Comparison of overhead factors in message-switched and packet-switched networks

Overhead factors for data networks using message-switching or packet-switching techniques are defined and analysed. The transmission lines are assumed to have only random errors and the acknowledgments are handled on a `stop-and-wait? strategy. For packet switching, the packet size used is such that, in each case, the overhead factor is minimised. Results show that the overheads for packet switching tend to be much higher than for message switching when the line error rates are low, i.e. better than 10?5. It is concluded that, although packet switching has several significant merits, these are achieved at the expense of a substantial increase in overheads.     

Design of a circulating-type analogue?digital convertor using bucket-brigade delay lines

A hardware implementation of a circulating-type analogue?digital convertor using bucket-brigade delay lines (b.b.d.l.) is described. The b.b.d.l. used is a 32-stage commercially available device. This large number of stages is not necessary, and, in practice, a b.b.d.l. with two or three stages would be satisfactory. Such an analogue?digital convertor can be implemented economically, and would be able to convert input signals in the frequency range 0?500 kHz. The principal advantage in using the b.b.d.l. is in the reduction of the number of analogue switches required. Experimental results show that 4?6 bit conversion is readily achieved, although a compensator network to compensate for the b.b.d.l. losses is required.     

Analysis of playout strategies for voice transmission using packet switching techniques

Due to variations in network delay, a stream of voice packets with deterministic interarrival times to a data network may not have deterministic interdeparture times at the destination. Two playout schemes which are designed to remove such variations in delay are considered. Analytic results for the performance of these two schemes are obtained. Numerical examples showing the effect of coefficient of variation of interdeparture time on performance are presented.     

Congestion control technique for an integrated services local area network

A congestion control scheme for implementation in an integrated services local area network is proposed. An outline is given of simulation studies that have been carried out to demonstrate the feasibility and effectiveness of such a scheme. The way that these studies can help identify the volume of real-time traffic that can be accommodated with an acceptable level of blocking is explained.     

Comment on the speed of binary multiplication using cellular arrays

A known recoding algorithm is considered and is shown to yield a speed improvement of approximately 33% for serial multiplication. However, in the case of cellular-array multipliers, such a recoding appears to offer no advantage; instead, it may even result in a speed penalty owing to the sequential nature of the recoding algorithm.     

An Analysis of One Direction of Window Mechanism

A queuing model is presented for the analysis of one direction of window mechanism as used in numerous data communication control procedures e.g., X.25, HDLC and SDLC. Two models of message length are considered viz. fixed length and exponential distribution. A novel approach is used to determine an effective service time to account for possible inserted idle time while waiting for the window to reopen. A set of delay characteristics is determined for different window sizes. A minor modification of the frame format is suggested to improve on the performance of the window mechanism.     

Simple design for UP/DOWN Gray-code counters

A simple design for a presettable UP/DOWN Gray-code counter is described. The design is such that the logic requirements for the UP mode are identical to those for the DOWN mode, a single AND/OR gate being the only addition to the entire counter. Furthermore, since the logic between any two consecutive stages remains unaltered, the counter can be readily extended for larger numbers of bits. A typical application of the UP/DOWN feature is in Gray-code addition/subtraction.     

Buffer analysis of an integrated voice and data terminal

Current research activities in computer networks tend to emphasize the integration of services such as voice and data by designing networks that take into account the performance requirements of these services. In some cases, however, design and implementation of a totally new network may not be feasible. An alternative approach in such cases would be to integrate the services at the terminal level. Such an integrated terminal could also be widely used in future office automation systems. The performance of such an integrated terminal is assessed with the assumption that the terminal is connected to a voiceband channel. The analysis indicates the performance limits on the application imposed by the buffer size at the terminal and the large fluctuation of delay performance for data traffic. A window-type flow control for data traffic is also considered. By appropriately choosing the buffers in the terminal and window size, both voice and data traffic service requirements can be met.