Prakash D. Vyavahare

Improved System Components for Secure Data Communication in MANETs using Secured DSR

Efficient and improved methods of secured route discovery and secured data transmission in Mobile Ad-hoc Networks (MANET) are most recent research issues for ensuring full applicability of deploying MANETs in wide range of applications, including strategic as well as non-strategic. Secured route discovery has been addressed by SRP/SMT, as a generic protocol independent technique for secured route discovery. In DSR the issue of secured route discovery is left to be addressed by other protocols. One such concept has been discussed in the proposal of S-DSR, by integrating SRP features in DSR, to achieve secured and multiple route discovery. S-DSR also proposes steps to achieve high throughput, cache management and sustained bandwidth. In this paper impact of proactive route discovery and periodic route validation (as suggested in S-DSR) on SMT/SSP has been analyzed. Authors of SMT/SSP have proposed algorithms for assigning route rating and survival probability. Our proposal in this paper suggests new techniques for assigning route rating and improving path survival probability. These proposed modifications simplify and enhance the functioning of SMT/SSP in terms of better and efficient cache management and improved path survivability.

Enhanced DSR with secured multi-path route discovery and concurrent data transmission

On-demand routing protocols for Mobile Ad-hoc Networks (MANET) include Dynamic Source Routing (DSR), which is capable of discovering multiple routes. Secured Routing Protocol (SRP) on the other hand provides mechanism for secured route discovery, even under adversarial conditions. The paper proposes integration of SRP features in DSR to gain security features for MANET routing requirements for the route discovery phase. Concurrent use of multiple paths for data transmission phase and its implications on performance is also evaluated.

Collision Resolution Schemes with Nonoverlapped Contention Slots for Heterogeneous and Homogeneous WLANs

CSMA/CA-based DCF of 802.11 MAC layer employs a best-effort delivery model, in which stations compete for channel access with the same priority. In a heterogeneous network, providing different priorities to different applications for required quality of service is a challenging task, since heterogeneous conditions result in unfairness among stations and degradation in the throughput. This paper proposes a class of collision resolution schemes for 802.11 having contention window control with nonoverlapped contention slots. In the first scheme, window ranges of two consecutive stages are nonoverlapped, and it is called nonoverlapped contention slots (NOCS) scheme. In the other scheme, termed as NOCS-offset, an offset is introduced between window ranges of two stages. Selection of a random value by a station for its contention with discontinuous distribution results in reduced probability of collision. Analytical and simulation results show that the proposed scheme exhibits higher throughput and fairness with reduced delay and collision probability in homogeneous and heterogeneous networks. Performance of the proposed scheme is evaluated for mix traffic and high data rate environment with advanced back-off management techniques to meet the requirements of the present applications.

Class of collision resolution schemes with multistep distribution for IEEE 802.11

Traditional Medium Access Control (MAC) algorithms for Distributed Co-ordination Function (DCF) in IEEE 802.11 extend window ranges after each packet collision in the next stage. As these window ranges intersect with window ranges of other stages, the system is collision prone. This paper proposes a class of collision resolution schemes for 802.11 contention window control with multistep distribution and called as Overlapped Contention Slots with Multistep Distribution (OCS-MD). It is shown that proper selection of different steps in the distribution of OCS-MD increases the mean value of back-off counter as compared to standard Binary Exponential Back-off (BEB). This reduces probability of window intersection of different stages, resulting in lesser probability of collision. The system model is formulated for the proposed schemes. Analytical results are validated against those obtained from simulation. The performance results show that the proposed algorithm enhances the system performance in terms of throughput and collision probability with comparable mean delay and fairness than existing scheme. Moreover these schemes can be easily incorporated with little modifications in existing 802.11 standard.

Saturation Throughput Analysis of 802.11 DCF in Presence of Different Receiver Combining Techniques

The Performance of IEEE 802.11 distributed co- ordination function (DCF), under ideal channel conditions degrades mainly due to packet collisions at MAC layer. However, studies reveal that MAC layer performance significantly reduces when wireless networks encounter channel fading at physical layer. Since fading results in burst errors, in addition to errors caused by collisions at MAC layer. In this paper, cross layer analysis is performed between MAC layer and physical layer. An analytical expression for throughput computation is used to investigate the effect of fading on the MAC layer performance which is verified through simulation. Further to enhance the network throughput in fading, receiver diversities are used. Selection Combining (SC), Equal Gain Combining (EGC) and Maximal Ratio Combining (MRC) diversity techniques are used at physical layer to calculate the enhancement of the system throughput of MAC layer. Outcomes for different diversities are compared and proposed that the same analysis can be useful for upcoming standards of WLAN using MIMO techniques.

