ISO/IEC 11573:1994 情報技術—システム間の電気通信と情報交換—プライベート統合サービスネットワークの同期方法と技術要件 | ページ 2

Foreword

ISO (the International Organization for Standardization) and IEC (the International Electrotechnical Commission) form the specialized system for worldwide standardization. National bodies that are members of ISO or IEC participate in the development of International Standards through technical committees established by the respective organization to deal with particular fields of technical activity. ISO and IEC technical committees collaborate in fields of mutual interest. Other international organizations, governmental and non-governmental, in liaison with ISO and IEC, also take part in the work.

In the field of information technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1. Draft International Standards adopted by the joint technical committee are circulated to national bodies for voting. Publication as an International Standard requires approval by at least 75 % of the national bodies casting a vote.

International Standard ISO/IEC 11573 was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology, Subcommittee SC 6, Telecommunications and information exchange between systems.

During the preparation of this International Standard, information was gathered on patents upon which application of the standard might depend. Relevant patents were identified as belonging to ALCATEL Business Systems. However, ISO and IEC cannot give authoritative or comprehensive information about evidence, validity or scope of patent and like rights. The patent-holder has stated that licences will be granted under reasonable terms and conditions and communications on this subject should be addressed to

ALCATEL Business Systems

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Introduction

When synchronous digital signals are being transported over a communications link, the receiving end must operate at the same average frequency as the transmitting end to prevent loss of information. This is referred to as link synchronization. When digital signals traverse a network of digital communications links, switching nodes, multiplexers, and transmission interfaces, the task of keeping all the entities operating at the same average frequency is referred to as network synchronization.

The design of a PISN requires specification of the timing sources and receivers for the synchronization network. Proper design requires that timing loops in the synchronization network be avoided. A timing loop occurs when a clock is using as its reference frequency a signal that is itself traceable to the output of that clock. The formation of such a closed timing loop leads to frequency instability and is not permitted. While it is relatively straightforward to ensure against timing loops in the primary synchronization reference network, care should be taken that timing loops do not occur during failure or error conditions when various timing references are rearranged.

When a PISN is not connected to the public digital network, synchronization can be achieved by having all PISN equipment derive timing from a single source.This source should be the highest quality clock available. Alternatively, if timing is derived from more than one class I clock, or public clock traceable source, the network is said to be operating plesiochronously.

If a PISN is connected to the public network at one or more nodes, the private network designer can coordinate with the public network provider to derive class I clock, or public clock traceable timing from the public digital network. More information is available in Annex A.

Annex G

Bibliography