Introduction

The purpose of this chapter is to explore the implications of some facts about prosody and intonation for efforts to create more general and higher quality speech technology. It will emphasize parallels between speech synthesis and speech recognition, because I believe that the challenges presented in these two areas exhibit strong similarities and that the best progress will be made by working on both together.

In the area of synthesis, there are now text-to-speech systems that are useful in many practical applications, especially ones in which the users are experienced and motivated. In order to have more general and higher quality synthesis technology it will be desirable (1) to improve the phonetic quality of synthetic speech to the point where it is as easily comprehended as natural speech and where it is fully acceptable to naive or unmotivated listeners, (2) to use expressive variation appropriately to convey the structure and relative importance of information in complex materials, and (3) to model the speech of people of different ages, sexes, and dialects in order to support applications requiring use of multiple voices.

Engineers working on recognition have a long-standing goal of building systems that can handle large-vocabulary continuous speech. To be useful, such systems must be either speaker-independent or speaker-dependent; if speaker-dependent, engineers must be trained using a sample of speech that can feasibly be collected and analyzed. Present systems exhibit a strong trade-off between degree of speaker independence on the one hand and the size of the vocabulary and branching factor in the grammar on the other.

Introduction

One of the most delightful features of a small symposium is that it allows for protracted discussions in which many people participate. Ample time for discussion was built into the symposium schedule throughout, but we allocated a special two-hour slot to challenge ourselves to identify the most significant problems capable of being solved in a five- to ten-year period. That they be solvable in that time frame challenges us beyond what we can see, but not beyond what we can reasonably extrapolate. That their solution be significant takes the discussion beyond questions of purely academic interest.

Furthermore, at the suggestion of one of the government representatives, we asked what applications should drive research (much as the application of natural language interfaces to database drove research in the 1970s and 1980s).

All attendees, including representatives of various governmental agencies, participated in this discussion.

To keep our thoughts large, we construed natural language processing (NLP) as broadly as possible, freely including such areas as lexicography and spoken language processing.

To direct the discussion without focusing it too tightly, we set forth the following questions:

1. What are the most critical areas for the next seven (plus or minus two) years of natural language processing? (“Critical” is taken to mean that which will produce the greatest impact in the technology.)

2. What resources are needed (such as people, training, and corpora) to accomplish the goals involved in that work?

3. What organization is needed (e.g., coordinated efforts, international participation) to accomplish those goals?

4. What application areas and markets should open up in response to progress toward those goals?

Introduction

If current natural language understanding systems reason about the world at all, they generally maintain a strict division between the parsing processes and the representation that supports general reasoning about the world. The parsing processes, which include syntactic analysis, some semantic interpretation, and possibly some discourse processing, I will call structural processing, because these processes are primarily concerned with analyzing and determining the linguistic structure of individual sentences. The part of the system that involves representing and reasoning about the world or domain of discourse I will call the knowledge representation. The goal of this chapter is to examine why these two forms of processing are separated, to determine the current advantages and limitations of this approach, and to look to the future to attempt to identify the inherent limitations of the approach. I will point out some fundamental problems with the models as they are defined today and suggest some important directions of research in natural language and knowledge representation. In particular, I will argue that one of the crucial issues facing future natural language systems is the development of knowledge representation formalisms that can effectively handle ambiguity.

It has been well recognized since the early days of the field that representing and reasoning about the world are crucial to the natural language understanding task. Before we examine the main issue of the chapter in detail, let us consider some of the issues that have long been identified as demonstrating this idea. Knowledge about the world can be seen to be necessary in almost every aspect of the understanding task.

Introduction

Although natural language processing (NLP) has come very far in the last twenty years, the technology has not yet achieved a revolutionary impact on society. Is this because of some fundamental limitation that can never be overcome? Is it because there has not been enough time to refine and apply theoretical work that has already been done?

We believe it is neither. We believe that several critical issues have never been adequately addressed in either theoretical or applied work, and that, because of a number of recent advances that we will discuss, the time is due for great leaps forward in the generality and utility of NLP systems. This paper focuses on roadblocks that seem surmountable within the next ten years.

Rather than presenting new results, this paper identifies the problems that we believe must block widespread use of computational linguistics, and that can be solved within five to ten years. These are the problems that most need additional research and most deserve the talents and attention of Ph.D. students. We focus on the following areas, which will have maximum impact when combined in software systems:

1. Knowledge acquisition from natural language (NL) texts of various kinds, from interactions with human beings, and from other sources. Language processing requires lexical, grammatical, semantic, and pragmatic knowledge. Current knowledge acquisition techniques are too slow and too difficult to use on a wide scale or on large problems. Knowledge bases should be many times the size of current ones.

