MN603 Wireless Networks and Security:Medium Access Control Protocol

Answer:

Introduction 

Medium Access Control or MAC protocol which is considered key element for various kinds of underwater acoustic networks that is UWANs. It has some special features like long delay in propagation, small capacity for channel, small reliability, proper dynamics for quality of channel [1]. It has some MAC designed strategies for which was originally designed for radio frequency based wireless networks (RWNs) which cannot work better in UWANs. In recent years’ human water activities in ocean have grown to a large extent with is inclusive of large number of sensor. Underwater wireless networking has become an important topic for more than one year [2]. It is similar to radio-frequency depended wireless network which is generally used in the environment of terrestrial on the contrary Medium Access Control is considered to be an important factor for underwater wireless networks.

 In the coming pages of the report a brief comparison has been provided regarding existing radio frequency based wireless networks with underwater acoustic network. After that an evaluation has been done regarding the Medium Access Control (MAC) for underwater acoustic network by considering the four factors namely long propagation delay, single depended reservation, scheduling which is based on MAC and receiver initiated protocol has been discussed in detail in the coming pages of the report.

Comparison of Radio Wireless Networks and Underwater Acoustic Networks 

Nowadays extensive research is seen in the field of research activity which is in relation to communication at underwater communication and sensors [3]. The main research in underwater communication focus on increasing distance and bandwidth with reduction in consumption of energy of underwater devices. Underwater research mainly focusses on optical signals, electromagnetic signals. System which works on the principle of optical communication can easily reach very high propagation. Systems which are based on acoustic waves tends to be less sensitive to fine particles suspended in water than optical waves. The advantage of using acoustic method is that they can reach larger distance. There is some disadvantage like it has low data rate which is generally limited by some important factors like attenuation, low carrier frequency and good reflection. In acoustic and ultrasonic communication, the goal of various researcher is to vary the type of modulation so that it can easily minimize the effects of reflection and a high rate of communication can be obtained [4]. Another problem associated in underwater communication is the low rate of data because of use low frequencies. Wireless sensors should be placed very close to each other so that proper measurement can be taken from the water during high communication of bandwidth.

On the contrary Radio Frequency (RF) technique is mainly used when high rate of data is required. In RF technique user are able to reach communication rate of 100 Mb/s in very short interval of time. Electromagnetic waves in the range of RF is generally considered to be better option for underwater communication system [5]. In comparison to underwater sensor networks, EM waves tends to be less sensitive to refraction and reflection in the shallower region of water. The speed of EM waves is much higher than acoustic.  The speed of EM waves generally depends on following parameters namely permeability, volume charge density, permittivity and lastly conductivity.  This parameter generally depends on the kind of water value of electrical conductivity associated with it. This ultimately depends on speed or rate of wave propagation and coefficient of absorption which directly depends on frequency of operation [6]. The main disadvantage of underwater communication is that it is dependent on EM waves for high attenuation which results from the conductivity of the water. The value of attenuation increases with the increases in the value of EM wave [7]. It can be concluded from above comparison that underwater communication in the band of 2.4 GHz is considered to be impractical and not so helpful as it is seen that water has a very high or greater value of attenuations.  

Evaluation of Medium Access Control (MAC) for UWANs 

Radio frequency based networks are generally used in terrestrial working environment and medium based access (MAC) is considered to be an important factor for underwater acoustics channels [8]. Due to peculiar nature of underwater channels like propagation delays, high dynamics, low channel capacity, low reliability MAC designed protocol and strategies RWNs are not widely used in UNAWs.

MAC protocol for UWAN design is faces a number of challenges like utilization of medium, efficiency of energy, fairness, Quality of services, support of mobility, method of validation of protocol. There are certain of characteristics for underwater channels like long propagation delay, small and crowded channel, vulnerable and changing channel,

 The speed of acoustic in seawater is considered to be magnitude five times slower than the speed of light. It is generally affected by a list of parameters of like temperature, salinity and depth which ultimately led to better speed of propagation. Long and variable speed of propagation delay generally results in creation of problems like temporal authority of uncertainty. Doppler effect tends to be more effective in the domain of mobile in UWANs because of the fact its magnitude is directly related the ratio of speed of transmitter and end of receiver and propagation speed of signal. This ultimately result in shifting of frequency and motion at induced rate of distortion which results in contribution of dynamics of quality of channel [9]. As a result of above mentioned feature of underwater acoustic channel, the MAC protocol of UWAN addresses a lot of challenges like medium utilization, it ultimately reflects the factor of efficiency a medium can work and transmit data of user.  As a result of small channel capacity and high medium of utilization is important factor but it is difficult issues with delay of long propagation. MAC is unable avoid collision at the end of receiver and sender after the proper acquiring of message which is undesirable in the case of long propagation of delay.

