April 2012
Intermediate to advanced
144 pages
4h 34m
English
Content preview from 802.11n: A Survival GuideBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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Part I. The PHY
In IEEE 802 networks, the physical layer, or PHY, is responsible for taking bits and turning them into a physical representation for transmission on the network medium. Every 802.11 physical layer has come with its own “big idea” that is the core driver for development. In the 1990s, the driver was just providing connectivity. Once that was established, 802.11b used Complementary Code Keying (CCK) to push the original 2 Mbps speed up to 11 Mbps. Simultaneous development on 802.11a used Orthogonal Frequency Division Multiplexing (OFDM) to establish the blazing speed of 54 Mbps. 802.11a required the use of a new spectrum, and the big idea in 802.11g was to bring the OFDM technology into the 2.4 GHz band to make higher speeds widely available.
In 802.11n, the big idea is MIMO. To transmit data, 802.11n can send multiple simultaneous data streams. Understand MIMO, and you understand the key to 802.11n. 802.11n broke new ground in other ways as well, most notably by standardizing wider channel bandwidths. In addition to the increase in the raw data rate from the PHY, it also included several efficiency enhancements in the MAC. Understanding 802.11n begins with understanding how the physical layer can move data so quickly.
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