The Short Message Service (SMS), often called text messaging, is a means of sending short messages to and from mobile phones. SMS was originally defined as part of the GSM series of standards in 1985^[1] as a means of sending messages of up to 160 characters, to and from GSM mobile handsets.^[2] Since then, support for the service has expanded to include alternative mobile standards such as ANSI CDMA networks and Digital AMPS, satellite and landline networks.^[3] Most SMS messages are mobile-to-mobile text messages, though the standard supports other types of broadcast messaging as well.

The term SMS is frequently used in a non-technical sense to refer to the text messages themselves, particularly in non-English-speaking European countries where the GSM system is well-established.

Overview

The service makes use of a Service Centre, which acts as a store and forward centre for short messages. The Service Centre communicates with the Public Land Mobile Network (PLMN) or PSTN via Interworking and Gateway MSCs.

Subscriber-originated messages are transported from a handset to a Service Centre, and may be destined for mobile users, subscribers on a fixed network, or Value-Added Service Providers (VASPs), also known as application-terminated. Subscriber-terminated messages are transported from the Service Centre to the destination handset, and may originate from mobile users, from fixed network subscribers, or from other sources such as VASPs.

The Short Message Service is supported on most digital mobile phones and some personal digital assistants with on-board wireless telecommunications.^[4] Text enabled fixed-line handsets are required to receive messages in text format. However, messages can be delivered to non-enabled phones using text-to-speech conversion.^[5]

History

The idea of adding text messaging to the services of mobile users was latent in many communities of mobile communication services at the beginning of the 1980s. Experts from several of those communities contributed in the discussions on which should be the GSM services. Most thought of SMS as providing a means to alert the individual mobile user, for example, of a deposited voice mail, whereas others had more sophisticated applications in their minds, such as telemetry. However, few believed that SMS would be used as a means for sending text messages from one mobile user to another.

As early as February 1985, after having already been discussed in GSM subgroup WP3, chaired by J. Audestad, SMS was considered in the main GSM group as a possible service for the new digital cellular system. In GSM document "Services and Facilities to be provided in the GSM System,"^[1] both mobile originated and mobile terminated short messages appear on the table of GSM teleservices.

The discussions on the GSM services were then concluded in the recommendation GSM 02.03 "TeleServices supported by a GSM PLMN".^[6] Here a rudimentary description of the three services was given:

1. Short message Mobile Terminated (SMS-MT)/ Point-to-Point: the ability of a network to transmit a Short Message to a mobile phone. The message can be sent by phone or by a software application.
2. Short message Mobile Originated (SMS-MO)/ Point-to-Point: the ability of a network to transmit a Short Message sent by a mobile phone. The message can be sent to a phone or to a software application.
3. Short message Cell Broadcast.

This was handed over to a new GSM body called IDEG (the Implementation of Data and Telematic Services Experts Group), which had its kickoff in May 1987 under the chairmanship of Friedhelm Hillebrand. The technical standard known today was largely created by IDEG (later WP4) as the two recommendations GSM 03.40 (the two point-to-point services merged together) and GSM 03.41 (cell broadcast).

The Mobile Application Part (MAP) of the SS7 protocol included support for the transport of Short Messages through the Core Network from its inception.^[7] MAP Phase 2 expanded support for SMS by introducing a separate operation code for Mobile Terminated Short Message transport.^[8] Since Phase 2, there have been no changes to the Short Message operation packages in MAP, although other operation packages have been enhanced to support CAMEL SMS control.

From 3GPP Releases 99 and 4 onwards, CAMEL Phase 3 introduced the ability for the Intelligent Network (IN) to control aspects of the Mobile Originated Short Message Service,^[9] while CAMEL Phase 4, as part of 3GPP Release 5 and onwards, provides the IN with the ability to control the Mobile Terminated service.^[10] CAMEL allows the gsmSCF to block the submission (MO) or delivery (MT) of Short Messages, route messages to destinations other than that specified by the user, and perform real-time billing for the use of the service. Prior to standardized CAMEL control of the Short Message Service, IN control relied on switch vendor specific extensions to the Intelligent Network Application Part (INAP) of SS7.

The first commercial SMS message was sent over the Vodafone GSM network in the United Kingdom on 3 December 1992, from Neil Papworth of Airwide Solutions^[11] (using a personal computer) to Richard Jarvis of Vodafone (using an Orbitel 901 handset). The text of the message was "Merry Christmas".^[12] The first SMS typed on a GSM phone is claimed to have been sent by Riku Pihkonen, an engineering student at Nokia, in 1993.^[13]

The first consecutive commercial deployments were by Acision with Telenor in Norway^[14] and BT Cellnet (now O2 UK) in 1993. The world's first web text messaging portal was created by e2sms in 1997.

