RF, channels and SSIDs for CCNA

A real-life situation

A small office has three access points (APs). Users complain that the Wi-Fi is slow near the meeting room, even with full signal bars. You look at the APs and find that all three use 2.4 GHz channel 6. Every device near the meeting room is waiting for every other device on the same channel. Nothing is broken; the radio plan is.

This lesson gives you the words and rules you need to plan and fix that: radio frequency (RF) terms, channels, the parts of a wireless LAN, and the 802.11 standards. If Wi-Fi is new to you, read Wi-Fi basics and Wi-Fi frequency bands first. This lesson goes to CCNA depth.

What RF is, in CCNA terms

Wi-Fi sends bits as radio waves. A wave repeats itself many times per second. The CCNA expects you to know these words:

On the way, signals are absorbed (walls, water), reflected (metal, glass), refracted (bent when passing through a material), scattered (rough surfaces) and diffracted (bent around corners). Each of these makes the received signal weaker or messier. That is why a site survey, where you measure the real signal, beats a guess.

Why Wi-Fi behaves the way it does

Radio is a shared medium. Every device tuned to the same channel hears every other device on it, like people in one room. A radio also can't listen properly while it is talking. So Wi-Fi is half duplex: on a channel, only one device sends at a time.

Wired Ethernet once used CSMA/CD (detect collisions). Radios can't detect a collision while sending, so 802.11 uses CSMA/CA (carrier sense multiple access with collision avoidance):

This is why channel planning matters so much. Two APs on the same channel within hearing distance of each other form one big shared "room". This is called co-channel interference: they take turns rather than work in parallel. Two APs on overlapping channels are worse: they corrupt each other's frames (adjacent-channel interference).

Bands and non-overlapping channels

2.4 GHz: only three clean channels

The 2.4 GHz band has channels 1 to 13 (1 to 11 in North America). Channel centres are only 5 MHz apart, but each signal is about 20 to 22 MHz wide. So neighbouring channels overlap. Only 1, 6 and 11 stay clear of each other.

In a building, you lay out APs like a honeycomb so that no two neighbours share a channel: AP1 on 1, AP2 on 6, AP3 on 11, AP4 back on 1 far away from AP1, and so on.

5 GHz and 6 GHz: many more channels

5 GHz has around 20 or more non-overlapping 20 MHz channels (the exact number depends on the country). They are numbered 36, 40, 44, 48 and up, each 20 MHz apart, so none of them overlap. Some 5 GHz channels are DFS channels (Dynamic Frequency Selection): the AP must move off them if it detects radar.

Channels can be bonded to 40, 80 or 160 MHz wide for more speed. Wider channels mean fewer channels to share between APs, so in busy buildings 20 or 40 MHz is often the better choice. The 6 GHz band (Wi-Fi 6E and Wi-Fi 7) adds many more clean channels and allows only WPA3 security.

The building blocks of a WLAN

802.11 has precise names for groups of wireless devices. The exam uses them, so learn them exactly:

One AP can advertise several SSIDs, for example Staff and Guest. Each SSID on each radio then has its own BSSID, usually the radio MAC with the last digit changed.

How a client joins and roams, step by step

Here is the ESS from the lab: AP1 and AP2 both advertise Staff, on channels 1 and 6, connected by SW1.

Notice who decides: the client chooses when to roam, based on what it hears. A good channel and power plan helps it make good choices.

The 802.11 standards

The IEEE 802.11 working group writes the standards. The Wi-Fi Alliance tests products and gives them friendly names like "Wi-Fi 6".

Real throughput is far lower than these numbers: half duplex, sharing the channel and protocol overhead often leave less than half.

Where you set this on a Cisco WLC

There are no IOS commands to type for RF on a lightweight AP. On a Wireless LAN Controller (WLC), covered in Autonomous, lightweight and cloud APs, the controller's Radio Resource Management (RRM) picks channels and power for you. On an AireOS WLC GUI:

How to verify it from a client

The quickest check is what the client itself sees. On Windows, netsh wlan show interfaces shows the SSID, the BSSID of the AP you are joined to, the standard, channel and signal:

C:\>netsh wlan show interfaces
There is 1 interface on the system:

    Name                   : Wi-Fi
    State                  : connected
    SSID                   : Staff
    BSSID                  : 70:6d:15:aa:10:21
    Network type           : Infrastructure
    Radio type             : 802.11ax
    Authentication         : WPA2-Enterprise
    Cipher                 : CCMP
    Band                   : 5 GHz
    Channel                : 36
    Receive rate (Mbps)    : 573.5
    Transmit rate (Mbps)   : 573.5
    Signal                 : 92%

What goes wrong and how to troubleshoot it

Common mistakes

• Thinking more transmit power always helps. It makes cells too big, so clients stick to far APs and APs hear each other more.
• Using 2.4 GHz channels like 3 or 9. They overlap both neighbours.
• Using 80 or 160 MHz channels everywhere in a busy building. You run out of clean channels.
• Mixing up SSID (name, shared) and BSSID (MAC, one per BSS).
• Thinking Wi-Fi is full duplex like a switched Ethernet port.

💡 Exam tip: know 1, 6 and 11 as the non-overlapping 2.4 GHz channels, and that 5 GHz channels don't overlap at 20 MHz. Be ready to match BSS, ESS, IBSS, BSSID and SSID to their definitions, and 802.11n/ac/ax to Wi-Fi 4/5/6 and their bands. Remember Wi-Fi uses CSMA/CA and is half duplex.

Key takeaways

• Higher frequency means shorter range but more channels.
• RSSI is measured in dBm; SNR is signal minus noise. Higher SNR means faster rates.
• 2.4 GHz: use only channels 1, 6, 11. 5 GHz: 20 MHz channels don't overlap.
• A BSS is one AP radio and its clients; an ESS is many BSSs with one SSID.
• Wi-Fi is a shared, half-duplex medium using CSMA/CA.

Check yourself