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If the drift to individual means of transport is to be avoided, the city bus must be made more attractive in its driving comfort and performance, and provide a maximum degree of safety. Vehicles of advanced design requiring a minimum of maintenance, and frames and superstructures constructed according to the latest conceptions, are absolutely necessary for an economic and reliable city bus. Easy-to-operate controls facilitate the driver's task and are an added safety measure. The standard O 305 regular service bus, designed in accordance with the regulations of the Public Transport Authority (Verband Öffentlicher Verkehrsbetriebe), meets these demands. The experimental electric bus with hybrid drive appears to be a solution to the exacting requirements of future traffic with regard to exhaust emission control.

Intelligent terminals are those which, by means of stored logic, are able to perform some processing on data which passes through them to or from the computer systems to which they are connected. Such terminals may vary widely in the complexity of the processing which they are capable of performing. The spectrum ranges from limited-capability point-of-sale terminals through moderately intelligent text-oriented terminals up to powerful interactive graphics terminals. The common thread that ties all these types of terminals together is their processing power and the questions relating to it.

We assume the reader is somewhat familiar with the ongoing development of Aloha techniques as described briefly in Abramson's FJCC 7 0 paper and Roberts's SJCC72 paper. In short, an Aloha system permits the (hopefully) occasional interference of data packet transmissions in a multi-access channel and provides for the retransmission of lost packets after some randomly distributed interval. The purposes behind permitting interference and providing recovery are (1) reduced central communication control to improve reliability and (2) improved use of communication facilites under bursty loads as found in interactive computer communication. Aloha techniques are finding applicability in various communication contexts (e.g., satellites), but here the emphasis is on large populations of (potentially mobile) interactive terminals. Slotted . In a slotted Aloha system, all terminals begin their packet (re)transmissions at the ' tick of some global clock. Slotting has been advocated by Roberts as a simple way to improve the limiting thruput of an Aloha channel by a factor of 2. Global ticking can be implemented in a (hopefully positive) number of ways and its feasibility is assumed in this paper. We rely on slotting to simplify our analysis, but suggest that the stability and control phenomena studied are characteristic of Aloha systems in general. Controlled . The notion of an optimal retransmission delay was introduced by Roberts. We extend the notion to include dynamic control of retransmission delay. A controlled Aloha system has the property that its terminals adjust their retransmission behavior as a function of perceived channel utilization. It will be shown that such adjustments are implementable in at least one way and that they improve system performance under heavy loads. Blocking and Thinking . When a user's terminal has a ready packet, it is assumed that no new packets can be generated. The user is said to be blocked . When a user is not blocked, he is said to be thinking . This assumption about user behavior departs from Abramson 's analysis and is thought to produce more realistic solutions. Recall that Abramson (in FJCC70) modelled users as unperturbable, realtime, Poisson sources of new packets. By adding blocking to an Aloha model, transmission delays feed back (as in real life) on the generation of transmission requests. User sessions are said to be dilated by transmissions delays. It is intended that think time account for (1) delays in central system response, (2) return transmission delays, (3) real user thinking time, and (4) type-in time. Block time accounts only for delays due to transmissions through the multi-access channel. The following analysis rests on some (very) simplifying assumptions about an Aloha system in equilibrium, in steady state. While having a large population of users tends to support our steady-state arguments, the dynamics do indeed require more detailed investigation.

This paper is one in a series of short papers highlighting research problems in the area of computer communication. Two previous papers have described problems in the design of data communication networks [CHOU 74] and selected research areas in computer communication [KLEI 74a]. In this paper, we will concentrate on problems which arise in the design of control procedures for computer networks. While some of the problems we are going to describe refer explicitly to packet - switched computer communication networks, others apply to packet-switched as well as line-switched systems. We will consider the following six problems: 1. Flow control and routing 2. Design and implementation of efficient protocols 3. Definition of network performance 4. Portability of communications software 5. Distribution of intelligence 6. Process synchronization with unrealiable components

In this paper, we discuss flow control in a resourcesharing computer network. The resources consist of a set of inhomogeneous computers called hosts that are geographically distributed and are interconnected by a store-and-forward communications subnet. In the communication process, messages pass between hosts via the subnet. A protocol is used to control the flow of messages in such a way as to efficiently utilize the subnet and the host resources. In this paper, we examine in some detail the nature of the flow control required in the subnet and its relation to the host flow control and subnet performance.

