Re: Erorr In Compile PDU

lin wrote:

> Hi John,
> Thanks for explaining how to decode PDU SMS that UD containing UDH, this
> confused me for a long time.

Thank you for your follow-up question to my post.

> I have a problem here, as you said "throw away the first 7 characters".
> Is it because the length of UDH(0500033A0202) is 6 bytes (48bits), so a
> fill bit needs to be added between UDH and SM to make SM starts from the
> septet boundary? (the length of UDH now is 7 bytes(49 bits), so the first
> 7 characters of the whole UD we decode are pseudo characters, meaningless)
> 
> If it is, as I know UDH can have different lengths, e.g.
> 1. 060804F42E0201
> 2. 080701030003080202
> The length of the UDH in the first example is 7 bytes (56 bits), so I can
> decode the SM directly, or decode the whole UD and remove the first 8
> characters. In the second example, UDL is 9 bytes (72 bits), so there are
> 5 fill bits to make UDH 77bits and then SM can start on septet boundary.
> So UDH uses 11 septets, after decoding the whole UD, I should remove the
> first 11 characters to get the real SMS content.
> 
> Do i understand correctly?

You do.

In your second example, the value of 5 bits 73 through 77 have no relevance 
to the decode.  Their "correct" value is undefined.

But they form part of the 11th septet (starting at the beginning of the 
whole UD).  It follows that the 11th 7-bit character from that decode is 
likewise undefined.

John