Bit-rate Selection in Wireless Network

Initially, 1997, only one base rate was supported in 802.11.

802.11 PHY layer supports multi-rate transmission capabilities by dynamically choosing the most appropriate modulation technique for the received signal strength.

Multirate Support

A method of negotiating a mutually acceptable data rate is necessary. Stations may also change speed frequently in response to the radio environment. Stations must adapt to the changing environment by alternating transmission speed as necessary. There are a few general rules that are required for all stations:

• Every station maintains a list of operational rates, which is the list of rates that are supported by both the station and the BSS.
• Every BSS has a basic rate set, which is a list of rates that must be supported by every station joining the BSS. Any frame sent to a group address must be transmitted as a basic rate.
• Some control frames must be sent at one rate of the basic rate set. 
• Unicast frame can be sent at any rate which is know to be supported by the destination.

Rate Selection and Fallback

Rate Control Algorithm

While the 802.11 standard defines the spec. for MAC and PHY parameters, it does not define any particular instance of Rate Control Algorithm(RCA) and is open to the device manufactures to implement it.
The RCA can be implemented in software driver  or chip.

Wireless protocol has many bit-rates from which transmitters can choose.
Each bit-rate uses a particular modulation to transform a data stream into a sequence of symbols.
All 802.11 packets contain a small preamble before the data payload.

Auto Rate Fallback (ARF) Algorithm

It selects the initial bit-rate to be the highest possible bit-rate.
Given the number of retries and whether the transmission is successfully acknowledged or not, the ARF adjusts the bit-rate based on the following criteria:

• move to the next lower bit-rate if the packets was never acknowledged
• move to the next highest bit-rate if 10 successive transmissions have occurred without any re-transmissions
• otherwise, keep at the current bit-rate

Onoe Algorithm

Developed by Atheros.
The Onoe algorithm keeps track of of the current bit-rate for the link and the number of credits that the bit-rate has accumulated.
It sets the initial bit-rate :

• 24 Mbps for 802.11g/a
• 11 Mbps for 802.11b

and sets the credits to be 0 for each bit-rate.

Then, start the following operations periodically:

• if no packet has succeeded, move to the next lower bit-rate
• if ( >10 packets have been sent ) and (the average number of retries /packet  > 1),  move to the next lower bit-rate
• if ( > 10% packets need a retry), decrement the number of credit of this bit-rate
• if ( < 10% packets need a retry), increment the number of credit of this bit-rate
• if the current bit-rate has >10 credits, increase the bit-rate 
• otherwise, keep at the current bit-rate

Receiver Based Auto-Rate (RBAR) Algorithm

RBAR chooses the bit-rate based on S?N measurements at the receiver.

When a receiver gets an RTS packet, it calculate the highest bit-rate that would achieve a BER based on the S/N of the received RTS packet.

The receiver piggypacks the bit-rate on the CTS packet, then the sender uses that bit-rate to send the data packet.