Analysis of 802.11b MAC A QoS, Fairness, and Performance Perspective

a r X i v :c s /0411017v 1 [c s .N I ] 8 N o v 2004Analysis of 802.11

b MAC:A QoS,Fairness,and Performance

Perspective

Srikant Sharma Department of Computer Science Stony Brook University Stony Brook,NY 11794-4400srikant@f0d6b5b369dc5022aaea002a Abstract Wireless LANs have achieved a tremendous amount of growth in recent years.Among various wire-less LAN technologies,the IEEE 802.11b based wireless LAN technology can be cited as the most prominent technology today.Despite being widely deployed,802.11b cannot be termed as a well ma-tured technology.Although 802.11b is adequate for basic connectivity and packet switching,It is evident that there is ample scope for its improvement in areas like quality of service,fairness,performance,se-curity,etc.In this survey report,we identify and argue that the Medium Access Controller for 802.11b networks is the prime area for these improvements.To enunciate our claims we highlight some of the quality of service,fairness,and performance issues related to 802.11b MAC.We also describe and an-alyze some of the current research aimed at addressing these issues.We then propose a novel scheme called the Intelligent Collision Avoidance ,seeking to enhance the MAC to address some of the perfor-mance issues in 802.11b and similar networks.1Introduction Although the concept of wireless LANs has existed since late 1970s,the WLAN technology started gaining its momentum only in late 1990s and today it has become a ubiquitous networking technology.The reason behind the recent explosive growth of this technology can be attributed to multiple factors,such as,techno-logical advances in error correcting codes,modulation techniques,processing power on network interfaces,

availability of unlicensed radio spectrum,and most importantly,the need for some kind of tetherless con-nectivity and mobility.

Today there exist multiple wireless LAN technologies,such as,Wi-Fi,Bluetooth,HiperLAN,HomeRF,etc.All of these technologies operate in the 2.4GHz ISM (Industrial,Scienti?c,and Medical)radio spec-trum.Each technology has its own niche depending on the deployment requirements of the wireless LANs.Bluetooth is mainly used as a cable replacement RF technology for short range communications.It is used to interconnect portable devices,such as,cellular phones,laptops,palmtops,etc.,without the need to carry interconnecting cables.It is capable of providing data rates upto 700Kbps and supports upto three voice channels at 64Kbps.HomeRF is used for wireless home networking for devices like,intelligent home appliances,laptops,smart pads etc.It has a range of upto 50meters which is adequate for short scale net-works.It supports data rates upto 1.6Mbps which is low by todays WLAN norms.HiperLAN is a family of four different wireless technologies classi?ed as types 1-4.HiperLAN-1operates in 5GHz radio spectrum

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is capable of supporting upto23Mbps at a range of50meters.One of the prime features of HiperLAN is its support for Wireless ATM.WATM is the extension of ATM capabilities,such as QoS features,etc.,to wireless networks.Wi-Fi technology is based on IEEE802.11b[1]standard.It operates in the unlicensed 2.4GHz radio spectrum,uses direct-sequence spread-spectrum(DSSS)for modulation,supports variable data rates upto11Mbps,and has a range of about50meters.

Out of all these technologies,IEEE802.11b or Wi-Fi is the technology which has received the widest market acceptance.The popularity of this standard is aptly re?ected in portable computer vendors’decision to integrate802.11b wireless network adapters with notebook computers.A market forecast by the Gartner [2]group predicts that by the end of year2005almost95%of notebook computers will be equipped with 802.11b cards.Further,by the end of year2002the802.11b penetration in corporate LANs is expected to reach up to50%,from the current level of around20%.Almost all PDA vendors are starting to support the 802.11b technology in the newer-generation PDAs on the market.The widespread availability of802.11b on wireless devices coupled with continuous cost reduction is also a strong indication of exponential growth of the802.11b technology.

IEEE802.11b LANs can be deployed in either ad hoc con?guration or infrastructure con?guration. The ad hoc con?guration refers to the peer-to-peer setup where a bunch of devices with802.11b network interface cards(NICs)can establish a network and communicate with each other without any infrastructural support.The connectivity of the nodes in this network is limited to their peers.On the other hand,the infrastructure or the access-point setup uses a central access-point(base-station)to form a network.The access-point is usually connected to a wired network as a bridge for next hop connectivity.Every packet transmitted by a wireless node is destined for the access-point which takes care of further routing/switching.

Most of the corporate and large scale wireless networks are setup in the infrastructure mode of oper-ation.There are two different classes of infrastructure operation.These are basic service set(BSS)and extended services set(ESS).In BSS con?guration each wireless node is associated with an access-point and this association remains unchanged inde?nitely,whereas,in ESS a mobile node can roam around and dis-associate from current access-point and associate with a new access-point or re-associate with the previous access-points.The ESS is basically meant to provide roaming support.

