Public Switched Telephone Network Research Articles

Performance evaluation of dynamic routing based on the use of satellites and intelligent networks

A dynamic routing scheme for public switched telephone networks is introduced which employs satellite broadcast to distribute network load data. The proposed network architecture closely resembles the IN (Intelligent Network) architecture, whereby the IN SCPs (Service Control Points) serve as so-called Routing Control Points (RCPs). The key functions of an RCP are (i) to execute the routing algorithm and issue routing instructions in response to routing queries it receives from its associated switches for calls which overflow from the default network links, and (ii) to monitor and evaluate the pattern of received routing queries to obtain an estimate for the traffic loads present in each RCP's domain of associated switches. Satellite broadcast is used to distribute the load information among all RCPs in the network in a periodic fashion. This paper also reports on the results of extensive call-by-call simulations. The objective of the simulations was to validate this new routing scheme and compare its performance with well-known existing schemes. Real traffic and network data as measured in the Austrian PSTN were used in the simulations. The main results are that, under all realistic network and traffic conditions including link and node failures, the proposed scheme yields lower blocking rates and significantly less routing and crankback attempts than the existing dynamic routing schemes. Note that this is achieved in the absence of any load measurements within the switches. As regards the periodic satellite based RCP-RCP broadcast for the PSTN studied, it was shown that an update period of 10 seconds yields an excellent routing performance which is close to the limit of a vanishing update period.

Cost per Billable Minute Metric for Comparing Satellite Systems

The cost per billable minute is the metric to compare the efŽ ciency of different mobile satellite system architectures. The cost per billable minute to achieve a speciŽ ed rate of return is determined from estimates of the useful system capacity and life cycle costs. To evaluate the useful system capacity, a computer simulation was developed to model the complex interaction between satellites, gateways, capacity limits, and an expected market model. The major capacity limiting factors addressed include frequency reuse, constellation design, link availability,spotbeam patterns, multiple access schemes, and satellite power. The life cycle costs were estimated for the satellites, launch vehicles, gateways, public switched telephone network hardware connections, operations, and launch insurance. The metric is evaluated for six model systems in low, medium, and geostationary Earth orbits to demonstrate the usefulness of the metric in the design process.

Interworking of a distributed architecture for wireless PCS networks with conventional networks

A distributed architecture for wireless PCS networks is appealing due to the separation of call and connection control and the use of operations which run in parallel that can result in reduced data management load, signaling load as well as reduced post-dial delay; the Distributed Architecture for Wireless PCS Networks (DAWN) we have recently developed is such an architecture. On the other hand, centralized architecture such as IS-41 has already been deployed in several wireless networks. Thus, due to imbedded base of conventional architecture such as the IS-41 architecture, the successful deployment of a distributed architecture such as DAWN is highly dependent on the ability to phase in the new architecture and, consequently, to provide seamless interworking capabilities so that a mobile user can move between any two areas where the architectures are different. In this paper, we address the interworking aspects of DAWN with IS-41 based networks. Various IS-41 call scenarios are presented to demonstrate the workings of the interworking function. Interworking with PSTN (public switched telephone network) and handling of intersystem handoffs is also discussed.

A multimedia gateway for phone/fax and MIME mail

Facsimiles have become the most popular form of text communication on the public switched telephone network (PSTN). At the same time, the e-mail is the well-liked textual delivery system on computer networks. The message exchange between the fax/phone and e-mail will improve their availability and increase their messaging capabilities. In the Distributed System Laboratory at the National Central University, ROC, a new system integrating the MIME (multipurpose internet mail extensions) mail and fax/phone has been proposed and implemented to fulfil the requirements of bidirectional multimedia messaging. A user can send mails from Internet to PSTN. Furthermore, our system allows transmitting multimedia messages from PSTN to Internet. The system has the following properties: (1) only use the usual current equipments, e.g. telephones, G3 fax machines and personal computers, (2) be conformable to the popular protocols and existing systems, (3) provide user-friendly interfaces, such as the voice guided help, and (4) provide a universal and transparent addressing scheme between facsimiles and e-mail. Finally, we believe that our system meets real user needs.

