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LayGO® DLM Software
LayGO® PXS User's Manual
LayGO® API
LayGO® Device Library
LayGO® Hardware Guide
Link Layer
LayGO® currently supports LAPB, LAPD and Frame Relay link layer protocols.
LAPB
In the ISO layered protocol model, the LAPB link layer provides a full duplex error-free point-to-point connection through a physical line. LAPB detects lost or corrupted frames and requests retransmission.
Advanced Relay's implementation conforms to the Frame Level CCITT X.25 Recommendation, 1988 ("Blue Book") and the European CEPT Net Certification Standard. Configurable LAPB parameters: T1, T2, T3, N1, N2, DTE, DCE, Loop-Back, modulo 8/128, window size. LAPB normally uses HDLC framing. LAPB can also be used on ISDN B-channel and if supported, on ISDN D-channel and may also be used in IEEE 802.2 LLC.
| LAPB Function Mapping | |
|---|---|
| ConnectRequest |
Sends a SABM/P=1 (Set Asynchronous Balanced Mode). If a UA/F=1 is received the Link enters the Data Transfer State. If a DM/F=1 is received the Link returns to the Open-state. If a SABM (SABM collision) is received, a UA response is issued and the Link enters the Data Transfer State. If modulo 128 sequence numbering is used, SABME (Set Asynchronous Balanced Mode Extended) is used instead of SABM in this and all other instances. To ensure that the Link is in a Disconnect State, LAPB can also be configured to start with a DISC/P=1 (Disconnect frame with Poll-Bit) every T1 seconds up to Max Retransmissions times. When and if a DM/F=1 (Disconnected Mode with Final-Bit) or UA/F=1 (Unnumbered Response) is received, a SABM/P=1 is then sent. |
| ConnectAccept | Not used. Since there is no negotiation, the link layer protocol rather than the API issues appropriate response to establish connection. If a SABM/P=1 is received, the Link Layer will respond with a UA/F=1 (see above), and inform the API that the Link Layer is connected. |
| ConnectReject | Not used. Since there is no negotiation, the link layer protocol rather than the API issues appropriate response to reject a connection. A DM/F=1 will be sent as response if the Link can not enter the Data Transfer State. |
| Listen | Sends a DM/F=1 (Unsolicited Response) every T1 seconds until a command is received from the remote, or lgo_ListenCancel() is used to return the service to the Open State. This unsolicited DM is a signal for the remote end to initiate Link Setup when it is ready. While in Listen State, if the remote sends unsolicited DMs of its own, sets up the link by sending a SABM/P=1. |
| DisconnectRequest | Sends a DISC frame every T1 seconds up to Max Retransmissions times. When and if a DM or UA response is received or if there is no response, the link will be shut down and returned to the Open State. |
| DisconnectConfirm | Not used. |
| Write | Sends an I-frame (Information-Frame) containing the data buffer passed. |
| WriteSpecial | Not used. Could be used for XID or V.25 bis synchronous dialing. |
| Xoff | Sends an RNR (Receiver Not Ready) frame. This asks the remote to stop sending I-frames until an RR (Receiver Ready) or REJ (Reject) frame is sent. The remote can still receive any frame (including I-frames), and can still send frames other than I-frames. RNR frames can be sent automatically if the system buffer pool starts to run out of buffers. |
| Xon | Sends an RR to indicate cancellation of the previous RNR. |
| ResetRequest | Sends a SABM/P=0 to reset the link. Unacknowledged I-frames can be lost in any reset whether user initiated or automatically initiated due to Link problems. |
| Reopen | Immediately returns the link to the open state. Link sends a DISC frame, but does not wait for any response. This should be used only as the last resort. |
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LAPD
Like X.25, LAPD allows for multiple virtual connections to be made over a single physical line (or "Major Device"). In LAPD, each virtual circuit is associated with a unique Terminal Endpoint Identifier (TEI).
