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SMS

The Telecom Source10 Slide Technology Series

IntroductionSMS stands for Short Message Service

SMS was first introduced in 1991 in Europe as a text messaging service based on European Telecommunications Standards Institute (ETSI) standards for mobile networks

SMS is being used in a wide range of social and business applications such as electronic voting, delivery of stock quotations, delivery of e-mail notification

SMS is currently supported on the major mobile network technologies including: GSM (Global System for Mobile communications) GPRS (General Packet Radio Service)CDMA (Carrier Division Multiplex Access)

SMS supports the sending and receiving of text, images, animation and sound

SMS messages are originated and received by Short Messaging Entities (SME). Examples of SMEs are: mobile phones; servers; personal computers

Basic Network Architecture

SMS-GMSC/SMS-IWMSC

HLR

SME MSSMSC MSC/SGSN

VLR

Outside the scope of GSM specifications

1. Short Message Entity (SME) – sends or receives short messages2. Short Message Service Centre (SC) – stores-and-forwards messages between the SME and the MS3. Gateway MSC For Short Message Service (SMS-GMSC) - receives messages from the SC, interrogates the HLR for

routing information and forward the messages to the MSC or SGSN• Home Location Register (HLR) - a database used for permanent storage and management of

user/subscriber profiles4. Inter-Working MSC For Short Message Service (SMS-IWMSC) - receives messages from the MSC or SGSN and forwards

them to the SC5. Mobile Service Centre (MSC) – performs switching functions for mobile stations in a geographical area

• Visitor Location Register (VLR) - a database that contains temporary information about roaming subscribers. The MSC and the VLR are always on the same platform.

6. Serving GPRS Support Node (SGSN) – performs packet switching functions for mobile stations in a geographical area. The SGSN is used instead of the MSC when SMS info is transferred over GPRS.

7. Mobile Station (MS) – a device on the mobile network capable of receiving and sending short messages

SMS Protocol Services and FeaturesSMS is a point-to-point store and forward technology with 2 basic services:

Short Message Mobile Terminated (SM-MT) - to transmit a message from the short message service centre to the mobile station. – SMS-DELIVER PDU (Protocol Data Unit)Short Message Mobile Originated (SM-MO) to transmit a message from mobile station to service centre – SMS-SUBMIT PDU

SMS allows message delivery to handsets either active/in-use or powered offMobile stations receive transport data protocol units (TPDU) denoted as SMS-DeliverMobile stations send transport data protocol units (TPDU) denoted as SMS-SubmitNote: the TPDU contains the user data (the short message)

SMS protocol permits request of message delivery confirmation report

SMS messages contain up to 140 octets which is equivalent to:160 Latin characters (7 Bit Coding) – Text Mode70 Unicode characters (double byte) e.g. Arabic Characters, Chinese CharactersSMS messages can also contain up to 140 octets of binary information

SMS messages are transported in the core network using SS7 (Signaling System 7)

SMS Service ElementsValidity Period is the service element that indicates the time period for which the SMSC will guarantee the existence of the short message when attempting to deliver it

Service Centre Time Stamp is the service element that indicates the time stamp of message arrival at the SMSC

Protocol Identifier is the service element in to indicate higher layer protocol or indicates inter-working with certain types of telemetic services (e.g. paging, email)

More Messages to Send is the service element by which a SMSC informs the MS that there is one or more messages waiting in the SC to be sent to the MS

Priority is the service element indicating that delivery of the message will be attempted regardless of the MS being temporarily absent or having no free memory

Message Waiting is the service element that enables the mobile network to provide the HLR, SGSN and VLR with the information that there is a message waiting in the originating SMSC for a MS

Alert Service Centre is the service element used by mobile networks to inform an SMSC that a previously unreachable MS (temporarily absent or no memory) is ready to receive messages

SM-MT Service – SMS-Deliver TPDU Structure

1-10 Octets 1 Octet 2-12 Octets 1 Octet 1 Octets 7 Octets 1 Octet 0-140 Octets

SCAPDU-Type OA PID DCS SCTS UDL UD

PDU Type:

Bits7 6 5 4 3 2 1 0

RP UDHI SRI MMS MTI

MTI bit 1 = 0bit 0 = 0

Parameter Description

SCA Service Centre Address - Telephone number of the Service Centre

PDU Type Protocol Data Unit Type

RP Reply Path - Parameter indicating that Reply Path exists

UDHI User Data Header Indicator - Parameter indicating that UD field contains a header

SRI Status Report Indication - Parameter indicating if the SME has requested a status report

MMS More Messages to Send - Parameter indicating whether or not there are more messages to send

MTI Message Type Indicator - Parameter describing the message type 00 means SMS-Deliver

OA Originator Address - Address of the originating SME

PID Protocol Identifier - Parameter indicating the SMSC how to process the Short Message (e.g. Fax)

