A STUDY ON MAC PROTOCOLS FOR WIRELESS AD HOC NETWORK

A STUDY ON MAC PROTOCOLS FOR WIRELESS AD HOC NETWORK S.Mythili #1, Dr.N.Rajendran *2 #1 Research Scholar, Periyar University, Salem, Tamilnadu, India. *2 Principal, Vivekanandha Arts and Science College for Women, Sankari, Tamilnadu, India Abstract--Wireless ad hoc networks are one of recent most interesting rising technologies. An ad hoc network consists of nodes that must communicate with each other without relying on any infrastructure or pre-defined hierarchy. Medium access control (MAC) protocols provide a means to nodes to access the wireless medium efficiently and collision free to the best of their ability. MAC layer protocols allow a group of users to share a communication medium in a fair, stable, and efficient way. Ad hoc wireless networks present even greater challenges than infrastructure wireless networks at the MAC layer. The absence of a centralized controller creates the need for distributed management protocols at the MAC layer, and possibly at higher layers of the network stack. This paper explains comprehensive survey of the MAC protocols for ad hoc networks. Keywords: Ad-hoc networks, MAC, Contention Based Protocols I. INTRODUCTION MAC protocols set defined rules to force distributed users/nodes to access the wireless medium in an orderly and efficient manner. MAC layer is sub layer of Data Link Layer involves the functions and procedures necessary to transfer data between two or more nodes of the network. It is responsible for error correction of anomalies occurring in the physical layer, framing, physical addressing, and resolving conflicts occurring in number of nodes to access the channel. Due to Mobility of nodes, in Ad-hoc wireless network, we may face the huge challenges of ad-hoc such as the lack of centralized control may get loss of connectivity, network partitioning, and bit errors. This involves bandwidth inefficiency utilization, not supported real time traffic, and hidden/exposed terminal problem express collisions. The design of a MAC protocol has to address on how reliably and efficiently data can be transmitted in this type of network [1]. MAC layer takes responsibilities of error correction for anomalies occurring in the physical layer, framing, physical addressing, flow controls and also address issues caused by mobility of nodes and an unreliable time varying channel [1]. Each type of network uses slightly different techniques and algorithms from each other in all aspects including MAC algorithms as well. It allows several users simultaneously to share a common medium of communication in order to gain maximum of channel utilization with minimum of interference and collisions [2]. MAC algorithm is similar to traffic regulations in the highway. Several vehicles cross the same road at a time but rules required to avoid collision e.g., follow the traffic lights, building the flyovers etc [3]. Wireless ad hoc networks are slightly different to the wireless network as in wireless network a central coordinator or access point exists in the network and all the connections are done through this central node where to configure a network in ad hoc mode, every couple of nodes can communicate with each other independent of the central coordinator by sharing a common radio broadcast channel [2] [3]. However the radio spectrum is limited, the bandwidth available for communication in such network is also limited [5]. That s why access to this shared medium should be controlled in such a manner that all nodes receive fair share of the available amount of bandwidth, and that the bandwidth is utilized efficiently [5]. Since ad hoc wireless network needs to address unique issues which are discuss in the next section, a different set of protocols is required for controlling access to the shared medium in such network and these are nothing but only MAC protocols [4] [5]. In this paper is organized various protocols involve in designing MAC protocols. Ad hoc network MAC protocols can be classified into three types (Figure 1): Contentionbased protocols, Contention-based protocols with reservation mechanisms and Contention-based protocols with scheduling mechanisms Figure 1: MAC Protocols 44

II. CONTENTION-BASED PROTOCOLS a) MACA(Multiple Access Collision Avoidance) : It extends for multiple access collision avoidance protocol and was proposed as an alternative solution of the issues arise in traditional carrier sense multiple access protocol. MACA does not make use of carrier sensing for channel access. It uses two additional signaling packets: the requestto-send (RTS) packet and clear-to-send (CTS) packet. When a packet wants to transmit a data packet, it first transmits a RTS packet to the intended receiver. On receiving the RTS packet, if the receiver node is not involving with other transmission and ready to receive the data packet, then reply back the sender node by sending a CTS packet [6]. Once the CTS packet is received by the sender without any error, it starts transmitting the actual data