on the move
A. K. Vanwasi
is fast gaining center stage as the preferred medium
for mobile messaging over GSM networks. Here's an
insight into SMS, how it works and the benefit it
is a paradigm shift happening in the world of telecommunication.
Fixed point-to-point telephony is giving way to global
cellular mobile communication system. Such a system
provides any-time, any-where connectivity.
Mobile communication provides both speech and text
communication. Speech communication provides real-time
interactive communication using scare frequency spectrum
inefficiently. Mobile data communication offers many
benefits to users. Information can be transported
quickly. This reduces the cost of air time usage.
Further, a written text message offers reliable and
unambiguous communication. Now, text messaging is
highly popular. Millions of users are sending billions
of messages across the globe.
'Global System for Mobile Communication' (GSM) is
a widely used mobile communication system. Today,
75 percent of all 400 million mobile communication
users use GSM system. GSM standard is optimized for
short message services, group 3 and group 4 fax, synchronous
and asynchronous communication system.
SMS is a technology that allows GSM mobile users to
send and receive short, concise and often abbreviated
text messages to other users. In SMS, length of a
single message is restricted
to 140 octets.
SMS messages can be sent and received simultaneously
with voice, data and fax calls. This is feasible due
to the fact that SMS uses low-bit rate signaling channel
transmission. This results in low cost, always-on
communication capability for short messages. SMS also
allows you to deliver messages both in real as well
as non-real time.
Here we try to understand how radio paging and SMS
works and finally
discuss the various issues pertaining
to these technologies.
Radio paging is the forerunner in the mobile data
services space. In the non-GSM world MOBITEX and CDPD
networks provide mobile data services
'Paging service network' provides for one-way (receive
only) messaging service to mobile users. One-way messaging
implies that a mobile user cannot
send a message. Such a service is also known as mobile
Paging enables transmission of numeric, alphanumeric,
tone and voice messaging from a paging
service center to a mobile user, in a given area.
Any one wishing to page a message conveys the message
by telephone/fax to the paging service operator. The
operator formats the message in POCSAG format and
sends in for broadcasting.
A message length is minimum 20 digits for numeric
and 32 digits/characters
for alphanumeric. A pager has capability to receive
20 digits and display at least 10 digits at a time.
Similarly, in alphanumeric mode, a pager can receive
32 digits and display
For incoming messages, there is provision of alert
tone/vibrator to alert the user.
A paging system has provision of group call facility.
Thus, the whole group will receive the same message
simultaneously. It is also possible that a subscriber
can be a member of more than one group.
A pager has provision of inbuilt memory to store the
message. The user can retrieve and display the message
as and when required. Further, it is possible to delete
a duplicate message.
A pager can receive data at 512/1200 bit/sec.
Limitations of paging
services have few limitations. Firstly, paging service
is an unconfirmed service. This means that no acknowledgement
is sent back to the message sender (this problem is
addresses by new two-way pagers, but the service provider
in your locality should provide this service). It
is one-way, mobile terminated messaging service. Also,
paging is an operator assisted service, this limits
its functionality. Then paging is restricted to a
particular city only, and does not offer the global
features provided by SMS.
MOBITEX is a popular and widely deployed cellular
wireless data network
in US. MOBITEX system provides
mobile data service to 7700 cities and towns. It covers
percent of America's urban business
MOBITEX specification defines both packet and circuit-switched
transmission facilities. The circuit-switched facilities
are meant for voice transmission but most operators
have restricted themselves to data transmission.
MOBITEX services are available in 450 MHz and 900
MHz band. 900 MHz band is mainly used in US and Canada.
450 MHz services are available in Europe and other
MOBITEX network deploys a hierarchical structure that
may contain up to six levels of network nodes. The
communication channels have 12.5 KHz bandwidth and
support a data rate of 8 K bit/sec.
The MOBITEX system offers
store-and-forward capabilities. Each registered user
has a mailbox in MOBITEX system. If a packet is undelivered,
then at the sender's discretion, it can be stored
in recipient's mailbox.
Cellular Digital Packet Data Network (CDPD)
CDPD enables an AMP/ D-AMP
system to offer packet transmission service also.
