Enhanced DCF of IEEE 802.11e to support QoS - Wireless by broverya76

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									      Enhanced DCF of IEEE 802.11e to Support QoS
                                                             Yang Xiao
                                                   Computer Science Division
                                         The University of Memphis, Memphis, TN 38152
                                                   E-mail: yangxiao@ieee.org

Abstract—In this paper, we will introduce the emerging IEEE         In this paper, we focus us on EDCF, but not HCF.
802.11e standard to support quality of service at medium access        Several priority studies have been reported in the literature
control level. One of the most important functions in 802.11e is    for DCF. Deng and Chang [2] proposed a priority scheme by
the contention-based channel access mechanism called enhanced
                                                                    differentiating the backoff window: the higher priority class
distributed coordination function (EDCF), which provides a
priority scheme by differentiating the inter-frame space and the    uses the window [0,2i+1 − 1] and the lower priority class uses
initial window size. We propose an analytical model to evaluate     the window [2i +1 ,2i + 2 − 1] , where i is the backoff stage. In [3],
the EDCF priority scheme. Saturation throughput and saturation      Aad and Castelluccia proposed a priority scheme by
delay are derived analytically. Simulations are also conducted to   differentiating inter-frame spaces (IFS’s), in which a higher
validate analytical results. Our study shows that differentiating
                                                                    priority class uses IFS, whereas a lower priority class uses a
the initial window size is better than differentiating the inter-
frame space in terms of total throughput and delay.                 space that equals the sum of IFS and the maximum window
                                                                    size. In [4], Veres and Campbell et al. proposed priority
   Keywords- IEEE 802.11 protocol, performance evaluation.          schemes by differentiating the minimum backoff window size
                                                                    and the maximum window size. In [22], Pallot and Miller
                      I.    INTRODUCTION                            proposed three priority schemes: static priority scheduling,
                                                                    prioritized DIFS time mechanism, and prioritized backoff time
The IEEE 802.11 enables fast installation, with minimum             distribution mechanism. In [23], Vaidya et al. proposed a fair
management and maintenance costs, and is a very robust              scheduling method. All the schemes [2-4, 22-23] are based on
protocol for the best–effort service in the wireless medium, but    simulations.
it is unsuitable for multimedia applications with Quality of           There have been many performance studies for IEEE
Service (QoS) requirements.                                         802.11 [5-18]: both simulation [18] and analytical [14] studies.
   IEEE 802.11 medium access control (MAC) employs a                Bing [12, 17] provided a good performance analysis by a
mandatory contention-based channel access function called           quantitative approach. Chhaya and Gupta [15] calculated the
Distributed Coordination Function (DCF), and an optional            throughput of CSMA/CA using a simple model with the
centrally controlled channel access function called Point           probabilities of capture and the presence of hidden stations.
Coordination Functions (PCF) [1]. DCF adopts a carrier sense        Bianchi [5, 11] and Ziouva et al. [6] presented an accurate
multiple access with collision avoidance (CSMA/CA) with             analysis to compute the saturation throughput performance.
binary exponential backoff. DCF does not support QoS, and           Huang, Ho, and Chen [13, 14] gave approximate models that
even cannot support any priority scheme. PCF can support            account for hidden terminals and capture effects. Cali, Conti
very limited QoS. It is an optional function, and has not been      and Gregori [9, 10] improved the performance by tuning
implemented in most of current products. To support MAC-            backoff strategies, and provided an excellent performance
level QoS, the IEEE 802.11 standardization committee is             analysis. Tay and Chua [15] provided a good approximation of
                                                                    saturation throughput. All theses schemes [5-18] are for
currently working on IEEE 802.11e [20], a supplement to the
                                                                    original IEEE 802.11 DCF.
original IEEE 802.11 MAC. The IEEE 802.11e MAC will
                                                                       There are very limited reports in the literature about IEEE
support multimedia applications such as voice and video over
                                                                    802,11e since it is new. Mangold and Choi et al. [21]
the IEEE 802.11 WLANs.                                              introduced IEEE 802.11e and provided performance
   The IEEE 802.11e MAC also employs a contention-based             evaluation via simulations.
channel access function called Enhanced Distributed                    In this paper, we will introduce EDCF priority scheme in
Coordination Function (EDCF), and a centrally controlled            section III, and based on Bianchi’s [5] and Ziouva’s [6]
channel access function called Hybrid Coordination Function         models, propose an analytical model to evaluate the EDCF
(HCF). EDCF provides a priority scheme by differentiating           priority scheme in section IV. Saturation throughput and
the inter-frame space and the initial window size. Such a           saturation delay are derived analytically. Simulation and
priority scheme may not provide guaranteed QoS but                  analytical results are studied in section V.
prioritized QoS due to the contention-based nature. Prioritized
QoS will be useful for those multimedia applications that can                            II.    CSMA/CA IN DCF
live without rigid QoS. One advantage of prioritized QoS is            We will briefly introduce DCF’s CSMA/CA mechanism,
that it is simple to implement and looks like DiffServ model.       which is useful for understanding our analytical model.



