Dynamic Bandwidth Scheduling for QoS Enhancement over IEEE 802.11 WLAN

Sangwook Kang, Sungkwan Kim, Mingan Wang, Sunshin An

Korea UniversityEuropean Wireless Conference 2004

Yi-Chi Cheng(JOE)

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Outline Introduction Dynamic Bandwidth Scheduling MAC

(DBSM) Protocol Update Procedure Nrt-data transmission Procedure Rt-data transmission Procedure

Simulation Conclusion

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Introduction (1/2) Distributed Coordination Function (DCF)

CSMA / CA Point Coordination Function (PCF)

Polling-based protocol in CFP

免競爭訊框(CFP)

超級訊框

Super Frame

需競爭訊框 (CP)

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Introduction (2/2)

Schedule Table Need more redundant data to compute

Distributed Fair Scheduling (DFS) The flow with larger weight obtains a hig

her throughput

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Interframe spaces of 802.11 Four types of Interframe spaces (IFS)

SIFS (Short IFS) PIFS (PCF IFS) DIFS (DCF IFS) EIFS (Extended IFS)

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Interframe spaces of 802.11 SIFS (Short IFS)

Is used as interframe between continue frame. Ex: RTS/CTS, ACK.

PIFS (PCF IFS) Is used in the PCF to gain priority access at the s

tart of the CFP. DIFS (DCF IFS)

Stations under the DCF to transmit data frames and management frames.

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Interframe spaces of 802.11 EIFS (Extended IFS)

Only used when the result of frame transition is error. ex: collisions

DIFS

Busy Medium

PIFS

SIFSDIFS

Contention Window

Back-off Window

Next Frame

Defer Access

Slot time

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Scheduled Table Format

STA ID: Station ID Link Type: 0 for real-time data; 1 for non-real time

data Last Success time: the time record in ms when

received ACK last time Extension Flag: has more data to transfer Idle Counter: If a station has no data to transfer then

counter + 1, if (counter == 3) this note is deleted from table

STA IDLINKType

Last SuccessTime

ExtensionFlag

IdleCounter

Reserved

8 1 7 1 2 3Bits

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Beacon Frame Format

Frame Control

Duration/

ID

資料

MAC Header

Other fields FCS

ProtocolVersion

TYPE SUBTYPE Otherfields

Timestamp Beaconinterval Information

Type: 00 ManagementSubtype: 1000 Beacon

2 2 260-2312 4

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Establish Schedule Table When a new station want to transmit

data, it sense the medium first. While Medium isn’t idle, station

sends a management frame to register itself in the CP.

If it never senses any message after a DIFS, it generates a schedule table and broadcasts to others though a beacon.

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Update Schedule Table

The first named station in the table broadcast a beacon contain “Timestamp”. Others in the area will synchronize by the “Timestamp”

Each active station listens to the medium and update the “last success time” when it received ACK frame.

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Delete Schedule Table The first station will be deleted after

received a “Link terminated” frame If a station has no data to transfer at

continued 3 times, this note is deleted from table

If the first station failed, it will be deleted and the second station acts as the first station to send the beacon frame

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Establish and Update table

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NRT-Data Transmission (1/3) When CYCLE is used for only nrt-data,

it permits one station to transmit data at one time.

The station having the smaller “last success time” value is earlier to transmit.

When CFP is over, station would stop to transmit frame whether it has more data or not.

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NRT-Data Transmission (2/3)

If a station finished its transmission in the CFP, the note will remove from table and next station start transmission after an ACK frame with “Link terminated”.

There is no problem of “monopolization” and “starvation”.

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NRT-Data Transmission (3/3)

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RT-Data Transmission (1/3) The rt-data of the each station is sorte

d by the “link type / last success time” in schedule table.

If a data transmitted successfully, an ACK would be received after a SIFS.

IF the station didn’t receive an ACK, the “Idle counter” increase by 1.If it reaches 3, the station would be deleted.

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RT-Data Transmission (2/3) IF “Last success time” + “Duration

of current frame” ≧ the smallest “Last success time” in table, current station sets “Extension Flag” and stops to transmit.

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RT-Data Transmission (3/3)

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Simulation (1/5)

Simulated both 802.11 standard protocol and DBSM protocol

Used Network simulator 2.26

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Simulation (2/5)

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Simulation (3/5)

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Simulation (4/5)

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Simulation (5/5)

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Conclusion

Advantage of DBSM Reduces the collision and improves the t

hroughput It’s robust, even if any station had failed Provides the fairness for nrt-data services Provides effective bandwidth reservation

for rt-data services