Understanding WiFi Speed Meaning: Throughput vs Bandwidth

This authoritative technical reference guide demystifies WiFi speed metrics for enterprise IT leaders, clearly distinguishing between link speed, bandwidth, and throughput. It provides actionable methodologies for measuring real-world performance, mitigating RF congestion, and optimising WLAN infrastructure across high-density venue deployments. IT managers, network architects, and venue operations directors will leave with concrete frameworks for aligning infrastructure investments with measurable business outcomes.

📖 8 min read📝 1,781 words🔧 2 worked examples❓ 3 practice questions📚 9 key definitions

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[0:00 - 1:00] Introduction & Context

Hello, and welcome to this executive briefing from Purple. I'm your host, and today we're tackling one of the most persistent challenges in enterprise networking: understanding what WiFi speed actually means. If you're an IT manager, a network architect, or a venue operations director, you've likely faced this scenario: you deploy a state-of-the-art wireless LAN, your vendor promises gigabit speeds, but your users or your point-of-sale systems are experiencing sluggish performance. Today, we're cutting through the marketing noise to differentiate between link speed, bandwidth, and throughput. We'll give you the actionable intelligence you need to design for capacity, mitigate risk, and ensure your infrastructure investments actually deliver the required business outcomes.

[1:00 - 6:00] Technical Deep-Dive

Let's get straight to the technical reality. The discrepancy between advertised speed and user experience stems from confusing three distinct metrics.

First, we have Link Speed, also known as the PHY rate. This is the number you see printed on the box of an access point — like 1200 Megabits per second. It is the maximum theoretical data transfer rate at the radio level. But here is the critical point: link speed is never achievable in practice. It's a gross rate that includes all the protocol overhead — management frames, acknowledgements, and inter-frame spacing. When a client device connects to an access point and Windows reports a connection speed of 866 Megabits per second, that figure represents the negotiated physical layer rate. It accounts for the modulation and coding scheme, the number of spatial streams, and the signal-to-noise ratio at that moment. It does not represent the speed at which your applications will receive data.

Second, we have Bandwidth. In radio frequency terms, bandwidth is the width of the channel you're using, typically 20, 40, or 80 Megahertz. Think of bandwidth as the number of lanes on a motorway. Wider channels mean a higher potential link speed. Doubling the channel width roughly doubles the potential data rate. But in high-density environments like a retail store, a hotel, or a stadium, using wide 80 Megahertz channels is often a critical design error. It dramatically increases the noise floor and causes what we call Co-Channel Interference. You run out of non-overlapping channels, and your access points start interfering with each other. In a hotel corridor with access points every 15 metres, deploying 80 Megahertz channels means every AP is fighting every other AP for airtime. The result is that each individual client gets a higher theoretical link speed, but the actual throughput delivered to each user collapses.

Third, and most importantly, is Throughput. Throughput is the actual payload data delivered to the application layer. This is the only metric your users care about. Because WiFi is a half-duplex medium — meaning only one device can transmit at a time on a given channel — actual TCP throughput will rarely exceed 50 to 60 percent of the link speed under the best conditions. This is what I call the Rule of Half. So, if a client negotiates an 866 Megabit per second link speed, your actual throughput ceiling is around 400 to 500 Megabits per second. If you have legacy clients dragging down the airtime, that number drops even further. Understanding this Rule of Half is essential for setting expectations with stakeholders and designing your network architecture correctly.

Let me give you a concrete example to illustrate this. Imagine a 400-room hotel. The IT team has deployed access points in the corridors, using 80 Megahertz channels on the 5 Gigahertz band. The controller dashboard shows link speeds of 866 Megabits per second for most clients. Yet during the evening peak, guests are complaining they can't stream video. What's happening? The airtime utilisation on each channel is running at 85 to 90 percent. The access points are causing severe Co-Channel Interference because they're all using the same channels. The solution is not to add more access points. The solution is to reduce the channel width to 40 Megahertz, which doubles the number of available non-overlapping channels in the 5 Gigahertz band, and to reduce the transmit power of each access point so that the cells don't overlap as aggressively. The link speed reported by each client will drop slightly, but the actual throughput delivered to each user will increase dramatically because the channel contention is resolved.

