UMTS/LTEDual IMSI Roaming Applet Application

Abhijit Hota

IntroductionTo enable Operator to roam in countries where there is no roaming partner.Subscribers are connected to Home network when at home using home network IMSI Subscribers are connected to preferred network in countries where Home network has bi-lateral agreements Subscribers are connected to Sponsor preferred network in countries where home network does not have bi-lateral agreements

Implementation: By means of a USIM Applet which detects when there is noHome Network partner in the country and switches IMSI to Sponsor IMSI.

AssumptionIn side the home network When the USIM is first used, HOME NETWORK IMSI is used by default After switching on the handset, if the handset cannot find HOME NETWORK within a specified time, the IMSI should switch.

Once registered on HOME NETWORK for more than the specified time HOME NETWORK IMSI should be always used and not switch until the handset is reset or power cycled

While overseas Last successfully registered IMSI is used after switching on the handset, if the handset cannot find any network within the specified time using current IMSI, the IMSI should switch Even after a successful registration, any loss of service for more than the specified time will trigger IMSI switching

EFs SwitchedThe following EFs in the USIM change when IMSI switches:

EF IMSI EF KI EF SMSP EF LOCI EF PLMN EF FPLMN - cleared

Proactive Command used TIMER MANAGEMENT REFRESH POLL INTERVAL PROVIDE LOCAL INFORMATION

USIM Architecture

Process Involved in IMSI switch1. Initialisation

The Applet starts when USIM receives PROFILE DOWNLOAD or STATUScommands after initialization.

POLL INTERVAL command is used to set the polling interval to a set time value.

Each STATUS command thereafter triggers the Applet to send PROVIDE LOCAL INFORMATION.

2. Triggering mechanism

The response to PROVIDE LOCAL INFORMATION will decide the next course of action for the Applet. No service – will indicate to the Applet that there is no network availabl

e and the Applet won’t do anything. Limited service – will indicate to the Applet that there is a network but

cannot register successfully. This will increase an internal counter in the Applet and eventually result in an IMSI switch, as it shows that thereis no roaming network with the current IMSI.

Full service – will indicate to the Applet that the registration with current IMSI is successful. Therefore the Applet won’t do anything.

If a response is not received by certain time (typically 2.5 minutes), a timer will expire and this will trigger an IMSI switch. This is to deal withexceptional cases where device doesn’t respond to USIM.

3. IMSI switchThe Applet switches IMSI by replacing the values of relevant EFs and issuing REFRESH command.Once REFRESH command is received, device must re-read EFs that have changed.

Roaming with Sponsor IMSI

Registration UE sends Location Updating Request message to a visited network. VLR of the visited network has no record of the IMSI. Since Sponsor network has a roaming agreement with the visited network,

VLR of the visited network sends a authenticate request to Sponsor . Sponsor recognises that the IMSI is assigned to Home network and forwar

ds it to home network. Home network rejects the request with a specific reject cause if it’s coming

from home network. Otherwise, it will check with home Network HLR. Home Network sends a response back to Sponsor. Sponsor masquerades the message as its own and forwards it to the visite

d network. At times, the request may time out on the visited network before a respon

se from Sponsor network is received. In this case, a specific reject cause issent back to the UE.

UE receives a response from the visited network.

Real World Scenarios:

A.In Home Operator Device will be switched on with Home Network IMSI, so it will register on

Home Network. Once registered on HPLMN, IMSI won’t change even if the device loses Ho

me Network coverage.

B. Going overseas where all operators are Home Network roaming partners Device will be switched on with Home Network IMSI, so it will register on

Home Network’s roaming partner. If the device loses coverage, since all operators have roaming agreement

with Home Network, device should be able to register on another network.

C. Going overseas where some operators are Home Network roaming partners andothers are Sponsor roaming partners.

Device will be switched on with Home Network IMSI, so it will try to register on Home Network roaming partner first.

If the device can’t find Home Network roaming partner after switch-on andresponds to a specific number of (not necessarily consecutive) PROVIDE LOCAL INFORMATION with limited service, the Applet will switch IMSI (to Sponsor IMSI). As FPLMN is cleared after an IMSI switch, device should findSponsor roaming partner immediately and register.

If the device loses coverage after a successful registration and responds toa specific number of (not necessarily consecutive) PROVIDE LOCAL INFORMATION with limited service, the Applet will switch IMSI.

D. Going overseas where all operators are Sponsor roaming partners Device will be switched on with Home Network IMSI, so it will fail to find a

ny network to register on. In countries without Home Network roaming partner, one response to PRO

VIDE LOCAL INFORMATION with limited service will trigger an IMSI switch.

With the Sponsor IMSI, the device will find a suitable network and registersuccessfully.

E. Coming back from overseas after roaming with Home Network IMSI As the device is switched on with Home Network IMSI, it will register on H

ome Network immediately.

F. Coming back from overseas after roaming with Sponsor IMSI Device is switched on with Sponsor IMSI and it won’t be able to register o

n any network as described in earlier When the device responds to specific number of (not necessarily consecut

ive) PROVIDE LOCAL INFORMATION with limited service, the Applet will switch IMSI (to Home Network IMSI).

With Home Network IMSI, the device will be able to register on Home Network.