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  1. 1. Mobile Computer Communication Systems 11. MESSAGING SERVICES Objectives The learning objectives of this section are • Understand the history of messaging pre text messaging • Understand the current technology and usage for Short Messaging Service (SMS) • Understand the components within the cellular network which allows messaging and how these interact with the current infrastructure to allow roaming. • Be able to recognise the replacement technologies for SMS and understand the possible benefits that could be offered to the end user and the position within the market place. I N TR O DUC TI O N The Short Messaging Service (SMS) facilitates transference small amounts of text information between two mobile devices. The popularity of this system has resulted in it being referred to by differing names which are used interchangeably, i.e. Text Messaging and Simple Messaging Service. Usage of SMS was considered part of the GSM standard, therefore defined in the standard ETSI GSM 03.40. The underlying rationale instigating the development of SMS was that it would serve as a replacement, or alternative to the pager system in widespread use. However, the SMS system was considerably more successful than had been originally envisaged and thus has further developed into new methods of sending text information with varying levels of success. Consequently the improved systems offer additional services and uses not considered within the original specifications. Page 1 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  2. 2. Figure 1 Standard Pager (Wikipedia, 2006) TEC HN O L O G Y As highlighted prior SMS was originated as a potential replacement for the pager network. A number of companies operate the pager network within the UK, notably Vodafone and O2. This system has proved to be very successful, attracting large numbers of business and emergency services users. The basis underpinning the pager network was that in the event of a message being required to be sent to a user by phoning an operator and conveying the message verbally they would then convert it into text message format and subsequently send it onto the appropriate pager. If the recipient wanted to respond to this message they would then find a phone and call the person back. The system encompasses many positive aspects, for example the communications were reliable, it used very little battery power; also the pager device itself was compact in size, as can be seen in Figure 1. However, the system also has limitations insofar as the device is restrictive to one way communication, which subsequently narrowed its appeal to a large user base. Page 2 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  3. 3. Nevertheless, despite the success of the SMS standards and its limitations, the pager network is still widely used in • Hospitals • Military • Police • Industry The development of SMS as a part of the GSM standard allowed for a small, but nonetheless important number of differences between the systems, which are as follows. • Point to Point communications • Two-way communications • Maximum message length of 160 characters Although the maximum number of characters supported is 160, this is based on the European character set with the transfer of ASCII text. ASCII allows for a standard format for the transfer of text across multiple computer systems. However, ASCII with its 7 bit representation of figures, does not support non European characters, therefore could be considered to be limited to a degree. Nevertheless it is possible to allow for non European characters by utilizing Unicode Transformation Format (UTF)-8, but this has the disadvantage of reducing the size of the message due to the nature of the set using 16 bits to represent a figure instead of only 7 bits with European characters. Exercise 1 What would be the maximum size of an SMS message if UTF-8 was used to transfer the message? The first text message sent was by a Vodafone engineer from a computer to a mobile device in December 1992, by Neil Papworth, the message read “Merry Christmas”. Within the GSM infrastructure there is also the transfer of out of bound control information facility in addition to the voice and data, which have been previously discussed. These control channels are not part of the main voice channels and will transfer information such as Page 3 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  4. 4. requesting a handover or an incoming call. Furthermore, a GSM device will listen to the control channel at a specified time to see if any information is being transferred to that device. If no information is of interest to that specific device, the receiver will be turned off until the next messaging time. This means it has the benefit of saving battery power, therefore has an advantage over the 1G TACS system which has to listen continuously for control messages. This channel for control information is referred to as the Digital Control Channel (DCCH.) One of the DCCH message packet types is the SMS point to point messaging, Paging Access Control Channel (SPACH) 0, 1 or 7 1-12 Octets 1 Octet 1 Octet 2-12 Octets 1 Octet 1 Octet 0-140 Octet Octet 1 Octet SCA PDU Type MR DA PID DCS VP UDL UD Figure 2 SPACH Protocol Format Exercise 2 How many bits are there in an Octet and why are the field lengths measured in Octets rather than bytes? SCA Service MR Message PID Protocol Centre Reference Identifier Address PDU Protocol Data DA Destination DCS Data Coding Type Unit Type Address Scheme VP Validity UDL User Data UD User Data Protocol Length Table 1 Definitions of fields used in the SPACH message type The figure above shows the breakdown of the fields in the SPACH packet type. The actual text message will be transferred in the User Data field, which is a maximum of 140 Octets, but can be smaller. To transfer the word “Justin” for example, this would be first converted into an Page 4 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  5. 