This paper models satellite communications systems by means of bipartite graphs. It assumes stochastic inputs and time delay is the factor of interest. The paper formulates the model and describes the routing and digital transmission schemes, assuming the network links to be synchronized. Messages may be transmitted only in the synchronized time slots, on a first-come first-served basis. The queuing problem is discussed for the single branch and for the satellite network cases, and approximate solutions are obtained for the case of satellites with finite memories.

In a recent paper a traffic assignment model has been constructed in which the cost on a link may depend not only on its load, but also on the loads on other links of the network. In this paper it is shown that this model is also capable of handling the case of several classes of users in the same transportation network each of which has an individual cost function and, at the same time, contributes to its own and other classes' cost functions in an individual way. Typical applications arise not only in street networks where vehicles of different types share the same roads (e.g., trucks and passenger cars) but also in other types of transportation networks (e.g., telephone networks). An algorithm is constructed for finding the system-optimizing flow pattern for such a multiclass-user transportation network.

This paper describes a proposed multi-channel mobile telephone communications system designed to meet Canadian Telephone Company system requirements. Some of the requirements satisfied by this system are maximum channel loading and automatic channel selection with a mobile equipment cost marginally higher than that of the standard mobile radio telephone. The paper will describe a mobile equipment designed for this system together with a description of the equipment modules required to utilize standard mobile and fixed station equipment in this system. The primary objective of A.M.T.S. is to allow maximum subscriber loading within the limits of the 1 to 11 contiguous channels available in the 150 mc mobile telephone service band. Because it has been possible to meet this objective without materially increasing the capital investment per subscriber, relative to present systems employing manual channel selection, it is felt that the A.M.T.S. concept may be of interest in other areas of North America.

In a previous paper, algorithms for optimally routing a single vehicle were presented. In this paper, the problem of routing a fleet of M vehicles serving fixed demand for transportation from many origins to a single destination is considered. The M vehicle General Routing Problem (abbreviated M-GRP) is the core of many real routing problems, such as school bus routing for a single school (many-to-one) and garbage truck routing to a central dump. It is shown that the M-GRP on a given graph is equivalent to a GRP on a modified graph. From this result, it is shown that single vehicle routing algorithms can be applied to the M vehicle routing problem.

Since mobile code can migrate from a remote site to a host and can interact with the resources and facilities of the host, security becomes the key to the success of mobile code computation. Existing mobile code security mechanisms such as access control are not able to fully address the import security properties of the host including confidentiality and integrity. And these practices tend to protect the host from potential attacks by confining the mobile code, thus will impair the function of mobile code. Information-flow policy is a technique that can ensure confidentiality, however the analysis of the information flow is practically difficult. This paper describes an innovative approach to provide Java mobile code system security by bytecode analysis. The key technique of the approach is the dependence analysis adapted to information flow analysis. A security model for mobile code system is also proposed in this paper. By this approach, two major properties of the host security– integrity and confidentiality can be protected while the additional restriction on mobile code can be greatly avoided. A prototype has been implemented, which can be applied to analyze Java class file, applet and mobile agent.

This paper presents a mathematical study of a time-sharing system with a single server and many queues. In each queue a customer receives one quantum of service and is then sent to the end of the next queue, provided his service demand has not been completely satisfied. When a quantum of service is completed, the server attends to the first customer in the lowest-index nonempty queue. The discipline of the highest indexed queue is “round-robin,” while all other queues obey the FIFO rule. Newly arrived customers join the end of the first queue. The paper derives mathematical expressions for the main performance measures and illustrates them by graphical means.

In this paper the authors study the queues that build up in a minor stream of traffic waiting to cross or merge with a major stream. The criterion for joining or crossing is that a gap between vehicles in the major stream be greater than or equal to some constant value. Several authors have studied the case where bulk service is provided for the minor stream queue with the appearance of each gap, in this paper the authors emphasize the higher flow rate situations where one car in the minor stream queue is released in a first-come first-serve fashion with the appearance of a gap. It is assumed that the minor stream is Poisson-fed while the intervehicle headways in the major stream are arbitrary but identically and independently distributed random variables.

This is the first of two papers dealing with single server systems. Two subjects are discussed in the present paper, (i) relations between such items as the probability of loss, probability of no delay, and the average number of customers served in a busy period, and (ii) the statistical behavior of a single server system in which no waiting for service is allowed.

