Question: A UE with a USIM (but no ISIM) needs to construct an XCAP Root URI. The UE's IMSI components yield an MCC of 123 and an MNC of 45. How should the UE format the domain part of the XCAP Root URI?
Option 1: ims.mnc045.mcc123.pub.3gppnetwork.net
Option 2: xcap.ims.mnc45.mcc123.pub.3gppnetwork.org
Option 3: xcap.ims.mnc045.mcc123.pub.3gppnetwork.org
Option 4: http://ims.mnc45.mcc123.3gppnetwork.org/xcap
Explanation: According to the specification, when a UE has a USIM and no ISIM, it constructs the XCAP Root URI with the home network identified as 'ims.mnc<MNC>.mcc<MCC>.pub.3gppnetwork.org'. If the MNC has fewer than three significant digits, a leading zero is added to fill the three-digit coding for MNC. Since the MNC is 45 (less than three significant digits), it should be padded with a zero, resulting in 'mnc045'. The correct format also includes 'xcap' as the first label and '.org' as part of the domain, leading to the correct answer being 'xcap.ims.mnc045.mcc123.pub.3gppnetwork.org'.