Delay related issues in integrated voice and data transmission: A review and some experimental work
SIGCOMM '79 Proceedings of the sixth symposium on Data communications
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The paper describes the work done thus far in the development of a means of statistically multiplexing data and encoded voice in a transparent and intelligent network called TI-NET. A review of previous work in packetized voice transmission in a conventional packet switched network (ARPANET) has revealed problems related to large fixed and variable transmission delays. These problems result in degradations in speech quality in the form of time scale distortion and gaps due to very late or lost packets. The paper shows that TI-NET has various features which counter the above problems, and thereby make it appear very suitable for encoded voice transmission. The paper describes a software implementation of a protocol for encoded voice transmission which takes advantage of the 60 to 65% idle time in conversations (one way) so that only active periods in speech need be transmitted; this is not possible with present frame synchronous vocoders which transmit continuously. The paper describes the present experimental TI-NET which consists of two nodes (PDP11's) joined by a 9.6 kb/s link. For convenience the protocol for encoded voice transmission has been implemented in the TI-NET nodes, and the transparent transmission of data and encoded voice has been demonstrated. With regard to packetizing synchronous messages within TI-NET, a new protocol is described (and has been implemented) for padding partially filled “minipackets” that are likely to occur at the end of most synchronous messages. In addition, the paper discusses the features of another aspect of TI-NET, i.e. satellite extension nodes, which are used to enable both local and remote regions of high data concentration to access the subnet through 12-14 GHZ satellite links. It is shown that the advantages of accessing TI-NET by satellite extension nodes (as compared to accessing a conventional packet switched network by terrestrial facilities), include lower communication costs, greater accuracy and security, and smaller entrance delay. Finally, the paper describes an experiment in which data and encoded voice was transmitted from a TI-NET node at Carleton University, Ottawa, at 9.6 kb/s in “multiuser” packets, over the Hermes (CTS) satellite to NASA AMES Research Center in California, where it was looped back and returned to the same TI-NET node. The significance of the work described in the paper is that it represents an important step in the development of a transparent and intelligent public network capable of transmitting encoded voice (at 9.6 kb/s and lower rates) and data. Such a network can serve as an integrated system for data and voice and provide cost benefits to its users through savings in bandwidth of the order of 50 to 1, by statistically multiplexing data with vocoded voice.