Sms Protocols Overview

15

ince the first Global Systemfor Mobile Communications (GSM) network started opera-tion in 1991, more than 100 countries have adopted the stan-dard. Over 20 million subscribers of GSM networks are nowoffered worldwide coverage, outstanding voice quality over awhole range of operating conditions, and a variety of value-added services. These services include voice mail, call han-dling facilities, call line identification, and Short MessageService (SMS).

With SMS, users are able to exchange alphanumeric mes-sages (up to 160 characters) with other users of digital cellularnetworks, almost anywhere in the world, within seconds ofsubmission. Even if the service was originally conceived as apaging mechanism for notifying the users of voicemail mes-sages, SMS is now increasingly used as a messaging service.The messages are typically created on mobile phone keypads,which is somewhat awkward. Fortunately, there are otherways to access the message centers, as discussed in this article.

Numerous applications are already available and make shortmessage reception and submission possible using a computer.Gateway architectures are also being widely implemented andconnect company’s e-mail or voicemail systems to the SMS.

The practical implementation of SMS and the differentprotocols for message submission are addressed in this article.The future of SMS and a brief review of the fields currentlybeing studied will conclude this article.

The Short Message ServiceDeveloped as part of the GSM Phase 2 specification, theShort Message Service, or SMS as it is more commonlyknown, is based on the capability of a digital cellular terminal

to send and/or receive alphanumeric messages. The shortmessages can be up to 140 bytes in length, and are deliveredwithin a few seconds where GSM coverage is available. Morethan a common paging service, the delivery of the message isguaranteed even when the cellular terminal is unavailable(e.g., when it is switched off or outside the coverage area).The network will hold the message and deliver it shortly afterthe cellular terminal announces its presence on the network.

The fact that SMS (through GSM) supports internationalroaming with very low latency makes it particularly suitablefor applications such as paging, e-mail, and voice mail notifi-cation, and messaging services for multiple users. However,the facilities offered to users and the charges for these facili-ties still mainly depend on the level of service provided by thenetwork operator.

There are two types of SMS available: cell broadcast [1]and point-to-point [2]. In cell broadcast, a message is trans-mitted to all the active handsets or mobile stations (MSs) pre-sent in a cell that have the capability of receiving shortmessages and have subscribed to this particular informationservice. This service is only one-way, and no confirmation ofreceipt will be sent. It can send up to 93 7-bit character or 828-bit characters, typically used to transmit messages abouttraffic conditions, weather forecast, stock market, and so on.

In point-to-point service, messages can be sent from onemobile to another or from a PC to a mobile and vice versa.These messages are maintained and transmitted by an SMSCenter (SMSC). The SMSC is an electronic form of ordinarymail postal service that stores and then forwards the messageswhen they can be delivered. Each GSM network must supportone or more SMSCs to sort and route the messages. EachSMSC checks, organizes, and sends the message to the opera-

IEEE Personal Communications • June 2000 1070-9916/00/$10.00 © 2000 IEEE

S

The Global System forMobile CommunicationsShort Message Service

Guillaume Peersman and Srba Cvetkovic,The University of Sheffield

Paul Griffiths and Hugh Spear, Dialogue Communications Ltd.

AbstractThis tutorial presents an overview of the Global System for Mobile Communications Short Message Service from the viewpoint of implementingnew telematic services. SMS offers the users of GSM networks the ability to exchange alphanumeric messages up to the limit of 160 characters.

The tutorial is motivated by an acute absence of research publications in this field. The information gathered in the tutorial was requiredconsidering the increasing potential SMS offers for integration with existing messaging services and its ability to offer a successful replacement

for the Transmission Control and Internet Protocols as far as low-bandwidth-demanding applications are concerned. Initially, the tutorial gives abrief overview of the building blocks of GSM networks — the mobile station, base station, and network subsystem — and then emphasizes the

SMS network and protocol architecture. The most widely used protocols for message submission are then introduced (text-based, SMS2000,ETSI 0705, TAP) and compared in terms of features provided and flexibility to handle extended alphabets or two-way messaging. Finally the

tutorial outlines a summary of current and future issues for further development and research in the light of novel features for submissionprotocols and telematic services.

16

tor. It also receives and passes on any confirmation messages toany GSM mobile on any network. However, in practice, thereare no agreements to allow SMS to travel between networks.

There are several ways in which a short message can besubmitted, depending on the interfaces supported by the GSMnetwork SMSC. Users can call a central paging bureau (i.e.,an operator), or directly create the message on the keypad oftheir handset. Typing the messages is made easier when usinga personal digital assistant (PDA) or a laptop connected tothe handset. A few SMSC equipment manufacturers and com-panies have also developed their own protocols for short mes-sage submission. Consequently, more and more GSMnetworks now offer access to their SMSC using these proto-cols over a variety of hardware interfaces: modem dialup,X25, and even the Internet.

