I have a basic question regarding digital communication , especially pertaining to wireless internet .
A packet is basically a stream of bits . And a bit is detected as zero or 1 depending on whether the amplitude of a signal of certain frequency is crossing a threshold or not . Now suppose a particular host or source $ A $ is sending this signals at a frequency $\ f $ to a receiver $ R $ Then it is possible that some device , $ B$ near the receiver can be placed to detect the frequency and regenerate a signal in that frequency and direct it towards the receiver .
In internet communication , I guess the packets will definitely be containing information about the source IP address, the IP of $ A $ but $ B $ can alter the bits so as to insert send its own IP address in the packets .
In such situations , how do real receivers , identify the correct source ?