More information about 802.11g playing nice, from Unstrung. There are 2 interoperability questions: between 802.11g devices from different vendors, and between 11g and 11b devices.
Interoperability between different 802.11g chipsets is certainly an issue. However, they are only typical interop questions, and there will be bumps along the way as they are ironed out, but there are no showstoppers.
Interop between 11g and 11b is another, more complex issue. The root cause of this problem is that 11b devices cannot "hear" an OFDM waveform, which 11g uses for higher speeds.
802.11 uses a mechanism called CSMA/CA, which stands for "Carrier Sense Multiple Access with Collision Avoidance". This is a complicated way to say "listen before you talk". This differs from wired Ethernet, which uses CSMA/CD -- "CD" means "Collision Detection", or "listen while you talk". 802.11 cannot listen while talking, because the receiver is overloaded by the transmitter.
So, when an 11b device tries to "listen", what if an 11g device is already transmitting a message using OFDM? The 11b device thinks the air is clear, and transmits. Both messages are corrupted to some degree, but the AP has already locked on to the OFDM message, and the AP will not be able to ACK the message from the 11b device.
This is particularly problematic when the 11b device has just been enabled, and it is actively scanning across channels looking for an AP. If an 11g device is hogging the AP, the 11b device will have little chance to "get lucky" enough to transmit a Probe Request successfully. Passive scanning may be more successful in locating APs, but the same problem exists when the 11b STA tries to authenticate or associate.
Using RTS/CTS can help the problem, but not eliminate it. The RTS/CTS sequence can be heard by 11b devices, but it is a very short time interval in the RTS/CTS/Frame/ACK exchange. A max-size packet, sent by an 11g device using 6 Mbps OFDM, takes slightly more than 2 milliseconds just to transmit the data. RTS, CTS, and ACK frames are short management frames without any data.
In the presence of high 11g traffic, an 11b device that is changing channels has a high percentage chance of listening in the middle of the OFDM data frame -- which it cannot hear.
RTS/CTS is an optional feature in 802.11. RTS/CTS adds additional overhead and reduces throughput. However, RTS/CTS alleviates the hidden node problem. And RTS/CTS is helpful in networks that mix 11b and 11g clients, but it does not fully address the interoperability issue.
For more information about 802.11g:
802.11g Starts Answering WLAN Range Questions
802.11g spec: Covering the basics
Posted by Patrick at January 28, 2003 10:57 AM | TrackBack