SMS history: how to send text by voice channel

SMS is so common that many people do not think about how messaging works in voice-oriented networks and do not remember why we are limited by such a short message length. Let’s remember where the short messages came from and what they are.

Development of SMS system as a service

This year, “short messages” will turn 25 years old. It is believed that the first SMS message was sent to the UK in 1992, and the commercial SMS service appeared in Finland in 1993.

It should be noted that until the emergence of the GSM standard, SMS did not exist, but to the current moment, i.e. After almost 25 years, the service has gained immense popularity, despite the fact that operators charge fees for outgoing and sometimes incoming messages.

Of course, the path to popularity for this service was pretty thorny. For the first time, the idea of a service that transmits short messages between telephones appeared in 1984. The German engineer Friedhelm Hillebrand, who in 1985 headed the non-voice GSM committee, already showed that 160 characters in the Latin alphabet (which corresponds to 140 bytes) is enough to transmit most household messages. At the heart of his statement were not very scientific experiments (the hand-sampling of casual household questions and answers, and also the observation that postcards and analyzed telex messages for business users almost always contain less than 150 characters), but they determined the nature of the service for scores years.

Is 160 characters sufficient? Of course, they do not fit the “Tolstoy” proposals, but before the on-screen keyboards (on devices with touch screens) or at least T9 to send a longer text did not seek and the users themselves. Moreover, at the launch, although the SMS was laid down in the GSM standard, it was not uncommon to lack the support for dialing and sending short messages in some devices.

From a technical point of view, SMS has not undergone almost any changes (with the transition to GSM 03.40- https://en.wikipedia.org/wiki/GSM_03.40 – the set of supported symbols in the message body has seriously changed). The commercial service has evolved in the networks of different operators approximately the same as the amendment for launch time, marketing and the approach to monetization.

Today, in addition to the user’s network, a huge number of service messages from operators (for example, delivery notifications) are transmitted, as well as service SMS from third-party service providers: banks, providers, Internet services. Moreover, they can be not only informative in nature (inform about the balance on the account, warn about the next payments under the contract, notify about the delivery of parcels), but also serve as one of the tools in two-factor authentication. In addition, short messages can be used to manage a number of services, for example, to send entries to Twitter.

Message format

The SMS message consists of text up to 140 bytes in length and a service information block:

Type of message – 2 bits (SMS-DELIVER – message delivery to mobile phone, SMS-DELIVER-REPORT – delivery confirmation, SMS-SUBMIT – sending message from mobile device, SMS-SUBMIT-REPORT – sending confirmation, SMS-COMMAND – changing parameters Or deleting a message stored in the SMS center, SMS-STATUS-REPORT – notification of the sender about the fate of his message).

Depending on the message type, subsequent fields may or may not be present, or may be interchanged. A detailed description of all the fields can be found in the GSM 03.40 specification, here we give only the main interesting from the point of view of parsing the process of sending messages:

  • The destination address is 2-12 bytes;
  • Date and time of sending (SMS center) – 7 bytes;
  • Protocol identifier – 1 byte;

Coding scheme – 1 byte: encoding, message class (affects the way the message is processed by the machine: displayed on the screen without saving, stored in the SIM or memory of the device), the request for automatic deletion after reading, the status of the flags corresponding to the unread fax or voice message, information About compression of message content, language (for broadcast SMS);

The length of the user data area is 1 byte;

The text of the message is up to 140 bytes. The length of the message “in letters” varies, depending on the encoding used. This is 160 characters for 7-bit encoding (Latin + basic characters – according to GSM 03.38 specification), 140 characters for 8-bit (latin + diacritics) and 70 characters for UCS-2 (Russian operators work with this encoding, Poet for Cyrillic characters, the length of the message – no more than 70 characters).

Initially (in the GSM phase 2 document), each SMS message was self-sufficient, but according to subsequent editions (GSM phase 2+), a message transmitted in this way can include several messages – in this case, each of them with Contains a note about the ordinal number (which reduces the length of the “body” of each message).

Send SMS

Between individual users of the mobile network, the message is sent via the SMS-center (SMSC or SMS-SC – Short Message Service – Service Center).

 

Along with the text of the message from the mobile phone, additional information is transmitted to the base station, including the address of the message center (in the early 2000s, this number had to be driven in by itself at the initial setting of the purchased device). If the message travels between networks, its delivery is primarily the SMS center of the sender’s network.

