ZyXEL Communications PLA-470 V2 - V3.0.5 Guia de Instalação descarregar pdf (Página 61)

frames coming from the source station from the response frames coming from

the destination station.

•

RGIFS (reverse grant interframe spacing). Used for frame separation in the

Reverse Grant network mode specific to the HomePlug AV standard.

Back-off Algorithm

As explained above, the PLC uses the CSMA/CA method to control access to the

transmission channel.

Since collisions cannot be detected due to the attenuation and noise on the elec-

trical medium, when a PLC station wants to transmit, it must wait until the medium

is available for transmission. The station must wait until an IFS is free for a random

period of time, called back-off time. As there is no guarantee that a collision will not

occur in the meantime, the source (transmitting) station waits for a positive

acknowledgment (ACK) frame from the destination station. The destination station

transmits a good receipt response if the data is received correctly. This ACK

response is transmitted in the next available IFS.

In PLC, the time is sliced into intervals or time slots. These time slots are man-

aged by a timer applied to transmissions and retransmissions of the various stations

so that they both have equal probability of accessing the medium.

The back-off algorithm defines a CW (contention window) or back-off window.

This parameter corresponds to the number of time slots that can be selected to calcu-

late the back-off timer.

It is between the CW

min

and CW

max

values predefined by the HomePlug standard.

This time slot number, called BC (back-off counter), is used by the back-off proce-

dure when the medium is busy or when the source station has not received an ACK

frame from the destination station. As soon as a station wants to transmit informa-

tion, it listens to the medium thanks to the PCS defined previously.

If the medium is not busy, it defers its transmission while it waits for an IFS.

When the IFS times out, and if the medium is still free, it directly transmits its frame

without using the back-off algorithm. Otherwise, since the medium is occupied by

another station, the station waits until it is free; it in other words defers its transmis-

sion.

To try to access the medium again, it uses the back-off algorithm. If several sta-

tions wait for transmission, they all use the back-off algorithm. A station ignores the

number of stations associated with the network. Without this mechanism, by which

each station potentially calculates a different back-off timer to defer its transmis-

sion, the stations would directly collide with each other as soon as the medium is

released.

The stations calculate their timer, or T

BACKOFF

, according to the following for-

mula:

T Random CW

BACKOFF

=×(, )0 time slot

Random(0,CW) is a uniform pseudorandom variable within the [0, CW –1]

interval. Therefore, T

BACKOFF

corresponds to a time slot number. This algorithm ran

domly extracts various timer values for each station.

42 Functionality

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