ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación descargar pdf (Pagina 57)

If managing the medium access is handled by the CCo centralizing device like in

the master-slave mode, the data is exchanged directly between the devices as in the

peer-to-peer mode.

Transmission Channel Functionalities

In PLC, the transmission channel is the electrical network. Since it was not originally

designed to support network applications, network functionalities had to be added

to so that the data link layer could be implemented correctly. Among them, medium

access and frame synchronization and frequency channel management processes on

the electrical wiring are specific to PLC technologies.

Access to the Medium Using CSMA/CA Techniques

CSMA/CA (carrier sense multiple access/collision avoidance) is a so-called random

access technique with listening to the carrier wave, which is used to listen to the

transmission medium before sending data. CSMA prevents several transmissions

from taking place over the same medium at the same time and reduces collisions but

does not prevent them completely.

In Ethernet, the CSMA/CD (carrier sense multiple access/collision detection)

protocol controls access to the medium of each station, and senses and handles the

collisions that occur when two or more stations try to communicate simultaneously

via the network.

In the case of PLC, the collisions cannot be detected. To detect a collision, a sta-

tion must be capable of listening and transmitting at the same time. In PLC systems

like in radio systems, the transmission prevents the station from listening at the same

time at the transmission frequency. Because of this, the station cannot hear the colli-

sions. Since a station cannot listen to its own transmission, if a collision occurs, the

station continues transmitting the complete frame, resulting in a global loss in net-

work performance.

With these specific characteristics in mind, PLC uses a slightly modified proto-

col compared with CSMA/CD, called the CSMA/CA protocol. The role of

CSMA/CA is not to wait for a collision to occur to react as with CSMA/CD but to

prevent collisions. Therefore, CSMA/CA tries to reduce the number of collisions by

avoiding their occurrence, knowing that a collision is most probable when the

medium is being accessed.

To avoid collisions, CSMA/CA uses various techniques, such as medium listen-

ing techniques introduced by the PLC; the back-off algorithm for medium multiple

access management; an optional reservation mechanism, the role of which is to limit

the number of collisions by making sure that the medium is free; and positive

acknowledgment (ACQ) frames.

The CSMA/CA used in the PLC is slightly modified compared with the one used

in Wi-Fi. Using a value that indicates the number of times that a station could not

emit in comparison with other PLC stations with the same medium access priority is

specified in the HomePlug standard. This value, called DC (Deferral Counter),

38 Functionality

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ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación Pagina 57