SMSC not only delivers and redirects messages, if necessary but also stores them. If the recipient’s phone is turned off when the message is sent, it remains in the SMSC before the device appears on the network. Typically, operators specify the maximum retention period for such messages (technically, this setting may be available to users of the mobile network). Also, SMSC sends reports on the delivery of messages if it is registered in the user settings.

In the process of sending messages from one user to another, not only SMSC is involved. In short, the process is as follows:

The request to send the message goes to the MSC (mobile switching center),

The MSC checks whether the sender’s work with messages on the GSM subscriber database (HLR / VLR – Home / Visitors Location Register) is possible and forwards the message along with the addresses of the sender and the recipient to the SMSC;

SMSC sends a receipt acknowledgment, assigns a temporary status “not delivered” to the message,

And he makes an attempt to deliver the text to the addressee.

Before delivery, the SMSC of the sender requests from the HLR a unique International Mobile Subscriber Identity (IMSI), which is necessary for carrying out any operations with the subscriber within the network, and also checks the absence of prohibitions on working with SMS. At the same time, the MSC address of the subscriber is requested;

Then, the location identification (LAI) is requested from the MSC and the subscriber’s temporary mobile subscriber identity (TMSI) and the connection to the device is initiated to send the message directly;

Upon completion of the transfer, the mobile phone sends a delivery confirmation (accordingly, the status of the SMSC message is changed to “delivered”). And in case of unsuccessful delivery, the status “not delivered” is retained, while the reason for HLR is the absence of memory or subscriber in the network. Re-delivery is initiated by the actions of the recipient himself (appearance on the network or clearing the device’s memory).

When you get to the recipient’s phone, the messages are displayed on the screen or saved on the SIM card / in the device’s memory, depending on the user settings and the properties of the message itself.

Gateways, floodgates …

For historical reasons, telephony, including GSM, operates on the basis of the protocols OKS-7 (Common Channel of Signaling – 7). In the terminology of this standard, SMS is transmitted via the signaling channel, and not via the traffic channel with voice data. It was designed to ensure that SMS can go smoothly in parallel with the transmission of voice traffic (during a call). In addition, the signaling channel is “idle” most of the time, so sending short messages over it is a very economical solution.

However, SMSC does not support ACS-7, so additional gateways must be used to communicate with other operating systems (HLR / VLR and MSC). To deliver messages to other operators, special gateways are also used. The first gateway appeared in the world only in 1999, we did not have a full-fledged message transfer between the operators and in the early 2000s (the gateways appeared a little later).

With the advent of the need to connect a communication network (fixed or mobile) to

IP-networks – at the turn of the new century – the SIGTRAN specification was created, which allows the transmission of OCS-7 streams over IP-networks. So appeared, including SMS-gateways, working on the Internet. These gateways can not only “start” content from the Internet in SMS, but also allow backup data transmission, for example, sending SMS messages from the phone to an e-mail address (true, the peak of development of similar services has already passed – with the advent of Internet access on phones, Built-in email clients, and applications under the smartphone OS, it’s much easier to send e-mail directly).

SMS-gateways can belong to the operator or exist independently, including as an addition to commercial corporate software for SMS-mailings. Poor security of such locks, as well as a very real opportunity to build a fraudulent gateway and lead to the spread of spam, hacking and other troubles associated with SMS. However, as far as their own strengths are concerned and, what’s more, under the pressure of the regulator, mobile operators are trying to deal with these problems.

Interesting facts about SMS

  • Like any popular technology, during its existence SMS have grown into a whole heap of legends and myths. Here are some interesting stories that managed to find confirmation:
  • In 2003, Malaysia recognized as legitimate the SMS-divorce between spouses;
  • In 2007, the number of SMS messages sent by the average American exceeded the number of calls made by him;
  • In 2008, SMS helped to conduct a complex surgical operation for a patient from the Congo: a volunteer surgeon received instructions from colleagues through text messages;
  • Abbreviations of the text within the framework of SMS communication become a new direction of language development: in 2011, the popular LOL and OMG in English were added to the Oxford Dictionary;
  • German engineer F. Hillebrand, who proposed the idea of SMS, eventually got only world renown – no licensing fees or patenting services developers did not provide.

Instead of the conclusion, I would like to note that although SMS has not technologically developed for a long time, the peak of popularity in the world has been a service recently – in 2012-2014. Considering that the large-scale take-off of technology occurred in the early 2000s, she stayed on the pedestal surprisingly long (from the point of view of the world, so avid for novelties). What is waiting for an SMS message tomorrow – a drop in popularity or a stable future? Time will show.