Sources of failure in the public switched telephone network

What makes a distributed system reliable? A study of failures in the US public switched telephone network (PSTN) shows that human intervention is one key to this large system's reliability. Software is not the weak link in the PSTN system's dependability. Extensive use of built-in self-test and recovery mechanisms in major system components (switches) contributed to software dependability and are significant design features in the PSTN. The network's high dependability indicates that the trade-off between dependability gains and complexity introduced by built-in self-test and recovery mechanisms can be positive. Likewise, the tradeoff between complex interactions and the loose coupling of system components has been positive, permitting quick human intervention in most system failures and resulting in an extremely reliable system.

The development of personal communication services under the auspices of existing network technologies

The concept of personal communication services (PCS) is only now beginning to take shape. In the past, PCS was described as services that were planned in certain frequency bands, such as in the 900 MHz narrowband licenses auctioned to the paging carriers (termed narrowband PCS) and the 1800 MHz frequencies auctioned for voice services. PCS has also been used as a synonym for certain technology standards like IS-136 TDMA, GSM (also known as DCS-1800), or IS-95 CDMA. However, the industry is now realizing that PCS is really a collection of voice and/or data services that meet the following minimum criteria: a single address or contact number to locate a user, the ability of the network to locate the recipient of the message, true universal coverage, and the independence of defined services from delivery mechanisms and technology. It can be argued that no single network or technology can address all these different attributes at an attractive price for all customers. This has caused existing service providers to invest in the internetworking of different wide area networks (WANs), establishment of standardized platforms for service creation, and development of content-aware smart applications. This article looks at several such developments in the context of the efforts underway at RAM Mobile Data USA LP to provide PCS using the Mobitex/sup R/ technology as its core. This article analyzes the Multi-Network Access Services (MNAS) which allow interconnection of several WANs, including satellite, circuit-switched cellular (CSC), public switched telephone network (PSTN), and paging networks. The approach is flexible enough to allow interconnection to additional networks that may be required in the future. This article also looks at the efforts on development of smart applications and services that are content-aware but independent of the underlying network technologies used. The proposed approach allows customers to use a single application to access similar services over multiple networks.

DVB specifications for broadcast-related interactive TV services

Following on from the early success of the Digital Video Broadcasting (DVB) Project, the next step is to enable DVB-compliant receivers to support an attractive range of interactive services. Various types of interactive services are described. The implications for standards are then listed and the specifications summarised. The specifications include a set of network-independent protocols, as well as specifications for the most popular types of networks for the interaction channel. These cover the public switched telephone network (PSTN) and its digital equivalent (ISDN), as well as cable TV networks. The opportunity exists to enhance the consumer experience by adding interactivity to digital broadcasting. DVB is putting in place some of the enablers to make this a reality.

Cellular digital packet data

Cellular digital packet data (CDPD) offers on-the-go users access to a low-cost, widespread, wireless data network. CDPD can be overlaid on existing analog cellular systems, and share their infrastructure equipment on a non-interfering basis. Basically, CDPD transmits packet data over idle cellular channels. It autonomously switches to another channel when the current channel is about to be assigned for voice usage. CDPD does not communicate with the underlining cellular network. However, CDPD does use its knowledge about the channel assignment algorithm of the cellular cell to predict the channels available for CDPD use. CDPD may serve as the wireless extension to other data networks (e.g., Internet) or public switched telephone networks. It supports connectionless network services which have packets called network protocol data units (NPDU). Every packet is routed individually based on the destination address of the packet and knowledge of the current network topology. Initially, CDPD provides two connectionless network services: the standard open systems interconnection connectionless network protocol and the Internet protocol.

