From: Frederic Dumas Date: Sat, 29 Jan 2000 18:38:01 +0100 A propos des SMS : http://ccnga.uwaterloo.ca/~jscouria/GSM/gsmreport.html (présentation générale pour débutants, mais style "universitaire") http://www.tcm.hut.fi/Studies/Tik-110.300/1998/Newtech/fast_gsm_1.html (permet de comprendre la différence entre SMS de 140 et 160 caractères) http://www.tcm.hut.fi/Studies/Tik-110.300/1998/Newtech/fast_gsm_2.html (protocoles MNP et RLP) Le premier possède un hyperlien qui pointe vers l'url d'origine du papier que je t'indiquais à une url italienne. Hélas: - l'url d'origine n'existe plus sur le serveur - le webmestre italien me répond par mail avoir remplacé le papier en anglais par sa traduction italienne. Bref, je copie à la suite le papier; il mérite d'être hébergé quelque part. -------------------------------------------------------------------- Short Message Service overview SHORT MESSAGE SERVICE OF GSM NETWORKS July -96 -------------------------------------------------------------------- Table of contents: 1. General 2. Short Message Service 3. Connections to SMS 4. Timings 5. New Features 6. Short Message Service Cell Broadcast 7. Detailed Information -------------------------------------------------------------------- 1. General This paper is a short introduction to Short Message Services (SMS) of GSM networks. The nature of this paper is informative, not to any extent complete. For the detailed information of the facilities of a certain GSM network, the reader is encouraged to negotiate with the operator of the network. The information presented here applies also to the GSM related networks (DCS 1800 and PCS 1900), which are basically GSM networks operating on a different radio frequency. So all these networks are here after referred with the acronym GSM. 2. Short Message Service There are two different kinds of SMS specified in GSM standards - Point to Point (SMS/PP) and Cell Broadcast (SMSCB). The SMS/PP will be presented in this section and the SMSCB has a section dedicated to itself later in this paper. From the normal GSM phone user's point of view, the SMS is delivering short text messages from a GSM phone to another. This is the SMS Point to Point. The text can be up to 160 characters long. The network delivers the message from the sender to the phone, the telephone number of which the sender sets when issuing the message. The sender does not know, if or when the receiver receives the message. This is analogue to sending mail: after one has left the envelope to the postal office, he expects it to be delivered to the receiver. In most cases this actually happens! Going into details reveals also more possibilities the SMS/PP offers, but first let us introduce ourselves some technical basics of the SMS/PP. The Short Message Service is provided by the subscriber's (user's) network operator. Without this subscription one cannot send or receive SMS messages. All the facilities provided by the operator are associated with the Subscriber Identity Module (SIM card) not with any phone. Of course the GSM phone itself must also have the SMS functionality. If any one of the aforementioned parts (subscriber's network operator, the SIM card and the GSM phone) do not support SMS, then the facility is not available for the user. The SMS is so called connectionless protocol, that is there is no connection being established between the sender and the actual receiver parts, which is done for voice and data calls. This declines the time needed for sending messages. Due to the same reason the sender cannot know when the message is received, nor is even the reception order of different messages guaranteed. Delivering a PP SMS message from a mobile phone to another, has to be considered as a concatenation of two separate tasks: sending the message from a mobile phone to a special entity in the network called Short Message Service - Service Centre (SMS-SC) and then from the SMS-SC to the receiving mobile phone. The first case is called Mobile Originating (MO) SMS and the latter Mobile Terminating (MT) SMS. In fact, this can be utilised simply by sending SMS messages between one (or multiple) mobile phone and a SMS-SC, this is discussed in more detail in chapter 'Connections to SMS'. Another useful outcome is that once the network has accepted the message, it can store the message until it can be delivered to the actual receiver. The maximum storage time is once again network operator dependent, but can be also affected with a special parameter of a Short Message. The SMS utilises six different Protocol Data Unit (PDU) types: SMS-DELIVER, SMS-DELIVER-REPORT, SMS-SUBMIT, SMS-SUBMIT-REPORT, SMS-STATUS-REPORT and SMS-COMMAND. The main functionality and direction of these different PDU types are shortly described in table 1. PDU type Direction Function SMS-DELIVER SM-SC => Mobile Delivers a short message. phone SMS-DELIVER-REPORT Mobile => SM-SC Delivers a failure cause phone (if necessary). SMS-SUBMIT Mobile => SM-SC Delivers a short message. phone SMS-SUBMIT-REPORT SM-SC => Mobile Delivers a failure cause phone (if necessary). SMS-STATUS-REPORT SM-SC => Mobile Delivers a status report. phone SMS-COMMAND Mobile => SM-SC Delivers a command. phone Table 1 The main task of SMS-DELIVER and SMS-SUBMIT is simply to deliver the actual message data and associated information between the SMS entities, phone and SM-SC. SMS-STATUS-REPORT carries information of whether or not the message was delivered to the actual receiver and when this happened. SMS-COMMAND contains a command to be executed to an earlier issued SMS-SUBMIT. The PDUs are sent via the control channels. During a call SMS PDUs are sent through Slow Associated Control Channel (SACCH), and otherwise through Stand alone Dedicated Control Channel (SDCCH). SMS is only one of the transferring tasks of these control channels, they are used also for transmission of such important information as network quality, location update and call establishment. 