This paper discusses two aspects of the performance and design of certain queued or buffered information transmission systems. Examples are buffered multiplexers and message-switching systems. The first part of the paper examines the theoretical performance of such a system as its output channel capacity and the number of its speed-limited information sources are increased proportionately. The aspects of performance considered are time delay and probability of overflow of the system's finite buffer capacity. Under the assumptions made (described in the text), system performance almost always improves as the size of the system is increased. The second portion of the paper considers the design of a communication system that must serve two classes of users, the two classes differing in average message length. Here the question is that of superiority of integrated or segregated communication system design under a channel-capacity constraint. Using average delay as a performance criterion it is shown that the integrated system is not always superior but becomes superior if there is sufficient excess capacity.

This is the second of two papers dealing with single server systems. Statistical problems associated with the busy periods, i.e., the periods during which the server is continuously busy, are considered in the present paper. The input and the service time distributions may be quite general, and the length of the waiting line is unrestricted. Among the new results is an asymptotic expression for the probability density of the lengths of the busy periods. This expression holds when the arrival rate is almost equal to the service rate and when the busy periods tend to be long. It is hoped that the methods used here will throw additional light on known results.

The unique multipoint interconnection capabilities of satellite communication networks place a burden on the communications system designer to achieve highly efficient transmission capacity in the face of a very wide range of network requirements. The ability of various types of multiple-access systems to meet these requirements is the subject of this paper. INTELSAT, recognizing that the frequency division multiplex-frequency modulation, frequencY division multiple access (FDM-FM-FDMA) method it currently uses has advantages in some applications but is not optimum for the multiple-access environment, has supported the development of two-digital multiple-access communications techniques: single channel per carrier frequency division multiple access (SPADE) and time division multiple access (TDMA). These two techniques are discussed in detail and the paper concludes with a presentation of a promising multiple-access technique of the future which includes time division switching on board the satellite.

This paper studies an M/G/1 queuing system with two classes of customers. The cost per unit time of holding a customer differs for the two classes. The server has the option of serving the first customer in line at no extra charge or paving a fee to earn the right to serve any higher-class customer further back in the queue. The paper finds an optimal policy for the simple case where this fee is independent of the position in the queue of the promoted customer. For the case where the server is restricted to serving either the first customer or the first higher-class customer further back in the queue, an appealing conjectured form of an optimal policy is shown to be invalid.

This paper is concerned with a communications system having m channels and serving a multitude of low-traffic sources. The system incorporates a waiting list having q places. Calls arriving when all m channels are busy, and up to (q - 1) other calls are already waiting, will be put on the waiting list and will be served in order of arrival. When the waiting list is filled, additional arriving calls will also be permitted to wait. When a place on the waiting list becomes available, one of these remaining waiting calls, chosen at random, will be admitted on this list. The delay distributions for this partly ordered service are investigated in this paper. Results are presented in a number of graphs.

Recent papers in this and other publications have addressed the current status of computer networks in some detail43, 14, 49, 11. This paper is intended to provide complementary material by updating portions of previous works concerned with the telecommunications aspects of computer networks. This is a notably difficult task for several reasons, including common carrier competition, the immaturity of the technology, and a very complicated and dynamic regulatory situation. Nonetheless we shall attempt to delve into those aspects of the current telecommunications scene likely to be of major significance to common carriers, users, service and equipment vendors, and people just generally interested in the evolution of computer networking.

This paper studies a priority queuing model in which the processing times of jobs are known upon arrival and preemption without loss of time or processing already accomplished is possible. Jobs are classified into K types according to their lengths, priority is assigned to them according to type and the length of processing remaining, and the paper obtains the expected conditional response times for jobs of each type.

This paper reviews the functions of private automatic branch exchanges (PABX's) and describes the organization of the IBM 2750 Voice and Data Switching System. This system has two main functional areas: (1) line switching, transmission and signaling, and (2) common control. The switching network uses a new integrated electronic crosspoint. The common control is performed by a built-in duplexed computer with stored programs dedicatedt o line switching. The IBM 2750 offers a variety of normal and advanced voice features, and some entirely new data features, and is designed for interconnection with an IBM System/360. Four companion papers describe the electronic switching network, the network control program, and the integrated crosspoint in greater detail.

