idle sense

Upload: blackspideymak

Post on 02-Jun-2018

235 views

Category:

Documents


0 download

TRANSCRIPT

  • 8/11/2019 Idle Sense

    1/56

    1 of 56

    Idle Sense:

    An Optimal Access Method for High

    Throughput and Fairness in Rate Diverse

    Wireless LANs

    Martin Heusse Franck Rosseau

    Romaric Guillier Andrzej Duda

    Presented by Nikki Benecke, October 10th, 2006 for CS577

  • 8/11/2019 Idle Sense

    2/56

    2 of 56

    Objective:

    Define an access methodoptimized for

    throughput and fairness, able to

    dynamically adapt to physical channelconditions, to operate near optimum for a

    wide range of error rates, and to provide

    equal time shares when hosts usedifferent bit rates.

  • 8/11/2019 Idle Sense

    3/56

    3 of 56

    Access method

    Way of deciding who can access the media

    at a given time

    In WLANs, CSMA/CA as implemented by

    DCF (required) or PCF

  • 8/11/2019 Idle Sense

    4/56

    4 of 56

    Optimized for throughput and

    fairness Throughput

    Not goodput

    Expect to maintain, not increase, with IS

    Fairness inIdle Sense:

    Jain Index

    Time Fairness

  • 8/11/2019 Idle Sense

    5/56

    5 of 56

    Dynamic channel adaptation

    Physical conditions vary wildly with time

    Frames received in error -> senders bitrate

    lowered to reduce error rate

    Idle Sense tries to intelligently decide when

    lowering the bitrate is worthwhile

  • 8/11/2019 Idle Sense

    6/56

    6 of 56

    Supporting a wide range of error

    rates Sort of superfluoushandled by dynamic

    channel adaptation

  • 8/11/2019 Idle Sense

    7/56

    7 of 56

    Equal time shares

    Step away from min-max fairness

    Keep slow senders from unnecessarily

    restricting fast senders

  • 8/11/2019 Idle Sense

    8/56

    8 of 56

    Motivation

    802.11 currently requires DCF as the access

    method

    Also allows PCF

    Idle Sense addresses some key problems

    with DCF

  • 8/11/2019 Idle Sense

    9/56

    9 of 56

    DCF: Operation

    [dcf in a nutshell]

    Martin Heusses presentation at SIGCOMM 05

  • 8/11/2019 Idle Sense

    10/56

    10 of 56

    DCF: Backoff

  • 8/11/2019 Idle Sense

    11/56

    11 of 56

    DCF: Bad Day Problem

    Bad transmission conditions -> host will lose many

    frames

    High error rate -> frequent backoffs

    CW is increased -> transmission attempt probability is

    lower

    So the host will try to send less often and may eventually

    starve!

  • 8/11/2019 Idle Sense

    12/56

    12 of 56

    DCF: Physical Layer Capture

    [physical layer capture]

    Martin Heusses presentation at SIGCOMM 05

  • 8/11/2019 Idle Sense

    13/56

    13 of 56

    Two Modifications to DCF

    1. No exponential backoff

    2. All senders have equal CW

  • 8/11/2019 Idle Sense

    14/56

  • 8/11/2019 Idle Sense

    15/56

    15 of 56

    Channel contention

    For a transmission to occur, one host must try to

    send and all others must be idle, so:

  • 8/11/2019 Idle Sense

    16/56

    16 of 56

    Channel contention

    - probability that no hosts are sending

    - probability of a collision

  • 8/11/2019 Idle Sense

    17/56

    17 of 56

    Channel contention

    - average number of consecutive idle

    slots

  • 8/11/2019 Idle Sense

    18/56

    18 of 56

    Channel contention

    Since all CW are equal:

  • 8/11/2019 Idle Sense

    19/56

    19 of 56

    Channel contention

    Throughput as a function of :

  • 8/11/2019 Idle Sense

    20/56

    20 of 56

    Channel contention

    Goal: maximize the throughput by

    minimizing the time spent in collisions and

    contention

  • 8/11/2019 Idle Sense

    21/56

    21 of 56

    Channel contention

    Maximizing is equivalent to

    minimizing defined as:

  • 8/11/2019 Idle Sense

    22/56

    22 of 56

    Channel contention

    Define as

    Take the first derivative of the cost function toget:

    is the unique solution for this equation

    that is in [0,1]

  • 8/11/2019 Idle Sense

    23/56

    23 of 56

    Formulas

  • 8/11/2019 Idle Sense

    24/56

    24 of 56

    value based on variant

    This is because it is based on the ts/tc ratio,

    which varies by flavor of 802.11

    = 0.1622 for 802.11b

    (important for later use)

  • 8/11/2019 Idle Sense

    25/56

    25 of 56

    Idle Sense: Principles

    Each host estimates and uses it to compute

    its CW

    By adjusting CW, a host makes

    converge to (common across hosts)

  • 8/11/2019 Idle Sense

    26/56

    26 of 56

    vs

    requires knowing N (# of hosts), which

    we would like to avoid

  • 8/11/2019 Idle Sense

    27/56

    27 of 56

    vs

    quickly approaches , though,

    so we can use this value as

    with little penalty

  • 8/11/2019 Idle Sense

    28/56

    28 of 56

    Channel adaptation

  • 8/11/2019 Idle Sense

    29/56

    29 of 56

    Channel adaptation

  • 8/11/2019 Idle Sense

    30/56

    30 of 56

    Adaptation example

    802.11b (nitarget = 5.68)

