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What is SMS and how does it work?
We’re all familiar with the SMS or standard text message. After all, it’s one of the oldest and most commonly used methods of mobile communication. While SMS is seemingly humdrum in the modern age, there’s a surprising amount of coordination and technology working in the background to send such seemingly simple messages. So let’s take a look at how it all works.
For a start – SMS stands for short messaging service. It’s a protocol used for sending short messages over wireless networks. Unlike many services in use today, such as MMS and other data-driven instant messaging services, SMS still works on the fundamental voice rather than the data part of the wireless network. It was originally built for GMS networks but continued to run on CDMA and HSPA in the 3G era, 4G LTE, and even modern 5G networks.
SMS allows for text messages of 160 characters (letters, numbers, and symbols) in length. Or for other alphabets with a wider range of characters, such as Chinese or Arabic, the maximum message size is limited to just 70 characters. Part of the reason for this is that SMS messaging was original considered as an afterthought added to the spare bandwidth available on wireless voice networks. There was always a limit on how large these messages could be.
The 160 limit was eventually decided upon by Friedhelm Hillebrand, who observed and tested the typical number of characters in the average sentence, combined with a compromise on the available bandwidth at the time. Nowadays bandwidth isn’t so much of a concern, and messages can easily be sent back to back and recompiled on the receiving handset. Long-SMS messages have been around for many years, allowing for back-to-back SMS messages to be combined into longer texts.
SMS messages can also send emojis, as they’re part of the official character list. But the standard doesn’t support images, gifs, video, and other features that we’ve come to expect from RCS messaging and other advanced messaging services.
The SMS standard explained
The SMS standard defines what information is sent in a text message, what bits of binary code make up each letter, and how this data is organized so that sending and receiving devices can communicate with each other. The actual data format for the message includes things like the length of the message, a timestamp, the destination phone number, and the actual message of course.
These details are described by the protocol description unit (PDU), which takes the form of a string of hexadecimal-octets and semi decimal-octets. Hexadecimal being in base 16, so that’s 16-bits of information per character of data sent. We won’t go into any more detail about SMS encoding. The important thing is what information the PDU includes. The format comprises where to send the message to, which short message center (SMC) to use, and the sender’s own number. The length of the information also has to be defined in the string, so that the receiver knows exactly what to look for.
After the sender and receiver information comes a protocol identifier and a tag to identify the data encoding scheme used in the message, allowing different receivers to know how to decode the actual message. This is important as it enables the use of unique character schemes used by languages around the world. There’s also a timestamp and information on the length of the user’s message before the user’s actual message is encoded.
As for the message itself, as already mentioned it can contain up to 160 characters, where each character is defined by the 7-bits GSM alphabet that covers Latin and Greek letters. A 7-bit alphabet results in 128 (2^7) available letters, numbers, and pieces of punctuation which can be used to create an SMS message. For example, 48656C6C6F is the GSM alphabet equivalent of the word Hello. The diagram below might help explain this whole standard a little better.
As you can see, there’s a lot more information sent with an SMS message than just a sentence or two. There are other vital pieces of information that will help deliver the message to the correct recipient and make sure that every device in the delivery line can properly understand what’s being sent.
Sending the data
As for the actual transmission of an SMS, the text message from the sending mobile device is stored in a separate channel called the short message service center (SMSC). Its primary job was forwarding messages to recipients and storing SMS messages if the recipient isn’t immediately available. In the original days of GSM and 2G networks, messages were then passed through the Enhanced Mobile Switching Center and Base Transceiver Station parts of the network before ending up at the receiving device. Storing message this way might sound simple enough but this was a novel set up at the time and the system stuck around in the 3G era too.
The move to 4G LTE presenting a problem, as the new network topology does away with the old switching and transceiver stations, and text data can’t be sent on the voice call plane. One option is to revert back to legacy networks to deliver SMS, which early smartphones and networks did but this wasn’t a long-term solution. To get around this, 4G networks introduce a new IP-SM-GW network element that communicates with SMSC directly to retrieve messages.
Modern 5G networks complicate the legacy messaging standard further, offering two ways to retrieve SMS data in addition to the original GSM method. The first is through SMS over IP, which essentially nabs the message through the 4G network layer. This is fine for early 5G deployments, but once 5G Standalone networks switch on they’ll have to implement SMS over NAS. SMS over NAS deploys a Short Message Service Function (SMSF) network element into the 5G Core Network, allow it to communicate directly with the SMSC.
The good news is that all of these implementations can run side by side, offering multiple pathways to send and receive text messages. But who would have thought sending 160 characters over the air could be quite this complicated?
Post SMS – The era of internet messaging
SMS has been the backbone of fast text communications for decades but that’s also one of the reasons its deployment is seemingly so complicated compared to data-driven services that leverage the internet. Unfortunately, the SMS feature set has also failed to keep pace with modern messaging features. It can’t send video, location data, read receipts, and other quality of life features we’ve become used to. The only benefit still in its favor is that SMS is ubiquitous across devices and countries around the globe.
Internet-based messaging apps such as Signal, Telegram, and WhatsApp offer capabilities that far exceed SMS. They’re also continuing to innovate all the time, giving their users additional communication and privacy tools that you won’t find with SMS. However, you’ll likely find yourself having to use multiple apps to keep in touch with everyone on your contacts list, which is far from ideal.
Rich Communications Services (RCS) is designed to bridge this gap, offering more advanced features than SMS while still providing widespread device support through the use of carrier network infrastructure rather than relying on third-party servers. However, support for RCS is still spotty in places around the world and the service doesn’t offer native support end-to-end encryption. A much sought-after feature in the modern age of snooping and digital privacy. Fortunately, Google now supports this feature through its Messages app for Android.
Despite its lackluster feature set, SMS remains an invaluable messaging system around the world for companies and individuals alike. The standard isn’t going anywhere, even as networks transition over to next-gen 5G technologies.