DCS Data Coding Scheme - Parameter identifying the coding scheme within the User Data (UD)

SCTS Service Centre Time Stamp - Parameter identifying the time when the SMSC received the message

UDL User Data Length - Parameter indicating the length of the UD-field

UD User Data - Data Field of the Short Message

SM-MO Service – SMS-Submit TPDU Structure

Bits

PDU Type:

MTI bit 1 = 0bit 0 = 1

Parameter Description

SCA Service Centre Address - Telephone number of the Service Centre

PDU Type Protocol Data Unit Type

RP Reply Path - Parameter indicating that Reply Path exists

UDHI User Data Header Indicator - Parameter indicating that UD field contains a header

SRR Status Report Request - Parameter indicating if the MS has requested a status report

VPF Validity Period Format - Parameter indicating whether or not the VP field is present

RD Reject Duplicates – parameter indicating if SMSC will accept a message with same MR and DA from the same OA

MTI Message Type Indicator - Parameter describing the message type 01 means SMS-Submit

MR Message Reference - Successive numbers (0…255) of all SMS-SUBMIT frames sent by the MS.

DA Destination Address - Address of the destination SME

PID Protocol Identifier - Parameter indicating the SMSC how to process the Short Message (e.g. Fax)

DCS Data Coding Scheme - Parameter identifying the coding scheme within the User Data (UD)

SCTS Service Centre Time Stamp - Parameter identifying the time when the SMSC received the message

UDL User Data Length - Parameter indicating the length of the UD-field

UD User Data - Data Field of the Short Message

6 5 1 0

VPF

1-10 Octets 1 Octet 1 Octet 2-12 Octets 1 Octet 1 Octets 0, 1 or 7 Octets 1 Octet 0-140 Octets

SCAPDU-Type MR DA PID DCS VP UDL UD

7 4 3 2

RP UDHI SRR RD MTI

Message Flow SM-MT

Message

Transfer sendRoutingInfo-

Delivery

ForShortMsg

forwardShortMessage

sendInfoFor-

MT-SMS

Message TransferDeliver Report

SM-Delivery

ReportStatus

Report

SMSC GMSC HLR MSC VLR

Page

Authenticate

MS

Note: ETSI/GSM MAP sendRoutingInforForShortMsg equivalent in IS41 (North American standard) is SMSrequest mechanism, while forwardShortMessage is Short Message Delivery-Point-to-Point (SMD-PP)

Message Flow SM-MO

Message

Transfer

Delivery

forwardShortMessage

sendInfoFor-

MO-SMS

Message Transfer

Delivery ReportDelivery

Report

Report

SMSC SMS-IWMSC

HLR MSC VLR

Access Request

Authenticate

MS

Note: ETSI/GSM MAP forwardShortMessage equivalent in IS41 (North American Standard) is Short Message Delivery-Point-to-Point (SMD-PP) mechanism

Short Message Service Centre - SMSCShort Message service Centre plays a central role in the management of SMS message origination and SMS message delivery

Interestingly, detailed functionality of SMSC is outside the scope of standardization, hence several vendor specific products and protocols:

EMI (External Machine Interface) – CMG, now LogicaCMGUCP (Universal Computer Protocol) – Logica, now LogicaCMGCIMD (Computer Interface to Message Distribution) – NokiaOIS (Open Interface Specification) – SEMA Group, now Airwide SolutionsComputer Access Service and Protocol – Ericsson

SMS Forum is an industry initiative to create a common standard “SMPP” (Short Message Peer-to-Peer) using Internet as the transport network

The ETSI/GSM standard does specify minimum mandatory SMSC requirements:Each SMS-Deliver to a MS must have unique time stamp with one second accuracyOnly one outstanding SMS-Deliver (i.e. message for which a report not yet received)If requested by MS or SME, initiate overwriting of previously received short messages

Additional Features and IssuesSMS standard allows concatenation of messages to enable transmission of longer messages (i.e. messages longer than 140 octets)

Short Messages may be compressed with algorithms described in GSM 03.42Compression only applies to user-data and excludes user-data-headerIf compressed message are greater than 140 octets, then the messages can be concatenated

In North America wireless subscribers can address text messages using 5-digit numbers (“short codes”) An example of the use of short codes is for SMS votingOne objective of short codes is to reduce or minimize Spam Short codes are being used to price and market services (e.g charge back to application provider)

Service gateway products are available that address multi-network (GSM/CDMA) and multi-protocol (e.g. EMI/OIS) inter-working

Security is an emerging concern similar to that with e-mail, for example: SPAM – sending of unsolicited messages and ads via SMS (e.g. to entice users to call numbers that have a high per minute charge)Virus - resend of message to all numbers in the phone’s address book (e.g. via a Trojan Horse)Identity Theft - retrieval of personal information from a SIM (Subscriber Interface Module) card

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