1

802.11 MAC Sublayer• MAC layer tasks:

–Control medium access–Roaming, authentication, power conservation

• Traffic services–DCF (Distributed Coordination Function)

(mandatory): Asynchronous Data Service• Only service available in ad-hoc network mode• does not use any kind of central control• exchange of data packets based on “best-effort”• support of broadcast and multicast

–PCF (Point Coordination Function) (optional): Time-Bounded Service• uses the base station to control all activity in its cell

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802.11 MAC Sublayer• PCF and DCF can coexist within one cell by carefully defining

the interframe time interval. The four intervals are depicted:– SIFS (Short InterFrame Spacing) is used to allow the parties in a single

dialog the chance to go first including letting the receiver send a CTS and an ACK and the sender to transmit the next fragment.

– PIFS (PCF InterFrame Spacing) is used to allow the base station to send a beacon frame or poll frame.

– DIFS (DCF InterFrame Spacing) is used to allow any station to grab the channel and to send a new frame.

– EIFS (Extended InterFrame Spacing) is used only by a station that has just received a bad or unknown frame to report the bad frame.

• The result MAC scheme used in 802.11 is carrier sensing multiple access with collision avoidance (CSMA/CA) that is based on MACAW.– Use NAV (Network Allocation Vector) to indicate the channel is busy.

3

The 802.11 MAC Sublayer Protocol

Interframe spacing in 802.11.

4

802.11 MAC Sublayer• Access methods

– DFWMAC-DCF (distributed foundation wireless medium access control- Distributed Coordination Function) CSMA/CA (mandatory)• collision avoidance via randomized „back-off“ mechanism• minimum distance between consecutive packets• ACK packet for acknowledgements (not for broadcasts)

– DFWMAC-DCF w/ RTS/CTS (optional)• avoids hidden terminal problem

– DFWMAC- PCF (Point Coordination Function) (optional)• access point polls terminals according to a list• Completely controlled by the base station. No collisions occur.• A beacon frame which contains system parameters is periodically (10 to 100

times per second) broadcasted to invite new stations to sign up for polling service.

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t

medium busy

DIFSDIFS

next frame

contention window(randomized back-offmechanism)

802.11 - CSMA/CA access method

• Station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment)

• If the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type)

• If the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time)

• If another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness)

slot timedirect access if medium is free DIFS

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802.11 - Competing Stations

t

busy

boe

station1

station2

station3

station4

station5

packet arrival at MAC

DIFSboe

boe

boe

busy

elapsed backoff time

bor residual backoff time

busy medium not idle (frame, ack etc.)

bor

bor

DIFS

boe

boe

boe bor

DIFS

busy

busy

DIFSboe busy

boe

boe

bor

bor

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802.11 - CSMA/CA access method• Sending unicast packets

– station has to wait for DIFS before sending data– receivers acknowledge at once (after waiting for SIFS) if the packet was

received correctly (CRC)– automatic retransmission of data packets in case of transmission errors

t

SIFS

DIFS

data

ACK

waiting time

otherstations

receiver

sender data

DIFS

contention

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802.11 – DFWMAC• Sending unicast packets

– station can send RTS with reservation parameter (transmission duration) after waiting for DIFS (reservation determines amount of time the data packet needs the medium)

– acknowledgement via CTS after SIFS by receiver (if ready to receive)– sender can now send data at once, acknowledgement via ACK– other stations set its net allocation vector (NAV) in accordance with the

duration field.

t

SIFS

DIFS

data

ACK

defer access

otherstations

receiver

sender data

DIFS

contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

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Fragmentation

t

SIFS

DIFS

data

ACK1

otherstations

receiver

senderfrag1

DIFS

contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

NAV (frag1)NAV (ACK1)

SIFSACK2

frag2

SIFS

The deal with the problem of noisy channels, 802.11 allows frames to be fragmented.

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DFWMAC-PCF

PIFS

stations‘NAV

wirelessstations

point coordinator

D1

U1

SIFS

NAV

SIFSD2

U2

SIFS

SIFS

SuperFramet0

medium busy

t1

A super frame comprises a contention-free period and a contention period.• D for downstream• U for upstream• CF for an end maker

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DFWMAC-PCF

tstations‘NAV

wirelessstations

point coordinator

D3

NAV

PIFSD4

U4

SIFS

SIFSCFend

contentionperiod

contention free period

t2 t3 t4

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802.11 MAC Frame format• Types

– control frames, management frames, data frames• Sequence numbers

– important against duplicated frames due to lost ACKs • Addresses

– receiver, transmitter (physical), BSS identifier, sender (logical)• Miscellaneous

– sending time, checksum, frame control, data

FrameControl

Duration/ID

Address1

Address2

Address3

SequenceControl

Address4 Data CRC

2 2 6 6 6 62 40-2312bytes

Protocolversion Type Subtype To

DSMoreFrag Retry Power

MgmtMoreData WEP

2 2 4 1FromDS

1

Order

bits 1 1 1 1 1 1

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MAC address formatscenario to DS from

DSaddress 1 address 2 address 3 address 4

ad-hoc network 0 0 DA SA BSSID -infrastructurenetwork, from AP

0 1 DA BSSID SA -

infrastructurenetwork, to AP

1 0 BSSID SA DA -

infrastructurenetwork, within DS

1 1 RA TA DA SA

DS: Distribution SystemAP: Access PointDA: Destination AddressSA: Source Address

BSSID: Basic Service Set IdentifierRA: Receiver AddressTA: Transmitter Address

Ad-hoc network: packet exchanged between two wireless nodes without a distribution system

Infrastructure network, from AP: a packet sent to the receiver via the access point

Infrastructure network, to AP: a station sends a packet to another station via the access point

Infrastructure network, within DS: packets transmitted between two access points over the distribution system.

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Special Frames: ACK, RTS, CTS• Acknowledgement

• Request To Send

• Clear To Send

FrameControl Duration Receiver

AddressTransmitterAddress CRC

2 2 6 6 4bytes

FrameControl Duration Receiver

Address CRC

2 2 6 4bytes

FrameControl Duration Receiver

Address CRC

2 2 6 4bytes

ACK

RTS

CTS

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802.11 - MAC management• Synchronization

– try to find a LAN, try to stay within a LAN– Synchronize internal clocks and generate beacon signals

• Power management– periodic sleep, frame buffering, traffic measurements – sleep-mode without missing a message

• Roaming for Association/Reassociation– integration into a LAN– roaming, i.e. change networks by changing access points – scanning, i.e. active search for a network

• MIB - Management Information Base– All parameters representing the current state of a wireless station and

an access point are stored in a MIB.– A MIB can be accessed via SNMP.

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Synchronization using a Beacon (infrastructure)

beacon interval

tmedium

accesspoint

busy

B

busy busy busy

B B B

value of the timestamp B beacon frame

Timing synchronization function (TSF) is needed for:• Power management• Coordination of the PCF and for synchronization of the hopping

sequence A beacon contains a timestamp and other management

information. The access point tries to schedule transmissions according to

the excepted beacon interval (target beacon transmission time).

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Synchronization using a Beacon (ad-hoc)

tmedium

station1

busy

B1

beacon interval

busy busy busy

B1

value of the timestamp B beacon frame

station2

B2 B2

random delay

The standard random backoff algorithm is also applied to the beacon frames in the ad-hoc networks.