UniFi - What is Zero-Handoff?


UniFi has always supported roaming according to base 802.11 standards. However with Zero-Handoff Roaming enabled, clients can freely roam between UAPs without incurring any latency penalty as a result of the roam. The basic premise is Zero-Handoff enabled UAPs appear as a single AP from the perspective of the client, thus eliminating disconnection.

Zero-Handoff Roaming is available starting with v3.x.

Non-ZH Roaming

When Zero-Handoff is disabled, a UAP doesn't do anything to assist or influence the station's roaming decision. In testing (v2.x) using WPA-PSK, UniFi developers sent flood pings from the laptop to a PC on the wired side, with the following results:

  • From the last ping to the original AP until association completion to the new AP is 45-155ms
  • From the last ping to the original AP until the first ping to the new AP is ~1s

For most internet applications (i.e. web-browsing), mobile activity would be seamless. For VoIP, observable delays may be noticed.

Seamless roaming as described by IEEE 802.11r (fast BSS transition) is AP-assisted client roaming. Some vendors require software-solutions for assisting in a fast-handoff solution.

ZH Roaming & Implementation

Zero Handoff Roaming works with any type of 802.11a/g/n client. However, all UAPs in the ZH-Enabled WLAN Group must be on the same layer-2 network (same subnet) and use the same wireless channel (e.g., Channel 1). Listed below are a few things to expect / avoid when deploying ZH-enabled UAPs.

RSSI Readings

RSSI readings of the AP arriving at the client may appear sporadic. In a non-Zero Handoff scenario where a client is associated to an AP with an RSSI of -75 and "sees" another AP in the same service set at -50 RSSI. The client decides to associate to the other AP. WIth Zero-Handoff Roam, the client sees the same SSID from both APs and may report differing RSSI values (first -75, then -50, back to -74, -51, -75, -49, etc.). Although this can be confusing from the perspective of the client, recall that with ZH-Roaming, the APs themselves handle roaming, not the client.

High Density

ZH Roaming should not be enabled on High-Density WLANs. This is because ZH requires the same channel be used on all access points across the BSSID. In addition, all associated wireless clients are using the same channel. So in a high density environment, when ZH is enabled, the channel faces overuse. Stations will experience higher latency and lower throughput. When planning a wireless network, decide whether the environment needs High Density or Zero-Handoff.