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Medium Access Control Operation

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Outline

• MAC sublayer and OSI• MAC address mechanism• Hidden node problem• MAC Frame exchange protocol• DCF (Distributed Coordination Function)• PCF (Point Coordination Function)• MAC Frame format

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Medium Access ControlMedium Access Control

• MAC sublayer and OSI (open systems interconnection) reference model

Network layer

802.2 logical link control

802.3CSMA-CD

802.5token ring

802.11wireless

LANs

otherLANs

Various physical layer

Network layer

Data Link layer

Physical layer

LLC

MAC

PHY

IEEE 802 OSI

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Medium Access Control

• MAC functionality- to provide a reliable data delivery service through frame exchange protocol at MAC level- to control access to the shared wireless medium distributed coordination function (basic access mechanism)‧ point coordination function (centrally controlled access)‧- to protect the data that is being delivered MAC provides a privacy service called Wired Equivalent Privacy‧ (WEP) encryption

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MAC ArchitectureMAC Architecture

Point CoordinationFunction

Distribution Coordination Function(CSMA-CA)

802.112.4-GhzFHSS

802.112.4-GhzDSSS

802.11Infrared

802.11a5-GhzOFDM

802.11b2.4-GhzDSSS

802.11g2.4-GhzDSSS,OFDM

MAC

Physical

Contentionservice

Contention-freeservice

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MAC Frame Exchange Protocol

• WLAN media are noisy and unreliable.- The source needs to make sure the frame has been received at the destination.

• At least, two frames are required:- a frame from the source to the destination- and an acknowledgment from the destination- these two frames are an atomic unit of MAC protocol- if the source does not receive the ACK, the source will resend the frame. (slower but more reliable)

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The Hidden Node Problem

Station A can communicate only with B

Station B can communicate with station A and C

Station C can communicate only with B

The frame from A to B can be corrupted by a transmission begun by

Station C

The station C may not know the ongoing transmission from A to B

A CB

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B

To Address the Hidden Node Problem

Two additional frames are added in MACRTS: Request To SendCTS: Clear to SendThe source sends a RTS to the destination; the destination returns a CTSto the source. The RTS and CTS will inform all stations in theneighborhood of A and B about the upcoming transmission from A to BWhen A receives the CTS from B, the real frame is sent from A to BIf the data is correctly received by B,B will return an ACK. Thus, fourframes are used in the MAC frame exchange protocol

AC

Area cleared by RTSCTS

RTS

Area cleared by CTS

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To Address the Hidden Node Problem(cont.)

A station in the neighborhood of A, receiving RTS from A, will delay

Any transmission of its own until if receives the frame announced by

RTS. If the announced frame is not detected, the station may use the

Medium.

Similarly, a station in the neighborhood of B, receiving CTS from B, will

Delay any transmission of its own until it receives the ACK frame. If the

ACK frame is not detected, the station may use the medium.

BA

C

Area cleared by RTSCTS

RTS

Area cleared by CTS

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More about the four-way MAC Frame Exchange Protocol

• Collision: if the frame exchange failed in the source station

-retransmit( under some rules for scheduling the

retransmission)

- to prevent the MAC from being monopolized

attempting to deliver a single frame, there are retry

counters and timers to limit the lifetime of a frame

• RTS and CTS may be disabled by the dot11RTSThreshold attribute in the MIB (Management

Information Base)

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More about the four-way MAC Frame Exchange Protocol (cont.)

• Regarding the dot11RTSThreshold attribute

- if frame_length > dot11RTSThreshold four-way frame exchange with RTS and CTS

- if frame_length < dot11RTSThreshold frame exchange without RTS and CTS

- if the stations are concentrated in an area where all can

hear from each other, the dot11RTSThreshold should be

set to a higher value. Thus, no bandwidth will be

consumed on RTS and CTS

- The default value for the threshold is 128

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More about the four-way MAC Frame Exchange Protocol (cont.)

