Physical carrier sense (CSMA/CA)
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Physical carrier sense (CSMA/CA)
- The first step that an 802.11 CSMA/ CA device needs to do to begin transmitting is to perform a carrier sense.
- This is a check to see whether the medium is busy. Think of it like listening for a busy signal when you call someone on the phone.
- There are two ways that a carrier sense is performed:
- virtual carrier sense
- physical carrier sense
- It is possible that a station did not hear the other radio transmitting and was therefore unable to read the Duration/ ID field and set its NAV timer. There could be numerous reasons why.
- CSMA/ CA utilizes another line of defense to ensure that a station does not transmit while another is already transmitting: The 802.11-2012 standard also defines a physical carrier sense mechanism to determine if the medium is busy.
- Physical carrier sensing is performed constantly by all stations that are not transmitting or receiving. When a station performs a physical carrier sense, it is actually listening to the channel to see whether any other transmitters are taking up the channel.
- Physical carrier sense has two purposes:
- to determine whether a frame transmission is inbound for a station to receive.
If the medium is busy, the radio will attempt to synchronize with the transmission. - to determine whether the medium is busy before transmitting.
This is known as the clear channel assessment (CCA)
- The CCA involves listening for RF transmissions at the Physical layer.
- The medium must be clear before a station can transmit.
- It is important to understand that both virtual carrier sense and physical carrier sense are always happening at the same time.
- Virtual carrier sense is a layer 2 line of defense, while physical carrier sense is a layer 1 line of defense.
Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.
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