packet. This whole mechanism is depicted in figure 2. Figure 2: Packet transmission in MACA Neighbor node hears the transmission of the RTS packet by the sender and defers its own transmission till the sender could receive the CTS packet. Both CTS & RTS packets carry the expected duration of the data packet transmission. If no CTS packet is heard by the node during its waiting period, it is free to transmit packet, once the waiting interval is over. Thus, Exposed terminal problem is solved in MACA. Similarly, a node near receiver, upon hearing the CTS packets, defers its transmission till the receiver receives the data packet. Hence, the hidden terminal problem is also overcome in MACA. Like IEEE 802.11 DCF, MACA too used binary exponential back-off (BEB) algorithm to determine lost / corrupt packets and to retransmit new packets. In this technique, if a packet transmitted by a node is lost, then the node back off for a random interval of time before retrying a new transmission. Each time a collision is detected, the node doubles its maximum back off Window. b) MACA-BI (Multiple Access Collision Avoidance-By Invitation) : MACA-BI [7] is a receiver-initiated protocol and it reduces the number of such control packet exchanges. Instead of a sender waiting to gain access to the channel, MACA-BI requires a receiver to request the sender to send the data, by using a Ready-To-Receive (RTR) packet instead of the RTS and the CTS packets. Therefore, it is a two-way exchange (RTR-DATA) as against the three-way exchange (RTS-CTS-DATA) of MACA [8]. Since the transmitter cannot send any data before being asked by the receiver, there has to be a traffic prediction algorithm built into the receiver so it can know when to request data from the sender. c) MACAW (Media Access Protocol for Wireless LAN): It introduces more optional control frames and an improved BEB algorithm to overcome some shortcomings arises in MACA. The BEB algorithm in MACAW is modified by launching a packet header as an additional field to carry the current back-off counter value of the transmitting node so that on receiving the packet the receiver node can copy this value into its own back-off counter [9]. This process helps to allocate bandwidth in a fair manner. Secondly the backoff counter value reset mechanism is modified by introducing a mathematical computation to avoid the large variation in the back-off values. In this technique, a Multiplicative Increase and Linear Decrease (MILD) backoff mechanism is used where upon a collision; the back-off is increased by a multiplicative factor and upon a successful transmission, it is decremented by one. It eliminates the rapid adjustment in the back-off counter values and helps to provide a quick escalation in the back-off values at high contention time. d) Floor Acquisition Multiple Access (FAMA): is another MACA based scheme that requires every transmitting station to acquire control of the floor (i.e., the wireless channel) before it actually sends any data packet [10]. Unlike MACA or MACAW, FAMA requires that collision avoidance should be performed both at the sender as well as the receiver. In order to acquire the floor, the sending node, sends out an RTS using either non-persistent packet sensing (NPS) or non-persistent carrier sensing (NCS). The receiver responds with a CTS packet, which contains the address of the sending node. Any station overhearing this CTS packet knows about the station that has acquired the floor. The CTS packets are repeated long enough for the benefit of any hidden sender that did not register another sending node s RTS. The authors recommend the NCS variant for ad hoc networks since it addresses the hidden terminal problem effectively. The objective of a FAMA protocol is to allow a station to acquire control of the Floor (channel) dynamically, and in such a way that no data packets ever colloid with any other packet. This can be viewed as a form of dynamic reservation. A FAMA protocol requires a station who wishes to send one or more packets to acquire the channel before transmitting the packet queue [10]. The channel is acquired using control packets that are multiplexed together with the data packets in the same link in such a way that, although control packets may colloid with others, data packets are always sent free of collision. III. CONTENTION-BASED PROTOCOLS WITH RESERVATION MECHANISMS a) D-PRMA (Distributed Packet Reservation Multiple Access) : D-PRMA is a TDMA [11] based scheme where the channel is divided into frames. Each minislot contains two control fields, RTS/BI Request To Send / Busy 45

