an overview of 802.11 mac entity-月會報告 - gicl...

An Overview of 802.11 MAC Entity

Hsung-Pin ChangDepart. Of Computer & Information Science

National Chiao-Tung University

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Standardsn IEEE 802.11

n Finalized in June of 1997 and revised in 1999n (1997 version) one MAC (DFW CSMA/CA) supports

multiple PHYs (DSSS, FHSS, Direct-modulated Infrared, Carrier-modulated Infrared and Multi-subcarrier-modulated Infrared)n 1&2 Mbps using either DSSS or FHSSn 1&2 Mbps using Direct-modulated Infraredn 4 Mbps using Carrier-modulated Infraredn 10 Mbps using Multi-subcarrier-modulated Infrared

n (In-building) Operation Range = 50~150 mn Quality of Service

n (optional) Point Coordination Function



Standards (Cont.)n IEEE 802.11

n IEEE Std 802.11an Orthogonal frequency domain multiplexing

(OFDM) radio in the UNII bands, delivering up to 54 Mbps data rates

n IEEE Std 802.11bn Extension to the DSSS PHY in the 2.4 GHz band,

delivering up to 11 Mbps data rates



802.11 Architecturen Basic Service Set (BSS): A group of

stations that can directly communicate with each othern Independent BSS (IBSS)n Infrastructure BSS

n Extended Service Set (ESS)



802.11 Architecture (Cont.)

Fixed-wirereplacement

Servercomputer

Portable-to-fixednetwork

Infrastructure

Portable-to-portablenetworkAd hoc



Independent BSS

n One and only one Basic Service Setn Ad hoc networkn Direct Communicationn Limited coverage area

Portable-to-portablenetworkAd hoc



Infrastructure BSS

n Infrastructuren Access Point

n Analogous the base station in a cellular communication network

n Provide specific services and range extension

Fixed-wirereplacement

Servercomputer

Portable-to-fixednetwork

Infrastructure



Extended Service Set

n Distribution Systemn Interconnect multiple BSSs to form an ESS, Extended Service

Setn Not part of 802.11 standard

n Could be IEEE 802.3 LANs, wireless, other networksn Only distributed System Services are defined

Distribution System

AP

AP



IEEE 802.11 Servicesn Station Services

n Authentication/Deauthenticationn Prove identityn Similar to the physically connecting to the

network cablen Privacy

n Provide an equivalent level of protection as that provided by a wired network

n WEP: wired equipment privacyn Data delivery



IEEE 802.11 Services (Cont.)n Distribution Services

n Association/Reassociation/Disassocationn Association service is usually invoked once, when the

mobile station enters the WLAN for the first time,n While reassociation service shall include the information

about the AP with which a mobile station has been previously associated.

n Distributionn An AP determines how to deliver the frames it receives

uses the distribution servicesn Integration

n Connect the IEEE 802.11 WLAN to other LANs



802.11 Specificationsn One MAC specification

n CSMA/CA

n Three physical specificationn Radio

n Frequency hopping spread spectrum (FHSS)n Direct sequence spread spectrum (DSSS)

n IR



States for BSS

n Authentication should be used in an Infrastructure BSS, but is option in an IBSS

n Association is never used in an IBSS



IEEE 802 System View



802.11 MAC Entity



802.11 Protocol Entities

MAC Sublayer

PLCP Sublayer

PMD Sublayer

MAC LayerManagement

PHY LayerManagement

LLC

MAC

PHY

StationManagement



MAC Functional Descriptionn 802.11 MAC

n Regulate the usage of the median Provide the same interface with other

802.x families to the upper LLC layer



MAC Functional Servicesn Data Service: carrier sense multiple

access with collision avoidance (CSMA/CA)n Distribution coordination function (DCF)

n Asynchronous data delivery services

n Point coordination function (PCF): optionaln Time bounded data delivery services

