Many of the replies above have outlined the various agencies that would talk to an aircraft flying from SFO
, and, for the most part, they are accurate as to the sequence of when the aircraft crew talks to which control agency.
I will try to explain "when" the "handoff" takes place between the various controlling agencies.
Each ATC agency has it's own geographic area of responsibility. Anything operating within that area of responsibility should be under the direct control of the agency that has been assigned control of that area.
Each control area is physically (geographically) next to another control area (Ground Control handles movement from the ramp to the runway, but not ON
the runway... Local Control handles aircraft ON
the runway, or those within a few miles of the runway on arrival or departure. Departure Control handles aircraft that have just departed Local Control's area but which have not yet reached the area controlled by an ATC Center. That area is usually defined as an altitude ceiling or as a distance-from-airport. ATC Centers handle everything at the higher altitudes where most IFR traffic cruises.)
Regardless of what the control area is, each control agency has specific, written policies concerning handoff procedures to adjacent agencies. We used to call these Letters of Agreement... they may have changed since I controlled traffic. These documents specify where, when, and how the one agency will hand off an aircraft to the other agency. The general rule of thumb is that the handoff must be COMPLETED prior to the aircraft leaving the control area of the losing agency.
The handoff consists of two distinct parts:
(1) The location and identification of the aircraft is brought to the attention of the gaining control agency by an ATC operator in the losing control agency, and the gaining control agency acknowledges that they understand the infomationg just given to them. This notification can be by landline, by radio, by computer link, or any other viable mether that is agreed upon between the two agencies.
(2) The losing control agency tells the aircraft to contact the gaining control agency on a specific radio frequency. When the aircraft acknowledges the instruction to change frequencies, the losing control agency effectively gives up control of the aircraft to the gaining control agency, even if the aircraft is still within the geographical boundaries of the losing control agency. (Should the gaining control agency need to tell the aircraft to gain/lose altitude or to change heading while the aircraft is still within the losing control agencies area of responsibility, the gaining control agency must coordinate such instruction with the losing organization prior to issuing the instruction to the aircraft.)
Regardless of the route structure or the ATC radar/non-radar facilities in use, the above method of effecting handoffs is pretty much universal. There may be slight variations, but the general idea will always be the same: make handoffs safe, accurate, and expeditious.
I hope this helps....
"There may be old pilots, and there may be bold pilots, but there are darn few green cows"