IEEE Paper Template in A4 (V1)

International Conference on IEEE, Advances in ICT for Emerging Regions (ICTer), Sri Lanka, ISBN 978-1-4673-5529-2

Concealed Data in Two Way Text Messaging: A Framework for Interactive Mobile Learning Environment H.K. Salinda Premadasa1, R.G.N. Meegama2 1

Centre for Computer Studies, Sabaragamuwa University of Sri Lanka Belihuloya, Sri Lanka [email protected]

2

Department of Statistics and Computer Science, Faculty of Applied Sciences, University of Sri Jayewardenepura Gangodawila, Nugegoda, Sri Lanka [email protected]

Abstract— A major limitation in two-way texting is sending back a part of received data with the reply message. This limitation results in users of a mobile learning environment being unable to reply back to the correct destination as the mobile communication gateway handles sending out information of several users through a single phone number. In this paper, we propose an open-source and secure messaging system that can be easily integrated to a learning management system (LMS) to provide an interactive learning experience to the user community. Initially, a database is integrated into the LMS that holds message information such as recipient’s phone number, message body, user data header (UDH), etc. A specific port associated with short messaging service (SMS) is used to send data relevant to a particular course unit concealed in the SMS body itself. Subsequently, software installed in the user’s mobile device captures this SMS and sends back the reply message to the appropriate course unit allowing both teachers and students to view messages sent and replied pertaining to a particular course unit. Results indicate the relevance and interoperability of the proposed technique. Keywords— SMS, UDH, PDU, LMS, concealed data

I. INTRODUCTION Short messaging service (SMS) is one of the best qualitative mediating tools adopted in the arena of mobile communication pedagogically, economically and technologically [1]. Recent reports support this claim in that more than 6.6 trillion SMSs were sent out over carrier networks worldwide in the year 2010 [2]. The use of mobile technologies for distance learning has become a popular tool in higher education across the globe due to its ever-increasing demand for student-centric learning. In this scenario, an SMS system can be established for university students as a supportive tool to capture students‘ retention in the existing LMS that provides a blended learning environment [3]. The teacher and the student are crucial entities in the learning process and moreover, supporting students in teaching is considered as an indirect form of supporting a student‘s learning ability [4]. A majority of college students use a mobile phone for routine voice and SMS communication. Consequently, short messaging service can be used as a

trusted technology to form a better learning environment having a huge potential in higher education with active (sending a message) and passive (reading a message) interactions [1, 5]. In mobile telephony, the Short Message Service Centre (SMSC) is the intermediate controlling unit in both parties (sender and receiver) when sending and receiving SMSs [23]. The mobile phone is handled by an inbuilt GSM (Global System for Mobile Communication) modem controlled by AT (ATTENTION) commands to pursue text messaging [6, 7]. As illustrated in Figure 1, this inbuilt modem also functions as a mediating unit between the GSM network and the processor of the mobile phone. A message received by the phone from outside is delivered to the operating system through the telephony stack, the software component that provides communication between the application processor and a modem. This telephony stack is comprised of a multiplexing layer, located at the bottom of the stack, which allows several applications to execute at the same time. While the upper layers of this telephony stack are used to place and answer voice calls and text messages, the multiplexing layer performs translation of API calls (Application Program Interface) to AT commands [8].

Fig. 1. Logical architecture of the mobile phone

Premadasa, H. K. S. and Meegama, R. G. N. “Concealed Data in Two Way Text Messaging: A Framework for Interactive Mobile Learning Environment”, International Conference on IEEE, Advances in ICT for Emerging Regions (ICTer), Sri Lanka, pp. 122-127, 2012.

