ADVANCED AND EFFICIENT MAC PROTOCOL FOR MOBILE AD-HOC NETWORK

Rahul Mukherjee. Assistant Professor, Electronics & Telecommunication Engineering Department, Rungta College of Engineering & Technology (RCET), Bhilai, India. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 12 September 2018 Final Accepted: 14 October 2018 Published: November 2018


ISSN: 2320-5407
Int. J. Adv. Res. 6(11), 1117-1129 1118 Hidden and Exposed Node Issue:-The transmission range of stations in wireless network is limited by the transmission power; therefore, all the station in a LAN cannot listen to each other. This means that normal carrier sense mechanism which assumes that all stations can listen to each other, fails. In particular, this gives rise to hidden node and exposed node problem. Consider stations A, B, C and D as shown in figure.

CSMA/CA:-
The most important part of a MAC protocol is Channel Access Mechanism. The channel access mechanism is way of regulating the use of physical channel among the stations present in the network. It specifies when a station can send or receive data on the channel. CSMA/CA (Carrier Sense Multiple Access) is derived from CSMA/CD (Collision Detection) which is the channel access mechanism used in wired Ethernets. Since the transmission range of wireless stations is limited, collision cannot be detected directly. This protocols tries to avoid the collision. On arrival of a data packet from LLC, a station senses the channel before transmission and if found idle, starts transmission. If another transmission is going on, the station waits for the length of current transmission, and starts contention. Since the contention is a random time, each station get statistically equal chance to win the contention. CSMA/CA is asynchronous mechanism for medium access and does not provide any bandwidth guarantee. It's a best effort service and is suited for packetized applications like TCP/IP. It adapts quite well to the variable traffic conditions and is quite robust against interference. 1119

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 are classified in three types; 1. Contention based protocols. 2. Contention based protocols with reservation mechanism. 3. Contention based protocols with scheduling mechanism.

Contention Based Protocols
These protocols follow a contention based channel access policy. A node doesn't make any resource reservation in priori. Whenever it receives a packet to be transmitted, it contends with other nodes for access to the shared channel. These are further divided into two types; 1. Sender initiated protocols 2. Receiver initiated protocols 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; 1. Synchronous protocols 2. Asynchronous protocols 1120 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.    . PCF is a centralized scheme, whereas DCF is a fully distributed scheme. We consider DCF in this paper.
Transmission range :-When a node is within transmission range of a sender node, it can receive and correctly decode packets from the sender node. In our simulations, the transmission range is 250 m when using the highest transmit power level.
Carrier sensing range:-Nodes in the carrier sensing range can sense the sender's transmission. Carrier sensing range is typically larger than the transmission range, for instance, two times larger than the transmission range. In our simulations, the carrier sensing range is 550 m when using the highest power level. Note that the carrier sensing range and transmission range depend on the transmit power level.
1122 Carrier sensing zone : When a node is within the carrier sensing zone, it can sense the signal but cannot decode it correctly. Note that, as per our definition here, the carrier sensing zone does not include transmission range. Nodes in the transmission range can indeed sense the transmission, but they can also decode it correctly. Therefore, these nodes will not be in the carrier sensing zone as per our definition. The carrier sensing zone is between 250 m and 550 m with the highest power level in our simulation. The IEEE standard 802.11 specifies the most famous family of WLANs in which many products are already available. This means that the standard specifies the physical and medium access layer adapted to the special requirements of wireless LANs, but offers the same interface as the others to higher layers to maintain interoperability.
System Architecture:-The basic service set (BSS) is the fundamental building block of the IEEE 802.11 architecture. A BSS is defined as a group of stations that are under the direct control of a single coordination function (i.e., a DCF or PCF) which is defined below. The geographical area covered by the BSS is known as the basic service area (BSA), which is analogous to a cell in a cellular communications network.
Conceptually, all stations in a BSS can communicate directly with all other stations in a BSS. However, transmission medium degradations due to multipath fading, or interference from nearby BSSs reusing the same physical-layer characteristics (e.g., frequency and spreading code, or hopping pattern), can cause some stations to appear hidden from other stations. An ad hoc network is a deliberate grouping of stations into a single BSS for the purposes of internetworked communications without the aid of an infrastructure network. Given figure is an illustration of an independent BSS (IBSS), which is the formal name of an ad hoc network in the IEEE 802.11 standard. Any station can establish a direct communications session with any other station in the BSS, without the requirement of channeling all traffic through a centralized access point (AP).

Dcf Operation
The DCF is the fundamental access method used to support asynchronous data transfer on a best effort basis. The DCF is based on CSMA/CA. The carrier sense is performed at both the air interface, referred to as physical carrier sensing, and at the MAC sub layer, referred to as virtual carrier sensing. Physical carrier sensing detects presence of Inter frame Spacing:-IFS is the time interval between frames. IEEE 802.11 defines four IFSs -SIFS (short inter frame space), PIFS (PCF inter frame space), DIFS (DCF inter frame space), and EIFS (extended inter frame space). The IFSs provide priority levels for accessing the channel. The SIFS is the shortest of the inter frame spaces and is used after RTS, CTS, and DATA frames to give the highest priority to CTS, DATA and ACK, respectively. In DCF, when the channel is idle, a node waits for the DIFS duration before transmitting any packet.
In figure, nodes in transmission range correctly set their NAVs when receiving RTS or CTS. However, since nodes in the carrier sensing zone cannot decode the packet, they do not know the duration of the packet transmission. To prevent a collision with the ACK reception at the source node, when nodes detect a transmission and cannot decode it, they set their NAVs for the EIFS duration. The main purpose of the EIFS is to provide enough time for a source node to receive the ACK frame, so the duration of EIFS is longer than that of an ACK transmission. As per IEEE 802.11, the EIFS is obtained using the SIFS, the DIFS, and the length of time to transmit an ACK frame at the physical layer's lowest mandatory rate, as the following equation : EIFS = SIFS + DIFS + [(8•ACKsize) + Preamble Length + PLCP Header Length] / Bit Rate other users by analyzing the activity in the channel through where ACK size is the length (in bytes) of an ACK frame, andthe received signal strength. Bit Rate is the physical layer's lowest mandatory rate. PreambleA station performs virtual carrier sense by Length is 144 bits and PLCP Header Length is 48 bits . Using a 1 examining the received MPDU (MAC Protocol Data Unit) Mbps channel bit rate, EIFS is equal to 364 μs.information in the header of RTS, CTS and ACK frames. The stations in BSS use this information to adjust their Network Allocation Vector (NAV), which indicates amount of time that must elapse until the current transmission is complete and the channel can be sampled again for idle status.

