802.11n

What this chapter covers

The 802.11n chapter is about the standard that made Wi-Fi fast enough for everyday heavy use.

It covers what 802.11n adds, how the data rates are built, how channels are bonded and frames are aggregated, how chains and power behave, and how to debug an n link.

The chapter follows one syllabus topic that lists seven items. It has three pages, and each groups items that belong together.

How the topics fit together

Every item in the topic is a part of one speed calculation.

The pages build on each other:

• Features and data rates explain MIMO, streams, MCS indexes and the n rate settings.
• Channel bonding and frame aggregation explain the two ways of getting more from the air.
• Chains, TX power and link debugging explain how to set the radio paths and find faults.

Reading a rate in the registration table needs all three. It tells you the streams, the width and the guard interval, and each one has its own cause when it is lower than you expect.

How to use these pages

Each topic page has the same shape, so you can skim for what you need.

• What it is: the idea in plain words.
• How it works: the mechanism, with diagrams and example commands.
• What to remember for the exam: the facts to memorise.
• Common mistakes: the classic traps.
• Check yourself: three questions to test your understanding.

The syllabus marks this topic as a lab. If you can, set up two n devices, read the rate they report, and then change one thing at a time: the number of chains, the width, the MCS list. Watch how the rate and the throughput change.

Learn the numbers in the tables, because they come up in questions. These are the MCS groups, the streams, and the meaning of the channel width values.