本末倒置S.Mangold et al.[8]present a comprehensive overview of these features.They evaluate various QoS support features by means of simulations.The simulations were mostly for802.11a network(a higher speed version of802.11operating at5GHz band).They analyzed the behavior of EDCF for different categories (mainly high,medium,and low priorities)and the measured throughput performance against the offered load.The simulation results obtained were consistent with the expected results.For high priority traf?c the throughput increased linearly with the increase in offered load.Whereas,the low and medium priority traf?c observed knee points after certain level.The knee point of low priority traf?c appeared ahead of medium priority traf?c.They also evaluated the performance of HCF for parameterized QoS.The metric chosen was the service delay for frames.The results were consistent with expectations.The delay probability decreased for higher values.Almost all the time the delay was within?xed bounds.The evaluations showed that the enhanced MAC protocols indeed provide the expected QoS which is the main objective of802.11e.
One of the major issues with QoS provisioning is how to percolate the QoS related information to perti-nent layers.For example,the QoS requirements are mainly application speci?c requirements,whereas,the actual bandwidth provisioning has to be done at the MAC layer in the LAN.This issue calls for a coordi-nation between the MAC and higher layers so that the applications can request their QoS requirement in an appropriate manner.For this purpose,802.11e de?nes two entities;Station Management Entity(SME)and
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MAC Layer Management Entity(MLME).SME is a logical entity in a node which is capable of communi-cating with all layers in the network stack,whereas,MLME deals with interacting with SME and managing MAC layer.The SME and MLME communicate with each other by means of intra-STA signaling.Differ-ent MLMEs in an infrastructure network communicate by means of inter-STA signaling.For example,the communication between HC and a node regarding TxOP duration etc.is part of the inter-STA signaling. The inter-STA signaling is used to setup,modify,and delete traf?c streams.This signaling carries a traf-?c speci?cation(TSPEC)element to characterize the QoS requirements of a node.As shown in Figure6, TSPEC carries various information like minimum data rate,burst size,etc.,which can be derived directly from higher layer requirements.Whereas,some?elds like polling interval,retry interval,etc.,are more MAC layer speci?c.Since SME is capable of interacting with all protocol layers,it is capable of obtaining QoS requirement for speci?c?ows.These requirements are then conveyed to the MLME which is respon-sible for establishment of?nal reservations.Sai Shankar et al.[9]describe the QoS signaling procedure for parameterized traf?c in the purview of RSVP.
As of beginning of year2003,the802.11e is still a draft and has not been standardized yet.The QoS mechanism seems to be geared more toward RSVP.(TSPEC is mostly RSVP speci?c).The draft is contin-uously being revised.For example,there are some documents which indicate that the TSPEC element is being replaced with a new Queue State element[10].
4Fairness
Wireless channel is a shared scarce resource.The MAC protocols used over wireless networks are distributed protocols which try to avoid collisions and provide the nodes in a network with an access to the channel in a fair manner.The ef?ciency of MAC protocols can be measured using two parameters:the probability of collision and fairness in the allocation of channel to competing nodes.The wireless LAN protocols,like any other randomized multiple access protocols,try to resolve the collision problem by following a binary exponential backoff(BEB).BEB is a very ef?cient mechanism in terms of reducing collision probability.It often reduces the collision probability to a fraction of transmissions,as low as1%.
Typically all variations of CSMA/CA protocol suffer from the fairness problem investigated?rst by Bhargavan et al.[11].A channel access protocol(MAC)is termed to be unfair if it fails to provide the channel access to individual nodes without giving preference to one node over others when there is no explicit differentiation.That is,when multiple nodes in a network are competing with each other for channel access,the probability of each node winning the contention should be equal.
Though the wired Ethernet protocol based on CSMA/CD is known to be fair,its wireless counterpart 802.11b based on CSMA/CA is proven to be unfair[12].The unfairness of wireless networks has roots in the fact that unlike wired networks,the collisions in wireless networks are asymmetric.It is not necessary in wireless networks that all nodes involved in collision suffer from packet loss.The collisions and hence the binary exponential backoff can occur primarily because of the following three reasons.