GSM Network ArchitectureThe layout of a generic GSM network with its several func-tional entities is shown in Fig. 1 [3]. The architecture can bedivided in three main components:• The subscriber holds the MS, namely the GSM terminal• The base station subsystem controls the radio link with the

MS• The network subsystem performs the switching of calls and

other management tasks such as authentication.

The Mobile StationThe MS and base station subsystem communicate across theUm interface, also known as the air interface or radio link.The base station subsystem communicates with the networksubsystem across the A interface. The MS consists of thephysical terminal and contains the radio transceiver, the dis-play and digital signal processors, and the Subscriber IdentityModule (SIM). The SIM provides the user with the ability toaccess their subscribed services regardless of the location andthe terminal used. The insertion of the SIM in any GSM cel-lular phone allows the user to access a network, make andreceive phone calls, and use all the subscribed services.

The International Mobile Equipment Identity (IMEI)uniquely identifies the mobile terminal according to the Inter-national Mobile Subscriber Identity (IMSI) contained in the

SIM. Because the IMEI and IMSI are independent, personalmobility is possible. The SIM can be protected against unau-thorized use by a personal identity number (PIN).

The Base Station SubsystemThe base station subsystem is composed of two parts, thebase transceiver station (BTS) and base station controller(BSC). They communicate across the specified Abis inter-face, thus allowing network operators to use componentsmade by different suppliers. The BTS houses the radiotransceivers that define a cell and handle the radio link pro-tocols with the MS. Depending on the density of the area,more or fewer BTSs are needed to provide the appropriatecapacity to the cell. Digital communications system (DCS)networks working at 1800 MHz need twice the number ofBTSs to cover the same area as GSM networks, but providetwice the capacity.

The BSC manages the radio resources for one or moreBTSs via the standardized Abis interface. It handles radiochannel setup, frequency hopping, and handovers. The BSC isthe connection between the MS and the mobile switching cen-ter (MSC). The BSC also takes care of converting the 13 kb/svoice channel used over the radio link (Um interface) to thestandardized 64 kb/s channel used by the public switched tele-phone network (PSTN).

The Network SubsystemThe MSC is the main component of the network subsystem.Its provides the same functionality as a switching node in aPSTN or integrated services digital network (ISDN), but alsotakes care of all the functionality needed to handle a mobilesubscriber such as registration, authentication, location updat-ing, handovers, and routing to a roaming subscriber. TheMSC also acts as a gateway to the PSTN or ISDN, and pro-vides the interface to the SMSC.

The international roaming and call routing capabilities ofGSM networks are provided by the home location register(HLR) and visitor location register (VLR) together with theMSC. The HLR database contains all the administrative infor-mation about each registered user of a GSM network alongwith the current location of the MS. The current location ofan MS is in the form of a Mobile Station Roaming Number

IEEE Personal Communications • June 2000

� Figure 1. The basic GSM network architecture.

BTS: Base transceiver stationBSC: Base station controllerSIM: Subscriber identity moduleME: Mobile equipment

AuC: Authentication centreEIR: Equipment identity registerHLR: Home location registerVLR: Visitor location register

Other base station subsystem

Base station subsystem

BTS

BSC

MSCOperation and

maintenance center

Data communicationnetworkSMSC

Network subsystem

VLR

Um Abis A

AuC EIR HLR

ME

SIM

Mobilestation

PSTN/ISDN

BTS

17

(MSRN), typically the SS7 number ofthe visited MSC, and used to route acall to the MSC where the mobile isactually located.

The VLR is usually located withinthe MSC to speed up access to theinformation required during a call andsimplify the signaling. The content ofthe VLR is a selection of the informa-tion from the HLR, basically all neces-sary information for call control andprovision of the subscribed services,for each single mobile currently locat-ed in the geographical area controlledby the VLR.

The network subsystem uses twoother databases for authentication andsecurity purposes. The EquipmentIdentity Register (EIR) contains a listof each MS IMEI allowed on the network. The authenticationcenter (AuC) database contains each single PIN stored in theMS SIM.

The GSM Signaling ProtocolThe exchange of signaling messages regarding mobility, radioresources, and connection management between the differententities of a GSM network is handled through the protocolarchitecture, as shown on Fig. 2.

