802.11g <–> 802.11 / 802.11b:
When 802.11g (ERP-OFDM) was released in 2003, we needed a way to co-exist with the large installed base of older 802.11 (DSSS) and 802.11b (HR-DSSS) based networks and equipment out there. The solution to the problem used RTS/CTS control frames to silence the channel before an 802.11g station used any of the higher modulations. Before transmitting a frame using OFDM modulation, the transmitting station would initiate an RTS / CTS exchange with the receiving station. The RTS/CTS exchange uses a PHY rate of 1 Mbps using DBPSK modulation as specified by DSSS, silencing all legacy stations in the coverage area. This is an awful co-existence mechanism and it chews up valuable airtime silencing the channel before any 802.11g station tries to talk!
As per CWNP.com’s CWAP text and the IEEE 802.11 2012 standard:
RTS/CTS protection is activated by all the STAs in a BSS when the “Use Protection” bit of the ERP Information Element is set to 1 in Beacons and Probe Responses. The following MUST trigger the “Use Protection” bit to be set:
- A legacy client that does not support ERP-OFDM associates to the BSS.
- A beacon from a neighbouring legacy BSS (that does not support ERP-OFDM rates) is detected.
- Any management frame (excluding a probe request) is detected coming from a neighbouring legacy BSS.
The above list is defines when the Use Protection bit MUST be set to 1. The IEEE 802.11 standard left it open to vendors to choose other scenarios when it is suitable to activate protection mode. For example, some vendors will set the Use Protection bit as soon as they receive a probe request from a legacy non-ERP station.
802.11g / OFDM-Only Mode
802.11g Radios can be configured to use only ERP-OFDM rates making them incompatible with DSSS / HR-DSSS radios. This is named differently per vendor but is usually something like “802.11g-only” or “OFDM-only” mode. This basically configures the radio to ignore probe requests from legacy clients that only support DSSS/CCK rates. The legacy clients will be prevented from associating to the BSS, removing the need for RTS/CTS for older client stations. It does not however absolve the OFDM-Only BSS from RTS/CTS completely, we still have to be polite to an legacy DSSS/HR-DSSS cells nearby as per points 2 and 3 above.
802.11n <–> 802.11a/g:
Later on with the introduction of 802.11n, the IEEE learned from the horror of RTS/CTS and implemented THREE new PPDU formats to enable more seamless compatibility with either 2.4GHz 802.11g (ERP-OFDM) or 5GHz 802.11a (OFDM) stations. The PPDU Frame formats are summarised as follows:
- non-HT Format: Used when communicating with either an 802.11g or 802.11a station. The Preamble and PLCP Header are exactly the same as the legacy communications. 802.11n stations can understand the older PPDU formats and keep quiet during transmission.
- HT-Mixed Format: Used when communicating with an 802.11n (HT-OFDM) station on either band in the presence of 802.11g or 802.11a stations. This PPDU format starts with the same preamble and PLCP Header as defined for 802.11a/g and then adds a second PLCP Header afterwards that enables 802.11n transmission of the MPDU. This allows 802.11a/g clients to recognise the impending frame transmission’s properties and keep quiet.
- HT-GreenField Format: Only has the HT-OFDM defined Preamble and PLCP Header. Used in networks that do not support backward compatibility. Does not allow older stations to recognise the impending frame transmission. This should only be used where no 802.11 a/g stations exist.
2.4Ghz 802.11n radios also interoperate with 802.11/802.11b radios by using RTS /CTS in the same way as 802.11g radios do and are capable of decoding DSSS modulated signals from legacy stations.
802.11n radios have the option of only working in “GreenFields” mode. This mode only allows 802.11n capable stations to join the BSS and does not use the non-HT or HT-Mixed PPDU formats. It also does not perform any RTS/CTS for legacy clients using 802.11 / 802.11b.
802.11ac <–> 802.11a/n
The 802.11ac standard actually only defines a single PPDU frame format for the 802.11ac standard called the VHT PPDU format. This frame format is compatible with 802.11a (OFDM), 802.11n (HT-OFDM) and 802.11ac (VHT-OFDM) stations all in one go. 802.11ac has no “11ac Only” or greenfield mode like 802.11n does. In my opinion forcing compatibility with 802.11a only stations was a little silly since they are so rare.