Signal propagation delay | EPFL Graph Search

Propagation delay is the time duration taken for a signal to reach its destination. It can relate to networking, electronics or physics. Networking In computer networks, propagation delay is the amount of time it takes for the head of the signal to travel from the sender to the receiver. It can be computed as the ratio between the link length and the propagation speed over the specific medium. Propagation delay is equal to d / s where d is the distance and s is the wave propagation speed. In wireless communication, s=c, i.e. the speed of light. In copper wire, the speed s generally ranges from .59c to .77c. This delay is the major obstacle in the development of high-speed computers and is called the interconnect bottleneck in IC systems. Electronics In electronics, digital circuits and digital electronics, the propagation delay, or gate delay, is the length of time which starts when the input to a logic gate becomes stable and valid to change, to the time that

Local in vivo optical measurements of neurometabolic and neurovascular coupling mechanisms in the rodent cortex

Elena Migacheva

This thesis introduces a novel approach for in vivo separation and quantification of spatio-temporal dynamics of optical coefficients (absorption, scattering), oxygen saturation (SO2) and haemoglobin concentrations within rat brain somatosensory cortex under baseline and neuronal stimulation conditions. The quantification of mentioned parameters became possible due to design of a novel optical setup for localized spatio-temporally-resolved (within ∼ 1 mm3 at 10 Hz) reflectance spectroscopy (STRReS) measurements at small source-detector separations. The possibility to automatically take into account the light propagation path lengths in tissue using Monte Carlo modeling, defines the novelty of this approach and its perspectives for brain research. This kind of corrections is currently limited or inaccurate with existing techniques like optical intrinsic signal imaging. In particular, the main aim of this thesis is to explore and better understand the local spatio-temporal characteristics of neurovascular and neurometabolic coupling mechanisms within in vivo rat cerebral cortex at initial and subsequent stages of neuronal stimulation with using STRReS technique. It has been proven theoretically (with Monte Carlo simulations) and experimentally (from in vivo rat brain cortex), that STRReS measurements are suitable from in vivo somatosensory cortex under neuronal stimulation conditions. As a part of this research, we have validated theoretically and experimentally the biological importance of new optical parameter ”gamma” (parameter of the phase function), which is nessesary to take into account for accurate optical coefficients extraction. On the other side, the STRReS technique was not found to be helpful to determine differential factors corrections for improvement of optical intrinsic signal imaging analysis due to differences in experiment’s geometries. Finally, it has been shown that temporal dynamics of reflectance is strongly influenced by light scattering variations within 500-900 nm range. Specifically, the shape of reflectance dynamics at 600 and 610 nm differs significantly from deoxy-haemoglobin concentration dynamics at initial stage of neural activation. For this reason, it is important always to take into account the variations of both components of reflectance signal: absorption and scattering. Additionally, an initial delay of 1 s was found in SO2 and haemoglobin concentrations dynamics. The interpretation of this delay, however, is strongly dependent on the origin of scattering signal. Our results are well correlated with ”metabolic” hypothesis of underlying neurovascular and neurometabolic coupling mechanisms, assuming that the scattering signal has a non-vascular origin at 500-599 nm. Nevertheless, the ”neurogenic” hypothesis is not completely excluded as far as the scattering signal has a vascular origin at 500-599 nm.

EPFL2012

A Note on the Fairness of TCP Vegas

Catherine Boutremans, Jean-Yves Le Boudec

We study the fairness of TCP Vegas. The latter is an alternative to the commonly used TCP Reno, and uses measures of the round trip time as feedback on congestion. We consider two cases that depend on the value of the two parameters alpha and beta controlling the window sizes` update. Our main conclusion is that TCP Vegas is unfair in several points. First, when alpha = beta, if the propagation delays are correctly estimated, TCP Vegas is known to be fair. However we show that any over-estimation of the propagation delay of a given connection results in an increase of its rate and hence leads to unfairness. This rate increase augments with the over-estimation factor. Moreover, the rate oscillations, whose amplitude increases with the rate value, are not sufficient to provide an accurate estimation of the propagation delay. Second, when alpha < beta, TCP Vegas is unfair even if the propagation delays are correctly estimated. In this case, the rate of a connection converges to a stable value that depends on the arrival order of all connections so that earliest established connections get more bandwidth. Also, in a more realistic scheme, later connections see their propagation delay over-estimated and thus they gain larger portion of the bandwidth. These two effects tend to counterbalance each other but the second tends to dominate. Future use of TCP Vegas in the context of TCP-friendly applications, should therefore rely on alpha = beta, but will require the propagation delays to be correctly estimated. Yet, this seems to be quite hard to achieve.

2000

Local in vivo optical measurements of neurometabolic and neurovascular coupling mechanisms in the rodent cortex

Elena Migacheva

EPFL2012