The recent introduction of new variable bit rate (VBR) speech coders has opened up new perspectives for the implementation of adaptive voice over IP (AVoIP) systems. The paper compares different VBR speech coding techniques in a scenario in which the rate of the single sources is dynamically adapted to the workload conditions. The coders compared are the AMR, the M/sup 3/R and the G.729. Using source and rate control mechanism models, performance was evaluated in terms of loss probability, offered throughput and mean CMOS (comparison mean opinion score) with varying numbers of sources and different background noise conditions. The use of header compression mechanisms was also evaluated.

The paper proposes a performance evaluation and comparison of G.729, AMR, and fuzzy voice activity detection (FVAD) algorithms. The comparison was made using objective, psychoacoustic, and subjective parameters. A highly varied speech database was also set up to evaluate the extent to which VADs depend on language, the signal-to-noise ratio (SNR), or the power level.

The paper proposes a performance evaluation and comparison of recent ITU-T and ETSI voice activity detection algorithms. The comparison was made using both objective and psychoacoustic parameters, so as to have reliable judgements that were close to subjective ones. A highly varied speech database was also set up to evaluate the extent to which VAD depend on language, the signal to noise ratio, or the power level.

This paper presents a hybrid multi-mode/multi-rate, toll quality CS-ACELP coder developed for voice over IP applications. The coder uses coding modes compatible with the three 6.4, 8, and 11.8 kbit/s coding schemes standardised by ITU-T in G.729. In particular, the algorithm presents 4 coding categories, with an average bit rate ranging between about 3 and 8 kbit/s, that adapt the rate to changes in network conditions.

Discontinuous transmission based on speech/pause detection represents a valid solution to improve the spectral efficiency of new-generation wireless communication systems. In this context robust voice activity detection (VAD) algorithms are required, as traditional solutions present a high misclassification rate in the presence of the background noise typical of mobile environments. The fuzzy voice activity detector (FVAD) proposed by Beritelli, Casale, and Cavallaro (see Contribution ITU-T, Study Group 16, question 19/16, Washington, September 1997), shows that a valid alternative to deal with the problem of activity decision is to use methodologies like fuzzy logic. In this paper we propose a multichannel approach to activity detection using both fuzzy logic and time delay estimation. Objective and subjective tests confirm a significant improvement over traditional methods, above all in terms of a reduction in activity increase for nonstationary noise.

In the field of mobile communications new robust Voiced/Unvoiced (V/UV) classification algorithms are required in that correct voicing detection is a crucial point in the perceived quality and naturalness of a very low bit-rate speech coding system. The paper shows that a valid and more convenient alternative to deal with the problem of voicing decision is to use methodologies like fuzzy logic which are suitable for problems requiring approximate rather than exact solutions, and which can be represented through descriptive or qualitative expressions. The Fuzzy Voicing Detector proposed is based on a pattern recognition approach in which the matching phase is performed using three fuzzy rules. The rules have been obtained using FuGeNeSys, a new hybrid learning tool based on Genetic Algorithm and Neural Networks. The fuzzy classifier is computationally very simple and more efficient than traditional methods, which are affected by misclassification errors, above all in the presence of background noise.

In the field of mobile communications correct voice activity detection (VAD) is a crucial point for perceived speech quality, reduction of co-channel interference, power consumption in portable equipment. This paper shows that a valid alternative to deal with the problem of activity decision is to use methodologies like fuzzy logic, which are suitable for problems requiring approximate rather than exact solutions, and which can be presented through descriptive or qualitative expressions. The fuzzy voice activity detector (FVAD) proposed uses the same set of parameters adopted by the VAD in Annex B to ITU-T G.729 and a set of six fuzzy rules automatically extracted through supervised learning. Objective and listening tests confirm a significative improvement respect the traditional methods above all for low signal-to-noise ratios.

Discontinuous transmission based on speech/pause detection represents a valid solution to improve the spectral efficiency of new generation wireless communication systems. In this context, robust voice activity detection (VAD) algorithms are required, as traditional solutions present a high misclassification rate in the presence of the background noise typical of mobile environments. This paper presents a voice detection algorithm which is robust to noisy environments, thanks to a new methodology adopted for the matching process. More specifically, the VAD proposed is based on a pattern recognition approach in which the matching phase is performed by a set of six fuzzy rules, trained by means of a new hybrid learning tool. A series of objective tests performed on a large speech database, varying the signal-to-noise ratio (SNR), the types of background noise, and the input signal level, showed that, as compared with the VAD standardized by ITU-T in Recommendation G.729 annex B, the fuzzy VAD, on average, achieves an improvement in reduction both of the activity factor of about 25% and of the clipping introduced of about 43%. Informal listening tests also confirm an improvement in the perceived speech quality.

The CCITT has defined the reference architecture for general network management, called telecommunication network management (TNM), as a logical definition and partition of functionalities. The TNM is assumed as a guideline, and a proposal for metropolitan area network (MAN) management in a public and private environment is examined. Such a hierarchical architecture is presented in a multiservices scenario, allowing the use of it in distributed queue dual bus (DQDB) MANs. The definition of functionalities for each management entity is given, and the way in which the entities work using the DQDB MAN parameters is discussed.>

FDDI II, a proposed standard for metropolitan area networks (MANs) that defines a hybrid protocol providing a mode of operation whereby both FDDI token ring operation and isochronous data transfer are integrated onto the same medium, is considered. A modification is presented of the method of dynamic bandwidth allocation for the FDDI II protocol, and its advantages in terms of bandwidth and simplification of the architecture of nodes in the network are pointed out. In comparison with the standard FDDI II, there is a gain in bandwidth of about 500 kb/s, deriving from simplification of the allocation procedure. This gain can be considered significant if the bandwidth is made totally available for asynchronous services, excepting a small part which has to be used for signaling the modification proposed.>

The design of a gateway that can interconnect teletex and OSI (Open Systems Interconnection) networks is shown. This goal is achieved by defining a functional block, the gateway mapping function (GMF), that provides a bridge between the procedures defined in CCITT Recommendation T.62 and the ISO OSI VT (virtual terminal) specification. The general architecture of the gateway is provided and the specification of its functions by means of SDL (specification and description language) diagrams are described.>

The authors analyze error-recovery mechanisms in local area networks (LANs) with token-ring access with particular reference to the problem of ring failure, i.e. (breaks in the ring or transmitter and/or receiver failure). They also suggest solutions to increase fault tolerance. The solutions suggested concern both the architecture (topology) and the access protocol (medium access control). In particular, a process capable of performing automatic network reconfiguration is proposed. The process is first described functionally and then specified through the finite-state machine.>