Tutorial: Roaming with a dual-radio client and PRP

A common way to increase the resilience of a WLAN infrastructure is to operate the various APs in different frequency bands. One way to implement this is for the physical WLAN interfaces of the APs to operate SSID-1 on the 2.4-GHz band and SSID-2 on the 5-GHz band, for example. A PRP-capable dual-radio client moving from the radio cell of one physical WLAN interface to a neighboring cell of the same infrastructure can experience uninterrupted cell switching thanks to PRP.

To do this, the dual-radio client using PRP initially connects its physical WLAN interface WLAN-1 to SSID-1 and WLAN-2 to SSID-2. If the reception for SSID-1 deteriorates and another radio cell with better reception is within range, the dual-radio client will perform a cell change. During the cell change the dual-radio client continues to send the data via WLAN-2 on SSID-2, while WLAN-1 already starts sending the same data with better reception on SSID-1. A PRP-enabled switch filters out the duplicate PRP packets before forwarding the data to the LAN.

Note: In this scenario, the APs in the WLAN infrastructure do not have to be configured to operate PRP.

Figure 1. Roaming of a dual-radio client in a PRP-based WLAN infrastructure

In order for the receiver to detect duplicate data packets, the APs in the WLAN infrastructure must be operating in client-bridge mode. The MAC address of the dual-radio client together with the RCT ensure that the receiver detects the duplicate packets. Without client-bridge support, an AP in the WLAN infrastructure would replace the MAC address of the dual-radio client with its own MAC address, so preventing the detection of duplicates.

Client-bridge support is enabled with LANconfig under Wireless LAN > General > Logical WLAN settings on the Network tab.

The PRP configuration of the dual-radio clients involves the following steps:

Under Wireless LAN > General > Physical WLAN settings, go to the Operation tab for each WLAN interface (WLAN interface 1, WLAN interface 2) and set the WLAN operation mode for each one to Client.

Specify the remaining WLAN parameters under Radio, Performance, Encryption and Client mode according to the requirements of the WLAN radio cells.

Important: In order for PRP to operate smoothly, the two instances of PRP must be operating on separate physical interfaces. If you are operating PRP on two logical interfaces of a single physical interface (e.g. "P2P-1-1" and "P2P-1-2"), then the device transmits the data sequentially. Apart from causing a loss of redundancy, this can also lead to delays in data transmission and a reduction in the bandwidth.
To enter the SSID, switch to the view Wireless LAN > General, click Logical WLAN settings and, for each WLAN interface, select network 1.
In the field Network name (SSID), enter the name of the WLAN which the WLAN interface is to be connected to.
Use WEBconfig to check whether Setup > WLAN > Dual-Roaming > Group is set to Off.

By deactivating the parallel roaming, you prevent the two physical WLAN interfaces from roaming at the same time or performing background scans. The result could be that both could lose connectivity to their radio cell.

When configured in this way, the dual-radio client can move past a line of APs and roam between the individual APs (see Figure 1).