Introduction to Tandem X.25 Capabilities

Data-Link-Layer Components
What is X.25?
065307 Tandem Computers Incorporated 1–27
For S-frames and U-frames, the control field contains the
command or response. Other components of the control field
are:
N(S) — Number Sent Variable or Send Sequence Number is
only included in the control field of I-frames. It identifies the
sequence of the current I-frame, so that the other end can
check to make sure that it has received all preceding
I-frames.
N(R) — Number Received Variable or Receive Sequence
Number is included in both I-frames and S-frames. It
indicates the expected sequence number of the next I-frame
to be received. N(R) indicates that the DTE or DCE
transmitting the N(R) has correctly received all I-frames
numbered up to and including N(R) -1.
P/F Bit — The Poll/Final Bit is used to solicit status, to
respond to a frame with the poll bit set, or to handle timer
recovery conditions. In command frames, the P/F bit is
referred to as the poll bit. In response frames, the P/F bit is
referred to as the final bit.
Information Field The information field contains the actual data or information to
be communicated (it contains the packet).
The size of the information field (packet size) can vary up to a
maximum that is agreed upon by the source and destination
computer systems. The information field is generally included
only in I-frames. However, a diagnostic information field is also
included in FRMR frames.
FCS The FCS (Frame Check Sequence) is used to check for
transmission errors. A standard 16-bit FCS is generally used, but
a 32-bit FCS can be used for particularly stringent error control.