Link Interfaces Reference
30
WAN Link Control-Signal Operation
CTS flow control is not supported.
1)
When a call is made to the bridge/router, RI will be asserted by the modem. The bridge/router responds by driving
DTR and RTS high to signal to the partner’s modem that it is ready to establish communications. The bridge/router
then waits (for the duration of the CD Wait Time, default 60 seconds) for the partner’s modem to respond with
incoming CD and DSR signals driven high. (With a dataset or DSU/CSU, RI is not applicable and the bridge/router
will respond when CD is high).
2)
After DTR goes high, if the incoming CD is not detected within the CD Wait Time then DTR and RTS are lowered for
5 seconds. This clears the link, hangs up the modem and causes the modem to redial in an attempt to establish the link.
(This “DTR toggle” polling is continuous when the link is inactive. When DTR is low, and RI is subsequently asserted,
then the bridge/router immediately drives DTR and RTS high and again waits for the duration of CD Wait Time for
CD to be asserted). NOTE: If the level 2 process times out, DTR toggle will be started.
3)
When incoming CD and DSR signals both go high, then it indicates that the local modem, etc. has established
communications with the remote modem and that communications may begin.
4)
If the link is up, and CD is subsequently interrupted for a period of 10 seconds, then the bridge/router assumes the
signal is lost and will display an Alarm. As stated previously, if the level 2 protocols can continue to send, they will be
allowed to do so. This prevents minor line glitches from taking the link down upon the loss of CD.
5)
If the RI signal is asserted while DTR is already high, the CD Wait Timer is restarted. Once the CD signal is received
by the called modem, the state of the RI signal is unimportant.
6)
In the Conditional Link mode, the secondary link will be inactive by holding DTR and RTS low (the modem, etc., will
normally be configured to drop the call when DTR and RTS are held low). When the secondary link needs to become
active, DTR and RTS will be raised and the modem will make the call.
