ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación descargar pdf (Pagina 64)

Data Transmission Example

When a source station wants to transmit data to a destination station, it makes sure

that the medium is not busy. If no activity is sensed during a time period corre-

sponding to a CIFS, the source station waits for the prioritization period then trans-

mits its data.

Figure 3.9 illustrates the role of the timers during the transmission of a data

frame and its acknowledgment.

If the medium is busy, the station waits until it is free. Once the medium is free,

the station waits during a CIFS then, after having checked that the medium is free,

initiates the back-off algorithm to defer once again its transmission in order to avoid

any collision. When the timer of the back-off algorithm times out, and if the

medium is free, the source station transmits its data to the destination station.

When two stations or more have simultaneous access to the medium, a collision

occurs. In this case, these stations reuse the back-off algorithm to have access to the

medium. If the sent data is received correctly—to know this, the destination station

checks the data frame CRC—the station involved waits during an RIFS time inter-

val and sends an ACK to confirm the correct receipt.

If this ACK is not sensed by the source station, if the data is not received cor-

rectly, or if the ACK is not received correctly, a collision has supposedly occurred

and the retransmission procedure is initiated.

The ARQ (Automatic Repeat Request) Process

When a source station transmits its data over the medium, it waits for an acknowl-

edgment frame from the destination station. This frame is potentially followed by a

procedure for the retransmission of non-received or erroneous data called ARQ

(automatic repeat request).

Transmission Channel Functionalities 45

Figure 3.9 Role of the timers in data transmission

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ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación Pagina 64