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Contention resolution with EIED backoff for bandwidth request in IEEE 802.16 networks
Int. J. Electron. Commun. (AEÜ) 67 (2013) 40–44
Contents lists available at SciVerse ScienceDirect
International Journal of Electronics and Communications (AEÜ) journal homepage: www.elsevier.com/locate/aeue
Contention resolution with EIED backoff for bandwidth request in IEEE 802.16 networks Rajesh Anbazhagan ∗ , Nakkeeran Rangaswamy Department of Electronics Engineering, Pondicherry University, Puducherry, India
a r t i c l e
i n f o
Article history: Received 14 December 2011 Accepted 2 June 2012 Keywords: Contention resolution WiMAX Media access control Contention efficiency Access delay
a b s t r a c t In this letter, we suggest contention resolution with exponential increase and exponential decrease (EIED) backoff for bandwidth request in worldwide interoperability for microwave access (WiMAX) networks. In EIED, setting of backoff factor to overcome collision due to contention is very challenging and hence we suggest a method to compute backoff factor with average contention window. Further, to reduce access delay, we estimate the response time based on probability of failure and average contention window. Simulations validate the proposed EIED backoff in terms of contention efficiency, capacity and access delay. The contention efficiency and capacity is improved by 47.50% (for q value of 0.25) and 28.57% (for 25 numbers of transmission opportunity), respectively, when bandwidth request is made with the proposed EIED backoff mechanism. © 2012 Elsevier GmbH. All rights reserved.
1. Introduction Worldwide interoperability for microwave access (WiMAX) defined by IEEE 802.16 standard enables the delivery of last mile and last inch broadband access as an alternative to digital subscriber line (DSL). In WiMAX network, the two stations of interest are base station (BS) and subscriber stations (SSs). The operations carried out by the SS during uplink are ranging and bandwidth request. The types of bandwidth request mechanism specified in 802.16 standards are polling (contention free access) and contention resolution [1]. In contention access, the SS contends for transmission opportunity (TxOP) to execute bandwidth request. If the SS is successful in obtaining the TxOP, then it proceeds in transmitting the bandwidth request. If unsuccessful, it proceeds with truncated binary exponential backoff (TBEB) mechanism. In literature, many backoff mechanisms have been proposed for wired and wireless systems [6–8]. We review the works related to WiMAX systems. The contention resolution with truncated binary exponential backoff (TBEB) under varying error rates, saturated and unsaturated traffic load conditions with grouping and non-grouping schemes were explored in [2]. The contention resolution mechanism with exponential increase and exponential decrease (EIED) backoff for initial ranging (IR) in WiMAX networks have been studied in [3]. In [9], the performance comparison between contention resolution (random access) and polling based
∗ Corresponding author. E-mail addresses: [email protected] (A. Rajesh), [email protected] (R. Nakkeeran). 1434-8411/$ – see front matter © 2012 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.aeue.2012.06.001
bandwidth request have been discussed in detail. The authors in [9] have suggested adaptive switching (based on traffic load) between these two bandwidth request mechanisms so as to improve the performance of the WiMAX system. Further, the performance of contention based bandwidth request owing to channel noise have been discussed for a given number of contending stations, maximum backoff exponent or stage, number of TxOP and uniform number of transmission attempts over the TxOP. To the best of our knowledge, no work has been carried out with EIED based bandwidth request for WiMAX network. The need for EIED in WiMAX network is due to the type of accessing mechanism. Unlike wireless local area network (WLAN), the SSs in WiMAX does not use carrier sense multiple access over collision avoidance (CSMA/CA) before transmitting a request. In WiMAX network, the backoff mechanism is carried without carrier sensing. Hence, the backoff counter is decremented without accounting the status of the channel or number of stations in the network. This increases the chance of overlapping of backoff counter among the stations that leads to high probability of collision with TBEB that has constant backoff factor. Since the backoff factor with EIED can be varied on collision and success, it was chosen for contention resolution in WiMAX network. In this letter, we developed an analytical model to calculate the backoff factors in EIED backoff. The analytical model derives expression for dynamic setting of increment and decrement back¯ ). off factors (ıI and ıD ) based on average contention window (W ¯ of each SS is estimated based on its transmission failure. The W Another main challenge in contention resolution mechanism is the setting of response time. In WiMAX network, there is no guarantee that the SS acquire its TxOP within the specified time and hence
R. Anbazhagan, N. Rangaswamy / Int. J. Electron. Commun. (AEÜ) 67 (2013) 40–44
41
increases the access delay. Hence, the model is extended to com¯ . Compared with the previous pute the response time (Tr ) with W techniques in the literature, the proposed method provides better performance in terms of contention efficiency, capacity, and access delay. The rest of the letter is organized as follows: Section 2 presents in detail the proposed analytical model for contention resolution with EIED backoff. In Section 3, the performance analysis of the proposed model is shown through simulation results and Section 4 draws the conclusion.
