I hereby certify that the thesis work entitled "Mechanism for Improving the Performance of the IEEE 802.11 MAC Protocol" submitted by Priyadarshini Sabut is a record of original research work carried out by her under my supervision and guidance in partial fulfillment of the degree requirement Master of Technology in Computer Science and Engineering with specialization in Computer Science from Department of Computer Science and Engineering, National Institute of Technology Rourkela. I would like to thank the administrative and technical staff of the Department for their cooperation. The performance of these networks depends on the performance and reliability of the medium access control (MAC) protocol used in such networks.
The performance of IEEE 802.11 is degraded by the presence of hidden and visible terminals. Although IEEE 802.11 DCF can overcome the hidden and exposed terminal problem, the throughput and channel utilization are lower due to the inability of the hidden and exposed node to transmit or receive. It is noted that the proposed scheme outperforms 802.11 DCF in terms of throughput and packet delivery ratio, with marginally increased control overhead.
Introduction
- Issues in Designing A MAC Protocol for Ad hoc Network
- Summary
- Thesis Organization
- Related Work
- Summary
The efficiency of ad hoc networks depends on the performance and reliability of the Medium Access Control (MAC) protocol applied to such networks [2]. Although the channel bandwidth has increased significantly with the IEEE 802.11b standard, the study of ad hoc network should still focus on bandwidth consumption. The following issues should be addressed when designing an aMAC protocol for a wireless ad hoc network [4].
Lack of central coordination: Ad hoc wireless networks do not have centralized coordinators, nor is it possible, since the nodes are constantly moving. Since the nodes in ad hoc wireless networks are mobile most of the time, the bandwidth reservations made or the control information exchanged may end up being of no use. An efficient MAC protocol through with mobile stations can share the broadcast channel is essential in ad-hoc network as the channel is a scarce resource.
The main problems in the design of MAC protocol for ad-hoc network were also identified. Section 2 discusses some related works on MAC protocol for ad hoc network. Several new MAC protocols are constantly being developed to optimize network performance.
This section presents some of the research being done in the field of Medium Access Control (MAC) in ad hoc networks. Since the CSMA protocol senses the channel state only at the transmitter, this protocol does not overcome the hidden terminal problem when the transmitter and receiver are not in range of each other.
IEEE 802.11
AN OVERVIEW OF THE IEEE 802.11 STANDARD
AN OVERVIEW OF THE IEEE 802.11 STANDARD IEEE 802.11
- IEEE 802.11 DCF
Before sending a packet, a station operating in RTS/CTS mode "reserves" the channel by sending a special short Request-to-Send (RTS) frame. The destination station acknowledges receipt of an RTS frame by returning a Clear-to-Send (CTS) frame, after which normal DATA packet transmission and ACK response occur. Since collisions can only occur on the RTS frame and are detected by the absence of a CTS response, the RTS/CTS mechanism makes it possible to improve system performance by reducing the duration of a collision, especially when the DATA packets being big.
If the channel is inactive for a period of time corresponding to a distributed interframe space (DIFS), the station transmits. Otherwise, if the channel is detected busy (either immediately or during DIFS), the station continues to monitor the channel until it is measured free for a DIFS. The node decrements its backoff counter by one for each free slot it detects on the channel.
The node freezes the deferral counter when a transmission on the channel is detected and reactivates when the channel is again found idle for more than a DIFS. When the backoff counter reaches zero, the node sends a Request-to-Send (RTS) packet. When a node receives an RTS packet, it responds, after a SIFS, with a Clear-to-Send (CTS) packet.
All neighboring nodes overhearing either RTS or CTS update their Network Allocation Vector (NAV) for the duration that the channel will remain busy, and these nodes postpone their transmission for this duration when NAV is set (Figure 3.2 ). This mechanism to defer transmission based on NAVis known as virtual carrier sensing and it effectively reserves the channel for the current dialogue.
Problem Identification
- Hidden Terminal Problem
- Exposed Terminal Problem
- RTS -induced and CTS -induced Problem
An exposed node is a node that is within range of the transmitter but out of range of the receiver. Hidden and exposed terminal issues can often occur in ad hoc networks, causing significant degradation in network throughput. Overcoming the problem of hidden and exposed nodes has become one of the important aspects of MAC protocol design.
To overcome the hidden and exposed terminal problem, IEEE 802.11 DCF uses a mechanism called Network Allocation Vector (NAV. Nodes that overhear RTS or CTS set their NAV and delay access to their channel for the expected time to complete packet transmission. Problems arise when the RTS or CTS packet packet is not properly received at the receiver or sender node, respectively, resulting in underutilization of the channel bandwidth due to the NAV setting.
