A+ Core Certification Practice Questions

Network Troubleshooting

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Q7. A customer purchases a new 802.11n router and connects it in his SOHO network. Some of the devices are in g/b compatible, and others are n-compatible devices. However, the customer says that he is not finding much speed advantage on his 802.11n laptop even after upgrading the wireless router to n-type. What is the most likely reason?

A. The wireless router is set to b/g mode.

B. The router is working is Mixed Mode

C. The laptop is not within the close range and signals are fading

D. 802.11n router is not compatible with 802.11g devices

Correct Answer: B


The 802.11n high throughput (HT) standard defines three modes of operation: a legacy (non-HT) mode, a greenfield (HT-only) mode, and a mixed mode where HT protection mechanisms ensure that transmissions can be detected by both old 802.11a/g/b devices and new 802.11n devices.

In mixed mode, HT protection requires that 802.11n devices send a legacy preamble, followed by an HT preamble. The legacy preamble lets 802.11a/b/g devices to avoid transmitting over HT frames sent by 802.11n devices.

These HT protection mechanisms significantly reduce an 802.11n WLAN's throughput, but they are necessary to avoid collisions between older 802.11a/b/g devices and newer 802.11n devices. If you knew that no legacy devices were present, you could configure your access point (AP) to operate in greenfield (HT-only) mode, eliminating this overhead. 

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