corresponding values of contention window. It is given by P(Wi ) = ¯ is given by, PPfi . Substituting Wi and P(Wi ) in (6), the W
2. Proposed contention resolution with EIED backoff for bandwidth request
¯ is computed by the SS before its transmission with its curThe W ¯ in (4) and (5), the backoff rent transmission failure. Substituting W factors, finally, take the expression,
¯ = W
(1)
where Pc denotes the probability of collision due to contention, q denotes the probability that the BS accepts a bandwidth request and Tr denotes response time or waiting time. With EIED backoff, the contention window size is exponentially decreased by a backoff factor ıD for every successful transmission, and is exponentially increased by a backoff factor ıI for each unsuccessful transmission. The value of ıI and ıD assumed to be constant in previous work [5] and it depends on the CWmin , CWmax , M, and n. The CWmin and CWmax correspond to minimum and maximum contention window size and, M and n are integers. The relation between ıI , ıD , CWmin , and CWmax can be expressed as follows [5]: M
CWmax = (ıI ) CWmin ı1 = (ıD )
n
M≥0
n≥1
(2) (3)
In the proposed model, the values of ıI and ıD are calculated ¯ be the average value based on average contention window. Let W ¯ the ıI and ıD of contention window. Eqs. (2) and (3) in terms of W can be written as,
¯ = W
(4)
¯ )1/(M×n) ıD = (W
(5)
As discussed earlier, the performance of contention resolution depends on the appropriate values of backoff factor. Hence, we proceed in estimating the backoff factor.
The average size of contention window avoids collision among SSs and longer waiting time since the SS estimate its contention ¯ is comwindow from its past probability of failures. The value of W puted by considering the maximum backoff stage ‘m’. The average value of contention window can be calculated as follows:
m
Wi P(Wi )
ıI =
ıD =
m
W ((1 − Pf )(1 − (ıPf ) )) (1 − Pfm )(1 − ıPf ) m
W ((1 − Pf )(1 − (ıPf ) ) (1 − Pfm )(1 − ıPf )
iff Pf = / 1, m > 1
(6)
i=0
where Wi is the size of contention window during ith backoff. With EIED, we define Wi as Wi = Wıi . Let P(Wi ) be the probabilities of the
(8)
1/M (9)
1/(M×n) (10)
The EIED backoff set its current contention window and hence the backoff counter with backoff factors in (9) and (10) on collision and success, respectively. 2.3. Estimation of response time (Tr ) After transmitting a request, the SS has to wait a specific period (waiting time or response time) for bandwidth grant from the BS. The response time (Tr ) is the time over which the SS waits for response from the BS after bandwidth request. The geometric probability distribution is considered in the proposed system and hence probability of transmission (P) with the average value of contention window is given by the expected value of random variable. The P ¯ and is given with average contention window becomes, P = 2/W by, P=
2(1 − Pfm )(1 − ıPf )
m
W (1 − Pf )(1 − (ıPf ) )
if W > 2
(11)
The probability of collision (Pc ) is the conditional probability that collision occurs when any of the ‘Nc − 1 station transmits when the station ‘Nc ’ is transmitting and it is expressed as follows,
1−
2(1 − Pfm )(1 − ıPf )
Nc −1
(12)
m
W (1 − Pf )(1 − (ıPf ) )
The response time can be expressed in terms of total probability of transmission failure (estimated from past transmission), probability of collision, probability of availability of bandwidth and number of contending stations. Since the events causing the transmission failure are assumed as independent, the Tr from (1) can be expressed as follows, (1 − Pc )(1 − q)Tr = Pf − Pc
¯ 2.2. Calculation of ıI and ıD in terms of W
¯ = W
m
W ((1 − Pf )(1 − (ıPf ) )) (1 − Pfm )(1 − ıPf )
Pc = 1 −
¯ )1/M ıI = (W
(7)
i=0
The main challenge in designing the backoff mechanism is to avoid overlapping of backoff counters among SSs with high probability of success. The probability of failure of the WiMAX system is characterized by collisions due to contention and unavailability of bandwidth [4]. The probability of failure (Pf ) is formulated by:
(Wıi )P(Wi )
where ı and W correspond to initial backoff factor and initial contention window size, respectively.
