Transmit Rate
2-6
Auto Rate
With the auto rate option, the PC Card in a client or AP automatically switches to the next
lower rate when data transmissions fail more than once. Shortly after completing the
transmission, the PC Card returns to transmitting data at the higher rate.
In most environments, Auto Rate allows the PC Card to use a higher rate for better data
throughput, yet the PC Card can still use the more reliable slower rate when transmissions
fail. A transmission can fail when the network experiences sporadic noise interference.
Also use Auto Rate if you have APs with 11 Mbit/s PC Cards and a mix of clients with
11 Mbit/s and 2 Mbit/s PC Cards. The AP can communicate with both types of clients, but
can communicate with the 11 Mbit/s clients at a higher rate than the 2 Mbit/s clients.
Fixed Rate
A fixed rate setting prevents the PC Card from retransmitting at a lower rate after a failed
transmission. One example of why you would do this is when a microwave oven in the area
produces noise in the same frequency as the wireless network (see Figure 2-1). The
interference only occurs when the machine is in use. The interference may temporarily
disrupt communications between a client and the AP.
After a transmission fails more than once, the AP retransmits at a lower rate. However, the
interference also prevents communication at the lower rate. Retransmitting at a lower rate
does not solve the problem and could decrease network performance. With fixed rate
enabled, the AP cannot retransmit at a lower rate.
Using a fixed low rate is useful in networks where range is more important than speed,
especially when network response times are affected by numerous retransmissions and the
communications quality is low due to a low signal level. Setting the transmit rate to a low
rate prevents the AP from slowing network response times by transmitting data
unsuccessfully at a higher rate then retransmitting at a lower rate.
A fixed transmit rate does not affect the receive rate. For example, an AP and a client both
have 11 Mbit/s PC Cards, but the client is fixed to only transmit at 2 Mbit/s. The AP can
send data at 11 Mbit/s to the client, and the client can respond by sending data at 2 Mbit/s.
You should not set the AP to a fixed rate of more than 2 Mbit/s if you have clients with
11 Mbit/s and 2 Mbit/s PC Cards. Otherwise, the 2 Mbit/s clients cannot communicate with
the AP. The 2 Mbit/s clients can only receive data at a maximum of 2 Mbit/s.