本末倒置Network interface Cards supporting these multiple data rates are capable of switching between different modulation techniques after assessing the channel characteristics.A user can choose to clamp the network interface at one particular rate or can enable the option of letting the device choose an appropriate rate. In order to improve the performance,the network interface cards can adapt to varying channel conditions and dynamically switch between different modulation techniques.This adaptation primarily involves two tasks,(1)sensing the channel quality and(2)selecting the appropriate technique and hence the data rate. Channel quality can be estimated by using several metrics,such as,signal to noise ratio,bit error rate, signal power,etc.A history of these metrics can be maintained which can be used to predict the channel conditions in future.But given the volatile nature of wireless channels,it is not clear how accurate and reliable these estimates can f0d6b5b369dc5022aaea002aing these estimates,one can select an appropriate rate for the estimated channel condition and transmit the data at the selected rate.For example,if the channel condition is excellent then the sender can select the highest possible rate for transmissions.On the other hand,if the bit error rate is high,the sender can choose to select a stronger encoding technique while dropping the rate.For worse channel conditions,the sender may resort to the lowest possible rate just to get the data across.
Auto Rate Fallback Scheme(ARF)
The channel quality estimation is a proactive approach and may not be accurate.Instead,one can imple-ment a simple reactive channel quality sensing mechanism which gauges the changing channel conditions based on success or failure of previous transmissions.
In IEEE802.11b,link level reliability is provided by explicit link level ACKs.Retransmissions are carried out if positive acknowledgments are not received.Lucent corporation’s Orinoco cards provide a simple result based Auto Rate Fallback(ARF)[26]mechanism which uses ACKs(or their absence)to estimate the channel quality.The ARF mechanism is a timer driven mechanism which keeps track of missed acknowledgments and works as follows.When an ACK is missed for the?rst time,after earlier successful transmissions,the?rst retransmission is carried out at the same rate.After second failure,the transmission rate is downgraded to the next lower data rate and a recovery timer is started.The transmission rate is upgraded back to the next higher data rate if the timer expires or10consecutive transmissions are successful. After this recovery if the very next transmission meets a failure,the system immediately reenters the fallback condition and resumes the normal operation.
While the ARF mechanism is good for link quality estimation between a?xed pair of nodes,it overlooks the fact that it is the receiver whose channel conditions need to be estimated and not the sender.A direct disadvantage of ARF scheme can be seen when there are multiple nodes communicating with each other in a wireless network.If a node moves to a location with bad radio characteristics,other nodes communicating with this particular node would experience transmission failures and as a result the transmission rate would be dropped.Consequently,it would take10successful transmissions or a timeout of the recovery timer to increase the data rate to the next higher level.This can result in pulling down the throughput for the entire network at lower data rates.If more than10%of the nodes in a wireless network are in bad radio
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characteristics zone and the communication traf?c between all nodes is more or less equal,the overall throughput of the network would always be clamped at the lower threshold.In addition,if some nodes are mobile and there are certain dark spots in the wireless network,the nodes moving in and out of dark spots would reduce the network throughput further.Thus,ARF would fail to perform ef?ciently in a network which comprises of zones with varying network characteristics.Which defeats the primary goal of ARF, that is to adapt to varying network conditions.
Receiver Based Auto Rate Scheme(RBAR)
Gavin Holland et al.[27]propose a rate adaptive scheme which uses feedback information from the re-ceiver to sense the channel condition rather than estimating it at the sender.They also observe that in cellular networks,most of the receiver based channel quality estimation techniques have following characteristics:?Channel quality information is estimated by the receiver and periodically fed back to the sender either on same channel or on a separate channel.
?The sender performs rate selection based on the feedback from the receiver.
?The estimation,feedback,and rate selection schemes often reside at the physical layer and are trans-parent to the higher layers(such as,even MAC).
It is dif?cult to apply same techniques to wireless LANs because conventional wireless LANs usually operate in half duplex mode?.This makes simultaneous feedback impossible.Moreover,wireless LANs use distributed contention based media access which need accurate time estimates for packet transmissions so that other nodes in the network can be informed about them.Dynamic encoding below MAC layer would hinder this operation and can impact ef?ciency.
Thus,one of the major problems in receiver based estimation schemes for wireless LANs like802.11b is the requirement of a mechanism to provide feedback information to the sender.Another desired feature about the feedback is that it should convey instantaneous information rather than history to be more effective.
Since,IEEE802.11b uses DCF for channel coordination,each DATA packet from sender is preceded by a short RTS packet.RBAR leverages on this RTS packet to estimate the channel quality and reception at the receiver f0d6b5b369dc5022aaea002aing this information the receiver selects an appropriate transmission rate.This rate information is then piggybacked on the CTS response to the sender.The sender then uses this information to carry out the actual transmission.
In802.11b DCF,RTS and CTS packets carry the duration information for which all nodes should set their NA V.RBAR proposes to replace this duration information with data rate and the size of the data packet. Other nodes overhearing RTS and CTS can calculate the duration from this information and still set their NA Vs to appropriate values.
The operation sequence and the4-way handshaking for RBAR is shown in Figure12.Whenever some node Src has some data to be sent to some node Dst,it performs the usual channel sensing for a period equal to DIFS.Once the channel is sensed to be idle for DIFS time it sends a short RTS message to DST. This RTS message carries the tentative transmission rate and the size f0d6b5b369dc5022aaea002aing this information, nodes in the vicinity of Src calculate the duration and set their NA Vs accordingly.The node Dst upon receiving RTS analyzes it for its signal quality,strength and other metrics and selects an appropriate rate for the transmission.This selected rate and the size of data is again encapsulated in the CTS response.
