You do have to know them. X-Modem and Y-Modem are different protocols for error detection. Check your textbook - it will tell you that:
X-Modem: Sends a packet and waits for the acknowledgement of the receiving computer before either retransmitting the same pack in response to a negative acknowledgement, or transmitting the next packet in response to an acknowledgement.
Z-Modem: Continiously transmits packets until it receives a negative acknowledgement from the receiving computer (which received a corrupt packet).
thanks, i had no idea about them as we had never done it and during my study last night i came across it. i would check my syllabus if i had got one, and i would check the text book for it but i couldnt find it in my textbook, they talk about using them but never say what it actually is