Establishing Links Outside the PBX
The systems outside the PBX are actually pretty simple once you understand the
internal systems.The voice world is made up of trunks, which interconnect public
or private switches.The basic functionality of these devices is no different for our
purposes.
However, there are a few things you should consider when thinking of
external PBX resources.These include the wide variety of phone numbers in the
international phone network, and the signaling protocol between switches in the
public network.
As you may know, calling internationally from your respective country can be
either a simple or difficult process.The administration of all the possible numbers is
also a daunting task. In either the legacy or AVVID environment, you’ll need to
work with these external-dialing plans to allow users to connect to other systems.
Consider your home telephone for a moment. In the United States, a call to
Israel would require calling 011 (the international escape code), 972 (the international
country code for Israel), 3 (the city code, similar to an area code), and the
local number, which may be six or seven digits. However, note that in some
countries, the city code may appear as 03. A call to Belarus would use a country
code of 375, and the city code and number may only contain five digits. A call
from another country to the US would require a three-digit area code and a
seven-digit number.As a PBX programmer, the system must be capable of handling
all the digits provided and routing the call to the correct destination.
Now, with the home phone, the routing of the call is simple—the phone
company takes care of it! But, when we enter the PBX, we may have multiple
paths to consider.Though this can become very complex, the basics might
involve the use of private links between systems (tie lines). Consider the United
States to Israel example again. It may be cheaper to route calls from Denver to Tel
Aviv through the private tie line terminating in Jerusalem rather than the public
network, and, although unlikely, it may be cheaper still to route calls for Mozyr,
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Old World Technologies • Chapter 1 11
Belarus, from Denver to Tel Aviv to Mozyr.This dialing plan addresses two factors:
call routing and call tariffing.
However, let’s presume our call to Mozyr is cheaper using the public network
and employing a link between New York and London. How does the network
understand our call and establish a path between Denver and Mozyr?
Well, this is the second point of external systems.The switches in the network
need to signal each other using a common protocol. In many networks, this protocol
is called Signaling System 7 (SS7).
Data network designers are probably used to in-band signaling, where the IP
address is part of each packet. No such mechanism exists in voice networks.
Rather, the signaling is out-of-band, or independent of the actual data. SS7 is
used between the switches to provide this dialog, and, in our call to Mozyr, the
Denver phone company switch might use SS7 to signal a path from Denver to
Chicago, and another link from Chicago to New York. Once the path is built
using SS7, a voice link is established and the call commences. Please note that this
does not occur with the PBX private connection to Jerusalem, as this is in-network,
and SS7 is typically not used in private switch-to-switch communications.