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MAC for WLAN. Doug Young Suh [email protected] Last update : Aug 1, 2009. IEEE802.11 Wireless MAC. MAC Service and functions MAC Addressing Fragmentation Carrier sense mechanisms Channel Access Protocols Distributed Coordination Function (DCF) Point Coordination Function (PCF)

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Mac for wlan

MAC for WLAN

Doug Young Suh

[email protected]

Last update : Aug 1, 2009

WLAN DCF PCF


Ieee802 11 wireless mac
IEEE802.11 Wireless MAC

  • MAC Service and functions

  • MAC Addressing

  • Fragmentation

  • Carrier sense mechanisms

  • Channel Access Protocols

    • Distributed Coordination Function (DCF)

    • Point Coordination Function (PCF)

    • Extended MAC for QoS (802.11e)

  • Power management

  • Security

Network layer

DLL

LLC sublayer

MAC sublayer

MSDU

PHY

WLAN DCF PCF


Mac service and functions
MAC service and functions

  • Transmitting/receiving MSDU between PHY and LLC

  • Specifying various PHY layer parameters

  • Unicast, multicast, broadcast data

  • Fragmentation and defragmentation

WLAN DCF PCF


Mac addressing
MAC Addressing

  • 48-bit IEEE 802 addresseⅹ4

  • Each BSS also has a BSS ID

    • the MAC address of the AP

    • If not, randomly generated

B

0b.cd.ef.gh.22.22

AP2

0b.cd.ef.gh.55.66

AP1

0b.cd.ef.gh.33.55

A

0b.cd.ef.gh.11.11

WLAN DCF PCF


Fragmentation defragmentation
Fragmentation/defragmentation

  • One MSDU  multi MPDUs

  • Even if an MPDU is lost, previous ones are valid.

  • <SA, MSDU seq #, MPDU fragment #>

  • Timer and aMaxMSDUtransmitLifetime

MSDU

MAC

HDR

C

R

C

MAC

HDR

C

R

C

MAC

HDR

C

R

C

MAC

HDR

C

R

C

Frame body

Frame body

Frame body

Frame

body

WLAN DCF PCF


Carrier sense mechanism
Carrier Sense Mechanism

  • The PHY-CCA.indicate primitive indicates to the MAC whether the channel is BUSY or IDLE.

  • NAV (Network Allocation Vector)

    • The length of time that is to elapse before the channel becomes IDLE (available).

    • The duration field in frame contains the length of time for which the transmitting station expects the channel to be busy.

WLAN DCF PCF



Principles of channel access protocols
Principles of Channel Access Protocols

  • Prioritized CDMA/CA

    • Random backoff for the transmission time

    • Priority : The higher, the shorter idle time.

  • IFS (Inter-Frame Space)

    • SIFS (Short IFS) between ACKS and consecutive fragments of the same MSDU

    • PIFS (PCF IFS), DIFS (DPCF IFS)

    • EIFS (Extended IFS) : for DCF after errors

    • SIFS < PIFS < DIFS

WLAN DCF PCF


Ifs inter frame space
IFS (Inter Frame Space)

Interframe spacing in 802.11.

WLAN DCF PCF


DCF

  • The station’s timer runs after DIFS.

DIFS

DIFS

BUSY

BUSY

A

B

C

D

  • At A, a frame from LLC, the station senses the medium busy, and calculates a backoff time of 12 slots.

  • From B, the station’s timer runs until the medium busy, and re-starts at C

  • At D, the timer expires and the station transmits.

WLAN DCF PCF


Backoff time
Backoff time

  • Random variable within [0, CW]

    • CW (Contention Window)

    • CW varies CWmin to CWmax

    • After a successful transmission, CW=Cwmin

    • At unsuccessful transmission, CW *=2

      • Note that worse station becomes to have worse chance for increasing overall throughput. This is problem for realtime multimedia service?

WLAN DCF PCF


Transmitting multiple fragments
Transmitting multiple fragments

SIFS

SIFS

SIFS

SIFS

SIFS

Fragment 1

Fragment 2

Fragment

3 (last)

A

C

K

A

C

K

A

C

K

  • ACKs are sent after only SIFS

  • Next fragment is sent if and only is an ACK is received.

  • A station may maintain control of the medium to transmit multiple fragments of the same MSDU.

* All fragments but last are in the same size.

WLAN DCF PCF


Rts cts
RTS/CTS

  • Seize the medium for a long unicast sequence

    • Request-To-Send, Clear-To-Send

  • RTS has the Duration field to set the total time required for the sequence and the last ACK.

  • All other stations receives the RTS/CTS to update their NAV and do not transmit until the end of the transmission.

  • Overcomes ‘hidden node’ problem

    • ABC : When C is not seen to A, for AB

  • No RTS/CTS for multicast/broadcast

DIFS

PIFS

SIFS

Contention window

NAV of others

SIFS

SIFS

SIFS

SIFS

SIFS

SIFS

SIFS

Fragment 0

Fragment 1

Fragment

2 (last)

RTS

ACK

ACK

CTS

ACK

WLAN DCF PCF


Point coordination function pcf

Point Coordination Function(PCF)

WLAN DCF PCF


Pcf overview
PCF Overview

  • In a Point Coordinator, part of an AP

  • Not in 11a, 11b, or 11g, but in 11e

  • Backward compatible to DCF in 11a, 11b, and 11g

  • Contention-free period and contention period

    • PCF during CFP, in every CFP repetition interval

    • DCF during CP

  • From the PC to other stations after PIFS

  • From other station to PC in response to a poll from the PC

  • Types of frames during the PCF : piggy-back allowed

    • Data, CF-Poll, CF-ACK, Null, CF-End

    • Null : negative response to CF-Poll

WLAN DCF PCF


Pcf frame sequence
PCF Frame Sequence

  • Dx = frames sent by Point Coordinator

  • Ux = frames sent by polled stations

Contention-free repetition interval

Contention-free period

PIFS

SIFS

SIFS

SIFS

SIFS

SIFS

SIFS

SIFS

PIFS

SIFS

SIFS

SIFS

beacon

D1+pol1

D2+ack

+pol2

D3+ack

+pol3

D4

+pol4

CF-End

U1+ack

U2

+ack

U4+ack

Contention period

No response

to CF-Poll 3

Reset NAV

NAV of other stations

CF_Max_Duration

WLAN DCF PCF


Cf polling
CF-Polling

  • CF-Pollable and Non-CF-Pollable

    • Every station registers to the AP to be a CF-Pollable station .

    • Non-CF-Pollable in the DCF rules.

  • Polling list

    * CF (Contention Free)

WLAN DCF PCF


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