5.1 Wi-Fi 4 HT and Wi-Fi 5 VHT

The jump from single-stream 802.11a/g OFDM to High Throughput and Very High Throughput is the point where 802.11 became a MIMO, variable-width PHY. This OpenExamPrep section is independent study material for CWNA-109 objectives 2.2.5 and 2.2.6: 802.11n HT (Wi-Fi 4) and 802.11ac VHT (Wi-Fi 5). You will be expected to know which bands each PHY may use, which channel widths exist, how many spatial streams the IEEE standards allow, how MCS works in each generation, what short guard interval and A-MPDU contribute, and the reliable trap that 802.11ac is a 5 GHz PHY only.

IEEE names describe the PHY. Wi-Fi Alliance generation names (Wi-Fi 4, Wi-Fi 5) describe certified products. On the exam and in the field you must map both vocabularies: Wi-Fi 4 is 802.11n HT; Wi-Fi 5 is 802.11ac VHT. Neither name is a claim that OpenExamPrep materials are an official CWNP product.

Wi-Fi 4: 802.11n HT on 2.4 GHz and 5 GHz

IEEE 802.11n added the HT PHY. Certified products were later marketed as Wi-Fi 4. HT is dual-band: the same HT clauses apply in the 2.4 GHz ISM band and in 5 GHz UNII bands. A two-radio access point can therefore run HT on both radios. When a later 5 GHz radio speaks 802.11ac, the 2.4 GHz radio in that chassis is still not VHT; it remains HT (or 802.11ax HE on a Wi-Fi 6 AP). If a question gives you a 2.4 GHz client at MCS 15, you are looking at HT, not VHT.

HT channel widths are 20 MHz and 40 MHz. Forty-megahertz mode bonds two adjacent 20 MHz channels and uses extra data subcarriers across the bonded pair. HT 20 MHz carries 52 data subcarriers instead of the 48 used by 802.11a/g, which is why MCS 7 at 20 MHz with the long guard interval is 65 Mbps rather than 54 Mbps. HT 40 MHz carries 108 data subcarriers. In 5 GHz, 40 MHz bonding is often acceptable because many 20 MHz channels exist. In 2.4 GHz, 40 MHz consumes two of the three classic North American non-overlapping 20 MHz slots (channels 1, 6, and 11) and is uncommon in multi-AP enterprise designs. Treat 2.4 GHz 40 MHz as a special-case isolated link, not as a campus default.

MIMO spatial streams are the other half of HT rate scaling. Each spatial stream is a distinct bit stream mapped onto a radio chain and recovered at the receiver using multiple antennas. The IEEE 802.11n standard allows up to four spatial streams. That number is a standard ceiling, not a typical client capability: many HT phones were 1x1:1 and many laptops were 2x2:2. A radio cannot originate more streams than it has chains. Notation such as 3x3:2 means three transmit chains, three receive chains, and two spatial streams. Four-stream HT is how the standard reaches its 600 Mbps peak; eight-stream MIMO is not an HT feature.

HT rate selection uses MCS indexes. For equal modulation, MCS 0 through 7 are one stream (BPSK up to 64-QAM), MCS 8 through 15 are two streams, MCS 16 through 23 are three streams, and MCS 24 through 31 are four streams. The MCS number therefore encodes modulation, coding rate, and stream count together. 256-QAM is not an HT constellation. Unequal-modulation MCS values exist in the standard (MCS 33 through 76) but are rarely the focus of administrator-level items. If you need a working memory model, remember MCS 7 / 15 / 23 / 31 as the top equal-modulation rates for 1 / 2 / 3 / 4 streams.

Two optional tools raise the number on the sticker. First, short GI: OFDM symbols normally last 4.0 microseconds (3.2 microseconds of useful waveform plus an 800 ns cyclic prefix). HT may use a 400 ns short guard interval, shrinking the symbol to 3.6 microseconds and increasing PHY rate by about 11 percent when delay spread is small. If reflections last longer than the GI, inter-symbol interference rises, MCS falls, and retries can erase the advertised gain. Second, A-MPDU aggregation: multiple MPDUs share one PHY preamble and one contention win, with a Block Ack bitmap confirming the burst. A-MSDU aggregation packs MSDUs inside one MPDU. High HT rates are not usable in real traffic without aggregation, because each small frame would still pay DIFS, backoff, preamble, SIFS, and ACK on its own. HT mixed-format PPDUs start with a legacy-readable preamble so 802.11a/g stations can set NAV duration, then continue with HT-SIG and HT data. Greenfield HT omits the legacy piece and is uncommon in mixed BSS. In 2.4 GHz mixed BSS, protection frames may still be required so HR-DSSS stations understand duration.

