Introduction to Tandem X.25 Capabilities

Packet-Layer Components
What is X.25?
1–12 065307 Tandem Computers Incorporated
Figure 1-8. X.25 Layers
008
X.25
Physical
X.25
Link
(Frame)
X.25 Packet
(Network)
DTE
User Defined
Processes
Layers 4 - 7
DCE
Peer Protocols
Packets
Frames
Electric Signals
Packet-Layer
Components
The Packet Layer (layer 3), as its name implies, is really the essence of communicating
in an X.25 packet-switching environment. Data from a user system is packaged into
packets for transmission through a packet-switching network to a specified
destination.
In a standard packet-switching network, a packet includes user data from only one
user. Packets from one source travel as a unit to a single destination.
In a multiplexed packet-switching network, a packet includes data from different
users. Packets include different data components, each of which can have a
different source and destination from other data components in the packet.
At the source or originating DTE, the Packet Layer receives application data from the
upper layers. This data is formatted into packets, which are sent to the data link layer.
The Data Link Layer surrounds the packet with a frame-layer header and closing
format, and transmits the frame across the data link. Only one packet can be
transmitted in a single frame (see Figure 1-9).
At the destination location, the packaging process is reversed. The Data Link Layer
strips the frame-layer header and closing format, and passes the packet up to the
Packet Layer.