[6:00 - 8:00] Implementation Recommendations & Pitfalls

How do we apply this in a real-world deployment? The primary objective is designing for airtime efficiency, not just coverage.

Step one: stop relying on internet speed tests to measure your wireless LAN. They introduce WAN variables. Use local iPerf3 testing to measure actual UDP and TCP throughput on your RF segment.

Step two: protect your airtime. Disable legacy low basic rates like 1 and 2 Megabits per second. Force clients to talk faster, which gets them off the air quicker. A single management frame sent at 1 Megabit per second consumes 54 times more airtime than the same frame sent at 54 Megabits per second. This single configuration change is the highest-impact, zero-cost improvement available to most enterprise WLAN deployments.

Step three: in high-density areas, default to 20 Megahertz channels on the 2.4 Gigahertz band, and 40 Megahertz on the 5 Gigahertz band. Capacity over coverage. You want more access points operating on clean, narrow channels rather than fewer access points shouting over each other on wide channels.

A common pitfall we see in hospitality is deploying access points in the hallways rather than in the rooms, and cranking up the transmit power. This creates massive Co-Channel Interference and destroys throughput, even if the link speed looks fine on the dashboard. Smaller cells, lower power, narrower channels — that is the formula for high-density performance.

[8:00 - 9:00] Rapid-Fire Q&A

Let's hit a few rapid-fire questions we hear regularly from CTOs and IT directors.

Question one: Why does my dashboard show 80 percent airtime utilisation but I only have a few clients connected? The most likely cause is that legacy basic rates are enabled, and the AP is sending management frames at 1 Megabit per second, consuming enormous amounts of airtime. A secondary cause could be non-WiFi interference from microwave ovens or AV equipment. A spectrum analysis will confirm the source.

Question two: Should we upgrade to Wi-Fi 6 to fix our throughput issues? Wi-Fi 6, or 802.11ax, is excellent for high-density environments because it introduces OFDMA, which allows an access point to serve multiple clients simultaneously on sub-channels. This significantly improves airtime efficiency. However, Wi-Fi 6 will not fix a fundamentally flawed channel plan or a network with legacy basic rates enabled. Fix your RF design first, then upgrade hardware.

Question three: Our users report fast speeds in the morning but slow speeds in the afternoon. What's happening? This is a classic capacity problem, not a coverage problem. As more users arrive and connect, airtime utilisation increases and throughput degrades. The solution is additional access points to distribute the load, combined with proper channel planning.

[9:00 - 10:00] Summary & Next Steps

To summarise the key takeaways from today's briefing.

Link speed is theory. Bandwidth is potential. Throughput is reality. Your job as a network architect is to engineer for throughput.

Remember the Rule of Half: expect actual TCP throughput to be approximately 50 percent of the advertised link speed under optimal conditions.

In high-density deployments, always prioritise capacity over coverage. More access points on narrower channels will always outperform fewer access points on wider channels.

Disable low basic rates to protect airtime. This single configuration change can deliver a significant improvement in WLAN performance with zero hardware cost.

Measure performance using local iPerf3 testing, not consumer internet speed tests. Track airtime utilisation and retransmission rates alongside throughput figures. And use the 70/80 rule: when sustained utilisation exceeds 70 percent, it is time to add capacity.

When you optimise for throughput, you enable the advanced services your business demands — whether that's reliable mobile point-of-sale in retail, seamless guest analytics in hospitality, or high-density connectivity at large events. Thank you for listening to this Purple executive briefing. For more detailed guides and architecture recommendations, visit the Purple resources hub at purple dot ai.

