88 CHAPTER 2: DIAL PLAN
Overview of
Virtual Tie Lines
A Virtual Tie Line (VTL) provides a way to make calls between NBX system
sites that are separated geographically but tied together by a Wide Area
Network (WAN) connection. An NBX 100 system can support up to 8
simultaneous VTL connections; a SuperStack 3 NBX system can support
up to 48. VTLs are a licensed feature of the NBX systems.
On any NBX system, any licensed VTL connection can be used either for
an incoming VTL call from any site or for an outgoing VTL call to any site.
A VTL connection is not dedicated in the same way as a physical tie line,
which always connects the same pair of sites. In the example in Figure 28
,
the VTLs on the Chicago NBX system can be used for any combination of
incoming and outgoing VTL calls to either Atlanta or Dallas.
The NBX system can reroute VTL calls that fail to reach their destination
on the first attempt. For details, see “Call Rerouting for Virtual Tie Lines”
later in this chapter.
■ You must implement either IP On-the-Fly or Standard IP on an NBX
system in order to use VTL connections to other NBX systems.
■ VTL connections cannot be configured to run through firewalls or NAT
routers.
■ When you calculate the number of devices on an NBX system, do not
include the number of VTLs.
There are two implementation techniques you can use: unique extension
ranges or site codes, as described next.
VTL Connections
Using Unique
Extension Ranges
If you can restrict the extension ranges on each of the NBX systems so
that they do not overlap, you can configure the dial plans to route calls
based only on the extension that is being dialed. The caller does not have
to dial any digits to specify the site.
Assess your growth plans for each site to verify that, as you add
telephones, you do not exceed the extension ranges that you have
defined.
Figure 28
depicts a configuration that uses unique extension ranges
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