802.11ax BBS Color or BSS coloring or Spatial Reuse

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Why do we need BSS Color mechanism?

Let us first understand the problem and then look into the need for BSS coloring.

1. Pre-802.11ax way (legacy mechanism):

 “Every Wi-Fi device has to follow CSMA/CA method and if the whole physical channel is free then it can transmit any frame in the air, when others are waiting for the medium to be freed“. For detailed CSMA/CA explanation, here is the link: https://wifisharks.com/2020/10/31/csma-ca/

2. OBSS:

Now, the problem occurs when one BSS device can hear the transmission of other BSS device operating in same channel. This is overlapping of two BSSs, often called as OBSS (Overlapping Basic Service Set).This is also referred as co-channel interference (CSI) -> Collocated two same channels are interfering each other.

Example: One AP1 operating in channel 11 and nearby another AP2 is also operating in channel 11. Here is one basic picture to understand OBSS problem.

For CCA/CS (channel assessment/carrier sense) way,

AP2, STA2 cannot transmit if AP1 is doing some transmission.

AP1, STA1 cannot transmit if AP2 is doing some transmission.

If we can select some optimal BSS packet detect (OBSS PD) threshold, then BSS-1 can transmit simultaneously with BSS-2.

What’s is problem in OBSS and solutions?

OBSS reduces the overall performance of WLAN.

Solutions:

1. A Quiet Element as specified in IEEE 802.11 (2007). In advertised Quiet Time, no can transmit in the same frequency. There are rarely any devices who supports this.
2. BSS Coloring was originally introduced in 802.11ah. There is no devices with BSS coloring in pre-802.11ax devices. From 11ax devices a procedure called spatial reuse operation is enabled. As per IEEE standard, two independent spatial reuse modes are shared.
   • OBSS PD-based.
   • SRP-based.

 

Basic knowledge on BSS Color:

Ideally, If AP supports BSS color feature, then for every 802.11ax BSS is given a numerical value called BSS color. This value is an identifier for that BSS.A different BSS color value should be chosen for other BSS.

To understand further, we should know about “Intra-BSS” and “Inter-BSS”.

“Intra-BSS”: If a received frame BSS color is same as receiver’s BSS color then the received frame is from intra-BSS. So, receiver should defer its transmission if it wants transmit any frame. Suppose: STA1 and STA2 are connected to AP1 .BSS color is 5. STA1 wants to transmit and it has received a frame [which is from STA2] with BSS color 5. Then STA1 cannot transmit immediately as the frame is from its own BSS.

 “Inter-BSS”: If a received frame BSS color is different from receiver’s BSS color then the received frame is from inter-BSS. So, receiver may not defer its transmission if it wants transmit any frame. Suppose: STA1 and STA2 are connected to AP1 .BSS color is 5. STA1 wants to transmit and it has received a frame [which is from STA3 from nearby BSS] with BSS color 6. Then STA1 may transmit as the frame is not from its own BSS.

There are some other parameter along with BSS color check. Let us understand in more details from below flow chart.

BSS color with Spatial Reuse Operation (SRO):

SRO is the technic used by Wi-Fi6 devices to apply adaptive CCA for detected OBSS. Let us understand this from another flow diagram and simples steps.

1. STA starts detecting RF energy.
2. CCA points if energy threshold > -82dBm or below.
3. If detected frame RSSI > -82dBm [RSSI > -82dBm means detected frame is having stronger signal than -82dBm] and station checks whether it can extract the traffic?
4. If yes, read packet header to check the BSS color.
5. If it’s same color, then its intra-BSS frame [same BSS frame] and will have to follow as usual CSMA/CA process.
6. If detected frame BSS color is not same as own BSS color.
7. Now, there is another RSSI threshold. If RSSI > -62dBm or RSSI < -62dBm.
8. If RSSI > -62, then received frame station is too nearby and medium will be declared as busy.
9. If RSSI < -62, then received frame station is too far, so STA will continue transmitting.

Will BSS color solve the co-channel interference (CCI) Problem?

It cannot solve completely. Some points to be remembered

• Legacy devices can not differentiate between BSS on same channel.
• It’s a real time challenge to implement adaptive SD threshold [min and max value] properly in 11ax devices.  So, in short this will take time to mature.

 

BSS Color fields format:

BSS color field is present inside HE operation element .This is generally sent by AP.

The BSS Color subfield value can be from 1 to 63.

The Partial BSS Color subfield = 1. This means an AID assignment rule depending on the BSS color.

The BSS Color Disabled subfield = 1. This indicates disable current BSS color.

BSS Color Change Announcement element:

This is used by an AP to advertise new BSS color for the current BSS .This is an ACTION frame. This element may be carried in Beacon, Probe Response, (Re) Association Response frame.STA cannot send this frame.

BSS color collision event:

The report is sent by STA and it carries information about the BSS color used by OBSSs.

When AP can change BSS color?

1. If AP detects OBSS AP also uses same BSS color.
2. AP receives autonomous BSS color collision event reports from its associated STAs.

Conclusion:

In this article we have learned some basic overview of BSS color feature in 802.11ax. There are many more information on BSS color to learn in future.

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