Louis Pouzin
ABSTRACT
Public packet networks are becoming a reality, and call for interface standards. Two levels of facilities have been proposed, virtual circuit (VC), and datagram (DG). The concepts of VC and DG are already well developed within computer networks. Their properties are reviewed, along with typical issues such as out-of-sequence and congestion problems.
Usually DG's are a sub-layer used as a transport facility by a VC protocol. They also provide the ability to extend switching functions within user systems. The characteristics of VC's considered by CCITT are examined critically, and related to experimental networks and manufacturer softwares.
VC's and DG's are compared from the viewpoint of adapting customer systems to public networks. When the customer is interested in a transport facility, DG's appear to have an edge. When a network becomes a terminal handler, adaptations are more complex and require character stream interfaces. Intelligent terminals would make this problem disappear, as they can use a DG interface.
Although various groups call for a DG interface, the carriers are opposed to it. Four carriers are rushing a VC protocol through CCITT. The carrier's goal is to take over terminal handling, and gradually other processing functions. DG's would leave too much freedom to the customer. The political implications of the carrier policy suggest that better boundaries be drawn up between carriers and data processing.
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