B vs. G Mixed Wlan Performance

B vs. G: Understanding mixed WLAN performance

6/3/14 9:28 PM

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B vs. G: Understanding mixed WLAN performance

Both 802.11a and 802.11g offer data rates up to 5 times faster than 802.11b. But 802.11g sales leap-frogged the slightly older 802.11a, due to backwards compatibility with legacy 802.11b products. Unlike A, which occupies the 5 GHz UNII band, B and G share the 2.4 GHz ISM band, letting G APs simultaneously communicate with B and G stations. However, as many WLAN owners have noticed, there is a price to be paid for backwards compatibility: the presence of B stations can degrade G performance. To get the most from your 802.11g AP, it can be helpful to understand the "protection" mechanism that enables B/G coexistence at the expense of performance. Why is protection needed? G achieves higher throughput using a more efficient modulation called Orthogonal Frequency Division Multiplexing (OFDM). For backwards compatibility, G products also support the older Direct Sequence Spread Spectrum (DSSS) modulation used by B products. According to the 802.11g standard, Extended Rate Physical (ERP) OFDM stations can use data rates up to 54 Mbps. Vendor-proprietary techniques like channel bonding can achieve higher rates, but that's irrelevant for standard B/G compatibility. Speaking both OFDM and DSSS is a good start, but it is not enough to enable peaceful coexistence. A radio channel is a shared medium; transmitters must cooperate to avoid collisions. To "take turns speaking," all 802.11 stations implement a standard Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) protocol. When an 802.11 station has data to send, it listens to the channel. If another 802.11 node is transmitting, the station waits until the channel is free. Compare this to meeting attendees who politely try to avoid speaking when someone else has the floor. In a large meeting, you might raise your hand and wait to be called upon. In CSMA/CA, this is done by...