1. Demand Based Bandwidth Assignment MAC Protocol for Wireless LANs K.Murugan, B.Dushyanth, E.Gunasekaran S.Arivuthokai, RS.Bhuvaneswaran, S.Shanmugavel

2. ABSTRACT The issues in wireless n/w to be considered are: Bandwidth utilization, cell capacity, propagation delay, power efficiency and quality of service of MAC Layer. IEEE 802.11 Specifications for WLANs use CSMA/CD scheme. This scheme is successful due to its simplicity but inefficient in utilizing the physical bandwidth. Many satellite networks have used Demand assignment multiple access-TDMA(DAMA- TDMA) where time slots are allocated dynamically.

3. ABSTRACT In this paper, a variation of this protocol has been proposed called Demand Based Bandwidth Assignment (DBBA) Protocol. Main challenge that this protocol resolves is to avoid collision to the maximum extent during the demand request contention period. To analyze DBBA, a test bench has been developed to simulate traffic.

4. ABSTRACT Based on simulation result, it has been observed that the proposed DBBA protocol has higher bandwidth utilization, supports larger no. of stations in one cell, lesser propagation delay, more power efficient and the ability to provide better QOS. Since there is no collision the efficiency of the system can be increased without affecting the overall throughput of the system.

5. 1.INTRODUCTION IEEE 802.11 supports up to 54Mbps of raw physical data. This is much higher compared to the 10Mbps wired networks and significantly smaller than 100 Mbps and Gigabit networks. It uses Aloha protocol to share the wireless medium. The drawback of this protocol being that it cannot work under high traffic load. Wireless media being highly bandwidth and power limited, a TDMA solution may help to increase the utilization of the channel bandwidth. The disadvantage of TDMA, being that the time slots are fixed and could limit the no. of nodes.

6. 1.INTRODUCTION DAMA-TDMA has been used by many satellite networks where the time slots are allocated dynamically. We propose a variation of this protocol in this paper called the Demand Based Bandwidth Assignment (DBBA) protocol. One of the challenges faced by this protocol is to avoid collision completely during the demand request contention period. The performance of this protocol can be assessed with throughput, latency and collision and the QOS capabilities can be compared with the existing ones.

7. 1.INTRODUCTION Time stamp based simulation is devised. The simulator generates packets at random time of random length. The scheduler processes and provides the packet transmission, collision and delay details to the monitor program. The monitor program collects the statistics to generate the simulation results at the end of the simulation run.

8. 2.IEEE 802.11 MAC PROTOCOL IEEE802.11 MAC uses CSMA/CA scheme. Full duplex solution is very expensive in wireless medium and so NIC cards are half duplex. Therefore MAC layer depends on acknowledgement packet to determine collision. The received station have to send an acknowledge packet before stipulated time interval after completion of the transmission. If the transmitting node does not receive the acknowledge packet within the timeout period, it assumes that the packet did not reach the destination and retransmits the packet.

9. 2.IEEE 802.11 MAC PROTOCOL The utilization of the bandwidth is very low due to MAC layer constraints. If the no. of nodes increases, the collision probability increases and lowers the overall network bandwidth. The latency of the packets depends on traffic conditions and cannot guarantee bandwidth to any node.

10. 2.1 MAC LAYER According to IEEE 802.11, stations access the channel using a basic access method, or an optional four way handshake access method with an additional Request-To –Send/Clear-To- Send (RTS/CTS) message exchange shown in fig1. Under the basic access method, the station when ready for a new data frame transmission senses the channel status. If the channel is found to be busy, the station defers its transmission and continues to sense the channel until it is ideal. After the channel is idle for a specified period of time called the Distributed Interframe space (DIFS) period, the station chooses a random number as a Back off timer.

11. 2.1 MAC LAYER

12. The time immediately after the DIFS Time period is slotted. The Backoff timer is decreased by one for each idle slot, stopped if the channel is sensed busy, and then reactivated if the channel is idle again and remains idle for more than a DIFS Time duration. When the Backoff timer reaches zero, the data frame is transmitted.

13. 2.1 IEEE 802.11 MAC PROTOCOL In the four-way handshaking access method, an additional operation is introduced on top of the basic access method before a data frame transmission taken place. When the Backoff timer of a station reaches zero, instead of transmitting the data frame as in the basic access method, the station with the four-way handshaking access method first transmits an RTS Frame too request for a transmission right. Upon receiving the RTS frame, the receiver replies with a CTS frame after SIFS period.

14. 3.DBBA-MAC PROTOCOL The DBBA MAC protocol considers a network, which has a primary controller node and multiple station nodes. The DBBA frame is made of n time slots. There are four basic types of time slots: a: Beacon time slot. b:Demand request time slot. C: Demand Assignment time slot and d: Data Time slots. The controller broadcasts a beacon packet during the beacon time slot at the beginning of each frame. The beacon frame contains information about the network SSID,frame information and timeslot information.

