Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter Guia Do Desenho
5-6
Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 5 Cisco Unified Wireless QoS
802.11 DCF
Figure 5-4
Distributed Coordination Function Example
The DCF steps illustrated in
are as follows:
1.
Station A successfully sends a frame; three other stations also want to send frames but must defer
to Station A traffic.
to Station A traffic.
2.
After Station A completes the transmission, all the stations must still defer to the DIFS. When the
DIFS is complete, stations waiting to send a frame can begin to decrement the backoff counter, once
every slot time, and can send their frame.
DIFS is complete, stations waiting to send a frame can begin to decrement the backoff counter, once
every slot time, and can send their frame.
3.
The backoff counter of Station B reaches zero before Stations C and D, and therefore Station B
begins transmitting its frame.
begins transmitting its frame.
4.
When Station C and D detect that Station B is transmitting, they must stop decrementing the backoff
counters and defer until the frame is transmitted and a DIFS has passed.
counters and defer until the frame is transmitted and a DIFS has passed.
5.
During the time that Station B is transmitting a frame, Station E receives a frame to transmit, but
because Station B is sending a frame, it must defer in the same manner as Stations C and D.
because Station B is sending a frame, it must defer in the same manner as Stations C and D.
6.
When Station B completes transmission and the DIFS has passed, stations with frames to send begin
to decrement the backoff counters. In this case, the Station D backoff counter reaches zero first and
it begins transmission of its frame.
to decrement the backoff counters. In this case, the Station D backoff counter reaches zero first and
it begins transmission of its frame.
7.
The process continues as traffic arrives on different stations.
CWmin, CWmax, and Retries
DCF uses a contention window (CW) to control the size of the random backoff. The contention window
is defined by two parameters:
is defined by two parameters:
•
aCWmin
•
aCWmax
The random number used in the random backoff is initially a number between 0 and aCWmin. If the
initial random backoff expires without successfully sending the frame, the station or AP increments the
retry counter, and doubles the value random backoff window size. This doubling in size continues until
the size equals aCWmax. The retries continue until the maximum retries or time-to-live (TTL) is
reached. This process of doubling the backoff window is often referred to as a binary exponential
backoff, and is illustrated in
initial random backoff expires without successfully sending the frame, the station or AP increments the
retry counter, and doubles the value random backoff window size. This doubling in size continues until
the size equals aCWmax. The retries continue until the maximum retries or time-to-live (TTL) is
reached. This process of doubling the backoff window is often referred to as a binary exponential
backoff, and is illustrated in
5
-1, and increases to 2
6
-1, on the next
backoff level, up to the aCWmax value of 2
10
-1.
DIFS
DIFS
DIFS
Station A
Station B
Station C
Station D
Station E
Frame
Frame
Frame
Frame
Defer
Defer
Defer
Defer
Defer
Defer
Defer
Defer
Backoff time
Backoff time remaining
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