Introduction to Tandem X.25 Capabilities

Physical-Layer Components
What is X.25?
065307 Tandem Computers Incorporated 1–35
FRMR-Frame Diagnostic Field
The diagnostic field of the FRMR indicates an error condition such as:
Receipt of a control field that is invalid.
Receipt of an information field that exceeds the maximum established length.
Receipt of an invalid N(R) number from the remote station. An invalid N(R)
number indicates an I-frame which has already been transmitted and
acknowledge previously, or an I-frame which has not been transmitted and is not
the next sequential I-frame pending transmission.
The diagnostic field of the FRMR notifies the other side of its current status regarding
the transmission and receipt of I-frames with the following variables:
V(S) — Indicates the sequence number of the next in-sequence I frame to be
transmitted by the DTE or DCE reporting the rejection. It also contains:
the control field of the frame being rejected
the reason for rejection
an indication showing whether the rejected frame was a command or a
response
V(R) — Indicates the sequence number of the next in-sequence I frame to be
received by the DTE or DCE reporting the rejection.
Physical-Layer
Components
The Physical Layer (layer 1) includes the standards for the electrical interfaces that can
be used to connect DTEs to PSDNs. Most users need to know only the names of the
electrical interfaces, so they can confirm that their hardware is compatible with the
standard. This subsection summarizes the characteristics of the standards used for
electrical interfaces compatible with X.25 connections. If you need to make your own
cables, you will need to ask your Tandem analyst for more detailed information about
pinouts.
X.21 bis The CCITT Recommendation for X.21 bis is the standard followed in the United States
when a terminal is connected to the telephone network via a dial-up connection.
According to CCITT Recommendation X.25, the preferred physical layer interface is
actually X.21, which provides a recommendation for digital circuit-switched public
networks. However, the availability of data communications equipment with X.21
capabilities is limited, especially in the United States. Therefore, X.21 bis is accepted as
an alternative to X.21.
The X.21 bis standard is functionally equivalent to the electrical interfaces
EIA RS-232-C and CCITT V.24, both of which assign a separate function to each
interchange circuit of the connection. The X21 bis standard also allows for the
provisions of CCITT V.35 and CCITT V.36.