[SI-LIST] What is meant by a differential signal

Dear Members,
        In my understanding unlike ethernet signals (Tx+ & Tx- and Rx+ & Rx-) 
which are true differntial the USB signals are not true differential. 
        Differential means current flowing in one signal has to return back 
through the counterpart. In the ethernet protocol Tx+ current will return 
through Tx- and vice versa. The same holds good for Rx pair also. In an ideal 
differential system current flowing through the midpoint (ground)  will be 
zero.  
       This is not the case for USB protocol. Here ground is the datum through 
which the return current flows and under no circumstances the ground current 
will be zero. Then why USB is identifed as a differential system? 
Bharath


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