2.1. EIED with varying backoff factor
Pf = Pc + (1 − Pc )(1 − q)Tr
m
(1 − q)Tr = Tr =
(13)
Pf − Pc 1 − Pc
(14)
log((Pf − Pc )/(1 − Pc )) log(1 − q)
(15)
Substituting the probability of collision in (15), and solving further, the response time can be estimated as follows, Nc −1
Tr =
¯ ) ¯ − 1) log((Pf − 1 + (1 − 2/W )/(2/W log(1 − q)
Nc −1
)
(16)
Hence, the contention resolution is performed with average contention window and response time. The backoff value with varying
42
R. Anbazhagan, N. Rangaswamy / Int. J. Electron. Commun. (AEÜ) 67 (2013) 40–44
Fig. 1. Contention efficiency with varying number of contending stations.
Fig. 2. Probability of success, idle and collision with varying number of contending stations.
backoff factor reduces the overlapping of TxOP among the stations and dynamic response time alters the channel waiting time. This technique of contention resolution improves the probability of transmission but the probability of success in WiMAX network depends on the fact that the BS has enough bandwidth to accept the request. 3. Performance evaluation and model validation 3.1. Simulation parameters In this section, we evaluate the performance of the proposed EIED model through simulations. The bandwidth request scheme for IEEE 802.16 network is implemented using MATLAB. The simulation results are obtained by averaging over 15 runs with each run iterated over 105 iterations. The simulation parameters are configured accordance to the standard [1] and shown in Table 1. The simulation is carried out in terms
of the contention efficiency, probability of success owing to contention, probability of idle due to contention, probability of collision, capacity (number of request per frame) and access delay. Table 1 Simulation parameters. Parameters
Details
Physical layer Duplexing Frame duration (ms) Symbol duration (us) Number of OFDM symbols Number of TxOP Minimum contention window Maximum backoff stage Maximum contending stations Distribution Simulation platform
OFDM Time division duplex (TDD) 5 72.8 69 15, 25, 45 8 6 45 Geometric MATLAB
R. Anbazhagan, N. Rangaswamy / Int. J. Electron. Commun. (AEÜ) 67 (2013) 40–44
43
Fig. 3. Number of successful request per frame with varying number of contending stations.
3.2. Observations The contention efficiency computed by varying the contending stations with varying q (the probability that the BS accepts the bandwidth request) is shown in Fig. 1. The contention efficiency increases with increase in the number of stations and attains saturation. At this saturation, the contention efficiency corresponds to 29.87%, 35.10% and 37.50% for q values of 0.15, 0.25 and 0.50, respectively. The prime design of any contention based mechanism is to increase the contention efficiency with increase in the number of contending stations. However, many mechanisms fail to attain saturated contention efficiency (due to increase in probability of collision, unfairness between SSs and unavailability of bandwidth) and their contention efficiency reduces with increase in the number of contending stations. The increase in contention efficiency with
the proposed system is due to faster retransmission attempt with dynamic response time. Further, the collision among SS is less with dynamic setting of backoff factors. In addition, the varying backoff causes fairness among SS on success and failure. In the proposed system, the maximum (saturated) contention efficiency depends on the value of q. Nevertheless, the q in WiMAX is controlled by call admission control at the BS. The performance curves for probability of success, probability of idle and probability of collision are shown in Fig. 2. The simulations were performed with q = 0.5 and m = 6. These curves verify our algorithm for different states (idle, success and collision) and with varying stations. With number of contending stations equal to 45, the collision due to contention is 8.50%, the success rate is 37.25% and idle time is 54.25%. It is evident that there is a steep decrease in the probability of idle time with increase in the number of stations.