5. ASCII binary representation of the letters as shown below, with each character being represented by 7 bits. Letter J u s t i n ASCII 1001010 1110101 1110011 1110100 1101001 1101110 This binary, which is in ASCII format, would then be converted into the 8 bits needed for transfer. Each character here is shown in a different colour to see where the different parts are reassembled. The method of making up the characters is to take the least significant bits from the following ASCII character binary value to ensure that each Octet has 8 bits. These ‘borrowed’ bits are then placed in the most significant position of the octet borrowing them. This is shown in Figure 3, with colours being used to show where the bits are coming from. To restore the original characters the reverse process is taken. So on this basis the first character will 'borrow’ 1 bit, the second will borrow 2 bits and so forth, until the process is reset again after borrowing 8 bits. J u s t i n 1 Octet 1 Octet 1 Octet 1 Octet 1 Octet 1 Octet 11001010 11111010 10011100 10011110 01110110 00000011 Figure 3 Conversion of the message into an 8-bit format In the event of the final character not giving a complete Octet the most significant bits will be set to zero to pad out the field. The alternative to this method would be to pad out each of the octets so that each 7-bit ASCII character would have an 8-bit value. However, this would be wasting 1 in every 8 bits, which when dealing with limited transmission capability of text message, i.e. 160 characters, would mean the maximum message size would be reduced to 140 characters, which ultimately could serve to hamper effective communication. In addition, the least and the most significant bits indicate a position within the binary value which is being stored. Considering the binary string, as highlighted below in , this clearly indicates a binary value. Furthermore, the most significant value is consigned to be the one having most effect in the event of changes occurring. Therefore, within this specific binary string this would be a value to the far left of the result. However, if this binary value were to Page 5 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  6. 6. be changed to 1 this would add 128 onto the value being stored. The alternative of the least significant value if changed will only in this instance remove a value of 1. Value 128 64 32 16 8 4 2 1 01110101 Most Least Significant Significant Bit Bit Figure 4 Positions within a binary string The use of SMS does not provide the users with delivery guarantees or a defined delivery period. Many users however consider that SMS will always be delivered within a few seconds of the message being sent. In reality, this is not always the case and longer timescales are possible, dependent upon the network and the time taken to transfer the specific message. For instance the transfer of SPACH packets will not take priority over other communications which may be taking place within the network. These delays can and do occur, especially at times when a network is heavily loaded e.g.. 23:59 on New Year ’s Eve. Furthermore, as stated above there are also no delivery guarantees of the SMS messages; they are delivered on a best-effort-basis. This means there is no guarantee that an SMS sent will actually be delivered to a recipient, which means this system has an element of uncertainty. For example, unforeseen events such as a SMSC crashing would effectively destroy a series of SMS messages. Additional hard ware is placed within the networks with backups to prevent problems like this, but the fact remains that SMS delivery guarantees are not integral to its standard’s. This is similar to IP used on the internet, but this relies on other mechanisms to guarantee delivery. These points should be considered when using SMS for time dependent uses, such as for example, an alarm reminder for an exam location sent by universities to students. S UCC ES S O F S MS The success of SMS was never planned for, nor intended. As was indicated previously, within the usage of SMS the technology enabled spare capacity to be utilized efficiently in regard to out of bound control messages which were transferring information to the mobile device. The success of SMS was an underground success, with no advertising or promoting of the Page 6 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  7. 7. technology from the standards groups or the operators. Users simply found this functionality within the devices and started to use, mostly the youth users sending messages. If the success of SMS had been planned for and realised, maybe different methods of delivery would have been implemented encompassing integral delivery guarantees. Exercise 2 Based on your knowledge of cellular networks, what would be the alternative to delivering the SMS messages from the current system? Indeed the number of messages transferred in the UK alone in December 2004 was 2.45 Billion messages. Clearly the operators were quick to realise the potential gained via having effective spare capacity; alongside also voice services. Consider the prospective of facilitating 2.45 Billion messages at an average price of an SMS at 8 pence. Page 7 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  8. 