wifi mac protocol

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WiFi MAC ProtocolWiFi MAC Protocol

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HistoryHistory WiFi is used to “describe WiFi is used to “describe

the underlying technology the underlying technology of wireless local area of wireless local area networks (WLAN) based networks (WLAN) based on the IEEE 802.11 on the IEEE 802.11 specifications specifications

WiFi developed by Kyle WiFi developed by Kyle BrownBrown

IEEE 802.11 “denotes a IEEE 802.11 “denotes a set of Wireless set of Wireless LAN/WLAN standards LAN/WLAN standards developed by working developed by working group 11 of the IEEE group 11 of the IEEE LAN/MAN Standards LAN/MAN Standards Committee (IEEE 802)” Committee (IEEE 802)”

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First WiFiFirst WiFi

Intended for cashiersIntended for cashiers Called WaveLANCalled WaveLAN

Vic HayesVic Hayes ““Father of WiFi”Father of WiFi” Helped design IEEE 802.11b, 802.11a and 802.11g Helped design IEEE 802.11b, 802.11a and 802.11g

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MAC ProtocolMAC Protocol

MAC stands for Medium Access ControlMAC stands for Medium Access ControlUsed to provide the data link layer of the Used to provide the data link layer of the

Ethernet LAN systemEthernet LAN systemJob is to add “a 14 byte header (Protocol Job is to add “a 14 byte header (Protocol

Control Information (PCI)) before the data Control Information (PCI)) before the data and append a 4-byte Cyclic Redundancy and append a 4-byte Cyclic Redundancy Check (CRC) after the data Check (CRC) after the data

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Network-Level ArchitectureNetwork-Level Architecture

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Network-Level Architecture Cont.Network-Level Architecture Cont.

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IEEE 802.11 StandardIEEE 802.11 Standard

PHY/MAC standard for wireless LANsPHY/MAC standard for wireless LANs First standardized in 1997First standardized in 1997 Meet great success starting in 1999Meet great success starting in 1999

Several working groupsSeveral working groups IEEE 802.11a: high speed extension to the 5GHz IEEE 802.11a: high speed extension to the 5GHz

bandband 802.11b/g: high speed extension to the 2.4GHz band802.11b/g: high speed extension to the 2.4GHz band 802.11e: Quality of service (QoS) enhancement (still 802.11e: Quality of service (QoS) enhancement (still

active)active) 802.11i: Security enhancement802.11i: Security enhancement 802.11s: Mesh-networking support802.11s: Mesh-networking support

Slide taken from: http://www.cse.ucsc.edu/classes/cmpe257/Spring05/lecture/lecture3.pdf#search='802.11e%20MAC%20protocol%2C%20powerpoint

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ProtocolsProtocols

ProtocolsProtocols Release Release DateDate

Op. Op. FrequencyFrequency

Date Rate Date Rate (Typ)(Typ)

Date Rate Date Rate (Max)(Max)

Range Range (indoor)(indoor)

LegacyLegacy 19971997 2.4 – 2.5 GHz2.4 – 2.5 GHz 1 Mbit/s1 Mbit/s 2 Mbit/s2 Mbit/s ??

802.11a802.11a 199919995.15-5.35/5.47-5.15-5.35/5.47-

5.725/5.725-5.725/5.725-5.875 GHz5.875 GHz

25 Mbit/s25 Mbit/s 54 Mbit/s54 Mbit/s ~30 meters ~30 meters (~100 feet)(~100 feet)

802.11b802.11b 19991999 2.4 – 2.5 GHz2.4 – 2.5 GHz 6.5 Mbit/s6.5 Mbit/s 11 Mbit/s11 Mbit/s ~50 meters ~50 meters (~150 feet)(~150 feet)

802.11g802.11g 20032003 2.4 – 2.5 GHz2.4 – 2.5 GHz 11 Mbit/s11 Mbit/s 54 Mbit/s54 Mbit/s ~30 meters ~30 meters (~100 feet)(~100 feet)

802.11n802.11n 2006 (draft)2006 (draft) 2.4 GHz or 5GHz 2.4 GHz or 5GHz bands bands 200 Mbit/s200 Mbit/s 540 Mbit/s540 Mbit/s ~50 meters ~50 meters

(~160 feet)(~160 feet)

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ProtocolsProtocols

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IEEE 802.11 Group StandardsIEEE 802.11 Group Standards

