Sharif Digital Repository

Sharding, i.e. splitting the miners or validators to form and run several subchains in parallel, is known as one of the main solutions to the scalability problems of blockchains. The drawback is that as the number of miners expanding each subchain becomes small, it becomes vulnerable to security attacks. To solve this problem, a framework, named as Ployshard, has been proposed in which each validator verifies a coded combination of the blocks introduced by different subchains, thus helping to protect the security of all subchains. In this paper, we introduce an attack on Ployshard, called the discrepancy attack, which is the result of malicious nodes controlling a few subchains and... 

Solid State Drives (SSDs) have recently emerged as a high speed random access alternative to classical magnetic disks. To date, SSD designs have been largely based on multi-channel bus architecture that confronts serious scalability problems in high-end enterprise SSDs with dozens of flash memory chips and a gigabyte host interface. This forces the community to rapidly change the bus-based inter-flash standards to respond to ever increasing application demands. In this paper, we first give a deep look at how different flash parameters and SSD internal designs affect the actual performance and scalability of the conventional architecture. Our experiments show that SSD performance improvement... 

Data center performance is affected by three main factors; bandwidth, latency, and reliability of intra-data center interconnection network. Bandwidth and latency are definitely improved by adopting optical technology for intra-data center communication, but fault tolerance of the corresponding optical networks has been raised less. Recently, we introduced two Torus-based, all-optical, and non-blocking networks, i.e. O-TF and O-FTF, addressing reliability of optical networks, and now, in this paper, to address the scalability problem, we propose a novel Optical Clos-based architecture which reduces minimum number of required wavelength channels, as well as, the switch size in each node.... 

In some applicable scenarios, such as community patrolling, mobile nodes are restricted to move only in their own communities. Exploiting the meetings of the nodes within the same community and the nodes within the neighboring communities, a delay tolerant network (DTN) can provide communication between any two nodes. In this paper, two analytical models based on stochastic reward nets (SRNs) are proposed to evaluate the performance of the epidemic content retrieval in such multi-community DTNs. Performance measures computed by the proposed models are the average retrieval delay and the average number of transmissions. The monolithic SRN model proposed in the first step is not scalable, in... 

In this paper, we extract analytical models for data transmission delay, power consumption, and energy dissipation of optical and traditional NoCs. Utilizing extracted models, we compare optical NoC with electrical one for varying values of link length and degree of multiplexing and calculate lower bound limit on the optical link length below which optical on-chip network loses its efficiency. Based on this constraint, we propose a novel hierarchical on-chip network architecture, named as H2NoC, which benefits from optical transmissions in large scale SoCs and overcomes the scalability problem resulted from lower bound limit on the optical link length. Performing a series of simulation-based... 

Traditionally, to handle security for stand-alone computers and small networks, user authentication and access control mechanisms would be almost enough. However, considering distributed networks such as the Internet and pervasive environments, these kinds of approaches are confronted with flexibility challenges and scalability problems. This is mainly because open environments lack a central control, and users in them are not predetermined. In such ubiquitous computing environments, issues concerning security and trust become crucial. Adding trust to the existing security infrastructures would enhance the security of these environments. Although many trust models are proposed to deal with... 

Traditionally, to handle security for stand-alone computers and small networks, user authentication and access control mechanisms would be almost enough. However, considering distributed networks such as the Internet and pervasive environments, these kinds of approaches confront with flexibility challenges and scalability problems. This is mainly because of that open environments lack a central control, and users in them are not predetermined, in such ubiquitous computing environments, issues concerning security and trust become crucial. Adding trust to the existing security infrastructures would enhance the security of these environments. Although many trust models have been proposed to... 

Importance of power dissipation in NoCs, along with power reduction capability of on-chip optical interconnects, offers optical network-on-chip as a new technology solution for on-chip interconnects. In this paper, we extract analytical models for data transmission delay, power consumption, and energy dissipation of optical and traditional NoCs. Utilizing extracted models, we compare optical NoC with electrical one and calculate lower bound limit on the optical link length below which optical on-chip network loses its efficiency. Based on this constraint, we propose a novel hierarchical on-chip network architecture, named as H2NoC, which benefits from optical transmissions in large scale...