ZyXEL Communications PLA-470 V2 - V3.0.5 Installationsanleitung PDF herunterladen (Seite 62)

Figure 3.7 illustrates the variation of the contention window (CW) and of the

transmission failure counter (DC) according to the number of retransmissions.

These values change from an initial value to a threshold value, which generally indi-

cates an overall problem with the network over which the station wants to transmit.

When the medium becomes free again, and after a CIFS and frame prioritization

phase, the stations make sure that the medium is still free. If this is the case, they dec-

rement their timer’s time slot by time slot until the timer of a station times out. If the

medium is still free, this station transmits its data by prohibiting access to the

medium to the other stations that block their timers.

The back-off procedure can be used even when no collision occurs. A station

increments its BPC (back-off procedure counter) as soon as a collision is detected or

when the BPC reaches zero. During the back-off, if another station transmits first,

the station checks it’s DC (deferral counter) and decrements it until it reaches zero.

After having decremented its DC, a station blocks its timer at the BPC value.

Once the station transmission is complete, the other stations still wait during a

CIFS and the priority phase. They check whether the medium is occupied during

and after the CIFS, then decrement their timers again where they had blocked this

timer until another station transmits data. However, they do not extract a new

timer value. Since they have already waited for medium access, they are more likely

to have access to it than stations just starting their attempts.

If the DC reaches zero, all the stations waiting for transmission must go through

a back-off procedure and defer the transmission of their data.

When calculating the timer, two or more stations may extract the same timer

value, which therefore times out at the same time, resulting in a simultaneous trans-

mission over the medium and causing a collision. After the back-off procedure, the

stations therefore reset the back-off algorithm for a new transmission if necessary

by obtaining a new CW and DC value. If a station receives a good receipt frame

(ACK), these values are reset to their minimum value.

Transmission Channel Functionalities 43

Number of

retransmissions

Number of

retransmissions

Figure 3.7 Contention window size variation according to the back-off algorithm

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ZyXEL Communications PLA-470 V2 - V3.0.5 Installationsanleitung Seite 62