IEEE 802.11: Wireless LANs

ALOHA, Slotted ALOHA

Carrier Sense Multiple Access (CSMA), CSMA/CD

MACA, MACAW, FAMA, DFWMAC

Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) Difficult to detect packet collisions (near-far problem) Optional RTS-CTS handshaking to avoid hidden node problem

IEEE 802.11 MAC

• Mandatory Distributed Coordination Function (DCF)

– For distributed contention-based channel

• Optional Point Coordination Function (PCF) – For centralized contention-free channel access

Distributed Coordination Function (DCF)

RTS

CTS

DATA

ACK

ACK... RTS ...NAV (RTS)

NAV (CTS)

DIFS

SIFS SIFS SIFS

DIFS

BACKOFF BACKOFF

STATION A

STATION B

OTHER STATIONS

Random Backoff ProcedureChoose a Backoff Time over the interval [0, CW] Backoff Time = Random() × aSlotTime

6

CW = 31(e.g., Backoff Time = 6 × 20 = 120 µs)

• If the medium is idle for a backoff slot, the backoff time is decremented by aSlotTime

• If the medium is determined to be busy during a backoff slot, the backoff procedure is suspended until the medium is determined to be idle for DIFS period

• Whenever the Backoff Timer reaches zero, a packet transmission begins

Random Backoff Procedure

• Successful Packet Transmissions : minimum contention window size

Frame

Frame

Frame

Frame

STATION A

STATION B

STATION C

DIFS DIFS DIFS

CW = 7, BT = 5 slots

CW = 7, BT = 2 slots

CW = 7, BT = 3 slots CW = 7, BT = 1 slot

CW = 7, BT = 2 slotsCW = 7, BT = 3 slots

Idle Backoff Slot

Deferred Backoff Slot

Random Backoff Procedure

• Retransmission Case (i.e., Collisions or Transmission Failures) : increase contention window size

Frame

Frame

STATION A

STATION B

STATION C

DIFS DIFS DIFS

CW = 7, BT = 2 slots

Frame

DIFS

CW = 7, BT = 2 slots

CW = 7, BT = 3 slots

CW = 15, BT = 5 slotsFirst Retransmission

CW = 15, BT = 3 slotsFirst Retransmission

CW = 7, BT = 1 slot

CW = 15, BT = 4 slots

CW = 15, BT = 2 slots

CW = 15, BT = 2 slots

CW = 7

PACKET COLLISION

Frame

Frame

Frame

CW = 7

Idle Backoff Slot

Deferred Backoff Slot

Binary Exponential Backoff

31 63127

255

511

1023CW max

CW min

More than 5 RetransmissionsFifth Retransmission

Fourth RetransmissionThird Retransmission

Second RetransmissionFirst Retransmission

Initial Attempt

Wasting Factors in Backoff Procedure

IDLE SLOTS : small # of active stations with large contention window size (ex, 2 stations in DSSS MinCW=31)

STATION A

STATION B

COLLISIONS : large # of active stations with small contention window size (ex, 100 stations in HSSS MinCW=15)

STATION A

STATION B

STATION C

STATION D

STATION E

STATION F

STATION G

STATION H

STATION I

STATION J

Example for Idle Slots

Frame

Frame

Frame

CW = 63, BT = 50, Idle Slots = 29

CW = 63, BT = 29, Idle Slots = 29

CW = 63, BT = 21, Idle Slots = 21

CW = 63, BT = 60, Idle Slots = 21 CW = 63, BT = 39, Idle Slots = 39

CW = 63, BT = 58, Idle Slots = 39

• Large minimum contention window size + small number of stations large wasting idle slots

Point Coordination Function (PCF)

• Two consecutive frames are separated by SIFS

• CFP lengths depend on traffic amount – Maximum length announced by

Beacon D1+Poll

NAV

SIFS

SIFS

U1+Ack

D2+Ack+Poll

SIFS

U2+Ack

SIFS

SIFS

CF-End

Uplink

Downlink

Contentio Free Period (CFP) for PCF

ContentionPeriod (CP)

for DCF

Contention Free Period Repetition Interval (CFPRI) or Superframe

Reset NAV

CF_MAX_DurationDx - downlink frame to STA xUx - uplink frame from STA x

PIFS

Problems of Legacy MAC

No notion of QoS and related signalingRestricted polling schedulingSuperframe with alternating CFP and CP needs to be short for short delay boundAP assuming the full control over the medium during CFP: overlapping WLANs?Uncontrollable/unpredictable frame transmission timesLarge wasting ilde slots when # of active stations is smallRapid performance degradation when # of active stations is large too slow to resolve collisions

Hybrid Coordination Function (HCF)

• Contention-based channel access– Enhanced Distributed Coordination Function (EDCF) for prioriti

zed QoS

– Variation of legacy DCF

– provide differentiated, distributed access to the WM for 8 user priorities

– By using different AIFS, CWmin,CWmax values,

• Controlled channel access– QoS is characterized by a set of parameters

– A traffic stream (TS) is set up between transmitter and receiver (and HC – located within QoS AP)

– Polling mode plus HC’s prioritized channel access for parameterized QoS

– Variation of legacy PCF

Access Category

AC0 AC1 AC2 AC3

Virtual Collision Handler

Backo

ff A

IFS[0]

BO

[0]

Backo

ff A

IFS[1]

BO

[1]

Backo

ff A

IFS[2]

BO

[2]

Backo

ff A

IFS[3]

BO

[3]

Transmission Attempt

• Access category (AC) as a virtual DCF

• 4 ACs implemented within a QSTA to support 8 user priorities

• Multiple ACs contend independently

• The winning AC transmits a frame

AIFS (Arbitration-time inter-frame space)

• AIFS is the deferral time for backoff count-down that is used to achieve QoS differentiation

• AIFS is an actual IFS of priority-dependent duration

For stations with classification i= 0,1,…AIFSi = aSIFSTime + aAIFSi x aSlotTime

where aAIFSi is the AIFS slot count for class i

Example: For the top-priority class aAIFS0 = 1 and AIFS0 = PIFS

For legacy stations aAIFS0 = 2 and AIFS = DIFS *

Default QoS Parameter Set

EDCF Inter-Frame Space

HC Controlled Channel Access

• Traffic Specification (TSPEC) Element

HC Controlled Channel Access

• During CFP– HC assumes the full control over the medium

– Similar to PCF

• During CP– HC can grab the channel after a PIFS idle time

• Polled TXOP can exist in both CFP and CP– Superframe size needs not be very small anymore

• QoS (+)CF-Poll specifies the polled TXOP limit• During a polled TXOP, the TXOP holder can transmi

t whatever frames it wants • NAV protects a polled TXOP

HC Controlled Channel Access

• HC scheduling– Mixture of downlink and polled TXOP scheduling

• QSTA scheduling– During a polled TXOP, schedule frame transmissions

• Admission control by HC– To decide whether to admit a TS or not