SMS -- Short but Sweet

by Tom Clements
February 2003
Short Message Service (SMS) is on the verge of becoming a big-time player in the wireless marketplace. Originally envisioned as a niche technology - as a way to squeeze more capacity out of under-utilized networks and marginally increase carrier profitability in the bargain - SMS has grown in popularity far beyond anyone's initial expectations. While chat has been the driving force behind this upsurge, there's more to SMS than transient teenage talk.
In this article, I'll describe the evolution of SMS as a wireless technology and point out both the promise and the pitfalls of SMS for location-based services and wireless enterprise apps.
SMS exists as a subset of the larger wireless domains such as GSM, TDMA, GPRS, and CDMA. Being small, it acts like a mouse in the wireless woodwork, peeking its head out periodically to see if the network coast is clear, scurrying quickly across the radio frequency floor, then disappearing back behind the wall of the frequency band when the analog big boys (circuit-switched voice) rumble through. It's so small, the network hardly knows it's there, and that's the beauty of the technology: it's small, cheap, and readily available across-the-air interface.
Generation after Generation
Because SMS exists within the larger context of second- and third-generation wireless technologies, it's instructive to take a quick look at where these technologies came from, where they're headed, and what they represent. Table 1 lists the key features and technical capabilities of the major carrier-dependent technologies.
Note: 2G and 2.5G networks like TDMA, CDMA, and GSM are inherently hybrid structures, with digital capabilities modulated over an analog network. For a more complete discussion of these technologies, see my articleMaking Sense of Cellular.
Gen Freq ~ Kbps Technology Emphasis Remarks
1 800
MHz range
9.6 AMPS Circuit-switched wireless analog voice. No data. Limited system capacity and little protection against fraud
2 800
900
1900
MHz range 9.6 to 14.4
TDMA
CDMA
GSM
Circuit-switched wireless digital voice and data
Better security and higher capacity
More support for data communications
SMS enabled
2.5 1900
MHz range 56 to 144 GPRS
CDMA2000-1X
EDGE circuit-switched wireless digital voice + new packet-switched data services.
GPRS is an "always on" air interface to the Internet Migration path to 3G
SMS and EMS enabled
3 G 2 GHz 144 vehicle,
384 outside,
2 Mbps indoors WCDMA
CDMA2000-MX
UMTS packet-switched wireless voice and data services,
encryption, high-speed multi-media SMS, EMS, MMS enabled
True 3G platforms expected 2003--2005
Table 1. Generation Gaps
As you can see from the Remarks column in Table 1, SMS evolves across these "generation gaps" from short messages to enhanced messages (EMS) to multimedia messages (MMS). EMS and MMS go well beyond the transmission of simple text-based messages and are expected to grow in importance and popularity as wireless networks evolve. Of more immediate concern to us in this article, however, is where SMS came from originally and why it's become so important.
Note: This chart omits Bluetooth and 802.11b (aka Wi-Fi) because they're local, ad hoc, and carrier-independent interfaces. For an imaginative (and somewhat startling) take on how 802.11b may make carrier-dependent networks obsolete, see Nicholas Negroponte's article in the October 2002 issue of Wired magazine.
SMS - Where, When, and Why
Europeans saw as early as 1982 that a single, transnational wireless telephone system was the only way to ensure seamless roaming of subscribers among countries. While America tried to wrap analog technology in a digital blanket - first TDMA, and later, in the 90s, CDMA - Europe cut to the chase and designed GSM (Global System for Mobile communications) from the ground up as a system for analog voice with modulated digital capabilities built in.
SMS took full advantage of these digital data capabilities to send short bursts of text-based messages across the GSM (and later TDMA and CDMA) networks.
SMS is consequently characterized by out-of-band packet delivery and low-bandwidth message transfer. It provides a point-to-point (and broadcast-to-point) mechanism for transmitting short messages (up to 160 text characters) from wireless handsets.
