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LayGO® DLM Software
LayGO® PXS User's Manual
LayGO® API
LayGO® Device Library
LayGO® Hardware Guide
Packet Layer
X.25
In the ISO layered protocol model, a network layer allows for multiple virtual connections to be made on a single physical line. Virtual connections are either Permanent Virtual Circuits (PVCs) or Switched Virtual Circuits (SVCs).
A PVC is set up at subscription time and is agreed upon by both sides. You know before hand who is on the other end of a PVC. SVCs, on the other hand, are set up as needed like a telephone call. Anyone could call you.
Function mappings are different for each type of X.25 device:
| Major Device Function Mapping | |
|---|---|
| Connect Request | Sends a Restart Request. When this is confirmed, the major service enters data transfer xon state. |
| ConnectAccept | Not used. |
| ConnectReject | Not used. |
| Listen | Awaits receipt of a Restart Indication. |
| DisconnectRequest | Sends a restart request. When this is confirmed, the major service enters open state. |
| DisconnectConfirm | Not used. |
| Write | Not used. |
| WriteSpecial | Not used. |
| Xoff | Not used. |
| Xon | Not used. |
| ResetRequest | Sends a restart request. When this is confirmed, the major service enters data transfer xon state. |
| Reopen | Reopen is not currently supported for the major device. |
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| PVC Function Mapping | |
|---|---|
| Connect Request | Sends a Reset Request. When this is confirmed, the PVC enters data transfer xon state. |
| ConnectAccept | Not used. |
| ConnectReject | Not used. |
| Listen | Awaits a Data packet or Reset Indication. |
| DisconnectRequest | Sends a Reset Request with Cause. When this is confirmed, the PVC service enters open state. |
| DisconnectConfirm | Not used. |
| Write | Sends a data packet |
| WriteSpecial | Sends a data packet, with the D (delivery), M (more), and Q (qualifier) bits set as requested. |
| Xoff | Sends an RNR packet. |
| Xon | Sends an RR packet. |
| ResetRequest | Sends a Reset Request with Cause. When this is confirmed, the PVC service enters data transfer xon state. |
| Reopen | Immediately returns the PVC to the open state. Sends a Reset Request packet, but does not wait for a response |
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| SVC Function Mapping | |
|---|---|
| Connect Request | Sends a Call Request. When this is confirmed with a Call Connected the SVC service enters data transfer xon state. |
| ConnectAccept | Sends a Call Accept Packet and enters data transfer xon state. |
| ConnectReject | Sends a Clear Indication and enters the open state. |
| Listen | Awaits Call Indication. |
| DisconnectRequest | Sends a Clear Indication. When this is confirmed, the SVC service enters open state. |
| DisconnectConfirm | Sends a Clear Confirmation and enters the open state. |
| Write | Sends a data packet. |
| WriteSpecial | Sends a data packet with the D (delivery), M (more), and Q (qualifier) bits set as requested. |
| Xoff | Sends an RNR packet. |
| Xon | Sends an RR packet. |
| ResetRequest | Sends a Reset Request. When this is confirmed, the SVC service enters data transfer xon state. |
| Reopen | Immediately returns the SVC to the open state. Sends a Clear Indication packet, but does not wait for a response. |
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Annex G
Annex G refers to a procedure for encapsulating X.25 traffic over Frame Relay. The procedure is described in Annex G of ANSI Standard T1.617a-1994, Integrated Services Digital Network (ISDN) - Signaling Specification for Frame Relay Bearer Service for Digital Subscriber Signaling System Number 1 (DSS1).
Using Annex G, a stream of LAPB/X.25 packets is encapsulated and transmitted transparently over Frame Relay.
LayGO® fully supports the Annex G specification.