Is SMS data 8 bits until transmitted?

Question

A lot of what I have recently read about SMS uses a specification of 140 octet characters, where most uses of SMS I am aware of use 160 septet characters. A UDH is 5 octets long, meaning if I want to send concatenated SMS I would only have 135 octet characters for my message data. This would allow me 154 septet characters after the UDH.

Do I take a 154 octet character message, append it to the 5 octet UDH, and send this to the modem as the message text, or do I have to encode my 154 message octet characters into a 7 bit character string, encode the UDH as a 7 bit string, concatenate the two, and send that text to the modem?

Answer 1

Most Content Interfaces (SMPP, EMI/UCP for example) to Mobile Networks are using 8 bit Character Set to submit messages, and are doing the bit stuffing somewhere deep in the SMSC.

An exception is the Modem AT+C Command interface, if you are using the PDU mode you have to do the bit stuffing on your own, and submit 140 hex bytes.

hth