Interestingly enough, SMS was initially bundled into GSM as a sop to network carriers, promising them a way to increase marginally the carrying capacity of a network. SMS messages traverse the less-populated, higher-frequency bands of the network (the wireless woodwork, to return to my earlier analogy) and therefore allow carriers to make full use of available bandwidth. No one, however, anticipated the huge volumes of calls that would eventually characterize the service.
SMS Applications
So, who's using SMS today, and why? The first and foremost answer to this question is: kids, for chat. Especially in Europe but increasingly in the United States, SMS is the main medium for short, pithy messages sent by teens and even twenty-somethings to their wireless counterparts: "Meet me backstage after the concert." "C U L8R." "Dude, where's my car!" That's the sort of thing that's interspersed (out-of-band, as they say, since that's the nature of SMS) on the wireless airwaves today. But what about tomorrow? Here are a few of the more sophisticated, enterprise-style applications either already in play or soon to be enabled:
Games People Play
Starting out with portfolios of primitive games like hangman and tic-tac-toe, European companies like Digital Bridges have had huge success creating and distributing titles such as Big Brother and Men in Black II to an ever-expanding group of wireless consumers. Games are the digital pastime of the new wireless youth.
Notification services
On the corporate front, SMS messages from business applications inform users of voice mail, email, or faxes that await their attention. "You have mail!" is typical: a small message announces the existence of one or more larger ones. Calendar services like reminders of meetings and business appointments fall into this category. Busy working parents may find themselves the recipient of an SMS alert: "You have kids! Don't forget to pick them up at day care tonight."
Email and paging internetwoking
In addition to notifying you of waiting email, some applications promise to integrate SMS and existing email services. For example, SMS could be used to scan streaming email headers to determine whether any messages demand immediate attention. Paging, whose very existence is threatened by the SMS upsurge, may instead find its life extended by integration with SMS services.
Information services
SMS is perfect for applications offering quick bursts of information on a periodic basis, such as weather reports and financial information - stock quotes, for example.
Location-based services
SMS is an ideal medium for many location-based services. Instant advisories of local traffic problems, pointers to films playing nearby, restaurant suggestions, telematics-related vehicle alerts ("Your car has moved beyond the specified geo-fence area"), and other short location-related messages may well be killer apps for SMS.
SMS Network Components
SMS applications are only as good as the networks that transport them. Following are the four crucial nodes that enable the transmission of SMS messages across the air interface.
Cell Tower or Base Station
A base station is the cellular relay station (or cell tower) that a cell phone talks to when initiating or receiving a wireless call. The base station's primary responsibility is to transmit voice and data traffic between mobile devices and a mobile switching center (described next). All transmissions are managed by the base station, which acts as a kind of clearinghouse for wireless communications.
MSC
A Mobile Switching Center (MSC) is the electronic field office of a cellular carrier, a computer-controlled switch for managing automated network operations. An MSC automatically coordinates and controls call setup and routing between mobile phones in a given service area. MSCs are connected to base stations by T1 landlines or microwave channels, and by landlines to the Public Service Telephone Network (PSTN).
MSCs maintain individual subscriber records, current status of subscribers, and information on call routing and billing in two subscriber databases called the Home Location Register (HLR) and the Visitor Location Register (VLR). The HLR contains subscriber profiles, while the VLR provides information relevant to roamers.
SMSC
SMS makes use of a Short Message Service Center (SMSC), which acts as a store-and-forward system for relaying short messages. Similar to an MSC, the SMSC guarantees delivery of text messages by the network. Messages are stored in the network until the destination cell phone becomes available, so a user can receive or transmit an SMS message at any time, whether a voice call is in progress or not.
GMSC
SMSCs communicate with TCP/IP networks via a Gateway Mobile Switching Center (GMSC). A GMSC is an MSC capable of receiving short messages from an SMSC. The GMSC interrogates the Home Location Register (HLR) for subscriber routing information and delivers the short message to the home MSC or roaming MSC of the destination mobile unit.
