4.2 Legacy 802.11 PHY Standards

Harborline still has a cage of 802.11b barcode scanners on the freezer dock. New office laptops speak OFDM. If you cannot tell those radios apart — band, occupied width, maximum PHY rate, and whether they are single-stream — you will misread a survey, enable the wrong protection mode, and blame the wrong amendment. This section covers CWNA-109 objectives 2.2.1 through 2.2.4 only: original 802.11 DSSS, HR-DSSS (802.11b), OFDM (802.11a), and ERP (802.11g). High Throughput (802.11n), VHT (802.11ac), and HE (802.11ax) wait for the next chapter. Do not import MIMO, 40 MHz bonding, or MCS indexes into these four PHYs.

Objective 2.2 asks you to explain and apply each PHY including supported channel widths, spatial streams, and data rates. For everything in this section the spatial-stream count is one: these are SISO (single-input, single-output) radios. One transmit chain, one receive chain, one spatial stream.

DSSS — original 802.11 (2.2.1)

The 1997 base standard defined more than one PHY (including frequency-hopping and infrared). CWNA-109's 2.2.1 item is DSSS — Direct Sequence Spread Spectrum — on 2.4 GHz ISM.

Spreading each data bit into 11 chips gives processing gain: the information rate is 1 or 2 Mbps, but the energy is smeared across a much wider slice of 2.4 GHz than a narrowband 1–2 Mbps signal would occupy. Neighboring access points still need about 25 MHz of center-frequency separation (the classic channels 1, 6, and 11 plan in the United States) because a 22 MHz-class emission does not fit neatly into a 5 MHz channel number.

Harborline's oldest dock printers still associate at 1 Mbps DSSS when the freezer door is open and the path is ugly. That is not a configuration error. It is the original PHY doing what it was designed to do: survive, slowly.

HR-DSSS — 802.11b (2.2.2)

High Rate DSSS (HR-DSSS), published as 802.11b in 1999, stayed in 2.4 GHz and kept the original 1/2 Mbps Barker rates for backward compatibility. It added 5.5 Mbps and 11 Mbps using Complementary Code Keying (CCK) instead of Barker spreading at those two rates.

CCK still uses an 11 million chips per second clock and still occupies a similar 22 MHz-class footprint. That is why 802.11b did not magically create extra non-overlapping 2.4 GHz channels. It made the same crowded band faster, not wider.

Optional historical extras such as PBCC existed in the amendment. For CWNA-109, remember CCK, 11 Mbps, 2.4 GHz, and 22 MHz-class occupied bandwidth.

Harborline's freezer scanners are HR-DSSS. They cannot demodulate 802.11g OFDM PPDUs. If those scanners share an SSID with OFDM laptops, the AP must protect the OFDM transmissions so the scanners can still set a Network Allocation Vector. That protection story belongs to 802.11g below.

OFDM — 802.11a (2.2.3)

802.11a (1999) left 2.4 GHz entirely. It is an OFDM PHY in 5 GHz UNII spectrum. Channel width is 20 MHz. Maximum PHY data rate is 54 Mbps. It remains SISO.

802.11a uses a 64-point FFT. Of those bins, 52 subcarriers are used: 48 data plus 4 pilots. A long-guard-interval OFDM symbol is 4 microseconds (3.2 µs FFT plus 0.8 µs cyclic prefix). Convolutional forward error correction at rates 1/2, 2/3, and 3/4 combines with BPSK, QPSK, 16-QAM, and 64-QAM to produce the eight named rates:

Check the math once so the table sticks: 48 data subcarriers × 1 bit (BPSK) × 1/2 coding ÷ 4 µs = 6 Mbps. The same skeleton produces 54 Mbps when each subcarrier carries 6 bits (64-QAM) with 3/4 coding.

Trap: 802.11a is not a 2.4 GHz PHY. If Harborline's 5 GHz radios are disabled for a DFS freeze, 802.11a clients have nowhere to go. They do not fall back onto channel 6 as 802.11b/g devices do.

