1. 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.

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

3. 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.

4. 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.

5. 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.

6. 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.

7. 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 b
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.

8. 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 b IFS and slottimes otherwise
Ronald Brockmann, et. al.

9. 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.

10. 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 b preamble so there is no CCA issue
Works smoothly alongside existing b equipment
Ronald Brockmann, et. al.