Short Message Service: What, How and Where?

by Puneet Gupta

Introduction

Short message service is a mechanism of delivery of short messages over the mobile networks. It is a store and forward way of transmitting messages to and from mobiles. The message (text only) from the sending mobile is stored in a central short message center (SMS) which then forwards it to the destination mobile. This means that in the case that the recipient is not available, the short message is stored and can be sent later. Each short message can be no longer than 160 characters. These characters can be text (alphanumeric) or binary Non-Text Short messages. An interesting feature of SMS is return receipts. This means that the sender, if wishes, can get a small message notifying if the short message was delivered to the intended recipient. Since SMS used signaling channel as opposed to dedicated channels, these messages can be sent/received simultaneously with the voice/data/fax service over a GSM network. SMS supports national and international roaming. This means that you can send short messages to any other GSM mobile user around the world. With the PCS networks based on all the three technologies, GSM, CDMA and TDMA supporting SMS, SMS is more or less a universal mobile data service.

à Note: The actual limit of size of SMS is 160 characters if Latin alphabets are used. If non-Latin alphabets like Chinese or Arabic are used, the limit is 70 characters.

How does SMS work

The figure below shows a typical organization of network elements in a GSM network supporting SMS.

The SMC (Short Message Center) is the entity which does the job of store and forward of messages to and from the mobile station. The SME (Short Message Entity) which can be located in the fixed network or a mobile station, receives and sends short messages.

The SMS GWMS (SMS gateway MSC) is a gateway MSC that can also receive short messages. The gateway MSC is a mobile network’s point of contact with other networks. On receiving the short message from the short message center, GMSC uses the SS7 network to interrogate the current position of the mobile station form the HLR, the home location register.

HLR is the main database in a mobile network. It holds information of the subscription profile of the mobile and also about the routing information for the subscriber, i.e. the area (covered by a MSC) where the mobile is currently situated. The GMSC is thus able to pass on the message to the correct MSC.

MSC (Mobile Switching Center) is the entity in a GSM network which does the job of switching connections between mobile stations or between mobile stations and the fixed network.

A VLR (Visitor Location Register) corresponds to each MSC and contains temporary information about the mobile, information like mobile identification and the cell (or a group of cells) where the mobile is currently situated. Using information form the VLR the MSC is able to switch the information (short message) to the corresponding BSS (Base Station System, BSC + BTSs), which transmits the short message to the mobile. The BSS consists of transceivers, which send and receive information over the air interface, to and from the mobile station. This information is passed over the signaling channels so the mobile can receive messages even if a voice or data call is going on.

Applications

Some of the common applications of SMS are:

• Exchanging small messages like "See you at 8.30 tonight at xyz". SMS is particularly suited for these kinds of short messages because SMS is much cheaper than calling some one and giving the same message. Calling some one to give the same message would invariably take more time and hence more cost.
• Many operators offer e-mail service over SMS. Every user is assigned an e-mail address at signup and any message delivered to that email is converted to short messages and delivered to the mobile.
• It is possible to send e-mail messages (less than 160 characters) from a mobile phone to any e-mail address via SMS.
• Information services like news, weather, entertainment and stock prices etc. can be availed just by sending a keyword like NEWS, WEATH etc to the short message center number.
• SMS can be used by the network operators to provide services like balance enquiry in case of prepaid cards using SMS.
• Mobile chatting is one more hot application of SMS
• SMS can be used to notify users that they have received new voice-mail or fax messages.
• It provides an alternative to alphanumeric paging services
• Using SIM-Toolkit, now a part of GSM specifications, SMS can be used to have on the air activation of features. By sending codes embedded in short messages from the server network operators can remotely provision the user's wireless terminal
• Internet e-mail alerts.
• Downloading new ring tones.

The present state of SMS

SMS has seen unprecedented growth in the last few years. In Europe SMS has already crossed three billion messages per month mark. The current figures and future projections in the European market below, clearly demonstrate the popularity of SMS in Europe. The international figures are not much different. (Data source http://www.gsmworld.com/gsmdata).

à The first short message was sent in December 1992 from a Personal Computer (PC) to a mobile phone on the Vodafone GSM network in the UK

In many parts of Europe people are spending more on SMS as compared to voice services. In Germany, UK, and Italy, SMS is very popular and the popularity especially among young people continues to rise.

Developments like WAP and SIM toolkit are greatly helping to increase the message usage. SMS can be used as a bearer service for WAP and until 2.5G services like GPRS become common, SMS would be increasingly used as a bearer for WAP. These protocols are also making it much easier for the users to access messaging services.

Limitations of SMS

There is no doubt that SMS has been very popular. The figures in the section above support this. What is more interesting to observe is that this popularity has been inspite of many limitations of SMS. Many of these limitations are the driving force behind the developments and initiatives being taken in the field of short messaging. Some of the limitations of SMS are:

• Messages are plain vanilla in nature. You can only send simple text messages. There is no scope for any graphics or audio.

However
As mentioned in the next section EMS would help fill this gap.
 
• The messages are limited by size. An SMS message can’t exceed 160 characters. (BTW this limitation is due to the limitation in the MAP protocol in GSM) In case of longer e-mails or information service messages like news, the messages need to broken down into more than one message. The need to break the messages into several smaller segments could make SMS comparatively costlier in comparison to GPRS (for the same kind of service). Also, This doesn’t look very appealing on a mobile device!

