Introduction to Tandem X.25 Capabilities

Physical-Layer Components
What is X.25?
1–36 065307 Tandem Computers Incorporated
RS-232-C / RS-232-D
Tandem RS-232-C is limited to data signalling rates of 20 kbps and a cable distance of
50 feet between the DTE and the DCE. (RS-232-D includes the specification of the
DB-25 connector used with RS-232-C.) RS-232-C is similar to CCITT’s V.24/V.28
interface and electrical characteristic recommendations used in Europe.
RS-449-1/ RS-422 and RS-423
RS-449-1 was intended to replace RS-232-C; however, it was not commonly used until
recently. RS-449 provides higher data signalling rates (up to 2 Mbps), and it permits
longer cable distances between the DTE and the DCE. Electrical standards RS-422
(equivalent to V.11) and RS-423 (equivalent to V.10) are used with the RS-449 interface.
CCITT V.36 recommendation is the equivalent to RS-449.
V.35
The CCITT Recommendation for V.35 is a standard for a wide-band 48 kbps modem.
V.35 is being replaced by a new V.36 standard, which is an equivalent to EIA RS-449.
X.21 This recommendation was designed for digital circuit-switched public networks.
Currently, this type of interface is available only for leased lines within the United
States, but it is used by some digital circuit-switched networks in other countries. X.21
makes it possible to achieve a faster call-connect time than X.21 bis, because an X.21
interface does not have to translate rotary or pulse tones into digital tones.