Infrastructure

The correct answer identifies MAC layer protocol overhead as the primary factor causing the 40% difference between the negotiated PHY rate (2400 Mbps) and actual application throughput (1450 Mbps).

In wireless networking, the PHY rate represents the raw physical layer data rate - the maximum theoretical speed at which bits can be transmitted over the air. However, actual usable throughput is always significantly lower due to MAC layer overhead, which includes:

• Frame headers and trailers (preambles, delimiters)
• Acknowledgment (ACK) frames required after each data frame
• Inter-frame spacing (SIFS, DIFS) - mandatory quiet periods between transmissions
• Contention window timing for CSMA/CA collision avoidance
• Management and control frames

The question specifically states that the team observed 'protocol overhead from frame headers, acknowledgments, inter-frame spacing, and contention window timing consumes substantial airtime' - this directly points to MAC layer efficiency as the culprit.

The scenario rules out other possibilities:
- Encryption overhead is only 3%
- Channel utilization is low at 18%
- No retransmissions are occurring
- Optimal MCS index with 1024-QAM is being used
- 4x4 MIMO is properly configured

This 40-50% efficiency loss from MAC overhead is a well-known and expected characteristic of Wi-Fi networks, even under ideal conditions. It is inherent to the 802.11 protocol design and explains why real-world throughput never matches advertised PHY rates.

The other options are incorrect because PHY layer encoding losses are already factored into the MCS rate calculation, TCP overhead would not account for such a large gap especially when the question focuses on MAC layer observations, and guard intervals are part of the normal OFDM operation already reflected in the negotiated PHY rate.

The correct answer is that the 2.4 GHz band configuration limits available channel bandwidth and throughput capacity compared to 5 GHz or 6 GHz bands.

This is the most likely cause of the significant throughput reduction for several key reasons:

1. The 2.4 GHz band has limited channel bandwidth - typically only 20 MHz wide channels are practical due to only three non-overlapping channels (1, 6, 11). In contrast, 5 GHz can use 40, 80, or even 160 MHz channel widths, and 6 GHz offers even more spectrum.

2. The maximum theoretical throughput on 2.4 GHz is significantly lower than 5 GHz or 6 GHz bands. Even with 802.11ax (Wi-Fi 6), 2.4 GHz connections are limited to roughly 100-150 Mbps in real-world conditions, which aligns with the 85 Mbps observed.

3. The scenario mentions IoT devices broadcasting on overlapping channels - the 2.4 GHz band is notoriously congested, and with only three non-overlapping channels, interference is common and reduces effective throughput.

4. The 400 Mbps near access points likely occurs because proximity allows for cleaner signal negotiation, but the fundamental 2.4 GHz limitations still apply at distance.

The other options are incorrect:

• OFDMA does not have a specific -55 dBm threshold requirement. The -65 dBm signal strength mentioned is actually considered good signal quality.

• While metal-reinforced concrete can cause multipath issues, the signal strength of -65 dBm indicates adequate signal is reaching the patient rooms. Multipath would more likely cause connection instability rather than consistent reduced throughput.

• While IoT devices sharing the network could impact performance, airtime fairness would affect all connected devices more equally. The specific pattern of high speeds near APs and reduced speeds at distance points more strongly to the 2.4 GHz bandwidth limitations.

The correct answer identifies that the client adapter's hardware limitations are the primary constraint on throughput.

In 802.11ax (Wi-Fi 6) networks, the actual connection speed is determined by the lowest common denominator between the access point and the client device. Even though Rachel's access points are configured for 160 MHz channels and 4x4 MIMO (which could theoretically achieve much higher speeds), the USB wireless adapters only support 2x2 spatial streams and 80 MHz channels.

This is a fundamental hardware limitation - the adapters physically cannot receive or process more than 2 spatial streams or decode signals wider than 80 MHz. With 2x2 MIMO and 80 MHz channels in 802.11ax, the maximum PHY rate is approximately 1.2 Gbps under ideal conditions, and real-world aggregate throughput of 720 Mbps is consistent with this limitation after accounting for protocol overhead and other factors.

The other options are incorrect:

• The firmware update suggestion is wrong because the limitation is hardware-based, not software-based. A 2x2 adapter has only 2 radio chains physically built into it - no firmware can add additional antenna chains or wider radio front-end capabilities.

• The multipath propagation theory is incorrect because the question states signal strength is excellent at -58 dBm and spectrum analysis shows clean channels. Additionally, multipath is actually beneficial for MIMO operation, not detrimental.

• The security throttling explanation is incorrect because the throughput limitation perfectly matches the technical capabilities of the 2x2/80 MHz adapters. There is no evidence of any policy-based throttling, and the performance aligns with expected hardware constraints.