The architecture consists of three layers: physical, datalink, and message. The physical layer and channel structureare described in detail by M. Mouly and M. Pautet [4]. Layer2 implements the data link layer using a modified flavor of theLink Access Protocol (LAPD) to operate within the con-straints set by the radio path. On the MS side, the messagelayer consists of three sublayers: connection management(CM), mobility management (MM), and resource manage-ment (RR). The CM sublayer manages call-related supple-mentary services, SMS, and call-independent supplementaryservices support. The MM sublayer provides functions toestablish, maintain, and release a connection between the MSand the MSC, over which an instance of the CM sublayer canexchange information with its peer. It also performs locationupdating, IMSI management, and Temporary Mobile Sub-scriber Identity (TMSI) identification, authentication, andreallocation. The RR sublayer establishes the physical connec-

tion over the radio link to transmitcall-related signaling information suchas the establishment of the signalingand traffic channel between the MSand the BSS.

On the MSC side, the message layeris divided into four sublayers. TheBase System Substation ApplicationPart (BSSAP) of the MSC providesthe channel switching functions, radioresources management, and internet-working functions. The MessageTransfer Part (MTP) and SignalingConnection Control Part (SCCP) pro-tocols are used to implement the datalink layer and layer 3 transport func-tions for carrying the call control andmobility management signaling mes-sages across the A interface. SCCP

packets are also used to carry the messages for SMS.Signaling between the different entity uses the Internation-

al Telecommunication Union (ITU) SS7, widely used in ISDNand current public networks. SS7 is currently the only elementof the GSM infrastructure capable of packet switching as wellas circuit switching. It is used to transport control signals andshort message packets for SMS. The protocol consists of theMobile Application Part (MAP), Transaction CapabilityApplication Part (TCAP), SCCP, MTP, and ISDN-User Part(ISUP) or Telephone User Part (TUP). Figure 3 depicts theSS7 protocol stack.

The ISUP provides the signaling functions needed to sup-port switched voice and data applications in the ISDN envi-ronment. The TUP provides the basic functionality for callcontrol functions for ordinary national and international tele-phone calls. The TCAP is an application layer protocol. Itallows an application at one node to invoke an execution of aprocedure at another node and exchange the results of suchinvocation. It isolates the user application from the complexityof the transaction layer by automatically handling transactionand invocation state changes, and generating the abort orreject messages in full accordance with ITU and AmericanNational Standards Institute (ANSI) standards. The MAPuses the TCAP services to provide the signaling capabilitiesrequired to support the mobile capabilities.

The MTP and SCCP (Fig. 4) correspond to the lower three


� Figure 2. The GSM protocol architecture.

CM

MS

MM

RM

LAPDm

TDMA/FDMA

LAPDm

RR

TDMA/FDMA

Message layer

MTP

BSS

SCCP

BSSAP

MTP

SCCP SCCP

MTP level 3MTP level 2MTP level 1

BSSAP

MM

CM

MAP

ISDNUPTUP

Um air interface

Data link layer Physical layer

A interface MSC

� Figure 3. The SS7 protocol stack.

TUP ISUPMAP

TCAP

MTP level 1

MTP level 2

MTP level 3

SCCP

18

layers of the open system interconnection (OSI) model (Fig. 4).The SCCP sublayer supports connectionless and connection-oriented services to transfer data and Global Title Translation(GTT) above MTP level 3 for voice, data, ISDN, and GSM ser-vices. The data transfer is reliable, independent of the underly-ing hardware, and transparent to users. The protocol employslogical signaling connections within the SS7 network to ensurereliability and integrity of the ongoing data transfer. The MTPis divided into three levels:• MTP level 1 defines the characteristics of the digital signal-

ing link and is equivalent to the OSI physical layer.• MTP level 2 is equivalent to the OSI data link layer and

provides a reliable sequenced delivery of data packetsacross MTP level 1.

• MTP level 3 provides congestion control, signaling manage-ment, and message discrimination, distribution,and routing in a similar way as the OSI networklayer.

Practical ImplementationSMS uses the SS7 signaling channel to transmit thedata packet [5], thus allowing a text message to bereceived when the user is making a voice or datacall. An active MS should be able to send andreceive a short message Transport Protocol DataUnit (TPDU) at any time regardless of whetherthere is a speech or data call in progress. A confir-

mation will always be returned to the SMSCindicating whether the MS has received the shortmessage or not. A confirmation will also bereturned to the MS from an SMSC indicatingwhether the TPDU has been received successful-ly. The software within the MS must be able todecode and store the messages.

SMS Mobile Terminated (SMS-MT) is theability to receive an SMS message from anSMSC and is more ubiquitous, while SMSMobile Originated (SMS-MO) is the ability tosend short messages to an SMSC. Messages canalso be stored on the SIM, which can beretrieved at a later time. When the phone is notwithin coverage or the SIM is full, the SMSCwill hold the message and deliver it shortly afterthe phone comes back into range or there isspace in memory.

The SMS Basic Network ArchitectureThe main components of the SMS network archi-tecture are shown in Fig. 5.