n Fragmentation/Defragmentation



DCF: CSMA/CAn CSMA: carrier sense multiple access

n Physical carrier sense: physical layern Virtual carrier sense: MAC layer

n Network Allocation Vector (NAV)

n CA: collision avoidancen Random backoff procedure



CSMA/CA: Without RTS/CTS

n NAVn Indicate the amount of time that must elapse until

the current transmission is completen Update the duration field of packets

n (Send, ACK) is an atomic operation

Ack

Data

NAV

Src

Dest

Other

Defer Access

CW

Backoff after Defer

DIFS

SIFS

DIFS



CSMA/CA: Without RTS/CTS (Cont.)n NAV setting

n RTS/CTS framen All frames sent during the CP other than

PS-Poll Control frames



"Hidden Node" Problem

• Transmitters contending for the medium may not "Hear "Hear each other" each other" as shown below.

• Separate Control frame exchange (RTS / CTS) between transmitter and receiver will Reserve the MediumReserve the Medium for subsequent data access.

AP

STA

STA

CTS-RangeRTS-Range

STA APRTS

CTS

Data

Ack

Stations do not hear each otherBut they hear the AP.



CSMA/CA with RTS/CTS

•• DurationDuration (=NAV)(=NAV) information is distributed by both transmitter and receiver through separate RTS and CTS Control Frames.

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

• NAV information is maintained in each station.

RTS

CTS Ack

Data

NAV Next MPDU

Src

Dest

Other

CW

Defer Access Backoff after Defer

NAV

(RTS)

(CTS)

DIFS



CSMA/CA with RTS/CTSn RTS/CST

n Resolve hidden node n Fast collision inference

n Cannot used for broadcast/multicast immediate address



"Hidden Node" Provisionsn NAV(Network Allocation Vector)

n All STAs receiving a valid frame shall update their NAV in the Duration Field.

n All of the RTS/CTS/DATA frames contains NAV field.

n Use of RTS/CTS is optional but must be implemented

n Use of RTS/CTS is controlled by a RTS_Threshold parameter per stationn To limit overhead for short frames



RTS/CTS Overhead Impactn RTS/CTS can be automatically disabled

by thresholding on frame length through dot11RTSThreshold.n IEEE 802.11 Std 1999 Edition Annex D

n The default value is 2347.

n IEEE 802.11 Handbookn A typical value for the threshold is 128.



Backoff Proceduren Random backoff procedure

n Decrement its backoff time by aSlotTime when free .n Stop the backoff procedure when busy .

Backoff Time = INT ( CW * Random( )) * aSlotTime

Node A

Node B

Node C(typical)

A completes the transmission ofits frame

B resenses the medium and, since it is quiet, starts to transmit its frame

C resense the medium but, since it now occupied, defers to B

B and C sense medium isnow free and both generatea random time interval

C resenses the medium and, since it is quiet, starts to transmitits frame

Node C(IEEE 802.11)

Virtual Carrier SenseVirtual Carrier Sense

SIFS/PIFS/DIFS



Backoff Procedure (Cont.)n When the medium becomes idle, resume the

backoff timer only after DIFS idle time.n Suspend the backoff timers if a transfer is

going on.



Backoff Procedure (Cont.)



Optional Point Coordination Function (PCF)

• Contention Free Service uses Point Coordination Function (PCF) on a DCF Foundation.– PCF can provide lower transfer delay variations to

support Time Bounded ServicesTime Bounded Services. – Point Coordinator resides in AP.

