Thursday, July 12, 2007

IEEE 802.11

Like many of us, I have a wireless router installed at home because I like the flexibility of portable network access. My Netgear router boasts pre-N technology meaning that the PHY is based on a draft version of IEEE 802.11n. But what does this mean?

IEEE 802.11 specifies 3 physical (PHY) layers which define how digital signals are modulated.

Protocol Frequency Modulation Max Rate
802.11a 5 GHz OFDM 54 Mbps
802.11b 2.4 GHz DSSS 11 Mbps
802.11g 2.4 GHz OFDM 54 Mbps


I've deliberately left 802.11n off the list because it's still a draft protocol. However, 802.11n will be interoperable with 802.11a/b/g and will support greenfield mode offering up to 248 Mbps between 802.11n peers.

Why does 802.11n have such a high maximum data rate? There are a few reasons. The first is that greenfield mode operates in the 5 GHz band which is relatively quiet compared to the 2.4 GHz band shared by microwave ovens and Bluetooth devices. The higher band means that channel bonding can be used to increase theoretical data rate. The second reason is that 802.11n supports OFDM, a multiplexing technique that significantly reduces the impact of transmission errors due to multipath and inter-symbol interference at high frequencies. The third reason is MIMO or multiple-input multiple-output, which is an antenna technology that increases channel utilization.

However, very few notebooks are fitted with pre-N wireless cards, so the highest data rate most of us can expect is 54 Mbps unless we install a non-WiFi certified card. Hence the problem with pre-N.

Pre-N wireless technology is based on a draft standard and interpretation varies among vendors. It's therefore very difficult for the WiFi-Alliance to certify interoperability for pre-N hardware. Furthermore, pre-N hardware is unlikely to be compatible with 802.11n hardware, meaning that the notebook you buy in 2008 with an 802.11n card won't support greenfield mode if peered with a pre-N router.

I actually run my pre-N Netgear router using 802.11g only and use WPA Personal and TKIP to create a robust security network in accordance with 802.11i. When configuring your wireless router, remember to steer clear of WEP because the encryption algorithm is cryptographically weak and is considered insecure.

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