Does G4 makes use of the two B channels?
Yes. Using the two B channels, most G4 machines can send and receive at the same time, so a single G4 can do the work of two G3s. For example, if while transmitting a fax someone tries to send you a fax, the G4 machine continues transmitting and spools the incoming fax to a buffer (hard disk, memory). When the first transmission is complete, the received fax is printed.