When routing a mobile originated short mes-sage, the SMSC forwards the short message tothe SMS-GMSC. The SMS-GMSC interrogatesthe HLR for routing information and sends theshort message to the appropriate MSC. TheMSC delivers the short message to the MS. Onthe other hand, when routing a mobile terminat-ed short message, the MS addresses the requiredSMSC according to its global title. If roamingabroad the visited public limited mobile network(PLMN) will route the short message to theappropriate SMS-IWMSC.

The SMSC identifies each short message unique-ly by adding a time stamp in the SMS-DELIVERTP-SCTS field. The short message arrival at theSMSC is accurate to the second. It is the SMSC’sresponsibility to assure that if two or more short

message arrive within the same second their time-stamps will bedifferent.

The MS has to be able to receive/submit a short messageTPDU, and then return a delivery report upon successfulreception. It is also responsible for notifying the networkwhen it has memory capacity available to receive one or moremessages, if it had previously rejected a short message becauseits memory capacity was exceeded.

Protocol ArchitectureThe protocol layer for SMS is shown in Fig. 6. The short mes-sage transfer layer (SM-TL) services the short message appli-cation layer (SM-AL) and enables it to exchange shortmessages with a peer as well as receive confirmation of recep-tion reports from earlier requests.


� Figure 4. The SCCP and MTP sublayers.

SCCP

MTPlevel 3

MTPlevel 2

MTPlevel 1

SCCP connection-oriented control

SCCProuting control

SCCP connectionlesscontrol

Signaling messagehandling

Signaling networkhandling

TCAP/ISUP/TUP

SCCPmanagement

MTPmanagement

Signaling linkfunctions

Signaling data linkfunctions

� Figure 5. The SMS network architecture.

SMSC SMS-GMSC /SMS-IWMSC

MSC

HLR VLR

MS

� Table 1. TPDU types.

SMS-Deliver Conveying a short message from the SMSC to the MS

SMS-Deliver-Report Conveying a failure cause

SMS-Submit Conveying a short message from the MS to the SMSC

SMS-Submit-Report Conveying a failure cause

SMS-Status-Report Conveying a status report from the SMSC to the MS

SMS-Command Conveying a command from the MS to the SMSC

19

The SM-TL exchanges PDUs with its peer entity. Theshort message relay layer (SM-RL) conveys the PDUs via theshort message link layer (SM-LL). Refer to GSM 03.40 [2] forfurther details.

SMS Protocol Data Unit TypesThere are six types of TPDU at the SM-TL, as listed in Table1. The elements of the SMS-Deliver and SMS-Submit TPDUare shown in Fig. 7 [2]. The main fields of the TPDU aredescribed in this document however for a complete descrip-tion of the TPDU please refer to GSM 03.40 [2].

TP-Data-Coding-SchemeThe data coding scheme field (TP-DCS) is used to iden-tify the coding scheme used by the user data, which canbe 7- or 8-bit or even Unicode [6], as defined in GSM03.38 [7].

TP-Validity-PeriodThe TP-VP field contains an information elementenabling an MS to specify a validity period for the shortmessage it is submitting. The value specifies how long anSMSC will guarantee the existence of a short messagebefore delivery to the recipient has been carried out.

TP-More-Message-To-SendThe SMSC uses the TP-MMS field to inform the MS thatone or more short messages are waiting to be delivered.

TP-User-Data-Header-IndicatorThe 1-bit TP-UDHI field indicates whether the TP-UDincludes an additional header as well as the short message.

TP-Protocol IdentifierThe TP-PID is used by the MS or SMSC to identify thehigher-layer protocol being used for internetworkingwith a certain type of telematic device (Telefax group 3or 4, Ermes, etc.)

TP-User-Data (TP-UD)The TP-UD field is used to carry the short message. Itcan store up to 140 octets of data for point-to-point SMS,together with a header depending on the setting of theTP-UDHI field. The amount of space taken by the headerreduces the amount of data the PDU can carry. Figure 8shows a representation of the layout of the TP-UD for 7-and 8-bit data schemes.

The header has at least three fields. The first field,the information element identifier, is used to identifyconcatenated short messages. Information data length(IDL) is used to indicate the length of the information

element data (IED) that follows. Each ofthese fields is 1 octet long.

In the user data, the message can be 7 bits,8 bits, or 16 bits. If 7-bit data is used and theheader does not end on a 7-bit boundary,padding bits are used. This is to ensure thatolder mobiles which do not support the TP-UDheader can still display the message properly.