(CSMA/CA )

ContentionService

Service

PHY

MACPCF

Optional

DCF

Contention FreeAsync

Time Bounded / Async



Optional Point Coordination Function (PCF) (Cont.)n Access method

n Virtual carrier-sense mechanism with access priority mechanism

n Virtual carrier-sensen PCF should distribute information within Beacon

frames to gain control of the medium by setting the network allocation vector in STAs

n Access priorityn Contention-free access



Contention Free operation

• Alternating Contention FreeContention Free and ContentionContention operation under PCF control.• CFP repetition interval• Contention Free Period

Contention Free Period Contention Period

CFP repetition interval

Variable Length

PCF Defers for Busy Medium

CF-BurstBusy mediumPCF DCF

(Optional)

Defer

NAV

Async traffic DeferPCF

"Reset NAV"

CFP repetition interval



Polling

n In a CPF, the PC sends poll frames to stations asking whether they have frames to send.

n If a polled station has a frame to send to the AP, it can send it to the AP now.



Priority Access

n SIFS/PIFS/DIFS to give priority to more urgent frames, such as the ACK frame following a DATA frame or the CTS frame followed an RTS frame.n SIFS = Short inter-frame spacen PIFS = Point-coordination function interframe spacen DIFS = Distributed coordination function interframe space

DIFS Contention Window

Slot time

Defer Access

Backoff-Window Next Frame

Select Slot and Decrement Backoff as long as medium is idle.

SIFS

PIFSDIFS

Free access when mediumis free longer than DIFS

Busy Medium



IEEE 802.11 Distributed Coordination Functionn To summarize,

n CSMA/CA (RTS/CTS) to prevent “ hidden node”

n ACK to DATA frame to provide reliable transmission

n Virtual Carrier Sense to avoid collisionsn duration information provided by RTS frame

and CTS frame



IEEE 802.11 Distributed Coordination Function (Cont.)n To summarize (Cont.)

n SIFS/PIFS/DIFS to give priority to more urgent frames, such as the ACK frame following a DATA frame or the CTS frame followed an RTS frame.n SIFS = Short inter-frame spacen PIFS = Point-coordination function interframe spacen DIFS = Distributed coordination function interframe

space

n Exponential Back-off Windows to prevent heavy load congestion.



Fragmentation

n Fragment Bursts which are individually acknowledgedn Random backoff and retransmission of failing fragment

when no ACK is returned.n Retransmission is applied to each fragment

Fragment 0

ACK 0

Src

Dest CTS

SIFSRTS

NAV (RTS)

NAV (CTS)

Other

PIFS

DIFS

Backoff-Window

ACK 1

Fragment 1

NAV (Fragment 0)

NAV (ACK 0)

SIFS



Fragmentation (Cont.)n Fragmentation is used to increase

reliabilityn Channel characteristics limit reception

reliability for longer frames

n Broadcast/multicast frames should not be fragmented even if their length exceed aFragmentationThreshold



802.11 Protocol Entities

MAC Sublayer

PLCP Sublayer

PMD Sublayer

MAC LayerManagement

PHY LayerManagement

StationManagement

LLC

MAC

PHY



MAC Management Layer

n Synchronizationn Timing Synchronization Function (TSF)

n Power Managementn sleeping without missing any messagesn Power Management functions

n periodic sleep, frame buffering, Traffic Indication Map

n Association and Reassociationn Joining a networkn Roaming, moving from one AP to another



Synchronization in 802.11

n All stations maintain a local timer.n Timing Synchronization Function

n keeps timers from all stations in synch.

n Timing conveyed by periodic Beacon transmissionsn Beacons contain Timestamp for the entire BSSn Timestamp from Beacons used to calibrate local

clocks



Synchronization in 802.11 (Cont.)

• APs send Beacons in infrastructure networks. • Beacons scheduled at Beacon Interval.• Transmission may be delayed by CSMA deferral.

– subsequent transmissions at expected Beacon Interval– not relative to last Beacon transmission– next Beacon sent at Target Beacon Transmission Time

• Timestamp contains timer value at transmit time.