| LAPD Major Device Function Mapping | |
|---|---|
| Connect Request | Responds with EVENT_CONNECTED. |
| ConnectAccept | Not used. |
| ConnectReject | Not used. |
| Listen | Waits for first frame. |
| DisconnectRequest | Responds with EVENT_DISCONNECTED. |
| DisconnectConfirm | Not used. |
| Write | Not used. |
| WriteSpecial | Not used. |
| Xoff | Not used. |
| Xon | Not used. |
| ResetRequest | Responds with EVENT_RESET_COMPLETE. |
| Reopen | Purges buffer queues and blocks all TEIs. |
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| LAPD TEI Function Mapping | |
|---|---|
| Connect Request | Sends SABME and waits for UA. |
| ConnectAccept | Not used. |
| ConnectReject | Not used. |
| Listen | Not used. |
| DisconnectRequest | Sends DISC and waits for UA. |
| DisconnectConfirm | Not used. |
| Write | Sends an I-frame. |
| WriteSpecial | Not used. |
| Xoff | Sends an RNR. |
| Xon | Sends an RR. |
| ResetRequest | Sends SABME and waits for UA. |
| Reopen | Sends DISC. |
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Frame Relay
As in X.25, Frame Relay 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 beforehand who is on the other end of a PVC. SVCs, on the other hand, are set up as needed like a telephone call.
Advanced Relay's Frame Relay does not currently support SVCs.
Frame Relay is a link layer protocol which is not error free. Three styles of management signaling are supported:
| No Management Signaling - PVC | |
|---|---|
| Connect Request | Responds with EVENT_CONNECTED. |
| ConnectAccept | Not used. |
| ConnectReject | Not used. |
| Listen | Waits for first frame, then responds with EVENT_CONNECTED. |
| DisconnectRequest | Responds with EVENT_DISCONNECTED. |
| DisconnectConfirm | Not used. |
| Write | Sends a frame. |
| WriteSpecial | No special flags recognized. |
| Xoff | Sets the BECN bit on all frames sent. |
| Xon | Clears the BECN bit on all frames sent. |
| ResetRequest | Responds with EVENT_RESET_COMPLETE. |
| Reopen | Purges buffer queues. |
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| LMI - PVC | |
|---|---|
| Connect Request | Responds with EVENT_CONNECTED if the PVC has been reported to be ACTIVE by the major device, otherwise responds with EVENT_DISCONNECTED. |
| ConnectAccept | Not used. |
| ConnectReject | Not used. |
| Listen | Waits for first frame, then responds with EVENT_CONNECTED. |
| DisconnectRequest | Responds with EVENT_DISCONNECTED. |
| DisconnectConfirm | Not used. |
| Write | Sends a frame. |
| WriteSpecial | No special flags recognized. |
| Xoff | Sets the BECN bit on all frames sent. |
| Xon | Clears the BECN bit on all frames sent. |
| ResetRequest | Resets the congestion control parameters to their configured values and responds with EVENT_RESET_COMPLETE. |
| Reopen | Purges buffer queues. |
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| Annex D - PVC | |
|---|---|
| Connect Request | Responds with EVENT_CONNECTED if the PVC has been reported to be ACTIVE by the major device, otherwise responds with EVENT_DISCONNECTED. |
| ConnectAccept | Not used. |
| ConnectReject | Not used. |
| Listen | Waits for first frame, then responds with EVENT_CONNECTED. |
| DisconnectRequest | Responds with EVENT_DISCONNECTED. |
| DisconnectConfirm | Not used. |
| Write | Sends a frame. |
| WriteSpecial | No special flags recognized. |
| Xoff | Sets the BECN bit on all subsequent frames. |
| Xon | Clears the BECN bit on subsequent frames. |
| ResetRequest | Resets the congestion control parameters to their configured values and responds with EVENT_RESET_COMPLETE. |
| Reopen | Purges buffer queues. |
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