Using the IEI allows sending and receivingof concatenated short messages. The IED fieldcontains all the necessary information for thereceiving entity to reassemble the messages inthe correct order, and is coded as follows:• First octet: short message reference num-

ber identifying the message within the sametransaction

• Second octet: specifies the maximum number of short mes-sages in the concatenated short message, which will notexceed 255

• Third octet: identifies the sequence number of the shortmessage within the concatenated messageThe minimum header length for concatenated message is 7

octets for 8-bit and 16-bit data and 8 for 7-bit data; leaving 133(140 – 7), 152 (160 – 8), and 66 ((140 – 7)/2) characters for theshort message. The maximum length of the message is thenincreased to 38,760 (255*152), 33,915 (255*133), or 16,830(255*66) depending on the character coding scheme used.


� Figure 6. The protocol layer for SMS point-to-point.

SMSC MSC MS

SM-RL SM-RLSM-RL SM-RL

SM-LL SM-LLSM-LL SM-LL

SM-AL SM-AL

SM-TL SM-TL

SMS-GMSC /SMS-IWMSC

� Figure 7. An SMS TL-PDU.

TP-message-type-indicator

TP-reject-duplicate

TP-validity-period format

TP-reply-path

TP-user-data-header-indicator

TP-message reference

TP-destination-address

TP-protocol-ID

TP-data-coding-scheme

TP-validity-period

TP-user-data-length

TP-user-data

SMS-submit

TP-message-type-indicator

TP-more-message-to-send

TP-reply-path

TP-user-data-header-indicator

TP-status-report

TP-originating-address

TP-protocol-ID

TP-data-coding-scheme

TP-service-center-time-stamp

TP-user-data-length

TP-user-data

SMS-deliver

20

Short Message Routing Considerations

In Fig. 9, user A in network is sending a short message touser B in network roaming in network . User A is usingthe SMSC in network to submit his short message [8].

The local cellular exchange routes the short message in anSCCP packet according to the SMSC global title as defined bythe E.164 numbering plan [9]. The SCCP packet is forwardedfrom exchange to exchange until it reaches the destinationSMSC (1). The routing has to be set up in all the SCCP switchesalong the route for the message to successfully reach the SMSCin network .

Once the SCCP packet carrying the message arrives at thedestination SMSC, a confirmation message is sent back to thehandset using another SCCP packet (2).

To deliver the short message to user B, the SMSC has toaccess the HLR database of his home network. A locationrequest SCCP packet, based on user B’s mobile number, issent by the SMSC (3).

This international SCCP network then routes the locationrequest SCCP packet to the appropriate HLR. When theHLR receives the request, it will return the location informa-tion in another SCCP packet to the SMSC (4).

The SMSC then sends the message to the VMSC of userB, based on the information received from the HLR (5).Finally, this VMSC interrogates the VLR (6, 7), and deliversthe message to user B (8). Upon successful delivery a confir-mation SCCP packet is sent back to the SMSC (9).

Throughout these routing procedures, the SCCP packetscan get lost if one of the cellular exchanges along the routedoes not know where to forward the SCCP packet. SCCProuting is based on the global title used for switches and theSMSC. The routing information has to be in place in theinternational SCCP transit switches for the messages to suc-cessfully reach their destination. Some international switchesonly check the country code prefix (e.g., 44 for the UnitedKingdom) and forward the packet to the next exchange, whileothers also check for the network prefix (e.g., 447976 forOrange). If the exchange routing table does not include allthe prefixes allocated to the subscribers, some messages willbe rejected. Incompatible implementation of the SMSC can

also lead to the short message not being understood andbeing rejected. All the above-mentioned problems can lead topackets getting lost along the way with different conse-quences:• A negative acknowledgment received by the sender (phone

displaying “message failed” or a similar message) althoughthe short message reaches its destination (loss of packet )

• Reception of duplicate short messages by user B (loss ofpacket or ); could also be due to the timeout valuebeing set too low in the SMSC

• Or, in the worst case the message might not be delivered atall (loss of packet , , , )Even if Fig. 9 shows the most complicated routing scenario

there is still much that can go wrong with short message rout-ing, and a lot of research is currently underway to overcomethese.

Protocols forShort Message Submission

The European Telecommunications Standards Institute(ETSI) specified a protocol for short message submission aspart of the overall GSM standard [10]. This specificationdefines three interface protocols for the transfer of SMS shortmessages between an MS and terminal equipment (TE) via anasynchronous interface. The protocols clearly overlap in func-tions, and it is not clear why three have been defined.

Block ModeThe block mode is a binary protocol which encapsulates theSMS PDU used for short message transfer between an MSand the SMSC defined in GSM 03.40 [2]. This protocolincludes error detection and is suitable for use where thelink between the application and the phone is subject toerrors. It will be of particular use where control of remotedevices is required. The application has to construct a bina-ry string including a header and the short message PDU(SMS-TPDU).

Once the application has requested the phone to enterblock mode a group of functions is available:

5431

95

1

1

1

43

1


� Figure 8. SMS-TPDU formats for 7-bit and 8-bit data content.

