B

Registrationfailure, no morein list

D

Move lastselected PLMNtemporarily into

(d) on list

NoAny

PLMNs

& Allowable?

availableSelect RPLMNor HPLMN if

none

Initiate HPLMNsearch

GHPLMNsearch inprogress

HPLMN not found

HPLMNfound

ESelect

HPLMN

F

Yes

C

Select firstavailable and

allowable PLMNin list

AG

No

Wait forPLMNs

to appear

PLMN availableand allowable

PLMN availableand allowable,which is notRPLMN

E A

* "List" consists of points i) to v) except in caseof a user re-selection in which case “list”consists of points i) to vi)

** Includes effective loss of coverage due to LAs being forbiddenin all potentially suitable cells

Registered

Switch Off

SIM removed or invalid SIM

User reselection

Through the selection and reselection of a PLMN, a UE can select and roam between a

GSM network and a WCDMA network with no need to upgrade the two networks.

Through the reselection of a PLMN, a UE can return to the WCDMA network from the

GSM network. To enable a WCDMA UE to automatically select the WCDMA network

when roaming from the GSM network to the WCDMA network, a carrier can set the

WCDMA and GSM networks as different PLMNs and set the WCDMA network as the

HPLMN in the USIM of the UE. In addition, the carrier can control the PLMN selection

time.

1.1.2 Selection of an ePLMN

ePLMN, short form for equivalent public land mobile network, provides a solution for

subscribers to reside and roam between the shared networks and home networks. Carriers

can configure ePLMNs, between which the communication network resources can be

shared. That is, the ePLMN feature allows subscribers to be served by other networks than

their home networks based on their selections. The network selected by a subscriber must

be saved in the USIM card and included in the ePLMN list provided by the carrier upon

subscription or location update.

An ePLMN is added to the USIM card of a UE in the following way: A carrier can

configure the equivalent relation between different PLMNs at the network side to make

these PLMNs equivalent. Thus, mobile subscribers homed to these PLMNs can select

which network to reside from the ePLMNs provided by the carrier. The carrier configures

the ePLMN list on the MSC server. One ePLMN list can contain at most five ePLMNs.

When sending a Location Updating Accept message to a UE, the MSC server includes the

related ePLMN list in the message, as shown in Figure 1-2. Then, the UE adds the PLMN

ID of the existing network to the ePLMN list and saves the list in the USIM card. The

ePLMN list is updated when the location update or combined location update request is

accepted next time.

Figure 1-2 ePLMN list contained in the Location Updating Accept message

If the UE is located in a VPLMN, it periodically tries to obtain services from the HPLMN

or PLMN with a higher priority. If the UE stores an ePLMN list, it selects one ePLMN

from the list as its serving VPLMN, which enjoys a higher priority than the other PLMNs

in the same country. Therefore, if the ePLMN is not supported and the VPLMN is not the

HPLMN (or PLMN with a higher priority) of the UE, the UE reselects an HPLMN; if the

UE stores an ePLMN list, the UE selects one ePLMN from the list as its serving VPLMN

and reselects a cell.

The following figure shows the ePLMN networking covering the 2G and 3G networks.

Figure 1-3 ePLMN networking covering the 2G and 3G networks

MSOFTX3000

RNC

BTS

UE/MS

HLR

NodeB

BSC

BTSNodeB

ePLMN2

MGW

ePLMN1

The UE/MS selects a network (for example, ePLMN1 or ePLMN2) from the ePLMN list.

1.2 Selection or Reselection of a Cell

After selecting a PLMN, the UE starts to select a cell providing the highest-quality signal

in this PLMN. If the UE finds that the reselected cell belongs to another LA or RA, the UE

initiates a location update procedure, so that the network can obtain the latest location

information about the UE. The system information block 1 (SIB1) contains CN common

GSM-MAP NAS system information or PS domain system information, which reveals

the LAC and RAC. The UE checks whether the LA or RA changes based on the LAC and

RAC.

Table 1-3 Content of CN common GSM-MAP NAS system information

8 7 6 5 4 3 2 1

LAC octet 1

octet 2

Table 1-4 Content of PS domain system information

8 7 6 5 4 3 2 1

RAC octet 1

Spare NMO octet 2

If the location registration or update of the UE fails (for example, the network rejects the

location registration or update request of the UE), or the UE leaves the current PLMN

coverage, the UE can reselect another available PLMN. The process of cell reselection is

as follows:

1. Searching for a cell

To find a certain PLMN faster, if the UE stores certain information (such as frequency or

scramble) about this PLMN, it first uses this information for cell search. This information

is saved in the USIM or non-volatile memory of the UE. The procedure of searching for a

cell is as follows:

1) The UE synchronizes with the timeslot of the cell by using the synchronization code

of the physical shared channel (PSCH).

