Introduction to Tandem X.25 Capabilities

Frame Types
What is X.25?
065307 Tandem Computers Incorporated 1–31
Frame Types The X.25 standard defines three basic frame types, which perform different
communications functions.
Information frames (I-frames)
Supervisory frames (S-frames)
Unnumbered frames (U-frames)
I-Frames I-frames transport information from the originating or source computer system to the
destination system. The information (the packet received from layer 3) is carried
within the I-field of an I-frame.
S-Frames Supervisory frames are used for flow control, recovery, and acknowledgement of
I-frames. There are three types of S-frames:
RR — Receiver-Ready (RR) frames indicate that the station is ready and able to
receive additional communications, to acknowledge frames received, and/or to
demand status. An RR frame is transmitted when the station has no data to send.
In Figure 1-20, the RR frames are transmitted to acknowledge frames received at
the end of the data transfer.
RNR — Receiver-Not-Ready (RNR) frames indicate that the station is busy (all
buffers are full). RNR frames include an acknowledgement of frames received
thus far.
REJ — Reject (REJ) frames request retransmission and/or demand status (see
Figure 1-21). REJ frames are sent because the receiving station detects an I-frame
lost during transmission (the frame’s sequence numbers indicated that a prior
frame was not received).