Media Access

Seite 1

Wireless networks use Carrier Sense Multiple

Access/Collision Avoidance (CSMA/CA) to control media access and avoid (rather

than detect) collisions. Collision avoidance uses the following process:

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.

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.

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.

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.

After

the time interval specified in the RTS and CTS has passed, other devices

can start the process again to attempt to transmit.

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.