Fig. 4. Access delay with varying number of contending stations.
44
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Table 2 Performance comparison (Nc = 40). Parameters
Yaser et al. [4] Proposed system Improvement (%)
4. Conclusion
Contention efficiency
Capacity (request per frame)
q = 0.25
q = 0.50
TxOP = 15
TxOP = 25
0.200 0.295 47.50
0.270 0.375 38.89
5 6 20
7 9 28.57
The prime reason is that with a fixed number of transmission opportunities, the channel idle time is less with lesser number of stations as very few collisions occur among SS. However, with increase in the number of contending stations, the channel idle time reduces as the retransmission attempt increases with increase in transmission failure. However, with the proposed method the probability of collision is maintained below 10% with adaptive estimation of response time and dynamic setting of backoff factor on collision and success. The capacity of the system is shown in Fig. 3. The capacity increases with increase in the number of stations and number of slots allocated for bandwidth request (TxOP). The BS successfully responds 16 requests per frame when number of stations and TxOP equal to 35 and 45, respectively. The access delay is decided by the effective contention mechanism since it resolves possible collisions due to contention. The access delay is calculated in terms of number of frames and it denotes the duration the SS waits to receive the response from the BS after sending the bandwidth request. Since the backoff counter is computed as a function of response time the system experiences lesser delay and the delay is further reduced with increase in the number of TxOP as shown in Fig. 4. The access delay is less than 40 ms when number of stations and TxOP equal to 45 and 15, respectively. The performance improvement of the proposed system (dynamic EIED backoff) over the conventional system (TBEB) is given in Table 2.
In this letter, contention resolution based bandwidth request mechanism using EIED backoff has been suggested for emerging IEEE 802.16 networks. An analytical model to compute the backoff factors and response time with EIED backoff has been developed. Performance evaluation with varying q (probability that the BS accepts the bandwidth request), transmission opportunity (TxOP) and contending stations concludes the effectiveness of developed model for next generation WiMAX network. References [1] IEEE standard for local and metropolitan area networks. Part 16: Air interface for broadband wireless access systems 2009. p. C1-2004. [2] Ni Q, Hu L, Vinel A, Xiao Y, Hadjinicolaou M. Performance analysis of contention based bandwidth request mechanisms in WiMAX networks. IEEE Syst J 2010;4(4):477–86. [3] Kwak B-J, Song N-O, Kwon DS. Enhancement of IEEE 802.16 wirelessMAN ranging performance with EIED backoff algorithm. In: IEEE 66th Vehicular Technology Conference (VTC-2007). 2007. p. 1902–6. [4] Pourmohammadi Fallah Y, Agharebparast F, Minhas MR, Alnuweiri HM, Leung VCM. Analytical modeling of contention-based bandwidth request mechanism in IEEE 802.16 wireless networks. IEEE Trans Vehicular Technol 2008;(5):3094–107. [5] Song N, Kwak B, Song J, Miller ME. Enhancement of IEEE 802.11 distributed coordination function with exponential increase exponential decrease backoff algorithm. In: IEEE 57th Vehicular Technology Conference. 2003. p. 2775–8. [6] Park H, Pack S, Kang C-H. Dynamic adaptation of contention window for consumer devices in WiMedia home networks. IEEE Trans Consumer Electron 2011;57(1):28–34. [7] Doirieux S, Baynat B, Maqbool M, Coupechoux M. An efficient analytical model for the dimensioning of WiMAX networks supporting multi-profile best effort traffic. J Comput Commun 2010:1162–79. [8] Abd-Elnaby M, Rizk MRM, Dessouky MI, El-Dolil SA. Efficient contention-based MAC protocol using adaptive fuzzy controlled sliding backoff interval for wireless networks. J Comput Electr Eng 2011:115–25. [9] Ni Q, Vinel A, Xiao Y, Turlikov A, Jiang T. Investigation of bandwidth request mechanisms under point-to-multipoint mode of WiMAX networks. IEEE Commun Mag 2007;45(5):132–8.
