Frame Relay Link Layer
Frame Relay is a link layer protocol which is not error
free. Three styles of management signaling are supported:
None, LMI, and Annex D.
No Management Signaling - Major Device.
| Function |
Mapping |
|---|
| Connect Request |
Responds with lgo_EVENT_CONNECTED. |
| Connect Accept |
Not used. |
| Connect Reject |
Not used. |
| Listen |
Waits for first I frame. |
| Disconnect Request |
Responds with lgo_EVENT_DISCONNECTED. |
| Disconnect Confirm |
Not used. |
| Write |
Not used. |
| Write Special |
Not used. |
| Xoff |
Not used. |
| Xon |
Not used. |
| Reset Request |
Responds with lgo_EVENT_RESET_COMPLETE. |
| Reopen |
Purges buffer queues and blocks all PVCs. |
LMI - Major Device.
| Function |
Mapping |
|---|
| Connect Request |
Sends STATUS ENQUIRY. |
| Connect Accept |
Not used. |
| Connect Reject |
Not used. |
| Listen |
Waits for first frame. |
| Disconnect Request |
Responds with lgo_EVENT_DISCONNECTED. |
| Disconnect Confirm |
Not used. |
| Write |
Not used. |
| Write Special |
Not used. |
| Xoff |
Not used. |
| Xon |
Not used. |
| Reset Request |
Sends a STATUS ENQUIRY. |
| Reopen |
Purges buffer queues and blocks all PVCs. |
Annex D - Major Device.
| Function |
Mapping |
|---|
| Connect Request |
Sends STATUS ENQUIRY. |
| Connect Accept |
Not used. |
| Connect Reject |
Not used. |
| Listen |
Waits for first frame. |
| Disconnect Request |
Responds with lgo_EVENT_DISCONNECTED. |
| Disconnect Confirm |
Not used. |
| Write |
Not used. |
| Write Special |
Not used. |
| Xoff |
Not used. |
| Xon |
Not used. |
| Reset Request |
Sends a STATUS ENQUIRY. |
| Reopen |
Purges buffer queues and blocks all PVCs. |
No Management Signaling - PVC.
| Function |
Mapping |
|---|
| Connect Request |
Responds with
lgo_EVENT_CONNECTED. |
| Connect Accept |
Not used. |
| Connect Reject |
Not used. |
| Listen |
Waits for first frame, then responds with lgo_EVENT_CONNECTED. |
| Disconnect Request |
Responds with lgo_EVENT_DISCONNECTED. |
| Disconnect Confirm |
Not used. |
| Write |
Sends a frame. |
| Write Special |
No special flags recognized. |
| Xoff |
Sets the BECN bit on all frames sent. |
| Xon |
Clears the BECN bit on all frames sent. |
| Reset Request |
Responds with lgo_EVENT_RESET_COMPLETE. |
| Reopen |
Purges buffer queues. |
LMI - PVC
| Function |
Mapping |
|---|
| Connect Request |
Responds with
lgo_EVENT_CONNECTED if the PVC has been reported to be ACTIVE
by the major device, otherwise responds with
lgo_EVENT_DISCONNECTED. |
| Connect Accept |
Not used. |
| Connect Reject |
Not used. |
| Listen |
Waits for first frame, then responds with lgo_EVENT_CONNECTED. |
| Disconnect Request |
Responds with lgo_EVENT_DISCONNECTED. |
| Disconnect Confirm |
Not used. |
| Write |
Sends a frame. |
| Write Special |
No special flags recognized. |
| Xoff |
Sets the BECN bit on all frames sent. |
| Xon |
Clears the BECN bit on all frames sent. |
| Reset Request |
Resets the congestion control parameters to their configured values and
responds with lgo_EVENT_RESET_COMPLETE. |
| Reopen |
Purges buffer queues. |
Annex D - PVC
| Function |
Mapping |
|---|
| Connect Request |
Responds with lgo_EVENT_CONNECTED if the PVC has been reported to be ACTIVE
by the major device, otherwise responds with lgo_EVENT_DISCONNECTED. |
| Connect Accept |
Not used. |
| Connect Reject |
Not used. |
| Listen |
Waits for first frame, then responds with lgo_EVENT_CONNECTED. |
| Disconnect Request |
Responds with lgo_EVENT_DISCONNECTED. |
| Disconnect Confirm |
Not used. |
| Write |
Sends a frame. |
| Write Special |
No special flags recognized. |
| Xoff |
Sets the BECN bit on all subsequent frames. |
| Xon |
Clears the BECN bit on subsequent frames. |
| Reset Request |
Resets the congestion control parameters to their configured values and
responds with lgo_EVENT_RESET_COMPLETE. |
| Reopen |
Purges buffer queues. |
Major Device
Parameters beginning with "Is DCE?" are
effective only when a management signaling protocol (LMI
or Annex D) is used. Otherwise, there is no link
integrity verification.
