本末倒置3
SIFS SIFS
SRC DST T0T1T2T3T4T5
SRC?NBR DST?NBR
DIFS Figure 1:Message exchanges for DCF.Each DATA packet is preceded by RTS and CTS messages.On hearing RTS,the nodes in the vicinity of sender set their NA Vs to the duration mentioned in RTS.On hearing CTS,the nodes in the vicinity of receiver set their NA Vs to the duration mentioned in CTS.This causes in establishment of channel reservation till the time the ACK is sent back to the sender.
After every successful transmission,the contention window is reset back to CW min .RTS,CTS,DATA,and ACK are separated by a time spacing of Short Inter Frame Space (SIFS).A timeline for DCF message ex-changes is shown in Figure 1.The sense period of DIFS is always larger than SIFS.This ensures that no new transmission attempts interfere with the ongoing transmission.
DCF provides a mechanism for collision avoidance by performing a virtual carrier sense through RTS-CTS message exchanges.This is necessary to solve the hidden node problem .However,a node may chose to resort to a 2-way handshaking mechanism where data packets are transmitted without RTS-CTS message exchanges.The packet is acknowledged back by the receiver by responding with an ACK message.This mechanism can be used for small packets where the overhead of RTS/CTS message exchange can be traded off for small probability of collisions.
2.2Link Management
Apart from channel access mechanism,802.11b also provides additional facilities such as power save fea-tures,security etc.Moreover,802.11b MAC is a distributed entity where decisions regarding various activ-ities are taken independently at various nodes.To distribute MAC level information among the constituent nodes,802.11b de?nes several messages which are transmitted as link level frames.The frames that are exchanged between different nodes of an 802.11b network are classi?ed into three categories as follows:
?Management
–{association,re-association,probe }{request,response },and disassociation:These frames are used for af?liation activity of any node to a particular cell and access-point.
–authentication and de-authentication:These frames are used for security purposes.
–beacon:This frame is usually sent by an access-point in infrastructure mode.In ad hoc mode,the ?rst node to initiate an ad hoc network sends the beacon.This frame carries important
management information like time-stamp,supported rates,traf?c indication maps etc.
–ATIM:Announcement Traf?c Indication Message is a frame sent after every beacon frame.This is used by nodes utilizing the power save features of 802.11b to indicate their traf?c pattern.
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Mobile hosts
Figure 2:IEEE 802.11b LANs are usually at the periphery of wired networks serving as the access networks.The servers are inside the wired domain and clients are in wireless domain exploiting the mobility advantage.
?Control
The frames like RTS,CTS,ACK,PS-Poll (power save poll)CF-Poll,CF-ACK,CF-END (Contention free channel information in PCF)are categorized as control frames.
?Data
The data frames can be piggybacked with frames like CF-ACK,CF-Poll etc.
IEEE 802.11b also speci?es a fragmentation mechanism using which an 802.11b node can divide data packets into smaller frames and transmit large packets when the frame error rates are high.The fragmen-tation threshold can be a con?gurable value or can be dynamically determined depending on the channel conditions.
To summarize,the 802.11b MAC layer provides,(1)Channel access mechanism,(2)Link management facility,(3)Rudimentary security,(4)Power management,and (5)Fragmentation.
3Quality of Service
IEEE 802.11b WLANs provide best effort service similar to their wired counterpart Ethernet networks.Best effort service essentially indicates that every data packet handed over to the 802.11b interfaces receives similar treatment as other packets in terms of delivery guarantees.Thus,from an application perspective,it receives no Quality of Service guarantees from the network in terms of available bandwidth,latency,jitters etc.Some applications like media streaming and conferencing are sensitive to packet latency and effective bandwidth characteristics of the underlying network.With the growing thrust on use of wireless networks for media streaming and conferencing,it becomes essential to provide a service that is not merely best effort service but a service that is deterministic at certain level.IEEE 802.11b WLANs are usually deployed as access networks to the wired infrastructures for mobile terminals as shown in Figure 2.In wireless LAN scenario,usually the media and conferencing servers reside in the faster and reliable wired domain,whereas,the clients reside in wireless domain exploiting the mobility features.Thus the importance of QoS is relatively higher in the infrastructure setup rather than the peer-to-peer setup.
On the wired network,IETF’s Integrated Services [4]and Differentiated Services [5,6]architectures are available to support guaranteed QoS and traf?c prioritization respectively above the link layer.IEEE’s 802.1p is a layer 2traf?c prioritization standard for switched Ethernet environments.However,there are very limited QoS solutions that exist for wireless LANs,particularly 802.11b networks.