A mobile platform, referred to as University Mobile Learning Environment (UMLE) [9] has the capability to offer its services to a larger segment of users than existing webbased learning environments via mobile devices. Although there are number of web-based paid SMS services available in the Internet at present that facilitate sending secure messages through an LMS, such paid services restrict the number outgoing messages as well as the number of users [10, 11]. Teachers who work with this LMS can send messages to a relevant course unit so that students who are currently logged in will receive this message on their mobile devices. This two-way interaction is complete if the student who receives a message relevant to a particular course is able to send back a reply to the same course unit from where the message was originated. A standard SMS does not fully support two-way texting when there is a necessity to include a part of the message received in the reply body. This has hampered two-way communication within a mobile community when one entity uses a mobile gateway to send out messages using a unique GSM number shared by all the users. In this paper, we propose a two-way text messaging system by concealing data in the message body using the UDH while an encoding and a decoding technique in the mobile phone is used in the background to support such a texting mechanism [12]. The proposed system is a combination of Java 2 Micro Edition (J2ME) [13, 14] and Personal Home Page (PHP) technologies. The reason for using such emerging technologies is that the PHP and J2ME combination facilitates integrating the proposed framework to a widely used LMS (such as Moodle). II. THEORETICAL FRAMEWORK It is traditionally known that an SMSC performs a ―store and forward‖ mechanism for sending and receiving messages [15]. The mobile originated (MO) SMS is transported from a mobile station to the SMSC where these MO-SMSs may be destined to other mobile stations or other services. In contrast, the mobile terminated (MT) SMS is transported from an SMSC to a mobile station. Two approaches, namely, PDU (Protocol Description Unit) and Text can be applied to send and receive SMSs using AT commands. The text mode (unavailable on some phones) is just an encoding of the bit stream represented by the PDU mode [16]. A majority of network services such as Wireless Application Protocol (WAP), Multimedia Messaging Service (MMS), mobile banking and Over-the-Air (OTA) makes use of the PDU messaging standard. Because this PDU mode is used to encode the SMS header and user data (payload) in hexadecimal or decimal semi-octet format, it is possible not only to manipulate the fields of an SMS header but also to modify the contents of user data. The MO-SMS is transferred from a mobile phone to SMSC by composing it with SMSSUBMIT PDU format and similarly, an MT-SMS is received at a mobile phone in SMS-DELIVER PDU format from the SMSC [17].

A. SMS-SUBMIT Protocol Description Unit (PDU) As depicted in Figure 2, the SMS-SUBMIT PDU can be expressed as with Service Centre Address (SCA) and the rest of Transport Protocol Data Unit (TPDU). The PDU format is a hexadecimal encoded binary format, which means that two hexadecimal digits represent a byte of data. A header with control information and user data make up a TPDU [18]. As some mobile phones do not supported the PDU format but only the TPDU format, the SCA is removed from the PDU and stored in the SIM card by the network operator. This information can be modified using AT commands and hence, the default SCA information (from the SIM card) can be used to implement SMS-SUBMIT to insert any type of SCA supported by the network operator. As a result of this implementation, the SCA part of the PDU can be replaced with a 00 Octet.

Fig. 2. SMS-SUBMIT protocol description unit (PDU)

B. SMS-DELIVER Protocol Description Unit (PDU) The mobile phone receives an SMS-DELIVER PDU from the SMSC as a Mobile Terminated (MT) message. For this architecture, the same method as described above performs both SMS-DELIVER and SMS-SUBMIT type messages [15]. The Data values contained in an SMS-SUBMIT PDU (with SCA) are represented in Figure 3.

Fig. 3. Data values within an SMS-SUBMIT protocol description unit (PDU)

Various fields that are associated with this SMS-SUBMIT PDU are shown in Table 1 and Table 2. These concepts allow us to understand the theoretical background of generating and sending an SMS [21]. C. User Data Header (UDH) As illustrated in Figure 4, the UDH is divided into two parts, the first part, which is only one byte in length, is identified as a User Data Header Length (UDHL) of the UDH. Following this UDHL, an Information Elements (IE) instructs the mobile phone to perform some action based on the information received. The most common Information Element Identifiers (IEI) are Port Number Addressing (representing byte 05) and Message Concatenation (representing byte 00). Each port number is addressed to a specific port in the phone that


