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International Journal of Computer & Organization Trends –Volume 3 Issue 11 – Dec 2013

An Effienct Collision Free MAC Scheme for Wireless LANS with Constant Contention Time P.NagaSrikanthKumar 1 , Mrs. Y.V.N.Tulasi 2 1 2

PG Student in cse department,Gudlavalleru Engineering College,Gudlavalleru,Krishna(dt) Assistant professor in cse department,Gudlavalleru Engineering College,Gudlavalleru,Krishna(dt)

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Abstract – IEEE 802.11 specifies a set of protocols for a wireless LAN defined by IEEE which covers the physical and data link layer. Nowadays, IEEE 802.11 based WLAN has a widespread and providing wireless connectivity to several electronic devices like cell phones, laptops, gaming devices and so on. All these devices operate on the same medium and contend with each other in order to access the medium. All devices using the IEEE 802.11 Standard adopt a basic MAC scheme called the distributed coordination function (DCF) whose key function is contention resolution.

In a slot, the stations probabilistically send a jam signal on the channel. The stations listening retire if they hear a jam signal. The others continue to the next slot. Over several slots, we aim to have one station remaining in the contention, which will then transmit its data. The DCF utilizes a Binary Exponential Back-off (BEB) scheme to reduce the collision probability by doubling the contention window (cw ) upon a packet collision. In this paper, we propose a new back-off algorithm to enhance the performance of DCF in IEEE 802.11 MANETs, which is based on the idea of using a threshold of the collision rate to switch between two different increasing functions . The performance results reveal that the improved DCF is able to achieve higher throughput as well as lower packet loss probability than the original DCF. Research will shows that DCF’s performance degrades especially with the large number of stations. Our scheme, which attempts to resolve the contention in a constant number of slots (or constant time).

Keywords – protocols.

Computer networks, wireless LAN, access

I. INTRODUCTION The application of wireless networks in everyday computing has long been a success story and new wireless technologies go on to emerge .Nowadays, wireless networks certainly are a necessary portion of computing world. It was brought to possibility from the IEEE 802.11 Standard that gives

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technical specifications for the wireless interfaces. Besides, the Wi-Fi Alliance was formed to certify interoperability of wireless products from various vendors. The Medium Access Control (MAC) scheme inside the standard that's most widely used happens to be the Distributed Coordination Function (DCF). Its function would be to arbitrate using the medium to multiple stations that are connected to one AccessPoint(AP) within the infrastructure mode. Besides, DCF work extremely well in the infrastructure-less, or unintended, mode in which there is virtually no AP. This paper supplies a study according to the MAC schemes. We propose a fresh scheme therefore we have a comparison between our scheme, DCF and a few other efficient schemes.The contention with DCF works like the following.

The stations use Contention Windows (CW) to randomize their access and take a look at to steer clear of collisions.

Initially, a station waits for DIFS (DCF Inter Frame Space) and transmits in the event the channel is idle[1].With in ad hoc wireless network where mobile units engage omnidirectional antennas, each communications channel is in facts hared by closely located mobiles. The sharing with this channel is controlled from the employed Medium Get access to Control (MAC) protocol. The network’s throughput efficiency will depend on the running of the MAC protocol.

Contention based random-access multiple get access to protocols have been widely used as they are really simple to implement. To further increase the efficiency of the operation, carrier sense based MAC algorithms are used, requiring the mobile terminal to first sense the channel to actually determine that in fact its's idle and simply then attempt its envelope transmission. The latter attempts is still able to ends in a collision event (as soon as the intended receiver detects a number of transmissions at such power levels that it also you won't be able to correctly receive any one them). Frequently used this sort of protocols include Carrier Sensing Multiple Access (CSMA) and CSMA with Collision Avoidance (CSMA/CA).

However, if the channel is busy, the CW is designed. The CW is initially assigned to a preset value, CWmin, which depends toward the physical layer. Then, a station sets a backoff (BO) in contact with a random value chosen from a uniform distribution from (0; CW). The station decreases the BO counter by one for any time slot the channel is idle. Any time

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International Journal of Computer & Organization Trends –Volume 3 Issue 11 – Dec 2013 a busy channel is detected, the BO counter is freezed plus the count down resumes beginning with the freeze value after the channel is idle to obtain duration of DIFS. The station transmits when its BO counter reaches zero. If several stations reach zero at the same time,t here is going to be a collision and the transmitted frames won’t be received correctly. The colliding stations will not receive an ACK frame and they're going to double their CW (until it reaches the utmost value equal to CWmax). Nevertheless, every time a station transmits an idea frame successfully, its CW is reset into the initial value CWmin. There are quite a few studies on DCF inside the literature. Some studies presented performance models of DCF to characterize its performance .