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PC
Figure3:A superframe in802.11b.Each superframe is divided into contention free and contention periods.PC polls nodes during CFP.If there is no response from a node,the PC polls next node after PIFS.The nodes respond to polls with either DATA+ACK frames or just ACK frames after SIFS interval.No nodes other than polled nodes can access the channel since it is never idle for DIFS period during CFP.The CFP ends when the PC sends CF-END frame.After this all nodes enter CP where DCF is used to access the channel.
3.1Point Coordination Function
IEEE802.11b standard provides a very rudimentary support for quality of service in its infrastructure mode of operation.This support is provided by the MAC layer in terms of Point Coordinated Function(PCF). PCF is a MAC coordination facility that may exist on access-points to differentiate between the traf?c?ows from different nodes.PCF is an optional capability for access points and its implementation is not manda-tory.Very few commercially available access-points for802.11b networks actually provide this facility. Moreover,there are no clear mechanisms for individual nodes to participate in PCF and exploit the quality of service mechanism provided by it.
PCF provides a very rudimentary form of QoS by allowing nodes to transmit frames in a contention free manner.The access-point of a cell acts as a coordinator called the point coordinator(PC)for that cell.All nodes in802.11b network obey the medium access rules of the PCF,since these are based on DCF which is followed by all nodes.The PC grants a contention free channel access to individual nodes by polling them for transmissions.On being polled,a node transmits a single frame destined for any node in the network.All nodes which need some quality of service are termed as CF-Pollable nodes.A node becomes CF-Pollable by indicating its interest in being polled during its association with an AP(PC).This results in the node being included in a CF-Pollable list maintained by the PC.A node can withdraw itself from polling process by performing a re-association.
In infrastructure mode of802.11b,the time is divided into periodic superframes which start with the so-called beacon frames.A beacon frame in802.11b is a management frame sent by an access point to carry out time synchronization and deliver protocol related information to all nodes.Beacon frames are periodically sent by the access points regardless of PCF functionality.Each superframe is divided into two units,namely,Contention Free Period(CFP)and Contention Period(CP).CFP is the period when contention free channel access is provided by the PC to individual nodes.CP is the period when all nodes contend for the channel using DCF.If the PCF functionality is not provided by the access point then entire superframe is the contention period.The extent of division of a superframe into CFP and CP is determined by the PC which can be arbitrary,but it is mandatory to have a CP of a minimum duration that allows at least one node to transmit one frame under DCF.
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Figure3shows the activity of a wireless network during a superframe.At the beginning of superframe the PC waits for a period PCF Inter Frame Space(PIFS)and then transmits the beacon frame.If the PC supports PCF and the list of nodes that are interested in being polled is not empty,the PC sends a CF-Poll (or DATA+CF-Poll)frame to one of the nodes after waiting for channel to be idle for SIFS.In response,the node can respond with a DATA+CF-ACK or just CF-ACK if no data is ready to be sent.The response is sent after sensing the channel to be idle for an SIFS period.If there is no response to CF-Poll frame,the PC sends CP-Poll to next node after waiting for an idle period of PIFS.At the end of CFP,the PC sends a CF-END frame to begin the contention period using DCF.Thus in CFP,each polled node transmits frames in a contention free manner.In CFP,RTS/CTS handshaking is not carried out.During the entire CFP the PC is in control because it accesses channel after sensing the channel to be idle for PIFS duration.PIFS is much smaller than DIFS which is the period for which every nodes in DCF should sense the channel to be idle.The shorter duration of PIFS compared to DIFS ensures that no node can contend for the channel except either the PC or the node that has been recently polled.
There are several problems with PCF that make it less attractive for QoS utilization.First and foremost, there is no guarantee of bandwidth or any other QoS parameters except a contention free transmission of a single frame.The channel share available to individual nodes cannot be speci?ed and it decreases with an increase in the number of pollable nodes in the network.Second,there is no de?nite bound on next poll being received by a node.The polls received from PC for a node may not be periodic.For?ows with tight delay bounds and periodic traf?c,this may be unacceptable.Third,there is no guarantee that the superframe itself is periodic.This is because the PC must sense the channel to be idle for PIFS duration before sending the next beacon.If the transmission of last frame from the previous superframe is prolonged then the beacon transmission may not be strictly periodic.Fourth,the size,rate and transmission time occupied by each frame is not constant.Some frames may be fragmented and may be arbitrarily long(upto2312bytes)and hence may take longer than usual transmission time.This will affect subsequent nodes.Some of the nodes may not be polled in a particular superframe.In short,there is no clear admission control and usage policy. Last,there is no clear way of interfacing an application with this mechanism.This makes PCF a facility with no use.