8. Figure 5 Text Usage figures from the website ( 2005) As highlighted in the above graph despite newer technologies being available, the usage of text messaging consistently increases monthly. N EW MES S A G I NG S YS TEMS The successful use of SMS, clearly indicated to operators the potential for large profits from a relatively simple method. As stated previously SMS was never really intended for the large scale use it received; also the capabilities of SMS are quite limited, with regard to sending plain text. Nonetheless the subsequent improvements now facilitate the movement of more than just plain text, as indicated below. Page 8 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  9. 9. Figure 6 Alternative Text Messaging Methods The Enhanced Messaging Service (EMS) was the first method after SMS for improving the messaging services. EMS allowed the users to send data beyond the normal ASCII information. An example can be seen in Figure 6, with the user sending the code to display a black and white picture, limited sounds and animations. EMS operates in the same store- and-forward technology used within SMS. The major difference being, due to limited data storage in a single SMS packet, the message is sent across several messages and then reassembled at the destination device. The usage of EMS however was very limited, subsequently it was replaced by the more versatile and suitable Multimedia Messaging Service (MMS). MUL TI MEDI A MES S A GI N G S ER VIC E ( MMS ) MMS is a messaging service which is intended to transfer more than ASCII text and as the name suggests will allow the transfer of Multimedia Information. The MMS standards define a number of types of multimedia in which a device must be able to receive and operate in order to be certified as MMS compatible. These are as follows: Page 9 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  10. 10. • Pictures • JPEG • PNG • GIF • Text • ASCII • Unicode 8/16 • Video • MPEG-4 • Sound • Adaptive Multi Rate (AMR) • SP-MIDI • Future Usage • Any multimedia type produced in the future The advantage of setting a default set of standards was that a device could recognise a number of different types of multimedia, and these could be transferred with the certainty that the recipient could display this information. But due to the time period in which this standard was generated it was recognised that not all of the current devices would have the capability of receiving the newer MMS messages. This posed a potential limitation of the technology, especially considered alongside the use of SMS, whereby a larger number of users had compatible devices. Therefore to allow the system to expand, despite attracting a limited user base, MMS made use of the Internet, which by this stage had become a widely used technology. Hence in the event of an MMS being sent to a recipient not having a compatible device, they would receive an Internet URL, which allowed them to see the message on a normal web browser. The MMS standard, in addition to Multimedia types, also defined a number of different page description languages which could also be utilized to format a message. The languages were XML, WML and SMIL. The use of Synchronized Multimedia Integration Language (SMIL), allowed for not only positioning on a page to be specified, but also timings. This would permit for example, a movie to be viewed at the same time as subtitle text. In the event of none of these methods being used the device itself had the capacity to make the decision on how best to present the information. Note also that in the standards for MMS in relation to what media it can receive, this element has been left open to accommodate future use. This allows the benefits to be reaped from any future developments of multimedia which are currently beyond today’s technology, but that in the future will be achievable. This has been demonstrated within the development of the World Wide Web; although originally text only, new developments and advances of computers meant they rapidly became more powerful becoming media rich, hence improving the experience for users. Likewise MMS was Page 10 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  11. 11. sanctioned to develop in a similar manner with the operating systems of the devices, being allowed to install ‘plug-ins’ in order to interpret the new data type. Consequently this has given rise to new operating systems for devices which will permit them to be updated by users for introduction of new functionality. Currently the most widely used operating systems are as follows: • Symbian • Microsoft Mobile • Palm OS This may change in the future, with a single OS dominating the market, as indeed has happened with personal computers. Exercise 3 Why are custom versions of an operating system written for a mobile device and why they use a cut down version of normal PC operating systems? The problem with allowing for unlimited expansion of the type of messages is the size of the message transferred. Unlike SMS, which fixed the size of the data units to a maximum of 140 octets; MMS has the potential to transfer unlimited amounts of data. The standard for MMS does not specify a maximum message size, mostly due to the potential for projected future usage – hence as throughput of the networks improve what is presently considered to be a large file stretching the capabilities of current network technology will