0-7803-7700-1/03/$17.00 (C) 2003 IEEE                           1291
Readers can refer to [1] for details. In DCF, a station with a                                   in the original IEEE 802.11 MAC. IEEE 802.11e allows a
frame to transmit monitors the channel activities until an idle                                  station to have fewer queues, but we will not address this
period equal to a distributed inter-frame space (DIFS) is                                        situation in this paper.
detected. After sensing an idle DIFS, the station waits for a                                        Assuming that a payload from a higher layer is labeled with
random backoff interval before transmitting. The backoff time                                    a priority value, it is pushed into the corresponding queue with
counter is decremented in terms of slot time as long as the                                      the same priority value. Each queue acts as an independent
channel is sensed idle. The counter is stopped when a                                            MAC entity and performs similar DCF function, introduced in
transmission is detected on the channel, and reactivated when                                    the previous section, with a different inter-frame space and a
the channel is sensed idle again for more than a DIFS. The                                       different initial window size.
station transmits its frame when the backoff time reaches zero.
At each transmission, the backoff time is uniformly chosen in
the range (0, CW − 1) , where CW is the current backoff                                                                      IV.      AN ANALYTICAL MODEL
window size. At the very first transmission attempt, CW equals                                      Assume that any queue in any station always has frames
the minimum backoff window size CWmin . After each                                               ready to send. If further assuming that there are n active
unsuccessful transmission, CW is doubled until a maximum                                         stations in a Basic Service Set (BSS) and each station
backoff window size value is reached. After the destination                                      implements all 8 queues, there are total 8n queue entities,
station successfully receives the frame, it transmits an                                         which are equivalent to 8n stations in the original IEE 802.11
acknowledgment frame (ACK) following a short inter-frame                                         DCF. We further lose above assumption so that we let
space (SIFS) time. If the transmitting station does not receive                                   ni (i = 0,..., 7) denote the number of active queues in the
the ACK within a specified ACK Timeout, or it detects the                                        priority i class. We only consider the basic access mechanism
transmission of a different frame on the channel, it reschedules                                 in this paper. The method in this paper can be easily applied to
the frame transmission according to the previous backoff rules.                                  the optional RTS/CTS mechanism.

         III.     ENHANCED DCF (EDCF) PRIORITY SCHEME                                             A. An Analytical Model
   The contention-based channel access function of IEEE                                              For a given station in the priority i class (i = 0,...7) , b(i, t )
802.11e is EDCF, in which multiple queues (up to 8) are used                                      is defined as a random process representing the value of
for different priorities (up to 8). Priorities are achieved by                                    backoff counter at time t , and s(i, t ) is defined as the random
differentiating the arbitration inter-frame space and the initial                                    process representing the backoff stage j , where 0 ≤ j ≤ m
window size. The traffic is classified into 8 priority
                                                                                                     and m is the maximum backoff stage. The value of the
classes: i = 0,...7 . For the priority i class, the minimum
                                                                                                     backoff counter is uniformly chosen in the range
backoff window size is CWmin [i ] , and the arbitration inter-
                                                                                                     (0,1,...Wi , j − 1) , where Wi , j = 2 j Wi ,0 and Wi ,0 = CWmin [i ] (we
frame space is AIFS [i ] . If the priority i class has a lower
priority than the priority              j   class, we have                                           use a different notation for convenience). Let pi (i = 0,...7)
CWmin [i ] ≥ CWmin [ j ] , and AIFS [i ] ≥ AIFS [ j ] , and at least one                             denote the probability that a transmitted frame collides, and
                                                                                                      pb denote the probability that the channel is busy. Similar to
of above inequalities must be a real inequality. In other words,
EDCF adopts AIFS [i ] ( i = 0,1,...,7 ) and CWmin [i ]                                               Bianchi’s model [5] and Ziouva’s model [6], the bi-
                                                                                                     dimensional random process {s (i, t ), b(i , t )} is discrete-time
( i = 0,1,...,7 ) instead of DIFS and CWmin , respectively. If one
                                                                                                     Markov chain under the assumptions that the probability pi
class has a smaller AIFS or CWmin , the class’s traffic has a
better chance to access the wireless medium earlier.                                                 and the probability pb are both independent to the backoff
    Fig. 1 shows the EDCF timing diagram, where 3 priorities                                         procedure. Therefore, the state of each station in the priority i
are shown: i , j , and k . We can see that AIFS [i ] > PIFS ,                                        class is described by {i, j, k } , where i is just for an index
where PIFS is point (coordination function) inter-frame                                              purpose standing for the priority i class, j stands for the
space.                                                                                               backoff stage and takes values (0,1,...m ) , and k stands for the
                                             AIFS[k]
                                                                                                     backoff delay and takes values (0,1,...Wi , j − 1) in timeslots.
  Immediate access when               AIFS[j]