Summary In a previous paper [4] the author studied the stochastic process {wn , n = 1,2, …}, recursively defined by with K a positive constant, τ1, τ2, … σ1, σ2, …, independent, nonnegative stochastic variables. τ1,τ2…, are identically distributed, and σ1,σ2,…, are also identically distributed variables. For this process the generating function of the Laplace-Stieltjes transforms of the joint distribution of Wn , σ2 + … + σ n and τ1 + … + τ n−1 has been obtained. Closely related to the process {wn , n = 1, 2,…} is the process {un , n = 1, 2,…} with {un = K + [wn + τ n − K]−, n = 1,2,…; these are dual processes. In the present paper we study the stationary distributions of the processes {wn , n= 1,2, …} and {un , n = 1,2, …}, and the distributions ot the entrance times and return times of the events “wn , n = 0” and “un = K” for some n, for discrete as well as for continuous time. For these events various taboo probabilities are also investigated. The mathematical descri ption of the processes {wn , n = 1,2, …} and {un , n= 1,2, …} gives all the necessary information about the time-dependent behaviour for the general dam model with finite capacity K, since the process {wn , n= 1,2, …} is the basic process for such dam models. In Sections 5, 6 and 7 the general theory is applied to the models M/G/1 and G/M/1. Complete explicit solutions are obtained for these models. The present theory also leads to new and important results for the queueing system or dam model G/G/1 with infinite capacity. For instance the joint distribution of the busy period (or wet period) and of the supremum of the dam content dunng this period is obtained.

The transmission of data, telegrams, or narrative messages (record material) over the facilities of one or more telecommunication networks requires agreed-to message formats and communication-link control procedures for traffic interchange over these networks. This paper is designed to be a definitive discussion of the principles to be used for developing message formats and control procedures. The paper is based primarily upon the current efforts of the American National Standards Institute (ANSI)Task Group X3S33, Message Header Formats and Task Group X3S34 Control Procedures.

A queueing system has restricted accessibility if not every customer is admitted to the system. For such a system the admittance of a customer will in general depend on the state of the queueing system at the moment of his arrival. In this paper queueing models will be studied for which the accessibility depends on the actual waiting time of the arriving customer. Various queueing situations encountered in the allocation of memory equipment for information processing systems may be described as a single server queueing system with restricted accessibility; the mathematical models for the involved storage problems belong to queueing theory and are discussed in the present paper.

Avi-Itzhak, Maxwell and Miller (1965) studied a queueing model with a single server serving two service units with alternating priority. Their model explored the possibility of having the alternating priority model treated in this paper with a single server serving alternately between two service units in tandem. Here we study the distribution of busy period, virtual waiting time and queue length and their limiting behavior. 1. Concepts and definitions We consider a queueing process with two service units, unit 1 and unit 2, and a single server. The server attends to the two units alternately according to some switching rule. A switching rule (Neuts and Yadin (1968)) is a rule describing how the server changes from one unit to the other. The server may change from one unit to the other either by a non-zero switching rule or by a zero switching rule. By a non-zero switching rule the server continues to serve in a unit until some upper number of consecutive services has been completed and then he switches to the other unit. By a zero switching rule the server stays in a unit until the queue in it becomes empty and then he switches to the other unit. In this paper we discuss a non-zero switching rule for unit 1 and zero switching rule for unit 2. Since the analysis of the non-zero switching rules for both units needs heavy notations, we omit the discussion of it here. The zero switching rules for both units is dealt with separately.

For a two-way communications network in which each link is assigned a cost per traffic unit, a minimum cost routing policy (MCR) routes the required traffic between two nodes along the path with least cost. This paper describes a new technique for computing the Composite Flow Requirement between any two nodes. This is the sum of the traffic requirement between the two nodes and all other traffic requirements whose paths pass through both of the nodes. Knowing the Composite Flow Requirement between any two nodes gives vital planning information about the traffic flow patterns in the network. The technique of this paper brings practical utility to the concept of Composite Flow Requirement by substantially reducing computation effort.

In modern telephone systems utilizing computers, the realization of a single server feedback queueing system often arises. In this paper, it is assumed that an originating or transferred call to such a system requires a number of different services before leaving the system as a completed or rejected call. A customer may also repeat a portion of his services, omit some services, or leave the system after any service with some assigned probabilities. In this paper, we present a theorem on the joint multi-variate queue length distribution, in a system with N services or queues, arbitrary priorities, and a general probabilistic transition matrix inter-connecting the various queues.

