Yes, I know, Google is my friend, but my “friend” isn’t telling me what I want to know.
When I call out on my phone, it searches for the nearest cell tower and grabs a frequency. It then connects to another tower, which connects to another, etc. etc. until a tower somewhere in southern California makes my friend’s phone ring.
That’s a string of hundreds of cell towers running across the country. How does the signal know to use that particular path? Or does it check every tower all around the world until it finds my friend’s phone? That seems like it would take an awful lot of bandwidth.
Cell phone calls do not jump from tower-to-tower across the country.
Your signal hits a cell tower which then puts the call through on a land line. The land line goes to a central station which routes your call. A computer asks “where” that number is and the call is routed via land lines and perhaps via satellite to a local routing station near your friend. The call is then routed to the cell tower your friend is currently connected to which broadcasts a signal with a code. All cell phones “hear” the code but only your friend’s phone, which has the matching code, will ring.
Your phone is constantly roaming - sending out a “ping” signal every few seconds to find the nearest tower and say “hello, I’m here”, and the tower responds. The tower relays that information back to the central computers for your phone company (also via land line). That way, the phone company can tell that either (a) you are not in service, or (b) where you are to route any calls.
Towers are connected by trunk lines to the telephone system.
Note the “ping” message every few seconds is pretty basic. There are 144 characters not used, so text messaging was developed to use those spare characters. Hence, really, a text message uses no resources whatever other than routing the message from sender’s tower to receiver’s tower. The airtime is free. Telcos that charge 10 cents or 5 cents a message (for both sender and receiver) are simply gouging.
Your phone doesn’t wait until you try to place a call to start communicating with the local tower. It has to be in contact with the towers frequently, so that if your friend tries to call you, the phone system knows which cell you’re in.
Also, it doesn’t grab a frequency - all the phones connecting to a certain cell site use the same frequency. If the cell site is GSM or EDGE, the phones take turns communicating with the tower in brief slices of time. With CDMA, WCDMA, HSPA, etc, all the phones are sending at the same time, and the tower sorts it out by applying a mathematical correlation function to separate them.
Since the question seems to be answered, can somebody enlighten me on this? Does any normal “everyday” communications go over satellites? I was under the impression that they were a very expensive way to communicate compared to fiber.
A standard cell site has many different radios, each on a different frequency. The cell site’s setup channel directs the mobile to retune to one of those frequencies. Then each frequency is multiplexed as you described above.
GSM phones don’t ping the cell every few seconds. Indeed the phone can be quiet for a very long time. Phones listen to the cells around them, and so long as they remain within one cell they don’t need to talk. When a phone is turned on, it negotiates with the cells it finds itself in, to establish one it is allowed to talk to (basically that you have a contract with the cell’s owner or a carrier that has a contract with that cells owner) This allows the carrier that your phone is registered with to know how to find you. Even when you are overseas. When you move between cells, your phone notices that it has moved, and initiates a handover protocol from one cell to the next. That requires that the phone initiate a conversation with the cells. Handover can also happen in mid call as well. Handover is one of the neat miracles of the protocols.
I remember in the early days of videoconferencing, attempting to use satellite for conversations. There was a significant delay - some for the travel, some for the code/decode back then. Satellites are 22,300 miles up (from the earth’s surface) so a round trip up and down is 44,600 miles. Light travels 186,000miles/sec (in a vaccum) so that adds almost a quarter-second to a transmission. Then their reply takes another quarter-second. A half-second may not seem like much, but it’s just enough that everyone waits-then-talks and conversation becomes difficult.
Plus, bandwidth is limited by the frequency used (and bandwidth allocated) and nowadays it is incredibly cheaper and much higher capacity and faster to use a direct fibre connection. The amount of fibre capacity across the USA and even underseas is immense compared to satellite capacity. Fibre can be poiint-to-point and parallel, a satellite ties up that bandwidth at least for a metropolitan area, depending on how precise the transmission cone is; and on the geostationary orbit thre are only so many slots before traffic for one satellite interferes with its neighbour - IIRC, the current separation is 2 degrees along the equator, down from 3 degrees a decade or so ago. Plus the backup for fibre is an alternative route, the backup for satellite is to order another one and a rocket and wait until it is launched.