IEEE802.11b technology has achieved a huge level of penetration in the wireless networking arena.It is being regarded as the de facto wireless standard for wireless LANs.Although the dependence on802.11b is growing,it cannot be termed as a very well matured wireless LAN technology.The technology,though adequate for basic connectivity and packet switching,falls short of expectations when it comes to issues like,quality of service,fairness,performance,security,etc.The wireless research community is persistently ?nding different ways to improve this technology and bridge the shortcomings.The channel access protocol (MAC)used by802.11b networks is known to have several performance related issues.The quality of service for applications using these networks is practically non existent.The protocol is known to exhibit unfairness for different streams in terms of channel allocation.The security aspect of these networks is known to have several problems.

This survey attempts to highlight the QoS,fairness,and performance issues in802.11b networks and various research related to MAC enhancements to address these issues.The report is organized as follows. In Section2,we give an overview of802.11b channel access protocol.In Section3,we brie?y look at the QoS efforts for802.11b networks.We examine the fairness problem of802.11b MAC in Section4. In Section5,we look at current performance related research for802.11b MAC.In Section6we propose and analyze enhancements to802.11b MAC protocol to improve throughput performance at the expense of power consumption.Finally,in Section7we present our conclusions and discuss the work that we are planning to pursue in future.

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2IEEE802.11b MAC overview

IEEE802.11b is a standard for Medium Access Control(MAC)and Physical Layer(PHY)speci?cations for wireless LANs.The PHY speci?cations deal with modulations techniques,error correcting codes,radio characteristics,physical layer convergence,and other signaling related issues.

IEEE802.11b MAC protocol is based on the CSMA/CA[3]protocol which uses physical carrier sense as well as virtual carrier sense to avoid collisions and packet loss.Physical carrier sense is used to avoid collisions at the sender,whereas,virtual carrier sense is used to avoid collisions at the receiver and address the hidden node problem present in wireless networks.The virtual carrier sense uses regular Request To Send(RTS)and Clear To Send(CTS)channel reservation mechanism.802.11b MAC improves the link layer reliability by including explicit ACKs for each data frame.Upon failure to receive an ACK,the data frame is repeatedly retransmitted till an ACK is received.The maximum number of retransmissions is a con?gurable parameter for each individual node and is usually set to seven.Thus each successful transmission follows the so-called4-way handshake protocol of RTS-CTS-DATA-ACK.A node may choose to disable the virtual carrier sense to reduce its overhead when the probability of existence of hidden nodes is known to be small.

802.11b MAC includes two coordination functions for channel access,namely,Distributed Coordination Function(DCF)and Point Coordination Function(PCF).The DCF speci?es channel contention mechanism for normal mode of operation,whereas,PCF speci?es a mechanism for channel access in a contention free fashion.PCF requires the presence of a point coordinator(PC)and can be used only in infrastructure mode of operation.We will be describing the details of DCF in the following part of this section.We will be looking at the details of PCF in Section3.

2.1Distributed Coordination Function(DCF)

In the normal mode of operation,the IEEE802.11b MAC uses a Distributed Coordination Function(DCF) for media access.DCF is an implementation of CSMA/CA protocol which follows the4-way handshaking protocol for data transmissions.In DCF,whenever a node is ready to transmit data,it senses the channel to be idle for a period of Distributed Inter Frame Spacing(DIFS).Following this,it generates a random backoff timer chosen uniformly from the a[0,ω-1]whereωis the contention window.Initially the con-tention window is set to CW min(16for802.11b).After the backoff timer expires,the node sends a short Request To Send(RTS)message to the intended receiver of data.If this message is received properly by the receiver and if it is able to receive any transmission,it responds back with a short Clear To Send(CTS) message.A node may not be able to receive any transmission if some other node in its vicinity has already reserved the channel for packet reception or transmission.Both RTS and CTS messages carry the duration information for which the channel is going to be occupied by the proposed data transmission.Upon hearing RTS and CTS,all other nodes in the vicinity of the sender and receiver update their Network Allocation Vectors(NA Vs)with the information about the duration for which the channel is going to be busy.NA V is essentially a channel reservation vector.Thus,all nodes in the vicinity of the sender and receiver defer their transmissions and receptions to avoid collisions.The CTS message is followed by the DATA transmission which is acknowledged by the receiver by sending an ACK message if the DATA is received successfully. The data is repeatedly retransmitted in the absence of ACKs till a threshold number of retransmissions are carried out.Once the retransmissions exceed the threshold,the transmission is assumed to be unsuccess-ful.After an unsuccessful transmission attempt,the sender follows a binary exponential backoff(BEB) and doubles its contention window size.This is done in order to reduce the channel contention between nodes.The contention window is not incremented further if it already equals CW max(256for802.11b).

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