Improving Performance of 802.11 MAC by Optimizing DCF in Mobile ad-hoc Network

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367

Improving Performance of 802.11 MAC by Optimizing

DCF in Mobile ad-hoc Network

P.S.Patheja

_{, Akhilesh A.Waoo}

_{, Vaishali Tiwari}

2_{Asso.Prof. M. Tech BIST, Bhopal India}

Abstract—Mobile Ad-hoc Network is a self configuring infrastructure-less network of mobile devices connected by wireless They have undergone rapid growth in the past several years because of their application in military and rescue service, disaster recovery operations, mobile conferencing and many other applications. The Media Access Control (MAC) data communication protocol sub-layer provides addressing and channel access control mechanisms that make it possible for network nodes to access common wireless channel through distributed coordination function (DCF). In the proposed work we will include analysis and implementation of conventional DCF protocol and proposes a new enhanced DCF protocol (EDCF) by improvements in the Binary Exponential Back-off Algorithm (BEB) and compare the performance of Enhanced DCF with conventional DCF protocol on different parameters.

Keywords—Mobile Ad-hoc Network, Wireless Network, 802.11 MAC, Distributed Coordinated Function (DCF), Point Coordination Function (PCF), Binary Exponential Back-off (BEB).

I. INTRODUCTION

Mobile Ad-hoc networks are collections of mobile nodes, dynamically forming a temporary network without pre-existing network infrastructure or centralized administration. These nodes can be arbitrarily located and are free to move randomly at any given time, thus allowing network topology and interconnections between nodes to change rapidly and unpredictably. As MANET networks are infrastructure less there exist no dedicated routers. Instead, every mobile node acts itself as a router and is also responsible for discovering and maintaining routes. Furthermore without centralized administration, MANETs can be called autonomous. To support this kind of autonomy, the routing protocol is required to automatically adjust to frequent environment changes. MANET suffers from temporary link failures and route changes. The IEEE project 802 recommend an international standard for Wireless Local area Network (WLAN’s) and provides detailed medium access control (MAC) and physical

(PHY) layer specification for WLANs. WLANs can operate in two modes namely infrastructure based and infrastructure-less mode or ad-hoc mode. In infrastructure based mode, a central coordinator or an Access Point (AP) is needed for operation of network. In the other mode of operation, known as the Mobile ad-hoc Network (MANET) nodes can communicate with each other directly without any central coordinator. This requires that all nodes must act as packet forwards to relay packets between two stations that are outside the radio coverage of each other. This provides greater flexibility and robustness. In WLAN, nodes transmit packets in an unsynchronized fashion. The protocol employed in the MAC layer responsible for coordinating access to the shared channel while minimizing conflicts. In the 802.11 protocol, the fundamental mechanism to access the channel is called Distributed Coordinated Function (DCF). This is a random access scheme, based on the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol. Retransmission of collided packets is managed according to binary exponential back-off rules.

II. CLASSIFICATION OF MAC PROTOCOL

MAC protocols for ad-hoc wireless networks can be classified into several categories based on various criteria such as initiation approach, time synchronization, and reservation approach. Ad-hoc Network MAC protocols [5][10] are classified in three types;

 Contention based protocols

 Contention based protocols with reservation mechanism

 Contention based protocols with scheduling mechanism

A. Contention Based Protocols

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368  Sender initiated protocols

 Receiver initiated protocols

B. Contention Based Protocol with Reservation Mechanisms

Ad-hoc wireless networks sometimes may need to support real time traffic, which requires QoS guarantees to be provided. In order to support such traffic, certain protocols have mechanism for reserving bandwidth in priori. These protocols are classified into two types;

 Synchronous protocols

 Asynchronous protocols

C. Contention Based Protocol with Scheduling Mechanisms

These protocols focus on packet scheduling at nodes, and also scheduling nodes for access to the channel. Node scheduling is done in a manner so that all nodes are treated fairly. Scheduling based scheme are also used for enforcing priorities among flows whose packets are queued at nodes.

