“Asynchronous Protocol Specification” is the title phrase of ITU-T Recommendation X.445 (04/95), not a universal name for every asynchronous protocol. It describes providing OSI connection-mode network service over telephone connections for asynchronous communication.
What ITU-T X.445 specifies
Published in April 1995, X.445 sets rules for asynchronous OSI communications over the telephone network. Its stated aim is to support reliable communication by optimizing protocols at the data-link and network layers. In plain language, it addresses how OSI connection-mode network service can be provided when the connection runs through telephone-network facilities.
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The recommendation identifies three environments:
- A direct connection over the telephone network.
- A telephone connection to a public switched data network (PSDN) through a packet assembler/disassembler (PAD).
- A telephone connection to a PSDN using packet mode.
These are the settings named in X.445’s scope; the title and scope alone do not describe the detailed procedures used in each one.
What “asynchronous” means here
In X.445, “asynchronous” belongs to the context of OSI communication over telephone connections. It should not be treated as a definition that applies to every protocol or technology using the same word.
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For example, IETF RFC 4130 uses “asynchronous” for a receipt returned in a different communication session from the one used for the original message. That is a distinct protocol context, not an explanation of how X.445 works.
Status and limits of the available description
The ITU-T record lists X.445 (04/95) as in force; that status was checked on October 4, 2026. A record’s in-force label does not by itself establish how widely the recommendation is deployed today.
The scope-level description identifies the service, purpose, and connection environments, but does not establish X.445’s encodings, negotiation, state machines, procedures, or conformance requirements. Those implementation details require consulting the relevant clauses of the full recommendation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to read the word “specification”
In standards references, a specification identifies documents that define a protocol; that is a different question from a protocol’s status or dependencies. RFC 1011 makes this distinction in a 1987 historical standards document. It is useful here only as terminology context, not as a current protocol registry.
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