Rack

A vertical or horizontal open support, usually made of aluminium or steel, that is attached to a ceiling or wall. Cables are laid in and fastened to the rack.

RAID

Redundant Array of Independent Disks (formerly Redundant Array of Inexpensive Disks)
 - A way of storing the same data in different places (thus, redundantly) on multiple hard disks. By placing data on multiple disks, I/O (input/output) operations can overlap in a balanced way, improving performance. Since multiple disks increases the mean time between failures (MTBF), storing data redundantly also increases fault tolerance.

Resistance

The property of a conductor that determines the current produced by a given potential difference. It impedes the flow of current and results in the dissipation of power as heat. Resistance is measured in ohms.

Router

A special purpose computer or software package that handles the connection between two or more packet switched networks. Routers spend all their time looking at the source and destination addresses of the packets passing through them and deciding which route to send them on.

RSVP

Resource Reservation Protocol
A protocol used to support QoS for different types of traffic in a network. Part of TCP/IP.