III. MAC SUB-LAYER ARCHITECTURE

The basic medium access method of the IEEE 802.11 protocol is the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). Figure 1 illustrates the MAC architecture indicating the Distributed Coordination Function (DCF) and Point Coordination Function (PCF).

Required for contention free service

MAC

Used for Contention Services and basis for PCF

Figure 1 MAC Sub-layer Architecture

IV.DISTRIBUTED COORDINATION FUNCTION (DCF)

A DCF is the basic medium access method using CSMA/CA and a random back-off time following a busy medium condition. Since a wireless station cannot hear its own transmission, it cannot detect collision therefore, the CSMA/CA algorithm is used.

A positive acknowledgement is also needed for each transmitted frame. If an acknowledgement is not received, a retransmission takes place [1] [11].

A. Carrier Sense Mechanism

Both physical and virtual carrier sense mechanism are used in determining of the medium. The medium is idle only when both mechanisms indicate such a condition. The physical layer performs a physical carrier sensing and forward the information to the MAC. The MAC layer uses the network allocation vector (NAV) to implement the virtual carrier sense mechanism, which reserves the medium for transmitting a data frame and its acknowledgement. Reserving the medium is accomplished in two ways: by using a Duration/ID field in the request-to-send (RTS) and clear-to-request-to-send (CTS) frames. or using the Duration/ID field in directed frames.

B. Inter-frame Space (IFS)

After sensing that the medium is idle, a station must wait for a certain interval of time called an inter-frame space (IFS). There are four different types of IFS. The first type is a short inter-frame space (SIFS), used in sending an acknowledgement, CTS, and the second or subsequent frames of a fragment burst. The second type is a PCF inter-frame space (PIFS). Except when responding to the poll, a station will use PIFS during the CFP. The third type is a DCF inter-frame space (DIFS), which is used under the DCF. DIFS is the longest inter-frame space. The fourth type of IFS is an extended inter-frame space (EIFS), used when the first attempt to transmit a frame has failed. Since the EIFS is shorter than DIFS, a retransmission has higher priority than a normal transmission.

C. DCF Access Procedure

Basic access is a core mechanism in accessing the medium. The back-off time counter is decremented as long as the channel is sensed idle “frozen” when a transmission is detected on the channel and reactivated when the channel is sensed idle again for duration larger than DIFS. The station transmits when the back-off time reaches zero figure 2 illustrates this operation [9]. Two station A and B share the same wireless channel.

Point Coordination

Function (PCF)

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369 At the end of the packet transmission, station B waits for a DIFS and then chooses a back-off time equal to 8 (uniformly chosen between 0 and CWmin), before transmitting the next packet. Assumes that the first packet of station A arrives at the time indicated with an arrow in the figure. After a DIFS the packet is transmitted. At the transmission time of the station A, the station B is in the middle of the slot time corresponding to back-off value equal to 5. As a consequence of the channel sensed busy, the back-off time is frozen to its value 5 and the back-off counter decrements again only when the channel is sensed idle for a DIFS [1][9].

STATION A DIFS DIFS

SIFS

STATION B DIFS

DIFS

8 7 6 5 4 3 2 1 0

Slot time 5: Frozen back-off time

Figure 2 DCF protocol with Basic Access Method [9]

The CSMA/CA doesn’t rely on the capability of the stations to detect a collision by hearing their own transmission. For that reason, an ACK is transmitted by the destination station to signal the successful packet reception. The ACK is transmitted immediately at the end of the packet, after a period of time called short interframe space (SIFS). As the SIFS is shorter than a DIFS, no other station is able to detect the channel idle for a DIFS until the end of the ACK. If the transmitting station does not receive the ACK within a specified ACK Timeout or it detects the transmission of a different packet on the channel, it reschedules the packet transmission according to the back-off rules.

D. RTS/CTS Access Mechanism

DCF defines an additional four-way handshaking technique to be optionally used for a packet transmission [9][11].

