Enterprise Dial Plans
Dial plans handle two types of calls: those within the enterprise, and those to outside
users using PSTN services. CallManager has many types of configurations
designed to handle these two, yet very extensible, configurations. Before talking
more about dial plans, let’s review the component parts of the dialing architecture,
in the order of influence and control:
Route pattern This is the layout of the number dialed that follows
that country’s numbering system.
Route list The route pattern is interpreted and then sent to a route
group, which is a group of devices that handle the actual call. Group 1
might have the best long distance rates, Group 2 the second best, and so
on, so calls can be sent out the best possible (or least costly) gateway.
Route group This is a collection of gateways, either H.323, PSTN,
Skinny Protocol, or MGCP. Devices within the route group can order
the delivery of calls in a preference list.
Devices Skinny Protocol like the DT24, Catalyst 6000, and analog
trunks; MGCP-based voice-capable gateway, the VG200; H.323-based
gateways, all Cisco IOS routers; H.323-only devices such as the
CallManager and NetMeeting endpoints.
The dial plan is the second most important topic within the VoIP environment
next to a properly installed CallManager.The dial plan should be taken into
very careful consideration and evaluated beside the current PBX solution.We’ll
refer back to Figure 11.1 as we design our dial plan.The three branch offices
along with the head office environment will also host mobile users on the Cisco
IP SoftPhone, conferencing, and features such as call park and call pickup. So, let’s
assign a group of numbers:
Head Office 6000 through 6999
Site A 7000 through 7099
Site B 7100 through 7199
Site C 7200 through 7299
Conference Calls 7990 through 7999
Call Services 7980 through 7989
These numbers provide for sufficient growth for all sites in question, at least
for the foreseeable future. Each of the four sites has their own local calling access,
so calling overhead has been reduced but not quite eliminated.With these numbers,
creating the initial route plans simply point to Site A if the dialed number
has the last four digits of 7000 through 7099, while the others follow the preceding
bullet points.The WAN is the first choice to find the destination, unless
the gatekeeper says that there’s not enough bandwidth to reach the destination.
According to the dial plan, if the IP WAN isn’t available or able to deliver the
traffic, the route group sends the traffic across the PSTN.
Let’s say that a call from the head office to Site A was attempted.The intended
extension was 7005 and was called from phone 6105 by simply dialing “7005.”
The head office phone would then contact CallManager to place the call, which
then contacts the gatekeeper to ensure that the desired amount of bandwidth
exists for the call to reach Site A. Gatekeeper reports back to CallManager that the
connection to Site A is not currently capable of supporting a g.711 64 Kbps call.
If you were at Site A, the area code is 703 and the prefix is 250-xxxx. Since
the gatekeeper told CallManager that the WAN connection can not support the
call, CallManager now looks to the route group for the next possible call routing
mechanism: the PSTN. CallManager uses a function called call transformations,
which handles the call by adding 91703250xxxx, where xxxx lets the 7005 be
inserted into the dialing string.Thereafter, the call is routed out the PSTN circuit.
How does this happen? Two other functions are used by CallManager to
choose the routing of a call: route partitions and the calling search space.Think of
a route partition as an IP subnet.This is a distinct logical block that requires a
router to send the IP packet to one place or the other.The calling search space is
the equivalent to an access control list, which says where this partition can be
routed to or from.
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Designing and Implementing
Yet another useful tool is called a locations definition. A location is just what
its name implies—a region of calling devices that can be controlled and modified
as desired. An example of this comes in controlling lobby and guest phones, from
which long distance calls should not be placed by someone visiting in the lobby
office (guests could call local numbers in the immediate city).These are two distinct
locations, defined as “city-only” and “employees,” where “city-only” could
make just local calls whereas employees could call anywhere.
All of these are issues that arise when defining the dial plan, and should be
designed and carefully thought out before any configuration tasks are completed
in CallManager.