1. For concatenated messages, the maximum length will be reduced by the length of the user data
header (UDH). 3GPP TS 23.040 defines two kinds of UDH length: 6 bytes and 7 bytes, so the two
kinds of <uid> are 8-bit (6 bytes) and 16-bit (7 bytes). AT+QCMGS uses 8-bit <uid>.
In the case of GSM 7 bit default alphabet data coding scheme, the maximum length of each
segment of a concatenated message is (140 octets - 6 )*8/7=153 characters.
In the case of 16 bit UCS2 data coding scheme, the maximum length of each segment is
(140-6)/2=67 characters.
In the case of 8-bit data coding scheme, the maximum length of each segment is 140-6=134
characters.
2. <mr> Message-Reference field gives an integer representation of a reference number of the
SMS-SUBMIT or SMS-COMMAND submitted to the SC by the MS, and it is used to confirm
whether the SMS-DELIVER has been received from SC duplicate or not.
<uid> The field of UDH. It is message identification of the concatenated SMS, which is different
from <mr>. Each segment in a concatenated message should have the same <uid>, but
<mr> must be incremented for each segment of a concatenated message.
3. AT+QCMGS does not support to send message in PDU mode (AT+CMGF=0).
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