WLAN Roaming

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Roaming in WLANs is most often a reference to the process used when a client disassociates with one access point, or a basic service set (BSS) and reassociates with another access point (AP) in the same Extended Service Set (ESS).

In modern wireless networks, seamless roaming between access points (APs) is critical for maintaining connectivity, especially for time-sensitive applications like Voice over Wi-Fi (VoWiFi). This article explores the mechanisms behind fast secure roaming, focusing on key technologies like PMK caching, opportunistic key caching (OKC), and Fast BSS Transition (802.11r).

The Roaming Process:

Layer 2 Roaming Basics

Roaming in wireless networks occurs when a client station moves from one AP to another while maintaining connectivity. The client makes the roaming decision based on factors such as:

• RSSI (Received Signal Strength Indicator)
• SNR (Signal-to-Noise Ratio)

The process involves:

1. The client sends a reassociation request frame to the target AP.
2. The target AP informs the original AP about the roaming event.
3. The target AP requests buffered packets from the original AP.

Fast Secure Roaming Mechanisms:

1. PMK Caching

• Pairwise Master Key Security Association (PMKSA) is established during initial authentication.
• PMK caching allows clients to skip reauthentication when roaming back to an AP where a PMKSA already exists.
• This is also known as fast secure roam-back but doesn’t address forward roaming.

2. Preauthentication

• A client can preauthenticate with a target AP while still associated with the original AP.
• This creates a PMKSA with the target AP in advance, reducing roaming latency.
• However, preauthentication can overload RADIUS servers and doesn’t scale well.

3. Opportunistic Key Caching (OKC)

• OKC allows PMKs to be shared among APs under administrative control.
• The PMK is forwarded from the original AP to potential target APs.
• The client calculates a new PMKID using the original PMK and the target AP’s MAC address.
• OKC is vendor-specific and not all clients support it (e.g., iOS devices do not).

4. Fast BSS Transition (802.11r)

• Defined in the 802.11r amendment, FT provides a standardized approach to fast roaming.
• Operates within a mobility domain—a group of APs supporting fast transitions.

FT Key Hierarchy:

1. PMK-R0: Derived from the Master Session Key (MSK) during initial 802.1X/EAP authentication.
2. PMK-R1: Derived from PMK-R0 and cached on APs.
3. PTK: Derived from PMK-R1 and used for encrypting unicast traffic.

FT Methods:

• Over-the-Air FT: The client communicates directly with the target AP using authentication and reassociation frames that carry FT-specific information.
• Over-the-DS FT: Uses FT Action frames over the wired infrastructure (less common).

Supporting Technologies:

802.11k (Radio Resource Measurement)

• Provides neighbor reports to help clients identify nearby APs for roaming.
• Reduces scanning time by informing clients about APs in the same mobility domain.

802.11v (Wireless Network Management)

• Enhances roaming performance by exchanging network condition information between clients and APs.

Voice Enterprise Certification

• Ensures interoperability for voice applications by testing 802.11k/r/v mechanisms.
• Requires latency < 50 ms and jitter < 50 ms for good call quality.

Layer 3 Roaming Challenges:

When clients roam across Layer 3 boundaries (different subnets), they must acquire a new IP address, disrupting connections. Solutions include:

• Mobile IP: Uses tunneling to maintain the client’s original IP address across subnets.
• Proprietary solutions: Some vendors implement Layer 3 roaming optimizations.

Common Roaming Issues:

1. Sticky Clients: Clients may cling to an AP with poor signal due to misconfigured power settings or driver issues.
2. High AP Power: Excessive transmit power can cause co-channel interference and hidden node problems.
3. Legacy Clients: Older devices may not support newer roaming mechanisms like FT or OKC, requiring separate SSIDs.

Best Practices for Roaming

1. Cell Overlap Design: Ensure secondary coverage at -75 dBm to facilitate smooth roaming.
2. Client Support: Verify client compatibility with fast roaming mechanisms (OKC, 802.11r/k/v).
3. Power Management: Avoid high transmit power settings to prevent sticky client issues.
4. VLAN Design: Minimize Layer 3 roaming scenarios or implement Mobile IP for seamless transitions.

Conclusion:

Fast secure roaming is essential for modern wireless networks, especially for real-time applications. Technologies like PMK caching, OKC, and 802.11r/k/v provide the foundation for seamless transitions. Proper design, client support, and adherence to best practices ensure optimal performance and user experience. As wireless networks evolve, backward compatibility remains a challenge, necessitating careful planning and testing.