Indication and CTS/BI Request To Send / Busy Indication. The competition for slots is a certain period from the beginning of every slot and it is reserved for carrier-sensing. The nodes compete for the first minislot in each slot. The winning one transmits a RTS packet through the RTS/BI part of the first minislot. The receiver responds by sending a CTS packet through the CTS/BI field. Thus, the node is granted all the subsequent minislots. In addition to that, this very same slot in the subsequent frames is reserved for the same node, until it ends its transmission. Within a time slot, communication between the source and destination nodes is done either by Time Division Duplexing (TDD), or by Frequency Division Duplexing (FDD). b) Collision Avoidance Time Allocation (CATA) Protocol: In this protocol, time is divided into frames, each frame into slots, and each slot into 5 minislots. The first four minislots are control ones, CMS, only the fifth is used for data transmission, DMS, and it is longer than the other ones. Figure 3: Time Division in the CATA protocol CATA supports broadcast, unicast, and multicast transmissions at the same time. CATA has two basic principles: First,The receiver of a flow must inform other potential source nodes about the reservation of the slot, and also inform them about interferences in the slot. and Second Negative acknowledgements are used at the beginning of each slot for distributing slot reservation information to senders of broadcast or multicast sessions. The CMS1 and CMS2 are used to inform neighbors of the receiving and the sending nodes accordingly about the reservation. The CMS3 and CMS4 are used for channel reservation [5]. c) Hop-Reservation Multiple Access (HRMA) : HRMA [12] is an efficient MAC protocol based on FHSS radios in the ISM band. Earlier protocols such as used frequencyhopping radios to achieve effective CDMA by requiring the radio to hop frequencies in the middle of data packets. HRMA uses time-slotting properties of very-slow FHSS such that an entire packet is sent in the same hop. HRMA requires no carrier sensing, employs a common frequency hopping sequence, and allows a pair of nodes to reserve a frequency hop (through the use of an RTS-CTS exchange) for communication without interference from other nodes. One of the N available frequencies in the network is reserved specifically for synchronization. The remaining N- 1 frequencies are divided into M = floor ((N- 1)/2) pairs of frequencies. For each pair, the first frequency is used for Hop Reservation (HR), RTS, CTS and data packets, while the second frequency is used for ACK packets. HRMA can be treated as a TDMA scheme, where each time slot is assigned a specific frequency and subdivided into four parts - synchronizing, HR, RTS and CTS periods. 46 d) FPRP (Five-Phase Reservation Protocol) : It is a single-channel time division multiple access (TDMA)- based broadcast scheduling protocol. Nodes use a contention mechanism in order to acquire time slots. This protocol is fully distributed, so it offers multiple reservations at simultaneously. And it assumes the availability of global time at all nodes. The reservation takes five phases: reservation, collision report, reservation confirmation, reservation acknowledgement, and packing and elimination phase. It has two frames: Reservation (RS) and Information Phase (IS). In order to reserve an IS, a node needs to contend during the corresponding RS where it composed by M reservation Cycle (RC). Nodes don t need to distribute the information other than one-hop neighbor nodes before reservation becomes successful. e) Multiple Access Collision Avoidance with Piggyback Reservations (MACA/PR) : Lin and Gerla proposed MACA/PR architecture to provide efficient rt multimedia support over ad hoc networks [13]. MACA/PR is an extension of IEEE 802.11 and FAMA. The architecture includes a MAC protocol, a reservation protocol for setting up rt connections and a QoS aware routing scheme. We will discuss only the MAC protocol here. In MACA/PR, nodes maintain a special reservation table that tells them when a packet is due to be transmitted. The first data packet in an rt data stream sets up reservations along the entire path by using the standard RTS-CTS approach. Both these control packets contain the expected length of the data packet. As soon as the first packet makes such a reservation on a link, a transmission slot is allocated at the sender and the next receiver node at appropriate time intervals (usually in the next time cycle) for the subsequent packet of that stream. The sender also piggybacks the reservation information for the subsequent data packet in the current data packet. The receiver notes this reservation in its reservation table, and also confirms this through the ACK packet. Neighboring nodes overhearing the data and ACK packets, become aware of the subsequent packet transmission schedule, and back off accordingly. The ACK only serves to renew the reservation, as the data packet is not retransmitted even if the ACK is lost due to collision. If the sender consecutively fails to receive ACK N times, it assumes that the link cannot satisfy the bandwidth requirement and notifies the upper layer (i.e., QoS routing protocol). Since there is no RTS-CTS exchange after the first data packet, collision prevention of rt packets is through the use of the reservation tables. For nrt data packet, MACA/PR uses IEEE 802.11 DCF. f) Real-Time MAC (RT-MAC) : In IEEE 802.11 protocol, packets that have missed their deadlines are still retransmitted, even though they are not useful any more. This causes bandwidth and resources to be wasted. Baldwin et al. [14] proposed a variation of the IEEE 802.11 protocol called RT-MAC that supports rt traffic by avoiding packet collisions and the transmission of already expired packets. To achieve this, RT-MAC scheme uses a packet transmission deadline and an enhanced collision

