Everything you need to know about concatenated SMS

August 26, 2015

concatenated SMS

Concatenate: to link or join together, especially in a chain or series – Collins English Dictionary.

SMS has an incredibly important part to play in business. Many high priority alerts, applications and notifications use SMS messages. The medium is also used widely in marketing, customer service and many value-add services offered by companies.  Used properly and carefully, SMS has an important part to play in business communications.

However, in terms of the character limit, there are some challenges with SMS. Limiting your message to 160 characters can be tricky at times, especially if you have much to say. Some organisations view it as a challenge, others view it not quite so favourably. So what happens if you simply cannot squeeze your message into 160 characters?

You use concatenated SMS.

What is concatenated SMS?

Traditional SMS is limited to 160 characters. So what happens if you write a text longer than that limit? If your provider supports it, your handset and the network will divide that message into two or more SMS and send them individually to the recipient. The recipient’s phone will collect and reassemble those messages into the same order you sent it to recreate the message.

Done right, this process is invisible to both sender and recipient and will be handled entirely by your phone and the network.

The GSM network protocol used to send an SMS has a payload limit of 1120 bits per message. Latin-based languages such as English, French or German use 7 bits per character which is where the 160 character limit comes in, 7 x 160 = 1120.

Where things get complicated is when non-Latin languages want to send a message. Languages such as Mandarin or Arabic have to use Unicode format (UTF -16) to build their SMS which uses 16 bits per character. So their maximum message limit is a mere 70 characters, 16 x 70 = 1120. Users of these languages really benefit from concatenated SMS!

How does concatenated SMS work?

As mentioned above, the splitting up and reassembling of multiple messages is all done automatically. As a user, you shouldn’t even notice that your messages are being divided and the recipient won’t even know. The phones at either end will disassemble and reassemble the message automatically.

The technical bit

Data packets that make up an SMS are made up of two elements, the header and the payload. The header is the part that tells the network what it is, the phone number to send the message to, the phone number sending the message and what kind of message it is. The payload is where the message itself is contained.

In that header is a binary UDH flag. UDH is User Data Header which contains protocol information and an indicator as to whether a message is part of a series, a concatenated SMS or not. If this flag is turned on the handsets at either end will know they have to handle multiple messages, and that they have to split the SMS up and reassemble it at the other end.

That UDH flag will include the order number within the header so the receiving phone knows where to put that individual message in order. So it will add a code that says “1” to the first message, “2” to the second and so on. Then, the receiving handset will know in which order to rebuild the message.
Each SMS will be sent sequentially, but variables such as network latency and multiple routes means not all messages will necessarily be received in that same order. It is necessary to add the order number so that users don’t receive a garbled message.

As concatenated SMS requires more data in the header, it takes up a little more of that 1120 bit limit. The UDH part of the header requires 48 bits, meaning there is only 1072 bits remaining for the message. So using the maths above, dividing 1072 by 7 bits leaves only 153 characters per SMS, 1120 – 48 = 1072. 1072 / 7 = 153.

What does this mean to me?

As we said earlier, as a user you won’t even notice that you’re using concatenated SMS because it’s all done behind the scenes. Your handset will break up the message when you send it, the network will send them all to the recipient and the receiving handset will automatically reassemble the messages in the correct order.

Depending on your handset, you can see anything up to 255 concatenated SMS in a single series. Much depends on the capabilities of the phones in question and the network. Some mobile networks allow the full range of concatenated SMS while others will limit the number of messages you can send at once. This is to both preserve network traffic and prevent bill shock.

It is important to note that your mobile network bills each SMS individually regardless of whether it is part of a concatenated SMS or not. So if you’re in the habit of sending longer messages, you’re going to pay for it. As the concatenation process is invisible, you won’t necessarily be aware that you’re using multiple SMS to send a single message.

Concatenated SMS is a clever way of stitching multiple messages together that overcomes the 160 character limit. With some clever protocol use in the background, all the hard work is done automatically and invisibly. It allows mobile users to send what they like, when they like, where they like and is all part of the service!

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