Wide Area Network Considerations
In centralized network designs, all CallManagers reside on the head office network
(as do associated solutions like Unity messaging) in a central location such
as the main head office backbone, not at field locations. Figure 11.1 illustrates a
typical centralized design. (Figure 11.1 will be used as the main reference point
in this section, and will be adjusted to reflect the amended designs explored
throughout the chapter.)
This is a balanced design, meaning that the capacity of the WAN circuits to
the branch offices equals the maximum capacity of the head office.This means
that if you add up the speed of the WAN links to all branch offices, the total does
not exceed the head office WAN connection to the frame cloud. Referring to
Figure 11.1, the three branch offices each use a 512 Kbps connection, which
totals 1536 Kbps. Since this is equal to the head office WAN connection speed,
the head office WAN connection cannot be over-subscribed.This is a very
important factor to consider when designing the VoIP solution.
The total VoIP seats in the branch offices cannot exceed the capacity of the
circuits, nor the centralized CallManager.Off-net calls are routed to, and placed
through, the Primary Rate Interface (PRI) to the telecommunications office that
is local to the head office network. In this manner, head office management can
negotiate the best rates for local and long distance calls, and also get the maximum
utilization out of the Frame Relay circuits by using the voice and data
paths together.
However, notice that the branch offices use a FXO connection to their local
telecommunications office to off-net their local calls instead of routing them
across the Frame Relay circuit to off-net them.The FXO ports use a standard
analog telephone line instead of a specialized PRI circuit, and the cost is dramatically
different. Also, standard analog lines are available in nearly every town in the
country. If you can get an analog line, this is the first step towards centralized
design.We’ll get into this more in the section about creating the off-net solution.
In this chapter, you’ll extend your knowledge of Chapter 10’s single site VoIP
solutions into a multisite corporate environment.We’ll be performing specialized
network designs geared to the AVVID functionality.These solution designs will
evaluate the benefits and detriments of a centralized design versus a distributed
environment for large environments, and will tackle the following design and
operational issues:
Providing cost-effective small site connectivity while providing required
CallManager redundancy.
Assuring a seamless growth path when a small site grows to consume
more network resources.
Ensuring that CallManager solutions are flexible in their coverage of the
corporate users.
Providing the network engineers and managers with adequate documentation
of the design, and showing how the various AVVID solutions
fit within each part of the design.
The solutions will first review what you’ve learned in the Chapter 10 single
site solutions, then expand each of those topics out to a full corporate system.
We’ll show you how to build redundancy and resiliency into each design, how to
build out clustered CallManager solutions. Lastly, you’ll learn how to deploy
other AVVID solutions in this same corporate environment.When you finish this
chapter, you’ll have at least a solid view of the minimum requirements of a Cisco
AVVID enterprise network.