Contents lists available at SciVerse ScienceDirect
International Journal of Electronics and Communications (AEÜ) journal homepage: www.elsevier.com/locate/aeue
Contention resolution with EIED backoff for bandwidth request in IEEE 802.16 networks Rajesh Anbazhagan ∗ , Nakkeeran Rangaswamy Department of Electronics Engineering, Pondicherry University, Puducherry, India
a r t i c l e
i n f o
Article history: Received 14 December 2011 Accepted 2 June 2012 Keywords: Contention resolution WiMAX Media access control Contention efficiency Access delay
a b s t r a c t In this letter, we suggest contention resolution with exponential increase and exponential decrease (EIED) backoff for bandwidth request in worldwide interoperability for microwave access (WiMAX) networks. In EIED, setting of backoff factor to overcome collision due to contention is very challenging and hence we suggest a method to compute backoff factor with average contention window. Further, to reduce access delay, we estimate the response time based on probability of failure and average contention window. Simulations validate the proposed EIED backoff in terms of contention efficiency, capacity and access delay. The contention efficiency and capacity is improved by 47.50% (for q value of 0.25) and 28.57% (for 25 numbers of transmission opportunity), respectively, when bandwidth request is made with the proposed EIED backoff mechanism. © 2012 Elsevier GmbH. All rights reserved.
1. Introduction Worldwide interoperability for microwave access (WiMAX) defined by IEEE 802.16 standard enables the delivery of last mile and last inch broadband access as an alternative to digital subscriber line (DSL). In WiMAX network, the two stations of interest are base station (BS) and subscriber stations (SSs). The operations carried out by the SS during uplink are ranging and bandwidth request. The types of bandwidth request mechanism specified in 802.16 standards are polling (contention free access) and contention resolution [1]. In contention access, the SS contends for transmission opportunity (TxOP) to execute bandwidth request. If the SS is successful in obtaining the TxOP, then it proceeds in transmitting the bandwidth request. If unsuccessful, it proceeds with truncated binary exponential backoff (TBEB) mechanism. In literature, many backoff mechanisms have been proposed for wired and wireless systems [6–8]. We review the works related to WiMAX systems. The contention resolution with truncated binary exponential backoff (TBEB) under varying error rates, saturated and unsaturated traffic load conditions with grouping and non-grouping schemes were explored in [2]. The contention resolution mechanism with exponential increase and exponential decrease (EIED) backoff for initial ranging (IR) in WiMAX networks have been studied in [3]. In [9], the performance comparison between contention resolution (random access) and polling based
∗ Corresponding author. E-mail addresses: [email protected] (A. Rajesh), [email protected] (R. Nakkeeran). 1434-8411/$ – see front matter © 2012 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.aeue.2012.06.001
bandwidth request have been discussed in detail. The authors in [9] have suggested adaptive switching (based on traffic load) between these two bandwidth request mechanisms so as to improve the performance of the WiMAX system. Further, the performance of contention based bandwidth request owing to channel noise have been discussed for a given number of contending stations, maximum backoff exponent or stage, number of TxOP and uniform number of transmission attempts over the TxOP. To the best of our knowledge, no work has been carried out with EIED based bandwidth request for WiMAX network. The need for EIED in WiMAX network is due to the type of accessing mechanism. Unlike wireless local area network (WLAN), the SSs in WiMAX does not use carrier sense multiple access over collision avoidance (CSMA/CA) before transmitting a request. In WiMAX network, the backoff mechanism is carried without carrier sensing. Hence, the backoff counter is decremented without accounting the status of the channel or number of stations in the network. This increases the chance of overlapping of backoff counter among the stations that leads to high probability of collision with TBEB that has constant backoff factor. Since the backoff factor with EIED can be varied on collision and success, it was chosen for contention resolution in WiMAX network. In this letter, we developed an analytical model to calculate the backoff factors in EIED backoff. The analytical model derives expression for dynamic setting of increment and decrement back¯ ). off factors (ıI and ıD ) based on average contention window (W ¯ of each SS is estimated based on its transmission failure. The W Another main challenge in contention resolution mechanism is the setting of response time. In WiMAX network, there is no guarantee that the SS acquire its TxOP within the specified time and hence
R. Anbazhagan, N. Rangaswamy / Int. J. Electron. Commun. (AEÜ) 67 (2013) 40–44
41
increases the access delay. Hence, the model is extended to com¯ . Compared with the previous pute the response time (Tr ) with W techniques in the literature, the proposed method provides better performance in terms of contention efficiency, capacity, and access delay. The rest of the letter is organized as follows: Section 2 presents in detail the proposed analytical model for contention resolution with EIED backoff. In Section 3, the performance analysis of the proposed model is shown through simulation results and Section 4 draws the conclusion.