TABLE I SMS-SUBMIT PROTOCOL DESCRIPTION UNIT (PDU) FIELDS

SMS-OCTETS

Field Name

06 91 49 17 00 00 F3 41

Address Length Type of address Address value First octet of SMS-SUBMIT

Length (bytes) 1 1 variable 1

00

TP-MR

1

0A 81 70 41 69 98 87 00 00

Address Length Type of address Address value TP-PID TP-DCS

1 1 Variable 1 1

A7

TP-VP

1

15 06 (05) 05 04 C350 0000 (00 03 01 02 01) 54747A0E4ACF4 16110945805B5C BF379F85C06

TP-UDL TP-UDHL

1 1

number of octets to follow SMSC number format (international format) address of an SMSC (947100003) shown in a Table 2 ID assigned to a SMS in SMS-SUBMIT format by a local GSM modem number of digits in the destination mobile number destination phone number format (national format) destination phone number (0714968978) informs about networking protocol and nature of SMS data user data encoding scheme validity period (amount of time that the SCA will hold submitted SMS if the destination address is unreachable size of the user data (payload) user data header length (Depicts in a Table 3)

TP-UDH

Variable

user data header (Depicts in a Table 3)

TP-UD

Variable

user data (PDU message), 19 octets = 21characters (TP-UD can be up to 140 octets = 160 characters maximum)

Description

TABLE II FIRST OCTET OF SMS-SUBMIT PROTOCOL DESCRIPTION UNIT (41 OCTET)

Binary representation Bit number

7

6

Fields

TP-RP (Reply Path)

TP-UDHI (User Data Header Indicator)

TP-SRR (Status Report Request)

TP-VPF (Validity Period Format)

Description

tells SMSC to route the reply of a SMS on the same path

indicates whether the user data contains optional headers

acknowledgement of the receiving device

specifies the format of Validity Period (VP) field

Reserved bits Value Value identification

1 0 reply path is not set

1 1 User Data Header (UDH) turned on

5

4

1 0

2 0

status report not requested

provides clear instruction on putting together two or more SMS messages to make a single message during concatenation of several SMSs. The second byte of the IE is the Information Element Data Length (IEDL) that tells the phone how many of the following bytes are part of the Information Identifier. The subsequent bytes of the IE are the actual information being conveyed and is referred to as the Information Element Data (IED) [15, 17]. D. Port Number Addressing and Concatenated SMSs Port number addressing can be used to deliver a particular SMS to an application running on a mobile phone where in most cases, these applications are developed using Java technology (J2ME). Such an application with a port

3

no validity period

0

2

1

0

TP-RD (Reject Duplicate)

TP-MTI (Message Type Indicator)

informs to SMSC to reject duplicate messages 1 0 duplicates accepted by SMSC

informs SMSC that the SMS is in SMS-SUBMIT or SMS-DELIVER PDU format 2 0 1 SMS SUBMIT

addressing technique allows us to specify a source port and a destination port number for a particular SMS message in the UDH resulting in port number information being transmitted through the UDH of the SMS message [22]. The concatenated SMSs (CSMS) are completely related to the User Data Header (UDH), a collection of bytes that can be placed at the beginning of an SMS [15]. To indicate that the SMS-SUBMIT PDU contains a UDH, a flag on the SMS, called TP-UDHI (shown as the 6th bit in Table 2), must be turned on. This signal instructs the mobile phone that it must separate the UDH from the rest of the PDU. For the CSMS, the UDHL (06 octets) specifies that the remaining six bytes are for the UDH. The IEI points out that the rest of the SMS is a CSMS (00 represents 8 bits message


in each SMS fragment. The total number of SMS fragments and current fragment number of the SMS must be indicated accordingly in the UDH-IED for each SMS fragment. This is achieved by setting up both fields to 02 and 01 consecutively (total number of SMS fragments and the fragment number of the current SMS fragment) allowing the receiving device to recognize that the concatenated SMS consists of just one fragment which is the current actual SMS. Table 3 illustrates the fields of the UDH with port number addressing or message concatenation [15, 17].