The theoretical HT maximum is 600 Mbps: four spatial streams, 40 MHz, 64-QAM rate 5/6, short GI. Two streams, 40 MHz, MCS 15, short GI is the widely printed 300 Mbps figure. One stream, 20 MHz, MCS 7, long GI is 65 Mbps; the same MCS with short GI is about 72.2 Mbps.

Wi-Fi 5: 802.11ac VHT, 5 GHz only

IEEE 802.11ac defined the VHT PHY. Certified products are Wi-Fi 5. VHT is specified only for 5 GHz. There is no 802.11ac 2.4 GHz PHY. Exam items that ask whether 802.11ac runs on 2.4 GHz are testing this single fact. Dual-band hardware labeled 802.11ac still uses HT or HE on 2.4 GHz. If you see 80 MHz, 160 MHz, 256-QAM, or eight streams on 2.4 GHz in an answer choice, treat that choice as a VHT feature incorrectly placed on the wrong band.

VHT widths are 20, 40, 80, and 160 MHz contiguous, plus 80+80 MHz non-contiguous. Eighty megahertz became the common 5 GHz operating width for that generation. 160 MHz doubles subcarriers again but occupies eight 20 MHz channels, often including DFS spectrum. 80+80 lets an AP use two 80 MHz segments that are not adjacent when a clean contiguous 160 MHz block is unavailable (for example, when a DFS hole sits in the middle of the band). Both 160 and 80+80 are 160 MHz-class transmissions from a rate-table point of view; they differ in whether the two 80 MHz pieces sit next to each other.

VHT MIMO supports up to eight spatial streams in the IEEE standard. Enterprise APs in that generation more often shipped three or four streams; eight is the protocol maximum, not a promise that a given AP has eight chains. Modulation rises to 256-QAM. VHT MCS indexes 0 through 9 describe only modulation and coding (MCS 8 and 9 are the 256-QAM rates). The number of spatial streams is a separate NSS field. Do not read a VHT MCS the way you read an HT MCS: VHT MCS 9 is not "nine streams." Not every MCS is valid for every width and NSS pair; CWNA-level items usually stay at the idea that 256-QAM exists and that NSS is separate.

Wave 1 VHT products typically offered up to 80 MHz, up to three streams, 256-QAM, and single-user MIMO. Wave 2 added 160 MHz in many designs, more streams in some radios, and downlink MU-MIMO: the AP beamforms different spatial streams to different clients in one transmit opportunity after sounding. 802.11ac MU-MIMO is downlink only. Uplink multi-user MIMO waits for 802.11ax. Wave 1 and Wave 2 are market generations, not IEEE clause titles, but they are the usual way administrators ask whether an 802.11ac AP can split streams across clients.

VHT transmissions wrap MPDUs in A-MPDU even when only one MPDU is present. Explicit beamforming sounding supports both SU and MU steering. RTS/CTS with bandwidth signaling allows dynamic fallback from 80 MHz to 40 or 20 MHz when a secondary channel is busy — a practical answer to overlapping BSS on wide 5 GHz channels.

Useful PHY rate checkpoints (order-of-magnitude, short GI): one stream, 80 MHz, MCS 9 is about 433 Mbps. Three streams at those settings are about 1.3 Gbps. Four streams at 160 MHz MCS 9 approach 3.5 Gbps. Eight streams at 160 MHz MCS 9 approach 6.9 Gbps. Those figures are PHY data rates, not TCP throughput. MAC overhead, IFS, Block Ack, retries, and contention all reduce what a user session actually delivers (see section 5.3).

How to read width, stream, and rate questions

Walk PHY identity first. 2.4 GHz rules out VHT. 160 MHz or 80+80 rules out HT. 256-QAM rules out HT. Five through eight streams rule out HT. Then apply the MCS rule for that PHY. Supported widths and stream maxima are facts you should recite. You rarely need to compute a rate from subcarrier counts under time pressure, but you should recognize why a second stream roughly doubles rate, why 40 MHz roughly doubles 20 MHz (plus extra data tones), why short GI adds about 11 percent, and why A-MPDU is required to approach those numbers at the MAC SAP.

When you configure radios, remember the split. 2.4 GHz HT at 20 MHz with one or two streams is still a capacity-limited band. 5 GHz VHT at 80 MHz with two or three streams is where 802.11ac delivered most of its user-visible speed. Do not enable 40 MHz on 2.4 GHz as a shortcut to make n faster in a multi-AP network. Do not assume an ac sticker on a dual-band AP means 256-QAM and 80 MHz on 2.4 GHz — that band never received the VHT PHY.