corresponding values of contention window. It is given by P(Wi ) = ¯ is given by, PPfi . Substituting Wi and P(Wi ) in (6), the W
2. Proposed contention resolution with EIED backoff for bandwidth request
¯ is computed by the SS before its transmission with its curThe W ¯ in (4) and (5), the backoff rent transmission failure. Substituting W factors, finally, take the expression,
¯ = W
(1)
where Pc denotes the probability of collision due to contention, q denotes the probability that the BS accepts a bandwidth request and Tr denotes response time or waiting time. With EIED backoff, the contention window size is exponentially decreased by a backoff factor ıD for every successful transmission, and is exponentially increased by a backoff factor ıI for each unsuccessful transmission. The value of ıI and ıD assumed to be constant in previous work [5] and it depends on the CWmin , CWmax , M, and n. The CWmin and CWmax correspond to minimum and maximum contention window size and, M and n are integers. The relation between ıI , ıD , CWmin , and CWmax can be expressed as follows [5]: M
CWmax = (ıI ) CWmin ı1 = (ıD )
n
M≥0
n≥1
(2) (3)
In the proposed model, the values of ıI and ıD are calculated ¯ be the average value based on average contention window. Let W ¯ the ıI and ıD of contention window. Eqs. (2) and (3) in terms of W can be written as,
¯ = W
(4)
¯ )1/(M×n) ıD = (W
(5)
As discussed earlier, the performance of contention resolution depends on the appropriate values of backoff factor. Hence, we proceed in estimating the backoff factor.
The average size of contention window avoids collision among SSs and longer waiting time since the SS estimate its contention ¯ is comwindow from its past probability of failures. The value of W puted by considering the maximum backoff stage ‘m’. The average value of contention window can be calculated as follows:
m
Wi P(Wi )
ıI =
ıD =
m
W ((1 − Pf )(1 − (ıPf ) )) (1 − Pfm )(1 − ıPf ) m
W ((1 − Pf )(1 − (ıPf ) ) (1 − Pfm )(1 − ıPf )
iff Pf = / 1, m > 1
(6)
i=0
where Wi is the size of contention window during ith backoff. With EIED, we define Wi as Wi = Wıi . Let P(Wi ) be the probabilities of the
(8)
1/M (9)
1/(M×n) (10)
The EIED backoff set its current contention window and hence the backoff counter with backoff factors in (9) and (10) on collision and success, respectively. 2.3. Estimation of response time (Tr ) After transmitting a request, the SS has to wait a specific period (waiting time or response time) for bandwidth grant from the BS. The response time (Tr ) is the time over which the SS waits for response from the BS after bandwidth request. The geometric probability distribution is considered in the proposed system and hence probability of transmission (P) with the average value of contention window is given by the expected value of random variable. The P ¯ and is given with average contention window becomes, P = 2/W by, P=
2(1 − Pfm )(1 − ıPf )
m
W (1 − Pf )(1 − (ıPf ) )
if W > 2
(11)
The probability of collision (Pc ) is the conditional probability that collision occurs when any of the ‘Nc − 1 station transmits when the station ‘Nc ’ is transmitting and it is expressed as follows,
1−
2(1 − Pfm )(1 − ıPf )
Nc −1
(12)
m
W (1 − Pf )(1 − (ıPf ) )
The response time can be expressed in terms of total probability of transmission failure (estimated from past transmission), probability of collision, probability of availability of bandwidth and number of contending stations. Since the events causing the transmission failure are assumed as independent, the Tr from (1) can be expressed as follows, (1 − Pc )(1 − q)Tr = Pf − Pc
¯ 2.2. Calculation of ıI and ıD in terms of W
¯ = W
m
W ((1 − Pf )(1 − (ıPf ) )) (1 − Pfm )(1 − ıPf )
Pc = 1 −
¯ )1/M ıI = (W
(7)
i=0
The main challenge in designing the backoff mechanism is to avoid overlapping of backoff counters among SSs with high probability of success. The probability of failure of the WiMAX system is characterized by collisions due to contention and unavailability of bandwidth [4]. The probability of failure (Pf ) is formulated by:
(Wıi )P(Wi )
where ı and W correspond to initial backoff factor and initial contention window size, respectively.