Centralized Flow Control Scheme for Rate-based Networks

This paper describes the Centralized Flow Control Scheme (CFCS) for a network that uses rate-based flow and congestion control. An example of such a network is an Asynchronous Transfer Mode (ATM) network using the Available Bit Rate (ABR) service. CFCS is a centralized control scheme in which a Network Controller Switch (NCS) maintains the current information of the network such as the number of Virtual Circuits (VCs) active at each switch, available capacity at each switch and rate assigned for each active VC. In order to do this it maintains a Rate Allocation Table (RAT). In our scheme, NCS is the main network controller switch with enough processing power and other switches in the network are the switches without processing power and they do not execute flow control algorithm. We call such switches as Passive Switches (PAS). PAS are controlled by NCS. Thus, in this scheme, the NCS controls the overall operation of the network by allocating rates to ABR connections with the help of RAT. CFCS can achieve zero cell loss, high utilization of links, maintains fairness in allocating bandwidth, it reduces transient response time. Moreover, it needs less feedback information for control and need less parameter to set by users and network administrators. We show the effectiveness of our scheme by simulation experiments.

Modified turbo codes for next generation wireless networks

Turbo Convolutional Codes (TCC) are widely used to reduce Bit Error Rate (BER) for Second Generation (2G) and Third Generation (3G) wireless networks. However, TCC require large decoding complexity. Recently, Low Density Parity Check Codes (LDPC) have been included in standards for 3G wireless networks. But encoding complexity of LDPC is larger than that of TCC. Modified Turbo codes (MTC) are low complexity turbo-like codes with error performance which is equivalent to that of Turbo codes. However, MTC require 2-dimensional interleavers with large spreading factor and dispersion. In this paper, it is shown that BER of MTC is almost equivalent to TCC. Moreover, MTC decoders require 50% less computations than those of TCC and LDPC. Spreading factor and dispersion for various interleavers are compared. It is observed that 2-stage interleaver achieves large spreading factor and dispersion.

Performance comparison of 802.11 DCF in fading with OFDM and diversity

Performance of IEEE 802.11 distributed co-ordination function (DCF) Medium Access Control (MAC) significantly degrades in fading channel conditions because of both collisions and fading. In order to enhance the system performance in these circumstances, multiple antenna systems and Orthogonal Frequency Division Multiplexing (OFDM) are used at physical layer. In this paper an interlayer analysis is performed between physical layer and MAC layer to investigate the throughput performance of WLAN. The system is simulated using Orthogonal Frequency Division Multiplexing (OFDM) without diversity, OFDM with Selection Combining (SC) and Maximal Ratio Combining (MRC). At physical layer Bit Error Rate (BER) for different physical (PHY) layers is simulated and calculated Packet Error Rate (PER) plotted as a function of bit energy to noise power (Eb/No). DCF is simulated taking account of channel fading and performance of MAC layer is evaluated in terms of throughput. Results find that OFDM with diversity gives better performance in terms of PER and throughput, as compared to physical layers without diversity. These outcomes can be used to select a particular PHY layer technique for WLAN under fading channel conditions.

Stochastic Modeling and Performance Evaluation of Fading Channel for Wireless Network Design

Characterization of temporal variations in wireless channel impairments plays an important role in the design of a reliable and efficient mobile communication system. Such channels are termed as fading channels since various random phenomena in the propagation path result in fading of the received signal envelope. Simulation models are often used to estimate channel behavior due to higher complexity in developing analytical models for such channels. In this paper, Finite State Markov Model is developed for the evaluation of multi-path fading channel. Cumulative states and frequency duration analysis approach is used for computing rate of transition between satisfactory states and outage states, and outage time, of the channel. Simulation results obtained for Rayleigh channel are then used to find correspondence between fade depth and outage time for a sample system. These results may be used in the estimation of bit error rate and deciding optimum sampling instant of the received signal. It will also assist in selection of appropriate channel code and interleaver design for a future wireless networks with enhanced channel capacity.

A Novel Centralized Flow Control Scheme for ABR Service in Wireless ATM Networks

In this paper, we propose a novel Centralized Flow Control Scheme (CFCS) for ABR service in WATM network. The goal is to provide Quality-of-Service (QoS) guarantee to the users and to reduce the computational complexity at switches. A novel concept of monitoring the network status in the centralized controller in the form of rate allocation tables for flow control is the main theme of our algorithm. CFCS operates in ATM switches placed in the backbone of WATM network. The Network Controller Switch (NCS) monitors the current status of VC and bandwidth availability as well as tracking the current number of active sessions contending for capacity, to adjust the explicit bound in the source transmission rates. The simulation results show that the scheme is stable and efficient in achieving maximum utilization of bandwidth occupancies, enhancing average throughput and achieving robustness under continuously varying network conditions. The simulation experiments also demonstrate the superiority of the proposed scheme to the other schemes in dealing with switch complexity.