subsequently seem diminutive with the advent of new technologies. Due to this reason MMS message are sent in multiple packets across the network, in a similar manner as TCP/IP on the Internet. The actual transport protocol used is the Wireless Application Protocol, (WAP), due to its capabilities within wireless networks. As a result of the packet switched nature of MMS and the limited capabilities of GSM devices in terms of data transfer, MMS is only available to GPRS and newer devices. However, a potential problematic issue exists insofar as there is no limit within the standards in respect of the size of the messages; therefore a MMS could potentially overwhelm1 a device receiving it. However, in order to adhere to the 1 1 Mb message transferred with only 512 Kb of storage on the device. Page 11 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  12. 12. requirements of the standards the device does need to have the functionality to receive and store MMS of at least 30 Kbytes. Also an upper size limit is imposed external to the standards by the device/network operators, specifically to prevent large amounts of data being transferred across the network. This in effect means that if a message is transferred and the device is unable to receive it, (for example a video message), it will then be streamed. The MMSC will therefore automatically stream the message directly to the phone, hence allowing the recipient to receive the message even if the device itself does not have the storage capability. Furthermore, data transfer for MMS will not take place using the control channels, as discussed already in regard to SMS, but will in fact take place across normal data channels, due to the large volume of data involved requiring transportation. Exercise 4 What problems could be caused by large scale streaming of MMS data to devices? S MS IN FR A S TR UC TUR E The infrastructure utilized for SMS largely incorporates the same concepts relevant to that within installation of GMS. This infrastructure is subsequently supplemented with a single additional server, i.e. the Short Messaging Service Centre (SMSC) Page 12 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  13. 13. PSTN OMC AUC BTS HLR MSC EIR BSC VLR BTS SMSC BSS NSS OSS Figure 7 SMS infrastructure The SMSC consists of a combination of hardware and software. The messages transferred are stored in the SMSC, in a “First In First Out” (FIFO) basis. This technology is referred to as store-and-forward technology. The SMS message is transferred from the device to the SMSC, which is defined in the SPACH header and will be determined by the network operator, the message is then stored at this point. In the event of the destination device being turned on the message will be transferred to that device and if requested a receipt will be transferred back to the source of the message. The status of the device and the location to forward the data to will be received from the Home Location Register (HLR), or alternatively for guests on the network from the Visitor Location register. If there is no entry in the HLR due to the device being currently switched off, the message will remain stored. But once the device is turned on again the HLR will inform the SMSC of this and the message will then be delivered to the new location. As with other GSM technology SMS is a roaming technology; provided there has been agreement between the operators involved. The agreement in the UK to allow for roaming between operators was achieved in 1998, with the ability to send messages between devices of different operators with the condition only the sender pays for the transfer. Furthermore, because the VLR is also part of the SMS infrastructure this facilitates international roaming in respect of the transfer of messages, Page 13 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  14. 14. providing of course there exists an agreement of transference of messages between the networks. In addition, although the original specification of SMS was intended to operate as part of the ETSI; developed GSM, SMS will also operate on other networks. This is due to its use of digital control channels; therefore the standard will operate on other 2G networks such as … • PCS – American • PDC – Japan SMS also has the capability to operate on circuit-based networks and the improved packet switched, without adjustment. MMS I N FR AS TR UC T UR E The infrastructure of the MMS system, in common with the SMS, builds upon the already installed system. However, unlike the infrastructure of SMS, due to the additional complexity of the MMS system, 3 servers are required. These servers are placed together and are collectively known as the Multi media Messaging Service Environment (MMSE) Page 14 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  15. 15. PSTN OMC HLR AUC BTS MSC VLR EIR BSC MMSE BTS PCU SGSN GGSN Firewall Firewall BSS NSS OSS Intranet Internet Figure 8 MMSE attached to a GPRS network The MMSE consists of a number of servers, which are listed below with basic functionality • Multimedia Messaging Service Centre (MMSC) o Similar functionality to the SMSC. o This node will store the MMS until the message can be delivered to a recipient. • MSS Relay o Allows delivery of the message across different packet switched networks o Based on using the Simple Mail Transfer Protocol (SMTP) o Records the usage of MMS for charging purposes o Converts the message if needed o Using WAP 2.0 standards and the capability of the device • MMS Valued Added Services (MMS VAS) o This is a server which contains a number of high quality Multimedia artefacts o These can be downloaded and used within a MMS message, at a cost Page 15 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  16. 