  Medium is free >= AIFS [i]       AIFS[i]                                                           The state transition diagram for the priority i class is shown
                                                   Contention Window from
      AIFS[i]                        PIFS          0 to CWmin[i]                                     in Fig. 2, where the state {i, −1, 0} stands for the state that the
                 Busy Medium
                                    SIFS
                                                       Backoff-Window           Next Frame           station senses the channel idle after AIFS [i ] and transmits
                                                       Slot time                                     successfully without activating the backoff stage.
                      Defer Access                Select Slot and Decrement Backoff as long             The non-null transition probabilities are listed as follows.
                                                                         as medium is idle
                                                                                                        Pr{i , −1, 0 | i , −1, 0} = (1 − pi )(1 − pb )
                               Fig. 1 EDCF timing diagram
  Normally, each station implements 8 queues, and each                                                  Pr{( i,0, k ) | ( i, −1,0 )} = 1 − (1 − pi )(1 − pb ) Wi ,0 , for 0 ≤ k ≤ Wi ,0 − 1
                                                                                                                                                             
queue supports one priority, behaving as a single DCF entity                                            Pr{( i, j, k ) | ( i, j , k )} = pb , for 1 ≤ k ≤ Wi , j − 1 and 0 ≤ j ≤ m




                                                                                              1292
    Pr{( i , j , k ) | ( i , j , k + 1)} = 1 − pb , for 0 ≤ k ≤ Wi , j − 2 and 0 ≤ j ≤ m                                                                                                   Substituting (8)-(9) to (7), we can solve unknown
    Pr{( i,0, k ) | ( i , j, 0 )} = (1 − pi ) pb Wi ,0 , for 0 ≤ k ≤ Wi ,0 − 1 and 0 ≤ j ≤ m                                                                                            parameters numerically. Then, we can calculate pb and pi
    Pr{( i, −1,0) | ( i, j,0)} = (1 − pi )(1 − pb ) , for 0 ≤ j ≤ m                                                                                                                     from (8)-(9).
    Pr{( i, j, k ) | ( i, j − 1,0 )} = pi Wi , j , for 0 ≤ k ≤ Wi , j − 1 and 1 ≤ j ≤ m                                                                                                 B. Saturation Throughput
    Pr{( i, m, k ) | ( i, m,0)} = pi Wi ,m , for 0 ≤ k ≤ Wi ,m − 1                                                                                                                          Let ps ,i (i = 0,...7) denote the probability that a successful
                                   (1-pi)(1-pb)
                                                                                                                                                                                        transmission occurs in a slot time for the priority i class. We
         (1-pi)(1-pb)
                                         i,-1,0
                                                                              [1-(1-pi)(1-pb)]/Wi, 0
                                                                                                                                                                                        have
                                                                                                                                                                                                                             7
                                                                                                                                                                                                                                          nτ
                                                                                                                                                                                           ps ,i = niτ i (1 − τ i ) n −1 ∏ (1 − τ h ) n = i i (1 − pb )
                 (1-pi )pb/Wi, 0
                                                                                                                                                                                                                                     i
                                                                                                                                                                                                                                                        (10)      h


                                                          1-pb                      1-pb
                                                                                                                                                                                                                        h = 0, h ≠ i     1 −τ i
                                                                                                             1-pb       1-pb                       1-pb
                                                                                                                    …
                                         i,0,0

                                                               pb
                                                                         i,0,1                    i,0,2                             i,0,Wi, 0-2                i,0,Wi, 0-1
                                                                                                                                                                                           Let Si (i = 0,...7) denote the normalized throughput for the
                                                                                        pb                                 pb                             pb

                                                          pi/Wi, 1
                                                                                                                                                                                        priority i class. Let δ , L , Ts ,i , and Tc ,i denote the duration
                                           .                              .                      .                                         .                          .
                                           .                              .                      .                                         .                          .                 of an empty slot time, payload size, the average time that the
                                           .                              .                      .                                         .                          .
                                                                                                                                                                                        channel is sensed busy because of a successful transmission
                                         i,j-1,0                          .                      .                                     .                          .                     for the priority i class, and the average time that the channel
                                                                                       pi/Wi, j

                                                          1-pb                      1-pb
                                                                                                                                                                                        has a collision for the priority i class, respectively. We have,
                                                                                                                                                                                                   E ( payload information transmitted in a slot time for the priority i class )
                                                                                                             1-pb       1-pb                       1-pb
                                          i,j,0                          i,j,1                       i,j,2          …               i, j,Wi, j-2               i, j,Wi, j-1
                                                                 pb                        pb                                                                                               Si =                                                                                 (11)
                                                                                                                               pb                         pb                                                               E ( length of a slot time )
                                                                                                                                                                                                                        p s ,i E ( L )
                                                                                                                                                                                              =
                                           .
                                                               pi/Wi, j+1
                                                                          .                      .                                         .                          .                            (1 − pb ) δ + ps ,iTs ,i +  pb − ps ,i  Tc,i
                                                                                                                                                                                                                                          
                                           .                              .                      .                                         .                          .
                                           .                              .                      .
                                                                                             pi/Wi, m
                                                                                                                                           .                          .                     Let TH , TE ( L ) , TACK , SIFS , L * , TE ( L*) , and γ denote the
                                                          1-pb                      1-pb                     1-pb
                                                                                                                    …
                                                                                                                        1-pb                       1-pb
                                                                                                                                                               i, m, Wi, m-1
                                                                                                                                                                                        time to transmit the header (including MAC header, PHY
                                       i, m, 0                        i, m, 1                   i, m, 2                         i, m, Wi, m-2