CDPD uses the existing 30 KHz channel for offering
subordinate packet data service. CDPD is a 2 G digital
packet transmission service. It is popular in the
CDPD is based upon an extension of the existing IP
technology. CDPD is designed to send and receive mobile
CDPD acts as an overlay that uses the dwell-time between
calls for packet transmission. The dwell-time can
be as high as 30 percent. During this time, small
packets are placed on the channel and routed in the
The CDPD packet size is restricted to 2048 bytes of
data. Longer size packets are fragmented into smaller
Short Message Service (SMS)
GSM is the most popular mobile cellular network. GSM
provides both interactive communication and short
messaging service to a mobile user simultaneously.
Thus, users can receiver or send short messages while
The short message service deploys low-bit rate signaling
channel for duplex mobile data transmission. It is
real-time communication. Thus, contrary to unacknowledged
paging service, the sender gets confirmation or failure
message about the message delivery.
SMS is a 'store and forward' service. This implies
that short messages are not sent directly from a sender
to recipient but always via a SMS service center instead.
The SMS provides for point-to-point text messaging
and cell broadcasting. Point-to-point short messaging
service enables a mobile user to send and receive
messages. This is two-way paging. Thus, both mobile
terminated and mobile originated messaging is possible.
Cell broadcasting service periodically delivers short
messages to all subscribers in a given area. This
is similar to the 'radio data system' in a car radio.
This feature is
also similar to 'Videotext' service on home television
sets, wherein a user can select certain pages to be
Point-to-point SMS Network
A point-to-point SMS network (Fig 1) is of two types:
mobile originated SMS and mobile terminated SMS. Here,
is a description of various network entities.
Short Message Entity (SME): SME is a device that originates
a short message. Examples of SMEs are:
1. A mobile station can originate and receive messages.
Typically, two mobile stations may belong to different
networks. However, SMS is transparent to user location.
2. A computer directly connected to service center.
This is a paging like service managed by an operator.
Here, a fixed network subscriber can dial-in and request
for a message delivery to a mobile station.
3. A computer/PDA connected to a fixed network. Such
a computer allows a user to dial-in directly into
center for message dispatch.
4. A mobile subscriber can subscribe to a news-service.
Such a mobile user can request for specific information.
Thus, computer sending
requested news item is also
Center: It is hub of SMS activities. It handles all
functions related to point-to-point SMS. SMEs send
messages to service center for forwarding to mobile
subscribers or to fixed network subscribers, as per
the request of SME.
'Service center' also stores the message
for time duration specified by the user or operator.
If required, it conveys originator about acceptance
or failure of a message delivery.
A service center can be multipurpose or application
specific. A multipurpose service center handles all
types of SMS applications. In case of application
specific service center, there could be more than
one service centers to handle various applications.
It is also possible that several operators may share
a common service center.
'SMS-Gateway Mobile-service Switching Center' (SMS-G-MSC)
SMS-G-MSC is a gateway between SMS service center
and GSM's mobile service switching center. It accepts
mobile terminated messages from SMS service center
and finds the current location of the mobile addressee.
Then, the message is delivered to the visiting MSC,
which in turn delivers the message to mobile addressee.
SMS-G-MSC is a dedicated function. It is implemented
Short Message Service-Inter Working-Mobile-service
Switching Center ( SMS-IW-MSC )
This gateway accepts mobile originated point-to-point
messages from 'Mobile-service Switching Center' and
forwards them to SMS service center. The short message
may be addressed to a fixed network subscriber or
to another mobile subscriber. The service center forwards
such messages to recipient's service center.
SMS-IW-MSC function is also implemented
within a MSC.
Home Location Register (HLR): HLR stores the identity
and user data of all the subscribers belonging to
a given MSC. For SMS, the relevant data in HLR includes
current location of the mobile recipient's location,
whether the mobile recipient's terminal is switched-on
and is available to receive short messages.
Mobile-service Switching Center (MSC): It is a complex
switching exchange. It is hub of GSM network. It routes
calls from fixed network subscribers and forwards
them to respective recipients in mobile network or
MSC is linked to other 'Public Land Mobile Network'
(PLMN) and periodically updates the current whereabouts
of a mobile station. This enables a MSC to forward
to a mobile recipient.
Transport of mobile terminated short messages
The following steps are involved in delivering a short
message from a fixed network SME to a mobile addressee.
A SME generates a short message and forwards it to
'short message service center'.
