HCIA-WLAN cheat sheet

802.11 PHY generations: 802.11b 2.4 GHz DSSS up to 11 Mbps; 802.11a 5 GHz OFDM up to 54 Mbps; 802.11g 2.4 GHz OFDM up to 54 Mbps; 802.11n (Wi-Fi 4) 2.4/5 GHz MIMO up to 600 Mbps; 802.11ac (Wi-Fi 5) 5 GHz only, 256-QAM, up to 6.93 Gbps; 802.11ax (Wi-Fi 6) 2.4/5 GHz, OFDMA, 1024-QAM, up to 9.6 Gbps.

2.4 GHz channels are 20 MHz wide with 5 MHz spacing; only channels 1, 6, and 11 are non-overlapping in the 1-13 regulatory set, giving three independent channels for co-channel planning.

5 GHz offers many more non-overlapping 20 MHz channels (e.g., UNII-1 36/40/44/48; UNII-3 149/153/157/161/165 in many regions); 80 MHz channels bind four 20 MHz channels, so wider channels reduce reuse capacity.

CSMA/CA is the 802.11 access method: station senses medium, waits DIFS, backs off a random number of slots, transmits, and waits for an ACK; a missing ACK triggers retransmission with a doubled contention window.

Hidden node problem: two stations that cannot hear each other but both hear the AP may collide; RTS/CTS exchange reserves the medium and mitigates it, at the cost of overhead.

802.11 frame types: management (beacon, probe request/response, authentication, association), control (RTS, CTS, ACK, Block ACK), and data frames; the beacon advertises SSID, supported rates, and capabilities.

Security evolution: WEP (RC4, broken) -> WPA (TKIP) -> WPA2 (AES-CCMP, 802.1X or PSK) -> WPA3 (SAE, PMF mandatory); 802.11i defines RSN with 4-way handshake for key derivation.

Roaming: a station decides to roam based on RSSI/SNR thresholds; 802.11r fast BSS transition pre-authenticates with the target AP to cut handover latency, and 802.11k provides neighbor reports to speed scanning.

Capacity rule of thumb: throughput per radio is shared among associated clients; adding SSIDs or wider channels does not add airtime, so co-channel interference and client count drive real capacity.

Huawei WLAN architecture roles: Access Controller (AC) manages APs via CAPWAP; Fit APs require an AC; Fat APs run standalone without an AC; cloud-managed APs register to a cloud management platform instead of a local AC.

CAPWAP runs over UDP: control tunnel uses UDP 5246 and data tunnel uses UDP 5247; AP discovery of the AC can use DHCP option 43, DNS, broadcast, or static AC address configuration.

AC capacity is licensed by number of managed APs; verify the AC model's maximum AP count and license before deployment, since exceeding the license blocks AP registration.

Huawei AP naming: model suffixes indicate radio count and standard, e.g., 2x2 or 4x4 MIMO; 'D' often denotes indoor, 'DN' distribution/outdoor variants; confirm antenna type (built-in vs external) from the datasheet.

PoE matters: 802.3af delivers about 12.95 W at the PD, 802.3at (PoE+) about 25.5 W; high-power Wi-Fi 6 APs with multiple radios may require 802.3at or a local power adapter.

AP power-up sequence: AP boots, obtains IP via DHCP, discovers AC address, establishes CAPWAP control tunnel (DTLS optional), downloads configuration and firmware, then starts radios.

Uplink design: APs typically use GE uplinks; aggregate AP traffic toward the AC/aggregation switch and account for AC throughput, not only AP count, when sizing the AC.

Huawei AC/AP management interfaces include the web NMS, CLI, and the eSight/cloud management platforms; WLAN configuration is organized under the wlan view with AP groups, profiles, and VAP profiles.

Profile binding chain: regulatory domain profile -> AP group -> AP; radio profile and VAP profile are bound to the AP group or AP; a VAP profile references an SSID profile and a security profile.

SSID profile defines the SSID name and whether it is hidden; security profile defines authentication/encryption (open, WPA2-PSK, WPA2-802.1X, WPA3-SAE) and the key or RADIUS server.

VAP profile ties SSID + security + service VLAN; the service VLAN determines which broadcast domain client traffic enters after the AP bridges it to the AC or locally.

Forwarding modes: tunnel forwarding sends client data to the AC over CAPWAP (centralized policy), while local forwarding (direct forwarding) bridges data at the AP/access switch and only control traffic goes to the AC.

DHCP for clients can be served by the AC, an external DHCP server, or the upstream switch; if the AC serves DHCP, configure the interface address pool and default gateway correctly.

CAPWAP source interface must be configured on the AC (often a loopback or VLAN interface); APs must be able to route to it, and DHCP option 43 should carry that address for AP discovery.

AP registration troubleshooting order: check AP got IP, check AP can reach AC (ping/UDP 5246), check AC license, check AP whitelist/authentication mode (MAC or no-auth), then check AP model support.

WLAN IDS/IPS and air scan can be enabled on radios; monitor mode radios scan channels for rogue APs and interference while serving clients, so plan radio roles accordingly.

Guest networks typically use a separate service VLAN and an ACL or traffic profile on the AC to restrict access to internal subnets while allowing internet.

Check AP state first: 'display ap all' shows AP name, MAC, state (fault, config, normal), and uptime; a 'fault' state usually means CAPWAP tunnel or license failure.

Client association troubleshooting: verify SSID broadcast, authentication method match, security key, DHCP address assignment, and whether the client reached the correct service VLAN.

Low RSSI/SNR causes retries and low rates; typical targets are RSSI better than -65 dBm and SNR above 25 dB for voice, with -70 dBm as a minimum data threshold.

Channel interference: use 'display wlan sta' and air scan/spectrum data to find co-channel and adjacent-channel interference; adjust channel and transmit power, favoring 5 GHz for clients.

High retransmission or low throughput often traces to weak signal, too many clients per radio, legacy 802.11b/g clients, or a mismatched channel width; check per-STA rate and retry counters.

Roaming issues: verify AP coverage overlap, check whether 802.11k/v/r are enabled, and confirm the client supports them; sticky clients may need minimum RSSI or band-steering settings.

CAPWAP tunnel flaps: check link stability, MTU/fragmentation on the path, DTLS, and AC CPU; intermittent tunnels cause mass client drops.

Log and trace tools: AC logs, AP logs, and packet capture on the AC or AP uplink; for authentication failures, check RADIUS reachability and shared secret before blaming the AP.

Firmware and configuration backup: keep AC and AP firmware consistent per vendor compatibility matrix, and back up the AC configuration before upgrades to allow rollback.