Messaging on the move
By A. K. Vanwasi

SMS 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 offers

There 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 becoming 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 for message 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

Radio paging is the forerunner in the mobile data services space. In the non-GSM world MOBITEX and CDPD networks provide mobile data services to users.

'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 terminated messaging.

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 16 digits.

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

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 messaging

features provided by SMS.

MOBITEX

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 over 90 percent of America's urban business population.

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 countries.

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 US.

CDPD is based upon an extension of the existing IP technology. CDPD is designed to send and receive mobile data.

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 same manner.

The CDPD packet size is restricted to 2048 bytes of data. Longer size packets are fragmented into smaller size packets.

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 talking.

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 feature 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 displayed.

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 the service 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 a SME.

Service 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 within MSC.

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 vice versa.

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 the message 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 addressee.

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 case.

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 delivery.

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 users.

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 is discarded

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.

References Free Web-based SMS

www.mtnsms.com
www.hooya.com
www.voicestream.com
www.blueskyfrog.com

For GSM-related literature visit www.gsmworld.com
For literature on mobile data services visit www.gsmdata.com

Short 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 its health.

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.

Technical glossary

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 service, etc.

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 services.

Octet: A binary sequence comprising of eight consecutive bits.

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 operator portability.

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.