Certain computer networks have been implemented using a radio broadcast frequency over which a large set of terminals are allowed to transmit packets of bits; one such example is the ALOHA computer network [1]. In such systems, a basic problem is that of the blocking of terminals whose transmission of a packet has overlapped in time with transmission by some other terminal. In this paper we consider a "slotted ALOHA" packet transmission scheme with an infinite set of terminals each transmitting at an infinitesimally small rate. We present a probabilistic model of such a system to show that the slotted ALOHA system with an infinite population is inherently unstable. This paper confirms the simulations and results of Kleinrock and Lam [8].

Packet radio communications provides an effective way to interconnect fixed and mobile computer resources. The ALOHA System at the University of Hawaii first introduced this capability in the context of a single-hop system using off-the-shelf RF equipment with all terminals within line of sight of the central station. The packet radio network described in this paper is 1) an extension of the basic Hawaii work to a geographically distributed system involving the use of repeaters to achieve area coverage beyond line of sight, and 2) provides added capabilities for authentication, antijam protection, and coexistence with other possibly different systems in the same band. An overview of the packet radio system concept is given in this paper.

This paper describes a controlling entity called the Network Manager which integrates and controls the activities of a network of distributed processes. The processes may be realized by special-purpose hardware devices, minicomputers, large-scale processing systems, or a combination of these. The Network Manager presents a well-defined hardware and software interface to the processes and provides control functions which are executive or operating system functions found in traditional software systems. Where appropriate, these functions are optimized by implementing them in hardware. The control functions are delineated in detail in this paper along with the basic hardware configuration of the Network Manager.

Developments of national and international communication served Data Processing Systems is dependent upon not only the availability of facilities, but on the economics involved. The paper shows in graphic form the differences in the tariff structures in the countries and also a comparison of break-even costs when it is economically justified to lease telegraph or telephone lines versus utilizing the telex or the telephone switched facility.There will also be a comparison of available facilities by country, leased point-to-point, leased multipoint, switched telephone and telex. Included in this paper will be a comparison of attachment policies and a summary status of the remote computing services.

The application of telephone techniques to a message switched system will be discussed in this paper. The first part of the paper will briefly review the traffic techniques employed. Then a model of the message switch is systematically developed based on the traffic analysis approach.

In this paper a computer network is defined to be a set of autonomous, independent computer systems, interconnected so as to permit interactive resource sharing between any pair of systems. An overview of the need for a computer network, the requirements of a computer communication system, a description of the properties of the communication system chosen, and the potential uses of such a network are described in this paper.

ABSTRACT Multipath, reverberation, and natural and man made noise limit the data rate and reliability of underwater acoustic command and telemetry links. This paper describes signal coding, gating, and receiver design techniques used in the RATAC(R) acoustic command and control system. Probability relationships for correct and error commands are derived and performance predictions made as a function of range. Test data on an offshore drilling platform is compared to theoretical performance predictions. 1.0 INTRODUCTION Acoustic telemetry techniques for control of underwater devices are limited by the constraints of the acoustic medium and its boundaries, and the sources of noise which are present in the underwater environment. Reliable command operation of these devices is a stringent requirement due to their inaccessibility and the slow communication data rates. Errors and undesired commands must be avoided in all types of noise and interference. This paper will discuss the signal processing and coding techniques, and the predicted and tested performance of the Raytheon RATAC (R) * acoustic command and control system. The system provides multichannel capability, and is designed to minimize the probability of a false command caused by natural or man made noise, or propagation effects. This is accomplished through the use of frequency diversity, time coding, and receiver gating. Test results at a drilling platform installation are included. 2.0 THE UNDERWATER ACOUSTIC ENVIRONMENT The background noise environment consists of several natural and man made noise sources. The most important natural noise sources are the usual sea state noise and the noise due to raindrops striking the surface.1 These spectra are shown in Figure 1. Biological noise sources are primarily concentrated at low frequencies, with the exception of snapping shrimp, which produces a noise level about equal to the rain noise spectrum.2 Interference due to porpoise and whale calls, which may appear similar to tones or frequency slides, is minimized by signal frequency coding and time gating techniques. Man made noises are generally localized, and generate most of their energy in the low frequency regions of the spectrum. The noise spectra produced by normal operation on a drilling platform have been chosen as a worst case. Two spectra are shown in Figure 1 for drilling platform noise.3 & 4 The first is the peak values of measured noise level from the CUSS I drilling barge; and the second is the noise level measured at a distance of 70 yards from a fixed leg platform, and referred to a standard reference distance of one yard. Spherical spreading was assumed as a worst case. The difference in measured levels is due to the presentation of peak values in the first case for the noise spectra, and the different platform noise characteristics. These curves include the maximum expected noise levels, with the exception of isolated events such as explosive pipe cutting. The noise spectrum consists of many line components as well as a broadband background. The signal decoding technique for a telemetry system must there for be able to discriminate against line frequency components as well as the more common white Gaussian noise background generally assumed for computational convenience.