Key Takeaways

✓Link speed (PHY rate) is a theoretical maximum that is never achieved in practice; it includes all protocol overhead and is not a reliable performance indicator.
✓Throughput is the only metric that matters to end users and applications; expect it to be approximately 50% of the advertised link speed under optimal conditions (the Rule of Half).
✓Bandwidth (channel width) is a design choice: wider channels increase peak link speed but reduce the number of non-overlapping channels and increase Co-Channel Interference in dense deployments.
✓In high-density enterprise environments — hotels, retail stores, stadiums — always design for capacity (more APs, narrower channels) rather than coverage (fewer APs, wider channels).
✓Disabling low basic rates (1, 2, 5.5, 11 Mbps) is the single highest-impact, zero-cost configuration change available to improve WLAN airtime efficiency.
✓Measure performance using local iPerf3 testing to isolate the RF network from WAN and ISP variables; track airtime utilisation and retransmission rate alongside throughput.
✓Sustained airtime utilisation above 70% is the primary trigger for capacity expansion; above 80%, user experience will degrade unpredictably regardless of link speed.

Executive Summary

For IT managers and network architects deploying enterprise WLANs, the discrepancy between advertised WiFi speeds and actual user experience is a persistent operational challenge. The root cause is almost always a misunderstanding of three distinct metrics: link speed (PHY rate), bandwidth, and throughput. While vendors market maximum theoretical link speeds — for example, 1200 Mbps on 802.11ax — the actual throughput delivered to an application is typically 40–60% of that figure due to protocol overhead, half-duplex radio operation, and environmental contention.

This technical reference guide provides a definitive framework for understanding wifi speed meaning in enterprise environments. It equips IT teams at hotels, retail chains, and large venues with the knowledge to accurately measure real-world performance, design for capacity rather than coverage, and align infrastructure investments with measurable business outcomes. By shifting the focus from theoretical maximums to sustained throughput and optimal bandwidth allocation, venue operators can deliver the reliable connectivity that modern Guest WiFi and WiFi Analytics platforms demand.

Technical Deep-Dive: Decoding WiFi Speed Metrics

To engineer a robust WLAN, IT professionals must distinguish between the theoretical capabilities of the RF medium and the practical delivery of data payloads. The three metrics — link speed, bandwidth, and throughput — are frequently conflated in vendor marketing, procurement discussions, and even internal IT reporting. Getting this right is foundational to every other optimisation decision.

Link Speed (PHY Rate): The Theoretical Ceiling

Link speed, or Physical Layer (PHY) rate, represents the maximum theoretical data transfer rate between an Access Point (AP) and a client device at the radio level. This rate is negotiated dynamically based on the modulation and coding scheme (MCS), the number of spatial streams, and the signal-to-noise ratio (SNR) at the time of association.

Crucially, link speed is never achievable in practice. It represents the gross bit rate, including all 802.11 management frames, control frames (RTS/CTS and ACKs), and inter-frame spacing (AIFS/DIFS). In enterprise deployments across Retail or Hospitality environments, a client reporting a 866 Mbps link speed on an 802.11ac network is actually capable of roughly 400–500 Mbps of actual data transfer under ideal, isolated conditions — and far less in a shared, multi-client environment.

Bandwidth: The RF Channel Capacity

Bandwidth refers to the width of the radio frequency channel allocated for transmission, typically measured in Megahertz (MHz). In the 5 GHz and 6 GHz bands, channels can be 20, 40, 80, or 160 MHz wide. Wider channels offer higher potential link speeds — doubling the channel width roughly doubles the potential data rate — but they also increase the noise floor by 3 dB per doubling and significantly reduce the number of non-overlapping channels available.

In high-density environments such as stadiums, conference centres, or hotel corridors, deploying 80 MHz channels often leads to catastrophic co-channel interference (CCI). Enterprise best practice therefore dictates using 20 MHz or 40 MHz channels to maximise spectral reuse and overall system capacity, rather than chasing peak individual speeds. This is a design philosophy that prioritises aggregate throughput across all users over the theoretical maximum for any single user.

Throughput: The Real-World Measurement

Throughput is the actual payload data delivered to the application layer (Layer 7), measured in Megabits per second (Mbps). This is the only metric that matters to the end user, and it is the only metric that should drive network design decisions.

Throughput is fundamentally constrained by the half-duplex nature of WiFi — only one device can transmit on a given channel at a time. When multiple devices compete for airtime, throughput drops proportionally. Furthermore, legacy clients transmitting at lower data rates consume disproportionate airtime, penalising faster clients sharing the same channel. Understanding the true cost of airtime consumption is critical when evaluating the impact of background data collection on your WLAN, as explored in depth in The Hidden Cost of Telemetry Data on Corporate WLANs .