SMS Network Diagram
The following diagram shows the various components that make up the SMS cellular network. Notice that gateways provide mobile devices access to the Internet across TCP/IP lines:

Figure 1. SMS Cellular Network
SMS Subscriber Services
The "short" in Short Message Services extends to the technology, which comprises two basic point-to-point (or broadcast-to-point subscriber services:
Mobile-terminated short messages (MT-SM)
Mobile-originated short messages (MO-SM)
MT-SMs are transported from the SMSC to the handset or from the SMSC to other sources on, for example, TCP/IP networks. A report is always returned to the SMSC, either confirming the message delivery or informing the SMSC of the SMS failure, along with the reason for the failure.
MO-SMs are transported from the handset to the SMSC, destined for other mobile users or for subscribers on fixed networks. As with MT-SMs, a report is also always returned to the handset.
For messages requiring immediate delivery, only one message delivery attempt is made per service request. For messages not requiring immediate delivery, one or more delivery attempts are made until an acknowledgment is received.
Currently, MO-SM messages are the weak link in the enterprise SMS value chain. Sending messages to handsets from an SMSC is a relatively staightforward process, but - from an application point of view - receiving messages that originate from a mobile device is an order of magnitude more difficult. Here's why:
Not to belabor the obvious but, when you want to contact a person by phone, you key in the person's phone number. The same is true when an application sends an SMS message to a mobile phone. The phone number targets the cell phone for which the mobile-terminated message is intended. But think about this from the other way around. How does a mobile device contact an application hosted on a server? What's the phone number for the app? The short answer is: There isn't any!
To get around this problem, carriers create and allocate short codes. According to Eddie Gonzales, an innovative engineer for an up-and-coming location-based startup called Telcontar, a short code is essentially a carrier-specific (and therefore carrier-dependent) phone number for applications routed through an SMSC. To send mobile-originated SMS messages to an application, your phone or handset must know its carrier-approved short code.
The downside of short codes is that developers can easily find themselves locked into the services of a specific carrier. The alternative is to work with a so-called consolidator or SMS broker.
SMS Brokers
An SMS broker is a kind of virtual network operator for short messages. SMS brokers negotiate deals with carriers, obtain thick pipe connections and provide an interface between SMS messages and the provider best prepared to deliver them to their intended destination.
Consolidators help developers get around carrier dependency by cutting deals with multiple carriers. This is an emerging market, however, and there are lots of "holes" in the network as consolidators struggle to ensure SMS developers ubiquitous coverage across disparate carriers. So far, no one consolidator has stepped up to meet this challenge with a sufficiently robust offering; therefore, mobile-originated messaging remains an iffy proposition.
Rather than reinvent the wheel, companies supporting wireless applications written in the Java programming language will most likely want to interact with an SMS broker, and call into a custom Java API for sending and receiving messages.
Here's a short list of companies who broker SMS: Simplewire, Quios, CMG, Annyway, Diax, Dr. Mantera, Inphomatch, NovelSoft, wapMX.com, MobileSys, Mobileway, and Winbox.com.
SMS Message Structure
As you might expect, SMS message packets are fairly simple in design. Each packet has the following structure:

Figure 2. SMS Message Structure
Note: On GSM-enabled phones, Subscriber Identity Modules (SIM) cards are used for activation, debit, and subscriber-profile purposes. SIMs are modular, interchangeable units that facilitate roaming through transnational GSM-based networks, mainly in Europe.
Service elements
Over and above packet structure, SMS includes the following three service elements relevant to short-message transmission and reception:
Validity period - indicates how long the SMSC will guarantee the storage of the short message before ultimate delivery. Priority - indicates the relative importance of the short message. Time-stamp reporting - indicates the time of submission of the message and informs the handset whether there are more messages to come.