• the dot11RTSThreshold value

- should be based on the bandwidth lost from the additional

overhead of the protocol and bandwidth lost from the

transmission being corrupted by hidden bodes

• No need to change the dot11RTSThreshold from the

default value in an AP.(AP should be heard by all stations)

- however, when APs are co-located and sharing a channel,

the RTS threshold value may be set for an AP

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Accessing the Wireless Medium

• Before transmitting a frame, MAC must gain access

to the medium using one of two modes: - Distributed coordination Function (DCF) : Carrier

sense Multiple Access with Collision Avoidance

(CSMA/CA)

- Point Coordination Function (PCF): a centrally

controlled access (priority-based access). A point

coordinator (PC) controls the PCF. The PC is always

located in an AP

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Distributed Coordination Function(DCF):CSMA/CA

• Carrier Sense Multiple Access - Similar to 802.3.MAC determines whether the medium is busy or idle based on both of physical (PHY) and virtual carrier sense mechanisms

• Virtual Carrier Sense - Network Allocation Vector (NAV): the amount time

that remains before the medium will become available to a stationoperates like a timer; initial NAV value = Duration field of last MAC frame; and counting down to zeroAfter NAV = 0,the station can transmit if PHY coordination indicates a clear channel

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CSMA/CANote:The period of time immediately followinga busy medium is the highest probabilityof collisions occurring.Many stations may be waiting for the medium to become idle and attempt to transmit at the sametime. Thus, whenever the station

sensing a busy medium, a random backoff time is used

This will also increment the retry counter associated with the frame. As the time elapsed, the station will check PHY

again

NAV=0?

MediumIdle?

Check PHY

TransmitFrame

Random Backoff

Time

Collision?

NO

YES

YES

YES

NO

NO

NO

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Random Backoff Time

• Binary exponential backoff algorithm - the amount of time (a pseudo-random number drawn from

a uniform distribution over the interval [0,CW]) must be

elapsed while the medium is idle before another

transmission

- The CW( Contention Window) doubles with every

attempt to transmit that is deferred until CW == CW

the initial value of CW is CW

- The CW and CW are attributes of MIB

max

max

min

min

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Error Recovery Mechansims

• Errors

- Station A send an RTS, but never receive the CTS

- Station A send a data frame but never receive the ACK

• Number of retransmissions

- shortRetryLimit (attribute in MIB): for short frames

(frame_length < RTSThreshold)

- longRetryLimit (attribute in MIB): for long frames

(frame_length >= RTSThreshold)

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Timing Intervals

• Time interval: the time from the medium is idle to time of beginning of transmission

• Five timing intervals: - short InterFrame Space (SIFS); shortest interval for highest

priority frames (e.g. ACK or CTS)

- slot time; longer than SIFS

- Priority InterFrame Space (PIFS); SIFS+ one slot time

- Distributed InterFrame Space (DIFS); SIFS+ two slot time

- Extended InterFrame Space (EIFS); the longest

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Timing Intervals (cont.)

• Priority InterFrame Space (PIFS); SIFS+ one slot time

- for stations use point coordination function (PCF)• Distributed InterFrame Space (DIFS); SIFS+ one slot time

- for stations use distribution coordination function (DCF)

• Extended InterFrame Space (EIFS); the longest

- for DCF-based stations have an incorrect FCS (frame check sequence) value in a frame transmission. This gives enough time for the receiving station to send an ACK frame

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DCF Operation

• MAC begins frame transmission - if both PHY and virtual carrier sense mechanisms

indicate the medium is idle for an interval of DIFS (of EIFS if previously received frame contained errors)

• If medium is busy during the DIFS interval - backoff interval is selected and increment retry counter

For each slot time, if medium is detected to be idle, decrement backoff interval;MAC begins to transmit if backoff interval is expired If the transmission is not successful (I.e. collision), CW is doubled and a new backoff interval is selected and countdown is begun, again. When to stop?