Basic Power Control Protocol:-
Here nodes A and B send RTS and CTS, respectively, with the highest power level so that node C receives the CTS and defers its transmission. By using a lower power for DATA and ACK packets, nodes can conserve energy. In the Basic scheme, the RTS-CTS handshake is used to decide the transmission power for subsequent DATA and ACK packets. This can be done in two different ways as described below. Let pmax denote the maximum possible transmit power level.
Suppose that node A wants to send a packet to node B. Node A transmits the RTS at power level pmax. When B receives the RTS from A with signal level pr, B can calculate the minimum necessary transmission power level, pdesired, for the DATA packet based on received power level pr, the transmitted power level, pmax, and noise level at the receiver B.
We can borrow the procedure for estimating pdesired from. This procedure determines pdesired taking into account the current noise level at node B. Node B then specifies pdesired in its CTS to node A. After receiving CTS, node A 1125 sends DATA using power level pdesired. Since the signal-to-noise ratio at the receiver B is taken into consideration, this method can be accurate in estimating the appropriate transmit power level for DATA.
In the second alternative, when a destination node receives an RTS, it responds by sending a CTS as usual (at power level p max). When the source node receives the CTS, it calculates p desired based on received power level, pr, and transmitted power level (p max), as whereRxthresh is the minimum necessary received signal strength and c is a constant. We set c equal to 1 in our simulations. Then, the source transmits DATA using a power level equal to p desired. Similarly, the transmit power for the ACK transmission is determined when the destination receives the RTS.

Deficiency Of The Basic Protocol:-
In the Basic scheme, RTS and CTS are sent using pmax, and DATA and ACK packets are sent using the minimum necessary power to reach the destination. When the neighbour nodes receive an RTS or CTS, they set their NAVs for the duration of the DATA-ACK transmission. When D and E transmit the RTS and CTS, respectively, B and C receive the RTS, and F and G receive the CTS, so these nodes will defer their transmissions for the duration of the D-E transmission. Node A is in the carrier sensing zone of D (when D transmits at pmax) so it will only sense the signals and cannot decode the packets correctly. Node A will set its NAV for EIFS duration when it senses the RTS transmission from D. Similarly, node H will set its NAV for EIFS duration following CTS transmission from E.
When transmit power control is not used, the carrier sensing zone is the same for RTS-CTS and DATA-ACK since all packets are sent using the same power level. However, in Basic, when a source and destination pair decides to reduce the transmit power for DATA-ACK, the transmission range for DATA-ACK is smaller than that of RTS-CTS; similarly, the carrier sensing zone for DATA-ACK is also smaller than that of RTS-CTS. Source and destination nodes transmit the RTS and CTS using pmax. Nodes in the carrier sensing zone set their NAVs for EIFS duration when they sense the signal and cannot decode it correctly.
The source node may transmit DATA using a lower power level, similar to the BASIC scheme.
To avoid a potential collision with the ACK (as discussed earlier), the source node transmits DATA at the power level pmax, periodically, for just enough time so that nodes in the carrier sensing zone can sense it.
The destination node transmits an ACK using the minimum required power to reach the source node, similar to the BASIC scheme.  The main difference between PCM and the Basic scheme is that PCM periodically increases the transmit power to pmax during the DATA packet transmission. With this change, nodes that can potentially interfere with the reception of ACK at the sender will periodically sense the channel as busy, and defer their own transmission.
Accordingly, 15 μs should be adequate for carrier sensing, and time required to increase output power (power on) from 10% to 90% of maximum power (or power-down from 90% to 10% of maximum power) should be less than 2 μs. Thus, we believe 20 μs should be enough to power up (2 μs), sense the signal (15 μs), and power down (2 μs). In our simulation, EIFS duration is set to 212 μs using a 2 Mbps bit rate. In PCM, a node transmits DATA at pmax every 190 μs for a 20μs duration. Thus, the interval between the transmissions at pmax is 210 μs, which is shorter than EIFS duration. A source node starts transmitting DATA at pmax for 20 μs and reduces the transmit power to a power level adequate for the given transmission for 190 μs. Then, it repeats this process during DATA transmission. The node also transmits DATA at pmax for the last 20 μs of the transmission. With the above simple modification, PCM overcomes the problem of the BASIC scheme and can achieve efficiency comparable to 802.11, but uses less energy.
The proposed power control protocol is modified such that in this the Data and ACK is transmitted at lower power level but after a certain duration it is transmitted at higher power level for a very fraction of time, in order to make the neighboring nodes understand that transmission is going on and they should restrict their transmission during that period so that collision does not take place hence saving power consumption. 1127

Conclusion:-
It is shown that the Basic scheme increases collisions and retransmissions, which can result in more energy consumption and throughput degradation. Hence, the proposed protocol is more efficient than Basic scheme and 802.11 yielding better throughput with much less power consumption, hence yielding better performance compared to the rest protocols.