WiFi Roaming: Understanding the Basics

This article explains the fundamentals of WiFi roaming and how it functions, along with mentioning a WiFi roaming test solution.

A typical WLAN (Wireless Local Area Network) consists of a STA (Station or client), an AP (Access Point, also known as a router or hotspot), and an AAS (Authentication Server). The WLAN network is also often referred to as a WiFi network. WiFi networks serve local STAs within a home or office, or they serve STAs across different rooms or even between buildings.

Based on this, WiFi roamings are of two types: internal and external.

• Internal Roaming: This happens when STA-1 moves from the area covered by AP-1 to the area covered by AP-2, both within the same network.
• External Roaming: This occurs when STA-1 moves from AP-1 or AP-2 to the area covered by AP-3, which is located in a different building or is far from the home WLAN network.

Let’s delve into how WiFi roaming works.

Figure 1: WiFi Roaming Types

As shown in Figure 1, STA-1 continuously monitors WiFi signals from nearby APs. When it detects that the signal from the AP it’s currently connected to is becoming weaker than a signal from a nearby AP, it initiates a process called handover or association.

WLAN APs act as RADIUS clients for 802.1x. As we know, 802.1x is the standard for access authorization between a STA and an AP. RADIUS is the standard used for communication between the AP and the AS (Authentication Server). The Access Point initiates authentication as soon as a Station (STA) attempts to associate with it. Once authentication is successful, the AP allows the STA to join and begin data communication.

The AAS plays the role of providing all the necessary authentication services.

Following are the roles of STA, AP and AS in Wi-Fi Roaming:

• STA: Acts as the Supplicant; it’s the roaming client.
• AP: Functions as the Authenticator; it forwards the roaming request to the AS.
• AS: The Authenticating Server.

The following procedure is followed in a typical WiFi roaming scenario:

• The AP requests identity from the STA using EAPOL.
• The STA sends its identity to the AP. The AP forwards the received STA identity to the AS via EAP.
• The AS and the STA have an EAP authentication dialog.
• Once the dialog is successful, the STA and the AS share a common session key.
• The AS transmits the session key to the AP in a RADIUS attribute as part of the “RADIUS accept message”.
• The AP enables the controlled port for the MAC address of the STA. It can also enable a WEP key.

WiFi Roaming Test Solution

Veriwave offers a WiFi roaming test solution. It provides complete automated tests to analyze WLAN networks of infrastructure type and mobile WLAN STAs roaming between APs. These tests offer the following measurements:

• Roaming Delay
• Network capacity
• Call quality during roaming
• Test results in various file formats (e.g., HTML/CSV)