What is UNII-2C?
UNII-2C, also called UNII-2 Extended, is the 5.470 to 5.725 GHz sub-band of 5 GHz Wi-Fi. It holds twelve 20 MHz channels, 100 to 144 in steps of 4, which makes it the largest block of channels in the 5 GHz band. UNII stands for Unlicensed National Information Infrastructure, the FCC's name for these bands. Every channel in UNII-2C requires DFS.
Where it sits in 5 GHz
| Sub-band | Frequency range | Channels | DFS required |
|---|---|---|---|
| UNII-1 | 5.150 to 5.250 GHz | 36 to 48 | No |
| UNII-2A | 5.250 to 5.350 GHz | 52 to 64 | Yes |
| UNII-2C | 5.470 to 5.725 GHz | 100 to 144 | Yes |
| UNII-3 | 5.725 to 5.850 GHz | 149 to 165 | No |
The 5.350 to 5.470 GHz gap between UNII-2A and UNII-2C isn't used for Wi-Fi, which is why the channel numbers jump from 64 to 100. In the United States the FCC also opened UNII-4 (5.850 to 5.925 GHz, channels 169 to 177), which fewer devices support.
DFS on UNII-2C
Radar systems share this spectrum, including weather radar near 5.6 GHz, in the middle of the sub-band. So an access point on channels 100 to 144 must:
- Listen for radar before using a channel (the channel availability check, commonly 60 seconds).
- Keep listening while it runs, and leave the channel quickly if it detects radar.
- Stay off that channel for a set time afterward, commonly 30 minutes.
Some older or low-cost client devices don't support DFS channels, so they can't see a network running on one.
Why use it
UNII-2C has room for six 40 MHz channels, three 80 MHz channels and one 160 MHz channel (channel 114, covering channels 100 to 128). Because some routers and clients avoid DFS channels, it's often less crowded than UNII-1 and UNII-3. The trade-offs are the start-up delay, occasional channel changes after radar detections, and clients that can't use these channels. Before settling on a DFS channel, check that every important device can still see the network.
For the CompTIA A+ exam
Core 1 (220-1201) objective 2.2 covers channel regulations and channel selection, including widths, frequencies and bands. UNII-2C isn't named in the objectives, but know the pattern: 5 GHz channels 52 to 64 and 100 to 144 need DFS, while 36 to 48 and 149 to 165 don't. UNII-2C has nothing to do with Bluetooth, which uses 2.4 GHz.