When an RTDS Simulator consists of three or more racks, a special purpose hub, referred to as a Global Bus Hub, GBH, is required to exchange synchronization signals between GTWIF cards. An RTDS Simulator consisting of a single rack operates independently and a two-rack RTDS Simulator can operate by making direct connection between the two GTWIF cards. For RTDS Simulators with 3 racks or more, each GTWIF connects to the GBH using a fibre optic Tx/Rx cable. The GBH is accessible from the rear of the cubicle. GBH communication is separate from the Ethernet communication used to exchange information between GTWIF and the host workstation.