Voice-Capable Gateways
As explained in Chapter 10, a voice-capable gateway is a Cisco router that runs the
MGCP IOS firmware that performs processing for voice calls on the local network
to local or external destinations.These routers are installed with PRI, FXO,
or FXS ports that form the external connectivity to a local telecommunications
carrier office.The voice-capable gateways for branch offices are:
Model 175x for small site gateways, for up to 10 users
Model 26xx for small sites, for up to 50 users
Mixed variations of these two devices
These two models are frequently used units; the Model 175x is the more
cost-effective unit, but has less flexibility than the 26xx series and VG200 gateways.
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396 Chapter 11 • Designing and Implementing Multisite Solutions
The field gateway router used for data only might also be an older 2500 or 3600
class router that has been at the branch office for quite some time. Also, newer
Model 1600 series routers may be positioned as small branch office gateways to
handle the data portion of the site. It is important not to not mix up these gateways,
and equally important to not try and use one gateway for both data and
voice combined.While such a combination has worked at times, it usually is not
a good idea to have all your eggs in one basket.
The important thing to understand is that voice-capable gateways exist to
provide external telecommunications connections at that site.The nonvoicecapable
gateways can still be used in a centralized environment where all calls are
passed through the central site, and there are no off-net local calls.While the central
site would then bear all telecommunications costs for the branch office, this
isn’t necessarily a bad thing. If the pre-VoIP design assessment found that 95 percent
of all calls were to the head office, then the cost of the remaining 5 percent
of calls could be routed through the head office backbone, resulting in that 5
percent being all long distance calls back to that branch office, but now coming
from head office and not the branch office. However, you must be aware that the
5 percent of rerouted calls could substantially increase your long distance toll call
costs, and thus should be a factor when deciding how to reroute calls like these.
This is just one example of how VoIP solutions must be approached in any
part of the design.The cost savings realized by not purchasing the voice-capable
gateways might be realized in that 5 percent of long distance calls.With long distance
calls now costing as little as four cents per minute from major carriers, this
might just be a negligible expense. Look at Figure 11.2, and you’ll see the
changes in removed external telecommunications costs.
This is possible if VoIP MGCP firmware is used, but the site will not have any
options to create external connectivity without replacing the router and adding
the new telco cards, causing site downtime. Notice that routers R4 through R6
have no external connectivity, nor do they have a gatekeeper at each site.This is
because the WAN circuit is powerful enough to centralize those functions and
still carry the data load as well.