The Cyber-Mainframe was the information network utilised by a race of Cybermen from another universe, seeking to conquer the multiverse. All cyber-units were connected to the Mainframe upon conversion, allowing them to download updates; these varied from locations of priority targets to weapons and shield enhancements. Cyber-Controllers were capable of creating updates and transmitting them to lower-ranking cyber-units.
In the case of virtually converted humans, in particular those who received cranial upgrades, connection to the Cyber-Mainframe could cause extreme mental stimulation and, if not fully converted after a certain amount of time, death. This was due to the immense size of the Mainframe, containing vast quantities of information on billions of cyber-units across entire universes; the sheer amount of information was too much for a human brain to process properly, with the only remedy being either full cyber-conversion (AUDIO: Code Silver) or if the virtual upgrades were reversed by means such as a corrosion virus. (AUDIO: Master of Worlds)
A major fundamental flaw of the Cyber-Mainframe was that it was also the means of which individual cyber-units received power, as electricity was generated by the various worlds assimilated into the Mainframe (such as one parallel Earth whose population were utilised as human batteries), then wirelessly broadcast through the network to individual units and cyber-technology. Because of this, electromagnetic interference could be used to sever a cyber-unit’s connection to the Mainframe, temporarily shutting them down as a result; this was used by Kate Stewart and Josh Carter to disable various Cybermen after they began invading the Fortress Island of the Earth of N-Space, although the Cyber-Controller leading the invasion restored its connection to the Mainframe via a secondary antennae in its chest unit. (AUDIO: Code Silver) Later, the Master created a larger electromagnetic field around the entire fort, temporarily disabling all the Cybermen within, and, after seeming to have been upgraded by a modified mobile conversion unit, boosted the dimension gate’s connection to draw power from every universe. As this included all the universes that the Cybermen failed to begin dimension seeding in, the Cybermen‘s power supply was poisoned, killing all those connected save for the Master and the millions of virtually converted humans, ending the Mainframe’s attempt to convert N-Space. (AUDIO: Master of Worlds)