Patent Publication Number: US-2005141467-A1

Title: Method and apparatus for evaluating performance of wireless LAN system

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a method and apparatus for evaluating the performance of a wireless LAN (local area network) system, and more particularly, to a performance evaluation method and apparatus which can predict the throughput and the like of a wireless LAN system even before the wireless LAN system is constructed.  
      2. Description of the Related Art  
      With the proliferation of wireless LANs, a need exists for techniques for evaluating the performance, for example, the throughput and the like, of wireless LANs. The wireless LAN suffers from a lower throughput due to errors on a radio transmission, i.e., errors on a physical layer caused by the use of the space as a transmission medium, and collisions resulting from simultaneous transmissions started on the MAC (Medium Access Control) layer, attempted by a plurality of terminals. Errors on the radio transmission include, for example, those caused by radio interference, extraneous noise, and the like.  
      To address this problem, there is a higher need for evaluating the performance, particularly, the throughput of wireless LANs than wired LANs. The performance of a wireless LAN may be evaluated by measuring the throughput and the like in the actual wireless LAN using a measuring instrument. However, this measurement-based approach disadvantageously encounters difficulties in providing conclusive results because this approach can be applied only to existing wireless LAN systems, and because performance values such as the throughput can largely vary depending on particular situations in which a wireless LAN is used. Taking into consideration the time and cost required to construct a wireless LAN system, the performance should be in many cases evaluated prior to the construction of the wireless LAN system rather than ex-post measurements made on the performance of the constructed wireless LAN system. For such a situation, the performance can be effectively evaluated by mathematical approaches or simulations.  
      For example, a performance evaluation approach based on a collision probability analysis in a wireless LAN has been proposed in G. Bianchi, “Performance Analysis of IEEE 802.11 Distributed Coordination Function,” IEEE Journal on Selected Areas in Communications, Vol. 18, No. 3, pp. 535-547, 2000. However, this approach makes an analysis premised on ideal radio channel conditions without transmission errors, thereby implying a problem that it provides evaluations too optimistic for actual systems.  
      Also, as described in A Doufexi, S. Armour, M. Buler, A. Nix, D. Bull, J. McGeehan, and P. Karisson, “A comparison of HIPERLAN/2 and IEEE 802.11a Wireless LAN standards,” IEEE Communications Magazine, Vol. 40, No. 5, pp. 172-180, 2002, an approach evaluates the influence exerted by transmission errors to the throughput, intended only for a single transmission/reception pair in consideration of radio conditions in the physical layer. However, this evaluation is based on a simple approach which involves subtracting an overhead as defined by the transmission standard, and a portion multiplied by an error ratio from a transmission rate as defined by the physical standard. Therefore, this approach is incapable of evaluating the influence on wireless LAN performance values such as the throughput exerted by collisions when there are a plurality of communication terminals.  
      Japanese Patent Laid-open Application No. 2001-168904 (JP, P2001-168904A) discloses a method of simulating the performance of a wireless LAN, which involves generating packets in accordance with a probability distribution, regarding some of generated packets as lost packets, and processing the remaining packets using a discrete event simulation to evaluate the throughput. JP, P2001-168904A, however, does not disclose how a simulation should be executed when a specific terminal topology is given.  
      Published Japanese Translation of PCT International Publication No. 2002-530956 (JP P2002-530956T), which corresponds to WO00/30384, discloses a method of predicting a radio condition in a CDMA (code division multiple access) based mobile communication system when positional information on radio stations are given. This method, however, supports a prediction of errors in a physical layer, so called in the LAN, and therefore does not take into consideration collisions of transmissions on the MAC layer, so that this method, as it is, cannot be applied to evaluation on the performance of wireless LAN.  
      As described above, there is no method for evaluating the throughput or the like of a wireless LAN before construction of the wireless LAN. Such an evaluation method should take into consideration both transmission errors and collisions of transmissions, and could make an accurate evaluation when a specific terminal topology is given.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to provide a method and apparatus which are capable of accurately evaluating the performance of a wireless LAN system in consideration of a transmission collision probability among a plurality of terminals, and a transmission/reception error rate or error probability associated with radio conditions of the respective terminals.  