The table below summarises the practical relationship between these three metrics:

Metric	Definition	Typical Value (802.11ax)	What IT Teams Should Do
Link Speed (PHY Rate)	Gross theoretical radio rate	Up to 9.6 Gbps	Use as a baseline indicator only; never as a performance target
Bandwidth (Channel Width)	RF channel width in MHz	20, 40, 80, or 160 MHz	Default to 40 MHz in enterprise; 20 MHz in high-density
Throughput	Actual application-layer data rate	300–500 Mbps per client (ideal)	This is the primary KPI for all WLAN performance assessments

Implementation Guide: Measuring and Optimising Performance

Transitioning from theory to practice requires rigorous measurement methodology and systematic tuning. The following steps reflect vendor-neutral best practices applicable across all major WLAN platforms.

Step 1: Establish Accurate Baselines

Do not rely on consumer internet speed tests (such as fast.com or Speedtest.net) to measure WLAN performance. These tests introduce WAN latency, ISP routing variables, and server-side bottlenecks that are entirely unrelated to your wireless network. Instead, deploy a local iPerf3 server on the same VLAN as the AP management interface to isolate the RF segment. Run UDP throughput tests to assess raw channel capacity, and TCP throughput tests to evaluate application-level performance — TCP is highly sensitive to packet loss and latency, making it an accurate proxy for real application behaviour.

Step 2: Design for Airtime Efficiency

Airtime is the most precious resource in any WiFi deployment. To maximise throughput across the venue, three configuration changes deliver the greatest impact:

Disable Low Basic Rates. Disable 802.11b rates (1, 2, 5.5, 11 Mbps) and mandate a minimum basic rate of 12 Mbps or 24 Mbps. This forces clients to transmit management frames faster, freeing up airtime for data payloads. A single management frame sent at 1 Mbps consumes 54 times more airtime than the same frame sent at 54 Mbps.

Enable Airtime Fairness (ATF). Where supported by the vendor, enable ATF to allocate equal transmission time to clients, rather than equal packet counts. This prevents slow legacy clients from monopolising the channel at the expense of faster, modern devices.

Optimise Channel Widths. Default to 20 MHz channels in the 2.4 GHz band (always channels 1, 6, and 11) and 40 MHz in the 5 GHz band for high-density enterprise deployments. Reserve 80 MHz channels for isolated, low-density environments only.

Step 3: Implement Modern Authentication and Security

Security protocols impact throughput via encryption overhead and roaming latency. Implement WPA3 where the client estate supports it, or WPA2-Enterprise (IEEE 802.1X) with Fast BSS Transition (802.11r) to minimise roaming delays below 50 ms. For guest networks, compliance with GDPR and PCI DSS requires robust network segmentation — guest traffic must be isolated from corporate and payment infrastructure via dedicated VLANs and firewall policies. Modern onboarding solutions that reduce authentication friction while maintaining compliance are discussed in How a wi fi assistant Enables Passwordless Access in 2026 .

Best Practices & Industry Standards

The following principles represent the consensus of IEEE 802.11 working group recommendations and enterprise WLAN deployment experience across Healthcare , Transport , and large venue environments.

Capacity Over Coverage. In modern enterprise environments, APs should be deployed to handle client density, not just to provide a signal. A strong signal (coverage) does not guarantee high throughput (capacity) if the channel is congested. The two are entirely different engineering objectives.

Band Steering. Aggressively steer dual-band and tri-band clients to the 5 GHz and 6 GHz bands to alleviate congestion on the narrow 2.4 GHz spectrum. The 2.4 GHz band offers only three non-overlapping channels (1, 6, 11) and is subject to significant interference from non-WiFi devices.

Minimum SNR Thresholds. Configure AP radios to reject client associations below a minimum SNR threshold (typically 20 dB). This prevents distant, weak clients from associating and transmitting at low MCS rates, which would consume excessive airtime.

