[Sis-dtn] practical size of the TC frame

Mon Mar 20 18:59:08 UTC 2023

Hi Jim,

Some answers below in blue.

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Marc Sanchez Net (332H)
Telecommunications Engineer
Jet Propulsion Laboratory
Cell: (617) 953-7977<mailto:(617)%20953-7977> | Email: marc.sanchez.net at jpl.nasa.gov<mailto:marc.sanchez.net at jpl.nasa.gov>
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From: SIS-DTN <sis-dtn-bounces at mailman.ccsds.org> On Behalf Of Jim.Bardgett--- via SIS-DTN
Sent: Monday, February 27, 2023 5:03 AM
To: Sis-dtn at mailman.ccsds.org
Subject: [EXTERNAL] [Sis-dtn] practical size of the TC frame

Hoping someone can point me in the right direction on this one. I'm assisting a colleague and we have questions about the frame size limitation:

  *   Upper TC frame size is 1019.  Is the size fixed to 1019? Can it be larger and what would the implications be?
The max size of a TC frame including header is 1024 bits. I believe this is due to the limited num. of bits in the header to encode the length of the transfer frame.
[cid:image001.jpg at 01D95B23.6012AD40]

  *   Lower TC Sync Ch Coding CLTU
Each TC frame must fit in one CLTU. Therefore, if using coding, you must fit the number of bits in a TC frame in an integer number of codewords at the Sync & Coding layer. For example, if using BCH (64,56) and a TC frame of 1024 bits, each CLTU will have ceil(1024/56)=19 BCH codewords, the last of which will have padding bits.

  *   Is there a size limitation for Lower TC protocol CLTU?
On the lower end, you need at least one codeword per CLTU, so depending on which code you use, the limitation will vary. For BCH (64,56), the min size of CLTU is 144 symbols, 16 for the start sequence, 64 for the BCH codeword (i.e., 1 BCH codeword), and 64 more for the tail sequence.
The upper limit on the CLTU size is given by the max TC frame size. Another factor is to ensure that the receiver maintains lock when start sequences (which act as sync markers) are increasingly further apart, which depends on SNR conditions, etc.

  *   In at least one document it says:

  *   TC transfer frame between the CLTU start/tail sequences has a max length of 1024 bytes

I'm believing these are the right references for my question. Is that in error?

Thanks and kind regards,

Jim Bardgett
Sr. Manager Cyber - Networks | Ch. Technologist Ntwk Engn
MS Digital Forensics, BSE Electrical, CISSP, CEH, JNCIS-ENT/SEC

Space and airborne systems / L3HARRIS TECHNOLOGIES
m +1 407 538 0431 / t +1 321 309 7164
L3Harris.com<mailto:L3Harris.com>  / jim.bardgett at L3Harris.com<mailto:jim.bardgett at L3Harris.com>
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