Introduction to Tandem X.25 Capabilities

Data-Link-Layer Components
What is X.25?
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Frame Structure Data is exchanged between the two stations in frames. The X.25 frame structure is
based on the HDLC frame format (Figure 1-18).
Figure 1-18. X25 Frame Format
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AddressFlag FlagFCSControl InformationFrame Level
Packet Level
Packet
Frames contain the following fields:
Flag The start of a frame is always indicated by a flag, which is a
sequence of 8 bits (01111110), six ones preceded and followed by
a zero.
Address Field The address field follows the opening flag and contains the data
link address information. Only two addresses are used on the
data link layer, 01 and 03, as shown in Figure 1-17 and Figure
1-19.
Control Field The control field has three basic formats, indicating a different
purpose for transmitting a frame. The control field identifies the
frames as either an I-frame (Information frame), S-frame
(Supervisory frame), or U-frame (Unnumbered frame).