Regular RF Audits. Conduct spectrum analysis and active throughput testing at least quarterly, and immediately following any significant change to the physical environment (new partitions, AV equipment, or tenant changes). The RF environment is dynamic; a channel plan that worked at deployment may be suboptimal six months later.

Troubleshooting & Risk Mitigation

When throughput degrades, IT teams must diagnose the RF environment systematically rather than immediately reaching for hardware upgrades. The majority of enterprise WLAN performance issues are configuration and design problems, not hardware limitations.

High Retransmission Rates. A retransmission rate above 10% typically indicates RF interference, hidden node problems, or poor client SNR. Use spectrum analysis tools to identify non-WiFi interference sources — microwave ovens, AV equipment, and neighbouring networks are common culprits in hospitality and retail environments.

Co-Channel Interference (CCI). If multiple APs on the same channel can hear each other at -85 dBm or louder, they share the same collision domain, drastically reducing throughput for all clients on that channel. Mitigate this by reducing AP transmit power, narrowing channel widths, and ensuring dynamic channel assignment (DCA) algorithms are functioning correctly.

Sticky Clients. Clients that fail to roam from a distant AP to a closer one maintain a low SNR, forcing the AP to use a low MCS rate and consuming excessive airtime. Mitigate with minimum RSSI thresholds for association, 802.11v BSS Transition Management, and 802.11r Fast Roaming.

Client Driver Issues. Outdated wireless drivers on end-user devices can cause incorrect MCS negotiation, failure to use MIMO spatial streams, or aggressive power-saving behaviour that disrupts throughput. Maintain a client device management policy that includes wireless driver version standards.

ROI & Business Impact

Optimising WiFi for throughput rather than theoretical link speed directly impacts the bottom line across every vertical. In Transport hubs and large venues, reliable connectivity is essential for operational efficiency — from mobile point-of-sale (mPOS) systems to digital signage and access control.

For venue operators, high-throughput networks enable advanced location-based services and analytics. Ensuring consistent, reliable connectivity is a prerequisite for features like those introduced in Purple Launches Offline Maps Mode for Seamless, Secure Navigation to WiFi Hotspots , which enhance the guest experience and drive measurable engagement. Purple's public sector expansion, detailed in Purple Appoints Iain Fox as VP Growth – Public Sector to Drive Digital Inclusion and Smart City Innovation , further underscores the importance of reliable, high-throughput public WiFi infrastructure as a foundation for smart city services.

The business case for throughput-focused WLAN design is straightforward: a network that delivers consistent 200 Mbps per client during peak hours is more valuable than one that delivers 866 Mbps link speed with 85% airtime utilisation and unpredictable real-world performance. By aligning IT metrics — throughput, airtime utilisation, retransmission rate — with business outcomes — guest satisfaction scores, mPOS transaction reliability, operational uptime — IT leaders can justify infrastructure investments and demonstrate clear, measurable ROI.

Key Definitions

Link Speed (PHY Rate)

The maximum theoretical physical layer data rate negotiated between a client and an AP, measured in Mbps. Determined by MCS index, spatial streams, and channel width.

Frequently cited in vendor marketing and procurement documents. IT teams must understand this is a gross rate that includes massive protocol overhead and is never achievable as application throughput.

Throughput

The actual rate of successful payload data delivery over a communication channel to the application layer, measured in Mbps.

The primary KPI for any WLAN performance assessment. The only metric that accurately reflects end-user experience and application performance.

Bandwidth (RF Channel Width)

The width of the frequency spectrum allocated for a transmission channel, typically 20, 40, 80, or 160 MHz in the 5 GHz band.

Determines the potential capacity of the channel. Wider bandwidths increase peak link speed but reduce the number of non-overlapping channels and increase susceptibility to interference in dense deployments.

Co-Channel Interference (CCI)

Performance degradation caused when multiple APs operate on the same frequency channel and can detect each other's transmissions, forcing them to share airtime via the CSMA/CA contention mechanism.

The primary cause of poor throughput in dense enterprise deployments. Mitigated by proper channel planning, reduced transmit power, and narrower channel widths.

Airtime Utilisation

The percentage of time a specific RF channel is occupied with transmissions (data, management, or control frames).