Upon hearing the RTS from node C, node B sets its NAV to the expected time required to complete the transmission.
Motivation
Proposed Solution
Summary
Assumption
The Proposed Mechanism
If both the receiver and the transmitter of the destination node are Free, then it does the following: Set the status of the transmitter and receiver of the source node to busy for the duration of the data transmission. Set their transmitter status to busy for the duration of the data transmission.
Set the status of destination node's sender and receiver to Busy for the duration of data transfer, and. Set the status of the receiver from source of the FCTS packet to Free for the duration of data transfer. The neighboring nodes of source S, upon receiving the DATA packet (before the timer expires), make the following changes:. a) Set the status of their own receiver to Busy for the duration of data transfer and also.
Notations Used
Proposed Algorithm
Example
- Analysis
The neighbors of the source node B, in our case node A (not the destination), set the transmitter and receiver of node B to Busy upon receiving the RTS packet. It also sets its own receiver to Busy for the duration of the data transmission and starts a timer for a duration of ∆t and starts listening for a data packet from node B. During the duration of ∆t, the source is expected to receive a CTS packet from destination C and start transmitting a data packet. If both the receiver and transmitter are free, then it sets Node B's transmitter and receiver and its own transmitter and receiver to Busy for the duration of the data transfer.
Nodes other than the source node, on receiving CTS packet, in current scenario node D, sets the transmitter and receiver of node C to Busy and also sets its own transmitter to Busy for the duration of data transmission. FCTS packet to its neighbor indicating that its sender is busy but receiver is Free for the duration of data transfer. Upon receiving the FCTS packet, the nodes, in the current scenario the node E, set the sender of node D to busy and the receiver of node D to free for the duration of data transmission.
The source node, when the CTS packet is received, sets the transmitter and receiver of node C and its own transmitter and receiver busy for the duration of data transmission and schedules data transmission, as shown in Figure 4.7. After the duration of transmission is over, each node sets the status of other nodes sender and receiver to unknown, this is shown in Figure-4.10. Solution for hidden terminal problem: As shown in Figure 4.5 upon receiving CTS packet from node C, node D (the hidden terminal of node B) sets its transmitter to Busy for the expected duration of data transfer.
Solution to the exposed terminal problem: In Figure 4.3, node A is the exposed terminal of node B, while node B is the transmitter and node D is the receiver. When listening to the RTS packet from node B, node A sets the receiver to Busy and the transmitter is free for the duration of the data transfer.
Summary
The next chapter emphasizes the performance evaluation of the proposed protocol and analyzes the simulation results. To evaluate the performance of our proposed protocol, we provide comprehensive simulations of system throughput, packet delivery ratio, and control packet overhead, and compare the obtained results with IEEE 802.11 MAC.
Simulation Model
Performance Metrics
Result Analysis
This is due to the fact that in the proposed scheme exposed nodes are allowed to transmit and hidden nodes are allowed to receive during the continuous transmission. It is seen from Figure 5.5 that the throughput increases with the increase in number of nodes in the proposed scheme. It is observed from the figure that the proposed scheme also has a higher packet delivery ratio compared to the traditional IEEE 802.11 MAC.
Summary
This thesis discusses the mechanism of 802.11 DCF and identifies the limitations of this mechanism, which prevent exposed nodes from transmitting and hidden nodes from receiving, as discussed in the literature. This work focuses on the bandwidth usage on the visible and hidden terminals, which is lost in traditional 802.11MAC. This is due to the fact that 802.11MAC does not keep any information about the ongoing transmissions in its environment.
The lack of overlapping transmission and reception on exposed and hidden nodes leads to throughput degradation due to inefficient channel usage. This work also focuses on two other problems, called RTS/CTS-induced problem, caused by unnecessary NAV settings on adjacent nodes even if the RTS/CTS packets are not exchanged properly.
Contribution
Future Work
Packet switching on radio channels: The i-carrier part senses multiple access modes and their throughput delay characteristics. Packet switching on radio channels: Part II–the hidden terminal problem in multiple access carrier sense and the busy voice solution. IEEE Transactions on Communications December 1975. In Ninth IEEE International Symposium on Personal, Indoor, and Mobile Radio Communications, 1998., volume 1, pages 147–152, September 1998.
In SIGCOMM ’95 Conference Proceedings on Computer Communication Applications, Technologies, Architectures, and Protocols, volume 25, pages 262–273, October 1995.
