How are signals sent by cellphones and through the internet identified?

[Bararontok]

The signals broadcasted by radio stations are identifiable because each station has its own frequency that no other station is allowed to use. Is this the same method used in cellphones and in the IP and MAC addresses of computers? Do they have circuits tuned to transmit data only at a set frequency?

 

[phyzguy]

On the internet - no. Data is transferred in packets of a fixed length. Each packet contains a header, which includes the source and destination IP addresses, and a payload, which includes the actual information being sent. So all of the packets can be sent in the same stream, and they get routed to the proper destinations by using the information in the header.

Cell phones are more complicated. Older cell phones used specific frequencies as you described, but over time several techniques have been developed to pack more cell phone data streams into the limited number of frequencies available. The most widely used technique today is called CDMA(Code Division Multiple Access), where multiple cell phone data streams share the same frequency, and each one has a different digital code that the receiver can use to pick out the one it wants.

 

[Bararontok]

So the information being sent is identified by automatically adding a fixed code to any data that is transmitted and since multiple signals are all traveling at once, when the person surfing the internet types the web address, the computer sends the request for the data and the server sends back the requested data, the computer accepts the data that has its IP and MAC address tagged onto it and it automatically blocks the data that is tagged with an IP and MAC address that is different from the one that it has.

In the case of the cellphone, the digital code that indicates the destination cellphone is added when the person selects the phone number that they would like to send data to. And the receiving cellphone blocks all the other signals that contain a different code than its own phone number.

But this system of adding a unique code to transmitted data works if the data transmitted is just being downloaded into the memory of the destination devices. But how is the connection for a call through a computer or cellphone maintained if the data is streamed live?

 

[phyzguy]

In a live call, the computer inside your cell phone digitizes the sound data (Using a device called an Analog-to-Digital to converter, or ADC) and then breaks the digitized data up into a series of digital packets. These packets are then sent over the internet as I described earlier, and the computer in the cell phone of the receiver reassembles the packets into a digital stream, which is then converted to analog sound waves by a Digital to Analog converter or DAC. Because there can be delays in the transmission of the packets, the computer buffers the digital data so that the receiver hears the sound data somewhat later than you speak it, Usually, this delay is unnoticeable, but sometimes you can hear it as an echo.

 

[Bararontok]

So this means that even the cellphone uses the internet to transmit data which is broken up into packets that are tagged by a destination code to ensure that only the destination phone receives the signal. If the cellphone is already using the internet network, then why are calls and texts charged and calls and texts through a desktop or laptop free?

Additionally, peer-to-peer file sharing also uses the same technique used when downloading data from a server. When the request for a certain file is made by running torrent files in a torrent program, the destination address of the CPU generating the request is tagged onto the address and location of the files of the computers containing the files. The computers containing the files then receive the request and tag the incoming destination address to the stream of data to be downloaded to the destination computer so that the file is downloaded to the correct computer.

 

[phyzguy]

So this means that even the cellphone uses the internet to transmit data which is broken up into packets that are tagged by a destination code to ensure that only the destination phone receives the signal. If the cellphone is already using the internet network, then why are calls and texts charged and calls and texts through a desktop or laptop free?

No, sorry - I oversimplified. What I described in the preceding post is really a description of what happens when you use a Voice over IP (VoIP) service, like Skype. When it is a direct cell phone-cell phone call, the data still gets digitized and broken into packets, but may be sent over the service provider's network and not use the public internet. However, I think the service providers still use the same techniques, and may use the public internet for sending the packets part of the way. For example, if you make a cell phone call from New York to someone in Pittsburgh, the digital packets are sent by radio from your cell phone to the cell phone tower in New York, but I think (not completely certain about this) that the packets then travel over the public internet from New York to the Pittsburgh cell phone tower, where they are then sent by radio to the receiver's phone.

As for what you are being charged for, it is both providing the network of cell phone towers that you can use, as well as keeping track of your phone so that the call can find you when someone calls.

 

[Bararontok]

Yes, this is correct. As described in a discovery channel documentary about surveillance, the cellphone that is to be called is tracked using signal triangulation where a signal that contains the phone number is sent to a passive RFID that reads the signal to verify that it matches the code contained in its memory card and then re-transmits a feedback signal to the network. The location is verified by the amount of time it takes for the signal to reach the network and how much the radiation has weakened in intensity as it travels through the air. Once the location is verified the phone number which is included in the message or call signal is used by the telephone exchange to send the signal to the correct location by triangulating the signal through the cell sites until the one closest to the cell phone is reached. Now aside from using telephone exchange switching technology to switch to the correct routes to the destination phone, the use of the phone number as a code to tag the signal is still necessary because the nearby cell site can still have multiple signals being transmitted and the unwanted signals would still need to be filtered out by the receiving phone.