                                                                    pb                     pb                                                              pb
                                                                                                                                                                                        header, and/or tail), the time to transmit the payload, the time
                                                                                                                                pb
                                                                                                                                                                                        to transmit the ACK, the time of SIFS, the length of the
                                                                 pi/Wi, m
                                                                                                                                                                                        longest frame in a collision, the time to transmit a payload
                Fig.2 The state transition diagram for the priority                                                                             i class                                 with length E ( L*) , and the time of the propagation delay,
    Let         bi , j ,k = lim Pr{s (i , t ) = j , b(i , t ) = k }                                                      be                    the stationary                           respectively. We have,
                                   t →∞

distribution of the Markov chain. In steady state, we can                                                                                                                                  Ts ,i = TH + TE ( L ) + SIFS + γ + TACK + AIFS [i ] + γ (12)
derive following relations through chain regularities.                                                                                                                                      Tc ,i = TH + TE ( L*) + AIFS [i ] + γ                                                                            (13)
   bi , j ,0 = pi j bi ,0,0 0 ≤ j ≤ m -1               (1)
                                                                                                                                                                                        C. Saturation Delay
                 pm
    bi ,m,0   = i bi ,0,0                                                                                                                                                      (2)        The conditional probability p s ,i pb ( i = 0,...7)                                                           is    the
               1 − pi
                                                                                                                                                                                        probability that a transmitted frame for the priority i class is
                   Wi , j − k                1
    bi , j ,k =                                  bi , j ,0 0 ≤ j ≤ m,1 ≤ k ≤ Wi , j -1                                                                                         (3)      successfully transmitted. One over this probability is the
                         Wi , j           1 − pb                                                                                                                                        average number of retransmissions for the priority i class, i.e.
                1 − (1 − pi )(1 − pb )                                                                                                                                         (4)       pb ps ,i . Let N c ,i (i = 0,...7) denote the random variable
    bi ,0,0 =                          bi , −1,0
                       1 − pb
                                                                                                                                                                                        representing the number of collisions before transmitting a
                        m Wi , j −1
                                                                                                                                                                                        frame for the priority i class. We have
    bi ,−1,0 + ∑              ∑b                            =1                                                                                                                 (5)
                                                                                                                                                                                           E ( N c , i ) = p b p s ,i − 1
                                               i , j ,k
                     j =0 k =0                                                                                                                                                                                                                 (14)
  Let α1 = 2 (1 − pb ) 2 (1 − pi )(1 − 2 pi ) and α 2 = [1 − (1 − pi )(1 − pb )] .                                                                                                         The average backoff delay depends on the value of a
Plugging (1)-(4) into (5), we have,                                                                                                                                                     station’s backoff counter and the duration when the counter
                                             α1                                    (6)                                                                                                  freezes due to others’ transmissions. Let X i ( i = 0, ...7) denote
   bi ,−1,0 =
              α1 + α 2  (1 − 2 pi ) + (1 − pi + 2m pi m+1 − pi m+1 2 m+1 )Wi ,0 
                                                                                
                                                                                                                                                                                        the random variable representing the time interval during
                                                                                                                                                                                        which the counter reaches zero without considering the case
    Let     τ i be the probability that a station in the priority                                                                                                                i      when the counter freezes. We have
class transmits during a generic slot time. We have                                                                                                                                                       m Wi , j −1                           bi ,0,0  Wi ,0 2 1 − pi − 3 pi ( 4 pi )  + 4 pi − 1
                                                                                                                                                                                                                                                                                      m
                                                                                                                                                                                                                                                                                                             (15)
                                                                                                                                                                                            E ( Xi ) = ∑                                                                                
           m
                                            2 (1 − pb )(1 − 2 pi )                                                                                                             (7)                             ∑ kb                      =
                                                                                                                                                                                                                                             6 (1 − pb )             (1 − 4 pi )(1 − pi )
   τ i = ∑ bi , j ,0 =
                                                                                                                                                                                                                          i , j ,k
                                                                                                                                                                                                          j = 0 k =1
                               (1 − 2 pi ) + (1 − pi + 2m pi m+1 − pi m+1 2m +1 ) Wi ,0 
                       α1 + α2 
         j =−1
                                                                                                                                                                                          Let Fi (i = 0,...7) and N F (i = 0,...7) denote the time that the
                                                                                                                                                                                                                      i
                          7
     pb = 1 − ∏ (1 − τ h )                                nh
                                                                                                                                                                               (8)      backoff counter of a station freezes and the number of times
                         h =0                                                                                                                                                           that the backoff counter freezes, respectively, for the priority
               i −1                               7               
     pi = 1 −  ∏ (1 − τ h ) nh  (1 − τ i ) ni −1  ∏ (1 − τ h ) nh                                                                                                          (9)       i class. The mean number of consecutive idle slot times is
               h =0                               h =i +1         




