Ch6

Mobile Computing

Chapter 6
SMS
Asoke K Talukder
Hasan Ahmed
© Tata McGraw Hill

Short Message Service (SMS)
 Most popular data bearer/service within GSM
 More than one billion SMS messages interchanged everyday
with a growth of more than half a billion every month on an
average
 Runs on SS7 signaling channels, which are always present but
mostly unused, be it during an active user connection or in the idle
state
 Each short message is up to 160 characters in length when 7-bit
English characters are used and 140 octets when 8-bit characters
are used

Strengths of SMS

 Omnibus nature of SMS: SMS uses SS7 signaling channel
which is available throughout the world.
 Stateless: SMS is session-less and stateless as every SMS
message is unidirectional and independent of any context. This
makes SMS the best bearer for notifications, alerts and paging.
 Asynchronous: SMS is completely asynchronous. In case of
SMS, even if the recipient is out of service, the transmission will
not be abandoned and hence, SMS can be used as message
queues. SMS can be used as a transport bearer for both
synchronous (transaction oriented) and asynchronous (message
queue and notification) information exchange.

Strengths of SMS

 Self-configurable and last mile problem resistant: SMS is self-
configurable and subscriber is always connected to the SMS
bearer irrespective of the home and visiting network
configurations.
 Non-repudiable: SMS message carries the Service Center (SC)
and the source MSISDN as a part of the message header through
which any SMS can prove beyond doubt its origin.
 Always connected: As SMS uses the SS7 signaling channel for
its data traffic, the bearer media is always on. Users cannot switch
OFF, BAR or DIVERT any SMS message. SMS message is
delivered to the Mobile Station (MS) without any interruption to
the ongoing call.

SMS Architecture

 Two types of SMS - SM MT (Short Message Mobile
Terminated Point-to-Point) and SM MO (Short Message Mobile
Originated Point-to-Point)
 SM MT is an incoming short message from the network and is
terminated in the MS
 SM MO is an outgoing message originated in the MS and
forwarded to the network for delivery
 For an outgoing message, the path is from MS to SC via the
VLR and the IWMSC (Inter Working MSC) function of the
serving MSC whereas for an incoming message the path is from
SC to the MS via HLR and the GMSC (Gateway MSC) function
of the home MSC

Short Message Mobile Terminated (SMMT)

 Message is sent from SC to the MS.
 For the delivery of MT or incoming SMS messages, the SC of
the serving network is never used which implies that a SMS
message can be sent from any SC in any network to a GSM phone
anywhere in the world.

Interfaces in SMMT

Short Message Mobile Originated

 For a MO message, the MSC forwards the message to the
home SC.
 MO message works in two asynchronous phases. In the first
phase, the message is sent from the MS to the home SC as a MO
message. In the second phase, the message is sent from the home
SC to the MS as a MT message.

Interfaces in SMMO

SMS as an Information Bearer

 For using SMS as an information bearer, we need to connect
the services running on the Enterprise Origin server to the SC
through an SME (Short Message Entity) or ESME (External Short
Message Entity).
 SME in any network is generally a SMS gateway.
 With respect to SMS, a GSM subscriber is always in control of
the SC in the home network irrespective of the serving network.
 If there is any SMS-based data service in the home network, it
will be available in any foreign network.

Operator Centric Pull

 Operators offer different information on demand and
entertainment services through connecting an Origin server to the
SC via a SMS gateway.
 Such service providers are known as Mobile Virtual Network
Operator(s) (MVNO).
 MVNOs develop different systems, services and applications to
offer data services using SMS.
 Many enterprises use MVNOs to make their services available
to mobile phone users.

Example of MVNO

 Let’s say few banks offer balance enquiry and other low
security banking services over SMS and customers need to
register for the service.
 During the registration, the customer needs to mention the
MSISDN of the phone which will be used for a banking service.
 Once a user is registered for the service, he enters ‘BAL’ and
sends the message to a service number (like 333) as a MO
message and then SC delivers this MO message to the SMS
gateway (known as SME-Short Message Entity) connected to this
service number.

Example of MVNO

 SMS gateway then forwards this message to the enterprise
application and response from the enterprise application is
delivered to the MS as a MT message from the SME.
 Even if the subscriber is in some remote region of a foreign
network within GSM coverage, he can send the same SMS to the
same service number in his home network and this makes the
home services available in the foreign network. Hence, operator-
centric SMS pull service is completely ubiquitous.