      A method of the present invention is provided for evaluating the performance of a wireless LAN system having a single access point and a plurality of transmission terminals, wherein the access point and the transmission terminals share the same radio channel. The method includes the steps of dividing the plurality of transmission terminals into groups depending on radio conditions, and calculating a transmission probability and a post-transmission failure probability for each group using a transmission rate, an error rate, and the number of terminals of the each group. Preferably, in the present invention, the method further includes the step of calculating a throughput based on the transmission probability and post-transmission failure probability.  
      Specifically, in the present invention, once a transmission rate and an error rate of each terminal, and the number of terminals belonging to a group are entered for each group, the method calculates the probability of transmission success/failure, when each terminal is continuously transmitting, in accordance with a predetermined calculation algorithm. Then, the method calculates performance values such as the throughput of each terminal and the overall system, and displays the result of the calculation.  
      An apparatus of the present invention is provided for evaluating the performance of a wireless LAN system having a single access point and a plurality of transmission terminals, where the access point and the transmission terminals share the same radio channel. The apparatus includes means for entering a transmission rate, an error rate, and the number of terminals of each of groups into which the plurality of transmission terminals are divided in accordance with radio conditions, and means for calculating a transmission probability and a post-transmission failure probability for each group using the transmission rate, the error rate, and the number of terminals of each group. Preferably, the apparatus of the present invention further includes throughput calculating means for calculating a throughput based on the transmission probability and post-transmission failure probability.  
      According to the present invention, the performance of a wireless LAN system can be readily found even before the construction of the wireless LAN system. Thus, the present invention significantly contributes to efficient system performance designing.  
      The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagram illustrating an exemplary topology of a wireless LAN system;  
       FIG. 2  is a diagram for describing an MAC layer protocol DCF according to IEEE 802.11 standard;  
       FIG. 3  is a block diagram illustrating the configuration of a wireless LAN evaluation apparatus according to one embodiment of the present invention;  
       FIG. 4  is a flow chart illustrating a procedure for evaluating the throughput using the apparatus illustrated in  FIG. 3 ;  
       FIG. 5  is a flow chart illustrating a procedure for estimating parameters r k , e k ;  
       FIG. 6  is a diagram representing a Markov chain of state transitions related to the number of times of packet re-transmissions, and the number of remaining slots until transmission of a backoff timer;  
       FIG. 7  is a block diagram illustrating the configuration of a wireless LAN evaluation apparatus according to another embodiment of the present invention; and  
       FIG. 8  is a block diagram illustrating the configuration of a wireless LAN evaluation apparatus according to a further embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       FIG. 1  illustrates the topology of a wireless LAN system which is subjected to an evaluation in a preferred embodiment of the present invention. Assume that in  FIG. 1 , the wireless LAN system comprises a single access point (AP) and a plurality of terminals for connection to the access point. In this wireless LAN system, all communications are made between the access point and terminals. A plurality of terminals are divided into K groups in accordance with radio conditions or the like, wherein terminals in each group have substantially the same radio conditions. As described later, the number of terminals belonging to group k (1≦k≦K) is represented by n k . For convenience of description, the access point is defined to belong to group 0. Therefore, n 0 =1.  
      While the evaluation method according to the present invention can be applied to a variety of wireless LAN systems, the following description will be made on the evaluation method applied to a wireless LAN system based on IEEE 802.11 standard which implements a MAC layer protocol DCF (Distributed Coordination Function) basic scheme. The DCF basic scheme is based on CSMA (carrier sense multiple access)/CA (collision avoidance)+ACK (acknowledge), the mechanism of which is illustrated in  FIG. 2 . For details on the DCF basic scheme, see, for example, IEEE Std. 802.1, “Part  11 : Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications,” 1997. In the following, the DCF scheme will be described as the premise for describing embodiments of the present invention.  
      A terminal or the access point which has data to be transmitted first checks a (radio) channel state. The state of channel is represented by “busy” when at least one of terminal/access point is involved in transmission of data on the channel, and otherwise by “idle.” The terminal or access point which attempts to transmit data, when sensing the channel as “idle,” transmits one packet when a continued idle time exceeds a fixed time interval referred to as “DIFS” (DCF interframe space). The terminal or access point, when sensing the channel as “busy,” first waits for DIFS after the channel becomes idle, and then waits for a random-length backoff period before it transmits data. If the channel again becomes busy before the end of DIFS, the foregoing process is repeated. If the channel becomes busy during a backoff period, a backoff timer is stopped until the end of next DIFS. When a transmitted packet is correctly received, an ACK packet is returned from a receiving terminal or access point for acknowledgement after an SIFS (short interface space) interval shorter than DIFS.  
