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Reliability considerations
Reliability considerations relating to backup trunk groups, use of non-PRI
trunks as backup, multiple PRI links, and DMS-250 switch loss and level
datafill are described in the following paragraphs.
Backup trunk group
To reduce the impact of PRI failures or degradations, backup trunk groups
can be set up in the DMS-250 switch routing tables. The backup groups can
be either PRI or AB-trunk groups.
Non-PRI trunks as backup
The backup trunk group may reside on the DTCI because it can support
non-PRI trunks. Route selection should be set up to select the PRI trunk
first, with rerouting to the non-PRI trunk(s) if the PRI is completely busy or
out of service. The route/trunk databases in the DMS-250 switch have to be
set up to accomplish this. The overflow from PRI could result in calls being
offered in both directions by means of a single two-way trunk group, or by
means of two one-way trunk groups.
The following is an example of a non-PRI backup trunk:
Table OFRT
When a user dials the code to reach the PRI trunk, the call is routed using
route OFRT 99. Calls are first offered through the PRI group
DMS2MSLPRI. If this is 100% busy, out of service, or experiences protocol
failure during call setup, the call is reoffered on the DAL trunk
DMS2MSLDAL. Calls incoming on DMS2MSLDAL are handled the same
way as calls incoming on DMS2MSLPRI.
Multiple PRI links
This does not apply.
DMS-250 switch loss and level datafill
PRI requires the addition of some tuples in table PADDATA. The following
example in Table 11-2 shows the recommended datafill for the DMS-250
switch central office to support the ISDN PRI loss and level plan. The key
port 1 in Table 11-2, PRIP, is the PAD group associated with the PRI on the
DMS-250 switch.
99
S D DMS2MSLPRI
S D DMS2MSLDAL
Digital Switching Systems DMS-250 ISDN Reference Manual MWC14 (CSP14)
Engineering recommendations 11-3
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