Saturday, December 3, 2016

Protection Mechanisms

In a heterogeneous network, clients of different standards (11b, 11g/11a, 11n) may be present. ion should use some protection mechanism to protect its communication. However, these mechanisms have overhead and should be used only when required. Its the responsibility of AP to inform its clients about presence of devices of different standards:

Presence of 11b Clients:

Presence of 11b client/s is indicated by the AP in the "ERP Information Element". If the "Use Protection" bit is set then stations should use the protection mechanism. The AP sets this bit if at least one 802.11b station is associated with BSS. The AP may, but is not required to, also set this bit if it detects the presence of 802.11b stations in a neighboring BSS. 
Note: ERP is synonymous to 11g  and hence non ERP means 11b stations. In the "ERP Information Element" there is another bit "NonERP Present" which is used to indicate (by AP & Stations) the presence of 11b clients).

Hence, any 11g/a or HT client should protect the communication when above mentioned bit is set. The method to protect is RTS/CT or CTS-to-self with a DSSS/CCK format at 11b rates.

Presence of 11g/a Clients:

Indication of associated non-HT stations (11g/a) in the BSS is given by "HT Protection" sub field in the HT Information element.  The AP may set the "OBSS Non-HT STAs Present bit" in the HT Information element if it detects non-HT stations on the primary or secondary channel that are not members of the BSS.
If there are HT stations associated with the AP that are not able to receive Greenfield format PPDUs then the AP will set the Non-Greenfield HT STAs Present bit to 1 in the HT Information element. 

However, HT transmissions are inherently protected against 802.11g and 802.11a (non-HT) stations through the use of the HT mixed format frame. This frame format includes a legacy compatible preamble, which allows non-HT stations to defer correctly for the duration of the frame.

There are few special exceptions:

  • RIFS Bursts: When spacing b/w frames in RIFS then non-HT stations may not defer correctly.  An AP can prevent RIFS from being used in a sequence by any station associated with the BSS by setting the RIFS Mode subfield in the HT Information element to 0. A station may only use RIFS bursting if the RIFS Mode subfield is set to 1. If RIFS bursting is permitted then the station may, but is not required to, protect RIFS sequences if there are legacy stations present (as indicated by "HT Protection" field). Given the overhead associated with protection it is likely that implementations would use SIFS rather than provide protection unless protection was needed for other reasons.
  • Presence of HT Greenfield Stations: The Greenfield format preamble is shorter and thus more efficient than the mixed format 
    preamble. However, the Greenfield format preamble is not compatible with legacy  
    stations and, support being optional, is also not receivable by some HT stations. When the Non-Greenfield HT STAs Present bit is set to 1 or the HT Protection field is set to 3 in the HT Information element then stations associated with the BSS must protect Greenfield format PPDUs.
  • 40 MHz transmission in 20/40 MHz BSS
For these exceptions, following protection mechanisms are available:
  • RTS/CTS:  The initiator begins a TXOP by transmitting a RTS frame, setting the Duration field in the RTS to the expected duration of the TXOP less the duration of the RTS frame itself. The responder sends a CTS frame setting the Duration field to the value seen in the RTS frame less SIFS less the duration of the CTS frame. The CTS frame is transmitted with the same modulation and coding as the RTS frame. Stations that successfully demodulate either the RTS or CTS frame or both frames have their NAV set for the duration of the TXOP and will not transmit during that time.
  • CTS-to-Self: 
    In most networks all stations are able to detect transmissions of all other stations and hidden nodes are rare. For such situations, the CTS-to-Self mechanism offered significantly reduced overhead compared with a full RTS/CTS exchange. At the beginning of a sequence, the initiator sends a CTS frame with the RA field set to 
    its own MAC address and the Duration field set to the expected duration of the sequence less the duration of the CTS frame itself.
Note:  In the case of a 40 MHz HT sequence the RTS/CTS exchange or CTS-to-Self transmission occurs on the primary channel.
  • a non-HT or HT mixed format PPDU soliciting a non-HT response PPDU (Data/ACK sequence):  A sequence that begins with a legacy compatible frame, for example a data frame, and that solicits a response frame, an ACK frame in this example, could be used to set NAV in nearby stations. In the below diagram, the data frame is likely transmitted using a high order MCS, however, the ACK frame is robustly modulated using a non-HT format PPDU and should be widely received. The ACK frame could thus be used to carry the NAV setting for nearby stations.

Q: When a data frame is send at 11n rates with mixed format preamble, will it be decoded successfully by non-HT clients?

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