[Peten]

I am really interested in intercepting cellular networks
I know how imsi works
I know what msc and hrl are
I know we can impersonate other devices
The real question is how do you decode sms
Or are they even decrypted while transmitting or you can read them ass a plain text??

[Interconnector]

Why would you need to decode them if you're able to enter the network? You could just switch SMSC and intercept it.

At most, the only thing you're going to have to "decode" is bytes.