7.1 802.11 Standards and CSMA/CA

The previous chapter introduced network appliances, including wireless access points. This chapter explains the wireless standards those access points implement: how 802.11 works, which frequencies and channels to use, and how to safeguard wireless traffic.

Why Wireless Networking Exists

Running Ethernet cable to every device is impractical in many situations. Maintenance technicians carry laptops between machines. Mobile HMIs move along production lines. Warehouse scanners roam across large areas. Wireless networking delivers connectivity where cables are impractical.

802.11 Standards

IEEE 802.11 is the family of wireless LAN standards. Each amendment defines a new generation with higher speeds or new frequency bands.

Standard	Frequency	Max Speed	Key Feature
802.11a	5 GHz	54 Mbps	First 5 GHz standard (1999)
802.11b	2.4 GHz	11 Mbps	First widely adopted (1999)
802.11g	2.4 GHz	54 Mbps	Backward compatible with 802.11b
802.11n (Wi-Fi 4)	2.4 / 5 GHz	600 Mbps	MIMO, dual-band
802.11ac (Wi-Fi 5)	5 GHz	3.5 Gbps	MU-MIMO, wider channels
802.11ax (Wi-Fi 6/6E)	2.4 / 5 / 6 GHz	9.6 Gbps	OFDMA, dense environments

MIMO (Multiple Input Multiple Output) uses multiple antennas to transmit and receive multiple data streams simultaneously. MU-MIMO (Multi-User MIMO) allows the AP to communicate with multiple clients at the same time.

CSMA/CA — How Wireless Handles Collisions

Wired Ethernet uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection): a device listens, transmits, and detects collisions during transmission. Wireless devices are unable to use CSMA/CD because a radio is unable to transmit and listen simultaneously on the same frequency.

Instead, 802.11 uses CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). The process works as follows:

1. The client listens to the channel. When the channel is busy, the client waits.
2. When the channel is idle, the client waits for a random backoff timer (a random number of time slots).
3. When the channel remains idle for the entire backoff period, the client transmits.
4. The AP receives the frame and sends an ACK. When the client receives no ACK, the client assumes a collision occurred and retries with a longer backoff.

The random backoff reduces the probability that 2 clients transmit at the same time. The backoff does not eliminate collisions entirely. More clients on the same channel means more contention and lower throughput.

The Hidden Node Situation

2 clients (A and C) both reach the AP but are unable to hear each other. Client A transmits. Client C, unaware of A’s transmission, also transmits. Both frames collide at the AP.

RTS/CTS (Request to Send / Clear to Send) solves the hidden node situation. Before transmitting, Client A sends a short RTS frame. The AP responds with a CTS frame that all clients hear. Client C hears the CTS and defers transmission for the duration specified in the CTS. The CTS reserves the channel for Client A’s transmission.

RTS/CTS adds overhead (2 extra frames per transmission). Enable RTS/CTS only when hidden node collisions cause excessive retransmissions.

Key Takeaways

CSMA/CA handles collisions, CSMA/CD detects collisions

Wireless uses collision avoidance because radios are unable to transmit and listen simultaneously. More clients means more contention.

Wi-Fi 6 handles dense environments

OFDMA allows the AP to serve multiple clients simultaneously, reducing latency in crowded spaces.

RTS/CTS solves hidden node situations

Enable RTS/CTS only when hidden node collisions cause excessive retransmissions. RTS/CTS adds overhead to every transmission.

What Comes Next

Understanding the standards is only part of the picture. The next page covers frequency bands, channels, channel bonding, and OFDMA — explaining how to plan wireless coverage without interference.

References

• IEEE 802.11-2020 — Wireless LAN Medium Access Control and Physical Layer Specifications