Operator Centric Pull

 Connectivity between SME and Origin server could be
anything like SOAP (Simple Object Access Protocol), direct
connection through TCP socket or through HTTP.
 There are applications where SMS is used in session oriented
transactions as ‘SMS chat’ and ‘SMS contests’ need to remember
the user context over multiple transactions.

Operator Independent Push

 Any push, which may be an alert, notification or even response
from a pull message generated by an application, can be serviced
by any network and delivered to any GSM phone in any network
without any difficulty.
 If appropriate roaming tie-ups are in place, an enterprise can
use SMS to send business alerts or proactive notifications to its
customer anywhere, anytime on his phone.

Operator Independent Pull

 For a SMS message to be routed to some enterprise SME
connected to external SC, SAT is used.
 SAT application running on the SIM card changes the SC
number during the transmission of the SMS and forces the SMS to
recognize a different SC of a different network as its home SC.
 Here, too, SMS is sent to the SME connected to the home SC.
If a SMS service is operator dependent, the cellular operator can
use this to its advantage.
 Enterprises need operator independent pull as enterprises have
customers around the world subscribing to different GSM
networks
 Above scenario can also be achieved through Intelligent
Network.

Value Added Services through SMS

 Value Added Services (VAS) can be defined as services,
which share one or more of the following characteristics:
1. Supplementary service (not a part of basic service) but
adds value to total service offering
2. Stimulates incremental demand for core services
offering
3. Stands alone in terms of profitability and revenue
generation potential
4. Can sometimes stand-alone operationally
5. Does not cannibalize basic service unless clearly
favorable


6. Can be an add-on to basic service, and as such, may be
sold at a premium price
7. May provide operational and/or administrative synergy
between or among other services and not merely for
diversification

 VAS over SMS are entertainment and information on demand
which is further categorized into:
1. Static information which does not change frequently
2. Dynamic information which changes in days
3. Real-time information which changes continually
 Some of the common VAS examples are:
1. News/Stock Quotes Service
2. Session-based Chat Application
3. Email through SMS
4. Health Care Services
5. Micro-Payment Services

Alert services through VAS

 Proactive alert services can be of the two kinds – Time based
and Watermark based
 Time based proactive alerts are sent to the mobile phone at a
pre-assigned time of the day
 Watermark based proactive alerts are sent when some event
occurs

Location based services through SMS
 Location based services could be road direction, restaurant guide,
shopping alerts, etc.
 In location based services, only the information relevant to the
current location of the mobile phone (or the subscriber) is provided.
 The location of a mobile phone can be determined either from the
network or from the device.
 The location of a mobile phone can be determined either from the
network or from the device.
 To find out the location from the device either of the following
technologies are used - Cell ID (CID) based system and Global
Positioning System (GPS) based system.

Cell ID based system

 CID of the current BTS is determined and then mapping of the
cell identifier to the geographical location is performed.
 For CID based system, the signal strength from all the
different CIDs are extracted from the device and sent to the
server through a SMS.
 Location of the user is determined using the signal strength
and triangulation algorithms.

GPS based system

 GPS is Global Positioning System.
 Location is determined through a GPS receiver installed
within the phone.
 GPS provides facility to compute position, velocity and time
of a GPS receiver.
 GPS based system is not dependent on the network operator.

Accessing the SMS bearer

 There are two ways through which SMS bearer can be
accessed:
1. Using a mobile phone as a GSM modem and connecting
it to the computer
2. Using the SMSC of an operator through SMPP or similar
interface

GSM Modem

 Normal cell phone can be used as a data modem which will be
in a position to access the network as a normal GSM phone.
 Once phone and computer is connected (through wired or
wireless means), cell phone can be used as an external GSM
modem and issue AT commands to transact data over the
GSM/SMS bearer. AT in Hayes terminology is known as
attention and are commands to the modem from the computer.
 AT commands can be for sending a SMS, reading a SMS,
checking battery power, writing a phone book entry, etc.
 To read a SMS from the GSM modem, we need to ensure that
the SMS is forwarded to the computer rather than the phone
local store and for this CNMI commands are used.

Example code for GSM
 Code is written in Visual Basic and uses Microsoftmscomm
controls. The mscomm controls use the COM1 port for
communication.
 Line 13 is for setting of the communication port and the interface
between the computer and the modem.
 Lines 14–24 are for initialization of the GSM phone as modem.
 Lines 28–46 are to send a SMS.
 Lines 50–52 are for reading SMS from the modem.

