User`s guide

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When this optional feature is included, the unit may be configured to pass X.25 calls or data
received in a TCP connection to another protocol or interface.
In addition, it is possible to specify a backup protocol or interface so that if an outgoing call
on one interface fails, then the backup interface is automatically tried. LAPB can be used to
switch to either ISDN or X.25 over serial depending on the configuration of the LAPB
instance chosen.
The logic used in the switching software is outlined in the flowchart below. The following
notes provide a more in-depth explanation of the actions taken in each of the numbered
boxes.
The unit will first look up the Called NUA/NUI in the Configuration - Network > Protocol
Switch > 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 unit 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 then the default IP address (IP Stream or XOT
Remote IP Address) is used.
The unit then determines from the source interface of the incoming call which interface type
it 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
parameter was set to LAPD, then the outgoing interface would LAPD.
If the outgoing interface is LAPD the unit changes the Calling NUA field of the incoming call
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 unit then searches the Configuration - Network > Protocol Switch > NUA Mappings table
to see if there are any matches for the Called or Calling NUA values on the specified
interface. In cases where there Interface Description Off/None Data will not be switched
from / backed-up from this protocol is a match, the NUA In value is substituted by the NUA
out value, i.e. the mapping is applied individually to both the Calling NUA and Called NUA
for the packet.
The unit then checks the leading characters of the Calling NUA to see if there is a match
with the Call Prefix parameter. If there is a match then 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 unit has determined that the outgoing interface is not LAPD, it checks if
the outgoing interface is LAPB. If it is, it then checks to see if the Called NUA field in the call
packet matches the LAPB 0 NUA parameter and if it does, 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 and if there is a match, 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
parameters then 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 unit then sets the Called NUA to the TE NUA
(LAPB 0) value. If the call is being switched over LAPB 1 the unit then sets the Called NUA
to the TE NUA (LAPB 1) value.
The Configuration – Network > Protocol Switch menu has the following sub-menu
options:
• CUD Mappings