Fig. 4. The architecture of the SMS with User Data Header (UDH)

reference number of CSMS) while the Information Element Data Length (IEDL) indicates the number of bytes (03) containing the IED. The first byte (01) is allocated to CSMS reference number, next byte (02) is for the total number of fragments of the CSMS and the final byte (01) is for the number of currently received fragment of the CSMS. TABLE III FIELDS OF USER DATA HEADER (UDH)

Fields

01

Description User Data Header Length UDHL(06 octets contained in this UDH) IEI: Application Port Addressing Scheme (16 bit address) Information Element Data Length (IEDL): Indicates the number of fields in UDH Mobile Handset Destination Port (Port 50000) Originating Port (Port 0) User Data Header Length UDHL(05 octets contained in this UDH) IEI: Concatenated SMS 8-bit (IEDL: Indicates the number of fields in UDH Concatenated message reference

02

Total message parts

01

Message part number

06 05 04 C350 0000 05 00 03

Port Number Addressing

III. SYSTEM FUNCTION AND ARCHITECTURE As illustrated in Figure 5, the proposed system enables a teacher to post messages to students to establish a discussion forum via a mobile browser interface through an Internetenabled mobile device. Once a teacher logs in to the system, a message relevant to a particular course (each course has a course code) is created with utmost 153 characters (this is a standard characters length for single text message with a UDH). Thereafter, this message is inserted into a database where an SMS daemon checks the database periodically and sends out stored messages through an SMS gateway to a GSM modem. Using ‗AT‘ commands, the modem in turn sends out SMS messages to all the recipients with access to that particular course unit through the SMSC. The SMS received by a user contains the course code generated automatically by the system. This course code information is extremely important to the recipient to identify from which course the message originated from as several courses are available in the LMS and moreover, a user may receive multiple messages from each course unit. Subsequently, the recipient is able to send a reply to the relevant course unit. These reply messages are initially delivered to a database to be categorized depending on the course unit (course code) by the system. Finally, an authorized user (teacher or student) can explore all the messages posted relevant to the particular course unit.

Message Concatenation

When transmitting a long SMS (more than 160 characters as user data) through a GSM network, the SMS is split into fragments of SMSs (153 characters for user data and the rest of 7 characters for UDH in each part of the SMS) [19]. Even though, these fragmented SMSs are concatenated as a single SMS in the recipient‘s mobile phone, the recipient is unable to recognize this back-end functionality. According to CSMS concept, the reference number of the UDH-IED must be same

Fig. 5. The architecture of the proposed system


The theoretical concept behind this system functionality is built upon the convention of Port Number Addressing and the UDH of the SMS PDU. In the standard format, in contrast to an email message, an SMS cannot enclose the full or part of the original text message in the reply message. The mobile device only recognizes the sender‘s phone number which is displayed in the reply SMS. As such, it is required to wrap information about the course code within the SMS and this piece of information should come back to the system with the reply SMS to be categorized into different course units. The port number addressing technique with a destination port number of the UDH can be used to perform this concealment of additional data in the SMS. Such an SMS is not directly saved in the standard INBOX of the mobile device. Rather, the mobile application software installed in the recipient‘s phone extracts this SMS having a specially defined destination port number. Course unit details (say ML 001) are added automatically by the system to the body of the SMS to be identified at the recipient‘s end. Based on that course unit, a news item, titled ―News Forum Started‖, can be expressed as in Figure 6 where the SMS-SUBMIT PDU converts an SMS to a PDU format using the Port Number Addressing (destination port 50000 is converted to C350 in Hex). This will be the PDU format of the SMS that is sent from the database by the SMS daemon. Details pertaining to the course unit are compared with the database to acquire a unique course code of the relevant course unit. While this function occurs in the background, the course code is appended to the message body by the system. At the next stage, the complete SMS-SUBMIT PDU is sent to the SMSC to be forwarded to the recipient within a valid time period. The recipient then generates a reply SMS with the course unit details (i.e. ML 001) and sends it back to the database. This is done by the J2ME application installed in the mobile device which captures the incoming SMS and

acquires the course unit information (i.e. ML 001) contained in the SMS body. While the recipient is ready to create a reply message, the application software automatically conceals course unit information, securely in the background, to the reply SMS. The recipient, however, only sees the phone number of the SMS originator as well as the reply text and does not see course unit information.

