802.11g Frame Sequence Variations - Simplified Analysis (802.11g Draft 2.1, January 2002)

1. 802.11g Frame Sequence Variations(based on 802.11g Draft 2.1 Jan-2002) Terry Cole AMD Fellow terry.cole@amd.com +1 512 602 2454 Terry Cole, AMD

2. Introduction • We provide many PPDU formats that can be used in 802.11gD2.1. • CCK long, CCK short, OFDM, CCK-OFDM, PBCC • Each frame, in general, may be transmitted using any format. • For this presentation, I have reduced the formats for simplicity: CCK and OFDM. • To achieve interoperability, everyone should support receiving all possibilities! Terry Cole, AMD

3. Transactions • Table 21 and 22 of 802.11 provides a list of all legal frame sequences. • We can analyze these to determine if 802.11g D2.1 to determine all the sequences that should interoperate. Terry Cole, AMD

4. Example #1: data(bc/mc) • We have 1 frame & 2 formats. 2 possibilities exist: • Data(bc/mc) (cck) – i.e. transmitted using a CCK format • Data(bc/mc) (ofdm) – i.e. transmitted using OFDM format • The standard requires broadcast and multicast at a rate in the basic rate set. • Case 1: A mixed environment • Defined by only 802.11b rates in the basic rate set • data(bc/mc) (cck) is the only sequence allowed. • Case 2: An 802.11g-only environment • Defined by at least one ERP rate in the basic rate set • Two sequences may be encountered, with selection by the transmitter (indicated by *, i.e., receivers always ready for either): • *data(bc/mc) (cck) • *data(bc/mc) (ofdm) Terry Cole, AMD

5. Example #2: PS-Poll Last ACK • There are 8 enumerable possibilities. • The standard requires: • The PS-poll control frame must be sent at a basic service set rate. • The ACK shall be OFDM if data is OFDM. No parallel for CCK exists. • Case 1: Mixed environment (i.e. 802.11b not excluded) • Two sequences allowed with selection up to AP (i.e. sleeping STA is ready for either): • PS-Poll (cck) *Last (cck) ACK (cck) • PS-Poll (cck) *Last (ofdm) ACK (ofdm)  not protected against preemption • Case 2: 802.11g only environment (i.e. 802.11b cannot join) • Three additional sequences allowed (waking STA makes first format choice, and AP makes second format choice): • PS-Poll (ofdm) *Last (cck) *ACK (cck) • PS-Poll (ofdm) *Last (ofdm) *ACK (ofdm) • PS-Poll (ofdm) *Last (cck-11) *ACK (ofdm-6) Terry Cole, AMD

6. Summary of Other Cases • There are quite a few frame sequences that have many possibilities in terms of using CCK & OFDM. • Fewest possibilities in contention period, and more in CP. • Fewer possibilities in mixed network; more in g-only. • Many options are eliminated for mixed networks. • Control, broadcast, and multiple recipients frames must use basic rate set. • Transmitter uses implementation specific knowledge to choose the “best” format for transmitting. • To maximize interoperability of the standard, receivers must be prepared to receive all allowable formats. Terry Cole, AMD

7. Receiver Multiple Format Sequences • Mixed Network (one not excluding 802.11b) • Contention period sequences:* indicates a multiple format frame choice. • {RTS CTS} [*Frag ACK] *Last ACK (4 format variations)  3 are not always safe! • PS-Poll [*Frag ACK] *Last ACK (4)  3 are not always safe! • Contention free period sequences: • *Mgmt(dir) • *Data+CF-Poll{+CF-Ack} *Data+CF-Ack {*CF-Ack} (8) • *Data+CF-Poll{+CF-Ack} *CF-Ack (4) • *CF-Poll{+CF-Ack} *Data+CF-Ack {*CF-Ack} (8) • *CF-Poll{+CF-Ack} *Data Ack (4) • *CF-Poll{+CF-Ack} *Data(null) (4) • *CF-Poll{+CF-Ack} Ack (2) Terry Cole, AMD

8. Receiver Multiple Format Sequences • 802.11g Only Networks • Contention period sequences:* indicates a multiple format frame choice. indicates a multiple format frame because of the special case of CCK-11 followed by OFDM-6 response. • *data (bc/mc) (2 format variations) • *mgmt (bc) (2) • {RTS CTS} [*Frag ACK] *Last ACK (8 + 19) • *PS-Poll ACK (2 + 1) • *PS-Poll [*Frag ACK] *Last ACK (8 + 10) • *DTIM(CF) [<CF Sequences>] {*CF-End} (4) • [<CF Sequences>] {*CF-End} (2) Terry Cole, AMD

9. Receiver Multiple Format Sequences • 802.11g Only Networks • Contention free period sequences:* indicates a multiple format frame choice. • *Beacon(CF) (2 format variations) • *data (bc/mc) (2) • *mgmt (bc) (2) • *mgmt(dir) (2) • *Data+CF-Poll{+CF-Ack} *Data+CF-Ack {*CF-Ack} (8) • *Data+CF-Poll{+CF-Ack} *CF-Ack (4) • *CF-Poll{+CF-Ack} *Data+CF-Ack {*CF-Ack} (8) • *CF-Poll{+CF-Ack} *Data Ack (4 + 2) • *CF-Poll{+CF-Ack} *Data(null) (4) • *CF-Poll{+CF-Ack} Ack (2 + 1) Terry Cole, AMD

10. Mixed Network Observations • In a mixed network during contention period, there are only a few multiple formats variations (8). • We should warn of the 6 combinations that are not always safe and strongly recommend the legacy beacon element use! • In a mixed network during the contention free period, a much larger number of possibilities arise (31). • Nearly half are because of the choice of PS-Poll format by the AP. • The other half are because of the choice of responding format for Data/ACK by the STA. Terry Cole, AMD

11. “G Only” Network Observations • In a “G only” network during contention period, anything goes (57 possibilities)! • There are 28 possibilities during CP because of the initiating frame format choices at each request, fragment, or data frame. • There are 38 choices during CFP because each frame may basically vary by format. • There are 32 additional possibilities because a control response may be OFDM-6 in response to CCK-11. Terry Cole, AMD

12. Straw Polls • Do so many receive possibilities threaten interoperability of the standard? or are the possibilities already fully comprehended in the scope of interoperability? • Should we allow CCK-11 frame followed by a OFDM-6 response in a g only network? Or should we make CCK responses parallel to OFDM responses? • Should we work to carefully decrease the possibilities without effecting decisions that enable smart devices to work in adverse conditions (e.g., fragment sequences all in one format)? Terry Cole, AMD