Acknowledgments allow senders to determine when to retransmit lost packets. There are also digital amateur TV repeaters that retransmit digital video signals. In either case, the receiver ignores the packet except to retransmit an acknowledgment. But they wait a calibrated time before they retransmit any packet. The sender would accordingly retransmit only the second segment with sequence numbers 2,000 to 2,999. Some stations use LPTV broadcast translators to retransmit to further areas. Duplicate acknowledgement is the basis for the fast retransmit mechanism. This greatly improves TCP's ability to retransmit the right segments. Also, radio stations cannot retransmit foreign programs without HAAC approval. With different codes, additional operators were required to translate and retransmit the message. Once, almost all of its stations also retransmit that network. Store-and-forward systems neither retransmit simultaneously, nor use a different channel.