?Transmissions from two nodes interfere with each other and hence their transmissions do not get acknowledged.The absence of ACKs is then treated as collisions by both the senders.
?In the4-way handshaking mode,if a node does not receive CTS response for its RTS request,it treats this as a collision and hence doubles its backoff window.This is irrespective of the status of the destination node.A node may defer to send back CTS if any other node in its vicinity has reserved the channel by sending an RTS or CTS to some other node.
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?If two nodes carry out simultaneous transmissions intended for the same destination,one of them may succeed because of higher power level.This is called the capture effect in wireless channels[13].
Out of these three collision scenarios only the?rst one is the real collision.The other two scenarios result in success for one node and failure for the other.In this case,only the failed one performs the binary exponential backoff.For subsequent channel contention,the node which succeeded recently has a higher probability of winning the contention because of its lower backoff window.This is essentially because of the dissimilar congestion view about the channel by different nodes.Nodes which are generally successful in accessing the media perceive it to be less congested compared to the nodes which encounter failures.This prompts the successful nodes to access the channel in a more aggressive manner(because of lower backoff window)than the failed nodes.This skewed notion of congestion leads to an unfair access of the channel.
The dissimilarity in wired and wireless networks arises because of the dissimilar nature of the media.In wired networks the media is indeed a shared media.If one node is accessing/using media all other nodes are aware of the media access.But the wireless media is a piecewise shared media.The reach of each node is limited by the transmission power and the local noise present in the region.This makes the media characteristics location dependent and hence a non-uniform nature of the media is perceived by constituent nodes.
The backoff procedure used in almost all wireless medium access protocols is essentially borrowed from the wired Ethernet where the non-uniform nature of media does not exist.So,the binary exponential backoff procedure which provides a fair media access in wired networks becomes the cause of unfairness in the wireless networks.
4.1Impact of Unfairness
The unfairness of MAC has a far reaching impact on the behavior of higher layer protocols and the appli-cations using the network.Application like audio/video streaming are sensitive to packet delays and jitters. When the underlying link behavior is unfair,some applications may be starved of bandwidth just because their share is unfairly distributed somewhere else.
Shugong Xu et al.[14]analyze the behavior of TCP protocol in f0d6b5b369dc5022aaea002aing simulations they show that TCP suffers from instability and unfairness problem in these networks.The instability causes the throughput of available wireless network to?uctuate because of interactions between different nodes carrying TCP-data and TCP-ACK traf?c.The unfairness problem leads to inde?nitely long timeouts causing multiple retransmissions and route breakups.
Koksal et al.[12]describe a scenario where TCP performance degrades because of short term unfairness exhibited by MAC protocols.Because of short term unfairness the TCP acknowledgments fail to reach the sender in a timely fashion.This results into a bursty traf?c.This bursty traf?c results into and ACK compression which aggravates the burstiness of the stream.The bursty traf?c has many disadvantages like packet loss in response to bursty traf?c and throughput loss because of idle links during two consecutive bursts.
4.2Achieving Fairness
Since the fairness problem is deep rooted in the MAC layer itself,it is reasonable to conclude that it can be solved by modifying the MAC in an appropriate way that achieves fairness.To this effect there have been several enhancements and modi?cations suggested to the MAC layer.Some of the examples are
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Figure7:A single cell con?guration.All nodes are within range of each other.Nodes N1,N2,and N3contend for channel to send data to node B.If some node has very small backoff window than others,it may end up grabbing the channel and as a result other nodes may fail to get their fair
share.Figure8:Node B is sending data to node N1and N2. Node N3is sending data to B.If the fairness criteria is de-pendent on share per node rather than share per stream, then nodes like base stations are starved for bandwidth as these are the nodes which have maximum number of streams for downstream traf?c.
MACAW[11],Estimation based backoff[15],Distributed Wireless Ordering Protocol[16],and Distributed Fair Scheduling[17].
MACA W
Vaduvur Bhargavan et al.[11]observe that to allocate media fairly,congestion level estimation should be a collective effort.In other words,the propagation of congestion information should be explicit rather than each node learning it on its own.