This paper reports the results of a study on traffic handling capacity of switching nodes and transmission lines in the circuit and packet switched networks, and on cost evaluation methods for the two types of networks.The two types of networks were compared by setting various parameters, including the communication density defined in the paper. The comparison results show that there are application regions favorable for each type of network.

This paper contains information on the general characteristics of the Control Data Corporation, MDM Communications Division M1000 Communications Message Switching System.It is the intent of this paper to discuss the analytic systems approach to the problem that led to the building of unique hardware and software to resolve the message handling functions. The M1000 hardware and software system features are discussed in the paper.

Efficient routing policies are required for both design and operation of distributed, packet-switched computer networks. In the network design process, deterministic policies, which are time invariant, easy to analyze and simple to represent, are generally used. For the routing of packets in a real network, on the other hand, adaptive policies, which can adjust to traffic fluctuations and component failures are implemented. Therefore, a computer network is typically optimized with respect to a deterministic policy and then operated with an adaptive policy.In the first part of this paper the two policies are compared using analytical and simulation methods, and it is shown that, in uniform traffic situations, the flow patterns generated are very similar. This result justifies the use of deterministic policy in the design phase.In the second part of the paper, the inadequacy of traditional adaptive policies in handling particular traffic situations is pointed out. As a solution, a new centralized adaptive policy is proposed, which combines the positive features of both deterministic and adaptive routing. The feasibility of such centralized policy implementation on a real network, and its efficiency with respect to the traditional implementation is discussed.

The first step in using Internet protocols in space is to establish the basic Internet datagram delivery service over satellite RF links. This paper discusses the low-level data link and data routing issues related to using Internet protocols to support spacecraft communications. It covers issues related to layer 1 (physical), layer 2 (data link), and layer 3 (network). It does not cover layer 4 (transport) and above. At the physical layer, the paper presents various applications of forward-error-correction (FEC) coding techniques, such as convolutional coding and Reed-Solomon. It describes approaches for using these techniques in ways that are independent of the protocols used at the data link layer and above. At the data link layer, common, commercially available framing schemes are discussed along with how they can be easily deployed. A rationale is provided for the selection of HDLC/frame relay framing along with IETF multi-protocol encapsulation. At the network layer, the Internet protocol end-to-end addressability and routing is discussed in the context of space-based applications. Standard solutions for dealing with the intermittent and mobile links of satellites are also discussed. These include a discussion of Mobile IP and mobile routing protocols. Finally, deployment of these protocols in both spacecraft and ground systems are discussed. Details of current implementations by the Operating Missions as Nodes on the Internet (OMNI) project at NASA/GSFC using operational space and ground systems such as UoSAT-12 and TDRSS are also provided.

In this paper, we discuss the use of chaotic signals in cryptography. A comparative analysis of three different analogue chaotic communication systems is presented. The paper concludes with a discussion of some possible measures that could improve the security of these systems.

This paper discusses the technical problems of traffic flow encountered in the overall design of a communication system for a specific purpose, namely the provision of air-traffic services to civil aircraft. The major part of the paper deals with (a) the evaluation of queueing delays on aeronautical mobile (aeromobile) speech channels, from which the number of channels required to meet a chosen delay criterion is estimated (b) the minimum signal/noise ratio required for satisfactory communication on aeronautical radio channels, and the transmitter power required to produce it. Statistics of operator loading, channel holding times and message-arrival rates are given, and the role of the human operator as a component of the system is evaluated.