Time Axis

Beacon Interval

X X X X

"Actual time" stamp in Beacon

Beacon Busy Medium



Synchronization in 802.11 (Cont.)n In infrastructure

n AP generates the beacon

n In ah-hocn Beacon generation is distributed among all

stations



Power Management

n Mobile devices are battery powered.n Power Management is important for mobility.

n Current LAN protocols assume stations are always ready to receive.n Idle receive state dominates LAN adapter

power consumption over time.n 802.11 Power Management Protocol:

n allows transceiver to be off as much as possible

n is transparent to existing protocols



Power Management-Infrastructure Network

n Allow idle stations to go to sleepn station's power save mode stored in AP

n APs buffer packets for sleeping stations.n AP announces which stations have frames bufferedn Traffic Indication Map (TIM) sent with every Beacon

n Power Saving stations wake up periodicallyn listen for Beacons

n If it has packets buffered, it then sends a power-save poll request frame to the AP.

n AP will send the buffered frame to the station. n The station can sleep again.



Power Management-Ah-hocn Similar to the infrastructure networkn However, the buffering scheme is achieved

by the sending stationn Sleeping station also wake up periodically to

listen beacon and ATIMn If it has data buffered

n Send an Ack and wake upn Sending station send the data to the sleeping

station



Wireless LAN Infrastructure Network

Access Point A

Access Point B

Station 4

Access Point C

Station 1 Station 2

Station 3

Station 5Station 6

Station 7

• Each Station is Associated with a particular AP– Stations 1, 2, and 3 are associated with Access Point A– Stations 4 and 5 are associated with Access Point B– Stations 6 and 7 are associated with Access Point C



Roaming

Access Point A

Access Point B

Station 4

Access Point C

Station 1 Station 2

Station 3

Station 5Station 6

Station 7

• Mobile stations may move*



Roaming

Access Point A

Access Point B

Station 4

Access Point C

Station 1

Station 2

Station 3

Station 5Station 6

Station 7

• Mobile stations may move*– beyond the coverage area of their Access Point



Roaming

Access Point A

Access Point B

Station 4

Access Point C

Station 1

Station 2

Station 3

Station 5Station 6

Station 7

• Mobile stations may move*– beyond the coverage area of their Access Point– but within range of another Access Point



Roaming

Access Point A

Access Point B

Station 4

Access Point C

Station 1

Station 2

Station 3

Station 5Station 6

Station 7

• Mobile stations may move*– beyond the coverage area of their Access Point– but within range of another Access Point

• Reassociation allows station to continue operation



Roaming Approachn Station decides that link to its current AP is poor using

the information ofn Carrier Sense and Energy Detectionn RSSI (Receiver Signal Strength Indicator)

n Station uses scanning function to find another APn Station sends Reassociation Request to new APn If Reassociation Response is successful

n then station has roamed to the new APn else station scans for another AP

n If AP accepts Reassociation Requestn AP indicates Reassociation to the Distribution Systemn Distribution System information is updatedn normally old AP is notified through Distribution System



Scanningn Scanning required for many functions.

n finding and joining a networkn finding a new AP while roamingn initializing an Independent BSS (ad hoc)

network



Scanning (Cont.)n 802.11 MAC uses a common mechanism for all

PHYn passive or active scanning

n Passive Scanningn Find networks simply by listening for Beacons

n Active Scanningn On each channel

n Send a Probe, Wait for a Probe Response

n Beacon or Probe Response contains information necessary to join new network.



Active Scanning Example

Steps to Association:

Station sends Probe.

Initial connection to an Access Point

Access Point CAccess Point A





Station sends Probe.APs send Probe Response.








Station selects best AP.









Station sends AssociationRequest to selected AP.










AP sends AssociationResponse.










AP sends AssociationResponse.

Initial connection to an Access Point- ReAssociation follows a similar process




Summaryn 802.11 Architecturen 802.11 Servicesn MAC function description: CSMA/CA

n DCFn PCFn Fragmentation/defragmentation

n MAC management descriptionn Synchronizationn Power Managementn Roaming



802.11 MAC

the end …



an overview of 802.11 mac entity-月會報告 - gicl...

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