Configure ISDN interfaces
Digi TransPort User Guide 245
Answering ISDN calls
Digi routers can answer incoming B-channel ISDN calls with three main protocols: rate adaption
protocols, LAPB, and PPP. Usually several instances of these protocols exist. This section explains
how answering priorities work for the different protocols.
Protocol entities
The following protocol instances are capable of answering an incoming ISDN call:
Adapt
Adapt instances provide rate adaptation protocols such as V.120 or V.110.
LAPB
LAPB instances allow the router to answer incoming X.25 calls over ISDN. These instances can
optionally connect the caller to a synchronous serial port, an asynchronous serial port bound to a
PAD, or switch the call to another interface.
PPP
IP data tunneled over PPP instances allows remote access to the router’s IP-based management
features and also facilitates onward IP routing through any of the router’s IP enabled interfaces.
The router automatically answers an incoming ISDN call if any of the following statements are
true, subject to the entity’s MSN, Calling Number and Sub-address parameters being set to
their default values:
• An Adapt instance is bound to an asynchronous serial port (ASY) and the answer ring count
(S0) for that serial port is set to 1
• A LAPB instance has its answering parameter set to On
• A PPP instance has its answering parameter set to On
If more than one of these protocols are configured to auto answer then the priority is as follows:
Adapt instances (normally V.120) will take priority over LAPB, which will take priority over PPP. If
an Adapt instance is bound to an asynchronous serial port (ASY port) but the answer ring count
(ATS0) is not set to 1 for that same serial port then Adapt entity will not answer automatically. If
any other protocol entities (such as LAPB, PPP or another Adapt instance) are configured to
answer then one of these protocol entities will answer the call. If no other protocol entities are
configured to answer then a repeating RING message will be sent out of the serial port and the
RS232 ring indicator control will be activated. If a terminal attached to the serial port sends ATA
followed by carriage return then the ISDN call will be answered by the Adapt entity and any
incoming data will be channeled out of the serial port and vice-versa.
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