      While a backoff length is calculated as (random number)×(basic slot length), where the random number follows a uniform distribution on a section [0, CW], where CW represents a window parameter. Window parameter CW is multiplied by two each time a transmission fails until a certain limited number of times m, as expressed by CW=[min(2 i , 2 m )]W−1, where i is the number of times a packet is re-transmitted, and W is a window size at the first transmission. While the foregoing parameters have values which vary in accordance with the standard of the physical layer, the slot length σ=9 μs, m=6, W=16 are defined in IEEE 802.11 a standard which employs radio frequencies in a 5-GHz band.  
      As described above, in the DCF scheme, each transmission terminal or access point independently sets its backoff timer at random to reduce the probability of transmission collisions. Due to the random nature, it is not simple to solve the performance of the overall wireless LAN system as well as the performance of individual terminals and access point. In the G. Bianchi&#39;s article, a mathematical analysis on the DCF scheme is made to investigate the influence on the throughput depending on the number of transmission terminals and access points under error-free ideal radio conditions.  
      However, the ideal radio conditions are not established in actual wireless LAN systems which entail possible transmission errors. In addition, every terminal does not generally have homogeneous radio conditions. In other words, the respective terminals and access point differ from one another in transmission rate and error rate. Bearing the foregoing in mind, the present invention intends to evaluate the throughput of a wireless LAN system under such heterogeneous radio conditions. We have published the result of a throughput analysis which was made based on the throughput evaluation method according to the present invention in H. Pan, S. Sato, and K. Kobayashi, “On the Throughput of an IEEE 802.11a Wireless LAN System with Terminals under Heterogeneous Radio Conditions,” Proceedings of the 18 th  International Teletraffic Congress, 2003, and Huanxu Pan, Shohei Sato, and Kazutomo Kobayashi, “Evaluation on Throughput of Wireless LAN in Consideration of Packet Collisions,” the Institute of Electronics, Information and Communication Engineers (Japan), 2003 Society Conference Proceedings, SB-6-5, 2003.  
       FIG. 3  is a block diagram illustrating the configuration of a wireless LAN evaluation apparatus according to one embodiment of the present invention. The illustrated apparatus generally comprises input device  10 , wireless LAN performance calculation unit  20 , and output device  30 . As input device  10  enters parameters, which reflects system conditions of a wireless LAN, into wireless LAN performance calculation unit  20 , wireless LAN performance calculation unit  20  calculates a wireless LAN performance characteristic value such as the throughput, and output device  30  displays the result of the calculation.  
      Input device  10  is provided for entering information on the number of terminals, information on the topology and the like related to the wireless LAN. Input device  10  has group number input unit  11  for entering the number K of terminal groups; terminal number input unit  12  for entering the number n k  of terminals in each group, where k is a number representing a group; transmission rate input unit  13  and error rate input unit  14  for entering a radio environment in each group; and system parameter input unit  15  for entering system parameters &lt;&lt;σ, m, W, DIFS, SIFS, Z&gt;&gt; related to the wireless LAN standard, packet size, and the like. Transmission rate input unit  13  receives transmission rate r k  of each group, while error rate input unit  14  receives error rate (i.e., error probability) e k  for each group. These values and parameters entered into input device  10  are passed from input device  10  to wireless LAN performance calculation unit  20 .  
      Wireless LAN performance calculation unit  20  comprises probability calculator  21  for calculating transmission probability τ k  of each terminal or access point in an arbitrary slot for each group based on a predetermined calculation algorithm, and failure probability f k  in the event of transmission; and throughput calculator  22  for calculating throughput S k  of each terminal or access point and throughput S of the overall system from the result of the probability evaluations. Wireless LAN performance calculation unit  20  calculates these throughput values which are passed to output device  30 .  
      Output device  30  comprises calculation result display  31  for displaying the results of the calculations.  