Towards Fault Recovery and Management inCommunication Networks

Society’s growing use of communication networks in both the public and private sector has led to an increased focus on reliability and survivability. Over the last decade, widely publicized network outages illustrated that a disruption of communications services can be very expensive to businesses and critical services (such as 911 service). The growing vulnerability of the public switched telephone network in the United States was initially discussed in 1989 by the National Research Council which noted a As we become more dependent on network, the consequences of network failure become greater and the need to reduce network vulnerabilities increases commensurately.o With the popularity of the Internet, a variety of network failures has surfaced in the recent years. It is clear that fault management will grow in importance for the foreseeable future. A variety of network failures are possible with typical events resulting in a failure being accidental cable cuts, hardware malfunctions, software errors, natural disasters (e.g., ® re, hurricane, earthquake), human error (e.g., incorrect maintenance), and malicious attack (both hardware and software). Broadly, we can classify the failures as physical-type failures and software-type failures. First we discuss physical failures in this context. Consider, a ® ber cable cut in a selfhealing ring in a metropolitan area telecommunication network; the failure can be restored in milliseconds by changing the direction of ow (with appropriate availability of capacity). However, using such a ring approach on a national or an international scale is likely to be cost prohibitive. This may lead to an approach which allows a combination of ring and digital cross-connect systems for transmission layer restoration, although it should be kept in mind that the spare capacity provided for such restoration is unused during normal network

Investment Estimation of Telecommunication Industry in Japan. A Study in the Economics of Information Society.

This work reports an estimate of demand for telecommunication services and an estimate of the investment cost of constructing telecommunication infrastructure in Japan for the period of 1996-2025. Economic models are formulated to describe main activities of the Japanese telecommunication industry including mobile communication, the Internet, Public Switched Telephone Network PSTN and Broadband Integrated Services Digital Network B-ISDN. Under a number of assumptions on macro economic and social indices, we make a forecast of the number of subscribers to telecommunication services, the amount of investment to satisfy the demand, the operating costs, the operating incomes and loses, and the cash-flow of the operator. Comparing some scenarios, we estimate the amount of public finance to promote the construction of the information infrastructure.

A TWO-LEVEL HIERARCHICAL MOBILE NETWORK: STRUCTURE AND NETWORK CONTROL

The increase in demand for mobile telecommunication systems, and the limited bandwidth allocated to these systems, have led to systems with smaller cell dimensions, which in turn led to the increase of control messages. In order to prevent controller bottle necks, it is desirable to distribute the network control functions throughout the network. To satisfy this requirement, a mobile network structure characterized by its hierarchical and decentralized network control is presented in this paper. The area served by the mobile system is divided into regions, and the regions are further divided into cells. Each cell is served by a base station, each base station is connected to a regional network through a base station interface unit (BIU). Each region has its own regional network. Connected to each regional network are the cellular controller, the home database, the visitor database, the trunk interface unit (TIU) and the gateway interface unit (GIU). The TIU connects the regional network to the public switched telephone network (PSTN). The GIU connects the regional network to other regional networks through the gateway network. This architecture distributes the network control functions among a large number of processing elements, thus preventing controller bottle necks — a problem faced by centralized controlled systems. The information and network control messages are transferred in the form of packets across this network. Processes inherent to the operation of this network structure are illustrated and discussed. These processes include the location update process, the setting up of a call, the handoff process (both the intra-region handoff process and the inter-region handoff process are considered), and the process of terminating a call.

