WiFi: Overview of the 802.11 Physical Layer

IEEE 802.11 Standards and Formats

IEEE 802.11-1997 ( Legacy Mode )

It specidied 3 alternative physical layer technologies:

• Diffuse infrared operation at 1 Mbps
• Frequency-hopping spread spectrum operating at 1 Mbps or 2 Mbps
• Direct-sequence spread spectrum operating at 1 Mbpsor 2 Mbps

MAC/PLCP Interactions

The short-format PLCP header was introduced as an optional performancing feature in the IEEE 802.11b-1999 stndard. IEEE 802.11g was the first 2.4GHz PHY to mandate support for the short-format PLCP header. IEEE 802.11a-1999 specified a shorter PLCP structure that is not comparable to the DSSS PLCP header.

STAs that use the Short Preamble option can spend more time in sending data and less time in waiting to do it. Therefore, the use of Short Preamble can improve the aggreate throughput of a BSS.

The Beacon must be understandable by any PHY operating in the 2.4GHz band, so it must be transmitted using the least-common-denominator PLCP header format, in other words, Barker/DBPSK modulation ( at a rate of 1 Mbps ) with a Long PLCP preamble. The Beacon's Capability IE indocates whether the Short preamble may be used within its BSS.

For backward compatibility with DSSS PHYs ( 1 Mbps and 2Mbps), Beacon must be transmitted using the long-format PLCP header. The original IEEE 802.11-1999 only specified the 1 and 2 Mbps DSSS RF PHYs, so the Beacons must still use the long-format PLCP preamble in IEEE 802.11b-1999 and 802.11g-2003 to maintain the backward compatibility.

Beacons are still sent using Long Preambles, and any IEEE 802.11b STA that wants to do so can use the Long Preamble. The indication from the AP ( in the Beacon and Probe Response ) that Short Preamble mode is enabled is not a directive that all STAs that support Short Preamble mode must send all their frames in that mode. The AP is giving permissions, not issuing an order.

The standard allows for a mode in which all STAs in a BSS must support the Short Preamble, since there is an error messages that may be used by an AP at Association time to indicate to a STA that its association was denied because it did not support the Short Preamble.

When a STA joins a WLAN, it will find out if the WLAN supports the Short Preamble option. However, before it has joined, it does not know whether this is the case. Now, any STA that supports the Short Preamble must implicitly also be able to receive the Long Preamble.

In order o permit WLANs to effectively mix Short and Long Preamble STAs, a rule has been defined that Long Preamble Probe Request will be answered with a Long Preamble Probe Response.

References

"A Field Guide to Wireless LANs for Administrators and Power Users" by Thomas Maufer