The Route of a Text Message, a Love Story

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The surprisingly complex journey a text message takes every time we hit 'send.'

Engineers would say that, when the phone senses voltage fluctuations over the ‘send’ button, it sends the encoded message to the SIM card (that tiny card your cell provider puts in your phone so it knows what your phone number is), and in the process it wraps it in all sorts of useful contextual data. By the time it reaches my wife’s SIM, it goes from a 140-byte message (just the text) to a 176-byte message (text + context).

My wife’s message is massaged into the 279-byte SS7 channel, and sent along to the local base transceiver station (BTS) near the bakery. From there, it gets routed to the base station controller (BSC), which is the brain of not just our antenna, but several other local antennas besides. The BSC flings the text to AT&T Pittsburgh’s mobile switching center (MSC), which relies on the text message’s SCA (remember the service center address embedded within every SMS? That’s where this comes in) to get it to the appropriate short message service center (SMSC). The SMS has to go from the SMSC to a global switchboard and then potentially bounce around the world before finding its way to my phone.

[Scott B. Weingart is an historian of science, a data scientist, and a librarian at Carnegie Mellon University.]


The Route of a Text Message, a Love Story