USIM IMSI Switching NAS Procedure Handling
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Solution Overview
Problem
In wireless communication systems, user equipment (UE) faces challenges in handling Non Access Stratum (NAS) procedures, particularly when receiving rejection messages with cause values, such as insufficient resources or congestion, leading to unclear operations when the International Mobile Subscriber Identity (IMSI) in the USIM is changed via data download or a 'REFRESH' command.
Innovation Solution
A communication device with a UMTS subscriber identity module (USIM) that stores multiple IMSIs, which executes instructions to access networks using the first IMSI, starts a timer upon receiving a NAS response, and changes to the second IMSI without removing the USIM, keeping the timer running and retransmitting NAS requests when the timer is active.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the UE stops transmitting NAS request messages when a timer is running or USIM is removed, then the UE avoids repeated rejections, but the UE cannot resume service when IMSI is changed via data download or REFRESH command
Solution Approach 1:
The patent applies dynamics by making the UE's NAS request transmission behavior adaptive based on real-time conditions. The UE dynamically adjusts its operation mode between 'normal mode' (transmitting NAS requests) and 'restricted mode' (not transmitting NAS requests) based on timer status and IMSI change events. This dynamic state management enables the UE to resume service when needed while avoiding repeated rejections during restricted periods.
Solution Approach 2:
The patent implements preliminary action by having the UE prepare and store multiple IMSIs in the USIM before service interruption occurs. When an IMSI change event is detected (via data download or REFRESH command), the UE already has the alternative IMSI ready to immediately resume NAS procedures, eliminating the need for re-subscription and enabling rapid service restoration.
2Reliability
If the UE does not transmit another NAS request message after receiving a NAS reject message with cause value, then the UE avoids further service denial, but the UE cannot access network when IMSI is changed via data download or REFRESH command
Solution Approach 1:
The patent implements preliminary action by pre-storing multiple IMSIs in the USIM and preparing the UE to switch IMSIs when needed. When a NAS reject message is received and the UE detects an IMSI change event (data download or REFRESH command), the UE can immediately switch to the alternative IMSI and resume network access without waiting for timer expiration or re-subscription, significantly reducing service interruption duration.
Solution Approach 2:
The patent uses the alternative IMSI as an intermediary to bridge the service interruption gap. When the primary IMSI is rejected or becomes invalid, the secondary IMSI acts as a mediator that enables the UE to re-establish network connection without complete service denial, allowing the UE to bypass the blocked state and resume normal operations.
3Adaptability or versatility
If the UE uses a single IMSI for all network access, then the UE simplifies identity management, but the UE cannot resume service when the IMSI is changed via data download or REFRESH command
Solution Approach 1:
The patent applies universality by designing the USIM to serve multiple functions: it stores not only the primary IMSI but also alternative IMSIs that can be activated when the primary IMSI is rejected or changed. This multi-functional USIM structure enables the UE to adapt to various network conditions (normal operation, rejection scenarios, IMSI changes) using a single unified module, enhancing versatility without requiring separate identity management systems.
Data Source
AI summary
A communication device of handling a Non Access Stratum procedure comprises instructions of accessing a first network by using the first IMSI; transmitting a first NAS request message to the first network by using the first IMSI; receiving a NAS response message comprising a timer value and a cause value from the first network; starting a timer for the first IMSI with the timer value; not transmitting a second NAS request message to the first network by using the first IMSI, if the timer is running for the first IMSI; changing from the first IMSI to the second IMSI without removing the USIM; keeping the timer running, after changing to the second IMSI; accessing a second or the first network by using the second IMSI; and transmitting a third NAS request message to the second or the first network by using the second IMSI, when the timer is running.