本末倒置3.2Rether for IEEE802.11b
If one were to revamp PCF and come up with a scheme that is tailored to provides QoS to applications, following enhancement would be required.
Interface:There must be some interface through which an application can specify its bandwidth require-ments to the underlying QoS mechanism.
Admission Control:There must be some admission control criteria so that the available meager resources, once granted to certain applications,are not hijacked back.
Periodicity:The channel access must be provided in a fairly periodic fashion for delay sensitive and peri-odic traf?c.
Isolation:Once an application(or its?ow)is admitted,the service provided to it must not be affected by quirkiness of other applications and nodes.
Bandwidth Guarantee:The channel access granted should re?ect into an appropriate bandwidth guarantee over a long term duration.
Rether[7]is a QoS mechanism for802.11b networks which provides bandwidth guarantees to individual
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Network Device Driver IP
UDP TCP
BSD Sockets
Application
Wireless Rether
Figure 4:Wireless Rether protocol layer.Wireless
Rether is implemented as a layer between the link-layer
hardware’s device driver and the IP protocol layer.With
this layering structure,Wireless Rether is able to exer-
cise QoS-related control over all outgoing network con-
f0d6b5b369dc5022aaea002awork Wired Edge Router Wireless Rether Server Access Point
Rether Clients Figure 5:Wireless Rether Architecture.Each edge network is augmented with a Wireless Rether Server (WRS),which acts as a bridge between the Layer-2ac-cess point and the Layer-3edge router.All wireless LAN hosts are equipped with Wireless Rether client software.
?ows.Rether borrows many salient features of PCF and is a software solution residing in the network stack of individual nodes.Rether addresses all of the above issues.In addition,it does not require any changes at the MAC layer and does not use the PCF capability which may not be available with every access point.
Like CFP and CP in PCF,Rether also has a notion of Real-Time (RT)and Non Real-Time (NRT)periods.Analogous to the superframe,Rether has a concept of a periodic cycle which corresponds to one set of RT and NRT period.Rether strives to guarantee a contention free channel access to all nodes during both RT and NRT period.During RT period,the channel access is granted to nodes with bandwidth reservations,and the NRT period is used to grant channel access to all nodes in the wireless network.Similar to the mandatory requirement of CP in PCF,Rether also has a notion of RT limit of bandwidth reservation beyond which no new ?ows are granted bandwidth guarantees.This is done to avoid the starvation of ?ows with no bandwidth reservations.Rether grants channel access to individual nodes by sending so-called tokens .The individual nodes signify their end of transmission by sending and explicit acknowledgment .Rether grants contention free channel access even in NRT period by circulating the token in a round robin fashion among the nodes without any bandwidth reservation.In order to avoid fairness issues,the round robin nature is persistent across consecutive NRT periods.
As depicted in Figure 4,Rether is implemented as a software module which resides between the IP layer and the device driver for the wireless interface.It intercepts all the packets that are handed over to the device driver and exercises control over their transmission which follows established QoS policies.Rether follows a client-server architecture.In an infrastructure wireless LAN,the Wireless Rether Server (WRS)is co-located with the access point.All nodes in the network understand the Rether protocol and are termed as Wireless Rether Clients (WRC).The WRS is primarily responsible for admission control and channel coordination.
Rether uses implicit bandwidth reservation mechanism based on port signaling.The advantage of this mechanism comes from compatibility with legacy applications which cannot be modi?ed to carry out explicit reservation requests.The reservation mapping is speci?ed in system policy ?les in terms of quintuples like :
{SrcAddr/Mask,DstAddr/Mask,SrcPrtRange,DstPrtRange,BW }
Rether module intercepts all outgoing packets on a node.Upon intercepting the ?rst packet correspond-ing to any ?ow,the WRC sends a reservation request to the WRS if a corresponding policy speci?cation exists.If the request is accepted all packets corresponding to the ?ow are maintained in a separate queue.The WRS circulates the token among all nodes who have established reservations.All WRCs dispatch pack-
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ets from their queues according to the bandwidth requirements upon reception of tokens.The dispatching is limited to bandwidth share or allocated slice of the cycle which ever occurs?rst.This limitation helps in providing isolation between nodes with bad radio characteristics and normal nodes.Once all nodes with reservations are served,WRS switches to NRT mode and the clients without any reservation are provided with tokens in a round robin fashion.
With this architecture,Rether provides an effective bandwidth guarantee scheme which can be readily used by applications.The guarantees are provided by granting a contention free channel access to all nodes transmitting packets.Further,Rether is an all-software protocol which does not need any modi?cation to the underlying MAC layer.
