The NYT has an article, What Carriers Aren’t Eager to Tell You About Texting , about new interest in understanding why charges for SMS service has been increasing even while volume is up and communication costs are down.
I learned one interesting thing from the article about the length of SMS messages. I’d never thought much about where the limit on the number of characters came from. According to the article, the limit is 160 (7 bit) characters because that’s what will fit into the control channel messages that mobile phones exchange with cell towers.
“The lucrative nature of that revenue increase cannot be appreciated without doing something that T-Mobile chose not to do, which is to talk about whether its costs rose as the industry’s messaging volume grew tenfold. Mr. Kohl’s letter of inquiry noted that “text messaging files are very small, as the size of text messages are generally limited to 160 characters per message, and therefore cost carriers very little to transmit.” A better description might be “cost carriers very, very, very little to transmit.”
A text message initially travels wirelessly from a handset to the closest base-station tower and is then transferred through wired links to the digital pipes of the telephone network, and then, near its destination, converted back into a wireless signal to traverse the final leg, from tower to handset. In the wired portion of its journey, a file of such infinitesimal size is inconsequential. Srinivasan Keshav, a professor of computer science at the University of Waterloo, in Ontario, said: “Messages are small. Even though a trillion seems like a lot to carry, it isn’t.”
Perhaps the costs for the wireless portion at either end are high — spectrum is finite, after all, and carriers pay dearly for the rights to use it. But text messages are not just tiny; they are also free riders, tucked into what’s called a control channel, space reserved for operation of the wireless network. That’s why a message is so limited in length: it must not exceed the length of the message used for internal communication between tower and handset to set up a call. The channel uses space whether or not a text message is inserted.”
There’s a lot more to the protocols, of course. The Wikipedia SMS article looks like a good place to start.