本末倒置16
fairness share.If the fairness shareφi is very small,there might be long duration of idle time because of this linear nature of backoff interval.If the number of nodes in the network is high,this might lead to dropped throughput of the network because of excessively long backoff intervals.This problem is circumvented by compressing larger backoff intervals into smaller exponential range.It is not clear how the channel shares φi is assigned to each node.
5Performance
Throughput performance of communication links is measured in terms of observed data rates.Looking from the link layer perspective,the performance is the effective data rate available for the raw bits exchanged be-tween nodes.Whereas,looking from the network layer perspective,it is the rate at which the network layer data is exchanged.This precludes the management data exchanged at the link layer.As one moves higher up the protocol stack,the performance of a link progressively reduces.The reason for this progressive perfor-mance reduction is almost always associated with lower level protocol overheads.Further,for shared media like wireless channels,where multiple nodes contend for the same channel,additional channel bandwidth is used up to resolve the contention and some bandwidth may even be lost when collisions arise.
5.1Performance Loss
For wireless networks like802.11b,it is never possible to achieve the theoretical available bandwidth at the application level.Since a wireless channel can never be devoid of noise,the?rst loss is at the physical radio link itself.Shannon’s equation gives the relation between available data rate R for a channel with bandwidth B with a Signal to Noise Ratio(SNR)as:
R=B log2(1+SNR)
This is the theoretical limit and there can be additional loss because of multipath fading,Doppler shifts, and bit errors[19].
The second rung of reduction in throughput occurs at the Physical layer.An802.11b frame is shown in Figure11.Each frame comprises of24bytes of Physical Convergence Protocol Layer(PLCP)preamble and header.This PLCP preamble is then augmented with rest of the MAC Protocol Data Unit(MPDU). The header is always transmitted at a slower rate of1Mbps and rest of the MAC Protocol Data Unit is transmitted at a variable rate.This slow rate transmission is necessary for compatibility and reachability with all nodes in the network.Thus,a signi?cant portion of transmission occurs at the lower data rates causing a reduced throughput.
IEEE802.11b uses CSMA/CA[3]protocol for media access.In this protocol each data packet transmis-sion is preceded by a channel reservation request(RTS)and a channel reservation response(CTS)between sender and receiver.The data packet is then followed by an acknowledgment from the receiver(ACK). The ACK is required for the reliability of link level transmissions.The transmission follows the so called 4-way handshake protocol of RTS–CTS–DATA–ACK in order to solve the hidden node problem.The ad-ditional bandwidth consumed by these RTS/CTS/ACK and several other management frames add up to the reductions in performance throughput.
The observed data rate for802.11b networks by network layer is around7Mbps in typical indoor environments and the bandwidth of802.11b networks is11Mbps.The next level of performance loss is because of complex interactions between protocol layers and other overheads.We will be examining some of the protocol performance issues.
17
Sync Start Frame Delimiter
Signal Service CRC Length MPDU variable length
128/56 bits 16 bits 8 bits 8 bits 16 bits 16 bits Figure 11:IEEE 802.11b frame format.Initial 24byte header is always transmitted at 1Mbps.Rest of the MAC Protocol Data Unit (MPDU)is transmitted at either 2Mbps or 5.5Mbps or 11Mbps.
5.2Performance of TCP
Performance of TCP/IP over wireless networks is a widely researched topic.TCP behavior is studied under various ?avors of wireless environments.The primary thrust of this research is on the behavior of TCP in response to the error conditions in wireless networks.TCP,which is known to be a very stable and robust protocol on wired networks does not perform as well on wireless networks.Some of the serious issues with TCP are that of performance degradation over wireless links.The primary reason for performance degradation is the assumption by TCP that all losses in the network are due to congestion [20].In almost all wireless networks,the transmissions are highly prone to frame errors which increase with packet size.For example,in 802.11networks,the Frame Error Rate (FER)doubles for every 300byte incre-ment in frame size [21].Thus,at higher packet sizes,the frame errors are higher and hence the packet loss is higher.This packet loss is wrongly treated as congestion by TCP and its congestion control mechanism is triggered.As a result,TCP reduces the congestion window size.The TCP congestion window size is the number of packet that can be sent with outstanding acknowledgments.Congestion window is the measure of minimum number of packets required to keep the link occupied increasing the channel utilization.This congestion control causes lower channel utilization even though in reality there is no congestion in link.
