Nortel ICS Install Manual page 122

122 / Dialing plans
If call-by-call services were not used, each Norstar system
might have to be equipped with the following trunks:
• 12 DID lines needed to handle peak incoming call traffic.
• eight Tie (E&M) lines needed to handle inter-office calls.
• eight lines needed to handle outgoing public calls
The total required is 28 lines. If the Norstar systems were using
T1 trunks, then two T1 spans would be required at each office.
Note that the total of 28 lines represents the worst case value
for line usage. In reality, the total number of lines in use at any
one time will generally be less than 28. For example, during
periods of peak incoming call traffic, the demand for outgoing
lines will be low.
With PRI call-by-call services, it is not necessary to configure
a fixed allocation of trunks. Each of the 23 lines on the PRI can
be used for DID, Tie, or outgoing public calls. This
consolidation means that it may be possible for each office to
use a single PRI span, rather than two T1 spans.
With PRI call-by-call services, the only limitation is that there
are no more than 23 calls in progress at any one time.
The dialing plan at each Norstar site is configured to determine
the call type based on the digits dialed by the user. If a user in
Toronto wishes to dial a colleague in New York, they dial the
four-digit private DN (such as 6221). The dialing plan
recognizes this as a private network DN, and routes the call
using Tie service with a private numbering plan.
Incoming Tie calls are routed to telephones based on the digits
received by the network, which in this case will be the four-
digit private DN.
If a user in either location wishes to dial an external number,
they dial 9, followed by the number (such as 9-555-1212). The
Modular ICS 7.1 Installer Guide N0130943 02