2) The UE synchronizes with the frame of the cell by using the synchronization code of

the secondary synchronization channel (S-SCH), and at the same time determines the

scramble group of the cell.

3) The UE obtains the primary scramble of the cell through the common pilot channel

(CPICH), and then the UE can read the broadcast channel.

If the UE has already known certain information (such as frequency and primary scramble)

about the cell, the preceding steps are completed faster.

2. Reading the broadcast channel

The UE reads the management information base (MIB) and checks whether the found

PLMN is the required one. If the found PLMN is the required one, the UE looks for the

other SIBs and obtains the content of these SIBs based on the dispatch information about

the other SIBs contained in the MIB. If the found PLMN is not the required one, the UE

searches for a cell from the next frequency. If finding a neighboring cell that meets the

requirements, the UE resides in this cell and reads the needed system information. Then,

the UE initiates a location registration procedure. If finding that no cell meets the

requirements, the UE considers that there is no network signal, and then continues to select

or reselect another PLMN.

The following figure shows the general process of cell selection and reselection.

Figure 1-4 Selection and reselection of a cell by a UE in idle mode

1

Initial

Cell Selection

Camped

normally

CellReselectionEvaluationProcess

Camped on

Any Cell

no cell information

stored for the PLMN

cell information

stored for the PLMN

go here whenever

a new PLMN

is selected

no suitable cell found

suitable cell found suitable cell found

Connected

Mode

suitable

cell found

no suitable

cell found

no

suitable

cell

found

Stored

Information

Cell Selection

Any CellSelection

Connected

Mode

(Emergency

calls only)

return to

idle mode

go here when no USIM in the UE

USIM inserted1

trigger

suitable

cell selected

no suitable

cell found

leave

idle mode

no acceptable cell found

Cell Selection

when leaving

connected

mode

leave

idle mode

return to

idle mode

an acceptable cell found

Cell Selection

when leaving

connected

mode

acceptable

cell found

NAS

registration

rejected

suitable

cell found1

CellReselectionEvaluationProcess

trigger

acceptable

cell selected

no acceptable

cell found

1.2.2 Cell Reselection Strategy and Cause

Table 1-5 Cell reselection strategy for the 2G/3G convergent network

Preferentially Select 3G Preferentially Select 2G

Differences in

technology

A UE can preferentially select the

3G network through PLMN or cell

reselection.

A UE can preferentially select the

2G network through PLMN or

cell reselection.

Requirements for

equipment

providers

To ensure that the 3G network is

preferentially selected, the MSC

server must support ePLMN or

adjustment at the wireless side.

UEs must support preferential

selection of a 3G cell.

To ensure that the 2G network is

preferentially selected, the MSC

server must support ePLMN or

adjustment at the wireless side.

UEs must support preferential

selection of a 2G cell.

Requirements for

and impact on

carriers

1. 3G UEs can preferentially

access the 3G network and enjoy

the 3G services and tariff.

2. 3G UEs can preferentially select

3G cells. 3. The wireless

equipment must ensure the

preferential selection of the 3G

network and prevent frequent

network handovers.

1. At the initial stage of 3G

network construction, the 2G

network coverage is better than

the 3G network coverage. If the

2G network is set as a

preferential selection, 3G UEs

cannot select the 3G network

after accessing the 2G network,

and thus cannot enjoy 3G

services and tariff.

2. 3G UEs can preferentially

select 2G cells.

Same as the PLMN selection strategy, the cell reselection strategy should also ensure that

UEs preferentially reside in the 3G network. In idle mode, a UE can roam between

different networks through cell reselection. The cell reselection strategy is as follows:

1. A UE reselects a 2G cell only when roaming out of the 3G network to ensure continuous

network coverage.

2. Once the UE returns to the 3G network, it reselects a 3G cell. The network selection

strategy of a UE upon power-on and the cell reselection strategy of the UE in idle mode

ensure that the UE preferentially resides in the 3G network, and take the advantage of the

2G network for better coverage.

The cell reselection strategy uses the 2G network to compensate the coverage defect of the

3G network at its initial construction stage, thus enabling 2G carriers to make the best of

their early investments on the 2G network and maintain their competitiveness in network

coverage. With only a minor change on the 2G network, the cell reselection strategy

enables a UE to return to the 3G network from the 2G network, ensuring 3G subscribers'

preferential access to the 3G network when the 3G network is available and carriers'

competitiveness in service provisioning. To enable a UE to return to the WCDMA system

from the GSM system through cell selection and reselection requires upgrading the GSM

system. In addition, the standards for measuring signal quality are different in the two

systems, and thus the parameters related to cell selection and reselection must be set

correctly; if any related parameter is set incorrectly, improper selection or frequent cycle

selection of cells might occur between the systems. In addition, a UE can return to the

WCDMA network from the GSM network through PLMN reselection, and thus cell

selection and reselection is not recommended to implement this function.