However
MMS (talked about later) would remove the limitation of small messages
 
• The limitation of easy input mechanisms in mobile devices makes it very uncomfortable sending messages larger than even 5-6 words.

However
Predictive text input algorithms implemented in a mobile phone can greatly help. Voice recognition systems can further help ease the situation
 
• Many proprietary protocols are used by SMS operators and application developers need to implement different interfaces for making their applications work with different SMS centers. X.25 is used as a popular protocol for connecting with SMS centers.

• SMS protocol data units as defined in GSM 03.40 are also not very efficient. The various header fields in the PDU are fixed which puts a constraint on the scenarios that can be indicated. 3G specifications are being looked up to look and address these constraints.

• Data rate and latency. GPRS and USSD provide better data rates and lower latency compared to SMS. This is because SMS uses the slow signaling channel, which is used for many other things also in GSM.

However
MMS will use data channels and hence higher rates and lower latency.
 
• The store and forward nature of SMS, though useful in many applications makes SMS not very suitable for WAP

SMS is a first generation GSM service. As described in the previous section, SMS has some inherent limitations. The majority of these are to do with the fact that SMS is plain vanilla in nature. Enhanced Messaging Service (EMS) is a mechanism by which you can send a comparatively richer message that are combination of text, simple melodies, pictures (simple, black and white) and animations to an EMS compliant handset. The service is very much based on the existing SMS. It basically extends the User Data Header (UDH) in SMS (UDH makes it possible to include binary information in the message header) and infact needs no upgrade to the network infrastructure. The handsets however need to be EMS compliant. The first EMS handsets should be available by mid of 2001.

à In EMS there are 10 different predefined sounds including low and high chimes and chords, Ding, TaDa, Claps, Drum and Notify.

à EMS standards are a part of 3rd Generation Partnership Project (3GPP) technical specification: 3G TS 23.040, "Technical realization of the Short Message Service (SMS)"

The next step in the evolution of SMS, which requires substantial changes in the network infrastructure, is the Multimedia Messaging Service (MMS) that allows a combination of text, sounds, images and video. MMS will support pictures and interactive video. It will be possible, for example, to send mobile greeting cards and visiting cards using MMS. MMS uses standardized protocols like WAP, MExE and SMTP. While EMS requires content reformatting for mobile devices, there is no such requirement for MMS. Theoretically MMS may run on any bearer service, but practically it would require GPRS or 3G bearer services before it starts appearing. AS opposed to SMS, MMS needs a dedicated channel. It would also require new network elements like Multimedia Messaging Relays, MMS Server and MMS User Databases. While MMS trials have been planned in mid 2001, the first MMS compliant handsets are expected to hit the market only in end 2002. MMS is being standardized by the 3GPP

à According to 3GPP standards MMS is a new service, which has no direct equivalent in the previous ETSI/GSM world or in the fixed network world.

SMS, WAP, and GPRS?

Where does SMS go with services like WAP and GPRS? Well, The first thing to understand is that SMS is a bearer service. It is a mechanism of sending short messages. WAP provides the user with services and protocols that can be used on top of SMS. With the increased use of WAP, the SMS traffic in networks should see considerable increase.

GPRS, on the other hand, is a packet based data service that provides much higher throughput. Unlike SMS, it provides a real time data bearer. The users always stay connected to the network. But GPRS and SMS don’t really compete with each other in the real sense. The costs involved in sending small messages for an end user in case of SMS are expected to be lower than that in GPRS (packet data) service because the responsibility for sending the message to the recipient in case of SMS lies entirely on the short message center as opposed to the user in GPRS. In GPRS there is no concept of storage. Confirmation of delivery is a unique feature of SMS because of the very nature of short message service. Simultaneous transmission with GSM voice, data, and fax services is another distinguishing characteristic of SMS.

True, GPRS will be a much better option to use for services like WAP, but the availability of GPRS and GPRS-compliant handsets will take some time to pickup. Also, SMS needs no special network elements and handsets. It is something that almost every mobile user has and can use to send messages to any other mobile user without worrying about the capabilities of that mobile and its network!

Conclusions

SMS, because of its very nature has unique advantages that other non voice services do not have. It provides a very convenient method of exchanging small bits of information between mobile users. The reasons for the enormous popularity of SMS have been the fact that this mechanism of sending and receiving messages not only saves time but costs less as well. In many situations one is relatively much more comfortable sending a message via SMS than talking over phone. With new information services and unique value added services being used by the operators the popularity of SMS is increasing further. SMS is also uniquely positioned as a very attractive advertisement medium. SMS should no longer be treated as a value added service in mobile networks. SMS is not only providing a useful mechanism for a host of innovative services over mobile networks but it acting as a point of entry for new data services like WAP in mobile networks.

Abbreviations

Reference:

• Mobile Station Application Execution Environment, www.mobilemexe.com
• For technical realization of SMS in GSM networks refer to SMS specifications at www.etsi.org
• www.gsmworld.com

About The Author: Puneet Gupta works for Bell Labs, Lucent Technologies in GSM/GPRS development. He has also been doing freelance technology writing for many magazines and newspapers. Puneet is currently focusing on areas related to mobile wireless data and associated technologies. He can be reached at puneetg@india.com.