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ANSI SCCP data length query

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merodio

Computer
Feb 7, 2005
1
Hi,
I'm having a bit of trouble working out exactly the allowed length of data in an SCCP LUDT message.

Standard T1.112.3-2001 Table 11B has the length of the "data" (Long Data) parameter as 3-3904. Section 4.1, bullet point 3, of the same document mentions that the value in the table includes "For type V parameters: the length, in octets, of the length indicator and of the parameter content". From this I'm concluding that the "Long data" parameter includes 2 octets of length (as stated in section 4.20 of T1.112.2-2001: The 'Long Data' parameter is a 'data' parameter with a two octet length indicator) and the remainder (3902) being the data.

Is this a correct assumption?

As an example to further convince me that my assumption is correct, if you refer to say the "segmentation" parameter in the same table, it is of size 6 which comprises of 1 octet for the parameter type indicator, 1 octet for the length indicator and 4 octets for the actual segmentation parameter. Hence I can only conclude that the table includes the lengths of the length indicators.

The problem I have is that for an XUDT, the length of the "data" parameter is from 2-251, of which I'm assuming 1 octet is for the length indicator, hence giving me 250 octets of actual user data. However, if this were the case then a segmented message (of which there can only be a max of 16 segments) could contain a maximum of 243 x 16 = 3888 octets (250 - 7 [for segmentation parameter (6 octets) and end of optional parameters (1 octet)] = 243). This is less than the 3902.

Also in T1-112-4-2001, section 4, footnote 11, the equation used is (251-7) * 16 = 3904. It's hence basing its calculation on the value of 251 which I'm assuming has 1 octet of length indicator, and also the 3904 contains my assumption of a 2 octet length indicator.

The ITU standard is unambiguous in this area. It notes that the "data" parameter contains (for XUDT) 254 octets + 1 octet for length. Therefore for a segmented message, the max size is (254 - 7 [segmentation parm + end of optional parm]) * 16 = 3952. The LUDT mentions that the data length is 3-3954, noting that this includes 2 octets of length, hence giving the size for actual data as 3952, which matches the above calculation!

I hope this all makes sense and you can see why I'm confused.

You enlightenment would be greatly appreciated.

Thanks
 
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