                                                                                                                                                                                     1293
(1 − pb )    pb . Therefore, we have,                                                                                       results match pretty well. As the number of active queues
                       E( X i )
                                                                                                                            increases, saturation throughputs for both classes decrease,
     ( )
   E N Fi =                           −1                                                                          (16)      and saturation delays for both classes increase. Fig. 4 also
                 max((1 − pb ) pb ,1)
                                                                                                                            shows that class 1 is more sensitive to the number of active
   Since     ps ,i     is the probability that a successful transmission                                                    queues than class 0. As illustrated in the figures, the class 0
occurs in a slot time, the probability that the transmitted frame                                                           has a much better saturation throughput and saturation delay
is successful is ps ,i pb . Therefore, we have                                                                              than the class 1. Therefore, the EDCF priority scheme is quite
                                                                                                                            effective.
                     p                 p                                                                        (17)
                 ( )
   E ( Fi ) = E N Fi  s ,i Ts ,i + (1 − s ,i )Tc ,i                                                                                            x 10
                                                                                                                                                        6



                      pb                pb          
                                                                                                                                            4




   Let Bi (i = 0,...7) denote the backoff delay of a station for                                                                           3.5




the priority i class before accessing the channel under busy                                                                                3




channel condition. We have                                                                                                                 2.5                                                           Priority
                                                                                                                                                                                                         Priority
                                                                                                                                                                                                                    0
                                                                                                                                                                                                                    0
                                                                                                                                                                                                                        class:
                                                                                                                                                                                                                        class:
                                                                                                                                                                                                                                 Simulation
                                                                                                                                                                                                                                 Analytic


   E ( Bi ) = E ( X i ) + E ( Fi )
                                                                                                                                                                                                         Priority   1   class:   Simulation

                                                           (18)
                                                                                                                                                                                                         Priority   1   class:   Analytic
                                                                                                                                            2




   Let Di (i = 0,...7) denote the random variable representing the                                                                         1.5




frame delay for the priority i class. Let To denote the time
                                                                                                                                            1



                                                                                                                                           0.5

that a station has to wait when its frame transmission collides
before sensing the channel again. Let Ttimeout denote the
                                                                                                                                            0
                                                                                                                                                 5          10    15         20         25          30              35                40      45
                                                                                                                                                                              Number of active queues



duration of the ACK timeout in time slots. We have                                                                                                          Fig. 4 Saturation Delay (µ sec.)

   E ( Di ) = E ( N c ,i )  E ( Bi ) + Tc ,i + To  + E ( Bi ) + Ts ,i
                                                                                                                (19)                    0.55




     To = SIFS + Ttimeout                                                                                         (20)
                                                                                                                                           0.5



                                                                                                                                          0.45



                                                                                                                                                                                                         Priority 0 class: Simulation

             V.                 NUMERICAL AND SIMULATION RESULTS
                                                                                                                                           0.4
                                                                                                                                                                                                         Priority 0 class: Analytic
                                                                                                                                                                                                         Priority 1 class: Simulation
                                                                                                                                                                                                         Priority 1 class: Analytic
                                                                                                                                          0.35

  The simulation models had been developed based on the
                                                                                                                                           0.3

IEEE 802.11e draft [20], the IEEE 802.11a standard [8], and
OPNET Wireless LAN simulation model version 8.0A.
                                                                                                                                          0.25




Parameters of IEEE 802.11a can be found in [7-8], as well as                                                                               0.2




how to calculate TH + TE ( L ) accurately [7]. Both the data rate                                                                         0.15
                                                                                                                                              15             20         25       30            35
                                                                                                                                                                                 CWmin of class 1
                                                                                                                                                                                                              40                    45        50




and the control rate are 6Mbps. The frame size is fixed as                                                                                       Fig. 5 Saturation Throughput (Normalized)
1024 bytes. For demonstration purpose, we adopt two priority                                                                 B. Effects of the initial window size
classes instead of 8 priorities.
                                                                                                                               Fig. 5 and Fig. 6 have following parameters:
A. Effectiveness of the EDCF priority scheme                                                                                 AIFS [0] = AIFS [1] = PIFS , W = 16 and n0 = n1 = 10 . Fig. 5 (Fig. 6)
                                                                                                                                                                  0,0


                     0.55
                                                                                                                            shows saturation throughputs (saturation delays) over the
                                                                                                                            minimum window size of class 1, W1,0 , which changes from
                                                                        Priority   0   class:   Simulation
                                                                        Priority   0   class:   Analytic
                                                                        Priority   1   class:   Simulation
                      0.5                                               Priority   1   class:   Analytic



                     0.45
                                                                                                                            16 to 48. As illustrated in Fig. 5 (Fig. 6), when                                                                      W1,0 = 16 ,

                      0.4
                                                                                                                            saturation throughputs (saturation delays) are the same for
                     0.35
                                                                                                                            both classes. As W1,0 increases, the saturation throughput of
                      0.3
                                                                                                                            class 1 decreases, the saturation throughput of class 0
                                                                                                                            increases, the saturation delay of class 0 decreases, and the
                                                                                                                            saturation delay of class 1 increases dramatically. Therefore,
                     0.25