3.3IEEE802.11e
The market thrust on wireless LAN technologies and the requirement of QoS support in these networks has led to the establishment of an IEEE standardization working group to enhance802.11to provide applications with QoS support.Upcoming standard802.11e tries to address the QoS problems faced by wireless LANs based on802.11speci?cations.
The QoS support in802.11e is provided in two forms.First,it supports a priority based best-effort service similar to diffserv.Second,it supports parameterized QoS for the bene?t of applications requiring QoS for different?ows.802.11e achieves this by enhancing the802.11DCF and PCF functionality,and by providing a signaling mechanism for parameterized QoS.
The enhanced MAC protocols in802.11e are EDCF and EPCF.Both EDCF and EPCF are commonly refered as the Hybrid Coordinated Functions(HCF).The priority based best effort service is provided by the EDCF.This is done by introducing so-called traf?c categories.Frames corresponding to different traf?c categories are now transmitted through different backoff instances.Each traf?c category has an associated independent backoff instance.The scheduling of frames for every traf?c category is done the same way as in DCF.The differentiation in the priority is achieved by setting different probabilities for different categories for winning the channel contention.The probability is changed by varying the so-called Arbitration Inter Frame Space(AIFS)which is the listen interval for channel contention.AIFS is analogous to the DIFS period in DCF.For each traf?c category,the value of AIFS determines the priority.With lower AIFS values,the listen interval required for channel contention is lower and hence the probability of winning the channel contention is higher.For compatibility with legacy DCF,AIFS should be at least equal to DIFS. The backoff procedure in case of collisions is similar to that in DCF.The main distinguishing factor is the method by which the contention window is expanded after collisions.In DCF,the contention window is always doubled.Whereas in EDCF,the contention window is expanded by a predetermined persistence factor(PF).For example,in DCF scenario the PF is always2since the contention window is always doubled.
A single node can have upto eight traf?c categories.These different categories are realized as eight different virtual nodes with varying parameters,such as,AIFS,CW,and PF.These parameters are responsi-ble for determining the priority of each traf?c category.If the backoff counters of multiple traf?c categories reach zero at the same time,there is a virtual collision within the same physical node but different traf?c cat-egories.This is resolved by a scheduler inside the node by allowing transmissions from the traf?c category with the higher priority.
Another enhancement in802.11e is the concept of a transmission opportunity(TxOP).TxOP is de?ned as the interval during which a node has the right to initiate transmissions.Thus,a node can initiate multiple transmissions as long as its TxOP has not expired.The value of EDCF-TxOP is unique throughout the network.
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Destnation
Element ID
(6 bytes)(6 bytes)(2 bytes)(1 byte)(1 byte)(1 byte)
(1 byte)(2 bytes)
Data
Rate
(2 bytes)(2 bytes)(2 bytes)(1 byte)(1 byte)(1 byte)(1 byte)
Polling
Interval
Nominal
Size Minimum Mean
Data
Rate
Burst
Maximum
Size
Tx Rate Reserved Delay Jitter
Bound
Bound
Length Source
Address Address TAID TS Info
Interval Interval
Inactivity
MSDU
Retry 30
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Figure6:TSPEC element in802.11e.This might be replaced with a new Queue State element.The?elds are mostly RSVP speci?c.
IEEE802.11e also facilitates parameterized QoS.This is provided by the enhanced version of PCF (EPCF).The HCF has a notion of Hybrid Coordinator(HC)similar to the PC in PCF.The HC can allocate TxOP to itself or any other node at any time but after sensing the channel to be idle for a duration of PIFS, which is shorter than DIFS.This ensures that the HC has the highest priority over all other nodes at any given time.The HC allocates TxOP to pollable nodes in contention free periods and sometimes even during contention periods.The main distinction between PCF and HCF is that in PCF a node can transmit only one frame after receiving CF-Poll frame.In HCF,a node can initiate the transmission of frames till the end of TxOP.The duration of TxOP is noti?ed to the node using the Poll frame sent by the HC.To provide TxOPs with appropriate duration and appropriate time,the HC needs to obtain the pertinent information from the individual nodes from time to time.For this purpose,the HC initiates the so-called controlled contention periods during which nodes send their resource requests to the HC without contending with other nodes which are sending only data traf?c.There can be eight more traf?c categories for parameterized traf?c in addition to the eight EDCF categories.The frames queued in these categories are transmitted after receiving poll frames from the HC.Thus,at most eight?ows on any node can be provided parameterized QoS.The ?ows are identi?ed by source and destination MAC addresses.
