How 11b Protection Works

For Access Points (AP) that support 802.11g, there is an Information Element (IE) in the 802.11 frame that describes Extended Rate Phy (ERP) data.  There are 2 fields relating to 802.11b protection:

•     If an AP of an IBSS detects one or more NonERP STAs that are part of the IBSS, then that AP sets the Use_Protection bit to 1 in the ERP IE.
•     The NonERP_Present bit is set to 1 when a NonERP STA is associated to the BSS.

The 802.11g specification also details in section some additional cases where these bits may be set.

These cases can be seen in the following examples from OmniPeek captures:

Capture 1 – An AP that has the Use_Protection bit set to 1 but the NonERP_Present bit is set to 0.  This means the AP doesn’t have 802.11b clients associated but has heard another AP advertising such clients that are associated or has heard an 802.11b client in the vicinity.

ERP Information
  Element ID:           42  ERP Information
  Length:               1
  ERP Flags:            %00000010
                        x... .... Reserved
                        .x.. .... Reserved
                        ..x. .... Reserved
                        ...x .... Reserved
                        .... x... Reserved
                        .... .0.. Not Barker Preamble Mode
                        .... ..1. Use Protection                    
                        .... ...0 Non-ERP Not Present

Capture 2 – An AP that has the NonERP_Present bit set to 1 and the Use_Protection bit set to 1.  This means that the AP has an 802.11b client associated.

ERP Information
  Element ID:           42  ERP Information
  Length:               1
  ERP Flags:            %00000111
                        x... .... Reserved
                        .x.. .... Reserved
                        ..x. .... Reserved
                        ...x .... Reserved
                        .... x... Reserved
                        .... .1.. Barker Preamble Mode
                        .... ..1. Use Protection
                        .... ...1 Non-ERP Present

On an Aruba controller, one can see if an AP is in protection mode with the following command:

(Aruba2400) #show ap debug radio-stats ap-name 1.1.1 radio 0  
Parameter            Value
---------            ----- -------------------  
General Per-radio Statistics
Total Radio Resets   2
Resets Beacon Fail   0
TX Power Changes     1
Channel Changes      2
Radio Band Changes   0
Current Noise Floor  101
11g Protection       1     ? Means Use_Protection bit is set to 1

Traffic When NonERP STAs are Present

The Use_Protection bit is used so that when ERP STAs coexist with NonERP STAs that there is a mechanism for NonERP STAs to know that ERP STAs are sending traffic so that collisions can be minimized.  In the absence of the Use_Protection bit, NonERP STAs would have no way of knowing ERP traffic is being sent and may send traffic at the same time, causing collisions that would hurt performance.

When the Use_Protection bit is present, then ERP STAs utilize protection mechanisms like Request to Send/Clear to Send (RTS/CTS) to notify NonERP STAs that traffic will be sent.  When sending RTS/CTS frames, the frames must be sent at data rates that NonERP STAs can understand.

Collisions will be minimized when protection mechanisms like RTS/CTS is used, but the extra overhead involved means that overall throughput is still affected.  So throughput when NonERP STAs are present can be considerably less than when ERP-only STAs are in the wireless network.

Disabling Protection

The Aruba OS in versions greater than 3.3.2.X has a knob for disabling 11b protection.  This knob should be used with extreme caution, since with protection disabled APs will not set the Use_Protection bit, even if NonERP STAs are present.  This means that if a NonERP STA is present and sending traffic, collisions will increment rapidly and seriously impact throughput performance.

To disable protection, :

(MC-02) #configure t
Enter Configuration commands, one per line. End with CNTL/Z

(MC-02) (config) #rf dot11g-radio-profile protect

(MC-02) (802.11g radio profile "protect") #?
dot11b-protection       Enable or disable protection for 802.11b clients.
                        WARNING: Disabling protection violates the 802.11
                        standard and may cause interoperability issues.

(MC-02) (802.11g radio profile "protect") #no dot11b-protection
WARNING: Disabling protection violates the 802.11 standard and may cause interoperability issues.

Version History
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Last update:
‎07-05-2014 03:59 AM
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