                      0.2
                                                                                                                            saturation delay is very sensitive to the minimum window size.
                     0.15
                            5     10   15   20         25          30              35                40      45             We also observe in Fig. 6, as W1,0 increases, the saturation
                                             Number of active queues


                            Fig. 3 Saturation Throughput (Normalized)                                                       delay of class 0 does not change a lot, and it is good for delay
   Fig.      3     and Fig. 4 have following parameters:                                                                    sensitive applications. An interesting observation in Fig. 5 is
[ AIFS [0], AIFS [1]] = [ PIFS , DIFS ] , W0,0 ,W1,0  = [16,32] , and n0 = n1 ,
                                                     
                                                                                                                            that the throughputs of class 0 and class 1 are symmetric along
                                                                                                                            a line parallel to x-axis. This phenomenon indicates that class
where                    PIFS = SIFS + SLOT = 25 µ sec.                     and
                                                                                                                            0 can steal throughput from class 1 as the initial window size
 DIFS = SIFS + 2* SLOT = 34 µ sec. [1, 8]. Fig. 3 (Fig. 4) shows                                                            of class 1 increases, whereas the total throughput of all classes
saturation throughputs (saturation delays) for two priority                                                                 does not change much. The reason that class 0 can steal
classes over the number of active queues ( n0 or n1 ). As                                                                   bandwidth from class 1 is stated as follows. As the initial
illustrated in both figures, analytical results and simulation                                                              window size of class 1 increases, there are two direct effects.



                                                                                                                         1294
The first effect is that entities in class 1 will delay accessing                                      the saturation throughput and the saturation delay of class 0
the channel so that the saturation throughput of class 1 will                                          will remain the same.
decrease and the saturation delay of class 1 will increase. The                                           We can conclude that differentiating the initial window size
second effect is that as the initial window size of class 1                                            is a better than differentiating the inter-frame space in terms of
increases, collision probabilities of class 0 will decrease so                                         total throughput and delay. However, differentiating the inter-
that the probability that a transmitted frame from class 0 is                                          frame space gives a very fast way to access the channel in
successful becomes larger. Therefore, the saturation                                                   favor of the class with a short AIFS.
throughput of class 0 will increase and the saturation delay of                                                    0.3405


class 0 will decrease. In summary, differentiating the initial
                                                                                                                                                                      Priority 0 class: Simulation
                                                                                                                                                                      Priority 0 class: Analytic
                                                                                                                                                                      Priority 1 class: Simulation


window size has both the function of reducing collisions and
                                                                                                                     0.34                                             Priority 1 class: Analytic




the function of providing priorities,                                                                              0.3395


                       6
                    x 10
              2.5                                                                                                   0.339




                                                                                                                   0.3385
               2


                                                                                                                    0.338


              1.5
                                                                                                                   0.3375




                                                                                                                    0.337
               1                                                                                                         25        26   27   28    29          30     31          32          33     34
                                                                                                                                                  AIFS[1] (class 1)

                                                            Priority 0 class: Simulation
                                                            Priority 0 class: Analytic                                  Fig. 7 Saturation Throughput (Normalized)
                                                            Priority 1 class: Simulation
              0.5                                           Priority 1 class: Analytic                                         5
                                                                                                                            x 10
                                                                                                                    4.995




               0                                                                                                     4.99
                15         20     25     30           35         40             45         50
                                         CWmin of class 1


                           Fig. 6 Saturation Delay (µ sec.)                                                         4.985




C. Effects of the arbitration inter-frame space (AIFS)                                                               4.98




  Fig. 7 and Fig. 8 have following parameters:                                                                      4.975



AIFS [0] = PIFS , W0,0 = W1,0 = 16 and n0 = n1 = 10 . Fig. 7                                                         4.97                                             Priority 0 class: Simulation
                                                                                                                                                                      Priority 0 class: Analytic
                                                                                                                                                                      Priority 1 class: Simulation

(Fig. 8) shows saturation throughputs (saturation delays) over
                                                                                                                                                                      Priority 1 class: Analytic

                                                                                                                    4.965


the arbitration inter-frame space of class 1, AIFS [1] , which                                                       4.96

changes from PIFS to DIFS . As illustrated in the figures,
                                                                                                                         25        26   27   28    29          30     31          32          33     34
                                                                                                                                                  AIFS[1] (class 1)



when AIFS [1] = PIFS = 25 µ sec. , saturation throughputs                                                                           Fig. 8 Saturation Delay (µ sec.)