This mechanism, which is called as RTS/CTS (Request-to-send/Clear-to-send), is shown in figure 3. A station that has a packet to transmit waits until the channel is sensed idle for a DIFS. Then instead of sending the data packet preliminarily sends a special frame called RTS. When receiving station detects an RTS frame it responds after a SIFS with a CTS frame. The transmitting station is allowed to transmit its packet only if the CTS frame is correctly received. The frames RTS and CTS carry the information of the length of the packet to be transmitted. This information can be read by any listening station and then that station can update a NAV containing the information of the period of time in which the channel will remain busy. Therefore when a station is hidden from either the transmitting or the receiving station, by detecting just one frame among the RTS and CTS frames, it can suitably delay further transmission, and thus avoid collision.

SIFS SIFS SIFS DIFS

Source

Destination

Others Delayed Medium Access Back-off

Figure 3 DCF with RTS/CTS Access Method [9]

V.POINT COORDINATION FUNCTION (PCF)

A PCF offers a guarantee of access to the medium for stations in a BS. This is beneficial for time-bound application such as voice or video. A PCF consists of a point coordinator (PC) and stations that can respond to the contention free (CF) polling frame. The PC controls the access of the medium during the contention free period. Once polled, a station may transmit only one frame to any station. All station, including the PC may “piggyback” the acknowledgement using data frame subtypes to increase the efficiency of the CFP.

Packet A

ACK

ACK RTS

CTS

DATA

ACK

NAV (RTS)

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370

VI.BINARY EXPONENTIAL BACK-OFF(BEB)ALGORITHM

The back-off procedure shall be invoked for a STA to transfer a frame when finding the medium busy as indicated by either the physical or virtual carrier sense mechanism [3]. The back-off procedure shall also be invoked when a transmitting STA infers a failed transmission. To begin the back-off procedure the STA shall set its back-off timer to a random back-off time. DCF adopts an exponential back-off scheme. At each packet transmission, the back-off time is uniformly chosen in the range (0, W-1). The value W is called contention window and depends on the number of transmission failed for the packet. At the first transmission attempt, is set equal to a value CWmin called minimum contention window. After each unsuccessful transmission W is doubled up to a maximum value CWmax = 2m * CWmin. The values and reported in the final version of the standard are summarized in table 1.

PHY Slot Time CWmin CWmax

FHSS 50µsec 16 1024

DSSS 20 µsec 32 1024

IR 8 µsec 64 1024

Table 1

Slot Time, Minimum and Maximum Contention Window values for three PHY specified by 802.11: Frequency Hopping Spread Spectrum (FHSS), Direct Sequence Spread Spectrum (DSSS), Infrared (IR)

The back-off time counter is decremented as long as the channel is sensed idle, “frozen” when a transmission is detected on the channel, and reactivated when the channel is sensed idle again for more than a DIFS. The station transmits when the back-off time reaches zero.

VII.PROPOSED GOALS

The main goal of this study is to analyze and enhancement of 802.11 MAC by optimization in DCF. The objective of this research is to evaluate proposed EDCF (Enhanced Distributed Coordinated Function) protocol by improvements in the Binary Exponential Back-off Algorithm.

In the old BEB algorithm, as the contention window is doubled, large back-off introduces unnecessary idle time on the channel and increase the average packet delay and reduces throughput as the number of nodes in the network increases. In the EDCF will try to monitor the performance of network by increasing the contention window with a back-off factor range.

To achieve the main goal the following have to be achieved:

 Get a general understanding of Ad-Hoc Networking

 Study of various MAC protocols in 802.11 for wireless Ad-Hoc Networks

 Understanding of NS2 simulator, where the implementation of EDCF protocol for wireless Ad-Hoc Network protocol by improvements in BEB algorithm will take place

 Analyze the protocol theoretically and through simulation.