UDL: User data length — 1 octetUDHL: User data header length — 1 octetIEIx: Information element identifier x — 1 octetIELx: Information element length x — 1 octetIEDx: Information element data x — 1 to n

Octets Octets

UDL UDHL IEIa IELa IEDa IEIb … IEIn Padding SM (7-bit data)

Septet boundaryTotal number of octets

Total number of septetsLength indicator

Length indicator

UDL UDHL IEIa IELa IEDa IEIb … IEIn SM (8-bit data)

Octet boundaryTotal number of octets

Total number of octetsLength indicator

Length indicator

21

• Submit a short message• Delete messages from the phone• List messages in the phone• Transfer all messages or one from the phone to the application• Set the phone so that the application is notified every time

a new short message is receivedEach of these commands contains a number of predefined

elements as described in the specification. For example, theInsert Message command format used to submit a short mes-sage is depicted in Table 2.

Text ModeText mode is a character-based protocol based on the ATcommand set modified for GSM. This mode is suitable forunintelligent terminals or terminal emulators, and for applica-tion software built on command structures like those definedin ITU V25ter. The application passes the message in plaintext to the phone that constructs the TPDU (Table 3). Thismeans that text mode offers a lot less functionality than block

or PDU mode. The text mode does not support or automati-cally pass incoming messages to the application (only notify it).

PDU ModePDU mode is very similar to text mode, except that it leaves tothe application the responsibility to build the short messageTPDU. This mode adds to the convenience of the AT commandset the possibility to construct more sophisticated PDUs (i.e.,allowing binary data to be transmitted, not just characters).

SEMA SMS2000Sema Group Telecoms developed SMS2000 as an implemen-tation of a GSM SMSC [11]. The specification mainlydescribes the delivery of short messages to MSs, but also spec-ifies the protocols for short message submission. The protocolhas been designed to operate over a variety of interfaces suchas X25, DECnet, and SS7. The SMS2000 SMSC is usuallyaccessed via the general X25 access gateway -either using aradio Packet Assembler Disassembler (PAD) or a dedicated

link to the message center.Once connected to the SMSC, an SME can

request any of the operations listed in Table 4. TheSMS2000 SMSC can also send the commands listedin Table 5 to an SME.


� Figure 9. International short message routing.

SMSCSMS-IWMSC

LocalSCCP switch

InternationalSCCP switch


Foreign SMSCnetwork

(x): Message number

(1)

BSC/BSCBSC/BSC VMSC

Originatornetwork

Recipientvisited network

User BUserA

(2) (5)

1

2

4

3

LocalSCCP switch

Recipienthome network

(1)

(4)

(9)(5)

(3)(2)

(1)

(4)

(9)(5)

(3)(4)

(9)(5)

(3)(4)(3)

(4)(3)

(2)HLR

VLR

LocalSCCP switch


(9)


LocalSCCP switch

(2)(1)

(2)(1)

(2)(1)

(9)(5)

(9)(5)

(9)(8)

(6)(7)

� Table 2. Short message submission using block mode.

Message type Insert SMS type: the value defined 1in the specification is 0x07

Insert type 1. Store in phone 12. Send, or3. Store and send

RP-dest. address Address of recipient as defined 1–12by GSM 04.01

SMS-TPDU As defined by GSM 03.40 Max. 140

Information element Meaning Length byte

� Table 3. Short message submission using text mode.

AT+CMGS=”44976123456”<CR> Send a message to 44976123456This is a text message<CR>^Z

+CMGS=3 Message accepted by the phone withOK a reference number 3

� Figure 10. Short message submission usingSMS2000.

SME

Submit SM

Invoke (submit SM)

SM submission status

SM delivery status report

SMSC

22

A transaction between the SME and the SMSC involvesone party sending a request with a status report sent back oncompletion or failure of the request. Figure 10 depicts thesubmission of a short message from an SME to the SMS2000SMSC.

The transaction is initiated by the SME when a Submit SMinvoke is sent to the SMSC. The SMSC responds with a resultmessage indicating that the short message has been acceptedand is being processed. Upon delivery the SMSC notifies theSME (if a status report has been requested). The SME thenacknowledges the SR, thus completing the transaction.

Since the SMEs connected to the SMS2000 SMSC areassumed to be trusted systems, a basic transaction will notinclude any exchange of login and password between the SMEand the SMSC. However a login facility is still provided inorder to access the SMSC from a different location (i.e., PAD).

Text-Based ProtocolsUsually these protocols are proprietary and developed as aninterface to the SMSC of a digital cellular network operator.The advantage of a text-based protocol is that the user doesnot need any special client software to submit a short mes-sage; they can dial the appropriate message center using anyterminal emulation software and submit short messagesusing the different options offered. Figure 11 describes thesubmission of a short message using the Telenote-text basedprotocol.

