Backup of data is such a primary issue that gross negligence is now assumed whenever server-based data is lost. RAID (dual mirrored drives) are the norm in such environments, and use of tape is still a nightly routine (or even four times per day). I work in an evironment where ALL server-based user data is backed-up every five minutes!! Finally, mission critical data, such as Am-Ex, or VISA transactions from around the world are continually stored in huge repositories six-stories underground in Texas to assure that, in the event of a thermo-nuclear exchange, you would still get your CC bill the moment the rubble was cleared away

What will happen if various pieces of the network lines go down?
The implications of an outage are dependent on the criticality of the line. It may mean no email for while, or it may inhibit department-wide process activities if data is sourced from other areas, such as call centers, as opposed to fulfillment centers.

RAID (Redundant Array of Inexpensive Disks) is a data storage structure that allows a system administrator, designer, builder, or user to combine two or more physical storage devices (HDDs, SSDs, or both) into a logical unit (an array) that is seen by the attached system as a single drive.
There are three basic RAID elements:

1. Striping (RAID 0) writes some data to one drive and some data to another, minimizing read and write access times and improving I/O performance.
2. Mirroring (RAID 1) replicates data on two drives, preventing loss of data in the event of a drive failure.
3. Parity (RAID 5 and 6) provides fault tolerance by examining the data on two drives and storing the results on a third. When a failed drive is replaced, the lost data is rebuilt from the remaining drives.

It is possible to configure these RAID levels into combination levels, called RAID 10, 50 and 60. The RAID controller handles the combining of drives into these different configurations to maximize performance, capacity, redundancy (safety) and cost to suit the user needs.