(saturation delays) are the same for both classes. As AIFS [1]                                      D. Effects of the number of active entities
increases, the saturation throughput of class 1 decreases                                              Fig. 9 and Fig. 10 have following parameters:
linearly, and the saturation delay of class 1 increases linearly.                                   [ AIFS [0], AIFS [1]] = [ PIFS , DIFS ] , W0,0 ,W1,0  = [16,32] , and n0 = 10 .
                                                                                                                                                         
An interesting observation is that both the saturation                                             Fig. 9 (Fig. 10) shows saturation throughputs (saturation
throughput and the saturation delay remain constant as
                                                                                                   delays) over the number of active queues of class 1, n1 , which
 AIFS [1] increases. This fact indicates that the arbitration
inter-frame space of class 1 does not affect the saturation                                        changes from 5 to 45. As illustrated in the figures, as n1
throughput and the saturation delay of class 0 as long as                                          increases, the saturation throughput of class 0 decreases, and
 AIFS [0] ≤ AIFS [1] . Compared to effects of the initial window                                   the saturation delay of class 0 increases. An observation is that
size in Fig. 5 and Fig. 6, effects of the arbitration inter-frame                                  the saturation throughput of class 1 increases as n1 increases.
space in Fig. 7 and Fig. 8 show very different characteristics.                                    The reason is that there are more active entities of class 1 so
The reason is that the initial window size and the arbitration                                     that the total saturation throughput of class 1 increases.
inter-frame space have different effects on priorities: the initial                                Another interesting observation is that the saturation delay of
window size has both the function of reducing collisions and                                       class 1 decrease at first and then increases later. From the
the function of providing priorities, whereas the arbitration                                      parameters, we know that class 0 has a higher priority than
inter-frame space has the function of providing priorities by                                      class 1, but how come that class 0 gets worse total throughput
accessing channel earlier/later, but not the function of                                           and delay than class 1 in Fig. 9 and Fig. 10, respectively, when
reducing collisions. Therefore, contrast to Fig.5 and Fig. 6, as                                   the number of active queues of class 1 is large? The reasons
the arbitration inter-frame space of class 1 increases, class 0                                    are stated as follows. There are more active entities of class 1
cannot steal bandwidth from class 1. The reasons are stated as                                     than class 0 so that the total saturation throughput of class 1
follows. As the arbitration inter-frame space of class 1                                           may be larger than that of class 0 even though the throughput
increases, entities in class 1 will delay accessing the channel                                    per entity in class 1 is smaller than that in class 0. To
so that the saturation throughput of class 1 will decrease and                                     understand the delay part becomes a little tricky. In fact, due
the saturation delay of class 1 will increase. However,                                            to different AIFSs and different initial window sizes for two
collision probabilities of class 0 will remain the same so that                                    classes, current windows of two classes have an overlap, and



                                                                                                1295
the size of such an overlap is changing dynamically. When the                                                                     [2]  D.-J. Deng and R.-S. Chang, “A priority Scheme for IEEE 802.11 DCF
                                                                                                                                       Access Method,” IEICE Trans. Communications., Vol. E82-B, No.1,
number of active entities of class 1 is very large, the number
                                                                                                                                       Jan. 1999, pp.96-102.
of collisions in the overlap portion become much more severe                                                                      [3] I. Aad and C. Castelluccia, “Differentiation Mechanisms for IEEE
than the non-overlap portion. Therefore, the delay                                                                                     802.11,“ IEEE INFOCOM 2001.
performance for class 0 depends on the ratio of the non-                                                                          [4] A. Veres, A. T. Campbell, M. Barry, and L.-H. Sun, “Supporting
                                                                                                                                       Differentiation in Wireless Packet Networks Using Distributed Control,”
overlap portion with the overlap portion in class 0. If the non-
                                                                                                                                       IEEE J-SAC, Vol. 19. No. 10, Oct. 2001, pp. 2081-2093.
overlap portion in class 0 is relatively small and the non-                                                                       [5] G. Bianchi, “IEEE 802.11--Saturation Throughput Analysis,” IEEE
overlap portion in class 1 is relatively large, the average delay                                                                      Communications Letters Vol. 2, No. 12, Dec.1998, pp. 318-320.
of class 0 could be larger than class 1.                                                                                          [6] E. Ziouva and T. Antonakopoulos, “CSMA/CA performance under high
                                                                                                                                       traffic conditions: throughput and delay analysis,” Computer
               0.6
                                                                                 Priority 0 class: Simulation                          Communications, 25 (2002), pp.313-321.
                                                                                                                                  [7] Y. Xiao and J. Rosdahl, “Throughput and Delay Limits of IEEE
                                                                                 Priority 0 class: Analytic
              0.55                                                               Priority 1 class: Simulation
                                                                                 Priority 1 class: Analytic

               0.5                                                                                                                     802.11,” IEEE Communications Letters, Vol. 6, No. 8, Aug. 2002, pp.
              0.45
                                                                                                                                       355-357.
               0.4
                                                                                                                                  [8] IEEE 802.11a WG, Part 11: Wireless LAN Medium Access Control
                                                                                                                                       (MAC) and Physical Layer (PHY) specification: High-speed Physical
              0.35
                                                                                                                                       Layer in the 5GHz Band, Sep. 1999.
               0.3
                                                                                                                                  [9] F. Calì, M. Conti, and E. Gregori, “Dynamic Tuning of the IEEE 802.11
              0.25                                                                                                                     Protocol to Achieve a Theoretical Throughput Limit,” IEEE/ACM
               0.2                                                                                                                     Trans. Networking, Vol. 8, No. 6, Dec. 2000, pp. 785-790.
              0.15
                                                                                                                                  [10] F. Cali, M. Conti, and E. Gregori, “IEEE 802.11 Protocol: Design and
                                                                                                                                       Performance Evaluation of an Adaptive Backoff Mechanism,” IEEE J-
                                                                                                                                       SAC, Vol. 18, No. 19, Sep. 2000, pp. 1774-1786.
               0.1
                     5              10     15      20           25           30                35             40      45
                                                 Number of active queues in class 1