Signal based Evaluation 

The radio frequency based wireless networks are generally used in terrestrial networks on the contrary medium access control is considered to be an important factor for underwater wireless networks. The fact should be taken into consideration that radio signal cannot easily propagate very well in the environment of underwater so to overcome issue acoustic communication is used. Due to specific feature of underwater acoustic channels like slow rate of propagation of signal, very small capacity of channel and high value of dynamics [10]. MAC protocol is specially designed in such a way that it can easily overcome design for underwater acoustic networks which addresses many kinds of challenges. The long delay of propagation is considered to be an important factor which makes the design of MAC strategy to be easily and widely adopted by RWNs and that is unsuitable for UWANs. The ultimate notion or goal of MAC protocol is to provide multiple user so that they can easily share common medium very efficiently. The major event in reception failure is the collision at the end of the receiver and the method of avoid it. The effect of Doppler tends to become more severe in the mobile UWANs due to the fact that magnitude is nothing but the ratio of speed of relative transmitter and receiver with propagation speed at the end. Most of UWAN and protocol of MAC are important factors and each of the value increases and tends to limit the capacity of channel for transmission in long range. Data channel for acoustic for a shorter range can easily have a value more than 100 kbit/s.  Multipath propagation is a well-known term which causes a signal from various source to meet at a common point through different paths and phase shift [11]. It is generally caused by reflection of acoustic signal from a surface. Apart from this the signal can result in inter-symbol interference in which a signal may result in inter symbol interference

Scheduling based on MAC 

The survey of UWAN MAC protocol can be easily divided into categories typical RWN protocol, new designed. It is much more easy to modify RWN based MAC protocol by providing peculiar features related to specific features of UWANs by providing proper methods to tackle it. The first type of can be easily differentiated into multiplexing and non-multiplexing [12].  The second part can be differentiated into scheduling, reservation, design of cross layered. Scheduling generally requires sender to make proper decision about the local time which is involved in transmission and aims which are required for deciding overcoming the impact which are generally done in the matter of propagation delay. A node generally calculates the time required for transmission so that they can assure collision free output and time involved in sleep or wakeup time which is spent in the saving of energy. A proper decision of scheduling depends on accuracy and availability of needed information [13]. At the global time of scheduling like TDMA depended protocol then SYN is needed. Scheduling in radio frequency based wireless networks (RWNs) is considered to be totally different from order of transmission order which support QoS instead of avoidance of collision like traffic scheduling. Several mechanism of algorithm is generally used for making changes in the protocol for improving adaptability of various protocol and efficiency of energy.

Receiver initiated Protocols 

Two schemes by making use of dedicated control channel with one ALOHA and other one RTS are proper analyzed or monitored. The first one sends a proper control over the DCC to provide proper information about the receiver of the channel which is selected for the upcoming transmission over the DCH and after that data is transferred over DCH [14]. The second one does not send data until and unless it has a RTS/ CTS shaking of hand when the receiver has selected the version of DCH. The output of RTS/CTS often provides ALOHA while on the contrary RTS/CTS are more prone to dynamic of ALOHA.  ALOHA is considered to be simplest form of MAC protocol and by the help of which a node can easily transmit frame of data at any interval of time [4]. The receiver needs to provide proper information to sender for every successful reception. If the sender cannot provide acknowledgement within a definite interval of time, it will retransmit the same frame after properly waiting which is known as backoff which is generally results in retransmission of data. In case of slotted of ALOHA that is (S-ALOHA) and in this method the time is easily divided into equal size of slot and provides a new attempt at the beginning phase which prevents new transmission from collision with an ongoing which frequently happens in the case of ALOHA [11]. A back off is generally added so that possible value of ALOHA can be easily reduced as it generally requires a node for proper transmission when the packet arrives.  Two of the well-known back off are mainly binary exponential and Poisson.    

Conclusion 

From the above pages it can be easily concluded that this report focus on Medium Access protocol which is considered to be a vital element for underwater acoustic network. In the above pages of the report a brief idea regarding the difference between radio wireless networks and underwater acoustic network has been discussed in details. After that an evaluation has been done on the medium access control for UWANs which focus on four important criteria namely delay during long propagation, evaluation based on signal, scheduling which is based on MAC, receiver based protocols has been discussed in brief in the above pages of the report. Research which are based on underwater generally focus on two important points that are optical signal and electromagnetic signal. Radio signal cannot pass or propagate through well through the environment of underwater so at present acoustic network is used. Due to some special feature like slow propagation of signal, small capacity of channel, low standard of channel, high dynamics of channel quality. MAC protocol which is designed for underwater acoustic networks which faces many kinds of new challenges. Various problems which are related to underwater communication has been discussed in brief in the above pages of the report. Two schemes which are generally used are AlOHA and RTS for receiver initiated protocol has been discussed in details in coming pages of the report.

References  

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[2]P. Wang, L. Zhang and V. Li, "A Stratified Acoustic Model Accounting for Phase Shifts for Underwater Acoustic Networks", Sensors, vol. 13, no. 5, pp. 6183-6203, 2013.

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[9]S. Yoon, A. Azad, H. Oh and S. Kim, "AURP: An AUV-Aided Underwater Routing Protocol for Underwater Acoustic Sensor Networks", Sensors, vol. 12, no. 12, pp. 1827-1845, 2012.

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