The basic link integrity verification algorithm used by
both LMI and Annex D is defined in ANSI T1.617-1991 Annex
D. Every T1 seconds, the DCE sends a keep
alive STATUS ENQUIRY message to the DCE. The DCE
responds with STATUS message. Both messages contain
sequence numbers. An error has occurred if any message is
received out of sequence, if the DCE fails to receive a
keep alive message within T2 seconds of the last one or
the DTE fails to receive a response to its message within
T1 seconds. If N2 out of N3 link verification events have
resulted in errors, a link integrity alarm is considered
to exist. The alarm is cancelled when N3 consecutive
events are error free.
Both LMI and Annex D provide for the exchange of PVC
status information during the keep alive polling. Every
N1th STATUS ENQUIRY, the DTE may request a full
status response containing the availability status
of each PVC on the link. LayGO uses this
information to control the state of the PVCs. Annex D
also provides for bidirectional polling in which both DTE
and DCE poll and are polled.
| Parameter |
Explanation |
|---|
| Number of PVCs |
The maximum number of PVCs open at one time. |
| Frame Size Maximum |
The maximum frame size in bytes |
| Write Maximum |
The maximum number of frames to enqueue to the physical layer. |
| Management Signaling |
Management signaling protocol to use:
0 None
1 LMI
2 Annex D
|
| Is DCE? |
TRUE if the side should be the DCE for a back-to-back connection (LMI and Annex D only). |
| Is bidirectional? |
TRUE if signaling is bidirectional (Annex D only). |
| Full Status Polling Counter N1 |
Link intergity verification parameter N1 |
| Error Threshold N2 |
Link intergity verification parameter N2 |
| Monitored Events Count N3 |
Link intergity verification parameter N3 |
| T1 Keep Alive timeout |
Link intergity verification timeout T1 in milliseconds. |
| T2 Polling Verification timeout |
Link intergity verification timeout T2 in milliseconds. Must be greater
than T1. |
PVC
The LayGO's Frame Relay PVCs use a variation of
the windowing algorithm described in ANSI T1.618-1991
Annex A to control the load offered to the Frame Relay
network. The algorithm is based on explicit congestion
notification. Explicit notification is considered to be
the receipt of any frame with the BECN bit set or the
receipt of more frames with the FECN bit set than cleared
during a period of time. The algorithm is based on the
assumption that the average frame size sent is relatively
constant. The value of the parameters should be based on
the committed information rate (CIR).
A current window size W is defined and set to an initial
value. During the period defined by the congestion
timeout, no more than W frames are transmitted to the
network. If, during that time, no explicit notification
of network congestion is received, W is increased by a
given factor. For example, if the factor is 3, the window
size is increased by one third. If W may be increased, it
is always increased by at least 1. On the other hand, if
explicit notification of network congestion is received,
W is reduced by a given factor as in the example above.
Again, if W must be decreased, it is always decreased by
at least 1. In no case is W increased above the given
maximum or decreased below the given minimum. The minimum
must be at least 1.
| Function |
Explanation |
|---|
| Minimum window size |
Minimum congestion window size. Must be greater than 0. |
| Maximum window size |
Maximum congestion window size. Must be greater than or equal to the minimum
size. |
| Initial window size |
Initial size of the congestion window. Must be in the range defined by window
minimum and maximum. |
| Factor to increase window size |
Factor by which to increase the window size if no explicit congestion
notification is received. Must be greater than 0. |
| Factor to decrease window size |
Factor by which to decrease the window size if
explicit congestion notification is received. Must be greater than 0. |
| Congestion timeout |
Time in milliseconds to adjust the window size based on explicit congestion
notification. |
Major Device
| Function |
Explanation |
|---|
| None |
|
PVC
| Function |
Explanation |
|---|
| Set DE bit |
Sets the DE (Discard Eligibility) bit in all subsequent frames. (This is the
default for Frame Relay PVCs.) |
| Clear DE bit |
Clears the DE (Discard Eligibility) bit in all subsequent frames. |