Wireless Media Access Facts

Wireless Media Access Facts

Wireless networks use Carrier Sense, Multiple Access/Collision Avoidance (CSMA/CA) to control media access and avoid (rather than detect) collisions. CSMA/CA uses the following process:

1. The sending device listens to make sure that no other device is transmitting. If another device is transmitting, the device waits a random period of time (called a backoff period) before attempting to send again.
2. If no other device is transmitting, the sending device broadcasts a Request-to-send (RTS) message to the receiver or access point. The RTS includes the source and destination, as well as information on the duration of the requested communication.
3. The receiving device responds with a Clear-to-send (CTS) packet. The CTS also includes the communication duration period. Other devices use the information in the RTS and CTS packets to delay attempting to send until the communication duration period (and subsequent acknowledgement) has passed.
4. The sending device transmits the data. The receiving device responds with an acknowledgement (ACK). If an acknowledgement is not received, the sending device assumes a collision and retransmits the affected packet.
5. After the time interval specified in the RTS and CTS has passed, other devices can start the process again to attempt to transmit.

Note: Using RTS and CTS (steps 2 and 3 above) is optional and depends on the capabilities of the wireless devices. Without RTS/CTS, collisions are more likely to occur.

Wireless communication operates in half-duplex (shared, two-way communication). Devices can both send and receive, but not at the same time. Devices must take turns using the transmission channel. Typically, once a party begins receiving a signal, it must wait for the transmitter to stop transmitting before replying.

 

Credit: Testout 640-802 CCNA Notes