Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: Improved PHY header for 15.4a. Date Submitted: Original (r0) wasNovember 15, 2006; Corrected (r2) was Feb 17, 2007. Source: Vern Brethour, Time Domain Corp Contact: Vern Brethour, Time Domain Corp Voice: +1 256.428.6331; E-Mail: vern.brethour@timedomain.com; Re: TG4a Abstract: Addition of a SECDED code to the PHY header. Purpose: To inform and enable comment resolution. Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15. Vern Brethour

2. The 15.4a header.(Coverage with a Hamming code.)The r2 revision fixes typos in the “Pr” (preamble length) designators. Vern Brethour

3. Why are we messing with this now? • In the course of studying comments on sponsor ballot, the 15.4a task group was forced to re-visit the risk of packet loss due to bit errors in the header. • To balance the error protection between bits in the header and bits in the payload, we will add a Single Error Correct, Double Error Detect (SECDED) code to the header. Vern Brethour

4. This is the header from draft 5 This is the header with SECDED 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Rt1 Rt0 L6 L5 L4 L3 L2 L1 L0 Rng Ext Pr1 Pr0 C5 C4 C3 C2 C1 C0 Preamble length Data rate Frame length field SECDED check bits Ranging flag Extension bit Vern Brethour

5. SECDED adds 6 check bits. First bit transmitted Last bit transmitted Transmit order 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Rt1 Rt0 L6 L5 L4 L3 L2 L1 L0 Rng Ext Pr1 Pr0 C5 C4 C3 C2 C1 C0 Preamble length Data rate Frame length field SECDED check bits Ranging flag Extension bit C0 = XOR ( Rt1, Rt0, L5, L4, L2, L0, Ext, Pr1 ) C1 = XOR ( Rt1, L6, L5, L3, L2, Rng, Ext, Pr0 ) C2 = XOR ( Rt0, L6, L5, L1, L0, Rng, Ext ) C3 = XOR ( L4, L3, L2, L1, L0, Rng, Ext ) C4 = XOR ( Pr1, Pr0 ) C5 = XOR ( Rt1, Rt0, L6, L5, C3, C4 ) Be careful reading the formula for C5: In effect, C5 is the XOR of all the non-check bits. C3 and C4 are just used as terms in C5 as an expedient way to pick up 9 of the bits. Vern Brethour

6. So what’s going on with the check bits? • The check bits on the previous slide are the bits for a standard Hamming SECDED code. • If no protection is desired, the check bits can simply be thrown away at the receiver. • The check bits can also be re-computed at the receiver from the other header bits received and compared to the received check bits. • Received check bits that do not match the re-computed values are the “syndrome bits”. Vern Brethour

7. Meaning of the syndrome bits: Syndrome Syndrome error error C5 C4 C3 C2 C1 C0 C5 C4 C3 C2 C1 C0 0 0 0 0 0 0 none 1 0 1 0 1 0 L3 1 0 0 0 0 1 C0 1 0 1 0 1 1 L2 1 0 0 0 1 0 C1 1 0 1 1 0 0 L1 1 0 0 0 1 1 Rt1 1 0 1 1 0 1 L0 1 0 0 1 0 0 C2 1 0 1 1 1 0 Rng 1 0 0 1 0 1 Rt0 1 0 1 1 1 1 Ext 1 0 0 1 1 0 L6 1 1 0 0 0 0 C4 1 0 0 1 1 1 L5 1 1 0 0 0 1 Pr1 1 0 1 0 0 0 C3 1 1 0 0 1 0 Pr0 1 0 1 0 0 1 L4 1 0 0 0 0 0 C5 0 any non-zero syndrome 2 errors Vern Brethour

8. So what goes in the Draft? • The (new) bit ordering of the header. • The equations for the 6 new check bits. • A statement (in text) that the check bits enable implementation (in the receiver) of a Hamming SECDED decoder covering all 19 bits of the header. • The error decoding table on the previous slide is not a secret (it’s posted on the IEEE802.15 document server) but since it’s a receiver thing, we will not put it into the standard. Vern Brethour