Appendix 1. CCITT recommendations (numbers and brief descriptions)

(Note: ter and bis are terms with Latin roots meaning 3 and 2, respectively. Recommendations with these terms in the number have 3 or 2 data rates, the system normally starting up in the higher rate but falling back to the lower speed under noisy conditions).

X 1 International user classes of service- in public data networks.
X 2 International user facilities in public data networks.
X 3 Packet assembly /disassembly facility in a public data network.
X 4 General structure of signals of International Alphabet No. 5 code for data transmission over public data networks.
X 20 Interface between data terminal equipment and data circuit terminating equipment for start-stop ...

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