avoidance scheme to determine the transmission station s next backoff value. When an rt packet is queued for transmission, a timestamp is recorded locally in the node indicating the time by when the packet should be transmitted. The sending node checks whether a packet has expired at three points: before sending the packet, when its backoff timer expires and when a transmission goes unacknowledged. An expired packet is immediately dropped from the transmission queue. When the packet is actually about to be sent out, the sending node chooses the next backoff value and records it in the packet header. Any node that overhears this packet then ensures that it chooses a different backoff value. This eliminates the possibility of collision. The range of values (i.e., contention window, CW) from which the backoff value is chosen, is made a function of the number of nodes in the system. Therefore, the number of nodes should be known or at least estimated in this scheme. IV. CONTENTION-BASED PROTOCOLS WITH SCHEDULING MECHANISMS a) Distributed Priority Scheduling (DPS): DPS uses the RTS-CTS-DATA-ACK packet exchange mechanism. The RTS packet contains the priority label for the DATA packet that is to be transmitted. The corresponding CTS contain it also. Neighbor nodes receiving the RTS packet update their scheduling tables with the node and its priority. When the DATA packet is sent, it contains piggybacked information about the priority of the next packet from this node and its priority, so the other nodes can record this information. Finally, when the acknowledgement comes, the nodes delete the entry for the data packet that is being acknowledged [5]. This mechanism enables the nodes to evaluate their priority in relation to the priority of the other nodes. Distributed priority scheduling (DPS) piggy-backs the priority tag of a node s current and head-of-line packets o the control and data packets The packets information are transmitted to its neighborhood, a node build a table (scheduling table) which maintains rank of position or priority of the packets and compare to the neighbor nodes. This rank is considered for back-off calculation and build schedule table with rank. b) Distributed Wireless Ordering Protocol (DWOP): The purpose of this protocol is to achieve a distributed FIFO schedule among multiple nodes in an ad hoc network. When a node transmits a packet, it adds the information about the arrival time of queued packets. All nodes overhear this information and record it in their local scheduling table. This information helps a node establish its relative priority in relation to the partial list of the nodes within its range, associated with their arrival times. According to DWOP, a node should contend for channel access only when it has the lowest arrival time of all the nodes within its range [6]. Entries in the table are deleted when the node hears the ACK packet. In case these ACK packets are not heard, we end up having false entries in the tables. This can be detected if a node notices other packets with lower priority being transmitted; it can solve the 47 problem by deleting the oldest entry. In case not all the nodes are within radio range of each-other, incomplete table information will lead to collisions, and will prevent a pure FIFO scheduling from happening [5,6]. c) DLPS (Distributed Laxity-based Priority Scheduling Scheme): Scheduling decisions are made based on the states of neighbouring nodes and feedback from destination nodes regarding packet losses Packets are recorded based on their uniform laxity budgets (ULBs) and the packet delivery ratios of the flows. The laxity of a packet is the time remaining before its deadline. It maintains Scheduler Table (ST) and Packet Delivery Ratio Table (PDT). ST maintains sorted information of the packets with priority index. PDT maintains count of the packets transmitted and ACK is received for every flow passing through the nodes. V. OTHER PROTOCOLS a) Rate-Adaptive MAC (RBAR): Some researchers have investigated the effects of dynamically tuning some parameters in 802.11 as well as general MAC protocols for ad hoc networks. Holland, Vaidya and Bahl present a Received-Based AutoRate (RBAR) protocol that adjusts the transmission rate according to latest channel condition. In the scheme, the channel quality estimation and rate selection are performed on the receiver side, because the authors believe the channel quality experienced by the receiver actually determines whether a packet can be successfully received. The channel quality estimation and rate selection are