Phonetically balanced cepstrum mean normalization for telephone speech

Cepstrum mean normalization is an effective method for recognizing distorted telephone speech. This method compensates for the difference of bias on cepstrum coefficients (CC) between training data and test data by subtracting the mean value of the CC calculated from a certain amount of given speech data. Such adaptation data are not phonetically balanced, which makes it difficult to get an accurate mean value for the CC. In this paper, a new approach to resolve this problem is proposed. Before recognizing speech, not only the mean value of the adaptation data itself must be calculated, but also the mean value from Gaussian distribution of continuous density HMMs must be calculated, whose phonemes appear in the adaptation data. When recognition occurs, the difference of these two mean values is subtracted from speech data. This proposed method using various telephone speech data has been investigated, i.e., speech data from an ordinal analog telephone, a code-less handset telephone, and a digital cellular phone (Japanese full-rate digital phone based on VSELP), which was recorded through the public switched telephone network. These experimental results show an advantage of the new method.

GPS-BASED LEO SATELLITE POSITION AND ATTITUDE DETERMINATION AUGMENTED BY PSEUDOLITES ON EARTH

GPS-based navigation and attitude determination of LEO satellites is presently considered as an alternative to the conventional systems which utilize earth sensors and magnetometers. The onboard GPS receiver determines the orbit position of the LEO satellite by the conventional system of linearized navigation equations, requiring the simultaneous reception of ranging signals from four GPS satellites by a single antenna. For attitude determination, pairs of antennae, suitably mounted on the satellite and feeding a common receiver, form several interferometric baselines. The baselines vectors, defined in a given coordinate system, determine the attitude of the satellite. For each baseline and each GPS satellite, the difference in phase of the received signal carriers is measured. The differencing operation eliminates the receiver clock bias. Solutions for the baseline vectors can be obtained with signals received from only three GPS satellites. If the coverage of a receive antenna is restricted to less than the hemisphere it will not have four GPS satellites in view all the time. It is demonstrated that a GPS pseudolite transmitter located on earth supplements the system, which then provides a usable geometric dilution of precision (GDOP) for position determination and an improved position dilution of precision (PDOP) for attitude determination. Pseudolites can be co-located with the gateways which provide access to the public switched telephone networks (PSTNs) for the LEO communication satellites.

Status of H.324—the videoconferencing standard for the Public Switched Telephone Network and mobile radio

The International Telecommunications Union (ITU) has prepared a series of draft recommendations defining a multimedia videoconferencing/ videophone terminal transmitting over the Public Switched Telephone Network (PSTN). Recommendation H.324 defines the overall system structure and therefore the device commonly known as an H.324 terminal. The author outlines the key technical characteristics of the entire set of five draft recommendations—H.324, G.723, H,263, H.246, and H.223. Of particular interest is the new H.263 video coding algorithm, which outperforms the H.261 video coder by approximately 2 or 3 to 1. The H.263 coder employs a wide range of advanced coding techniques, including half-pel motion compensation, 8 x 8-pel motion vectors, overlapped block motion compensation, and PB frames. It is likely that the H.263 recommendations will be used for other applications at higher bit rates as well as for the H.324 PSTN. The ITU has also begun work to adapt the H.324 PSTN terminal for mobile radio applications. The primary effort is the addition of error control measures to the multiplexer (and possibly the source coders) to address the more challenging error characteristics in the mobile environment.

Multimedia networks and the public switched telephone network

The coherence of the public switched telephone network (PSTN) was based on the understanding that, to further common interests, the terminals and networks were to be coordinated to allow the transparent flow of traffic. Innovations were evaluated based on the overall system performance and the potential number of connections. The evolution of public networks and technological advancements, of course, have allowed traffic types other than speech to be carried over the PSTN. However, because stakeholders in multimedia communications often have conflicting requirements, they usually promote application-specific approaches to code source information and multiplex the different media streams. Rarely do they ponder over possible ways to merge all these approaches in a harmonious way. Multimedia communications straddle the traditions of both telecommunications and information technologies. As the technology evolves, it is becoming increasingly difficult to identify the boundary of networks.