Mandatory rates in 802.11a are 6, 12, and 24 Mbps; 9, 18, 36, 48, and 54 Mbps are valid OFDM rates you will still see in BSS Basic Rate Sets and in client statistics.

ERP — 802.11g (2.2.4)

Extended Rate PHY (ERP), published as 802.11g in 2003, moved the 802.11a OFDM rate set into 2.4 GHz. Maximum PHY data rate is again 54 Mbps. OFDM channel width is 20 MHz. Spatial streams remain 1 (SISO).

ERP is a compatibility PHY, not a green-field-only PHY:

• ERP-OFDM carries 6, 9, 12, 18, 24, 36, 48, and 54 Mbps in 2.4 GHz, using the same modulation and coding pairing as 802.11a.
• ERP-DSSS/CCK still offers 1, 2, 5.5, and 11 Mbps so an 802.11b station can join the same BSS.

When a non-ERP (802.11b) station is associated or detected, ERP protection is required. 802.11b devices cannot decode OFDM PPDUs, so they cannot read Duration/ID from those frames and cannot set NAV. The ERP AP (or a client) therefore sends a DSSS/CCK-encoded protection frame first — commonly CTS-to-Self or a full RTS/CTS exchange — so Barker/CCK radios set NAV and stay quiet while the OFDM data PPDU occupies the air. CTS-to-Self costs less airtime than RTS/CTS but does less for hidden-node problems. Mixed BSS operation is the reason Harborline's freezer SSID felt slow after IT allowed office laptops onto the same 2.4 GHz warehouse WLAN: protection frames and slow CCK acknowledgements tax OFDM stations even when those laptops are capable of 54 Mbps.

Slot time is another mixed-mode detail. ERP can use a 9 µs short slot when the BSS is OFDM-only. When 802.11b stations are present, the BSS falls back toward the 20 µs long slot used by DSSS/CCK. That is MAC timing, not a new PHY rate, but it is part of why a mixed 802.11b/g cell never matches a clean 802.11g cell.

Channel widths and the 2.4 GHz packing problem

Do not call DSSS a 20 MHz OFDM channel. Original 802.11 and 802.11b occupy a 22 MHz-class spectrum mask. 802.11a and 802.11g OFDM use 20 MHz. In 2.4 GHz the channel numbers are still 5 MHz apart, so you still plan non-overlapping centers (1, 6, 11 under FCC-style 1–11 plans) rather than packing adjacent 20 MHz OFDM channels the way 5 GHz UNII can.

Five gigahertz 802.11a 20 MHz channels do not use the 1/6/11 story. They use UNII channel numbers with 20 MHz spacing. Regulatory which-channels-are-legal is Domain 2.6; this section only requires that you know 802.11a uses 20 MHz OFDM channels in 5 GHz.

Data rate versus what Harborline's users feel

PHY data rate is the over-the-air bit clock of the PPDU, not TCP throughput. A 54 Mbps ERP-OFDM association in a protected mixed BSS can deliver a small fraction of that number to a file copy after MAC headers, acknowledgements, protection frames, and contention. Throughput versus data rate is its own later objective. For 2.2.1–2.2.4, memorize the PHY maxima: 2, 11, 54, and 54 Mbps.

What this section does not teach

802.11n HT, 802.11ac VHT, and 802.11ax HE are not legacy PHYs in the 2.2.1–2.2.4 list. They add spatial streams, 40/80/160 MHz widths, and MCS tables. If a question about original DSSS, 802.11b, 802.11a, or 802.11g offers MIMO or 40 MHz as the reason for 54 Mbps, that answer is inventing a later amendment.

Independent OpenExamPrep teaching for these four PHYs stops at SISO, the bands in the table, 22 MHz-class versus 20 MHz, CCK versus OFDM, and ERP protection for 802.11b.