The Archangel Network was the main network on Earth from early 2007 onwards. It was a series of 15 satellites in stationary orbit around the planet, and all other networks were carried by Archangel.
It was designed by the Saxon Master in the guise of British Minister for Communications "Harold Saxon". Publicly, it was initially conceived as a secure defence communications system for Britain's armed forces, before growing into a public-private partnership whereby the satellites would also carry the first 4G mobile network; a network which was advertised as offering a more reliable service and cheaper calls.
Among the mobile phones which received the Archangel Network was Martha Jones' phone.
However, the Master had an ulterior motive for putting the satellites in orbit, that is for the purposes of mind control, which was composed of a weak signal of four drumbeats, in which were coded a message telling humans to trust Saxon and vote for him in the upcoming 2008 UK general election. Vivien Rook had figured this out, but was killed by the Toclafane. She had planned for this, however, having an email be sent to Torchwood containing all of her gathered knowledge on Archangel, enabling Jack Harkness, Martha Jones and the Tenth Doctor to learn of the Master's plan. The Doctor eventually ensures the network was deactivated after defeating the Master. (TV: The Sound of Drums [+]Loading...["The Sound of Drums (TV story)"])
When Earth was endangered by a signal being transmitted worldwide, the KOSAT 5 satellite launched by South Korea was scanned by UNIT for any suspicious activity only to come up clean, with Shirley Anne Bingham saying it was nothing like the Archangel Network. (TV: The Giggle [+]Loading...["The Giggle (TV story)"])
Other realities
The Year That Never Was
In an alternative timeline, one which was erased a year after it diverged, the Network advanced to give humanity a global consciousness by using a low-level psychic field, and was utilised to make all fear the Master and so be subservient to him. However, this telepathic field also caused the Master's demise, as when all the slaves on Earth revolted against the Master shouting cries and thoughts of "Doctor!" due to the stories told to slaves by Martha Jones, the Tenth Doctor (who had been aged 900 years by the Master's laser screwdriver) was rejuvenated, became a temporary sort of god. The timeline was then erased when the Master was distracted by Jack Harkness's destruction of the paradox machine in the Doctor's TARDIS. (TV: Last of the Time Lords [+]Loading...["Last of the Time Lords (TV story)"])
Behind the scenes
The Visual Dictionary
The non-narrative source Doctor Who: The Visual Dictionary gave further information about the Archangel Network.
It was a worldwide, mobile phone network that, from its 15 satellites, broadcasted the rhythm which allowed the Master to hypnotise Earth's population, giving him control over the people.