RDKB BandSteering - Design Approach

Introduction

Band Steering is a solution ensures that clients are connected to the best radio. Dual Band supported Gateway can transmit SSIDs in both 2.4GHz and 5GHz frequency band. Band Steering solution does upgrade steering ( from 2.4 GHz to 5 GHz) and downgrade steering (5 GHz to 2.4 GHz) which allows optimal utilization of both bands. The Steering of the Wireless clients are based on the following parameter,

• Wireless Client Capacity
• Signal Strength in current and target band
• Bandwidth Utilization in current and target band
• Steering History
• Wireless Client Activity (Inactive Client/ Active Client)

The purpose of Band Steering feature is to maintain Wireless Clients in band best suited to them. For instance, a 5GHz capable client would be maintained in faster & wider-channeled 5GHz band but if the Received Signal Strength is lesser than the threshold value, the Client is moved to a wider-range 2.4GHz band.

Design Considerations

Decision Overview

The most basic prerequisite of Band Steering is both the 2.4GHz and the 5GHz frequency band must have the same SSID and passphrase. The wireless Clients will be steered to a better band only if SSID and password are same. So before performing Band Steering operations, same SSID, Authentication Mechanism and  passphrase are set to both 2.4GHz and 5GHz band. The enable & disabling of the Band Steering feature is done by dmcli command. Changes are done in WiFiHal and meta-cmf-raspberrypi layer.

WiFi HAL Changes

Modified Wifi Hal APIs related to Band Steering are as follows,

wifi_setBandSteeringEnable

On Enabling, checks if the Device is dual band capable and makes sure that the SSID, passphrase and Authentication mechanism is same in both 2.4GHz and 5GHz. Sets syscfg value for "band_steering_enable" as "1"

When disabled, resets the configured SSIDs, passphrase and Authentication Mechanism in 5GHz band. Also sets syscfg value for "band_steering_enable" as "0".

wifi_getBandSteeringEnable

Returns the Band Steering status of the device, this is done by using the syscfg get "band_steering_enable" variable.

wifi_getBandSteeringCapability

Returns true if the device can transmit SSID in both 2.4GHz and 5GHz band, else return false. The return type is the device capability to support Band Steering Feature.

Other Component Changes

A compilation flag is defined to include WiFi Band Steering related calls in the CcspWifiAgent component.

System default is added with name "band_steering_enable" to track band steering status, whether enabled/disbaled.

Band Steering Use cases

• RSSI Threshold for client devices which support both 2.4GHz and 5GHz -

Pre-association Band steering:

a. When RSSI of client probe request received by AP/Gateway in 2.4GHz band is more than the threshold, then AP blocks the 2.4GHz band for the client and informs client to probe request to 5GHz.

b. If the client device continues to probe request to 2.4GHz band, AP blocks the response in 2.4GHz until client device is ready to connect to 5GHz.

c. Sometimes client devices continues to probe request to 2.4GHz though AP blocks its response, in this case, after blocking a considerable number of times during a time period, AP enters backs off period and allows client to connect to 2.4GHz. For this period, AP does not try to steer the client to 5GHz again.

Post-association Band Steering:

a. Client has associated with a non-preferred band, RSSI is checked if it is above threshold. If it is above threshold, client is disassociated from the band and Steered to preferred band using Pre-association procedure.

Post-association Band Steering using RSSI Threshold

Description

RSSI is the Received Signal Strength of the probe request sent by the Client to the APs for connection establishment. RSSI will always be a negative value when converted to dbm.

As we already know, 5GHz Band has bigger channel Bandwidth and more number of channel occupancy when compared to 2.4GHz band. 5GHz provides good network connection to all the clients connected for a shorter distance, unlike 2.4GHz which is suitable for longer range networks.

RSSI which determines the signal strength of the client towards AP, can have a threshold value which will help in directing/deciding as to which band the client needs to be connected to. Signal strength indirectly indicates the distance of the client from AP. It is understood that as RSSI decreases, the distance of client device from the AP increases.

However, there are certain clients which are not capable of connecting to 5GHz band, as they do not support 5GHz.

For this, the events generated when the clients probe requests to gateway have to be captured. If the clients probe request to 2.4GHz band always, they are considered to be capable of connecting only to 2.4GHz. If the clients are found to be probing requests on both 2.4GHz and 5GHz band, then they are capable of connecting to 5GHz network.

Approach

Command to obtain Signal Level value for multiple client:

a. save the Mac addresses

iw dev <interface name> station dump | grep Station | cut -d ' ' -f2 | sort | uniq

b. check the signal level for a particular Mac address

iw dev <interface name> station dump | sed -n -e '/Station <Mac address>/,/Station/ p' | grep signal |cut -d ' ' -f3 | awk '{print $1}'

API to obtain the frequency client is associated:

wifi_getRadioSupportedFrequencyBands

RSSI threshold:

RSSI threshold is set to -100 by default, based on the Real time Box behavior.

RSSI range:

-100 to 0 (To Be confirmed)

Sequence diagram

Band Steering using Utilization Threshold (To be updated further)

Description

Band Utilization is the number of channel bands occupied by clients. The Utilization threshold is set to a default value. When the Number of Clients connected to 5GHz band are more than the threshold value, the clients have to be steered to 2.4GHz band. This is done to improve the network quality to the connected clients.

Assuming, all wireless devices that support both 2.4GHz and 5GHz band will be automatically directed, to be connected to 5GHz initially. Here, if the number of clients connected to 5GHz is more than the set threshold value, i.e., 10, then network quality may degrade for the connected clients because many clients will be sharing the available band. For this, once the number of connected clients crosses threshold, few of the clients have to be steered to 2.4GHz.

Depends on:

a. Density/Number of Clients in the Band

b. Assuming client is in 2.4 GHz, Downlink Phy rate > = Phy Rate threshold , then switch to 5GHz

Assuming client is in 5GHz, Downlink Phy rate < Phy Rate threshold, then switch to 2.4GHz

c. Uplink signal

Combining:

Before the clients are steered to 2.4GHz from 5GHz band, there has to be a check to decide which clients can be steered to a preferable band. If the clients whose signal strength is low, i.e., they are far from AP are retained in 5GHz and the clients with high signal strength are directed to 2.4GHz, the clients may lose connectivity.

Hence, the RSSI threshold and Utilization threshold implementations have to be combined to achieve a better Band Steering.

Approach

a. Density/Number of Clients in the Band

iw dev <interface-name> station dump | grep Station | cut -d ' ' -f2 > /tmp/wifi_5Gstations.txt

iw dev <interface-name> station dump | grep Station | cut -d ' ' -f2 > /tmp/wifi_2-4Gstations.txt

cat /tmp/wifi_5Gstations.txt | wc -l

cat /tmp/wifi_2-4Gstations.txt | wc -l

b. Downlink Phy rate

Maximum speed of data in downlink

Need the information from driver

c. Uplink signal

Signal to Noise ratio in uplink

Need the information from driver

Sequence Diagram

implementation

Architecture

• Layout

Implementation of Band Steering using RSSI Threshold:

Data Model

Limitations

• Band Steering feature is limited only to dual/tri band radio capable devices.
• Steering of 5 GHz capable wireless clients can be done only when SSID and security parameters are same for both 2.4GHz and 5GHz.
• Band Steering using RSSI signal works well only for clients capable of connecting to 5GHz. Clients capable of only 2.4GHz will not work.

Future Enhancements

• Band Steering using RSSI signal for clients not capable of connecting to 5GHz needs to be implemented.
• Steering of Clients are based on Utilization threshold, RSSI threshold. These use cases need to be implemented.