Subscriber trunk dialling requires routing to be divorced from numbering. The paper discusses the various signalling problems which arise with register-translators. Basic principles are analysed rather than details of design, and a signalling philosophy is formulated as a basis of design for national and international networks. A main problem is the choice between the same or separate line equipment for the supervisory and number signalling; the paper details the merit of supervisory signalling by per-line equipment and number signalling by common-register equipment. End-to-end and link-by-link signalling, the signalling facilities to be given and the number signalling code all influence the signalling technique, and are analysed. Conclusions are given on the merits of link-by-link line signalling for flexibility of application, of end-to-end register number signalling for fast call set-up time and short post-dialling delays, and of fast coded self-checking multi-frequency signals for number transmission. The transmission features of line networks will have considerable influence on signalling-system design, particularly with end-to-end signalling for number transmission. Satisfactory design solutions are possible with limited end-to-end signalling. Speech-interpolation equipment (t.a.s.i.) applied to intercontinental circuits modifies the conclusions reached for the non-t.a.s.i. condition if account is to be taken of minimum degradation of the t.a.s.i. advantage, and the basic problem is the relative assessment of call set-up time and t.a.s.i.-advantage considerations.

This paper considers a multiserver queue with heterogeneous servers in which arriving customers are assigned to servers according to an autonomous Markov chain. Under conditions ensuring stability, stationary distributions are computed for the waiting times and the system response times of successive customers. Limits are also obtained for the expected system workload and the expected number of customers in the system.

This paper describes a study of transmission control procedure for computer-to-computer and computer-computer-to-terminal communication through a public packet-switched network. Four types of basic data link are defined over a packet travel path in the network, and the functions required for the data link are discussed. A process number and its associated control method are proposed as a method of packet-interleaved communication. By applying the result of the discussion and introducing the concept of a data link and a process number, a transmission control method and a call control method between packet-switched network and computers are proposed for the packet-interleaved communication on the basis of the HDLC procedure (high-level data link control procedure).

A hierarchical queuing network is a layered network of queues in which all arrivals receive initial service from a primary server, and some customers receive additional service jointly from a primary and a secondary server. This paper considers the simplest such network, involving two levels and a single secondary server who functions as a consultant to several primary servers and always works in conjunction with the primary server who refers the customer to him. An approximate analysis of this hierarchical queuing network is carried out for three operating disciplines governing the consultation process. The analytical results compare favorably with the results of computer simulations and are therefore used to explore several important issues in the design of such networks. Hierarchical queues are common in public-service systems, including new medical-care systems known as “telemedicine.”

This paper deals with some important practical applications of teletraffic theory to field engineering problems. It is shown that the following tasks concerning line switching telephone networks can be mastered by means of a few handy tables: loss calculation and dimensioning of trunk groups for full or limited access; very accurate loss approximation of link systems with an arbitrary number of stages, operating as group selection or traffic concentrating arrays with point-to-group selection mode (regarding this, and also optimal link structures, wiring, etc., see [7] in this issue); dimensioning groups with limited or full access for offered peaked overflow traffic; cost minimizing partition of the traffic in local or toll networks with alternate routing to the various trunk groups; and computerized dimensioning is of course also possible.

Packet switching over broadcast channels with random access schemes is of current interest for local distribution system and for satellite channels. This mode of operation is useful when the communicating devices are mobile and when the ratio of the peak to average data rate requirement of each device is high. Such systems have been analyzed for the case in which all communicating devices are within an effective transmission range of each other; either directly or through the satellite. In this paper, we address broadcast networks in which originating devices cannot directly reach the destination receiver. Thus, devices are introduced which receive these packets and repeat them to the destination. The capacity (maximum throughput) of such systems is determined, and design problems related to the number of repeating devices and the usefulness of directional antennas are resolved.

This paper presents an approximate procedure for computing selected performance characteristics of an urban emergency service system. Based on a recently developed hypercube queuing model, the procedure requires for N servers solution of only N simultaneous equations, rather than 2N as in the exact model. The procedure relies on the theory of M/M/N queues in which servers are selected randomly and without replacement until the first available (free) server is found. The underlying model is intended for analyzing problems of vehicle location and response district design in urban emergency services, includes interdistrict as well as intradistrict responses, and allows computation of several point-specific as well as area-specific performance measures.

This paper considers the problem of locating bus stops in the context of a set covering problem. Zero-one integer programming models are suggested for use in the location of bus stops on new routes and for use in the location of express bus stops on current routes. The models may be used to locate the minimum number of (express) bus stops required to ensure that no passenger need walk more than a specified distance to reach an (express) bus stop. A modified version of the model is presented which enables the router to locate a specified number of (express) bus stops in such a manner that the total distance walked by all boarders is minimized.