IEEE 802.11 The original 1 Mbit/s and 2 Mbit/s , 2.4 GHz RF and IR standard (1999)

IEEE 802.11a 54 Mbit/s, 5 GHz standard (1999, shipping products in 2001)

IEEE 802.11b Enhancements to 802.11 to support 5.5 and 11 Mbit/s (1999)

IEEE 802.11c Bridge operation procedures; included in the IEEE 802.1D standard (2001)

IEEE 802.11d International (country-to-country) roaming extensions (2001)

IEEE 802.11e Enhancements: QoS, including packet bursting (2005)

IEEE 802.11f Inter-Access Point Protocol (2003) Withdrawn February 2006

IEEE 802.11g 54 Mbit/s, 2.4 GHz standard (backwards compatible with b) (2003)

IEEE 802.11h Spectrum Managed 802.11a (5 GHz) for European compatibility (2004)

IEEE 802.11i Enhanced security (2004)

IEEE 802.11j Extensions for Japan (2004)

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IEEE 802.11 Group Standards Cont.IEEE 802.11 Group Standards Cont.IEEE 802.11k Radio resource measurement enhancements

IEEE 802.11l (reserved and will not be used)

IEEE 802.11m Maintenance of the standard; odds and ends.

IEEE 802.11n Higher throughput improvements using MIMO (multiple input, multiple output antennas)

IEEE 802.11o (reserved and will not be used)

IEEE 802.11pWAVE - Wireless Access for the Vehicular Environment (such as ambulances and passenger

cars)

IEEE 802.11q (reserved and will not be used, can be confused with 802.1Q VLAN trunking)

IEEE 802.11r Fast roaming Working "Task Group r"

IEEE 802.11s ESS Mesh Networking

IEEE 802.11T Wireless Performance Prediction (WPP) - test methods and metrics Recommendation

IEEE 802.11u Interworking with non-802 networks (for example, cellular)

IEEE 802.11v Wireless network management

IEEE 802.11w Protected Management Frames

IEEE 802.11x (reserved and will not be used)

IEEE 802.11y 3650-3700 Operation in the U.S.

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802.11 Protocol Entities802.11 Protocol Entities


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MAC in DetailMAC in Detail

Channel access mechanismChannel access mechanismDistributed Coordination Function (DCF)Distributed Coordination Function (DCF)

Carrier sense multiple access (CSMA) with Carrier sense multiple access (CSMA) with immediate MAC-level ACKimmediate MAC-level ACK

RTS/CTS(4-way handshaking) exchange (optional)RTS/CTS(4-way handshaking) exchange (optional) RTS stands for Request-to-SendRTS stands for Request-to-Send CTS stands for Clear-to-SendCTS stands for Clear-to-Send

Point Coordination Function (PCF)Point Coordination Function (PCF)Polled access through AP and distributed accessPolled access through AP and distributed accessContention-free period (CFP) and contention Contention-free period (CFP) and contention

period (CP)period (CP)Seldom implemented in practiceSeldom implemented in practice


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CSMA/CA ExplainedCSMA/CA Explained

Reduce collision probability where mostly neededReduce collision probability where mostly needed Stations are waiting for medium to become freeStations are waiting for medium to become free Select Random Backoff after a Defer, resolving contention to avoid Select Random Backoff after a Defer, resolving contention to avoid

collisionscollisions Efficient Backoff algorithm stable at high loadsEfficient Backoff algorithm stable at high loads

Exponential Backoff window increases for retransmissionsExponential Backoff window increases for retransmissions Backoff timer elapses only when medium is idleBackoff timer elapses only when medium is idle

Implement different fixed priority levelsImplement different fixed priority levels To allow immediate responses and PCF coexistenceTo allow immediate responses and PCF coexistence


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CSMA/CA + ACK ProtocolCSMA/CA + ACK Protocol

Defer access based on Carrier SenseDefer access based on Carrier Sense Clear Channel Assessment (CCA) from PHY and Clear Channel Assessment (CCA) from PHY and Virtual Carrier Virtual Carrier

SenseSense state state Direct access when medium is sensed free longer then Direct access when medium is sensed free longer then

DIFS, otherwise defer and backoffDIFS, otherwise defer and backoff Reciever of directed frames to return an ACK Reciever of directed frames to return an ACK

immediately when CRC correctimmediately when CRC correct When no ACK received then retransmit frame after a random When no ACK received then retransmit frame after a random

backoff (up to maximum limit)backoff (up to maximum limit)Slide taken from: http://www.cse.ucsc.edu/classes/cmpe257/Spring05/lecture/lecture3.pdf#search='802.11e%20MAC%20protocol%2C%20powerpoint

