Cisco Cisco Aironet 3700i Access Point 백서

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However, it must be noted that only air time in the downlink direction (access point to client) can be controlled

accurately by the access point. Although air time in the uplink direction (client to access point) can be measured, it

cannot be strictly controlled. Although the access point can constrain air time for packets it sends to clients, the
access point can only measure air time for packets that it ‘hears’ from clients, as it cannot strictly limit their air time.

As mentioned, a goal for ATF is to establish air time limits (defined as a percentage of total air time) and to apply

those limits on a per-SSID basis (where the SSID is used as a parameter to define a group of clients. Other

parameters can be used as well to define groups of clients, discussed later in this paper).

Furthermore, a single air time limit (defined as a percentage of total air time) can be applied to individual types of

clients.

It is important to note that if the air time limit for an SSID (or client) is exceeded, packets will be dropped. However,

only packets in the downlink direction are dropped. This is because dropping downlink packets (access point to
client) frees up air time, whereas dropping uplink packets (client to access point) doesn’t do anything to free up air

time since the packet has already been transmitted over the air by the client (once the packet has been on the air,

that air time cannot be reclaimed).

Of course, this can mean that a client which is transmitting a large amount of uplink traffic can exhaust air time (this

is essentially a denial-of-service attack that is beyond what ATF is meant to address; other mitigation is available

for this scenario).

Air Time Fairness per SSID

Next, consider how ATF is configured from the network administrator’s perspective (Figure 7).

The first step is to create a “time slice.” A time slice typically represents a specified relative maximum weight of the

available downstream air time. Although this can be thought of as indirectly assigning a not-to-exceed percentage

of airtime, the numeric assignment of a weight should not be interpreted as a literal percentage.

The second step is to assign an SSID to a time slice. It is worthwhile to note that multiple SSIDs can be assigned

to the same time slice (this generally assumes a single customer that requires multiple SSIDs for some business-

related reason).

All traffic from a given SSID utilizes the assigned time slice for transmissions. Downlink traffic destined for different

SSIDs will share the same time slice, and will share it in a first-come-first-served manner.