Introduction to Tandem X.25 Capabilities

Packet-Layer Components
What is X.25?
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Packet Type Identifier (PTI) — Indicates the specific packet type being
transmitted. There are five general types of packets, which are described in the
next subsection.
M bit — Indicates that more user data is coming in some following packets.
When the packet is carrying user data, the third octet of the packet header can
contain an M bit. The M bit identifies a sequence of related packets passing
through the network.
User Data
The user data is the data from the higher layers. Packets that convey only lower-layer
control information do not contain user data. The content of the user data field
depends on the type of packet being used.
Call User Data
In call-setup packets (see next subsection for more information on call-setup packets),
an optional call user data (CUD) field may be present. The call user data field can be
used to transmit higher-level protocol information. Do not confuse the call user data
field with the user data field. The user data field appears in data packets; the call user
data field appears only in call setup packets (call request and incoming call). The first
4 octets of the CUD are sometimes used to specify the protocol ID.
Figure 1-12 illustrates the CUD field within the format of a call setup packet.