Dilip Kr. Saikia

Significance of group delay functions in signal reconstruction from spectral magnitude or phase

In this paper we discuss the problem of signal reconstruction from spectral magnitude or phase using group delay functions. We define two separate group delay functions for a signal, one is derived from the magnitude and the other from the phase of the Fourier transform of the signal. The group delay functions offer insight into the problem of signal reconstruction and suggest methods for reconstructing signals from partial information such as spectral magnitude or phase. We examine the problem of signal reconstruction from spectral magnitude or phase on the basis of these two group delay functions and derive the conditions for signal reconstruction. Based on existing iterative and noniterative algorithms for signal reconstruction, we propose new algorithms for some special classes of signals. The algorithms are illustrated with several examples. Our study shows that the relative importance of spectral magnitude and phase depends on the nature of signals. Speech signals are used to illustrate the importance of spectral magnitude and picture signals are used to illustrate the importance of phase in signal reconstruction problems. Using the group delay functions, we explain the convergence behavior of the existing iterative algorithms for signal reconstruction.

Enhancing User Identity Privacy in LTE

Identity privacy is a security issue that is crucial for the users of a cellular network. Knowledge of the permanent identity of a user may allow an adversary to track and amass comprehensive profiles about individuals. Such profiling may expose an individual to various kind of unanticipated risks, and above all may deprive an individual of his right to privacy. With the introduction of sensitive services like online banking, shopping, etc. through cellular phones, identity privacy has now become a bigger security issue. In GSM and UMTS, the problem of user identity privacy vulnerability is proven to exist. In both these systems, there are situations where the permanent identity of a subscriber may get compromised. Long Term Evolution (LTE), which evolved from GSM and UMTS, is proposed by 3GPP for inclusion into the fourth generation of cellular networks. Although security of LTE has evolved from the security of GSM and UMTS, due to different architectural and business requirements of fourth generation systems, LTE security is substantially different and improved compared to its predecessors. However, the issue of identity privacy vulnerability continue to exist in LTE. In this paper, we discuss how the security architecture of LTE deals with identity privacy. We also discuss a possible solution that may be utilised to overcome the problem of user identity privacy in LTE.

Two ranking schemes for efficient computation on the star interconnection network

A node ranking scheme provides the necessary structural view for developing algorithms on a network. We present two ranking schemes for the star interconnection network both of which allow constant time order preserving communication. The first scheme is based on a hierarchical view of the star network. It enables one to efficiently implement order preserving ASCEND/DESCEND class of algorithms. This class includes several important algorithms such as the Fast Fourier Transform (FFT) and matrix multiplication. The other ranking scheme gives a flexible pipelined view of the star interconnection network and provides a suitable framework for implementation of pipelined algorithms.

RTT Based Congestion Control and Path Switching Scheme for SCTP

SCTP is an emerging transport protocol that has TCP like congestion control mechanism. The occurrence of packet loss due to congestion is part of SCTP as its congestion control mechanisms rely on packet loss. Loss of packets leads to retransmissions causing excessive delay jitters and increasing traffic burden on the network reducing the throughput. As a result the packets suffer more delays. We have employed a RTT based congestion control technique [5] on SCTP which avoids occurrence of extreme congestion situations. With this scheme the queue-occupancy of the packets and unnecessary packet losses can be controlled. As a result the packet latency and jitter decreases and the utilization efficiency of the links improve. Further, SCTP uses the alternate path only for the retransmission of the packets and failover cases. We propose use of the alternate path for diversion of the traffic based on detection of congestion in the primary path using RTT. This allows good control over queue occupancy of the packets. Result is significantly improved throughput and fairness and reduced latency and packet loss. Experimental results have been obtained to that effect.

Embedding torus on the star graph

In this paper, we present a scheme for efficient embedding of torus of any dimension on a star graph. The dilation of the embedding is four. The expansion is small. Congestion depends upon the routing scheme used. With one routing scheme, the congestion is bound by a small constant (/spl ap/2) with an increase in expansion cost. For a second routing scheme, the congestion is O(n), for an n-star, with bounded expansion.

Order preserving communication on a star network

Abstract We present a scheme for ordering nodes in a star network such that it requires only a constant time for communication between a pair of nodes of the same rank in different sub-stars. This enables us to implement order preserving ASCEND/DESCEND type of divide and conquer algorithms such as the Fast Fourier Transform (FFT) efficiently on the star network. We also present a scheme to model a star network as a pipeline of sub-stars. This gives us a convenient framework for implementation of pipelined computations on a star network.

An Efficient TDMA MAC Protocol for Multi-hop WiFi-Based Long Distance Networks

Multi-hop WiFi-based long distance (WiLD) network is considered to be a cost-effective solution for extending Internet to rural underserved areas. In such gateway-based mesh architecture, interference-free scheduling of transmission is critical in optimizing overall network performance. In this paper, we propose an efficient TDMA-based MAC protocol for gateway-based multi-hop WiLD networks for enhancing end-to-end throughput and delay performances. The possibility of collision which exists in token-based MAC protocols has been reduced significantly by employing inherent node synchronization mechanism. The proposed protocol enhances network performance by utilizing the available bandwidth through maximization of TDMA slots overlapping. It shows clear improvement over relevant MAC protocols in terms of saturation throughput as well as average end-to-end packet delay over multiple hops.

UMTS user identity confidentiality: An end-to-end solution

In all mobile communication scenario, it is envisaged that user identity remains confidential. Universal Mobile Telecommunication System (UMTS) is no exception to this. Every Mobile Station (MS) in UMTS is assigned a unique International Mobile Subscriber Identity (IMSI). If this IMSI is compromised, so will be the user identity. There is restriction on transmission of this IMSI over radio link. Unfortunately, circumstances when such transmission happens can be created by an adversary, thereby compromising anonymity of users. In this paper, we build on our End-to-End User Identity Confidentiality (E2EUIC) solution [1], an extension to 3GPP-AKA that not only takes care of identity confidentiality vulnerability over the wireless link, but goes one step ahead to ensure the same over the wired network as well.

End-to-End User Identity Confidentiality for UMTS networks

A standard security feature in any wireless network is user identity confidentiality. In Universal Mobile Telecommunication System (UMTS), there are circumstances where this important security feature is compromised due to transmission of the International Mobile Subscriber Identity (IMSI) in clear-text over the radio interface. Such situations defy anonymity of users. In addition, it is assumed that there is no threat from the agents in the wired network. In this paper, we introduce End-to-End User Identity Confidentiality (E2EUIC), an extension to 3GPP-AKA which not only takes care of identity confidentiality vulnerability over the wireless link, but goes one step ahead to ensure the same over the wired network as well.

Packet loss free congestion control in TCP for controlled packet latency and optimal throughput

The occurrence of packet loss due to congestion has been a part of TCP, as existing TCP control mechanisms rely on packet loss to detect congestion. Packet loss leads to retransmission, causing excessive delay jitter. Retransmission also has the effect of increasing the traffic burden on the network, reducing throughput efficiency. Moreover, when congestion is allowed to approach the extreme, the router queue lengths reach their maxima. As a result, packets suffer more delays. We have devised a congestion control scheme for TCP based on RTT, which avoids the occurrence of extreme congestion situations. With this scheme, the queue length can be controlled to maintain queuing delays within desired limits. As packet losses are avoided, the resulting retransmissions are also eliminated. As a result, the utilization efficiency of the links and their throughput are enhanced. Simulation experiment results have been obtained to that effect.