The Mark 1 Digital Subscriber Switch (DSS) was the first to be introduced. It is a time-space-time switch setup with a theoretical maximum matrix of 96x96 Time Switches. In practice, the maximum size of switch is a 64x64 Time Switch matrix. Each time switch is duplicated into two security planes, 0 and 1. This allows for error checking between the planes and multiple routing options if faults are found. Every timeswitch on a single plane can be out of service and full function of the switch can be maintained, however, if one timeswitch on plane 0 is out, and another on plane 1 is out, then links between the two are lost. Similarly, if a timeswitch has both plane 0 and 1 out, then the timeswitch is isolated.