      Next, the operation of the apparatus in this embodiment will be described. As mentioned above, in this embodiment, the wireless LAN system under evaluation is formed of a single access point and a plurality of terminals which share the same radio channel. The terminals are divided into K groups in accordance with radio conditions, where terminals belonging to the same group share the same radio conditions. In a general wireless LAN implementation, this means that terminals belonging to the same group have the same transmission rate and error rate. The error used herein refers to a transmission failure due to transmission errors caused by insufficient radiowave conditions. In contrast to this, there is another transmission failure caused by collisions resulting from simultaneous transmissions from a plurality of terminals and/or access point. In the following description, the terminals and access point are both called the “station” unless they need to be particularly distinguished from each other. Assume that all stations always have data to be transmitted.  
      Paying special attention to a particular station in group k, c k  represents the probability that a packet transmitted from the station can encounter a collision, and e k  represents the probability that the packet can corrupt, that is, the packet error rate (PER). Probability c k  can be found by a mathematic analysis according to the present invention, while probability e k  depends on signal-to-noise ratio (C/N) k  and transmission rate r k  which serve as indicia of the radio conditions. Also, n k  designates the number of stations belonging to group k. Group 0 exclusively includes the access point, and n 0 =1 because the wireless LAN system under evaluation has a single access point.  
       FIG. 4  is a flow chart illustrating an operational procedure in this embodiment.  
      First, at step  400 , it is determined whether or not a variety of parameters on the system standard, and a parameter for average packet size Z of transmission data have been entered. A variety of parameters on the system standards can be, for example, the aforementioned parameters σ, m, W, DIFS, SIFS, and the like. Many of these system parameters are common among different wireless LAN systems, and therefore can be reused for evaluating other wireless LAN systems, so that if they have been entered, they need not be entered again. If the system parameters have not been entered, these parameters are entered through input device  10  at step  410 .  
      Next, at step  402 , the number K of terminal groups, and the number n k  (1≦k≦K) of transmission terminals in each group are entered through input device  10 . At step  403 , it is determined whether or not parameters (r k , e k ) representative of the radio conditions of the wireless LAN system under evaluation can be applied directly from a measuring instrument or the like. If the parameters can be directly supplied, the procedure proceeds to step  405 . If the parameters cannot be directly supplied, parameters r k , e k  are estimated at step  404 , followed by transition of the procedure to step  405 . At step  405 , the peculiar parameter values r k , e k  (1≦k≦K) for the wireless LAN system are entered through input device  10 .  
      Now, description will be made on how parameters r k , e k are estimated at step  404 . For accommodating parameters r k , e k  which cannot be directly supplied, the wireless LAN performance evaluation apparatus illustrated in  FIG. 3  is provided with input preparation unit  50  for calculating parameters r k , e k  and supplying input device  10  with the calculated r k , e k . Input preparation unit  50  estimates parameters r k , e k  as illustrated in a flow chart of  FIG. 5 .  
      With a general radio measuring instrument used for measuring the parameters, if an access point alone has been installed in the system even without wireless LAN terminals, the measuring instrument can measure a signal-to-noise ratio (C/N) k  on the assumption that wireless LAN terminals are installed. Therefore, input preparation unit  50  determines at step  500  whether or not radio condition (C/N) k  can be measured for each terminal group k, and estimates parameters r k , e k  from the measured (C/N) k  at step  502  when it can be measured. These parameters may be estimated as described in the aforementioned article by A. Doufexi et al.  
      On the other hand, there is no radio measuring instrument available for measuring the signal-to-noise ratio in the wireless LAN system, or when the access point has not even been installed at a preparatory stage in the construction of a wireless LAN system, input preparation unit  50  uses a radiowave propagation model (see, for example, Recommendation ITU-R P. 1238-2, “Propagation Data and Prediction Methods for the Planning of Indoor Radiocommunication Systems and Radio Local Area Networks in the Frequency Range 900 MHz to 100 GHz,” 2001) at step  501  to estimate (C/N) k  from communication distance d k  between the access point and each terminal belonging to each group in the wireless LAN system which is scheduled to be constructed. Then, at step  502 , parameters r k , e k  are estimated from estimated (C/N) k  in a similar manner to the foregoing.  
      Turning back to  FIG. 4 , after execution of step  405 , probability calculator  21  calculates average transmission rate r 0  and error rate e 0  of the access point from the foregoing entered values in the following manner on the assumption that a transmission can be made from the access point to each terminal at uniform opportunity:  
                 r   0     =         ∑     k   =   1     K     ⁢     n   k           ∑     k   =   1     K     ⁢       n   k       r   k             ,           ⁢       e   0     =         ∑     k   =   1     K     ⁢       n   k     ⁢     e   k             ∑     k   =   1     K     ⁢     n   k                   (   1   )             
 