A critical operational metric. Sustained utilisation above 70–80% indicates severe congestion and impending throughput collapse. Should be monitored per-radio and per-SSID.

Half-Duplex

A communication mode where data can be transmitted in both directions, but only one direction at a time on a shared medium.

The fundamental characteristic of WiFi that limits throughput to significantly below the theoretical link speed. Unlike wired Ethernet (full-duplex), WiFi requires all devices to take turns transmitting.

Spatial Streams (MIMO)

Multiple independent data signals transmitted simultaneously using Multiple Input Multiple Output (MIMO) antenna technology, increasing throughput without requiring wider bandwidth.

A key differentiator between 802.11ac (up to 8 spatial streams) and 802.11ax (Wi-Fi 6). Effective only when both the AP and client device support multiple antennas.

Basic Rates

The mandatory data rates that all clients must support to associate with a BSS. Management and control frames are transmitted at the lowest enabled basic rate.

Disabling low basic rates (1, 2, 5.5, 11 Mbps) is a standard and highly effective IT configuration practice. A frame sent at 1 Mbps consumes 54 times more airtime than the same frame at 54 Mbps.

MCS (Modulation and Coding Scheme)

An index value that defines the combination of modulation technique (e.g., 256-QAM, 1024-QAM) and forward error correction coding rate used for a given transmission.

Higher MCS indices deliver higher throughput but require a stronger signal-to-noise ratio. The AP and client negotiate the highest feasible MCS based on current RF conditions.

Worked Examples

A 400-room hotel is experiencing guest complaints about slow WiFi speeds during the evening peak (7 PM – 10 PM). The IT manager notes that the APs are reporting link speeds of 866 Mbps, but guests are struggling to stream video. The network uses 80 MHz channels on the 5 GHz band with APs deployed in corridors at maximum transmit power.

Conduct an airtime utilisation assessment during peak hours using the WLAN controller's built-in analytics or a dedicated tool such as Ekahau Sidekick. Expect to find utilisation above 80% on the primary 5 GHz channels, confirming Co-Channel Interference (CCI). 2. Reconfigure the WLAN controller to reduce channel widths on the 5 GHz band from 80 MHz to 40 MHz. This doubles the number of available non-overlapping channels from 6 to 12 in the UNII-1/UNII-3 bands, significantly reducing CCI. 3. Reduce AP transmit power to approximately 11–14 dBm to shrink cell sizes and reduce the number of APs that can hear each other on the same channel. 4. Enable dynamic channel assignment (DCA) to allow the controller to optimise channel allocation automatically. 5. Implement per-client bandwidth throttling (e.g., 15 Mbps downstream per device) to prevent individual users from monopolising the internet uplink during peak hours.

Examiner's Commentary: This scenario highlights the central fallacy of chasing high link speeds. By using 80 MHz channels in a dense hotel environment with high-power APs, the deployment created a large number of APs all competing on the same channels — effectively turning the entire hotel into a single collision domain. Reducing channel width lowers the theoretical peak speed per client but drastically increases aggregate throughput and consistency across all users by eliminating CCI. The fix is entirely configuration-based, with zero hardware cost.

A large retail chain is deploying mobile Point-of-Sale (mPOS) tablets across 50 stores. The tablets require reliable, low-latency connections for payment processing, but are frequently dropping sessions when staff move between aisles. The WLAN uses WPA2-Personal with default basic rates enabled.

Implement IEEE 802.11r (Fast BSS Transition) on the corporate mPOS SSID to reduce roaming authentication delays from 300–500 ms to under 50 ms. This is critical for session-sensitive payment applications. 2. Adjust the AP minimum mandatory basic rate to 12 Mbps. This reduces the effective cell size, encouraging tablets to roam to closer APs sooner rather than maintaining a weak connection to a distant AP (sticky client behaviour). 3. Migrate the mPOS SSID from WPA2-Personal to WPA2-Enterprise (802.1X) with certificate-based authentication to meet PCI DSS requirements for cardholder data environments. 4. Apply WMM (Wi-Fi Multimedia) QoS tags to the mPOS SSID, prioritising traffic in the Voice or Video queue to protect throughput during periods of high guest network usage. 5. Implement 802.11k (Neighbour Reports) and 802.11v (BSS Transition Management) to assist tablets in identifying and roaming to optimal APs proactively.