SMS over SMTP
Some wireless carriers expose a Simple Mail Transport Protocol (SMTP) interface to send SMS. According to one scenario, the email address "is" the device's phone number or pager identification number prepended to the carrier's specific domain; for example, 4155551234@messaging.sprintpcs.com.
SMTP is free to the sender. SMS brokers offer software development kits for implementing the protocol.
Some disadvantages of SMS over SMTP include: lack of speed, uncertain (that is, non-guaranteed) delivery, and inconsistent or non-existent error checking. Email simply wasn't designed to provide quick, guaranteed delivery.
Third-Party SMS Support
Third-party solutions hide the intricacies of SMS from developers, and range from shrink-wrapped software to service-provider gateways.
Simplewire is one such third-party platform, that acts as an operating-system layer between the Internet and wireless devices of all types. Simplewire offers a free JDK for SMS development. Most solutions, however, entail licensing or usage fees, because carriers set their own pricing. Providers of third-party solutions are often called SMS brokers, even though their offerings are significantly different.
Wireless Messaging API (WMA)
WMA enables applications to send and receive short text and binary messages over wireless connections.
WMA is an optional package within the Java 2 Platform, Mobile Edition (J2ME). It's based on the Generic Connection Framework (GCF) defined by the Connected Limited Device Configuration (CLDC). WMA can extend profiles based on both CLDC and CDC, and consequently is available to any J2ME application. Users might, for example, opt to receive stock quotes by way of an SMS broadcast.
Problems with SMS
After all is said and done, several problems persist in the SMS space, holding back full-scale implementation of SMS. Here are some of the more conspicuous problems, according to Joe Lauer of SimpleWire and Jack Gold of the Meta Group:
SMS is a description of a service provided by wireless carriers, rather than a description of a method of delivery. As a consequence, methods of delivery differ and interoperability difficulties prevail.
Enterprises should look on SMS as a convenience, not as a high-reliability interaction mechanism. SMS is not a particularly robust system - users may get messages minutes or hours later if they're roaming, or out of range of an SMSC. Companies should rely on SMS as a notification mechanism only when guaranteed delivery and real-time customer interactivity are not issues.
Fragmentation of carriers. Over and above inconsistent transports, carriers do not typically allow public access to their networks. Many SMS developers opt to bypass the carrier and implement their own solutions. Alternatively, developers opt to ally themselves with an SMS broker and incur additional fees for service.
Unsatisfactory interoperability among carriers. Difficulties are often encountered in sending SMS messages among carriers because of implementation differences. Payment models also differ significantly among carriers. This difference is especially stark when you compare carriers in North America to carriers in Europe, where only the sender is charged for each SMS message. Europeans find it curious that Americans have to pay for incoming messages on their cell phones!
Differing capabilities of devices. Some devices support one-way text messaging, others support two-way. Some can receive only 140 characters per message. Others support concatenations of SMS messages. Some can download binary data, others can't. Developers must maintain extensive information about each carrier and every device.
Conclusion
Problems with SMS aside, it seems safe to say that mobile handsets will do for data in the 21st century what the transistor radio did for music in the 1950s. And SMS is poised to be an important part of this mix, spreading a new and different form of text-based information and entertainment to the masses. With the inclusion of support for binary data in EMS and MMS, a brave new world of text, images, cartoons, audio and video clips transmitted instantly to users on the move may give a whole new meaning to, not just short messages, but short attention spans as well.
Resources
Joe Lauer, Sending A Wireless Text-Message With Java, http://www.simplewire.com/developers/knowledge/articles/issue/010428.html, April, 2001
Tom Clements,Making Sense of Cellular, July 2002
Jack Gold, META Group, http://www.metagroup.com/
Handspring SMS Tutorial, http://www.handspring.com/developers/AN-18UsingtheSMSLibrary_100.pdf
Scott Guthery and Mary Cronin, Mobile Application Development, 2002, McGraw-Hill