User`s guide

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ANSWERING ISDN CALLS
Digi routers are capable of answering incoming B-channel ISDN calls with 3 main protocols.
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 unit to answer incoming X.25 calls over ISDN. They 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 tunnelled over PPP instances allow remote access to the unit’s IP-based
management features and also facilitate onward IP routing through any of the unit’s IP
enabled interfaces.
The unit will automatically answer an incoming ISDN call if any of the following statements
are true (subject to the entity 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 (e.g. 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 channelled out of the serial port
and vice-versa.
Multiple Subscriber Numbers
An MSN (multiple subscriber number) is an alternative number provided by the telephone
service provider which when dialled will also route through to your ISDN line. It is possible
to purchase several MSNs for an ISDN line. This means that in effect one ISDN line can
have several ISDN numbers.
Every entity in the router which is capable of answering an ISDN call (Adapt, LABP and PPP)
has an MSN parameter.