How a Text Message Gets to Another Phone
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Message Sorting
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In addition to connecting people using voice, most modern cellphones also have the capability of sending text messages. This feature is called "Short Message Service," or SMS. While these SMS messages use the same cell networks as voice calls, they operate in a completely different way.
When a SMS text message is composed from a phone, it does not transmit directly to its destination. Instead, it travels first to an "SMSC," or Short Message Service Center. Each cellphone network, such as AT&T or Verizon, has a separate Short Message Service Center. This location sorts all incoming messages from the network and can handle incoming data from thousands of individual phones at once. At this center, text messages are stored and then forwarded toward their intended destinations. The SMSC serves as the "mail center" for all messages.
Message Switching
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Not all cellphones operate on the same network. Even though phones may be from different companies, all messages must be able to reach their correct destinations. Ensuring that messages are routed toward the proper places is the job of the "GMSC," or Gateway Mobile Switching Center.
Once the Gateway Mobile Switching Center has received a message that has been forwarded by the SMSC, it checks its database of network details. This database contains information on cellphones from all different carriers, including the general location where the destination cellphone was last used. These details allow the GMSC to forward the text message to the correct location throughout the country.
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Message Delivery
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When the GMSC switching center has located the correct geographic destination and passed the message on, the text message delivery process is nearly finished. To complete the final step, however, the message must be broadcast in the proper location and received by the destination cellphone.
A "BSS," short for Base Station System, transmits the text message over the air using radio frequencies into the area where the destination phone is located. If the cellphone is on, it receives the message and displays it to the user. If the phone is off or out of range, a copy of the message is re-sent automatically at another time.
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