Why SMS is just 160 char ?

After knowing the way how to send SMS, we continue to explain why we just send SMS in 160 characters / message. From where does it from ?

To understand this case, we’ve to know about PDU. PDU ( Protocol Description Unit ) is one way to send SMS beside using SMS text mode.
PDU can send binary information SMS in 7 bit or 8 bit format.
The text mode is just an encoding of the bit stream of  PDU mode. If we read SMS from our computer application, we need AT commands.
If  we use phone, it will encode for reading incoming SMS message with can use text mode or PDU mode. If we used text mode, it was limited by phone ability.

For default alphabet, we use 7 bit / characters, and for another purpose like smart messaging like images and ringing tones or chinese and  japanese we user 8 bit.
For other purposes 16 bit will be used like for Flash SMS which show blinking or alerting SMS.

The standard SMS for Digital code scheme is 140 byte. It means we have 140×8 = 1120 bit data. For SMS code 7 bit we just have = 1120 / 7 bit = 160 characters.
Same like that for 8 bit we have = 140 characters and 70 characters for 16 bit.
Thats why for standard SMS we just have 160 characters maximum for one messages.

PDU string consist of message and meta information about the sender, SMS Center, time stampt, protokol identifier etc.

For example if my number +622199719478 sending hello to others, here is the examples of PDU :

07 91261299174987 040BC87238880900F10000993092516195800AE8329BFD4697D9EC37

This octet sequence consists of three parts: An initial octet indicating the length of the SMSC information ("07"), the SMSC information itself : ("91261299174987"), and the SMS_DELIVER part for the next part.

All the octets are hexadecimal 8 bit octet, except for Service center mobile, sender number, time stamp. They are using decimal semi octet. The format of semi octets are decimal, and support for number even. So if the number is odd we have to add value F in the end of octet data before swapping it.

So if my number is +6221997194787 we just add like this :

91261299174987F7

So now we undertsand why we just have 160 characters for maximum sending one messages. But it is possible we extend it if we user long concatenated SMS.

As generally, concatenated SMS or known as long sms can be sent using multiple SMS or more than one SMS just like as usual.
We use User Data Header (UDH) which contains information segmentation.
So if we use UDH, we need additional byte inside payload which decrease amount of maximum characters in one message.

For 7 bit we have 153 char, 133 for bit and 67 for 16 bit encoding.
So if we use UDH, the maximum characters in one message is not 160 char anymore !

In receiver, the handset can reassembly and make them into one long messages, based on phone capabilites. We will discuss it further in the next post Insya Allah : )

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