2. Interaction with multiple underlying systems to give NL systems the utility and flexibility demanded by people using them. Single application systems are limited in both usefulness and the language that is necessary to communicate with them.

3. […]

Introduction

This chapter concerns a dispute about the relationship of sentences to the events they describe, and how that relationship is manifested in sentences with adverbial modifiers. The two sides to the argument might be called the “Davidsonian position” and the “situation semantics position”; the former being chiefly represented by Donald Davidson's well-known paper “The Logical Form of Action Sentences” (Davidson, 1980) and the latter by John Perry's critique of Davidson's view, “Situations in Action” (Perry, unpublished manuscript).

The issue turns on Davidson's analysis of how a sentence such as (1) is related to a similar sentence with an adverbial modifier, such as (2).

1. (1) Jones buttered the toast.

2. (2) Jones buttered the toast in the bathroom.

Stated very informally, Davidson's position is this: (1) claims that an event of a certain type took place, to wit, a buttering of toast by Jones, and that (2) makes a similar claim but adds that the event took place in the bathroom. Put this way, an advocate of situation semantics could find little to complain about. Perry and Barwise themselves say rather similar things. The dispute is over the way that (1) and (2) claim that certain events took place. Davidson suggests that the event in question is, in effect, a hidden argument to the verb “butter”. As he would put it, the logical form of (1) (not analyzing the tense of the verb or the structure of the noun phrase) is not

1. Buttered (Jones, the toast)

but rather

1. ∃X(Buttered(Jones, the toast, x)),

where the variable x in (4) ranges over events.

Introduction

The present work investigates the contrastive discourse functions of a definite and a demonstrative pronoun in similar contexts of use. It therefore provides an opportunity to examine the separate contributions to attentional state (Grosz and Sidner, 1986) of two linguistic features – definiteness and demonstrativity – independently of pronominalization per se. The two pronouns, it and that, have clearly contrastive contexts of use, explained here in terms of distinct pragmatic functions. Certain uses of it are claimed to perform a distinctive cohesive function, namely, to establish a local center (that modifies rather than replaces the notion of a center). The crucial distinction between a local center and the Cb (backward-looking center) of the centering framework (cf. Sidner, 1983; Grosz et al., 1983; Grosz et al., 1986; Kameyama, 1986) is that there is only a single potential local center rather than an ordered set of Cfs (forward-looking centers). The local center is argued to constitute a reference point in the model of the speech situation in a manner analogous to 1st and 2nd person pronouns. In contrast, a deictic function is posited for apparently anaphoric uses of that whereby the attentional status of a discourse entity is changed, or a new discourse entity is constructed based on non-referential constituents of the linguistic structure. Because it is impossible to observe attentional processes directly, I present an empirical method for investigating discourse coherence relations. I analyze statistically significant distributional models in terms of three types of transitions in the cognitive states of conversational participants: expected transitions, unexpected transitions, and transitions with no relevant effect.

INTRODUCTION

This case study is a formal description of a protocol for a local area network, using the specification language PSF.

One approach to local networking is the ring network. Although various types of rings have been proposed and built, we will study one of the more popular organizations, the token ring network. In such a network, a token circulates around the ring, which can be captured by one of the components. The component guarding the token is allowed to transmit a message.

The protocol specified in this paper is based on the token ring described in [IEEE85b] as an IEEE standard. This description is given partly in informal, natural language and drawings, and partly by means of state transition systems. The intention of this chapter is to apply a Formal Description Technique in order to give a formal specification of the protocol. In contrast to the protocols in the previous chapters, we try to provide a specification, that resembles an existing standard as much as possible.

TOKEN RING NETWORK, AN INTRODUCTION

A ring consists of a collection of ring interfaces connected by point-to-point links that form a circle, as shown in Figure 7.1. Point-to-point links involve a well-understood and field-proven technology. Due to the sequential ordering of the stations attached to a ring, a ring-based protocol is in general fair in the sense that each station eventually will get control of the ring. In a token ring, each station has a known upper bound on channel access. The ring network standardized in [IEEE85b] is called a token ring and in this section we will take a closer look on what this is.

