Digi TransPort WR31 User Manual page 598

Configure legacy protocols
The router first looks up the Called NUA/NUI in the Configuration > Network > Protocol Switch
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> NUA to Interface Mappings mapping table to determine the IP address to use in the event
that the call ends up being switched to a TCP or XOT interface. If a match is found on the Called
NUA/NUI the router assigns the matching IP address from the table to the call. If IP address
mapping table does not contain an entry for the Called NUA/NUI and the call is eventually
switched to a TCP or XOT channel, the router uses the default IP address (IP Stream or XOT
Remote IP Address).
The router then determines from the source interface of the incoming call which interface type it
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should be switched to, from the Switch from parameters on the Protocol Switch page.
For example, if the call arrived via a LAPB 0 interface and the Switch from LAPB 0 to
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parameter was set to LAPD, then the outgoing interface would LAPD.
If the outgoing interface is LAPD the router changes the Calling NUA field of the incoming call
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to the D-Channel NUA value (as defined on the Protocol Switch page). If the outgoing
interface is NOT LAPD processing proceeds as at step 6.
The router then searches the Configuration > Network > Protocol Switch > NUA Mappings
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table to see if there are any matches for the Called or Calling NUA values on the specified
interface. When the Interface Description is Off/None data is not switched from or backed-up
from this protocol is a match, the NUA In value is substituted by the NUA Out value, as the
mapping is applied individually to both the Calling NUA and Called NUA for the packet.
The router checks the leading characters of the Calling NUA to see if there is a match with the
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Call Prefix parameter. If there is a match, the prefix digits are removed before the outgoing X.25
call is made. Otherwise, the call is made anyway, and the switching process is complete for this
call.
If after step 3, the router has determined that the outgoing interface is not LAPD, it checks if the
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outgoing interface is LAPB. If it is, it checks to see if the Called NUA field in the call packet
matches the LAPB 0 NUA parameter. If it does, it selects LAPB 0 as the outgoing interface. If the
Called NUA field does not match LAPB 0 NUA, it checks for a match with LAPB 1 NUA. If there is
a match, it sets the outgoing interface to LAPB 1.
If the Called NUA field in the calling packet matches neither the LAPB 0 NUA or LAPB 1 NUA
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parameters, the outgoing interface is set to the interface specified by the relevant Switch from
parameter.
If the call is being switched over LAPB 0. the router sets the Called NUA to the TE NUA (LAPB 0)
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value. If the call is being switched over LAPB 1, the router sets the Called NUA to the TE NUA
(LAPB 1) value.
Digi TransPort WR Routers User Guide
Configure Protocol Switch software
598