Initial growth was slow, with customers in 1995 sending on average only 0.4 messages per GSM customer per month.^[15] One factor in the slow takeup of SMS was that operators were slow to set up charging systems, especially for prepaid subscribers, and eliminate billing fraud which was possible by changing SMSC settings on individual handsets to use the SMSCs of other operators.

Over time, this issue was eliminated by switch-billing instead of billing at the SMSC and by new features within SMSCs to allow blocking of foreign mobile users sending messages through it. By the end of 2000, the average number of messages per user reached 35,^[15] and by 2007 an average of 9 million texts were sent every hour during New Year's Day in the UK alone.^[16]

It is also alleged that the fact that roaming customers, in the early days, rarely received bills for their SMSs after holidays abroad had a boost on text messaging as an alternative to voice calls.

SMS was originally designed as part of GSM, but is now available on a wide range of networks, including 3G networks. However, not all text messaging systems use SMS, and some notable alternate implementations of the concept include J-Phone's SkyMail and NTT Docomo's Short Mail, both in Japan. E-mail messaging from phones, as popularized by NTT Docomo's i-mode and the RIM BlackBerry, also typically use standard mail protocols such as SMTP over TCP/IP.

SMS derives its benefit from two absolute advantages compared to any other form of communication. SMS is the fastest form of communication if measured by actual communication throughput including instances such as the counterpart not being able to take a call, being out of radio coverage, listening to voicemail, put on hold etc. SMS at its worst is a few seconds slower than a direct voice call or Blackberry wireless e-mail etc, but in the best case is faster by hours or even days than any other form of communication. SMS messages tend to be read within 30 minutes where an email message tends to be read in 48 hours.

Commercially SMS is a massive industry in 2006 worth over 80 billion dollars globally.^[17] SMS has an average global price of 11 cents and maintains a near 90% profit margin.

Technical details

GSM

Main article: Short message service technical realisation (GSM)

The Short Message Service - Point to Point (SMS-PP) is defined in GSM recommendation 03.40.^[2] GSM 03.41 defines the Short Message Service - Cell Broadcast (SMS-CB) which allows messages (advertising, public information, etc.) to be broadcast to all mobile users in a specified geographical area.^[18] Messages are sent to a Short Message Service Centre (SMSC) which provides a store-and-forward mechanism. It attempts to send messages to their recipients. If a recipient is not reachable, the SMSC queues the message for later retry. Some SMSCs also provide a "forward and forget" option where transmission is tried only once. Both Mobile Terminated (MT), for messages sent to a mobile handset, and Mobile Originating (MO), for those that are sent from the mobile handset, operations are supported. Message delivery is best effort, so there are no guarantees that a message will actually be delivered to its recipient and delay or complete loss of a message is not uncommon, particularly when sending between networks. Users may choose to request delivery reports, which can provide positive confirmation that the message has reached the intended recipient.

Transmission of short messages between the SMSC and the handset is done using the Mobile Application Part (MAP) of the SS7 protocol. Messages are sent with the MAP mo- and mt-ForwardSM operations, whose payload length is limited by the constraints of the signalling protocol to precisely 140 octets (140 octets = 140 * 8 bits = 1120 bits). Short messages can be encoded using a variety of alphabets: the default GSM 7-bit alphabet (shown below), the 8-bit data alphabet, and the 16-bit UCS2 alphabet.^[19] Depending on which alphabet the subscriber has configured in the handset, this leads to the maximum individual Short Message sizes of 160 7-bit characters, 140 8-bit characters, or 70 16-bit characters. Support of the GSM 7-bit alphabet is mandatory for GSM handsets and network elements,^[19] but characters in languages such as Arabic, Chinese, Korean, Japanese or Slavic languages (e.g. Russian) must be encoded using the 16-bit UCS2 character encoding (see Unicode). Routing data and other metadata is additional to the payload size.

GSM 03.38
	x0	x1	x2	x3	x4	x5	x6	x7	x8	x9	xA	xB	xC	xD	xE	xF
0x	@	£	$	¥	è	é	ù	ì	ò	Ç	LF	Ø	ø	CR	Å	å
1x	Δ	_	Φ	Γ	Λ	Ω	Π	Ψ	Σ	Θ	Ξ	ESC	Æ	æ	ß	É
2x	SP	!	"	#	¤	%	&	'	(	)	*	+	,	-	.	/
3x	0	1	2	3	4	5	6	7	8	9	:	;	<	=	>	?
4x	¡	A	B	C	D	E	F	G	H	I	J	K	L	M	N	O
5x	P	Q	R	S	T	U	V	W	X	Y	Z	Ä	Ö	Ñ	Ü	§
6x	¿	a	b	c	d	e	f	g	h	i	j	k	l	m	n	o !7x |p||q||r||s||t||u||v||w||x||y||z||ä||ö||ñ||ü||à

Larger content (known as "long SMS", "concatenated SMS", multipart or segmented SMS) can be sent using multiple messages, in which case each message will start with a user data header (UDH) containing segmentation information. Since UDH is inside the payload, the number of characters per segment is lower: 153 for 7-bit encoding, 134 for 8-bit encoding and 67 for 16-bit encoding. The receiving handset is then responsible for reassembling the message and presenting it to the user as one long message. While the standard theoretically permits up to 255 segments, 6 to 8 segment messages are the practical maximum, and long messages are often billed as equivalent to multiple SMS messages.