Examiner's Commentary: Retail mPOS requires sustained throughput and seamless roaming, not peak bandwidth. The combination of 802.11r, 802.11k, and 802.11v — collectively known as 802.11kvr — is the industry standard for enterprise roaming optimisation. Disabling low basic rates addresses the sticky client problem by shrinking the cell size, ensuring tablets maintain a high SNR and therefore a high MCS rate. The PCI DSS requirement for 802.1X is non-negotiable in a cardholder data environment and should be treated as a compliance baseline, not an optional enhancement.

Practice Questions

Q1. You are designing the WLAN for a high-density university lecture theatre with 300 seats. Your goal is to maximise aggregate throughput for all users simultaneously. The venue has 8 APs deployed in the ceiling. Should you configure the 5 GHz radios to use 20 MHz, 40 MHz, or 80 MHz channel widths?

Hint: Consider the number of non-overlapping channels available in the 5 GHz UNII-1 and UNII-3 bands, and the impact of Co-Channel Interference in a single open room with multiple APs.

View model answer

Use 20 MHz channels. In a high-density, single-room environment with 8 APs, you need each AP to operate on a distinct, non-overlapping channel to avoid CCI. The 5 GHz band offers approximately 24 non-overlapping 20 MHz channels (in regions with full UNII band access), but only 6 non-overlapping 40 MHz channels and 3 non-overlapping 80 MHz channels. With 8 APs using 80 MHz channels, at least 5 APs would be sharing channels, creating severe CCI. By using 20 MHz channels, you can assign unique channels to all 8 APs, allowing them to transmit simultaneously without contention. The individual link speed per client will be lower, but the aggregate throughput across all 300 users will be dramatically higher.

Q2. A client complains that their new 802.11ax (Wi-Fi 6) laptop only achieves 480 Mbps on a local iPerf3 test, despite Windows reporting a link speed of 1.2 Gbps. The client believes the AP is faulty. How do you assess and explain this situation?

Hint: Apply the Rule of Half and consider the relationship between PHY rate and TCP throughput in a half-duplex medium.

View model answer

The AP is almost certainly functioning correctly. The 1.2 Gbps is the negotiated Link Speed (PHY rate) — the gross theoretical radio rate. Because WiFi is half-duplex, and because the 802.11 protocol requires significant overhead (management frames, ACKs, inter-frame spacing), actual TCP throughput is typically 40–60% of the link speed. 480 Mbps from a 1.2 Gbps link represents a 40% efficiency ratio, which is within the expected range and indicates the network is performing well. To confirm, check the retransmission rate (should be below 5%) and airtime utilisation (should be below 50% for a single-client test). If both are healthy, the result is excellent and the AP should not be replaced.

Q3. During a site survey in a busy retail warehouse, you notice the airtime utilisation on channel 6 (2.4 GHz) is consistently at 88%, but there are only 6 active clients connected to the AP. The AP is a modern 802.11ax device. What are the two most likely causes, and what is the remediation for each?

Hint: Think about how legacy data rates affect airtime consumption, and consider sources of non-WiFi interference common in warehouse environments.

View model answer

Cause 1: Legacy basic rates are enabled. If the AP is transmitting management frames (beacons, probe responses) at 1 Mbps, each frame takes 54 times longer than at 54 Mbps, consuming enormous amounts of airtime even with few clients. Remediation: Disable 802.11b rates and set the minimum basic rate to 12 Mbps or 24 Mbps. Cause 2: Non-WiFi interference in the 2.4 GHz band. Warehouses commonly contain microwave ovens, Bluetooth devices, and older industrial wireless equipment that generate broadband interference in the 2.4 GHz band, artificially inflating airtime utilisation figures. Remediation: Conduct a spectrum analysis using a tool such as Ekahau Sidekick or a dedicated spectrum analyser to identify the interference source, and where possible migrate clients to the 5 GHz band.