15. 3.DBBA-MAC PROTOCOL When a node wants to transmit, it sends a packet called the demand request packet to the controller during the demand request time slot. A demand request packet has information about the source node id, destination node id, date size and quality of service requirement. The controller processes all the requests and comes with the bandwidth assignment table. The next time slot is the Demand Assignment time slot. The controller broadcasts acknowledgement to all the accepted requests, requested during the Demand Request time slot.

16. 3.1 FRAME STRUCTUTRE

17. 3.1 FRAME STRUCTURE A typical duration of a frame ranges from 10 millisecond to 50ms. As shown in the diagram a frame has a beacon time slot, demand request time slot, bandwidth assignment time slot and n data time slots. The controller assigns special tome slots to every super frame, which could be between 500ms to 10 seconds. These special time slots allow the new users to enter the network and existing stations to acknowledge their presence. On powers up, the node tries to locate the controller. If it succeeds in finding the controller, it associates with the controller after successful authentication. If node fails to find the controller, it takes possession of being a controller in ad-hoc mode.

18. DBBA NETWORK SIMULATION MODEL

19. 4.SIMULATION In order to assess the performance of the proposed DBBA protocol, a simulator has been devised. In this section the complete details about the simulator is provided. Simulation Model: The Scheduler, Modem and packet generator are three classes used to simulate the MAC protocol for IEEE 802.11 Wireless Local Area Network.

20. 4.SIMULATION Scheduler: Scheduler is the main truck of the simulator. Scheduler collects the desired transmission time from all stations. The modem in turn runs the required methods and submits the desired transmit times. The scheduler then determines the modem references, which has the lowest desired transmission time stamp. The scheduler detects for collision and then if it detects, it sends message to all stations about the collision and the duration of the collision.

21. 4.SIMULATION Modem: Modem class depicts the behavior of stations. It gets information from the packet generator. Modems returns the packet timestamp, length, destination and other details to scheduler. If the packet is transmitted successfully, it gets a new packet from the packet generator. The modem waits for acknowledge of the transmitted packet from the scheduler. If the scheduler fails to give the acknowledge, the modem assumes that the packet has collided or has not reached the destination properly.

22. 4.SIMULATION Packet Generator: Packet Generator Class depicts a user generating traffic data. Every modem will have one instance of the Packet generator class. The length of packet is generated using three uniform distribution packets. The three different types of data packets are small, medium and large packets.

23. 4.2 FLOW CHART FOR DBBA NODE PROCESSING DATA Stations will ensure that channel is available before transmitting it. Every station other than the transmitting station will recompute the start of their packet transmission. The station does the following to simulate the condition. 1.Conputes the time at which the current packet will finish. 2.Adds the delay for acknowledgement. 3.Adds the random time to account for the back off time. 4.Submits the new desired time of transmission to the scheduler.

25. 5.SIMULATION RESULT In this paper 802.11 MAC was compared with a new protocol DBBA in terms of throughput,collisions,and average latency. In the 1 st set of experiments,the throughput and collisions of both 802.11 and DBBA were studied with different no. of nodes.

26. 5.SIMULATION RESULT According to figure, as the number of nodes increases the throughput of DBBA slightly increases due to higher traffic loads.

27. 5.SIMULATION RESULT In the figure the number of collisions is 5 % to 8% lesser than 802.11 as the number of nodes increases.

28. 5.SIMULATION RESULT In the second set of experiments, the throughput and collisions of both 802.11 and DBBA is studies withy different traffic loads. As shown in fig,the throughput of DBBA increases as traffic load increases, because it uses a TDMA frame with dynamic time slot allocation.

29. 5.SIMULATION RESULT In this figure, the amount of collisions is 5% to 8% lesser than 802.11 as the traffic load increases.

30. 5.SIMULATION RESULT In the third set of experiments, the average latency of DBBA and 802.11 is studied with increasing number of nodes and different traffic loads. As shown in fig, the delay is little more than the 802.11 as the number of nodes increases with traffic load 0.8

31. 5.SIMULATION RESULT In this figure, under different traffic load conditions, the average latency of DBBA protocols is higher than the other one.

32. 6.CONCLUSION In this paper, a variation of Demand Assignment Multiple Access-TDMA protocol has been proposed called Demand Based Bandwidth Assignment(DBBA) protocol. It has been observed that the DBBA protocol has higher bandwidth utilization,supports larger number of stations in one cell,lesser propagation delay, more power efficient and the ability to provide better quality of service.

33. 6.CONCLUSION Since there is no collision, the efficiency of the system can be increased without affecting the overall throughput of the system. Further to be considered are the allocation of dedicated slots for a complete session, which can reduce the delay for the constant bit rate services.