In the world of mobile messaging, the character limit for a standard SMS (Short Message Service) is capped at 160 characters. While this may seem restrictive, most modern devices allow you to send much longer messages without any issues. How does that work? The answer lies in a behind-the-scenes process known as SMS concatenation.
In this blog post, we’ll dive into how SMS concatenation works, why it's necessary, and the mechanics that make it seamless for users.
What is SMS Concatenation?
SMS concatenation refers to the process of splitting a longer message into smaller parts, sending each part as an individual SMS, and then reassembling them on the recipient’s phone to display as one continuous message. This allows users to send texts longer than 160 characters without disruption.
Each segment in a concatenated SMS is transmitted as a separate message, but they are tagged in a way that allows the recipient's device to know how to piece them back together in the correct order.
Why is Concatenation Necessary?
The original SMS protocol was developed for simplicity and efficiency, and intended for brief communications. Given the limited bandwidth available for text messaging in the 1990s, the 160-character limit was chosen to maximize efficiency while ensuring messages could be delivered quickly over mobile networks.
However, as messaging evolved, people wanted to communicate more than just brief thoughts. The 160-character cap became restrictive. Instead of overhauling the entire SMS system, concatenation was introduced as a clever workaround, ensuring that users could still send longer messages by essentially stitching smaller messages together.
How Does SMS Concatenation Work?
Here’s a breakdown of the SMS concatenation process:
Message segmentation
When a user sends a message that exceeds the 160-character limit, the message is split into multiple parts. Each part is typically up to 153 characters long. This allows for 7 characters to be used for metadata, which is necessary for the phone to reassemble the message.
UDH (User Data Header)
Each segment of the message contains a User Data Header (UDH), which holds crucial information for concatenation:
A unique reference number, so the phone knows all the segments belong to the same message.
The number of segments in the message.
The segment’s position in the overall sequence.
This metadata ensures that even if the segments are received out of order, the recipient's device can still display them correctly.
Reassembly
Once all the parts of the message are received, the recipient’s device uses the UDH data to combine the segments into the correct order. The result? The user sees a single, coherent message as if it were sent as one long SMS.
Delivery considerations
Since each part of a concatenated SMS is sent as a separate message, carriers typically charge for each individual segment. So, a long message that spans three segments would be billed as three separate SMS. However, on most modern messaging apps, this process is invisible to users, who only see the combined message.
Example of SMS Concatenation
Let’s say you want to send this message:
“Hey, just wanted to remind you about the meeting tomorrow. Make sure to bring the presentation notes and don’t forget to follow up with the client.”
This message is 159 characters long, so it could be sent as a single SMS.
Now, imagine you add a bit more:
“…Also, we need to finalize the budget by Friday, and the deadline for the report is next week. Let’s meet earlier in the day to go over everything in detail.”
Now your message is 258 characters. In this case, SMS concatenation will come into play. The message will be split into two segments:
Part 1: “Hey, just wanted to remind you about the meeting tomorrow. Make sure to bring the presentation notes and don’t forget to follow up with the client." (153 characters)
Part 2: “…Also, we need to finalize the budget by Friday, and the deadline for the report is next week. Let’s meet earlier in the day to go over everything in detail.” (105 characters)
These two parts will be sent as separate SMS messages but will appear as one message on the recipient's phone.
Technical Details of Concatenation
Character encoding
SMS supports different types of character encoding, which affects how many characters can fit into one segment. The most common types are:
GSM-7: The default encoding that uses 7 bits per character, allowing up to 160 characters per message or 153 characters per segment when concatenated.
UCS-2: Used for languages that require more complex characters (such as Chinese, Arabic, etc.). This encoding uses 16 bits per character, reducing the maximum characters per message to 70, or 67 characters per segment when concatenated.
SMS limitations
While SMS concatenation allows for longer messages, there are still practical limits. Most devices can handle concatenation of up to 6–8 segments, meaning messages longer than 918–1224 characters may be truncated or sent as separate parts that do not reassemble properly. In such cases, using MMS (Multimedia Messaging Service) or messaging apps (like WhatsApp or iMessage) is recommended.
Advantages and Disadvantages of SMS Concatenation
Pros:
Seamless Communication: Users can send longer messages without worrying about character limits.
No Loss of Information: The message is reassembled in full, ensuring nothing is lost in transit.
Cons:
Costs: Each segment counts as a separate SMS, leading to higher costs if you're sending long messages frequently.
Delivery Issues: If one or more segments fail to deliver, the entire message may be incomplete or out of order, especially if the phone’s message app can’t handle reassembly properly.
Conclusion
SMS concatenation is an essential feature that has enabled longer text communication within the constraints of the SMS protocol. Although SMS is less dominant today, with modern messaging apps largely replacing it, concatenation ensures that people can still send extended messages via standard SMS without breaking them into awkward 160-character chunks.
Understanding how SMS concatenation works can help you manage your text communications better and be mindful of potential costs or technical issues when sending long messages.
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