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DCF Operation Timing

DIFS

Contention window

Next Transmission

Slots

Wait for reattempt time

End of previousTransmission

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Example of Backoff Intervals

1 After packet arrival at MAC, station 3 senses medium free for DIFS, so it starts transmission

after a random backoff interval

2 For station 1,2, and 4, their DIFS intervals are interrupted by station 3.Thus, the backoff

Intervals for station 1,2, and 4 ,are generated randomly(i.e.9,5,and7,respectively)

3 After transmission of station2,the remaining backoff interval of station1 is (9-5)=4

4 After transmission of station2,the remaining backoff interval of station4 is (7-5)=2

5 After transmission of station2,the remaining backoff interval of station1 is (4-2)=4

busy

busy

busy

busystation1

station2

station3

station4

Packet arrival at MAC

DIFS DIFS DIFS DIFS

Backoff=2Backoff=7

Backoff=5

1

2 3 5Backoff=4Backoff=9 Backoff=2

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Point coordination Function(PCF)

• PCF, priority-based access, provides contention frame transfer

• A point coordinator (PC) controls the PCF. The PC is always

located in an AP.PC gains access to the medium by using the normal DCF procedures

– Stations request PC to register them on a polling list– PC regularly polls (CF-poll frame) the stations for traffic– The PCF uses the PIFS(<DIFS) to seize control of the

medium. Then, PC begins a contention-free period (CFP)– At the beginning of CFP, PC transmit a Beacon frame

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Point coordination Function(PCF)

• Beacon Frame– When stations receive the beacon, they update their NAVs with the CFPMaxDuration value found in the CF Parameter Set of beacon of frame

• This NAV prevent stations from accessing (independently) the medium during the CFP• PC announces the end of the CFP by transmitting a contention-free end (CF-End) frame. This cause stations to reset their NAVs

• Why PCF?• In most Cases, the DCF is good enough; however, PCF can support the transmission of time-bounded of formation, such as audio and video. PCF may impose overhead due to pollng

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(PCF)TimingPC sends a data frame to a station

and polls the same station.This is a

piggyback to reduce the

network overhead.

Data+CF-pollData+CF-Ack from station 1

SIFS

Data+CF-Ack+CF-pollTo station2

ACK fromStation 2

PC sends Data and CF-Poll for station

2;CF-Ack can be for different station

CF-pollTo station n

Data+CF-PollTo station n+1

PIFS

……… CF-End

End of CFPThe PC will expect the responding frame (ACK or a data frame in response to CF_Poll) , Within a SIFS interval .If no response, the PC Will transmit its next frame before a PIFSInterval expires after the previous transmission.

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PCF&DCF

• PCF and DCF can coexist.• Two ways to prevent stations to access medium The primary mechanism is to use the NAV. The beacon

frame, from PC, contains the information about the expected length of the CFP. Every station uses this information to set its NAV.

The secondary mechanism used by PC is to transmit its next frame on PIFS interval (in case ,stations do not receive the beacon and may try to access the medium).

PIFS<DIFS. This can prevent staions to access the medium.

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MAC Frame Structure

MAC accepts MSDU(MAC service data unit) from higher layers in the protocol stack and adds headers and trailers to create a MAC protocol data unit (MPDU).MPDU (aka MAC frame) is then passed to PHY to be sent over the medium.

MAC may fragment MSDUs into several frames to increase the probability of successful delivery.

MAC frame types : management, control and data.

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MAC Frame Format

Frame

Control

Duration/ID

Address 1

Address 2

Address 3

Sequence control

Address 4

Frame body

FCS

(CRC)

MAC header(bytes)

2 2 6 6 6 2 6 0-2312 4

Protocol version

type Sub type

To DS

From DS

More frag

Retry Pwr mgt

More data

WEP order

2 2 4 1 1 1 1 1 1 1 1

0, for Current version of the std. 00:mgt.

01:control10:data11:rsvd.

0,last frag . of the data or mgt. frame. control frames are not fraged.