Fig. 6. The theoretical concept behind the system functionality

In order to implement the proposed framework, it is essential to have mobile devices that support the UDH concept. When the mobile application is installed, another SMS INBOX, in addition to the standard inbox, appears in the menu. All SMSs originating from the proposed system are delivered to this newly created INBOX. Figure 7 depicts that the mobile phone interfaces of the functionality. IV. RESULTS AND DISCUSSION The proposed algorithm is tested with 100 SMSs sent by the system to a series of smart phones as well as basic phones (i.e. Nokia 5800, 6720, 3120, 6210) having connectivity to several GSM networks (GSM provider I – 38 SMSs, GSM provider II – 28 SMSs, GSM provider III – 20 SMSs and GSM provider IV – 14 SMSs). These SMSs are delivered continuously

Fig. 7. The mobile phone interfaces of the functionality


Fig. 8. Ddelivery (left) and delay times between SMS messages

within 342 seconds through the SMS gateway without any significant time variations and interruptions. As seen in Fig. 8, the minimum and maximum delay times between one SMS to another is 3 and 5 seconds, respectively. In addition, the results show that the number of occurrences of minimum delay times (3 seconds) is 63 while the number of occurrences of maximum delay times (5 seconds) is 7. Randomly selected (out of all learning stream i.e. science, commerce and arts) 100 of students were participated for the evaluation process. All of them (100%) were received SMSs sent by the system within the network maximum period. Subsequently the twoway communication was completed after receiving 100 of reply SMSs with concealed data by the database without any interruption. This result shows that the proposed technique can be utilized anywhere to perform this two-way texting by concealing data (i.e. SMS quizzes) The SMS transmission is normally performed over the non secure communication channel. However, the proposed mechanism is based on the UDH concept and functioning over the specially implemented mobile device software with port addressing technique. Hence the student wouldn‘t be able to hack the communication by modifying the UDH in any stage. Initially, the system was tested with ―Gnokii‖, an open source SMS gateways. However, this gateway crashed while inserting a large amount of SMSs (i.e. 100 SMSs) to the database. The reason is that Gnokii can hold only 9 pending SMSs in the database at any given time. Moreover, it utilizes only a single database table to store both to-be-sent and sent SMSs. Considering the technological aspects of the UDH, we concluded that Gnokii does not adequately support UDH texting. The ―Gammu‖ open source SMS gateway, on the other hand, accommodates UDH efficiently, maintains two database tables to store to-be-sent SMSs (Outbox table) and sent SMSs (SentItems table) and does not crash while inserting large amounts of SMSs into the queue. Due to these reasons, Gammu was selected as the SMS gateway installed in Fedora 12 operating system. It is observed that if the mobile phone is busy with another application, such as a phone call or camera, the SMSs do not get delivered to the INBOX specially created using J2ME

application. Instead, messages get delivered to the standard SMS inbox making it difficult to the recipient to create a reply message destined to the correct course code. This research should be extended to mitigate this factor to allow uninterrupted reception of short messages to the newly created INBOX. Furthermore, the J2ME application is allocated 160 characters (This is a single text message without UDH) for the reply SMS generated by the student. Hence, the student should be paid for a single SMS in this process and this would be a great opportunity to minimize the cost for this mechanism. However if the student needs to send more than single SMS to perform this news forum, he has an opportunity to do that. In such case the student needs to pay for the cost of SMSs which is sent by him. The other important fact is any kind of mobile phone which can be installed J2ME application would be able to use for this mechanism. This result shows that the interoperability of different kind of mobile phones usage. V. CONCLUSION In this paper, we have presented a novel SMS messaging technique, integrated into a learning management system, offering two-way texting. The proposed framework provides a more flexible and convenient way of passing messages between the users than a traditional web-based E-learning platform. The pilot survey carried out with 100 SMSs reveals performance of the system in sending messages with minimum delay. As statistical data [23] confirms extensive usage of mobile phones for texting, the proposed two-way texting mechanism will certainly provide an ideal opportunity not only in the education sector but also in other entities which require a reply message to be sent to a particular user out of a large number of users who share a single phone number.


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