3. Connections to SMS There are multiple ways to send and receive Point to Point Short Messages. Sending messages from a mobile phone to another is only the most basic one. A very effective way of utilising the SMS is sending messages between a phone and a SMS-SC and visa versa. The possibilities depend on the network operator, what kind of services it can provide with its SMS-SC. Some possibilities are sending Short messages as e-mail, fax, voice or paging messages. Many operators can also provide a leased line connection to their SMS-SC, in case of which the connected equipment can easily handle multiple simultaneous and successive messages. One protocol allowing this kind of connection is Computer Interface to Message Distribution (CIMD) of Nokia Telecommunications. The messages do not need to be text information only, they can be sent also as eight bit binary data. By doing so the phone is prevented to show the messages on its display and some kind of mapping or compression can be easily introduced to increase the information capacity of Short Messages. With eight bit data the maximum message length is 140 bytes. The maximum message length of 160 characters with normal text messages (seven bit data) is achieved with a SMS specific packing. 4. Timings As the PP SMS is a connectionless protocol, the timings are not so exact and the delays are typically longer than in a connection oriented protocol. This does not necessary mean that delivering information as SMS messages would be slower than delivering the information using data calls, it is simply a case sensitive matter. Sending a SMS message one way, that is between a phone and SMS-SC no matter which direction, the delay caused by the network is from 3 to 5 seconds. As sending from a phone to another is actually concatenation of two deliveries the delay is cumulative and thus from 6 to 10 seconds. Whether or not the phone is capable of issuing the message to the network, will be resolved from 3 to 40 seconds. As the SMS messages are sent via the controlling channels the through put rate of the messages is heavily affected by the other signals employing the same channels. This also has the effect that the highest SMS message issue rate of one phone is one every five second. The SMS-SC can, of course, send multiple simultaneous messages. As a comparison, the creation of a modem like data connection will take approximately 40 seconds before any data can be delivered. For example, if there are multiple messages to be sent to various destinations, it can be easily calculated which way is more effective. 5. New Features The evolution of GSM is divided into phases. The phases describe among other things what kind of services the networks and the phones have to offer to the users. There are many amendments done to the SMS specifications in the new phases, phases 2 and 2+. Three new SMS/PP specific features will be described here, they are `more messages to send', 'alphabet extensions' and 'concatenated Short Messages'. 'More messages to send' is a phase 2 feature, with which the delivery time of multiple Short Messages in MT direction can be reduced. When there are multiple messages or status reports for one phone, the connection to the phone will be kept until all the messages and status reports have been transferred. 'Alphabet extensions' is a phase 2+ feature increasing the number of characters supported by SMS. In the earlier phases only one character set was specified that is the seven bit SMS default alphabet. The new alphabet is unicode (16-bit ISO/IEC 10646). Also a compressed from of the unicode will be supported. The compression is likely to be done using Huffman coding. 'Concatenated Short Messages' is a phase 2+ facility that allows concatenation of various Short Messages to form a longer message. There is information included so that the receiver can re-assemble the Short Messages in the correct order. It is specified that up to 255 messages could be concatenated, this yields the maximum length of a concatenated message to be 38760 characters (seven bit data) or 34170 bytes (eight bit data). 6. Short Message Service Cell Broadcast Short Message Service Cell Broadcast (SMSCB) functionality permits a number of unacknowledged general messages to be broadcast to all receivers within a particular region. Cell Broadcast (CB) messages are broadcast to defined geographical areas, one or more cells. CB service can be used for delivering weather, traffic, advertising or other local information. The CB message comprises of 82 octets (93 characters). Up to 15 of these messages can be concatenated to form a macromessage. To permit phones to selectively display or discard CB messages, the CB messages are assigned a message class that categorises the type of information they contain and the language in which the message has been compiled. 7. Detailed Information Detailed information can be received from the GSM standards of European Telecommunications Standards Institute (ETSI). The most essential standards describing SMS/PP and SMSCB are listed below. European digital cellular telecommunications system; Alphabets and language-specific information, (GSM 03.38) European digital cellular telecommunications system; Technical realization of the Short Message Service (SMS) Point to Point (PP), (GSM 03.40) European digital cellular telecommunications system; Technical realization of Short Message Service Cell Broadcast (SMSCB), (GSM 03.41) GSM specific standards are reachable e.g. via the ETSI World Wide Web home page: http://www.etsi.org