There are, however, key disadvantages with text-based pro-tocols: they offer limited support for extended character sets,and only work one way, to name a few. The user is only ableto send messages and receive confirmation of submission. TheSMSC is unable to notify the end user of successful delivery.

Telocator Alphanumeric Protocol

Developed by Telecom Securicor Cellular Radio Limited, theTelocator Alphanumeric Protocol [12] provides greater flexi-bility and more features than text-based protocols. The overallperformance is also significantly more efficient.

In its fully featured implementation, the protocol allowsthe user to perform the following operations:1. Submit a short message and receive confirmation of accep-

tance.2. Submit a short message and receive status of the first deliv-

ery attempt.3. Query the current status of a message submitted by 1 or 2.4. Delete a message submitted by 1 or 2.5. Replace a message submitted by 1 or 2, unless the message

has already been delivered to the mobile.6. Update a message submitted by 1 or 2. If the message is

still in the SMSC. it is replaced; otherwise, a new messageis sent to the mobile.

TAP is a session-based protocol, as opposedto a permanently connected one. Each sessioncomprises a logon, a number of transactions, anda logoff, as shown on Fig. 12.

Other Submission ProtocolsMany others protocols have been designed andoperate over a wide range of hardware interfaces(UCP, CIMD, SMPP, etc.). Most of the protocolscan be classified as dumb or smart based onwhether they provide notification of deliveryand/or advanced functionality (message deletion,replacement, etc.). We chose to discuss the pro-tocols used in the United Kingdom as an exampleof the different possibilities offered to subscribersand software developers.

Current IssuesSMS is increasingly popular; the barrier of 100million messages in a month was recently reachedin the United Kingdom. As growing numbers ofusers start to realize that GSM has more to offerthan crystal clear voice calls, the services offeredare likely to evolve even more rapidly, and it isquite likely that data calls and short messages willsoon represent an important part of the overallGSM traffic.

New protocols are now being designed thatwill allow even more advanced functionality to beimplemented in client software. Some of the fea-tures under development include:• Support for message concatenation to allow up

to 255 messages to be submitted as part of the


� Table 4. SMS2000 commands.

Submit SM Send an SM to an MS

Delete SM Delete a previously submitted SM

Replace SM Replace a previously submitted SM to an MS.

Delete all SM Delete all previously submitted and undelivered SM to an MS

Enquire SM Request status of a previously submitted SM

Cancel SRR Cancel all status report requests (SRR) about a previously submitted SM.

Alert SME request Request to be alerted when a specified SME becomes registered.

Retrieve Request Request transmission from the SMS2000 SMSC of any pending SM or SR.

Login For X25 general access when accessing from a different location

Change Password For X25 general access when accessing from a different location

� Table 5. Additional SMS2000 commands.

Alert SME Indicates a MS has registered with the GSM network

Status Report Indicates successful delivery or failure of a previously submitted SM.

Incoming SM Indicates an incoming SM is being held by the SMS2000 SMSC

� Figure 11. Short message submission using Telenote.

PLEASE ENTER THE MOBILE NUMBER ININTERNATIONAL FORMAT, E.G.44385XXXXXX, FOLLOWED BY RETURN, (ORRETURN TO QUIT).

>44374164828

PLEASE TYPE YOUR TELENOTE, (MAXIMUM160 CHARACTERS), FOLLOWED BY RETURN.

>This is a test message

MESSAGE ACCEPTED

23

same transaction• Support for distribution list creation and modification with-

in the SMSC to greatly increase short message throughputLast but not least, some research projects are studying the pos-

sibility of using SMS as an alternative layer to TCP/IP, thusadding mobility to low-bandwidth applications. Intel’s NarrowBand Socket specification first tried to define how applicationscould benefit by using SMS as an alternative to TCP/IP; however,this work is now in the process of being superseded by the Wire-less Application Protocol (WAP).

References[1] ETSI GSM 3.41, “Digital Cellular Telecommunication System (Phase 2);

Technical Realisation of Short Message Service Cell Broadcast (SMSCB),”v. 5.2.0, May. 1996.

[2] ETSI GSM 3.40, “Digital Cellular Telecommunications System (Phase 2+)Technical Realisation of the Short Message Service Point-to-Point,” v.4.13.0, May. 1996.

[3] J. Scourias, “A Brief Overview of GSM,” Univ. of Waterloo; http://ccnga/uwaterloo.ca/~jscouria/GSM/gsmreport.html

[4] M. Mouly and M. Pautet, “The GSM System for Mobile Communica-tions,” 1992.

[5] W. Roth, “Data Service on the GSM Platform,” GSM Summit HongKong, Mar. 1993.

