Active scanning in WiFi is a method used by a wireless device (like a laptop or smartphone) to actively search for available wireless networks by sending out probe requests. This contrasts with passive scanning, where the device simply listens for beacon frames broadcast by access points.
Here's a breakdown:
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Probe Requests: Instead of just listening, the device broadcasts a probe request frame. This frame essentially asks, "Is there a network here with this SSID (network name)?" The SSID can be specific (asking about a known network) or a wildcard (asking about any available network).
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Access Point Response: If an access point (router) receives a probe request that matches its configuration (either a specific SSID or a wildcard request), it will respond with a probe response frame. This frame contains information about the network, such as its SSID, supported data rates, security settings, and other capabilities.
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Active Engagement: The key difference from passive scanning is the active role the device takes. It's not just passively listening; it's actively querying the environment.
How Active Scanning Works:
- The device wants to find a WiFi network.
- It sends out a probe request.
- The probe request contains either a specific SSID or a wildcard.
- Access points within range receive the probe request.
- If the access point's SSID matches the probe request (or it receives a wildcard request), it sends back a probe response.
- The device receives the probe response and learns about the network.
- The device can then choose to connect to the network.
Advantages of Active Scanning:
- Faster discovery of networks: Active scanning can be faster than passive scanning because the device doesn't have to wait for a beacon frame. It proactively asks.
- Ability to find hidden networks: If a network is configured not to broadcast its SSID (a "hidden" network), a device using passive scanning will not discover it unless it knows the SSID in advance. Active scanning, using a probe request with the specific SSID, can find these networks.
Disadvantages of Active Scanning:
- Increased power consumption: Sending probe requests consumes more power than simply listening for beacon frames.
- Potential privacy concerns: Broadcasting probe requests can reveal information about the networks a device has previously connected to, potentially allowing tracking of the device's location.
- Increased network traffic: Active scanning generates more network traffic than passive scanning. While this is usually minimal, in very dense environments it can contribute to congestion.
Table Comparing Active and Passive Scanning
| Feature | Active Scanning | Passive Scanning |
|---|---|---|
| Method | Sends probe requests to actively search for networks. | Listens for beacon frames broadcast by access points. |
| Speed | Faster network discovery, especially for known or hidden networks. | Slower network discovery; relies on the frequency of beacon frame broadcasts. |
| Power Consumption | Higher power consumption due to transmitting probe requests. | Lower power consumption as it only involves listening. |
| Hidden Networks | Can discover hidden networks if the SSID is known and included in the probe request. | Cannot discover hidden networks unless the SSID is already known. |
| Privacy Implications | May reveal previously connected networks through probe requests, raising privacy concerns. | Generally less privacy-invasive as it doesn't actively transmit identifying information. |
| Network Traffic | Increases network traffic due to probe requests. | Generates less network traffic. |
In summary, active scanning allows a device to proactively search for available WiFi networks by sending out probe requests and listening for responses from access points. While it offers faster discovery and the ability to find hidden networks, it also consumes more power and raises some privacy considerations.
