THOMSON Gateway - Implementing Multiple SSIDs

E-DOC-CTC-20060609-0001 v2.0
Chapter 6
Virtual Access Points
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6.2 Multiple SSIDs
Support of multiple SSIDs
A physical AP can support multiple SSIDs by the use of one of the following approaches:
> Multiple SSIDs per beacon, single beacon, single BSSID
> Single SSID per beacon, single beacon, single BSSID
> Single SSID per beacon, multiple beacons, single BSSID
> Single SSID per beacon, multiple beacons, multiple BSSIDs
Multiple SSIDs per beacon, single beacon, single BSSID
The AP uses a single BSSID and sends a single beacon with the standard beacon interval. The AP includes
multiple SSID Information Elements (IEs) within the beacon or probe response.
This approach has the following characteristics:
> Limited interoperability: many wireless station implementations assume that there can only be a single
SSID IE within a management frame. As a result, they may not react well to multiple SSIDs within a single
beacon or probe response.
> Single BSSID: the AP uses a single BSSID in all management frames. As a result, wireless stations receive
traffic from broadcast domains they do not belong to. This traffic is then discarded as a decrypt error,
since the wireless station only obtains the default key that corresponds to the associated SSID.
> Limited flexibility: each SSID offers the same set of capabilities.
Single SSID per beacon, single beacon, single BSSID
The AP uses a single BSSID and sends a single beacon with the standard beacon interval. The AP includes
only one SSID IE within the beacon or probe response. The beacon only includes the “primary” SSID IE and
the AP also responds to probe requests for the broadcast SSID with a probe response including the
“primary” SSID IE. However, the AP responds to probe requests for “secondary” SSIDs with a probe
response including the corresponding SSID IEs.
This approach has the following characteristics:
> Good interoperability: this approach is interoperable.
> Single BSSID: the AP uses a single BSSID in all management frames. As a result, wireless stations receive
traffic from broadcast domains they do not belong to. This traffic is then discarded as a decrypt error,
since the wireless station only obtains the default key that corresponds to the associated SSID.
> Good flexibility: each virtual AP can have a different SSID and set of capabilities.
> Increased roaming times: in order to learn all supported SSIDs and capability sets, a wireless station
must send a probe request for each of the “secondary” SSIDs.
> No discovery: this approach does not allow the discovery of new SSIDs and capability sets, except by
snooping of probe responses.
i
The IEEE 802.11 standard does not provide advice on which of these approaches is appropriate. As
a result, different approaches have been chosen by different vendors. Unfortunately, several of
these approaches result in interoperability problems.
i
This approach is used in R.6.2 of the Thomson Gateway.