      Average transmission rate r 0  thus calculated represents an average rate for a transmission of data in a fixed size. The probability of post-transmission failure (error or collision) at a particular station in group k, designated by f k , is expressed by: 
 
 f   k   =c   k +(1 −c   k ) e   k , 0 ≦k≦K   (2) 
 
      Attention is paid only to a time point at which the number of remaining slots changes before the transmission of the backoff timer at a particular station. The state of the station at that time is defined by (i, j), where i represents the number of times of re-transmission of a current packet (i=0 for the first transmission), and j represents the number of remaining slots until the transmission of the backoff timer. With this definition, the state transition can be represented by a Markov chain as illustrated in  FIG. 6 . The representation of the state transition by the Markov chain in this manner is described in the aforementioned article by the present inventors. Solving a stationary solution of the Markov chain, the probability that the particular station transmits in an arbitrary slot is calculated as follows:  
                 τ   k     =         ∑     i   =   0     m     ⁢     P     i   ,   0     k       =       2   ⁢     (     1   -     2   ⁢     f   k         )             (     1   -     2   ⁢     f   k         )     ⁢     (     W   +   1     )       +       f   k     ⁢     W   ⁡     [     1   -       (     2   ⁢     f   k       )     m       ]                 ,     0   ≤   k   ≤   K             (   3   )             
 
      On the other hand, collision probability c k  is calculated from {τ k .} as follows:  
                 c   k     =     1   -       1     1   -     τ   k         ⁢       ∏     l   =   0     K     ⁢       (     1   -     τ   l       )     nl             ,     0   ≤   k   ≤   K             (   4   )             
 
      Combining Equations (2) and (3), the resulting f k  is expressed by:  
                 f   k     =     1   -       1     1   -     τ   k         ⁢       ∏     l   =   0     K     ⁢       (     1   -     τ   l       )     nl         +         e   k       1   -     τ   k         ⁢       ∏     l   =   0     K     ⁢       (     1   -     τ   l       )     nl             ,     0   ≤   k   ≤   K             (   5   )             
 
      At step  407 , probability calculator  21  solves simultaneous equations composed of Equations (3) and (5) to find post-transmission failure probability f k  and transmission probability τ k .  
      Next, at step  408 , throughput calculator  22  defines parameters τ, π k , α k , β k , γ for convenience of calculating the throughput, and evaluates these parameters.  
      Parameter τ represents the probability that at least one or more stations transmit in an arbitrary slot, and is expressed by:  
             τ   =     1   -       ∏     k   =   0     K     ⁢       (     1   -     τ   k       )     nk                 (   6   )             
 
      Parameter τ k  represents the probability that a particular station in group k successfully transmits in an arbitrary slot, and is expressed by:  
               π   k     =           τ   k     ⁡     (     1   -     e   k       )         1   -     τ   k         ⁢       ∏     l   =   0     K     ⁢       (     1   -     τ   l       )       n   l                   (   7   )             
 
      Parameters α k , β k  represent average slot lengths when a station in group k successfully transmits and fails to transmit, respectively, and can be evaluated by the physical layer parameters &lt;&lt;σ, m, W, DIFS, SIFS, Z&gt;&gt; (see the aforementioned articles by the present inventors).  
      When there is no transmission in a slot, the probability of such a situation is represented by 1−τ, and the time length of this slot is equal to σ.  
      When a transmission is successful in a slot, the transmission success probability of group k is represented by n k τ k , and average slot length α k  in this event is expressed by: 
 
α k   =DIFS+SIFS+ 2 PHY+ 2δ+(2 HPHY+HMAC+ACK+Z )/ r   k  
 
 where PHY represents the physical layer overhead including a preamble and a PLCP header; HPHY represents remaining bits in the physical layer header and tail; HMAC represents the MAC layer header and FCS (frame check sequence) bits; δ represents a propagation delay; and ACK represents the length of a MAC layer ACK frame. 
 
