Migrating X.25 over PSI to XOL

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X.25 over PSI
In accordance with CCITT X.25 Recommendations, three layers of protocol (Layer 1, Layer 2 and
Layer 3) are functional when X.25 is used over PSI cards. Figure 2 shows the three protocol layers in
X.25 over PSI.
Figure 2. Three layers of protocol in X.25 over PSI
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Logical Link Control (LLC) Overview
The LLC sub-layer is the upper sub-layer in the data link layer and is common to the various medium
access methods. The International Standard IEEE Std. 802.2 describes the functions, features,
protocol, and services of the Logical Link Control (LLC) sub-layer in the ISO/IEC 8802 LAN Protocol
Standard.
LLC1 (Type 1 operation): LLC1 provides a connectionless-mode service across a data link with
minimum protocol complexity. This operation is useful when higher layers, instead of the data link
layer, provide essential recovery and sequencing services. This operation is also useful in applications
where guaranteed delivery of every data unit in the data link layer is not critical.
LLC2 (Type 2 operation): LLC2 provides a connection-mode service across a data link, comparable to
the data link control procedures provided in HDLC. This operation supports the sequenced delivery of
data units in the data link layer and includes a comprehensive set of data link layer error recovery
techniques.
LLC3 (Type 3 operation): LLC3 provides an acknowledged-connectionless-mode data unit exchange
service across a data link. This operation enables a station to concurrently send data and request
acknowledgment for the receipt of data. Although the exchange service is connectionless, this
operation supports guaranteed and sequenced delivery of data sent by the initiating station.