Airtime fairness in DCF
Last Post: July 10, 2009:
This is related to scenario when multiple clients are connected to an AP which follows DCF.
Two clients are associated to an AP and a PC is behind the AP. If from the PC, traffic is being sent to the two clients, what is the sequence of frames including the IFS intervals, AP follows & how will the air time will be utilised among the two clients.
Both incase of single direction as well as Bi-directional traffic IF
1.Both clients have same rates/Modulations
2.Clients have different rates/Modultaions
What I have observed is incase of (1) the airtime is equally distributed but my doubt is that in case of DCF, only the client that has got the access should be served by AP. Please clarify
In DCF, the sequence and IFS's don't change depending on direction of traffic or number of clients. For all Wi-Fi devices (AP's and STA's) that are on the same channel in the same RF area, the sequence is always as follows:
Hey... I've decided to write this as a BASIC program. Wow... geeky.
20 Randomize Backoff Time
30 Decrement Backoff time by one slot
40 IF backoff timer = 0 (and NAV is 0 and CCA is pos) then goto 50 ELSE goto 30
50 Transmit one frame
60 SIFS (waited by receiver)
70 ACK from frame receiver
80 Any device that has something to transmit, goto 10
I didn't include what would happen if the CCA became negative. The answer is simple, the device just stops the process and defers until the DATA SIFS ACK sequence of the other device completes.
Can you explain this detail.
Like how the packets destined to two clients are treated in the AP's queue and if the packets are bursty how will be the impact on fairness. I am expecting the answer interms of packet by packet analysis. What are the chances that a single is always given the major chunk, given that both the clients are operating at the same rate and are mutually exclusive.
There is no 802.11 standard as to how to handle the queue inside of the AP. I've run a lab quite a few times that have been with a wired server connected to an AP and multiple client STA's pulling from the server.
Running bandwidth tests almost always shows an even distribution of data between all of the clients. This would show that the AP operates in a FIFO (First In First Out) order.
(Edit - Everything I'm describing is in an Non-QoS system)