Short messages can also be used to send binary content such as ringtones or logos, as well as Over-the-air programming (OTA) or configuration data. Such uses are a vendor-specific extension of the GSM specification and there are multiple competing standards, although Nokia's Smart Messaging is by far the most common. An alternative way for sending such binary content is EMS messaging which is standardised and not dependent on vendors.

The SMS specification has defined a way for an external Terminal Equipment, such as a PC or Pocket PC, to control the SMS functions of a mobile phone. The connection between the Terminal Equipment and the mobile phone can be realized with a serial cable, a Bluetooth link, an infrared link, etc. The interface protocol is based on AT commands. Common AT commands include AT+CMGS (send message), AT+CMSS (send message from storage), AT+CMGL (list messages) and AT+CMGR (read message).

Today, SMS is also used for machine to machine communication. For instance, there is an LED display machine controlled by SMS, and some vehicle tracking companies use SMS for their data transport or telemetry needs. SMS usage for these purposes are slowly being superseded by GPRS services due to their lower overall costs.

Non-GSM

Premium-rated short messages

Short messages may be used to provide premium rate services to subscribers of a telephone network.

Mobile terminated short messages can be used to deliver digital content such as news alerts, financial information, logos and ring tones. The VASP providing the content submits the message to the mobile operator's SMSC(s) using a TCP/IP protocol such as the Short message peer-to-peer protocol (SMPP) or the External Machine Interface (EMI). The SMSC delivers the text using the normal Mobile Terminated delivery procedure. The subscribers are charged extra for receiving this premium content, and the amount is typically divided between the mobile network operator and the value added service provider (VASP) either through revenue share or a fixed transport fee.

Mobile originated short messages may also be used in a premium-rated manner for services such as televoting. In this case, the VASP providing the service obtains a Short Code from the telephone network operator, and subscribers send texts to that number. The payouts to the carriers vary by carrier and the percentages paid are greatest on the lowest priced premium SMS services. Most information providers should expect to pay about 45% of the cost of the premium SMS up front to the carrier. The submission of the text to the SMSC is identical to a standard MO Short Message submission, but once the text is at the SMSC, the Service Centre identifies the Short Code as a premium service. The SC will then direct the content of the text message to the VASP, typically using an IP protocol such as SMPP or EMI. Subscribers are charged a premium for the sending of such messages, with the revenue typically shared between the network operator and the VASP. Limitations of short codes include the limitation to national borders (short codes have to be activated in each country where the campaign takes place), as well as being expensive to sign up together with mobile operators.

An alternative to inbound SMS is based on Long Numbers (international number format, e.g. +44 7624 805000), which can be used in place of short codes for SMS reception in several applications, such as TV voting, product promotions and campaigns. Long Numbers are internationally available, as well as enabling businesses to have their own number, rather than short codes which are usually shared across a lot of brands. Additionally, Long Numbers are non-premium inbound numbers.

Quality in SMS

The University of Duisburg-Essen, in partnership with mobile messaging provider Tyntec, have developed the study QoS Monitoring for Professional Short-Message-Services in Mobile Networks for SMS messaging to enable the detailed monitoring of SMS transmissions to ensure a greater degree of reliability and a higher average speed of delivery. The new parameters can be used by mobile network operators, third party SMS gateways and mobile network infrastructure software vendors to monitor the transmission of SMS messages and to detect network transmission problems quickly and accurately.

Vulnerabilities

In October 2005, researchers from Pennsylvania State University published an analysis of vulnerabilities in SMS-capable cellular networks.

See also

Details

• Short Message Service Centre (SMSC)
• SMS gateways (sending texts to or from devices other than phones)
• Reverse SMS billing

Related protocols

• 3rd Generation Partnership Project (3GPP)
• Enhanced Messaging Service (EMS)
• Multimedia Messaging Service (MMS) (a newer standard)
• Short message peer-to-peer protocol (SMPP)
• External Machine Interface (EMI), an extension to the Universal Computer Protocol (UCP)
• WAP Push

Related technology

• BlackBerry
• Instant messaging
• Mobile dating