1, the frame is a retrans-mission.

0,the station is in active mode.1, the station will enter pwr mgt mode( no more communication).Must be the same value for a single frame exchange.(2-way or 4-way)

1,the frame body is encrypted (only for data frames or mgt,frames of subtype authentication.)

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Type and Subtype of Frame Control

Type value Type description Subtype b7 b6 b5 b4 Subtype description

00 management 0000 Association request

00 management 0001 Association response

00 management 1000 Beacon

01 Control 1011 Request to sent( RTS)

01 Control 1100 Clear to send( CTS)

01 Control 1101 Acknowledgment( ACK)

01 Control 1110 Contention free( CF)-end

01 Control 1111 CF-end +CF-ACK

10 data 0000 Data

10 data 0001 Data + CF-ACK

10 data 1000-1111 reserved

11 reserved 0000-1111 reserved

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Subfields: More Data, Order• More data It is set to “1” when there is at least one frame buffered at the AP for

the mobile station. During the CFP, station( which is polled by PC) can use this field to

inform the PC that there is at least one frame buffered at the station to be sent to the PC.

In multicast frames, the AP can set this field to one to indicate that there are more multicast frames buffered at the AP.

• Order It is set to one when the content of the data frame was provided to

the MAC with a request for strictly ordered service. This information is given to the AP and DS to ensure the service.

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Address Types

• Four types of address used in 802.11 Source address( SA), destination address( DA), receiver

address( RA) and transmitter address( TA). SA and TA must be individual address; DA and RA can

be either individual address or a group address. BSS Identifier( BSSID): In an infrastructure BSS, the BSSID is the MAC address(

48 bits) of the AP. In an independent BSS, the BSSID is locally

administered, individual address that is generated randomly by the station that starts the IBSS.

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Address Fields

• Address 1 field: Is used to perform receive address matching decisions.• Address 2 field: Is used to identify the sender of the frame, so an ACK

can be send back to the sender.• Address 3 field: Is used as SA if the frame is from an AP, as DA if the

frame is being sent to AP( for DS forwarding decision).• Address 4 field : Is to used only as a frame is forwarded from one AP to

another AP ( wireless DS).

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Address Field FunctionsFunction To

DS

From

DS

Address 1 Address 2 Address 3 Address 4

IBSS

(frame from station to station within a BSS)

0 0 RA=DA SA BSSID

N/A

From the AP

(frame exiting the DS)

0 1 RA=DA BSSID

SA N/A

To the AP

(frame destined for the DS)

1 0 RA=BSSID

SA DA N/A

Wireless DS

(From one AP to another AP)

1 1 RA TA DA SA

the address of the receiver

The address of the sender

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Duration/ID field• 16-bit long Duration information for updating the NAV Short ID, association ID( AID), used by a station to retrieve frames

that are buffered for it at the AP. Only the power-save poll( PS-poll) frame contains the AID.

• As short ID The AID is aligned in 14 bits of LSB. Two MSB are set to one.• As duration MSB is set to zero. Bits 14-0 represent the remaining duration of a

frame exchange. Stations receive this frame will update the NAV and will not begin a transmission.

• During the CFP, bit 15 is one and all other bits are zero( 32768) in all frames. This will inform newly joined stations about the ongoing CFP.

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Sequence Control Field• 16-bit long, comprising two subfields 4-bit fragment number and 12-bit sequence number.• Sequence number Assigned sequentially by the sending station to each MSDU.( 0-

4095-0-4095…) If MSDU is fragmented, this number is sent with each frame

containing a fragment of the MSDU• Fragment number The first, or only, fragment of an MSDU is assigned a fragment

number of zero. The next fragment will have a number of 1. Please recall that there is a “more frag” subfield in the frame control field.

• Both are constant over all transmissions and retransmissions of a particular frame (MSDU or Fragment)

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Frame Body Field

• The frame body field is used in data or management frames. It is variable length.