16. Exercise 5 Why does the MMSE connect to the HLR and the VLR within the infrastructure? The MMSE is referred to as an ‘environment’ implying all the servers are located within a single network, but in fact it is also perfectly possible for servers to be located outside of the network. The obvious candidate server for this would be the MMS VAS, with the potential for other operators offering multimedia to use within the messages. FU TUR E US A G E O F MES S A GI N G The future usage of messaging seems to be assured; indeed as has been indicated already within this section the growth rate of SMS shows no signs of plummeting. SMS has also become an integral part of British culture, with such as television programs and advertisements consistently asking users to text for information. This is a convenient method of communicating and allows people to communicate even without a receiver device being turned on because the message is conveyed as soon as it is switched on again. The MMS however has not been so successful, with a limited take up of this technology. This is despite operators spending large quantities of money on advertising campaigns. A short article by the BBC discusses this more fully (BBC, 2004), and briefly considers potential reasons for the lack of take up. The development of MMS was intended to increase the profit of the networks and the expectation of the operators was that MMS would be as popular as SMS. Some reasons given for the lack of success of this system concerned the initial problems surrounding charging for the messages, which was not as ‘clear cut’ as with SMS, which operated on the simple principle of only the sender paying. The usage of MMS however it is envisaged will continue to grow as pricing structures are agreed and more users purchase phones having video/camera functionality and subsequently find this technology to be beneficial in differing situations. The Mobile Data Association (MDA) is predicting that 2005 will see a large growth in the use of MMS as over 75% of UK based mobile devices will be able to use at least GPRS for data services. Page 16 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  17. 17. C O NC L US I O N This section of the material has considered the messaging services used within the cellular environments. The dominance of SMS usage today equates with successful technology, which despite being superseded by ‘better’ newer technologies still manages to maintain its strong market position. The technologies and the reasons for the development have also been discussed, focusing on the perceived benefits of the newer technologies and rationale underpinning design decisions. Page 17 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  18. 18. GL O S S A R Y O F TER MS 0G - Pre First generation mobile phone technology 1G - First Generation 2G - Second Generation 2.5G - Two and Half Generation 3G - Third Generation ASCII - American Standard for Code Information Interchange BS - Base Station BSS - Base Station Support System Byte - 8 bits generally equal one byte ETSI - European Telecommunications Standards Institute FDMA - Frequency Division Multiple Access GGSN - Gateway GPRS Support Node GSM - Global System for Mobile Communications GPRS - General Packet Radio Service Kilobyte - In storage 1024 bytes indicates 1 kilobyte NSS - Network Support System Mbps - Mega Bits per Second = 1000 ms - Millisecond = 1 thousandth of a second MSC - Mobile Switching Centre PCU - Packet Control Unit PSTN - Publicly Switched Telephone Network QOS - Quality of Service SGSN - Serving GPRS Support Node SMS - Short Messaging Service TDMA - Time Division Multiple Access Page 18 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  19. 19. R EVI EW Q UES TI O N S Question 1 Try generating your name in ASCII and then converting it to a format to transfer this name in a SPACH user data field. Question 2 A local college has recognised they have a problem with students either not attending, or being late for examinations. This has resulted in the college having poor results for courses, which is reflecting badly upon its reputation. Traditionally the exams are announced on a piece of paper which is hung up outside of the office. The college are considering changing this and removing the paper notice and replacing this with an automated SMS message unique to each student with the details of their specific exams. The message will be sent out initially out 2 weeks before the exam and then again 30 minutes before an exam starts. You have been asked to offer advice on this proposal, what would you advise? Question 3 Why does the MMSE need to connect to the GGSN? Question 4 What is the advantage of transferring a SMS message via control channels rather normal data communications? Page 19 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  20. 20. R EFER ENC ES BBC, 2005, “Mobile multimedia slow to catch on”, Downloaded on the 15th of August 2005, 2005, “Text Messaging growth”, intPageId=750 downloaded on the 22nd of August 2005 Wikipedia, 2006, “DME Pager Patron.jpg”, downloaded on the 3rd of January 2006 Page 20 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  21. 21. US EF UL L IN K S Text it Association SMS text message Generator Mobile Data Association Page 21 of 22 Version 1.0 ©Staffordshire University 20.07.2006
  22. 22. Page 22 of 22 Version 1.0 ©Staffordshire University 20.07.2006