Cisco Cisco Aironet 3700i Access Point 백서

 

 

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Figure 1 shows that for any given packet size, the airtime consumed is dependent on the over-the-air PHY rate, 

which is itself dependent on RF link characteristics. As an example, the RF link characteristics are different based 

upon the distance of the client from the access point. A client that is nearer to the access point typically operates at 

a higher data rate than a client farther from the access point. This is because the access point and client are 

deliberately designed to adapt their transmission rates in order to maintain an optimal quality of the RF link. (This is 

normal behavior between the access point and clients since the client devices are neither expected to remain at a 

constant distance from the access point, nor are expected to be at equal distances from the access point.) 

In brief, the RF environment is dynamic. Therefore, the airtime consumed by any single client will dynamically 

fluctuate over time. 

As a consequence, two devices, each assigned a 10 Mbps rate limit and sending equally sized frames, would 

consume unequal airtime when their RF link characteristics are different This could mean that one device is 

consuming more airtime than it is entitled to, even though that device is not consuming more than its limit of bps. 

Figure 1.    Airtime Consumption Dependencies 

 

Thus, SLAs made between a WLAN operator and its customers are difficult to fulfill if only rate limiting is used. 

Policing usage on an airtime basis is the optimal way to assure that all parties have access to the resource that 

they are entitled to. 

In order to control or constrain airtime on a percentage basis, the airtime, which includes both uplink and downlink 

transmissions of a client or SSID is continuously measured. 

However, note that only airtime in the downlink direction (access point to client) can be controlled accurately by the 

access point. Although airtime in the uplink direction (client to access point) can be measured, it cannot be strictly 

controlled on a cumulative basis. The access point can constrain the airtime for any data frame it sends to or 

 a client through configuration of transmission opportunity (TXOP). However, the access point can 

only measure airtime for data frames that it ‘hears’ from each client, as it cannot strictly limit the number of data 

frames any individual client sends.