2.1. EIED with varying backoff factor
Pf = Pc + (1 − Pc )(1 − q)Tr
m
(1 − q)Tr = Tr =
(13)
Pf − Pc 1 − Pc
(14)
log((Pf − Pc )/(1 − Pc )) log(1 − q)
(15)
Substituting the probability of collision in (15), and solving further, the response time can be estimated as follows, Nc −1
Tr =
¯ ) ¯ − 1) log((Pf − 1 + (1 − 2/W )/(2/W log(1 − q)
Nc −1
)
(16)
Hence, the contention resolution is performed with average contention window and response time. The backoff value with varying
42
R. Anbazhagan, N. Rangaswamy / Int. J. Electron. Commun. (AEÜ) 67 (2013) 40–44
Fig. 1. Contention efficiency with varying number of contending stations.
Fig. 2. Probability of success, idle and collision with varying number of contending stations.
backoff factor reduces the overlapping of TxOP among the stations and dynamic response time alters the channel waiting time. This technique of contention resolution improves the probability of transmission but the probability of success in WiMAX network depends on the fact that the BS has enough bandwidth to accept the request. 3. Performance evaluation and model validation 3.1. Simulation parameters In this section, we evaluate the performance of the proposed EIED model through simulations. The bandwidth request scheme for IEEE 802.16 network is implemented using MATLAB. The simulation results are obtained by averaging over 15 runs with each run iterated over 105 iterations. The simulation parameters are configured accordance to the standard [1] and shown in Table 1. The simulation is carried out in terms
of the contention efficiency, probability of success owing to contention, probability of idle due to contention, probability of collision, capacity (number of request per frame) and access delay. Table 1 Simulation parameters. Parameters
Details
Physical layer Duplexing Frame duration (ms) Symbol duration (us) Number of OFDM symbols Number of TxOP Minimum contention window Maximum backoff stage Maximum contending stations Distribution Simulation platform
OFDM Time division duplex (TDD) 5 72.8 69 15, 25, 45 8 6 45 Geometric MATLAB
R. Anbazhagan, N. Rangaswamy / Int. J. Electron. Commun. (AEÜ) 67 (2013) 40–44
43
Fig. 3. Number of successful request per frame with varying number of contending stations.
3.2. Observations The contention efficiency computed by varying the contending stations with varying q (the probability that the BS accepts the bandwidth request) is shown in Fig. 1. The contention efficiency increases with increase in the number of stations and attains saturation. At this saturation, the contention efficiency corresponds to 29.87%, 35.10% and 37.50% for q values of 0.15, 0.25 and 0.50, respectively. The prime design of any contention based mechanism is to increase the contention efficiency with increase in the number of contending stations. However, many mechanisms fail to attain saturated contention efficiency (due to increase in probability of collision, unfairness between SSs and unavailability of bandwidth) and their contention efficiency reduces with increase in the number of contending stations. The increase in contention efficiency with
the proposed system is due to faster retransmission attempt with dynamic response time. Further, the collision among SS is less with dynamic setting of backoff factors. In addition, the varying backoff causes fairness among SS on success and failure. In the proposed system, the maximum (saturated) contention efficiency depends on the value of q. Nevertheless, the q in WiMAX is controlled by call admission control at the BS. The performance curves for probability of success, probability of idle and probability of collision are shown in Fig. 2. The simulations were performed with q = 0.5 and m = 6. These curves verify our algorithm for different states (idle, success and collision) and with varying stations. With number of contending stations equal to 45, the collision due to contention is 8.50%, the success rate is 37.25% and idle time is 54.25%. It is evident that there is a steep decrease in the probability of idle time with increase in the number of stations.
