A Scalable MAC Protocol forNext-Generation Wireless LANs

Zakhia (Zak) Abichar, J. Morris Chang, and Daji QiaoDept. of Electrical and Computer EngineeringIowa State University

IEEE International Symposium on a World of Wireless, Mobile and Multimedia Networks (WoWMoM), June 2006

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Outline Introduction System model Limitations of DCF in Next-Generation WLANs Group-Based Medium Access Control (GMAC) Simulation Results

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Introduction The performance of current wireless LANs is not sufficient

for our needs– With higher rates, we can enable new applications

Multimedia-oriented (video, voice)

IEEE 802.11n– Currently a draft (Pre-N products)– Aims at a throughput higher than 100 Mbps– Backed by Enhanced Wireless Consortium (EWC), an industry group– Requirement: 802.11n and 802.11b/g can operate in the same WLAN

Simply increasing Tx rates doesn’t provide a higher throughput– The overhead of the MAC should be reduced

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System Model Infrastructure-based WLAN with high data rates

– Rates up to 270 Mbps A large number of users in a cell

– Enough bandwidth to serve many users Hidden nodes Stations are able to localize themselves

– Error upperbound δ

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Limitations of DCF DCF has a high overhead especially in next-generation WLANs

Overhead of DCF:– Interframe spaces (IFS), backoff slots, control packets– Weight of overhead becomes magnified with high rates

Collision rate becomes high when the number of station is large

CTSRTS DATA ACKCurrent WLANs

CTSRTS ACKNext-GenerationWLANs

DATADIFS

DIFS

contention

contention

SIFS SIFS SIFS

SIFS SIFS SIFS

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Overhead of DCF Control packets are still

transmitted at low rate– They should be received by

all the stations– Maintain interoperability with

802.11b/g stations

In the figure, there are no collisions– i.e. assume there is one

station only

Cases:– Rate: 27, Throughput: 15– Rate: 81, Throughput: 25– Rate: 135, Throughput: 28– Rate: infinity, Throughput: 36

Throughput upperbound of DCF

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Collision Rate of DCF Collision rate of DCF is high

with a large number of stations

In the figure:– collision rate with no hidden

nodes

10 stations: coll. rate=15% 30 stations: coll. rate=26% 100 stations: coll. rate=40%

Collision rate of DCF

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Other Limitations of DCF Hidden nodes

– Use RTS/CTS (further control overhead)

Fairness in a multi-rate environment– With DCF each station transmits one packet upon access:

Throughput-based fairness

– Stations with low rate occupy the channel for a long time– We use time-based fairness: a station is allocated a time to

transmit one packet at the lowest data rate Stations with high rates aggregate multiple MPDUs

CTSRTS DATA ACKLow-rate station

CTSRTS ACKHigh-rate station DATA DATA DATA

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Group-Based Medium Access Control (GMAC)

Stations are divided into groups– Each group has a leader– Groups are free of hidden nodes

Only group leaders contend using CSMA/CA– A winning leader reserves time for all its group (RTS/CTS)– Then it transmits a polling packet

Non-leader stations transmit following the polling packet Interoperable with 802.11b/g

AP

Group leader:Non-leader:

Contention

Group 1

Group 2

Group 3

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Details of GMAC Group formation Contention of group leaders Polling of non-leaders Group maintenance

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Group formation Groups should be free of hidden nodes Group leaders broadcast their location in the polling packet A station joins a group if:

(d(sta,leader)< R/2 - 2δ) && (groupSize < maximumSize) – Otherwise, the stations becomes a leader

A station indicates a leader in the Association Request Implementation:

– Several localization schemes based on Received Signal Strength (RSS)

– Localization based on GPS

Polling Packet

Group formation

Groupleader R/2-2δ 2δ

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Contention of Group Leaders Group leaders contend using CSMA/CA Reserves time using RTS/CTS

– Time to transmit one data packet at the lowest rate– Stations aggregate multiple packets if they have a high rate

Leader monitors all the transmissions– Check if some stations skip

Release the reserved time early by the End-NAV packet

Reserved time for 7 stations

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Polling of Non-leaders A winning leader polls all the stations in its group Non-leaders transmit

– SIFS between consecutive transmissions Within a group, no hidden nodes

– A station can detect when other stations have missed transmission

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Group Maintenance Leader withdraws: group is disbanded

– Form a new group according to the initial procedure– We cannot relegate the role of leader to another station, this cannot

guarantee the group remains free of hidden nodes Non-leader withdraws: it is removed from the polling list Leader fails:

– Group leader appends its backoff counter– Non-leaders can track the transmission of their leader and detect

failure

Non-leader fails:– The leader uses a timer Timer-Skip-Max– If a station skips many times, its timer is reset– Station is removed from the polling list

Data of Leader 4Backoff used in next contention

CTSRTS

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Simulation Results Custom MAC-level simulation for GMAC and DCF PHY characteristics from 802.11n draft Two configurations for the data rates:

– Single-stream rates. Two antennas at each station. Rates from 13.5 Mbps to 135 Mbps

– Double-stream rates. Four antennas at each station. Rates from 27 Mbps to 270 Mbps

Nodes uniformly distributed in the WLAN cell

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Throughput with High Data Rates Number of stations: 20

– Low collision rate Did we reduce the control

overhead?

Single-stream rates (Mbps)– Average rate: 13.5 to 62– GMAC: 8.9 to 47.3– DCF: 4.5 to 6.1

Double-stream rates (Mbps)– Average rate: 27 to 125– GMAC: 19 to 94– DCF: 5.4 to 6.5

Single-stream rates

Double-stream rates

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Throughput with a Large Number of Stations Large number of stations Did we reduce the collision rate?

Single-stream rates (Mbps)– Average rate: 56 to 49– GMAC: close to 40– DCF: 6.2 to 4.1

Double-stream rates (Mbps)– Average rate: 113 to 98– GMAC: 81 to 74– DCF: 6.5 to 4.3

Single-stream rates

Double-stream rates

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Dependency on the Average Data Rate Number of stations: 20 Single-stream rates

Simulation with various positions of the stations– Average rate changes

The throughput is sensitive to the average rate

Single-stream rates

Throughput

Average rate

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Conclusion We need new MAC protocols for Next-Generation

Wireless LANs– Focus on reducing the overhead and collision rate

In GMAC:– A hierarchical approach reduces the overhead– Maintains a high throughput

Scales with data rates Scales with the number of stations

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Thank you