THOMSON Gateway Configuration Manual page 51

Chapter 6
Virtual Access Points
Single SSID per beacon, multiple beacons, single BSSID
The AP uses a single BSSID, but sends multiple beacons, each with a single SSID IE. If there are N virtual APs,
the beacon interval of each virtual AP is N times the standard beacon interval. The AP responds to probe
requests for supported SSIDs (as well as the broadcast SSID) with a probe response including the capabilities
corresponding to that SSID.
This approach has the following characteristics:
>
Moderate interoperability: wireless station implementations have a timer to determine the time during
which information is stored. If this time is too short compared to the time needed to discover all SSIDs,
interoperability problems may occur. Instead of discovering multiple virtual APs, a wireless station will
only discover a single AP flipping between capability sets.
>
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.
Single SSID per beacon, multiple beacons, multiple BSSIDs
The AP uses multiple BSSIDs, i.e. each virtual AP uses its own BSSID. For each virtual AP, beacons are sent
with the standard beacon interval. Each beacon or probe response includes only a single SSID IE. The AP
responds to probe requests for supported BSSIDs (as well as the broadcast SSID) with a probe response
including the capabilities corresponding to that BSSID.
This approach is the recommended approach and has the following characteristics:
>
Good interoperability: each virtual AP uses its own BSSID. As a result, virtual APs are indistinguishable
from multiple physical APs. There are no interoperability problems with existing wireless stations.
>
Broadcast domain
broadcast traffic between broadcast domains. If a wireless station receives traffic from BSSIDs that it
does not recognize, this traffic is filtered in hardware without first decrypting it.
>
High flexibility: each virtual AP may have a different BSSID, SSID and set of capabilities.
>
Roaming times: this approach does not require probe requests for each individual BSSID. As a result, it
does not increase roaming times.
>
Discovery: the AP responds to a probe request for the broadcast SSID with all probe responses for each
BSSID. This approach allows the discovery of new SSIDs and capability sets.
>
Capabilities
can advertise different sets of capabilities.
>
SSID routing: each virtual AP uses its own BSSID. As a result, the selected SSID can be understood from
the BSSID to which pre-authentication frames are directed. This allows virtual APs to distinguish their
pre-authentication traffic.
i
This approach will be used in R.7.2 of the Thomson Gateway. Each virtual AP operates with a
different locally unique BSSID. This BSSID is derived from the Thomson Gateway unique MAC
address, which is assigned at manufacturing time. The algorithm for derivation of a BSSID is
chosen to make it highly unlikely that a neighbouring AP would exist that applies an identical
BSSID.
E-DOC-CTC-20060609-0001 v2.0
separation: each virtual AP uses its own BSSID. As a result, there is no leakage of
advertisement: each virtual AP can send its own beacons and probe responses. Therefore, it
45