INTRODUCTION

The Amoeba Distributed Operating System uses a Transaction Protocol for the communication between different processes running under the supervision of the Operating System. A transaction is a basic form of information exchange between two processes, consisting of a request followed by a reply. Contrary to a Connection Oriented Protocol a Transaction Protocol does not establish a permanent (logical) connection between two communicating processes. For each transaction a connection is built up. As soon as the transaction is finished the connection is broken. The choice of a Transaction Protocol in favour of a Connection Oriented Protocol is based on the observation that in a distributed operating system most communications within a network do not imply massive data transport during a long time. As a result the overhead costs of building up and maintaining a permanent connection between two processes will be (too) high.

In the Amoeba Operating System transactions take place between a Client process and a Server process. A Client process sends a request to the network. This request can be answered by a Server process with a reply. In order to increase the performance and the fault tolerance of the operating system several Server processes may provide the same service. When a specific Server crashes or is temporarily busy another one can take over its task.

As in all communication protocols, acknowledgements are needed for reliable communications. In the Amoeba Transaction Protocol, abbreviated to ATP in the sequel, an acknowledgement message from Client to Server is used to report the reception of a reply. The reply itself serves as an acknowledgement of the reception of a request.

AIM AND SCOPE

An important reason why formal description techniques are not appreciated as widely as wished by the developers of such techniques, is that people who actually design and implement software have relatively little knowledge of formal methods. The acceptance of formal techniques not only depends on the existence of techniques that are easy to understand and easy to use, but also on the training of potential users. This implies that there is a need for text books and case-studies. We think that a collection of formal specifications in a restricted area of application may help to get a better understanding of the use of formal techniques. Although the method we use is well suited for formal verification, we concentrate on the act of specification. A first requirement for a formal correctness proof is a formal specification.

We restrict ourselves in this book to a collection of specifications concerning one application area, the field of communication protocols. Although this seems to be an area with a relatively high acceptance of formal techniques, most of the protocols that are actually in use are specified in natural language, if ever specified otherwise than by the actual implementation. Even well-known and accepted standards, such as the token ring protocol, do not have a rigorous formal definition. Informal specifications in this area may lead to misinterpretations and, thus, to different implementations that will not be able to work together. Formal techniques are especially needed for communication protocol design, since these protocols describe distributed systems which have a high degree of non-determinism.

INTRODUCTION

Sliding Window Protocols are used to provide reliable data communication between two computers in a network environment. A Sliding Window Protocol is connection oriented: a logical connection between the computers is established before data are transferred. Establishing a connection is not part of a Sliding Window Protocol. The connection is supposed to be a point-to-point connection without an intermediate network station. Sliding Window Protocols are situated in the Data Link Layer of the ISO OSI layer model.

In Tanenbaum ([Tan89]) three Sliding Window Protocols are presented. In this chapter a formal specification of these protocols is given. In the remainder of this section we give a general and informal description of a Sliding Window Protocol. In sections 4.2 to 4.4 the different Sliding Window Protocols are introduced and specified in PSF. The communication between Host processes and a Sliding Window Protocol is specified in section 4.5.

GENERAL DESCRIPTION OF A SLIDING WINDOW PROTOCOL

A Sliding Window Protocol (SWP) manages the communication on a point-to-point connection between two computers in a network at the Data Link Layer level in the OSI terminology. A SWP is a full-duplex protocol. This means that data can be transmitted simultaneously from station <I>A to station <I>B and vice versa. On both sides a SWP process is active, taking care of correct transmission. A SWP process contains a sending and a receiving part, managing outgoing and incoming data respectively. As we shall see in the sequel, these parts are not fully separated.

The specifications in this book are the result of a number of case studies performed by researchers from the Programming Research Group at the University of Amsterdam. The primary goal was to study the use of the techniques developed by the Programming Research Group for the specification of real-life protocols. From the pool of available case studies we made a selection that focuses on communication protocols, which we present in an order well suited for use in education. We hope that this book provides a first step towards a methodology for the design of communication protocols using PSF.

The following people have contributed to this book: Jacob Brunekreef, Henrik Jacobsson, Sjouke Mauw, Gert Veltink and Jos van Wamel.

Other people have helped in initiating and creating this book. The editors would like to express their gratitude for their help in various ways to Jan Bergstra, Jacob Brunekreef, Bob Diertens, Casper Dik, Hans Kamps, Hans Mulder and Jos van Wamel.