Detection and correction of transmission errors in facsimile images

Today, facsimile is recognized as the primary communication tool for both printed and written materials. Most facsimile machines operating on public switched telephone networks use the Group 3 (G3) facsimile compression standards, in which images are entropy-coded. Although the synchronization codeword end of line (EOL) is employed, a transmission error in a codeword may cause the current codeword, the subsequent codewords in the current line, even the codewords in the subsequent lines to be misinterpreted, resulting in a great degradation of the received image. The objective of the proposed error detection and correction approaches is to completely or partially eliminate transmission errors in G3 facsimile images, requiring no extra transmission bit rate and without changing the transmitter and the receiver. The proposed approaches are based on the error checking conditions derived from the relationship between the current line and the previous line as well as the constraints on compressed image data. A corrupted line is detected if any of the error checking conditions is satisfied. When a corrupted line is detected, a sequence of bit inversions and redecoding operations are performed on the current corrupted line and/or its previous lines so that at least one possible (feasible) redecoding solution can be found. Then, the best solution is selected by using some selection criterion. Based on simulation results, the proposed approaches can recover the original or high-quality facsimile images from their corresponding corrupted facsimile images. This shows the feasibility of the proposed approaches.

Signaling System Number 7

In a public switched telephone network (PSTN), the control information is exchanged among the network elements by signaling. The early telephone system used circuit associated signaling or inband signaling where the same transmission channel was shared by both signaling and user information (voice or data). Another signaling method is common channel signaling (CCS). Unlike the circuit associated signaling, CCS uses a separate out-of-band signaling network to carry signaling messages. CCS consists of supervisory functions, addressing, and providing call information. A CCS channel conveys messages to initiate and terminate calls, check on the status of some part of the network, and control the amount of traffic allowed. Signaling System No. 7 (SS7) is a CCS system developed to satisfy the telephone operating companies' requirements for an improvement to the earlier inband signaling systems. The advantages of CCS are listed.

Internet Voice, 'Cyberbypass', Competitive Efficiency

This paper considers the competitive issues raised by the continuing evolution of public switched telephone network development toward an open data network. Adopting a platform hierarchy set forth by the National Research Council, the paper agrees that some higher level services may indeed reduce traditional telephone company revenues, but argues that it would be counterproductive for regulators to devise bans or adjust prices if they Could Regulators faced with unbundling may ensure any necessary revenue recovery through tariff rebalancing among facilities providers which provide competitive carriage through rebalancing strategies that remain competitively neutral. Suggested strategies include cost-based usage charges for data calls, fixed cost write-offs and increases in subscriber line charges. Copyright 1996 by Oxford University Press.

Screen-Based Multimedia Telephony

Today, telephone users are already realizing the multimedia dream as the AT&T VideoPhone 2500, Picasso® and Vistium™ products are introduced into consumer's homes and onto office desktops. A major, common element of these products is a display screen that provides a visual perspective. In addition, end-users now have a choice about what they can view at any given instant. The quality of this choice depends on the product itself, the feature set being used, and the bit rate of the telephone transmission medium. This bit rate varies from the very low speeds of the public switched telephone network (PSTN) to the higher speeds of basic-rate ISDN and ultimately to the very high speeds of broadband and asynchronous transfer mode technologies. This paper discusses these factors, the overall architecture of the three AT&T products, as well as their rich sets of features. In addition, key markets and applications are highlighted.

Phoneme based speech recognition

A flexible vocabulary speech recognition system is provided for recognizing speech transmitted via the public switched telephone network. The flexible vocabulary recognition (FVR) system is a phoneme based system. The phonemes are modelled as hidden Markov models. The vocabulary is represented as concatenated phoneme models. The phoneme models are trained using Viterbi training enhanced by: substituting the covariance matrix of given phonemes by others, applying energy level thresholds and voiced, unvoiced, silence labelling constraints during Viterbi training. Specific vocabulary members, such as digits, are represented by allophone models. A* searching of the lexical network is facilitated by providing a reduced network which provides estimate scores used to evaluate the recognition path through the lexical network. Joint recognition and rejection of out-of-vocabulary words are provided by using both cepstrum and LSP parameter vectors.