Here are the information elements added in 802.11n (HT)

CWAP-HT-IE-01In this post we will see the details of HT Operations information element. HT Operation Element is found in Beacon, Reassociation Response & Probe Response frames transmitted by an AP.

CWAP-HT-OPIE-1Here is a frame capture of a beacon frame shows these fields. HT Operation Element ID is 61 & length is set to 22 bytes.

CWAP-HT-OPIE-2Here is a brief description of each field that can be found in HT Operation Element.

1. Primary Channel
Indicates the channel number of the primary channel.

2. Secondary Channel Offset (2 bits)
Indicates the offset of the secondary channel relative to the primary channel.
1 (SCA) -secondary channel is above the primary channel
3 (SCB) -secondary channel is below the primary channel
0 (SCN) -no secondary channel is present
2 -reserved

3. STA Channel Width (1 bit)
Defines the channel widths that may be used to transmit to the STA.
0 – 20 MHz channel width
1 –  allows use of any channel width in the Supported channel width set

4. RIFS Mode ( 1 bit)
Indicates whether the use of reduced interframe space is permitted within the BSS. RIFS can be used only when a HT Greenfield network is in place.
0 – RIFS is prohibited
1– RIFS is permitted

5. HT Protection
Indicates protection requirements of HT transmissions.
0 – no protection mode (Green Field mode)
1 – nonmember protection mode
2 – 20 MHz protection mode
3– non-HT mixed mode

When HT protection is enabled within an HT BSS, an HT STA will precede HT transmissions with either an RTS/CTS control frame exchange or a CTS-to-Self control frame using modulation and coding understandable to the STAs that are being protected against. The Duration ID within these control frames causes STAs to update their network allocation vector (NAV), preventing them from initiating a new transmission until the end of the transmitting HT STA’s TXOP.

When protecting the transmission of 40 MHz HT frames against clause 17 (OFDM) radios and clause 19 (ERP) radios, protection mechanism control frames can be sent over the 40 MHz channel using non-HT duplicate transmissions. Non-HT duplicate transmissions allow the two identical 20 MHz non-HT control frames to be transferred simultaneously on both the primary and secondary channels as shown below (CWAP Study Guide – page 422).

CWAP-HT-Operation-026. Non Greenfield HT STAs present
AP indicates if any HT STAs that are not HT-greenfield capable have associated. Determines when a non-AP STA should use HTgreenfield protection. Present in Beacon and Probe response frames transmitted by an AP or mesh STA. Otherwise reserved.
0 – all HT STAs that are associated are HT-greenfield capable or all HT peer mesh STAs are HT-greenfield capable.
1 – one or more HT STAs that are not HT-greenfield capable are associated or one or more HT peer mesh STAs are not HT-greenfield capable

7. OBSS Non-HT STAs Present
Indicates if the use of protection for non-HT STAs by overlapping BSSs is determined to be desirable.
1 – use of protection for non-HT STAs by OBSSs is determined to be desirable.
0 – otherwise

8. Dual Beacon
Indicates whether the AP transmits an STBC beacon.
0 – no STBC beacon is transmitted
 1 – STBC beacon is transmitted by the AP

9. Dual CTS Protection
Dual CTS protection is used by the AP to set a NAV at STAs that do not support
STBC and at STAs that can associate solely through the STBC beacon.
0 – dual CTS protection is not required
1 – dual CTS protection is required

10. STBC Beacon
Indicates whether the beacon containing this element is a primary or an STBC beacon. The STBC beacon has half a beacon period shift relative to the primary beacon. Defined only in a Beacon transmitted by an AP. Otherwise reserved.
0 – primary beacon
1 – STBC beacon

When implementing STBC, the received signal may be improved by up to 8 dB, resulting in greater range shown in the below figure (CWAP- Page 423). An 8 dB increase in signal strength can yield up to 69 percent more range. This increased range will only apply to STBC frames and therefore does not automatically mean an increased BSS size for all STAs.

CWAP-HT-Operation-0311. L-SIG TXOP Protection Full Support
Indicates whether all HT STA in the BSS support L-SIG TXOP protection.
0 – one or more HT STA in the BSS do not support L-SIG TXOP protection
1 – all HT STA in the BSS support L-SIG TXOP protection

L-SIG TXOP protection is an optional Physical layer protection mechanism, which uses the L-SIG (Legacy Signal) fi eld in the HT mixed PPDU header.

CWAP-HT-Operation-0412. PCO Active
Indicates whether Phased Coexistence Operation(PCO) is active in the BSS Present in Beacon/Probe Response frames transmitted by an AP. Otherwise reserved. Non-PCO STAs regard the BSS as a 20/40 MHz BSS and may associate with the BSS without regard to this field.
0 – PCO is not active in the BSS
1 – PCO is active in the BSS

This is an optional mode of operation that divides time and alternates between 20 MHz and 40 MHz transmissions. The HT access point designates time slices for 20 MHz operations in both primary and secondary 20 MHz channels and designates time slices for 40 MHz transmissions.

CWAP-HT-Operation-05The main advantage of PCO is that no protection mechanisms are needed during the 40 MHz operational phase. PCO might improve throughput in some situations. However, switching back and forth between channels could increase jitter, and therefore PCO mode is not recommended when VoWiFi phones are deployed.

13. PCO Phase
Indicates the PCO phase of operation Defined only in a Beacon and Probe  Response frames when PCO Active is 1. Otherwise reserved.
0 – indicates switch to or continue 20 MHz phase
1 – indicates switch to or continue 40 MHz phase

14. Basic MCS Set
Indicates the MCS values that are supported by all HT STAs in the BSS. Present in Beacon, Probe Response, Mesh Peering Open and Mesh Peering Confirm frames.  Otherwise reserved.
The Basic MCS Set is a bitmap of size 128 bits. Bit 0 corresponds to MCS 0.
A bit is set to 1 to indicate support for that MCS and 0 otherwise

The last field in the HT Operation Element is the Basic MCS Set. This field has a similar format to the RX MCS Bitmask subfield in the supported MCS Set field of the HT Capabilities Element. Unlike the RX MCS Set subfield, which shows the MCS supported by the AP, the Basic MCS Set field shows only MCS values that are supported by all HT STAs within the BSS.

1. CWAP Official Study Guide – Chapter 10
2. IEEE 802.11-2012 standard.

Related Posts

1. CWAP – Introduction to 802.11n
2. CWAP – HT Control Field
3. CWAP – HT Capabilities IE
4. CWAP – HT 20/40 BSS Coexistece IE
5. CWAP – HT Overlapping BSS Scan IE