How to correctly represent message class in SMPP

Question

I am currently trying to figure out how sms classes are correctly represented in SMPP. However I am by now completely confused by the standard and it's documentation.

In normal sms we have

• Class0: Flash sms, which are shown on the display
• Class1: Normal Sms to be stored on the sim or internally in the device

Looking at the SMPP spec, I first find the parameter data_coding in the submit_sm operation, which is used to set the DCS sent via MAP. As far as I understand this, if we want to explicitly set the message class we need to set the first four bits of this parameter to ones, then two bits indicating the coding and then another two bits indicating the message class. So for Class1 Sms, we would set 1111xx01. Is this correct so far?

If we try to set this DCS, however currently we also set the data coding to "8-Bit data". It seems, several phones are not able to understand this. Is this specified anywhere, and can we just change this, or is a special coding needed when sending other message classes.

More confusion arises, when we try to use the SMPPv3.4 recommended way of setting the Message class. Since 3.4 there is an optional parameter in the submit_sm operation, called dest_addr_subunit. According to the standard this parameter should be set to 0 for unknown, 1 for MS-Display, 2 for Mobile equipment, etc. If I look at this, it seems the parameters are shifted by one compared to GSM message classes. Class0 is encoded as 1, Class1 is encoded as 2 and so on. Is this correct or is there any more complicated mapping behind this?

Also, if we set dest_addr_subunit, do we still have to set DCS as well, or can we just leave this parameter at it's default value?

Answer 1

I recommend to read 3GPP TS 23.038 specification with detailed DCS (Data Coding Scheme) description.

In case of DCS bits 7654 are 00xx, you should check DCS for bit 4 value.

• bit 4 == 0 - no message class for this message (bits 1 and 0 are reserved)
• bit 4 == 1 - bits 1 and 0 contains message class

So you should set data_coding SMPP parameter in accordance to 3GPP TS 23.038 specification to handle message_class properly.

By default GSM SMS message has no message_class and this is not the same as message_class = 1.