Dynamic priority protocols for packet voice

Since the reconstruction of continuous speech from voice packets is complicated by the variable delays of the packets through the network, a dynamic priority protocol is proposed to minimize the variability of packet delays. The protocol allows the priority of a packet to vary with time. After a discussion of the concept of dynamic priorities, two examples of dynamic priorities are studied through queueing analysis and simulations. Optimal properties of the oldest customer first (OCF) and earliest deadline first (EDF) disciplines are proven, suggesting that they may be theoretically effective in reducing the variability of packet delays. Simulation results of the OCF discipline indicate that the OCF discipline is most effective under conditions of long routes and heavy traffic, i.e., the conditions when delay variability is most likely to be significant. Under OCF, the delays of packets along long routes are improved at the expense of packets along short routes. It is noted that more complex and realistic simulations, including simulations of the EDF discipline, are needed     

A clock-free chip set for high-sampling rate adaptive filters

As digital signal processing systems become larger and clock rates increase, the typical design approach using global clock synchronization will become increasingly difficult. The application of asynchronous clock-free designs to high-performance digital signal processing systems is one promising approach to alleviating this problem. To demonstrate this approach for a typical signal processing task, the system architecture and circuit design of a chip set for implementing high-rate adaptive lattice filters using the asynchronous design techniques is presented.This research was sponsored in part by the Semiconductor Research Corporation and by DARPA.     

Timing Recovery in Digital Subscriber Loops Using Baud-Rate Sampling

Sampled-data techniques are the most practical means of obtaining the necessary signal processing functions for timing recovery in the VLSI implementation of a digital subscriber loop transceiver. The sampled-data timing recovery techniques described in this paper are applicable to both echo cancellation and time-compression multiplexing systems. Timing recovery using baud-rate sampling in conjunction with a special pulse-shaping and timing function fulfills all the objectives for timing recovery in this application. It recovers a timing phase that has minimum precursor intersymbol interference, and makes possible the combination of decision feedback equalizer and echo canceler, reducing the convergence time and increasing the step size. The pulse-shaping function can be performed either in the transmitter by means of digital coding, or in the receiver by means of analog filtering. In the latter case, the transmitted pulse is compatible with more conventional approaches. The proposed partial-response line coding, a special form of AMI coding, is less susceptible to line impairments if detected as a two-level signal. Performance by analysis, simulation, and experimental measurements is reported on a variety of cable configurations, some including bridged taps. Analysis of jitter performance leads to design techniques for reducing the jitter magnitude.     

Back to the user [open source]

Open source is incomplete as a software creation process, somebody has to understand needs, design an architecture accommodating long-term evolution; create a working prototype as starting point; provide a talented; centralized person or group to coordinate and govern; and provide support, training, certification. These critical tasks hardly differ between open source and traditional processes. Application often have a diversity of uses and legitimate variations in needs that evolve over time. Efficiently capturing commonalities while accommodating variation and evolution is crucial to expanding the application base. Open source can't accommodate this gracefully. So open source fails organizational and nonprogrammer users.     

Architectural techniques for eliminating critical feedback paths

The authors demonstrate architectural techniques, for small-state feedback circuits that significantly improve the throughput without requiring circuit design efforts or advanced technologies. The method is flexible in terms of achievable implementations and speedups. The authors discuss a new high-throughput solution for systems with finite-level feedback values. As an example, the authors consider coding and signal processing systems for optical communications, which usually have very simple feedback. The authors demonstrate the method by realizing a 2 micron CMOS layout of a bimode 3B4B line coder. Simulation results show that, using standard cell design, the chip achieves a coding rate of 1.4 Gb/s. Other design options are discussed     

Timing Recovery in Digital Subscriber Loops

Tradeoffs in the design of the timing recovery functions in a subscriber loop receiver are analyzed. The techniques considered are applicable to both the echo cancellation (EC) and time compression multiplexing (TCM) methods of full duplex transmission. Emphasis is on those techniques that lend themselves to implementation in MOSLSI technology, where the objective requirement is that timing recovery be implemented on a sampled-data signal (with the minimum possible sampling rate where EC is used). The wave difference method (WDM) for timing recovery appears to be the best candidate. A detailed study of its performance is carried out analytically and by computer simulation for the case of binary and alternate mark-inversion (AMI) line coding. A closed form expression describing the binary jitter performance of the WDM and its continuous time counterpart, the spectral line technique, is used to compare the two techniques. Analytical and simulation results for recovered phase and jitter are presented for various cable pulse responses carefully chosen to represent worst-case or nearly worst-case conditions. Two methods for including frequency detection in the WDM, the quadricorrelator and the rotational detector, are also simulated.     

Capacity penalty due to ideal zero-forcing decision-feedbackequalization

We consider the capacity C of a continuous-time channel with frequency response H(f) and additive white Gaussian noise. If H(f)|^-2 behaves like a polynomial of order ρ at high frequencies, we show that the per-symbol capacity approaches ρ/2 nats per channel use at high signal powers. If the receiver uses an ideal zero forcing decision-feedback equalizer (DFE) consisting of a sampled whitened-matched filter followed by a zero-forcing tail canceler that is free of error propagation, the overall system is free of intersymbol interference and has a well-defined capacity C_ZF. By comparing this capacity with the capacity C of the underlying channel, we quantify the loss of information inherent in the tail-canceling operation that typifies zero-forcing DFE and zero-forcing precoding systems. For strictly bandlimited channels, we find that the capacity penalty approaches zero in the limit of large signal power. On the other hand, for nonstrictly bandlimited channels, the asymptotic penalty is nonzero; however, with bandwidth optimization, the asymptotic penalty is at most 0.59 dB, and the asymptotic ratio C_ZF/C is at least 93.6%, depending on the asymptotic order ρ of the channel response