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CSMA/CDCSMA/CD

CSMA stands for Carrier Sense Multiple CSMA stands for Carrier Sense Multiple AccessAccess

CD stands for Collision DetectionCD stands for Collision DetectionAllows for Retransmission Back-OffAllows for Retransmission Back-Off

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RTS/CTS Based AccessRTS/CTS Based Access

Duration field in RTS and CTS frames distribute Medium Duration field in RTS and CTS frames distribute Medium Reservation information which is stored in a Net Reservation information which is stored in a Net Allocation Vector (NAV)Allocation Vector (NAV)

Defer on either NAV or “CCA” indicating Medium BusyDefer on either NAV or “CCA” indicating Medium Busy Use RTS/CTS is optional but must be implementedUse RTS/CTS is optional but must be implemented Use is controlled by a RTS_Threshold parameter per Use is controlled by a RTS_Threshold parameter per

stationstation To limit overhead for short framesTo limit overhead for short frames


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Frame FormatsFrame Formats

MAC Header format differs per Type:MAC Header format differs per Type: Control Frames (several fields are omitted)Control Frames (several fields are omitted) Management FramesManagement Frames Data FramesData Frames

Includes Sequence Control Field for filtering of Includes Sequence Control Field for filtering of duplicates caused by ACK mechanismduplicates caused by ACK mechanism


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Address Field DescriptionAddress Field Description

Addr 1 = All stations filter on this address.Addr 1 = All stations filter on this address. Addr 2 = Transmitter Address (TA)Addr 2 = Transmitter Address (TA)

Identifies transmitter to address the ACK frame toIdentifies transmitter to address the ACK frame to

Addr 3 = Dependent on To and From DS bitsAddr 3 = Dependent on To and From DS bits Addr 4 = Only needed to identify the original Addr 4 = Only needed to identify the original

source of WDS (Wireless Distribution System) source of WDS (Wireless Distribution System) framesframes


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ApplicationsApplications

Wireless Access Point (WAP)Wireless Access Point (WAP)Wireless RoutersWireless RoutersWireless Ethernet BridgeWireless Ethernet BridgeRange ExtenderRange Extender

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Gaming ApplicationsGaming Applications

Nintendo DSNintendo DSSony PSPSony PSPXbox 360Xbox 360Playstation 3Playstation 3Nintendo WiiNintendo Wii

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Problems / Performance IssuesProblems / Performance Issues

Interruption by other devicesInterruption by other devicesHigh power consumptionHigh power consumptionWhen setup the network defaults to open When setup the network defaults to open

modemodeLarge area coverageLarge area coverage

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Security Issues Security Issues

WEP encryption is breakableWEP encryption is breakableSpoofing authorized MAC addressesSpoofing authorized MAC addressesWardriving and WarchalkingWardriving and Warchalking

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Question Summary / ConclusionQuestion Summary / Conclusion What is WiFi MAC Protocol?What is WiFi MAC Protocol?

It is the underlying technology of wireless local area It is the underlying technology of wireless local area networks (WLAN) based on the IEEE 802.11 specifications networks (WLAN) based on the IEEE 802.11 specifications that provides the data link layer of the Ethernet LAN that provides the data link layer of the Ethernet LAN system.system.

Who came up with WiFi MAC Protocol?Who came up with WiFi MAC Protocol? The IEEE Task Groups and created the IEEE 802.11 or The IEEE Task Groups and created the IEEE 802.11 or

IEEE 802.11x Committee, however IEEE only sets IEEE 802.11x Committee, however IEEE only sets specifications. WiFi Alliance runs the certification specifications. WiFi Alliance runs the certification program.program.

What is the difference between DCF and PCF?What is the difference between DCF and PCF? DCF uses carrier sense multiple access with immediate DCF uses carrier sense multiple access with immediate

MAC-level ACK and contains RTS/CTS optional exchange. MAC-level ACK and contains RTS/CTS optional exchange. PCF uses polled access through AP and distributed PCF uses polled access through AP and distributed access; cotains a contention-free period and a contention access; cotains a contention-free period and a contention period; and is seldom implemented in practice.period; and is seldom implemented in practice.

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