carried out on a per-packet basis through modified RTS/CTS packets, which contain the desired transmission rate and the size of the data packet rather than the duration of the reservation. Other nodes hearing the RTS/CTS packets calculate the reservation duration based on the provided information. With such modifications, RBAR protocol can be incorporated into 802.11. According to the simulation results, RBAR improves the network throughput over previous proposed rate adaptive protocols. The disadvantage of RBAR is computation overhead. Since the channel quality sensing/estimation is performed per packet, the overhead will be considerable under heavy traffic and eventually leads to longer packet delay. VI. CONCLUSION In our survey of ad hoc network MAC protocols, it has become evident that the overwhelming majority of these protocols was derived heuristically and was aimed at optimizing a particular set of measures under a particular set of operating conditions. However, most of these heuristics lacked generality and were not tested in a deployed network. Establishing a principled framework for optimizing ad hoc network behavior is challenging since there is clearly a wide range of applications and potential physical-layer technologies that have different considerations. To address both of these issues, this will allow the network to optimize cost based on the requirements of various settings. The cost function equips

the MAC protocol with the means to exercise admission control and coordinate effectively between the physical layer and higher layers. A promising direction for future work would be to integrate the cost function into an ad hoc network MAC protocol that follows suitable guidelines. REFERENCES [1] Rajesh Yadav, Shirshu Verma, N.Malaviya, A survey of MAC protocols for Wireless Sensor Networks, UbiCC Journal, Volume 4, November 3, pp. 827-833, August 2009. [2] M. Rubinstein, I. Moraes, M. Campista, L.K. Costa, O.B. Duarte, A Survey on Wireless Ad Hoc Networks, in: G. Pujolle (Ed.) Mobile and Wireless Communication Networks, Springer US, 2006, pp. 1-33. Dr.N.Rajendran received his Ph.D Degree from Periyar University, Salem in the year 2011. He has received his M.Phil, Degree from Bharathiar University, Coimbatore in the year 2000. He has received his M.C.A Degree from Madras University, Chennai in the year 1990. He is working as Principal of Vivekanandha Arts and Science College for Women, Sankari, Salem, Tamilnadu,. He has 24 years of experience in academic field. He has published 21 International Journal papers and 15 papers in National and International Conferences. His areas of interest include Digital Image Processing and Networking. [3] Maali ALBALT, Qassim NASIR, Adaptive Backoff Algorithm for IEEE 802.11 MAC Protocol, International Journal of Communications, Network and System Science, Vol. 4, pp. 249-324, 2009. [4] V.K Sing, Sanjay Ghatak, Lekhika Chetri, Biswaraj Sen, Effect of Exponential Back-off Mechanism in MACA and MACAW for MANETs: A Survey, International Journal of Latest Trends in Engineering and Technology, Vol.1, Issue 2, pp. 53-57, July 2012. [5] C.Siva Ram Murthy, B.S. Manoj, Ad hoc Wireless Network: Architecture and Protocol, PEARSON Publication, 2004 [6] P. Karn, MACA a new channel access method for packet radio, In Proceedings of the ARRL/CRRL Amateur Radio 9th Computer Networking Conference September 22, 1990. [7] S. Kumar, V. Raghavan, and J. Deng, 2006 Medium Access Control protocols for ad hoc wireless networks: A survey. [8] A. Abbas and O. Kure, 2010 Quality of Service in mobile ad hoc networks: a survey, International Journal of Ad Hoc and Ubiquitous Computing,vol. 6 [9] V. Bharghavan, A. Demers, S. Shenker, L. Zhang, MACAW: a media access protocol for wireless LAN's, in: Proceedings of the conference on Communications architectures, protocols and applications, ACM, London, United Kingdom, 1994, pp. 212-225. [10] C. L. Fullmer and J. J. Garcia-Luna-Aceves, Floor Acquisition Multiple Access (FAMA) for Packet-Radio Networks, Proc. ACM SIGCOMM, Cambridge, MA, Aug. 28 - Sep. 1, 1995. [11] Marek Natkaniec,., Szymon Szott., 2013 A Survey of Medium Access Mechanisms for Providing QoS in Ad-Hoc Networks. [12] Z. Tang and J. J. Garcia-Luna-Aceves, Hop-Reservation Multiple Access (HRMA) for Ad-Hoc Networks, IEEE INFOCOM, 1999. [13] C. R. Lin and M. Gerla, "MACA/PR: An Asynchronous Multimedia Multihop Wireless Network, Proc. IEEE INFOCOM, March 1997. [14] R. O. Baldwin, N. J. Davis IV and S. F. Midkiff, A Real-time Medium Access Control Protocol for Ad Hoc Wireless Local Area Networks, Mobile Computing and Commun. Rev., Vol. 3(2), 1999, pp. 20-27. AUTHOR S PROFILE S.Mythili received her M.Phil(C.S) Degree from periyar University,Salem in the year 2006. She has received her M.Sc.,(CS) Degree from Periyar University, Salem in the year 2004. She is working as Assistant Professor, Department of Computer Science, Siri PSG College Of Arts and Science for Women, Sankari, Tamilnadu, India. Her areas of interest include Data Mining and Networking 48