From greg.j.kazz at nasa.gov Wed Aug 5 13:26:20 2026 From: greg.j.kazz at nasa.gov (KAZZ, GREG J. (MSFC-HP27)[MOSSI2]) Date: Wed, 5 Aug 2026 17:26:20 +0000 Subject: [Sls-slp] Enabling SPDU Type 5 to exceed 120 bits Message-ID: Dear SLP members, I am looking forward to your feedback on the following technical issue. Please respond back to me by Aug 12. We need a way to increase the maximum size of the Type 5 SPDU. Currently, the maximum size of a variable length SPDU is 120 bits due to the limit of the 'length of SPDU data field' to 16 bits. There is a RID against 235.1 that recommends that the maximum length of this SPDU be expanded. The RID points out that often more than 120 bits of directive data will be used for hailing and comm change e.g., Set Fixed-Length Frame (fwd) 24 + Set Fixed-Length Frame (rtn) 24 + LEC(rtn) 96 + Ranging 96 + LEC (fwd) 96= 336 bits are used. Below is my thinking and two proposed options below: The 120-bit maximum size variable length SPDU is too constraining, because only 4 bits are assigned to the 'Length of SPDU data field'. 120 bits works for SPDU Types 1 though 4, so I suggest we modify Table 3-2 to contain only those SPDUs and delete types 5 through 8 from that table. Then Types 5-8, can each be uniquely defined, whenever there is a need to define them. We currently only have a need for S/Ka band Lunar directives for Type 5. Option 1: My suggestion for Type 5, is to expand the SPDU Header to 2 octets, fixed: SPDU Format ID (1 bit) = '0' SPDU Type ID (3 bits) = '100' (Type 5) Length of SDPU Data Field (12 bits) = ( 0 to 4094 octets) ... from 4 bits to 12 bits So with this new Type 5 format, all 336 bits from the example above would easily fit into a single SPDU within a single P-frame. See the attached pink sheet to enable that change as a Word file. Option 2: Explicitly state that a P-frame may contain one or more variable length SPDU without intervening bits between them. This is currently the case, but it may be somewhat obscure to the user. The current method of accommodating more than 120 bits in a hail frame or "comm change" frame would be to concatenate multiple SPDUs back-to-back with no interleaving bits within a P-frame. However, it would be less efficient and convey no additional useful information if the SPDU header is duplicated for every 120 bits of directive data used. Please let me know which option you prefer. Thanks much, Greg SLP WG Chair -------------- next part -------------- An HTML attachment was scrubbed... URL: -------------- next part -------------- A non-text attachment was scrubbed... Name: Larger_SPDU_Type5.docx Type: application/vnd.openxmlformats-officedocument.wordprocessingml.document Size: 16434 bytes Desc: Larger_SPDU_Type5.docx URL: