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what is the difference between ISIM and USIM in VoLTE/LTE/4G scope? What is its use in SIP REGISTRATION process? Why we need two type of it, and does it depend on operator?
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IM Services Identity Module (ISIM) is an application which resides on a mobile devices SIM card (UICC). It performs security procedures such as authenticating the network, and contains user identity and security keys used to encrypt the information transmitted over-the-air.
The USIM is the functional equivalent of the SIM in a GSM device. It is the entity that stores subscriber-related information and implements the security functions pertaining to authentication and ciphering on the user side in UMTS and LTE (3G and 4G) devices.
Universal Subscriber Identity Module (USIM) card is a SIM card for 3G services. USIM provides enhanced features than compared to SIM like enhanced security. USIM Module is also a type of SIM card but specifically for WCDMA/UMTS (3G) phones.
Android 9 introduces a new SystemApi interface called ImsService to help you implement IP Multimedia Subsystem (IMS). The ImsService API is a well-defined interface between the Android platform and a vendor or carrier-provided IMS implementation.
USIM make use of a new algorithm which protects from unauthorized access to one’s phone line. Encrypted calls and data exchanges of USIMs make use of strong keys than keys used by SIMs. Comparison between SIM and USIM Card: SIM is a memory chip which is used in mobile phones.
Since it does not require physical space for an eSIM chip or removable SIM card, the iSIM can reduce the footprint of a device. With the iSIM functionality built directly into the base cellular module hardware, the result is savings from the elimination of one or more components like SIM trays, SIM cards or eSIM chips.
SIM cards (Subscriber Identity Module) are used to communicate on GSM networks. With the introduction of UMTS or 3G, it’s highly recommended to use a USIM card to access UMTS network. Although it’s possible to access it with a simple SIM card, the USIM have many advantages as compared to the SIM:
SIMs are basically Universal Integrated Circuit cards (UICC) which contain both account information and memory. USIM cards store subscriber information and authentication information.
USIM - Universal Subscriber Identity Module
ISIM - IP Multimedia Services Identity Module
ISIM and USIM are applications which run in your simcard. Both application are responsible (among other things) to perform security procedures such as authenticate the network. They also carry your identity and security keys used to encrypt the information transmitted over-the-air.
ISIM carry a lot of information(files) used in SIP protocols such as your IP Multimedia Private Identity (IMPI), domain, IP Multimedia Public Identity (IMPU) and cypher keys (used to encryp information. So, this application is used for SIP/IMS procedures - consequently, VoLTE calls.
USIM does not have those kind of information. It has instead IMSI, SPN, Cyphering and Authentication keys used during registration in UMTS/LTE networks.
In a simple way, ISIM is used during SIP/IMS procedures as long as USIM is used during NAS/RRC/MM/GMM procedures.
Even then, I believe that you don't need a ISIM to register in IMS (and make a VoLTE call). 3GPP defines standards which make possible to create a IMPI/IMPU using keys from your USIM.
Even then, as I observed, most part of network carriers replace their old SIMCard which has only the USIM application by a new SIMCard which has both applications (since they can co-exist in same SIMCard).
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