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-raspberypi 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

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.

Approach

RSSI is determined by the iwconfig command

Signal Level=37/100 is the RSSI in arbitrary unit, has to be converted to dbm.

Command to obtain Signal Level value:

iwconfig wlan0 | grep Signal | cut -d '=' -f3 | cut -d ' ' -f1

Based on Cisco chart for conversion of arbitary unit to dbm:

RSSI above 67: dbm = 1.2*RSSI – 123

or

RSSI between 36 and 66: dbm = 1.1*RSSI – 114.6

or

RSSI below 35: dbm = 1*RSSI - 112

RSSI threshold:

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

RSSI range:

-100 to 0 (To Be confirmed)

Sequence diagram

Architecture

• Layout

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.

Future Enhancements

• Band Steering status should be retained even after reboot. Currently, this is not yet implemented.
• Steering of Clients are based on Utilization threshold, RSSI threshold. These use cases need to be implemented.