The 127-Byte Frame Budget

Ada re-derives this chapter’s own numbers step by step, at full precision

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Ada ADA · CALCULATION AUDIT

The 127-Byte Frame Budget

An 802.15.4 data frame has a hard 127-byte cap: a compact in-PAN frame with short addresses leaves 116 payload bytes, but switching both endpoints to 64-bit addresses costs 12 bytes and enabling MAC security costs another 9, dropping the compact frame to 107 bytes. That is already under the “about 110 bytes” the chapter warns about, before any 6LoWPAN or routing header is added. This audit adds the header up byte by byte and asks whether the frame budget survives real addressing and security, or quietly goes over before the first capacity claim.

Companion to the chapter 802.15.4 Specifications and Roles — every number here comes from that chapter.

Ada: The chapter says a compact in-PAN data frame leaves 116 payload bytes, that long addresses cost 12 of them, and that MAC security takes another 9. Let me add the header up byte by byte and check each figure against the 127-byte cap.

  • Compact overhead with PAN-ID compression and short addresses: FCF 2 + sequence 1 + dest PAN 2 + dest short 2 + source short 2 + FCS 2 = 11 bytes, so payload space is 127 - 11 = 116 bytes.
  • Switch both addresses to the 64-bit EUI-64 and each grows from 2 to 8 bytes: 11 + (8 - 2) x 2 = 11 + 12 = 23 bytes of overhead, leaving 127 - 23 = 104 bytes. Short addressing therefore recovers exactly 116 - 104 = 12 bytes per frame.
  • Enable MAC security with a 5-byte auxiliary header and a 4-byte message integrity code: 5 + 4 = 9 bytes, dropping the compact frame from 116 to 116 - 9 = 107 bytes.

Every number checks out, and the audit exposes the real squeeze: the compact frame starts at 116 bytes but security alone takes it to 107, so the “about 110 bytes” payload the chapter warns about is already over budget once a 6LoWPAN or routing header is added - which is exactly why the frame budget must be counted before any capacity claim.

Every number above is taken from the chapter’s own material and re-derived step by step.