July 2001 doc.: IEEE 802.11-01/387r2

IEEE P802.11
Wireless LANs

ACK and NACK responses to FEC DATA MPDUs

Date: July 2001

Author: Adrian P Stephens
Mobilian Corporation
7431 NW Evergreen Pkwy, Hillsboro, OR 97124
Phone: (503) 681 8600
e-Mail:

Abstract

This document describes MAC-layer processes for acknowledging FEC-encoded DATA MPDUs

Note, this document in 11-01-387r0 proposed normative text for the generation of positive and negative acknowledgements. The need for negative acknowledgement was based on a false understanding of the mandatory nature support for CFB in an ESTA. This no longer applies. So this section describes how it is possible to decide to send an ACK in order to inform the debate.

1.1  7.5.1 Generating Acknowledgements to FEC-encoded MPDUs (informative)

Support for the behavior described in this section is optional.

1.1.1  7.5.1.1 Introduction

The use of separate RS codewords to protect the MPDU header and payload allows errors in the received header and payload to be discovered separately.

A station that receives an FEC-encoded DATA MPDU sent using DCF rules can use this property to select between one of two response behaviors:

·  generate an ACK MPDU (acknowledgement),

·  generate no response.

1.1.2  7.5.1.2 Procedure at the Receiver

Having received an MPDU, a station that implements this behavior selects between the two response behaviors defined in Table 1 using the decision logic defined in Figure 1. The conditions used to make decisions are defined in table Table 2.

In what follows, the term correctable means that an RS codeword can be determined to contain less than some implementation-dependent number of errors before a response MPDU must be transmitted.

Table 1 - Outcomes of Rx Response Decision

Outcome / Definition
Generate ACK / Generate an ACK using the rules defined in section xxx
Do not generate a response / No response MPDU is transmitted.

Figure 1- Rx Response Decision Logic

Table 2 - Conditions Affecting Receive Response Behavior

Condition / Definition
PHY length < min for RS? / The length of the MPDU as reported by the PHY RXVECTOR length field is less than the minimum FEC-encoded DATA MPDU length
To Me? / The MPDU header RA field contains this stations MAC address
Outer FCS OK? / The MPDU’s outer FCS indicates no errors in the received MPDU (without FEC encoding)
MPDU Header RS correctable? / The MPDU header is assumed to be FEC-encoded as described in section xxx.
Based on this assumption, the MPDU header RS codeword defined in xxx is determined to be received correctly or is correctable.
Corrected To Me? / The correctly-received or corrected MPDU header receiver address field contains this stations MAC address
Payload RS Correctable? / Every RS codeword following the MPDU header is received correctly or is correctable.

1.1.3  How good is the prediction “RS Correctable”

The RS-decoder indicates that a codeword has less than some number of errors. This indication is not always correct – i.e. it wrongly believes that there are less than some number of errors. The false positive error rate depends on the BER.

This is called here the false positive error rate. This section presents results for the false positive error rate as well as the uncorrected error rates for various sizes of payload.

Table 3 - Packet Error Rates for various payload lengths

Packet Error Rate for packets of payload length (Bytes)
Bit Error Rate / 256 / 400 / 1500
0.001 / 0.8535 / 0.9593 / 1.0000
0.0001 / 0.1747 / 0.2739 / 0.6988
0.00001 / 0.0190 / 0.0315 / 0.1131

Table 4 - False Positive Error Rates from RS Decoder

Submission page 1 Adrian Stephens, Mobilian Corporation