THOMSON Gateway - Understanding Virtual Access Points

E-DOC-CTC-20060609-0001 v2.0
Chapter 6
Virtual Access Points
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6.1 What is a Virtual Access Point?
Concept
A virtual access point (AP) is a logical entity that exists within a physical AP. A single physical AP can support
multiple virtual APs. Wireless stations cannot distinguish a situation with multiple physical APs from a
situation with multiple virtual APs within a single physical AP.
The following illustration shows the concept of multiple physical APs:
The concept of multiple virtual APs within a single physical AP is shown in the following illustration:
Beacon frames
Beacon frames are transmitted periodically by physical APs. The wireless stations use these beacon frames
to identify the APs. The beacon frames contain among other things the following information:
> BSSID: the BSSID uniquely identifies an AP.
> SSID: the SSID is a name that wireless stations associate with an expected service or service network.
According to the 802.11 standard, a wireless station may only associate with a single AP and only a single
SSID may be included within an association or reassociation request frame. Hence, a station can only be
associated with a single SSID at a time.
> Set of capabilities: the set of capabilities includes e.g. the supported data rate, the supported security
mechanism,...
When a wireless station has received a beacon frame with an SSID that announces the desired service, the
station can associate with this SSID. To this end, the station sends an association request frame to the AP that
has transmitted the beacon frame, using the BSSID of this AP.
For further details on the BSSID and SSID of an AP, please refer to “ Basic Service Set Identifier (BSSID)” on
page 7 and “ Service Set Identifier (SSID) or Network Name” on page 7.
Access Point A
Channel 6
Access Point B
Channel 6
Station
Beacon
SSID: A
Rates: 1, 2 , 5.5, 11 Mbps
Security: WPA
Beacon
SSID: B
Rates: 1, 2 , 5.5, 11 Mbps
Security: WEP
Access Point
Channel 6
Station
Beacon
SSID: A
Rates: 1, 2 , 5.5, 11 Mbps
Security: WPA
Beacon
SSID: B
Rates: 1, 2 , 5.5, 11 Mbps
Security: WEP