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MDG: Measurement-Driven Guidelines for 802.11 WLAN Design. Ioannis Broustis, Konstantina Papagiannaki, Srikanth V. Krishnamurthy, Michalis Faloutsos, Vivek Mhatre ACM MOBICOM 2007. Goal: Improve WLAN network performance. Three functions to improve network performance in dense WLANs

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Mdg measurement driven guidelines for 802 11 wlan design

MDG: Measurement-Driven Guidelines for802.11 WLAN Design

Ioannis Broustis, Konstantina Papagiannaki, Srikanth V. Krishnamurthy,

Michalis Faloutsos, Vivek Mhatre


Goal improve wlan network performance
Goal: Improve WLAN network performance

  • Three functions to improve network performance in dense WLANs

    • Frequency selection

      • Provides spatial separation between interfering APs

    • User association

      • Provides load balancing among APs

    • Power control

      • APs shrink their cells to facilitate higher spatial reuse of spectrum

Frequency selection



Frequency Selection

  • Neighbor Access Points (APs) select different frequencies in order to mitigate interference from each other

Cell of B

Cell of A

Channel 6

Channel 11





Channel 6

Channel 6

Inter-cell Contention: A cannot transmit

because B is transmitting

User association
User Association

  • Users select less-loaded Access Points (APs) in order to get more throughput






Power control
Power Control

  • APs shrink their overlapping cells in order to reduce interference and improve spatial reuse





Problem statement
Problem statement

  • Which are the functions that should be applied in a specific scenario?

  • In what sequence should they be applied?

    High level objective: to maximize a fair notion of aggregate network throughput

Previous work
Previous Work

  • Most studies have tried to optimize one of these functions

    • No studies on the interdependencies between allthree functions

      • A. Mishra, V. Shrivastava, D. Agarwal, S. Banerjee. Distributed Channel Management in Uncoordinated Wireless Environments. MOBICOM 2006.

      • N. Ahmed, S. Keshav. SMARTA: A Self-Managing Architecture for Thin Access Points. CoNEXT 2006.

      • K. Sundaresan, K. Papagiannaki. The Need for Cross-Layer Information in Access Point Selection Algorithms. IMC 2006.

      • A. Mishra, V. Brik, S. Banerjee, A. Srinivasan, W. Arbaugh. A Client-Driven Approach for Channel Management in Wireless LANs. INFOCOM 2006.

      • A. Kumar, V. Kumar. Optimal Association of Stations and APs in an IEEE 802.11 WLAN. NCC 2005.

      • T. Korakis, O. Ercetin, S. V. Krishnamurthy, L. Tassiulas, S. Tripathi. Link Quality Based Association Mechanism in IEEE 802.11h Compliant Wireless LANs. RAWNET 2005.

      • B. Leung, K. Kim. Frequency Assignment for IEEE 802.11 Wireless Networks. VTC 2003.

      • Y. Bejerano, S. Han, L. Li. Fairness and Load Balancing in Wireless LANs Using Association Control.

      • MOBICOM 2004.


  • We perform an extensive experimental study on Testbed-A

  • We quantify the interplay of the three functions

    • We employ 3 previously proposed algorithms for these functions

    • We identify the conditions which make the topology conducive to each one of these functions

  • We develop the MDG framework (Measurement Driven Guidelines)

  • We validate the effectiveness of MDG on a different Testbed-B!

    • Testbed-B is significantly different from Testbed-A

    • We observe that MDG provides the best strategy in all cases

The structure of this talk
The Structure of This Talk

  • Background on the 3 algorithms

  • Part 1. Experimental Study on Testbed-A - Derivation of conditions

  • Part 2. Building the MDG framework

  • Part 3. Validating MDG on Testbed-B

Our choice for the algorithms
Our choice for the algorithms

  • All 3 algorithms based on Gibbs sampling

    • Fully-saturated downlink traffic

  • Frequency selection algorithm (FS) [Kauffmann et al. ‘07]

    • Finds the channel allocation with minimum total interference

  • User Association Algorithm (UA) [Kauffmann et al. ‘07]

    • Finds the state of minimal potential delay of clients

    • Depends on AP channel access time, AP-client link quality and number of clients per AP

  • Power Control algorithm (PC) [Mhatre et al. ‘07]

    • Finds the state of minimal potential delay by jointly tuning PTXand Clear Channel Assessment threshold (CCA)

[1] B. Kauffmann et al. “Measurement-Based Self Organization of Interfering 802.11 Wireless Access Networks”. INFOCOM 2007

[2] V. Mhatre, K. Papagiannaki, F. Baccelli. “Interference Mitigation through Power Control in High Density 802.11 WLANs”. INFOCOM 2007

Algorithmic requirements
Algorithmic requirements

  • The Access Points (APs):

    • Measure: channel power, load etc

    • Exchange: information with other APs

    • Advertise: information to the clients

  • We require minimal functionality from the client

    • Clients tune their Tx power, CCA threshold

    • Clients pick Access Point (AP) for association as per the optimization criteria

Implementation and experimental set up
Implementation and Experimental set-up

  • The 3 algorithms are implemented

    • for both APs and clients

    • on Intel 2915 prototype driver and firmware

  • Testbed A : U Cambridge, UK

    • 21 APs, 30 client

  • Technical Characteristics

    • Nodes: Soekris net4826,

    • Wireless cards: Intel 2915 a/b/g

      • 5-dBi omnidirectional antennae

  • Experiments late at night, to avoid external interference

    • Both in 802.11a and 802.11g

Experiments at a glance
Experiments at a glance

  • We study each function in isolation

    • To understand the capabilities of each

  • We study all pairwise combinations

    • To undestand how each affects the other

  • We study the effect of all three of them

  • Methodology

    • Activate APs and clients in random order

    • Apply the algorithms

    • Run throughput measurements to observe gain due to each combination

Frequency selection fs in isolation
Frequency Selection (FS) in isolation

  • Should we always apply FS?

