DCF Method ( 802.11 MAC Layer)
DCF implies Distributed Coordination Function
A station that wishes to transmit
(1) Performs a clear channel assessment (CCA) by sensing the medium for a fixed duration, the DCF* inter-frame space (DIFS).
(1.1) If (medium is idle) è begin a frame exchange sequence.
(1.2) Else if (medium is busy) è waits for the medium to go idle, defers for DIFS, and waits for a further random backoff period.
A station that wishes to transmit
(1) Performs a clear channel assessment (CCA) by sensing the medium for a fixed duration, the DCF* inter-frame space (DIFS).
(1.1) If (medium is idle) è begin a frame exchange sequence.
(1.2) Else if (medium is busy) è waits for the medium to go idle, defers for DIFS, and waits for a further random backoff period.
Once a station has gained access to the medium, it
maintains control of the medium by keeping a minimum gap, the short inter-frame
space (SIFS), between frames in
a sequence. Other station will not gain access to the medium during that
sequence since it must defer for a fixed duration that is longer than SIFS.
DCF uses both physical and virtual carrier sense functions to determine the state of the medium.
Physical carrier sense (PHY) uses energy detect and preamble detect with frame length deferral to determine when the medium is busy.
Virtual carrier sense (MAC) uses reservation information carried in the Duration field of the MAC headers announcing impeding use of the medium. Which is called the Network Allocation Vector (NAV).
The medium is determined to be idle only when both the physical and virtual carrier sense mechanisms indicate it to be so.
The DCF also makes use of immediate sending an ACK frame in response to the initiator’s data or management frame. Not receiving the ACK response frame is a likely indication that the initiator’s transmission was not correctly received, either due to collision or poor channel conditions at the time of the data transmission.
To further minimize the chance of collisions, and as a more robust collision detect mechanism è the initiating station may using robustly modulated RTS/CTS frames. è Sets the NAV in the stations surrounding both the initiator and responder, some of which may be hidden nodes unable to detect the more remote station’s transmissions and thus only able to defer for frame transmissions from nearby nodes.
Benefit of DCF Method in 802.11 : provides stations compete for channel access without the need for a central coordinator or arbiter.
DCF uses both physical and virtual carrier sense functions to determine the state of the medium.
Physical carrier sense (PHY) uses energy detect and preamble detect with frame length deferral to determine when the medium is busy.
Virtual carrier sense (MAC) uses reservation information carried in the Duration field of the MAC headers announcing impeding use of the medium. Which is called the Network Allocation Vector (NAV).
The medium is determined to be idle only when both the physical and virtual carrier sense mechanisms indicate it to be so.
The DCF also makes use of immediate sending an ACK frame in response to the initiator’s data or management frame. Not receiving the ACK response frame is a likely indication that the initiator’s transmission was not correctly received, either due to collision or poor channel conditions at the time of the data transmission.
To further minimize the chance of collisions, and as a more robust collision detect mechanism è the initiating station may using robustly modulated RTS/CTS frames. è Sets the NAV in the stations surrounding both the initiator and responder, some of which may be hidden nodes unable to detect the more remote station’s transmissions and thus only able to defer for frame transmissions from nearby nodes.
Benefit of DCF Method in 802.11 : provides stations compete for channel access without the need for a central coordinator or arbiter.
留言
張貼留言