If there is a disturbance to the connection with the Internet provider or to a remote network, a backup line can act as a temporary replacement for the normal data line. This requires the existence of a second physical connection which can be used to contact the remote site. Examples of backup lines are typically:
- An ISDN line as a backup for DSL Internet access
- An ISDN line as a backup for VPN network coupling
- A modem connection (GSM or analog) as a backup for DSL or ISDN lines and VPN connections