The many faces of 802.11

by Matthew Gast


Recent press has trumpeted the release of "802.11g" products, which is not incorrect. 802.11g is still in draft form, and currently shipping products cannot comply with a standard that does not yet exist. The following table should answer the most common questions about the differences between the many radio layers that have been standardized for use with the 802.11 MAC.










































Spec

Abbreviation

Speed (Mbps)

Modulation

Radio band

802.11

FHSS

1 or 2


frequency hopping

2.4 GHz (ISM)

802.11

DSSS

1 or 2


direct sequence

2.4 GHz (ISM)

802.11a

OFDM

6, 9, 12, 18, 24, 36, 48, or 54


orthogonal frequency division multiplexing

5 GHz

(U-NII)

802.11b

HR/DSSS

5.5 or 11


direct sequence with complementary code keying

2.4 GHz (ISM)

802.11g (draft)

unknown

up to 54


orthogonal frequency division multiplexing

2.4 GHz (ISM)



Other protocols:



  • 802.1x is a specification for LAN authentication on IEEE 802 networks, and can be used on 802.11. It is an adaptation of the IETF's Extensible Authentication Protocol (EAP) to IEEE 802 layer-2 network technologies.


  • 802.11i is a forthcoming specification that will clear up a number of security problems in 802.11. It specifies the temporal key integrity protocol (TKIP), "IV mixing" to blunt attacks against WEP's initialization vector, a new "message integrity code" to ensure frames are not tampered with (Michael), and an AES-based privacy protocol. It is not ratified yet, but it is expected in late 2003.


  • Wireless Protected Access (WPA) is not an IEEE 802.11 standard. It is the Wi-Fi Alliance's marketing term for the parts of 802.11i that are done now (TKIP, Michael, 802.1x authentication framework). It leaves out management frame authentication and encryption and AES-based privacy.





A few notes:



  1. 802.11g has not been finalized, and is still a draft. You can buy draft-complaint hardware, but there is no guarantee that it will be interoperable with equipment that is developed against the final version of 802.11g, expected later this year.


  2. Several vendors have proprietary speed enhancements over the standardized rates in the table. Some vendors use the Texas Instruments chipset that allows extended "802.11b" gear to boost speed to 22 Mbps by using packet binary convolution coding (PBCC) instead of complementary code keying, but only when talking to other gear that uses the TI chipset. Similar approaches are used in extended "802.11a" equipment that either uses the OFDM pilot carriers for a 72 Mbps top speed, or bonds two 802.11a channels together for a 104 Mbps speed.


  3. 2.4 GHz is a band set aside for microwave ovens, but not because 2.4 GHz is a resonant frequency of water. The selection of 2.4 GHz is due to the period between vibrations, not a natural affinity of water for 2.4 GHz microwave energy.