Protection Mechansims in 802.11g

Need for Protection 

In a WLan BSS with both 802.11b and 802.11g devices,  the AP ( 802.11g)  has to serve both 802.11g and 802.11b clients. Due the difference in chipset implementations of 802.11b and 802.11g devices, a 802.11b device cannot decode  high rate  transmissions from a 802.11g device.  Unaware of a 802.11g device's transmission, if a 802.11b client transmits at the same time, it will cause interference to 802.11g's transmission.  So 802.11g specifies certain protection mechanisms to ensure that 802.11g's transmissions are not interrupted by the legacy devices.

These protection mechanisms  are there to make sure that the legacy devices are  aware when a 802.11g device is transmitting so that , legacy devices can defer their transmission.

Types of Protection Mechanisms in 802.11g

802.11g specifies two types of protection mechanism. 

1) cts-to-self protection mechanism: In this , when ever a 802.11g station has a ofdm data frame to send, it sends a CTS-TO-SELF frame are a lower data rate  using a modulation that can be understood by all the stations . The destination address of the cts-to-self frame is the same as that of the sending station. The sending station will update the duration value to that of the time required to transmit the cts, ofdm frame and the ACK frame. All stations in the network will listen to this CTS frame and update the NAV value accordingly and will not transmit during this period.

2)  Full RTS/CTS exchange : In this, whenever a 802.11g client intends to transmit, it sends a RTS frame to the receiver, and the receiver sends a CTS frame in response. Here the RTS/CTS frames are sent at the rate and modulation that can be understood by all stations in the network. The 802.11b stations will update the NAV based on the duration value read from the RTS and CTS frame. This mechanism is more efficient in the presence of hidden nodes

Protection frames are transmitted based on the rules for 802.11b . These frames are sent at 1Mbps or 2Mbps with Barker Chip codig or using 5.5Mbps or 11 Mbps using CCK coding.

When DSSS-OFDM  phy is used, no protection mechanism is necessary since header is modulated using DSSS modulation , this can be understood by 802.11b devices

When and how is protection activated?

Within an ERP BSSS , if there are 802.11 or 802.11b stations, these are known as non-erp stations

When is protection activated:

1. Within a ERP BSS, if a non - ERP station ( 802.11 or 802.11b) station associates with ERP AP then protection is activated. 

2.  If an ERP AP hears a beacon from 802.11b or 802.11 AP  , a co-channel AP, then protection is activated. 

How is protection activated?

The station responsible for sending the Beacon is also responsible for deciding whether to activate protection. In IR network, the Access Point takes care of activating the protection mechanism. Once the AP decides to enable /activate the protection mechanism, it needs to inform or indicate the other erp stations to use the protection mechanism.

ERP Information Element is present in the beacon frame, this is used to control the protection mechanism in the network. There are two fields  in the IE ,Non-ERP present and use protection. These 2 bits are set to 1, when non-erp station assocites with erp AP or ERP AP hears beacon from a 802.11/802.11b AP in the same channel.

When this bit is set, ERP stations in the BSS will use the protection mechansim when sending ERP OFDM frames









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