One observation from the results shows that DCF’s performance degrades significantly using an increase in the volume of stations. Although this wasn’t a difficulty for the inception of DCF, eliminate plus much more people use wireless connections which certainly becomes a limitation practically. The decrease of performance in this case is as a consequence of the large number of collisions when using the rise in multitude of stations. Other evaluations of DCF demonstrate that its delay might be very large with busy traffic conditions. Finally, the fairness s of DCF continues to be considered and it was shown that DCF doesn’t have a great fairness inside the short-term, although its fairness increases clearly as the stations contend for longer periods. In this paper, we present a MAC scheme that provides access by resolving the contention between stations. The main feature of our scheme is that it attempts to resolve the contention within the same number of slots each and every time. Our scheme, which attempts to resolve the contention inside a CONstant TIme, is termed CONTI. The contention resolution has several slots. For the first slot, all the stations with frames to deliver contend.

II. LITERATURE SURVEY The scheme Prioritized Repeated Eliminations Multiple Access was proposed in . PREMA is basically a jammingbased scheme. It really works just as below are. Contending stations transmit a simple jam, whose length in slots is drawn coming from a geometric distribution with parameter q. Just after the very last jam slot, the stations do one slot relating to carrier sense. If and when they hear another ongoing slot, they’re using this contention. If it is not, this means they passed this elimination. The stations along with the longest burst will survive the elimination.

Following, they certainly another elimination basically choosing another random number beginning with the same distribution and jamming and then one slot relating to carrier sense. The volume of eliminations is a parameter called h.

The authors of PREMA utilize the parameters h = 4 and q =0.5. We use majority of these parameters at times when we compare our help PREMA.

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CONT1 algorithm:

Figure1: Contention resolution using CONTI The contention of n stations is resolved using CONTI over k contention slots. Each of the stations uses the same probability vector p. All of the stations go through the following procedure. Before a contention slot si, a station chooses signal 1 with probability pi or signal 0 with probability 1 _ pi.

During a contention slot, the station will transmit a pulse on the channel if it has signal 1. Otherwise, the station will listen to the channel[1,2].

During a contention slot, it is better to eliminate the largest number of stations possible. This means that the contention resolution is occurring quickly and the amount of time spent on contention resolution is minimized.

Figure2: Contention slot for signal retire and access

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International Journal of Computer & Organization Trends –Volume 3 Issue 11 – Dec 2013 Existing system drawbacks: 

High collision rate due to contention resolution between stations.

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Probability of estimation uses two possible values ie uniform distribution random values of 0 and 1.

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More time spend for contention resolution.

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Lack of time slots allotment dynamically.

(a)Collision Rate – When two nodes/stations try to transmit data packets over the same network at the same time, their packets collide and are lost or destroyed. This is known as collision. Collision rate is defined as the ratio of collisions to total transmissions. An increased rate of collision means higher number of retransmissions and inefficient use of channel bandwidth. Every mechanism aims to reduce the probability of collision during transmission.

Percentage Collision = (Total no. of sent RTS – Total no. of received CTS)/ Total no. of sent RTS (b)Throughput – This parameter indicates the average rate of successful packet transmission over a channel. In other words it is the percentage of time that is actually expended in sending data bits. In our thesis, we use the following formula to calculate throughput.

III. PROPOSED SYSTEM Improvements in Existing Work:

IV. RESULTS IEEE 802.11 DCF adopts an exponentially back-off algorithm to handle the collision between any two wireless stations. However, the DCF will not consider the mobile station situation whether it is under heavy traffic loads or not. To handle this issue, this paper proposes an improved DCF with a dynamic back-off algorithm to control the wireless medium, which uses the collision rate for being threshold to change between two different increasing functions (exponential and quadratic) in situations when of collision. The improved DCF uses the exponential increasing function in the event the mobile station is under light loaded traffic while uses the quadratic increasing function in the event the mobile station is under heavy loaded traffic.