VIII.SIMULATION ENVIRONMENT

Simulation is a fundamental tool in the development of MANET protocols, because the difficulty to deploy and debug them in real networks. The simulation eases the analyzing and the verification of the protocols mainly in large scales systems. It offers flexible testing with different topologies, mobility patterns, and several physical and link layer protocols. To test the new protocol, Network Simulator (NS2) will be used. The simulation will be run for existing conventional DCF and will be run for enhanced protocol (EDCF) under the same environment to see the performance differences against various parameters. The performance matrices will be tested are Average end-to-end delay, throughput, average jitter and total packet loss ratio.

IX.CONCLUSION

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371 Different values for back-off factors will be tested and compare against the conventional IEEE 802.11 DCF protocol which can give better performance.

X.FUTURE ENHANCEMENTS

There are number of enhancements can be made over Enhanced DCF proposed in this study. This will involve certain changes in the message format and access procedures of DCF algorithm. In the proposed work we will consider IEEE 802.11b mode of operation also known as Wi-Fi. However, IEEE is coming up with several different versions of 802.11 such as a/b/c/d/e/f/g/h/n etc. and it becomes important to observe the medium access techniques of all these technologies and suggest a common MAC protocol for them which improves efficiency and increases throughput, at the same time is compatible with other LAN technologies.

Some suggested areas of improvements are:

A. Enhancement in EDCF to support real time traffic B. Modifications in the DCF algorithm to support

priority access for different traffic types

C. Enhancements in DCF to support battery power conservation for mobile stations

REFERENCES

[1] Dr. Jyotsana Sengupta and Er. Gurpreet Singh Grewal , Reader department of computer science, Punjabi university patiala and lecturer in department of information and technology, Ludhiana “Performance Evaluation of IEEE 802.11 MAC layer in supporting delay sensitive services”. International journal of wireless and mobile networks, vol2, no 1, February 2010.

[2] “Contention Window Optimization: an enhancement to IEEE 802.11 DCF to improve Quality of service”, Sedrati Maamar, Bilami

Azeddine, Benmohammed Mohamed, Computer Science

Department. IJDIWC 1(1): 273-283 The Society of Digital Information and wireless Communications, 2011(ISSN 2225-658X).

[3] “Performance Analysis of the Binary Exponential Backoff Algorithm for IEEE 802.11 Based Mobile Ad Hoc Network”, Yi Hua Zhu, Xian-Zhong Tian and Jun Zheng, School of Computer Science and Technology, IEEE Communications , IEEE ICC 2011.

[4] Bhanuprakash Batula, Satya Prasad and Mohammed Maulana, A.P. India “Performance analysis on IEEE 802.11 non-saturated DCF”, IJCSI, vol. 8, issue 3, no. 1, may 2011.

[5] MAC classification ref. BANGLADESH UNIVERSITY OF

ENGINEERING AND TECHNOLOGY Tunable Parameters for IEEE 802.11 based Ad-hoc network.

[6] IEEE 802.11e/D11.0, Draft Supplement to part: Wireless Medium Access Control (MAC) and Physical Layer (PHY) specifications: Medium Access Control (MAC) Enhancements for Quality of Services (QOS), October 2004.

[7] Protocol Enhancement for IEEE 802.11e EDCF, Networks, 2004 (ICON 2004), 12th _{IEEE Conference on Nov 2004.}

[8] Imrich Chlamtac, Macro Conti, Jennifer, Mobile ad hoc networking : imperatives and challenges, School of engineering, University of Texas at Dallas, USA, Ad-Hoc Networks 1 (2003).

[9] Qiang Ni, Lamia Romdhani, Thierry Turletti, “A survey of QoS Enhancements for IEEE 802.11 Wireless LAN”. Journal of Wireless Communications and Mobile Computing, Wiley, 2004, vol. 4, issues 5.

[10]Vadiya, Nitin H., “Mobile Ad Hoc Networks: Routing, MAC and Transport Issues”, MobiComm 2001 Tutorial, Rome, IT, pp. 1-431, July 2001.

[11]IEEE 802.11 standard, “Wireless LAN Medium Access Control (MAC) and Physical Layer specifications” IEEE std. June 1999.