Final answer:
If line supervision is unavailable, at least two independent means of alarm signal transmission need to be provided to the monitoring location to ensure redundancy and reliability.
Step-by-step explanation:
If line supervision is unavailable, at least two independent means of alarm signal transmission need to be provided to the monitoring location. This is done to ensure redundancy and reliability in case one means of transmission fails. For example, if the primary method of alarm signal transmission is through a phone line, a secondary method like cellular communication or internet connection can be used as a backup.