The service center stores the message and checks the
validity period. It adds a timestamp to message. It
performs necessary protocol conversion. It also checks
if other messages are to be forwarded to the same
The 'Service Center' notifies the SMS-G-MSC about
the waiting short message for a mobile addressee.
MSC checks whether the mobile addressee belongs to
this MSC. In case, the message pertains to other MSC,
then it is forwarded to home 'MSC'.
Now, SMS-G-MSC asks HLR about the current location
of mobile addressee. If the mobile station is switched
off or there is another problem then the service center
is notified. HLR also sets a flag that indicates that
the message delivery attempt was not successful. This
situation will be cleared only upon successful message
delivery or expiry of validity period. With precise
knowledge about the mobile addressee, the SMS-G-MSC
forwards the short message to visiting MSC.
The visited MSC checks the VLR to know the location
area. The message is forwarded to related 'Base Transmission
System' where the mobile addressee is roaming. The
currently visited 'Base Transmission System' establishes
a channel and delivers the message to the mobile.
The mobile terminal also acknowledges the correct
receipt of message to BTS which in turn confirms to
MSC. In case of failure in reception, the corresponding
message is sent back. Finally, service center is informed
about message delivery status.
Mobile originated short message delivery
In mobile originated message case, the message flow
is in opposite direction to that in mobile-terminated
The short message is prepared in the mobile station.
Alternatively, message is transferred from an external
PDA/computer to mobile station. The MS via BSS sends
message to mobile switching center. The MSC passes
the message to SMS-IW-MSC of the addressed service
center, which in turn forwards to the related service
center. Now, service center is responsible for message
Short message cell broadcast
Short message cell broadcast is a value-added messaging
service to all users, within a cell, an area or a
network. Message may pertain to weather alert, train-timing,
location-based services, commercial advertisements,
traffic information and news highlights etc. It is
an unacknowledged service. This implies that there
is no guarantee that the message is received by all
A single broadcast message contains 88 octets. Here,
82 octets represent data and 6 octets are overhead
bytes. The data is coded using 7-bit coding. This
represents 93 characters per message.
Six overhead bytes furnish serial number, message
identifier, data coding scheme, etc. One single message
is divided into 4 blocks. Each block has 22 octets.
All four blocks are transmitted consecutively. If
a mobile station does not receive all four blocks,
then the message is treated as a corrupt message and
Network entities in cell broadcast
Short messages are broadcasted in a cell using dedicated
cell broadcast channel or the radio channel. This
facility is to be enabled on a mobile terminal.
The cell broadcast messages are generated in 'Cell
Broadcast Entity' (CBE). The CBE includes all aspects
of formatting the message as well as splitting the
message into various short messages.
The 'Cell Broadcast Center' handles all GSM-related
functions of cell broadcast system. The cell broadcast
center gets input from more than one CBEs. The 'Cell
Broadcast Center' output is connected to multiple
'Base Station Systems' (BSS). A BSS comprises 'Base
System Controller' and multiple base transmitter systems.
The BSS system broadcast the short message to all
subscribers roaming within the cell area.
Issues with SMS
Current SMS support European languages only. Because
SMS system has explosive growth in Asian countries,
there is move to add Arabic and Chinese character
sets. For this purpose a dedicated 16-bit coded alphabet
will be used. The coding scheme is known as 'Universal
Coding Scheme 2'. With 16-bit code, it is possible
to have up to 65,536 different unique characters.
SMS is a useful service. However, each operator has
proprietary implementation. Thus, interoperability
between operators is a big issue. Prior to 'mobile
number portability' (MNP), it was easier to identify
the network of a mobile user. However, with MNP it
is impossible to identify the home network of a user.
To resolve the billing related issue, one solution
is to establish an independent third party platform.
All intranetworking and the internetworking SMS traffic
are routed to this third party. It charges a fee for
every message and also extends billing service to
related network operator. Such a solution is implemented
in Hong Kong, where third party is extending billing
service to six operators.
Mobile spam has potential of blocking SMS. However,
for a telecom operator, it is not easy to stop messages
from a particular customer to other customer.
One suggested solution to resolve mobile spam problem
is to create a neutral agency. This agency maintains
a centralized opt-out database, so that all messages
can be screened before being sent out.
