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Canada was the first country to implement SMS (Short Message Service) interworking between wireless carriers, regardless of the digital wireless technology of the sender or receiver of the message. Since April, 2002 Bell Mobility (CDMA), Telus (CDMA and iDen), Rogers (TDMA, GSM/GPRS) and Microcell (GSM/GPRS) have allowed short messages to be exchanged between any two mobiles on their combined systems through a CMG Interoperability gateway. This significant advance in messaging was a major factor in pushing major US carriers in the same direction.
SMS interworking is not an issue in Europe, where GSM originated and is still the dominant wireless technology. To these carriers, there is only GSM, and consequently only one SMS format. At every protocol layer, interoperability is assured.
Partly because of the absence of technology barriers, messaging became an important additional revenue stream for European carriers, accounting for more than 10% of their revenue by some estimates. Messages have increased from about 4 billion per month in January 2000 to about 24 billion in May 2002. The GSM Association estimates that 360 billion text messages will be sent throughout 2002.
North America has had to face interoperability challenges since TDMA (Time Division Multiple Access – IS-54 and ANSI-136) came on the scene in the late 1980’s to challenge the then dominant analog system (AMPS), followed in a few years by GSM (in the PCS band) and CDMA (in both the cellular and PCS bands). The standards had various levels of support for SMS, originally supporting only the receipt of messages. After a few revisions of the standards, all had comparable (but not compatible) support.
For several years, North American wireless carriers were forced to sit by and watch message usage and revenue grow elsewhere, while their customers were frustrated by an inability to exchange messages with friends and colleagues who had chosen a different technology.
There is no reason why SMS message formats need to be different between different technologies. It just happened that way because of a lack of coordination between standards organizations. GSM was originally developed as a pan-European standard, with no perceived need to interwork with other technologies. In North America, analog (AMPS) could not support messaging, and proponents of CDMA and TDMA were far from cooperative in the early years of the ‘technology wars’.
Gradually, carriers began to realize that one technology would not reach total dominance, and that as they globalized their networks, they would be faced with interworking if they wanted to achieve a reasonable level of integration of their operations.
After a proliferation of technologies in the 1990’s, standardization has now settled down to two major competitors as 3G slowly emerges – 3GPP defining GSM and UMTS standards, and 3GPP2 defining CDMA2000 standards. Furthermore, these organizations do have a considerable number of cooperative projects underway, including some cooperation in the development of the next generation of messages – Multimedia Messaging Service (MMS). Technology advocates realize that while they may still fight for dominance in some areas, they must cooperate in areas where customers are demanding interoperability. Cooperation can, in some cases, increase the total demand for wireless services, making the costs of inter-working easy to justify. Messaging is one of those cases.
This desire for interworking resulted in the development of the SMPP protocol. Originally a proprietary Aldiscon (now Logica) specification, it was turned over as an open standard to the SMS forum (http://www.smsforum.net), which now maintains it. It defines an intermediate format for exchange of messages between SMSCs and gateways, and between gateways. Any technology with SMS capabilities can have a mapping into and out of SMPP. This made possible the development of inter-technology SMS gateways which need to include only one conversion algorithm for each technology they support.
Mobile to mobile SMS is conceptually the simplest form of messaging, but is one of the most complex to implement. Figure 1 illustrates how a short text message is originated by one mobile (step 1) and sent to a Base Station/MSC using, for example, a TDMA (ANSI-136) or CDMA (ANSI-95 or IS-2000) interface. The message (step 2) is forwarded to the mobile’s home SMSC (Short Message Service Center, also known as MC) using the ANSI-41 protocol running over SS7. The SMSC reformats the message in the SMPP format and sends it to the SMS Gateway (step 3) using TCP/IP over public or private internet links.
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In Canada, the SMS Gateway is operated by CMG as a service bureau. In some cases the message may need to be forwarded to another regional gateway, again using SMPP over TCP/IP (step 4). Eventually the message reaches the destination mobile’s SMSC (step 5), where it is reformatted, in this example, into GSM MAP format. The message is then forwarded to the MSC currently serving the mobile (step 5), which then transmits it over the GSM radio interface. Delivery acknowledgements will follow the reverse path.
Carriers currently connect to CMG using dedicated T1 (1.5 Mbps) lines. A cheaper alternative is to use the public internet, through a VPN (Virtual Private Network) for security, although this will not guarantee the same quality of service when the internet is busy. Another possibility as traffic grows is to move to Frame Relay. No matter what physical connections are used, the higher level protocols will be IP, TCP and SMPP. Figure 2 illustrates the network layers that are used as a short message flows from mobile to mobile.
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SMS interworking has some restrictions. Messages must be shorter than the maximum size allowed by both carriers or the interoperability device will have to segment them, and they may be delivered in separate pieces. Several capabilities that are technically possible have not yet been implemented (such as delivery notification), and all capabilities require support by both the originating and destination carriers.
Server to mobile traffic often takes a different approach. An airline informing their customers of a delayed flight or a broker sending out stock quotes may enter the wireless network via an email gateway. This allows the airline to connect not only with wireless systems using, but also with email as well. As volume grows, submitting these messages via an SMS interoperability gateway may provide greater efficiency, while still avoiding the need for servers to know all the different SMS formats.
There are several special applications for SMS beyond the exchange of text messages, including:
Ring tone downloads are a good example. Although it would be technically possible to perform this between carriers, there is no way at present to bill for this service, and carriers may compete to provide the coolest ring tones to their customers. Consequently, technology agnostic adaptations of services like this will have to wait until carriers have specific contracts in place to allow for the exchange of billing data and the associated revenue exchange – if they even decide that this is desirable.
According to William H. Dudley, North American Product Manager for CMG, the major priority for the future is increasing the number of interconnected carriers. There are still some of the smaller Canadian wireless carriers that are not connected.
CMG is planning ‘peering’ agreements with other interoperability providers, such as Infomatch and Verisign. These will also use the SMPP protocol. This will probably first allow messaging with the United States, where large carriers such as AT&T, Cingular, Verizon and Sprint have also implemented interoperability. Interconnectivity with Europe, Asia, Mexico and South America will not be far behind. It is likely that, in a couple of years, Canadians will be able to text message to virtually anyone with a wireless phone anywhere in the world. Other challenges will come from the need to keep pace with the developments of new air interfaces, and new messaging capabilities.
Interoperability is starting at a basic level, providing reliable delivery of text messages between two mobiles. There are a number of services that could be provided by interoperability gateways in the future. These include:
Wireless carriers have learned through their SMS experience that sometimes it pays to cooperate, rather than compete. Some services can be provided within the walled garden of one carrier’s network, but when the service is a form of communication the value increases with the number of possible destinations. SMS is no longer seen as a gimic, it is now a utility, and should be available to every pair of subscribers.
Canadian consumers are learning that SMS is a useful complement to their voice services. Messaging may be used instead of a voice call in some cases, but more often it will be used to communicate when a voice call is not appropriate. SMS is more useful to reach someone in a meeting, in class, on an airplane, or in a totally different time zone, and probably sleeping at present. The ‘store and forward’ nature of SMS gives wireless consumers more choices. Now that the technology is in place, Canadians will soon show their carriers what they really want to do with their new messaging opportunities.
In a few years people will find it difficult to believe that there ever was a time when you could only send text to people in the same country using the same technology, just as teenagers now find it difficult to believe that there was a time when you had to make sure that you weren’t renting a Beta tape when you had a VHS VCR.
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