• It may be as long as 2304 bytes without WEP encryption or 2312 bytes when the frame body is encrypted using WEP.

• Why 2304?• 2048-byte data+256 bytes of upper layer

protocol headers and trailer( e.g. LLC PDU =LLC header+[ SNAP header+ information])

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Frame Check Sequence Field

• 32 bits long.

• It contains the result of applying the CCITT CRC-32 polynomial to the MAC header and frame body.

G( x)=X32+X26+X23+X22+X16+X12+X11+X10+X8+X7+X5+X4+X2+X+1

Same as the 802.3 ( CRC, Cyclic Redundancy Check)

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Control Frame Subtypes

• Six control frame subtypes RTS, CTS, ACK, power save poll( PS-Poll),

contention-free end (CF-End), and CF-End+ ACK.

Frame control

Duration RA TA FCSRTS

2 2 6 6 4

Frame control

Duration RA FCS

Frame control

Duration RA FCSCTS

ACK

The duration is measured inmicrosecond.

The RA value in taken from the address 2field immediatelypreceding data, management, orPS-Poll frame.

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Control Frame Subtypes( cont.)

Frame control

AID BSSID TA FCS

Frame control

Duration RA BSSID FCS

PS-Poll

The purpose of this frame is to request that an AP deliver a frame that has been buffered for a mobile station while it was in a power saving mode. (AID is given to the station upon association with the BSS)

PS-PollACK

SIFS

Station (TA)

AP (BSSID)

NAVAll stations

The NAV value is not part of the PS-Poll frame, But is set by every station.

2 2 6 6 4

CF-End /CF-End +CF ACK

2 2 6 6 4

These frames are sent by PC as the last frame in the CFP. The RA is the broadcast group address, since it is to be received by every station in the BSS. The duration value is zero. The BSSID is the MAC address of the PC (AP).

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Transmission of MPDU

• Without RTS/CTSDIFS

Data

Source

Destination

ACK

SIFS

Other NAV

Defer access Wait for reattempt time

DIFS

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Transmission of MPDU• With RTS/CTS (a 4-way frame exchange

handshake)

Source

Destination

Other

RTS

CTS

DIFS

SIFS

Data

SIFS ACKSIFS

DIFSNAV( RTS)

NAV( CTS)

NAV( data)

Defer access

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More about ACK Frame

• ACK frame

Case1: It is used to acknowledge the immediately previous data, management or PS-Poll frame( more fragment subfield=0) that the frame was received correctly. ACK( Duration 0;)

Case 2: ACK frame is used to transmit the duration information for a fragment burst to those stations close to the receiving station. (similar to the CTS frame). In the fragment burst mode, frames’ more fragment subfield is equal to 1. ACK( Duration != 0). Of course, for the last fragment, it will be same as case 1.

Frame control

Duration RA FCSACK

2 2 6 4

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Fragment Burst Mode

RTS

CTS

Frag1 Frag2

ACK1 ACK1

SIFS SIFS SIFS SIFS SIFS

NAV(RTS)

NAV(CTS)

NAV(Frag1)

NAV(Ack1)

NAV(Frag2)

DIFS

For ACK2

Other

Destination

Source

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Point Coordination Frame Transfer

BD1+Poll

U1+ACK

D1+ACK+Poll

U2+ACK

CFEnd

NAV

Contention-free period (CFP) repetition interval

SIFS SIFS SIFS SIFS SIFS

Contention period

TBTT

Reset NAVPIFS

CF_MAX_duration

TBTT: Target beacon transmission time

D1, D2: frames sent by point coordinator

U1, U2: frames sent by polled station

B: beacon frame (sent by AP)

CFP repetition interval (CFP_Rate) determines the frequency with which the PCF occurs. The PCF is required to coexist with the DCF.

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Deferred Beacon

B B B B

PCF Data

Beacon Interval Delay

SIFS Deferred Beacon

Note: a deferred beacon frame may cause some issues in maintaining the QoS.