      While a station in one group k transmits in a slot, the transmission may fail due to an error in reception. The probability of such a transmission failure is represented by n k π k e k /(1−e k ), and an average slot length β k  in this case is expressed by: 
 
β k   =DIFS+PHY+δ +( HPHY+HMAC+Z )/ r   k  
 
      When a collision occurs due to transmissions made by two or more stations in a slot, the probability that n k   c  stations from group k are involved in the collision is expressed by:  
               ∏     k   =   0     K     ⁢       (           n   k               n   k   c           )     ⁢       (     τ   k     )       n   k   c       ⁢       (     1   -     τ   k       )         n   k     -     n   k   c                   (   8   )             
 
      The average slot length in this case is represented by:  
               γ   ⁡     (       n   0   c     ,     n   1   c     ,   …   ⁢           ,     n   K   c       )       =       max     (     k   ❘       n   k   c     &gt;   0       )       ⁢     {     β   k     }               (   9   )             
 
      It is understood from the foregoing that the throughput can be expressed as the ratio of the average number of bits successfully transmitted in an arbitrary slot to the average time length of the slot (note that since a slot involving transmission includes such times as a transmission, DIFS and the like, the average time length of the slot is not equal to σ which is a fixed length). In other words, the system throughput represented by S, and the throughput of an individual station in group k represented by S k  are calculated by:  
             S   =       Z   ⁢       ∑     k   =   0     K     ⁢       n   k     ⁢     π   k                       (     1   -   τ     )     ⁢   σ     +       ∑     k   =   0     K     ⁢       n   k     ⁢     π   k     ⁢     σ   k         +       ∑     k   =   0     K     ⁢           n   k     ⁢     π   k     ⁢     e   k         1   -     e   k         ⁢     β   k         +                 ∑         n   0   c     +     n   1   c     +   …   +     n   K   c       &gt;   1       ⁢       γ   ⁡     (       n   0   c     ,     n   1   c     ,   …   ⁢           ,     n   K   c       )       ⁢       ∏     k   =   0     K     ⁢     [       (           n   k               n   k   c           )     ⁢       (     τ   k     )       n   k   c       ⁢       (     1   -     τ   k       )         n   k     -     n   k   c           ]                           (   10   )                 S   k     =         π   k         ∑     l   =   0     K     ⁢       n   l     ⁢     π   l           ⁢   S             (   11   )             
 
      At step  409 , throughput calculator  22  calculates throughputs S, S k  in the foregoing manner. In Equation (10), Z represents an average packet size in bits.  
      Finally, at step  410 , output device  30  displays the results of calculations expressed by Equations (10), and (11).  
      In this way, according to this embodiment, the wireless LAN evaluation apparatus can find the performance provided by a wireless LAN system, which is constructed in future, such as the throughput, even prior to the construction of the wireless LAN system. Thus, the wireless LAN evaluation apparatus facilitates efficient designing of a wireless LAN system, and installation of access points and terminals in the wireless LAN system. In addition, since the method used in this embodiment takes into consideration the probability of collisions among a plurality of terminals as well as the transmission/reception error rate associated with the radio conditions of the respective terminals, and is based on mathematical approaches such as Markov analysis, the throughput can be accurately evaluated.  
      Next, another embodiment of the present invention will be described with reference to  FIGS. 7 and 8 .  
      In the foregoing description, when transmission rate r k  and error rate e k  cannot be directly supplied as shown at step  404  in  FIG. 4  and in  FIG. 5 , input preparation unit  50  can be provided for estimating r k , e k .  FIGS. 7 and 8  each illustrate the configuration of a wireless LAN evaluation apparatus which comprises such input preparation unit  50 .  
       FIG. 7  illustrates the configuration of the wireless LAN evaluation apparatus when a radio measuring instrument can be used to measure signal-to-noise ratio (C/N) k . In this scenario, input preparation unit  50  may comprise an input unit  51  through which signal-to-noise ratio (C/N) k  is entered, as measured for each group k, and estimation unit  52  for calculating transmission rate r k  and error rate e k  from (C/N) k . Here, (C/N) k  is entered through input unit  51 , and estimation unit  52  estimates r k , e k  in accordance with the aforementioned step  502 .  
       FIG. 8  illustrates the configuration of the wireless LAN evaluation apparatus when signal-to-noise ratio (C/N) k  cannot be measured. In this scenario, input preparation unit  50  may comprise input unit  53  through which entered is distance d k  from each terminal group to the access point; first estimation unit  54  for estimating (C/N) k  on a group-by-group basis from the entered communication distance between the access point and terminal; and second estimation unit  55  for calculating transmission rate r k  and error rate e k  using estimated (C/N) k .  
      While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.