Multi-Bssid And Essid Access Ports - Symbol WS5000 Series System Reference Manual

AC Parameters
Packets are then added to one of four independent transmit queues (one per AC; i.e., voice, video, best effort,
or background) in the AP. The AP has an internal collision resolution mechanism to address collision among
different queues, which selects the frames with the highest priority to transmit. The same mechanism deals
with external collision, to determine which client should be granted the "Opportunity to Transmit" (TXOP).
The collision resolution algorithm that is responsible for traffic prioritization is probabilistic and depends on
two timing parameters that vary for each AC.
• The minimum interframe space, or Arbitrary Inter-Frame Space Number (AIFSN)
• The Contention Window (CW), sometimes referred to as the Random Backoff Wait.
Both values are smaller for high-priority traffic.
For each AC, a backoff value is calculated as the sum of the AIFSN and a random value from zero to the CW.
• The value of the CW varies through time.
• Initially the CW is set to a value that depends on the AC (CWmin)
After each collision, the CW is doubled until a maximum value (CWmax), also dependent on the AC, is reached.
• After successful transmission, the CW is reset to its initial, AC dependant value.
The AC with the lowest backoff value gets the TXOP.
• As frames with the highest AC tend to have the lowest backoff values, they are more likely to get a TXOP.
Once a client gains a TXOP, it is allowed to transmit for a given time depending on the AC and the PHY rate.
• TXOP limit ranges from 0.2 ms (background priority) to 3 ms (video priority) in an 802.11a/g network, and
from 1.2 ms to 6 ms in an 802.11b network.
• This bursting capability greatly enhances the efficiency for high data rate traffic, such as AV streaming.
• Also, the devices operating at higher PHY rates are not penalized when devices that support only lower
PHY rates (e.g. because of distance) contend for medium access.

1.3.5 Multi-BSSID and ESSID Access Ports

In a networked wireless environment, multiple access ports are connected to a WS5000 Series Switch to
provide RF connectivity to MUs. Each access port radio sends and receives RF signals over a range of space,
the Basic Service Set (BSS). The BSS coverage area is identified by a Basic Service Set Identifier (BSSID).
The access port beacon contains its BSSID, which enables the MU to recognize the access port and associate
with it. Extended Service Sets (ESS) are a logical group of BSSs. ESSs virtualize or increase the number of BSS
radio signals.
The beacon contains information about the access port and the network, which enables the MU to rank access
ports based on the received signal strength. The beacon can optionally include the Extended Service Set
Identifier (ESSID). MUs associate with the most preferable access port in the coverage area.
After association, the MU continues to scan for other beacons to ensure that it is receiving the best, continuous
signal strength, in case the signal from the currently associated access port becomes too weak to maintain
communications as the MU moves through the area.
Most access ports support multiple BSSs (see
as a separate radio signal. Access ports with multiple BSSs solve performance and security issues by isolating
broadcast traffic on a specific BSS rather than sending broadcasts to all BSSs. This enables MUs to save
WS5000 Series Switch Overview
Access Port Support on page 1-4). MUs sense each unique BSS
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