  • FS is always beneficial

    • FS outperforms Random Channel Selection (RCS) by 48% in 802.11a and by 65% in 802.11g

User assoc ua in isolation
User Assoc. (UA) in isolation

  • We observe that the performance due to UAis largely dependent on the level of contention

    • If the contention among APs is high, there is notmuch for clients to gain

    • Less contention = more throughput due to UA, when AP load is not balanced

  • Contention is lower in 802.11a than in 802.11g

    • UA is more favorable in 802.11a than in 802.11g

User assoc ua and freq selection fs
User Assoc. (UA) and Freq. Selection (FS)

  • We apply FS before UA

  • The combination of UA and FS is always beneficial!

    • The total network throughput becomes higher than the sum of throughputs in the isolated cases!

    • Much more in 802.11a than in 802.11g

Power control helps in some scenarios
Power Control helps in some scenarios

  • How does topology affect the ability of PC to shrink cells?

    • Five topological cases

  • Cases where PC improves performance

    • Case a

      • AP-client link strong (RSSI>-55 dBm)

      • AP-AP link weaker by at least k dBm (k = 15 to 20)

    • Case b

      • Both AP-AP and AP-client links strong (RSSI>-55 dBm)

        • Reduction in power not feasible

        • Increasing CCA makes APs ignore each others’ transmissions --> parallel transmissions possible

Power control does not always help
Power Control does not always help!

  • Cases where PC has no effect

    • Case c

      • AP-AP link stronger than AP-client link

        • Isolation is not possible

    • Case d

      • AP-client weak and AP-AP evenweaker

        • Power reduction reduces the AP-client link quality

        • With CCA increment, AP is disconnected from client

    • Case e

      • AP-client link stronger by k dBm than AP-AP link, k < 15 dBm

        • No cell isolation is possible

Power control pc and frequency selection fs
Power control (PC) and Frequency Selection (FS)

  • PC usually does not provide benefits without FS

    • Many co-channel links under cases (c) and (e)

    • With FS, remaining co-channel APs have reduced AP-AP link qualities

  • FS + PC is more beneficial in 802.11g than in 802.11a

    • After FS there is still significant contention in 802.11g, due to fewer channels

  • Power control pc and user association ua
    Power Control (PC) and User Association (UA)

    • PC in conjunction with UA, is usually not beneficial without FS!

      • UA may create long AP-client links

        • As long as a user discovers a lightly loaded AP that is going to provide lower delays, the client will associate to that AP

        • This reduces the AP-client link quality (RSSI) even more



    Applying all 3 functions
    Applying all 3 functions

    • Blindly applying all three algorithms may hurt the performance !

    24% throughput degradation

    when applying all 3 algorithms!!

    The need for a systematic approach
    The need for a systematic approach

    • We develop MDG (Measurement Driven Guidelines)

      • A framework for deciding when to apply each function

      • Based on the empirical observations

      • Measurement-based inputs:

        • Whether overlapping cells exist, so as to apply FS

        • Whether overloaded APs exist, so as to apply UA

        • Whether AP-AP and AP-client links are conducive for PC

    • Intuitively, MDG:

      • First mitigates interference

      • Second balances the load

    Building mdg
    Building MDG

    • Initial steps:

    • 1. Check if FS is beneficial

    • 2. If not, check if UA is beneficial

    Checking for contention after fs
    Checking for contention after FS

    • If FS resolves all interference, PC is not needed

      • If after FS there still exists contention among APs, then further steps depend upon whether the network employs 802.11a or 802.11g.

    802 11a ua or pc
    802.11a: UA or PC ?

    • In 802.11a, FS+UA is more beneficial than FS+PC

      • Applying FS almost eliminates cell overlaps

      • When contention is limited, it is preferable to apply UA rather than PC

    802 11g ua or pc
    802.11g: UA or PC ?

    • In 802.11g, FS+PC is more beneficial than FS+UA

      • FS does not eliminate cell overlaps, due to the limited number of channels

      • FS+PC boosts the network performance

    Mdg measurement driven guidelines for 802 11 wlan design

    • The MDG diagram looks really cool, but does it really work? :-)

    Validating mdg on testbed b
    Validating MDG on TestBed-B

    • We validate MDG on a second, different network

      • UC Riverside Wireless Testbed

      • Different scale and environmental factors

      • 8 APs, 20 clients

    • Validation procedure

      • Apply MDG

      • Evaluate the performance

    Validating mdg 802 11a
    Validating MDG: 802.11a

    MDG discovers the path that provides the highesttotal network throughput in 802.11a

    Validating mdg 802 11g
    Validating MDG: 802.11g

    MDG discovers the path that provides the highesttotal network throughput in 802.11g

    Further testing of mdg performance
    Further Testing of MDG Performance

    • Does external interference affect MDG?

      • MDG performs well even in the presence of other LANs (during daytime)

    • MDG is better than any random network configuration tested

      • Random channel selection

      • Random client affiliation

      • Random PTX and CCA, constant C = PTX * CCA

    MDG provides the best performance, compared to 40 other random configurations


    • MDG maximizes the synergy between frequency selection, user association and power control

    • MDG is measurement driven

      • Relying on the fundamental understanding of the inter-dependencies between the three functions/algorithms

      • Grounded on conditions that make the topology conducive to each function

    • We validate the efficiency of MDG on a different network

    • MDG is useful for network management in WLANs in practice

    Thank you
    Thank you!

    • Questions ?