                 Fig. 9 Saturation Throughput (Normalized)                                                                        [11] G. Bianchi, “Performance Analysis of the IEEE 802.11 Distributed
                         x 10
                                5
                                                                                                                                       Coordination Function,” IEEE J-SAC, Vol. 18, No. 3, Mar. 2000, pp.
                 18
                                                                                Priority   0   class:   Simulation                     535-547.
                                                                                                                                  [12] B. Bing and R. Subramanianb, “A novel technique for quantitative
                                                                                Priority   0   class:   Analytic
                 16                                                             Priority   1   class:   Simulation
                                                                                Priority   1   class:   Analytic


                 14
                                                                                                                                       performance evaluation of wireless LANs,” Computer Communications,
                                                                                                                                       Vol. 21, No. 9, July 1998, pp.833-838.
                                                                                                                                  [13] K. C. Huang and K.-C. Chen, "Interference analysis of nonpersistent
                 12


                 10
                                                                                                                                       CSMA with hidden terminals in multicell wireless data networks", Proc.
                     8                                                                                                                 IEEE PIMRC, Toronto, Canada, Sept. 1995, pp. 907-911.
                     6
                                                                                                                                  [14] T. S. Ho and K. C. Chen, "Performance evaluation and enhancement of
                                                                                                                                       the CSMA/CA MAC protocol for 802.11 wireless LAN's", Proc. IEEE
                     4
                                                                                                                                       PIMRC, Taipei, Taiwan, Oct. 1996, pp. 392-396.
                     2                                                                                                            [15] H. S. Chhaya and S. Gupta, "Performance modeling of asynchronous
                     0
                         5           10    15      20           25           30            35                40      45
                                                                                                                                       data transfer methods of IEEE 802.11 MAC protocol", Wireless
                                                 Number of active queues in class 1                                                    Networks, vol.3, pp. 217-234, 1997.
                                    Fig. 10 Saturation Delay (µ sec.)                                                             [16] Y. C. Tay and K. C. Chua, “A Capacity Analysis for the IEEE 802.11
                                                                                                                                       MAC Protocol,” Wireless Networks 7, 2001, pp. 159-171.
                                          VI.      CONCLUSIONS                                                                    [17] B. Bing, “Measured Performance of the IEEE 802.11 Wireless LAN,”
                                                                                                                                       IEEE LCN’99.
   In this paper, we introduced the emerging IEEE 802.11e                                                                         [18] G. Bianchi, L. Fratta, and M. Oliveti, “Performance evaluation and
standard to support quality of service at medium access                                                                                enhancement of the CSMA/CA MAC protocol for 802.11 Wireless
control level. An analytical model was proposed to study                                                                               LANs,” Proc. PIMRC 1996, Taipei, Taiwen 1996, pp. 392-396.
                                                                                                                                  [19] Y. Xiao and J. Rosdahl, “A Performance Analysis of IEEE 802.11a
EDCF priority scheme in terms of saturation throughput and                                                                             Wireless LAN,” Proc. of The 6th World Multi-Conference on
saturation delay. Our study shows following results:                                                                                   SYSTEMICS, CYBERNETICS AND INFORMATICS, (SCI 2002),
• The EDCF priority scheme is quite effective.                                                                                         July 14-18, 2002, Orlando, Florida, U.S.A, pp. 243-248.
                                                                                                                                  [20] IEEE 802.11 WG, Draft Supplement to Part 11: Wireless Medium
• Differentiating the initial window size is a better than                                                                             Access Control (MAC) and physical layer (PHY) specifications:
     differentiating the inter-frame space in terms of total                                                                           Medium Access Control (MAC) Enhancements for Quality of Service
     throughput and delay. However, differentiating the inter-                                                                         (QoS), IEEE 802.11e/D2.0, Nov. 2001.
                                                                                                                                  [21] S. Mangold, S. Choi, P. May, O. Kein, G. Hiertz, and L. Stibor, “IEEE
     frame space gives a very fast way to access the channel in
                                                                                                                                       802.11e Wireless LAN for Quality of Service,” Proc. European Wireless
     favor of the class with a short AIFS.                                                                                             ’02, Florence, Italy, February 2002.
• Simulation results match analytical results very well.                                                                          [22] X. Pallot and L. E. Miller, "Implementing message priority policies over
   The results of this paper are beneficial in designing good                                                                          an 802.11 based mobile ad hoc network," IEEE MILCOM 2001.
                                                                                                                                  N. Vaidya, P. Bahl, and S. Gupta, “Distributed Fair Scheduling in a Wireless
prioritized QoS parameters.                                                                                                            LAN,” ACM MOBICOM 2000.

                                                REFERENCES
[1]   IEEE 802.11 WG, Part 11: Wireless LAN Medium Access Control
      (MAC) and Physical Layer (PHY) specification, Aug. 1999.




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