Fig. 4. Access delay with varying number of contending stations.
44
R. Anbazhagan, N. Rangaswamy / Int. J. Electron. Commun. (AEÜ) 67 (2013) 40–44
Table 2 Performance comparison (Nc = 40). Parameters
Yaser et al. [4] Proposed system Improvement (%)
4. Conclusion
Contention efficiency
Capacity (request per frame)
q = 0.25
q = 0.50
TxOP = 15
TxOP = 25
0.200 0.295 47.50
0.270 0.375 38.89
5 6 20
7 9 28.57
The prime reason is that with a fixed number of transmission opportunities, the channel idle time is less with lesser number of stations as very few collisions occur among SS. However, with increase in the number of contending stations, the channel idle time reduces as the retransmission attempt increases with increase in transmission failure. However, with the proposed method the probability of collision is maintained below 10% with adaptive estimation of response time and dynamic setting of backoff factor on collision and success. The capacity of the system is shown in Fig. 3. The capacity increases with increase in the number of stations and number of slots allocated for bandwidth request (TxOP). The BS successfully responds 16 requests per frame when number of stations and TxOP equal to 35 and 45, respectively. The access delay is decided by the effective contention mechanism since it resolves possible collisions due to contention. The access delay is calculated in terms of number of frames and it denotes the duration the SS waits to receive the response from the BS after sending the bandwidth request. Since the backoff counter is computed as a function of response time the system experiences lesser delay and the delay is further reduced with increase in the number of TxOP as shown in Fig. 4. The access delay is less than 40 ms when number of stations and TxOP equal to 45 and 15, respectively. The performance improvement of the proposed system (dynamic EIED backoff) over the conventional system (TBEB) is given in Table 2.
In this letter, contention resolution based bandwidth request mechanism using EIED backoff has been suggested for emerging IEEE 802.16 networks. An analytical model to compute the backoff factors and response time with EIED backoff has been developed. Performance evaluation with varying q (probability that the BS accepts the bandwidth request), transmission opportunity (TxOP) and contending stations concludes the effectiveness of developed model for next generation WiMAX network. References [1] IEEE standard for local and metropolitan area networks. Part 16: Air interface for broadband wireless access systems 2009. p. C1-2004. [2] Ni Q, Hu L, Vinel A, Xiao Y, Hadjinicolaou M. Performance analysis of contention based bandwidth request mechanisms in WiMAX networks. IEEE Syst J 2010;4(4):477–86. [3] Kwak B-J, Song N-O, Kwon DS. Enhancement of IEEE 802.16 wirelessMAN ranging performance with EIED backoff algorithm. In: IEEE 66th Vehicular Technology Conference (VTC-2007). 2007. p. 1902–6. [4] Pourmohammadi Fallah Y, Agharebparast F, Minhas MR, Alnuweiri HM, Leung VCM. Analytical modeling of contention-based bandwidth request mechanism in IEEE 802.16 wireless networks. IEEE Trans Vehicular Technol 2008;(5):3094–107. [5] Song N, Kwak B, Song J, Miller ME. Enhancement of IEEE 802.11 distributed coordination function with exponential increase exponential decrease backoff algorithm. In: IEEE 57th Vehicular Technology Conference. 2003. p. 2775–8. [6] Park H, Pack S, Kang C-H. Dynamic adaptation of contention window for consumer devices in WiMedia home networks. IEEE Trans Consumer Electron 2011;57(1):28–34. [7] Doirieux S, Baynat B, Maqbool M, Coupechoux M. An efficient analytical model for the dimensioning of WiMAX networks supporting multi-profile best effort traffic. J Comput Commun 2010:1162–79. [8] Abd-Elnaby M, Rizk MRM, Dessouky MI, El-Dolil SA. Efficient contention-based MAC protocol using adaptive fuzzy controlled sliding backoff interval for wireless networks. J Comput Electr Eng 2011:115–25. [9] Ni Q, Vinel A, Xiao Y, Turlikov A, Jiang T. Investigation of bandwidth request mechanisms under point-to-multipoint mode of WiMAX networks. IEEE Commun Mag 2007;45(5):132–8.