No official Samsung firmware (stock ROM) exists for GT-I9100 with Android 5.1.1. Samsung ceased official support for this device at Android 4.1.2 (Jelly Bean) . Any Android 5.1.1 firmware for GT-I9100 is custom, unofficial, and community-developed (primarily LineageOS-based or CyanogenMod 12.1).

Gt-i9100 Firmware 5.1.1 Download Site

No official Samsung firmware (stock ROM) exists for GT-I9100 with Android 5.1.1. Samsung ceased official support for this device at Android 4.1.2 (Jelly Bean) . Any Android 5.1.1 firmware for GT-I9100 is custom, unofficial, and community-developed (primarily LineageOS-based or CyanogenMod 12.1).