Introduction to Tandem X.25 Capabilities

Comparing X.25 with other Networking Standards
What is X.25?
065307 Tandem Computers Incorporated 1–5
Figure 1-3. Comparison of X.25, OSI, and SNA
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Transaction
Services
Presentation
Services
Data Flow
Control
Transmission
Control
Path Control
Datalink Control
Physical Control
Application
Presentation
Session
Transport
Network
Datalink
PhysicalPhysical
Link
(Frame)
Packet
(Network)
X.25
OSI SNA
Layers
7
6
5
4
3
2
1
Using X.25 for Direct
Connections
Although X.25 was developed to facilitate communications to remote locations via
PSDN, it is now used also for direct connections that do not involve a PSDN. X.25 is
used for direct connections, because it provides a common platform for the
lower-layer communications functions. Thus, X.25 allows a terminal to communicate
with an OSI-type application; or allows an OSI-type application on one system to
communicate with an OSI-type application on another system. Some benefits of using
X.25 for direct connections are:
A standard lower-layer communications interface for OSI-type applications
A standard communications interface for connecting to a LAN
A single bit-synchronous line rather than multiple asynchronous lines to connect
multiple local terminals to the host system