Transfers were receiver-driven; the transmitter would not send any data until an initial <NAK> was sent by the receiver. This was a logical outcome of the way the user interacted with the sending machine, which would be remotely located. The user would navigate to the requested file on the sending machine, and then ask that machine to transfer it. Once this command was issued, the user would then execute a command in their local software to start receiving. Since the delay between asking the remote system for the file and issuing a local command to receive was unknown, XMODEM allowed up to 90 seconds for the receiver to begin issuing requests for data packets.