Last Post: October 2, 2006:
This is a very worthwhile audio clip to listen to. In hopes that every explanation can be improved upon and later errors thereby avoided, I offer the following.
Preparing to explain how 802.11n products will have other PHYs built in, the speaker asserts that, "the 802.11g PHY only specifies modulation and coding schemes above and beyond 802.11 classic (DSSS) and 802.11b, and it is left to vendors to combine PHYs to support data rates of classic 1 and 2 Mbps, 802.11b 5.5 and 11 Mbps, and 802.11g 6, 12, and 24 Mbps." This is not a word for word quote, but I did my best to use his terms.
Equating a PHY with the modulation and coding schemes (MCS) the PHY introduced, or with the letter of the amendment (if any) that introduced the PHY, represent classic mistakes that I believe result from a failure to plainly understand the amendments, the PHYs, and the MCSs. Because marketeers choose to sell 802.11b or 802.11bg or 802.11abg network cards, too many of us choose to believe those products must implement one, two, and three PHYs respectively when in fact they implement one (HR/DSSS), one (ERP), and two (ERP and OFDM) PHYs respectively. In the last case typically only one PHY is operational at a time.
The DSSS PHY was introduced by the IEEE 802.11-1997 document. It must support data rates of 1 and 2 Mbps in the 2.4 GHz band. IEEE 802.11 section 15.1: "The DSSS system provides a wireless LAN with both a 1 Mb/s and a 2 Mb/s data payload communication capability. The DSSS system uses baseband modulations of differential binary phase shift keying (DBPSK) and differential quadrature phase shift keying (DQPSK) to provide the 1 Mb/s and 2 Mb/s data rates, respectively."
The OFDM PHY in the 5 GHz band was introduced by the IEEE 802.11a-1999 amendment. It interoperates with no other standard PHY (yet). It must support data rates of 6, 12, and 24 Mbps in the 5 GHz band. IEEE 802.11 section 17.1: "This clause specifies the PHY entity for an orthogonal frequency division multiplexing (OFDM) system. The OFDM system provides a wireless LAN with data payload communication capabilities of 6, 9, 12, 18, 24, 36, 48, and 54 Mb/s. The support of transmitting and receiving at data rates of 6, 12, and 24 Mb/s is mandatory."
The HR/DSSS PHY was introduced by the IEEE 802.11b-1999 amendment. It is required to interoperate with DSSS PHYs using common MCSs, unless blocked by configuration. IEEE 802.11 section 18.1.1: "This extension of the DSSS system builds on the data rate capabilities, as described in Clause 15, to provide 5.5 Mb/s and 11 Mb/s payload data rates in addition to the 1 Mb/s and 2 Mb/s rates. To provide the higher rates, 8-chip complementary code keying (CCK) is employed as the modulation scheme. The chipping rate is 11 MHz, which is the same as the DSSS system described in Clause 15, thus providing the same occupied channel bandwidth. The basic new capability described in this clause is called HR/DSSS. The basic High Rate PHY uses the same PLCP preamble and header as the DSSS PHY, so both PHYs can co-exist in the same BSS and can use the rate switching mechanism as provided."
The ERP PHY, or simply the ERP, was introduced by the IEEE 802.11g-2003 amendment. It is required to interoperate with DSSS and HR/DSSS PHYs using common MCSs, unless blocked by configuration. IEEE 802.11 section 19.1.1: "The ERP builds on the payload data rates of 1 and 2 Mb/s, as described in Clause 15, that use DSSS modulation and builds on the payload data rates of 1, 2, 5.5, and 11 Mb/s, as described in Clause 18, that use DSSS, CCK, and optional PBCC modulations. The ERP draws from Clause 17 to provide additional payload data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mb/s. Of these rates, transmission and reception capability for 1, 2, 5.5, 6, 11, 12, and 24 Mb/s data rates is mandatory."
The HT PHY, anticipated to be introduced by the IEEE 802.11n amendment no later than 2008, is expected to interoperate with DSSS, HR/DSSS, ERP, and OFDM PHYs using common MCSs, unless blocked by configuration. IEEE P802.11n/D1.03 section 21:1: "Clause 21 specifies the PHY Entity for a high throughput (HT) orthogonal frequency division multiplexing (OFDM) system. The HT STA shall be compliant with PHY specifications as defined in clause 17 for operation in the 5 GHz bands, and clauses 18 and 19 for operation in the 2.4 GHz bands. The HT features are applicable to operation in either the 2.4 GHz band or the 5 GHz bands, or both, as specified in 21.3.14."
Let's hope the marketeers call these "802.11 HT cards" rather than "802.11abgn cards."
I hope this helps. Thanks. /criss