|Fig. 1: Sending an SMS on a blackberry smart phone. (Source: C. Durkin)|
Short Message Service (SMS) is the world's most common form of text messaging, the exchange of brief electronic text between two or more short message entities (SMEs). SMEs are communication devices used for composing and disposing short messages, including devices such as mobile phones, pagers, and personal computers. Common applications of SMS are short written conversations (Figure 1), reminders, alerts, subscriptions, polling, and charitable donations. 
Finnish electrical engineer Matti Makkonen originally developed SMS texting in the 1980's while working at Finland's telecoms authority.  In 1984, German Friedhelm Hillebrand and Frenchman Bernard Ghillebaert proposed SMS texting to the Global System for Mobile Communications (GSM, previously Groupe Special Mobile) telecommunications standards body, who drafted technical specifications for SMS by 1987.  The first SMS message "Merry Christmas" was sent from a personal computer to a mobile phone via the United Kingdom's Vodafone network in 1992.  Today, SMS texting is a staple of modern communication. In 2008, between three and four trillion SMS texts were sent, generating revenue of $80-100 billion. 
SMS utilizes a secondary radio channel on mobile networks that previously was used only for communicating reception strength and information about incoming calls. Because this data lane already existed and had mostly free capacity when SMS was first proposed, it represented an inexpensive space to support small data communications. 
SMS text messages are limited in size to 140 octets, or 1120 bits, which translates to a maximum of 160 7-bit characters (GSM character set, Latin alphabet), 140 8-bit characters, or 70 16-bit characters (Cyrillic alphabet, Chinese, Japanese, Korean, Arabic).  The 1120 bit size constraint for SMS is not a technical limitation, but rather a personal choice by SMS creator Hillebrand. He decided on 160 characters after testing common phrases on his typewriter and analyzing average character counts on postcards.  Messages of greater lengths than 160 characters will be split into several segments and sent as multiple SMSs. 
When person A sends an SMS to person B, the transmission occurs as shown in Figure 2. The Mobile Switching Center (MSC) is responsible for routing both voice telephone calls and SMS texts and the Visitor Location Register (VLR) is a database that temporarily store information about subscribers that have roamed into its jurisdiction, including the subscriber's identity number (IMSI), phone number (MSISDN), mobile station roaming number (MSRN), authentication data, and HLR address. The Home Location Register (HLR) is a permanent database that is used for the storage and management of subscription and service profiles. The Short Message Service Center (SMSC) is an element in the mobile phone network that is a store and forward service, which allows the message to reach the recipient even if his or her device is unavailable, out of service, or switched off. In addition to forwarding the text message to person B, an acknowledgement that the SMS has been successfully sent is relayed to person A. [5, 6]
|Fig. 2: Transmission of an SMS over a mobile network. [5,6]|
SMS is brief, informal, and discreet compared to a phone conversation and texts can be sent to many people at once rather than just one-on-one communication. One advantage of SMS is that it is a store-and-forward service, so the receiving device does not have to be on or active when the message is sent.  There are also no long distance charges for SMS, which are usually charged per text message or paid for through an unlimited subscription fee. Additionally, because SMS consists of written rather than verbal dialogue, it is a communication platform for the deaf and hearing-impaired.
Disadvantages of SMS include the fact it is only for sending text; pictures, video, sound, and other media are not supported. A similar technology, Multimedia Messaging Service (MMS), supports these larger data transfers. Some users also are charged per text message, which can become expensive if sending hundreds of texts each month. Because SMS operates on "best effort" delivery, text messages are not guaranteed to be sent and received. Reliability depends on carriers and their services. 
© Claire Durkin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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