Protection Mechanism and Ripple Effect

1. Protection Mechanism and Ripple Effect IEEE 802.11

2. IEEE 802.11 Access to the Medium • CSMA/CA is the mechanism by which 802.11 STAs share the transmission medium. • An 802.11 radio does not know that the frame being transmitted has collisioned because it can not transmit and receive at the same time • An 802.11 radio only knows that a frame has been delivered when the ACK comes back

3. IEEE 802.11 Access to the Medium • The rules of 802.11 Distributed Coordination Function DCF state that a radio must listen on the channel for a period defined by the DIFS (DCF Inter-frame Space). (basically the time after the ACK frame) • If the channel is idle, the radio may attempt to transmit • If the channel is busy then the radio must defer its transmission and wait a random back-off interval before trying again. • If a radio station has transmitted a frame but no ACK is received in return, then a collision is assumed to have happened. • When a collision is presumed the radio station must do the random back-off with an increased interval • This is meant to get a transmit opportunity TxOP fairly allocated among all the radio stations

4. IEEE 802.11 Access to the Medium • 802.11 CSMA-CA includes Virtual Carrier Sense mechanisms. • The frames transmitted must show a DURATION field that indicated the estimated total time required for this particular frame delivery including the time of the corresponding ACK to be sent back as a receipt of reception. • The other stations set their counters according to this DURATION information. The name of the counter is NAV Network Allocation Vector.

5. Mutual Interference in Wi-Fi Ripple effect in 802.11

6. IEEE 802.11g/b Mixed Mode • The main idea of coexistence in the IEEE 802.11g specification is to protect the WLAN from interferenceby other 802.11 radios. • 802.11g radios function in the same band (2.4 GHz) than 802.11b but they do not use all the same velocities, modulations and coding systems. • The RF signals do interfere even if they are different systems. • There must be a system to minimize the disruptions.

7. IEEE 802.11g Amendment • 802.11g radios and 802.11b are only compatible at the lowest, most basic transmission sets. • An 802.11b radio transmits at 1, 2, 5.5, and 11 Mbps using DSSS and CCK. • An 802.11b radio does not understand OFDM. • 802.11g radios understand the basic rates transmission methods of 802.11b, but they would prefer to transmit in the faster OFDM that can yield 54 Mbps. • The problem is when the two radios coexists, they would interfere to each other even if they use different transmission systems.

8. IEEE 802.11g Amendment • An 802.11g access point AP should signal all associated client stations that there is the presence of an 802.11b station nearby. • This operation is called “Protection Mechanism” • This signalling must be done in the lowest data bit rate available so all stations, 802.11b and 802.11g, can understand it. • This signalling is done with Beacon frames

9. IEEE 802.11g Amendment • When an Access Point detects the presence of an 802.11b transmitter (it could be either another AP or a client station) the AP will signal Beacons with the “NonERP_Present” bitUse_Protection bit set to 1 • That means that the 802.11g stations should say “RTS Request to Send” before they transmit. • This RTS is transmitted in the most basic, lowest velocity rate that any station can understand either 11b or 11g. • The answer to a RTS is a Clear to Send CTS sent back by the AP to the client station

10. IEEE 802.11g Amendment • All stations read the RTS and CTS. • These frames contain the Duration field set to the estimated time that the RF medium will be occupied. • So the stations set their Network Allocation Vectors NAV to such time and then they wait (this is the principle of Distributed Coordination Function for Wi-Fi MAC Medium Access Control )

11. Extended Service Set Scenario • This scenario is a properly designed service set with non-adjacent channel reusing and a high rate HR data bit rate capability (11g or 11n) in the 2.4 GHz band • A lower, more basic rate station (11b) is present nearby • This station associates with one AP that is in Mixed mode 11g/11b Associated IEEE 802.11b 1 11 11 6 1 1 11 6 6 1

12. Extended Service Set Scenario • The AP can be heard (low signal level) by other APs in the same channel 1. • When the AP detects the presence of the 11b client, it activates the protection mechanism. • That means other pure 11g (or 11n, same case) stations start using RTS and CTS. Associated IEEE 802.11b AP1 channel1 11 11 6 1 1 11 6 6 1

13. Extended Service Set Scenario • Since the other APs in the set can hear the faint signals from AP1, they also activate their protection mode mechanism. • Other APs can hear the faint signals from these APs. • The activation of the protection mechanism cascade throughout the whole. • The presence of an 11b station in the perimeter of the set has a ripple effect. Associated IEEE 802.11b AP1 channel1 11 11 6 1 1 11 6 6 1

14. IEEE 802.11g/b Mixed and 11g Only Mode • The 802.11g specification defines the mechanisms of coexistence with 802.11b radios. • The option 11g/b MixedMode acknowledges the potential presence of an 11b transmitter. • Mixed mode activates a Protection Mechanism. • This mechanism requires 11g radios to send signals that 11b can understand. This result in a lower overall performance for the wireless service set but that is the price for the coexistence. • The option 11g Only just ignores that an 11b radio might be present. Only Mode implies no Protection Mechanism so it should only be used when there is the certainty than an 11b radio will not be ever present.

15. Protection Mechanism Ieee 802.11n

16. IEEE 802.11n • The main idea of coexistence in the IEEE 802.11n specification is to protect the WLAN from interference from other IEEE 802.11 radios. • 802.11n can function in both bands 2.4 and 5 GHz so it must coexist with 11a, 11b, and 11g • The 802.11n specification defines the mechanisms by which HT High Throughput radios can coexist with non-HT radios such as 802.11a/b/g

17. IEEE 802.11n Coexistence • 802.11n radios face the same problem than 802.11g radios. • The solutions are similar for 802.11n also. • 802.11n radios are ERP and they also coexists with HR/DSSS (802.11b) radios.

18. 802.11 Radio Preambles • A radio preamble and a PHY Layer header precedes 802.11 Frames • The radio preamble is used by radio receivers to tune to the signal correctly. • The radio receivers circuitry adjust factors such as gain, antenna diversity, and timing or synch.

19. 802.11 Radio Preambles • The PHY Layer header (or PLCP Physical Layer Convergence Procedure) contains information such as: • Modulation system • Coding system • Length of transmission

20. PHY Layer

21. PHY Layer Compatibility

22. PHY Layer • General Rule: • Later amendment 802.11 must have a basic notification system to advice previous 802.11 about their presence • Basically, certain fields in the frames must be visible and readable by all radios in the same shared radio band.

23. Clause 20 802.11n • Preamble Types • 802.11n specifies three PHY Preamble Types: • Non-HT (for OFDM 11a/g) • HT Mixed Mode • HT Greenfield (pure HT)

24. Non-HT Mode • Mandatory • For operation with legacy 11a/b/g

25. HT Mixed Mode • 11n stations with non-HT stations

26. HT Mode • Greenfield • Only HT stations are present

27. 802.11n Protection Rules • Clause 20 radios (referred as HT radios also) will use the Beacon and Probe Response frames to carry the HT Information Element. • The HT Information Element contains these fields: • Operating Mode • Non-Greenfield STAs Present • OBSS Non-HT-STAs Present

28. Operating Mode Field • 0: All stations present are HT • 1: there are Non-HT stations present in one of the bonded channels • 2: At least one HT station supports only 20 MHz channels • 3: there are Non-HT stations in the BSSS

29. 11n • 802.11n not only detects other radio systems but also bonded channels that might be overlapping with its own channel allocation.