Wi-Fi Roaming

What Is Wi-Fi Roaming?

Wi-Fi roaming allows stations (STAs) to move seamlessly within the coverage areas of access points (APs) belonging to the same extended service set (ESS) without experiencing service interruptions. This feature is essential in wireless local area networks (WLANs) as it enables STAs to move freely without being tethered by physical media.

When multiple APs in an area share the same service set identifier (SSID), STAs can connect to the same SSID even if they move between different APs. Wi-Fi roaming ensures that the transition of STA services between APs is smooth and uninterrupted, akin to how a mobile phone call continues seamlessly when moving between cellular towers.

Why Is Wi-Fi Roaming Necessary?

AP radio signals cover only a limited area. When a STA moves away from its associated AP, the signal weakens, leading to eventual disconnection from the WLAN without Wi-Fi roaming. This disconnection can severely affect user experience, especially for services like voice and video.

Wi-Fi roaming addresses this issue. For instance, when a STA moves from the coverage area of AP1 to AP2, it seamlessly associates with AP2 while disconnecting from AP1. This process ensures that the network connection remains intact, much like a mobile phone call that continues without interruption when moving between base stations.

The primary advantage of WLANs is the ability of STAs to move without physical restrictions. Wi-Fi roaming enhances the user experience by:

• Retaining STA IP addresses, allowing continued access to the initially associated network and services.

• Preventing packet loss or service interruptions caused by lengthy authentication processes.

How Does Wi-Fi Roaming Work?

Wi-Fi roaming typically involves several phases:

• Triggering: A STA triggers the channel scanning process upon detecting that the downlink RSSI is below a certain threshold.

• Scanning: The STA discovers available APs through active or passive scanning, measuring network information to select the best AP.

  • Active Scanning: The STA sends a Probe Request frame and receives a Probe Response frame from APs.

  • Passive Scanning: The STA listens to Beacon frames sent by APs to detect their presence.

• Selecting: The STA selects an AP as the roaming target based on the scanned information.

• Handover: The STA and network capabilities determine the appropriate roaming mode.

Wi-Fi Roaming Classification

Wi-Fi roaming can be categorized as Layer 2 or Layer 3 roaming, depending on whether the STA roams within the same subnet. In Layer 2 roaming, the STA remains in the original subnet. In Layer 3 roaming, the STA moves between different subnets.

Roaming can also be classified as intra-AC or inter-AC roaming, depending on whether the STA associates with the same wireless access controller (WAC) before and after roaming. Intra-AC roaming is recommended for APs on the same floor or building, while inter-AC roaming is suitable for APs in different buildings on medium- and large-sized campuses.

Development of Wi-Fi Roaming

Fast Roaming: Fast roaming technologies reduce roaming handover time and improve the success rate. Two primary mechanisms include:

• PMK Caching: Defined in IEEE 802.11i, this method uses cached pairwise master keys (PMKs) to expedite the roaming process.

• 802.11r Fast Roaming: The 802.11r protocol reduces information exchanges during STA roaming, eliminating the need for 802.1X authentication or key negotiation.

Smart Roaming: Smart roaming assists STAs with low roaming aggressiveness, known as sticky STAs, to connect to APs with better signals, thereby improving load balancing and user experience.

AI Roaming: AI roaming enhances smart roaming by using personalized policies and coordinated measurements to improve roaming sensitivity and success rate.

Suggestions on Wi-Fi Roaming Deployment

Effective WLAN planning is crucial for a good Wi-Fi roaming experience. Professional planning and deployment documents, including recommended AP models, deployment solutions, and Wi-Fi roaming KPIs (e.g., roaming success rate, average roaming delay, in-roaming packet loss rate), are essential for optimizing network performance in various scenarios.