[6] ISO/IEC10646, “Universal Multiple Octet Coded Character Set (USC),UCS2, 16 Bit Coding.”

[7] ETSI GSM 3.38, “Digital Cellular Telecommunications System (Phase 2+):Alphabets and language-specific information,” v. 5.2, May. 1996.

[8] K. Holley, http:/ftp.labs.bt.com/people/holleyka[9] CCITT E.164, “Numbering Plan of the International Telephone Service,”

v. 5, 1997.[10] GSM 07.05, “Digital Cellular Telecommunications System (Phase 2);

Use of Data Terminal Equipment — Data Circuit terminating; Equip-ment (DTE - DCE) interface for Short Message Service (SMS) and CellBroadcast Service (CBS),” draft, May 1996.

[11] SEMA Group Telecommunications, “SMS2000 v. 4.0, Open InterfaceSpecification,” INS/FS/28.

[12] Telocator Alphanumeric Protocol (PCIA) v. 1.2 Functional Spec for TAP-AIM ver 2.6 (Aldiscon)

Additional Reading[1] M. Rahnema, “Overview of the GSM System and Protocol Architecture,”

IEEE Commun. Mag., vol. 3, no. 4, Apr. 1993, pp. 92–100.

BiographiesGUILLAUME PEERSMAN ([email protected]) graduated from the Insti-tut Superieur d’Electronique de Paris (ISEP) in 1996 with a double M.Eng.in electronics and computer networks. He then joined the University ofSheffield, and is currently reading for a Ph.D. degree in the Computer Sci-ence Department. His research interests focus on the development and per-formance analysis of two-way messaging gateways for the GSM ShortMessage Service. He also recently extended his field of research to theWireless Application Protocol gateway design.

SRBA R. CVETKOVIC ([email protected]) completed his B.Sc. (withFirst Class Honours) and Ph.D. degrees in electronic and electrical engineer-ing at City University, London (1983) and University College London (1987),respectively. In June 1987, he joined the Department of Electronic and Elec-trical Engineering at the University of Surrey as a lecturer in telecommuni-cations and satell ite systems. In September 1992 he moved to theDepartment of Electrical Engineering and Electronics, Brunel University,West London, to a senior lectureship in data communications. In Septem-ber 1995 he took up the post of senior lecturer in multimedia communica-tions systems in the Department of Computer Science at the University ofSheffield. In January 1996 he established the Centre for Research, Educationand Development Online (CREDO). He now leads the Research Group in Com-munications and Distributed Systems (CDSRG), which he co-founded in Octo-ber 1997 with Prof. Colin Smythe. CDSRG (including CREDO) is currentlyinvolved in projects of value in excess of US$8.5million and has over 35 full-time members of academic, research, and support staff. In October 1997 hewas promoted to reader (in telematics) and in June 1999 to a personal chairin mobile systems, a joint position between the Departments of Electronic andElectrical Engineering, and Computer Science. His research interests includemultimedia/broadband communication systems, satellite communications andDSP systems, as well as numerical modeling and measurements of electro-magnetic fields. More recently, his focus has been on protocols for supportingmobile systems (Mobile IPv6, GSM/SMS) and knowledge management. Forfurther information visit http://www.dcs.shef.ac.uk/~srba

PAUL GRIFFITHS ([email protected]) graduated from Sheffield City Poly-technic in 1986 with a B.Sc. Hons. in business studies. He then joinedFretwell Downing Data Systems as a software developer where he worked onvarious projects between 1998 and 1991. Between 1991 and 1994 heworked on secondment with Sheffield Hallam University to conduct researchinto the development of GUI environments. In 1994 he helped to found Dia-logue Communications Limited who developed a product for GSM shortmessaging called pagemail which has since sold over 500,000 copies. He hassince been involved with both business and technical aspects of developingserver products for GSM SMS. He is now co-director of Dialogue Communica-tions Ltd., a specialist SMS software manufacturer which has recorded anaverage growth rate of 70 percent for the last three years.

HUGH SPEAR ([email protected]) received an M.Sc. with distinction inSoftware Systems Technology from Sheffield University in 1991. Between1991 and 1995 he worked as a systems analyst and project leader withFretwell-Downing Data Systems in Sheffield. In 1995 he co-founded Dia-logue Communications Limited, a mobile messaging business which devel-ops and markets products and services for messaging over GSM SMSandpaging services. By the end of 1999 Dialogue had over 250,000 customersand products in over 20 countries.


� Figure 12. Short message submission using TAP.

<CR>

SME

<ESC>PG1<Password><CR>

<STX><MISDN><CR><Message><CR><ETX><Checksum<CR>

ID=

<CR><ACK><CR>

<ESC>[p<CR>

<Message response><CR><ACK><CR>

SMSC

Sms Protocols Overview

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