All experiments were performed with the configurations Intel(R) Core(TM)2 CPU 2.13GHz, 2 GB RAM, and the operating system platform is Microsoft Windows XP Professional (SP2).

Initialization Cw=cw_min; In case of Collision Function cw_inc() Begin Collision++; If(col_rate<Threshold) Cw=(cw+1)*2-1; Else Cw=(cw+1)^2-1 If(cw>cw_max) Cw=cw_max End After a successful transmission: Function cw_reset() Begin Number_packet_sent++; Col_rate=collision/number_packet_sent Cw=cw_min; End

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Figure3: Home view of WirelessLan

Figure4: Configure wirelesslan Pc

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International Journal of Computer & Organization Trends â&#x20AC;&#x201C;Volume 3 Issue 11 â&#x20AC;&#x201C; Dec 2013

Figure5: Interface between nodes of wirelesslan

Figure8:start tcp server to listen client port

Figure6: Configure network setting to each node Figure9: Send message through wirelesslan tcp server port

Figure10: Client to server communication through wireless ip port

Figure7: InitialSetup

Figure11: Client to server communication through wireless server port

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International Journal of Computer & Organization Trends –Volume 3 Issue 11 – Dec 2013 Table1: Graph between collision rate vs data framesize.

300 200

Proposed Existing

100

Figure12: Sending message to server from client 0

Collision rate

Framesize

Proposed

0

245

Existing

14

230

Table2: Data framesize vs Time taken in existing and proposed work

800 600 Proposed 400

Existing

200

Figure13: Message sending status information

0

Time(ms)

Framesize

Proposed

124

245

Existing

584

230

V. CONCLUSION AND FUTURE SCOPE In the existing work the fairness was more in the small frame sizes and in the smaller number of stations. If we increase the number of stations then the fairness will be reduced and Collision will occur if number of nodes increased then throughput will reduced this decreases the performance. To overcome this, the proposed mechanism is used to provide the efficient throughput, good performance and to reduce the delay.

Figure14: Contention statistics Performance Analysis: Contention time for each packet deliver is analyzed. In the existing work wireless creation setup and initializing the parameters takes more time . Collision rate reduces to 0.

Overall time taken to build and sending message in this work is optimal compare to existing approach.

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REFERENCES 1) Constant Time Contention Mechanism For Efficient Throughput In 802.11 MAC Protocol, M.Devisuriya, International Journal of Communications and Engineering. Volume 04– No.4, Issue: 04 March 2012.

2) A. Kumar, E. Altman, D. Miorandi, and M. Goyal, “New Insights from a Fixed Point Analysis of Single Cell

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International Journal of Computer & Organization Trends –Volume 3 Issue 11 – Dec 2013 IEEE 802.11 WLANs,” Proc. IEEE INFOCOM,Aug 2005.

3) M. Carvalho and J. Garcia-Luna-Aceves, “Delay Analysis of IEEE 802.11 in Single-Hop Networks,” Proc. 11th IEEE Int’l Conf. Network Protocols (ICNP), March 2003.

4) E.Ziouva and T.Antonakopoulos, “CSMA/CA Performance under High Traffic Conditions: Throughput and Delay Analysis,” Computer Comm., vol. 25, no. 3, pp. 313321,Jan 2002.

5) N.H.Vaidya, P.Bahl, and S.Gupta, “Distributed Fair Scheduling in a Wireless LAN,” Proc. ACM MobiCom, Aug. 2000.

6) J.L. Sobrinho and A.S. Krishnakumar, “Quality-of-Service in Ad Hoc Carrier Sense Multiple Access Wireless Networks,” IEEE J. Selected Areas in Comm., vol. 17, no. 8, pp. 1353-1368, Aug. 1999.

7) A Medium Access Control Scheme for Wireless Lans with Constant-Time Contention Zakhia Abichar, IEEE Transactions on Mobile Computing, VOL. 10, NO. 2 Feb 1997.

8) P.Jacquet,P.Minet, P.Muhlethaler,and N.Rivierre, “Priority and Collision Detection with Active Signaling—The Channel Access Mechanism of HIPERLAN,” Wireless Personal Comm., vol. 4, no. 1, pp. 11-26, Jan. 1997.

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