Monitoring cell broadcast channel reduces the battery
life of a mobile station in standby mode. The introduction
of discontinuous reception of cell broadcast messages
will bring relief. Now, mobile phones are becoming
smaller and lighter. It is common experience that
it is difficult to read small display. Further, typing
a short message is even more difficult because 3 letters
are allotted to a single key.
The problem is resolved by providing an attached display
and keypad. Mobile terminals are connected with attachments
by wire line, wireless or infrared links.
In the near future, it is expected that SMS will continue
to grow. Further, in the long run, other mobile data
services will gradually take over
from SMS. These mobile data
services will be based on the Internet services like
e-mail and Web-based services.
For GSM-related literature visit www.gsmworld.com
literature on mobile data services visit www.gsmdata.com
Message: A single short message may contain up to
140 octets of text. The message can comprise words,
numbers, or alphanumeric. Alternatively, non-text
message such as binary data can also be sent. Using
7-bit representation for a character, it is possible
to compress 160 characters in 140 octet space.
A long message of more than 140 octets can be sent
by representing it as a string of multiple short messages.
This process is known as 'concatenation'.
Using 7-bit code, it is possible to encode 128 characters.
However, currently SMS specifies only one alphabet.
It supports total of 28 characters. This is sufficient
to cover all European languages.
Visiting Location Register (VLR): This register contains
all relevant data on all mobile visiting subscribers
in a serving MSC.
Base Station System (BSS): It accepts messages from
central MSC and transmits them over radio interface
to a mobile station.
Mobile Station: It is either an originator or recipient
of short messages. It also informs the network about
For transporting messages between BSS and mobile station,
low-bit rate signaling channel is used. For dedicated
signaling channel, the bit rate is 760 bit/sec. When
slow associated control channel is used for signaling,
then the channel bit rate is 350 bit/sec.
Teleservice: These are standalone basic services
provided by a network. To provide teleservice, no
external equipment/software is needed. Examples of
teleservices are voice telephony, fax, short message
Bearer Service: Such services provide data-pipe
between access points. They require terminal equipment
where data pipe is terminated.
Supplementary service: Such services supplement
teleservices and bearer services. These are subscription
services. These services enable a user to exercise
control over the conditions and circumstances. Examples
are call forwarding, call restriction, number identification
and multiparty conferencing.
Roaming: This feature enables a subscriber
to communicate in a cellular system other than its
home registered one.
Mobile station: It consists of physical equipment
and associated hardware and software. It is used by
mobile subscriber to gain access to telecommunication
Octet: A binary sequence comprising of eight
POCSAG (Post Office Code Standardization Advisory
Group): It is a popular one-way radio paging protocol.
It was originally developed for tone-only pagers and
short numeric message. It is a low-speed paging protocol.
Number portability: When a user moves his telephone
services from the existing operator to a new operator,
the problem of handling number changes is a difficult
one. Number portability allows a user to retain his
existing number. It can take two forms: operator portability
and geographical portability.
Operator number portability: It allows subscribers
to retain their numbers when changing to a new operator.
Geographical number portability: When a subscriber
changes the geographical address, geographical portability
enables him to retain his original number. This service
is available at much more limited scale compared to
Teletext: In Teletext a text message is sent
along with a TV signal. In a TV transmission 25 frames
are sent in one second. Further, each frame comprises
of two fields. Thus, 50 fields are sent in a second.
AMP (Advanced Mobile Phone system): It is first
generation analogue cellular telephone networks. It
uses frequency modulation.
D-AMP (Digital-Analog Mobile Phone system):
It is a second generation wireless system. It uses
digital modulation techniques and advanced call processing.
It has same frequency plan as AMP system. It is also
known as IS136.
Global System for Mobile communication (GSM):
GSM is the European digital cellular communication
system. It is a circuit-switched technology. It operates
in 900 MHz and 1800 MHz band. A GSM system has 200
full-duplex channels per cell. Each channel consists
of a down-link and up-link frequency. Each frequency
band is 200 KHz wide that supports eight time-slots.
Discounting overhead, each connection can send compressed
voice signal or data at 9.6 K bit/Sec. GSM provides
SMS using low-bit rate signaling channel.
Author A. K. Vanwasi is GM(R&D)
ITI Ltd. Naini, Allahabad. He can be reached at Vanwasi_nni@itiltd.co.in.