The Essential service reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Essential service

In telecommunication, an essential service (critical service) is a network-provided service feature in which a priority dial tone is furnished.

• Essential service is typically provided to fewer than 10 % of network users.
• Essential service is recommended for use in conjunction with NS/EP telecommunications services.

Source: from Federal Standard 1037C

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