Experiments with packet switching of voice traffic

There has been much interest recently in integrated services digital networks carrying both voice and data traffic. Packet switching is being used to carry data in an attempt to make better use of trunk capacity than with circuit switching. In a telephone conversation, for most of the time only one person is talking, and it has been suggested that packet switching can lead to economies in carrying voice traffic also. In view of the variable delays associated with store and forward switching, buffering is usually required at the receiver to enable received speech to be reconstituted at the proper rate. Simulation experiments of packet switching of voice traffic with fixed packet routing have been carried out. The results of these simulation experiments, which are described in this paper, show that, for a single link between two exchanges, 22 conversations can be carried by packet switching with reasonable delay. For the same inter-exchange-link capacity, only 15 conversations can be carried by circuit switching. For a larger network with more exchanges and links per path, a similar advantage is also found with packet switching. The results show that the standard deviation of interpacket delay for successive packets of the same talkspurt is an order of magnitude less than the standard deviation of packet transit time for all packets. This suggests correlation of flows of packets within the same talkspurt. The wider variation of transit delay applies to each talkspurt as a whole and all packets within the talkspurt have correlated transit times, and hence interarrival times. The fact that the standard deviation of interpacket delay is small as compared with the standard deviation of packet transit time suggests that the receiver buffering requirement is less than that indicated by the standard deviation of the packet transit time.