Short Message Service: What, How and Where?
by Puneet Gupta
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.
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
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.
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
SMS can be used to notify users that they have
received new voice-mail or fax messages.
It provides an alternative to alphanumeric paging
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).
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
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.
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!
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
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.
Initiatives and future
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
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.
In EMS there are 10 different predefined sounds including low and high
chimes and chords, Ding, TaDa, Claps, Drum and Notify.
EMS standards are a part of 3rd Generation Partnership Project (3GPP) technical
specification: 3G TS 23.040, "Technical realization of the Short Message
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
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!
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.
||Simple Mail Transport
||Mobile station application
||General Packet Radio
||Time division multiple
||Code division multiple
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