    N = 5

    CW = 60

    = 1.2

    = 0.001

  • 8/11/2019 Idle Sense

    31/56

    31 of 56

    Adaptation example

    1. CW = 60 -> = 0.033, Pi = 0.847,

    = 5.53

    Ni < 5.68, so increase CW

    CW = = 1.2 x 60 = 72

    (Multiplicative decrease)

  • 8/11/2019 Idle Sense

    32/56

    32 of 56

    Adaptation example

    2. This increase leads to = 6.71

    6.71 > 5.68, so decrease CW

    CW =

    CW = 69(Additive Increase)

  • 8/11/2019 Idle Sense

    33/56

    33 of 56

    Adaptation example

    3. Ni is now 6.41, still greater than 5.68.

    So decrease again, CW gets value 67.

    As this continues, we will oscillate around

    (Additive Increase)

  • 8/11/2019 Idle Sense

    34/56

    34 of 56

    Time-fairness

    Argument: using min-max fairness restricts

    fast hosts by making them send as slowly as

    slow hosts

  • 8/11/2019 Idle Sense

    35/56

    35 of 56

    Time-fairness

    Using time-fairness eliminates two

    problematic situations:

    (i) Slower hosts limit the throughput of fasterhosts

    (ii) Slow hosts suffer starvation because an

    access point will not switch to a slowerrate

  • 8/11/2019 Idle Sense

    36/56

    36 of 56

    Time-fairness

    They say TF is better for both slow hosts and

    fast hosts.

  • 8/11/2019 Idle Sense

    37/56

    37 of 56

    Time-fairness

    InIdle Sense, accomplish TF by controlling

    the access probability for the hosts

    Slow host transmitting at bit rate rcurr

    receives a modified CW, CW =

    CW x , so scales down.

  • 8/11/2019 Idle Sense

    38/56

  • 8/11/2019 Idle Sense

    39/56

    39 of 56

    Idle Sense: Performance

    Developed discrete-event simulator thatimplements 802.11 DCF andIdle Sense

    No source code provided

    Three variants of 802.11

    802.11b, 802.11g, theoretical 100Mb/s 802.11

    Idle Senseparameters:

    = 68.17 for 802.11b= 31.0 for 802.11g

    = 19.3 for 100Mb/s 802.11

  • 8/11/2019 Idle Sense

    40/56

    40 of 56

    Performance

    Simulations run for 106transmissions

    Experimentally determined parameters

    = 0.001

    = 1.2

    Ntrans= 5 (see figure 6)

  • 8/11/2019 Idle Sense

    41/56

    41 of 56

    Throughput

    Compare 802.11b DCF, Slow Decrease,

    Asymptotically Optimal Backoff (AOB)

    andIdle Sensefor an increasing number ofhosts

    Throughput is the average of the throughput

    for all hosts active in the network

  • 8/11/2019 Idle Sense

    42/56

    42 of 56

    Throughput

    [figure 7]

  • 8/11/2019 Idle Sense

    43/56

    43 of 56

    Throughput

  • 8/11/2019 Idle Sense

    44/56

    44 of 56

    Throughput: Conclusion

    Authors expected throughput to stay around

    that of DCF

    Throughput actually improves slightly

  • 8/11/2019 Idle Sense

    45/56

  • 8/11/2019 Idle Sense

    46/56

    46 of 56

    Delay

    K = # of intertransmissions between other

    hosts between two transmissions of a given

    hostFor larger K, hosts experience greater delays

  • 8/11/2019 Idle Sense

    47/56

    47 of 56

    Delay

    [table 5]

    Hosts should experience significantly less

    delay!

  • 8/11/2019 Idle Sense

    48/56

    48 of 56

    Collision Overhead

    [table 3]

    of DCF

  • 8/11/2019 Idle Sense

    49/56

    49 of 56

    Convergence Speed

    Start out with 5 greedy hosts, add another 5 at 2000, drop 5 at 3000

    Stabilizes quickly after every change

  • 8/11/2019 Idle Sense

    50/56

    50 of 56

    Time fairness

    One 1Mb/s host and N-1 11Mb/s hosts

    By TF, fast hosts should get 11 x the

    throughput of the slow host in Idle Sense

  • 8/11/2019 Idle Sense

    51/56

    51 of 56

    Time fairness

    In DCF, all hosts get the slow hosts

    throughputtput much lower than in IS!

  • 8/11/2019 Idle Sense

    52/56

  • 8/11/2019 Idle Sense

    53/56

    53 of 56

    Possible weaknesses?

    No info provided on actual simulation

    tool/we cant recreate experiments

    Some experimentally derived parameters

    How is this actually implemented?

    What happens if some senders have IS and

    others dont?

    Is time-fairness really fair?

  • 8/11/2019 Idle Sense

    54/56

  • 8/11/2019 Idle Sense

    55/56

    55 of 56

    Questions/Comments

  • 8/11/2019 Idle Sense

    56/56

    56 f 56

    Acknowledgements

    Most figures taken directly from the paper

    Slide 10 is from the 1999 801.11 technical

    specifications (Section 9.2, DCF)

    Slides 9 & 12 and the Bad Day and

    physical capture ideas are from Martin

    Heusses presentation ofIdle SenseatSIGCOMM 05