SMPP
 Short Message Peer to Peer (SMPP) protocol
 Open, industry standard protocol designed to provide a flexible
data communications interface for transfer of short message data
between a Message Center (SC or SMSC) and a VAS application
such as a WAP Proxy Server, Voice Mail server, E-Mail Gateway or
any other Messaging Gateway
 SMPP client is termed a External Short Message Entity (ESME)
and is connected to the SC
 SMPP release v3.4 presently supports Digital Cellular Network
technologies which include GSM, IS-95 (CDMA), CDMA
1X/CDMA 2000, ANSI-136 (TDMA) and IDEN

SMPP
 SMPP supports a full featured set of two way messaging
functions such as:
1. Transmit messages from an ESME to single or multiple
destinations via the SMSC
2. An ESME may receive messages via the SMSC from other
SMEs
3. Query the status of a short message stored on the SMSC
4. Cancel or replace a short message stored on the SMSC
5. Send a registered short message
6. Schedule the message delivery date and time

SMPP
7. Select the message mode such as datagram or store and
forward
8. Set the delivery priority of the short message
9. Define the data-coding type of the short message
10. Set the short message validity period
11. Associate a service type with each message such as voice mail
notification

SMPP
 Open message transfer protocol that enables short message
entities (SMEs) outside the mobile network to interface with an
SC and non-mobile entities that submit messages to, or receive
messages from an SMSC are known as External Short Message
Entities (ESMEs)
 The SMPP protocol defines operations and data as:
1. Set of operations for the exchange of short messages between an
ESME and an SMSC
2. Data that an ESME application must exchange with an SMSC
during SMPP operations

SMPP
 Subscribers to an SMS capable Cellular Network may receive
short messages on a Mobile Station (MS) from one or more
ESMEs
 Examples of such ESME applications can be:
1. Voice mail alerts originating from a VMS (Voice Messaging
System)
2. Numeric and alphanumeric paging services
3. Informative services
4. Calls directly dialed or diverted to a message-bureau operator,
who forwards the message to the SMSC, for onward delivery to
a subscriber’s handset

SMPP
5. Fleet management applications that enable a central station to
use the SMSC to determine the location of its service vehicles
and notify the closest vehicle of a service request in their area
6. Telemetry applications
7. WAP Proxy Server

Kannel

 Open source SMS gateway
 Kannel gateway also supports WAP and MMS (Multi Media
Messaging) interfaces
 Offers HTTP interface for message transfer and administrating
of the gateway
 Kannel divides its various functions into different kinds of
processes (figure ahead) called boxes, based on what kinds of
external agents it needs to interact with
 Bearerbox implements the bearer level of SMS and as a part of
this, it connects to the SMS centers
 Definitions of different TCP/IP ports, usernames, passwords,
etc. are required to be defined for Bearerbox connection

Kannel
 Smsbox implements the rest of the SMS gateway functionality
and as a part of this it receives textual SMS messages from the
bearerbox and interprets them as service requests and responds
to them in the appropriate way
 All the services are handled and managed by Smsbox
 There can be only one bearerbox, but any number of Smsboxes
in a single Kannel instance
 While it is possible to have each SMS center served by a
different process, it has been deemed not to give enough extra
reliability or scalability to warrant the complexity
 Each box is internally multithreaded

Pull messages in Kannel
 User enters a message with a keyword and then sends the same
to a service number.
 During binding of the SMS gateway, we intimate the SC that we
are listening for a service number. Therefore, all the messages
sent to a service number will be routed to our SMPP gateway.
 In the Kannel configuration file, we mention that whenever
there is a message with a particular keyword, it should be
forwarded to a HTTP URL.
 To service the user with appropriate response, we need to know
the request with all the parameters and the MSISDN number of
the phone. These are transferred from Kannel gateway to the
URL through %a and %p.

Pull messages in Kannel
 The response of the http request will be forwarded directly to
the user by Kannel gateway.
 If the response from the content/origin server is more than 160
characters, Kannel splits the message into multiple messages.
 The max message parameter defines the limit of maximum
number of messages as response. If we set the max messages to
0, no response will be sent to the user, though there could be
some response coming from the HTTP request.

Push messages in Kannel
 Message is sent through HTTP interface as well and an
application uses an HTTP URL to communicate with the Kannel
gateway and to send SMS messages
 Kannel delivers these messages to the SC
 To offer certain level of security, Kannel allows the user
authentication through user identifier and a password to access
such URLs

Next Chapter

General Packet Radio Service (GPRS)

Thanks

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