Avoiding Collisions In Wireless Networks - Siemens SIMATIC NET System Manual

1.5.8

Avoiding collisions in wireless networks

CSMA/CA with RTS/CTS
Ethernet uses the bus access method CSMA/CD. This acronym stands for Carrier Sense
Multiple Access with Collision Detection. After the node wanting to send has listened to the
line and identified it as being free (Carrier Sense CS), the data is sent.
While sending, the sending node can recognize a collision (Collision Detection, CD) with
other nodes sending at the same time (Multiple Access, MA) based on a disturbed level and
end the transmission.
This mechanism is used in just the same way in a wireless network apart from the fact that
collisions are deliberately avoided (Collision Avoidance, CA) to avoid reducing the net data
throughput unnecessarily. For this reason, wireless LANs do not use the CSMA/CD method
with which collisions can occur and be detected, but rather the CSMA/CA method (Carrier
Sense Multiple Access with Collision Avoidance).
Instead of physically listening in on the channel, a communications protocol is used that
reserves the channel for a specific time. Before sending, a node checks whether or not the
medium is free.
In this so-called RTS/CTS method, the node wishing to transmit sends a short test signal
("Ready To Send" - RTS). The actual transmission begins after the recipient has replied to
this with "Clear To Send" (CTS). If a collision occurs, the retransmission follows after a
pause not selected at random but according to priority. With this strategy, communication
remains deterministic.
Industrial Ethernet
System Manual, 09/2019, C79000-G8976-C242-10
Basics of communication with Industrial Ethernet
1.5 Wireless LAN
59