Why the TGg compromise will work a MAC perspective November 2001 November 2001 Why the TGg compromise will work a MAC perspective Ronald Brockmann (Intersil) Wim Diepstraten (Agere) Maarten Hoeben (Intersil) Sid Schrum (Texas Instruments) Ronald Brockmann, et. al. Ronald Brockmann, et. al.

Goals Use OFDM concurrently with CCK in the presence of 802.11b November 2001 Goals Use OFDM concurrently with CCK in the presence of 802.11b Efficient mixed 802.11b/802.11g operation PBCC and CCK-OFDM options Don’t break 802.11b 802.11g product achieves in a mixed environment same or higher performance than 802.11b Works for current MAC with or without TGe enhancements Ronald Brockmann, et. al.

November 2001 Assumptions 802.11b PHY may not correctly see a pure OFDM frame as a busy channel 802.11g PHY can acquire on both Barker and OFDM preamble on every packet 802.11b rates are basic rates Control frames, multicasts and broadcasts are sent using Barker/CCK Ronald Brockmann, et. al.

How to protect OFDM frames? November 2001 How to protect OFDM frames? Use of RTS/CTS and protections of NAV An existing MAC mechanism to deal with hidden node situations The NAV of the RTS or CTS protects the OFDM Data Ronald Brockmann, et. al.

How about the performance? November 2001 How about the performance? Example Data frame of 1500 octets + additional MAC overhead: 802.11b – { Data – Ack } 1213цs + 10цs + 106цs = 1330цs 802.11g – { RTS – CTS – Data (24 Mbps) – Ack } 124цs + 10цs + 106цs + 16 цs + 532цs + 16цs + 106цs = 910цs Ronald Brockmann, et. al.

Further enhancements Don’t use RTS/CTS for shorter frames November 2001 Further enhancements Don’t use RTS/CTS for shorter frames The OFDM Data frame + preamble may be shorter than the RTS itself A TCP ACK frame 24 Mbit/s is just 2 slot times long! Use CCK rates for shorter frames Allow Acknowledgements to be sent on rates other than a basic rate. 11 Mbps RTS/CTS + 24 Mbps Data – Ack is as efficient as 11 Mbps Data – Ack MAC enhancements to increase efficiency of the MAC for certain PHYs may be incorporated by TGe or standardized by a separate TG Ronald Brockmann, et. al.

Centralized solutions November 2001 Centralized solutions The PCF allows efficient OFDM frame exchanges protected by the Contention Free Period No additional overhead is necessary to protect the OFDM frame exchanges from 802.11b How about the HCF HCF is based on the same mechanisms No known issues with current HCF draft Corner cases may need further evaluation Ronald Brockmann, et. al.

How to solve timing issues November 2001 How to solve timing issues General rules of thumb: Use OFDM SIFS after an OFDM frame on which a response follows. Use the SIFS associated with the received frame modulation Use 802.11b IFS and slottimes otherwise Ronald Brockmann, et. al.

Some timing examples <DIFS>[slots…] Data<SIFS>Ack November 2001 Some timing examples <DIFS>[slots…] Data<SIFS>Ack <DIFS>[slots…] RTS<SIFS>CTS<SIFS>Data<SIFS>Ack CFPoll<SIFS>Data<SIFS>CFAck+CFPoll… CFData+CFPoll<PIFS>CFPoll<SIFS>Null… Ronald Brockmann, et. al.

November 2001 Conclusions There is enough evidence to believe that the proposed compromise TGg PHY can be made to work efficiently with the existing MAC and the proposed TGe enhancements Both PBCC and CCK-OFDM use existing 802.11b preamble so there is no CCA issue Works smoothly alongside existing 802.11b equipment Ronald Brockmann, et. al.