One of the ?rst proposed solutions for TCP performance degradation on wireless links is Indirect-TCP (I-TCP)by A.Bakre et al.[22].I-TCP works by splitting the transport connection at the wired-wireless boundary,usually on base stations.I-TCP maintains two separate TCP segments,one is over the wired network between the base station and the wired end of TCP connection,the other is over the wireless between the base station and the wireless host.This way,the losses occurring on wireless network are hidden from the wired nodes.To cover up for the losses on wireless segment the base station carries out retransmissions.Though an ef?cient solution,this approach violates the end-to-end semantics of TCP.For example,in this approach even if the sender receives an acknowledgment,it does not mean that the receiver has indeed received the packet.This violation of end-to-end semantics can have serious impact on applications.It may so happen that the wireless host may crash before it receives the packet that has been acknowledged by the base station.This is against TCP semantics and may not be acceptable to certain applications.
Hari Balakrishnan et al.[23]try to address this problem by proposing a snoop module that resides in the routing protocol stack of the base station.This snoop module snoops on all the packets that are sent to the mobile hosts and the acknowledgments that are transmitted by the mobile nodes.It also caches the packets that are transmitted over the wireless network.This enables the base station to detect packet losses in the wireless network and to retransmit the lost packets,thus avoiding the triggering of TCP’s congestion control
18
Data Rate Modulation Bits/Symbol
11(Barker Sequence)1MSps
2Mbps QPSK2
8CCK 1.375MSps
11Mbps QPSK8
Table1:Different possible data rates for802.11b networks.
and avoidance mechanisms.
The802.11b provides a fragmentation oriented solution for this at the MAC layer.Since the frame error rate increases with frame size,the reliability of transmissions can be increased by reducing the frame size.Al most all802.11b network interface cards support an option of enabling the fragmentation.In the event of bad channel conditions and increased frame errors,the MAC unilaterally fragments the larger frames into smaller frames of optimal size.This is done in a manner transparent to the higher layers.The receiving node then defragments the frames to obtain a complete data packet which can be handed over to the higher protocol layer.Since the fragmentation occurs at the frame level,there is no higher protocol overhead associated with the fragments.There is a small overhead of additional MAC f0d6b5b369dc5022aaea002apared to the alternative of loosing the frame and reducing the overall throughput,a small overhead is a fair trade off.
Another problem with TCP in wireless LANs is that of Self Collision.George Xylomenos et al.[24] describe a scenario where the TCP data packets contend for the channel with the ACKs for the previous packets.This contention results in self collision for the TCP connection causing overall performance to degrade.This self collision is caused because of the half duplex nature of wireless networks.Haitao Wu et al.[25]propose a modi?cation(described in section5.3)to the Distributed Coordination Function(DCF), which is the MAC protocol for802.11b,to alleviate this self collision problem.
Typically most of the solutions for the issues faced by higher protocols like TCP are localized solutions. The problems are tackled within a protocol layer and no effort is made to make other protocol layers aware of the changing situation.While this transparent approach is good from modularity and simplicity of design point of view,it severely impacts the performance.It is evident that in order to optimize the network performance,all layers in the protocol stack should adapt to the variations in the wireless link appropriately. Further,this should be done while considering the adaptive strategies at other layers[19].
5.3Performance Improvements
The performance improvement research for IEEE802.11b can be broadly categorized into three classes based on the locality of the optimizations involved.These categories are mainly:MAC performance opti-mization,Network or Transport layer enhancements,and Infrastructural arrangements.
5.3.1MAC Performance Optimization
Recent performance optimization research in802.11b MAC aims at improving or enhancing the perfor-mance by modifying the MAC while retaining backward compatibility with existing speci?cation.We give an overview of some schemes seeking to modify the DCF channel access protocol to bring in performance enhancements.
19
Multi-Rate802.11b
IEEE802.11b supports data transmission facility at multiple rates.These multiple data rates are possible because of different modulation techniques which are optimized for different channel conditions.Tech-niques like Quadrature Phase Shift Keying(QPSK),with8bit Complementary Code Keying(CCK)error correction codes,provide a bandwidth of11Mbps but need very high Signal to Noise Ratio(SNR)in chan-nel.Whereas,techniques like QPSK,with11bit Barker Sequence,provide2Mbps of bandwidth but are capable of operating in noisy environments.Table1describes various data rate speci?cations for IEEE 802.11b.
