Interpreting PBX Terminology
The world of telecommunications and PBX systems includes a vocabulary unique
unto itself.You may find that many of the words and acronyms are familiar and
common if your background is based in the data world. Nevertheless, there are a
number of new terms and concepts that need to be understood before tackling the
integration of voice and data systems. In addition, some acronyms have multiple
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In the previous sidebar, “What Voice Designers Do,” we discussed
the design and deployment considerations of a modern PBX. We also
saw the similarities between data systems and voice PBXs. These similarities
include redundancy, cost/performance, and design limitations.
PBX systems augment these similarities with a few distinct differences,
including:
Power Electrical requirements in PBX systems are frequently
48 volt DC. Data network devices are often 120 volt AC.
Wiring It is rare that a PBX system will require Category 5
cabling for connectivity, unlike Ethernet. In addition, it is
uncommon to terminate voice grade wiring on patch panels.
Rather, voice wiring uses punch-down blocks that hold each
bare wire onto a clip. Requirements such as maintaining
twists and staying under 100 meters are not part of the typical
voice installation.
Dial Plan Unlike IP routers, voice systems rely on static
routing tables when forwarding calls. Calls are routed based
on a match with the destination number—unlike data networks,
the source address is rarely used for call routing. The
static route map will define a preferred path, an alternate
path, and, sometimes, tertiary paths for each number within
the environment.
Circuits In the data world, most circuits are billed at a flatrate
per month. These charges can be distance insensitive (as
in the case in Frame Relay), or distance sensitive (common in
leased line connections). In voice, it is common to use leased
line connections and the associated tariffs, which can allow
for significant savings when traffic is carried on alternative
paths. These paths may be the connection to the long-distance
provider, or may be a private leased line between
PBX systems.
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meanings depending on whether you’re discussing voice or data. For example, the
acronym CDP, to a Cisco router guru, likely means Cisco Discovery Protocol. In the
voice world this term refers to Coordinated Dial Plan.
So, what are the common PBX terms you may encounter? Well, the first
is a T-1.A T-1 circuit is capable of carrying up to 24 voice channels (DS-0),
depending on provisioning.The total available bandwidth is 1.544 Mbps, although
the Integrated Services Digital Network (ISDN) Primary Rate Interface (PRI),
which uses T-1 framing, takes one DS-0 for upper layer signaling.The European
standard is called E-1. It provides, however, 2.048 Mbps of bandwidth, or 32 channels.
An E-1-based PRI, on the other hand, uses two of these channels for signaling
and framing, and thus, allows for 30 user-based voice channels. In addition
to the T-1 ISDN PRI , the circuit may also be configured as channel associated
signaling (CAS) or ear-and-mouth (E&M).
It is warranted to expand on ear-and-mouth technology slightly in this
forum, as E&M ports are found on the Cisco hardware platforms and many
interconnections will make use of this specification. E&M can also stand for earth
and magneto, amongst other variations, and is simply another signaling methodology.
E&M, like FXO and FXS, is an analog specification, unlike ISDN, which is
digital. In addition, FXO is available for PSTN or PBX connections, whereas
E&M is for trunk or tie lines between switches—they are network-to-network
links. As such, some Cisco installations use the VIC-2E/M interface for connections
to voice mail or legacy PBX systems. Please note that this module supports
both the two and four wire specifications of E&M for types I, II, III, and V.
These links may also be loop start, in which removing the receiver from the
hook closes a circuit and creates a loop, allowing connections. Or they may be
ground start, where an earth ground is needed to complete the loop and allow
connectivity.
NOTE
It is important to remember that voice services are based on time division
multiplexing, or TDM. This is the basis for most connections in the
voice world, just as it is for T-1 signaling. A DS-0 is a single voice digital
channel of traditional voice bandwidth—8Hz at 8 bits per sample.
The term central office is a legacy description of the local telephone company’s
termination point for all numbers in a given area, and commonly connects to
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14 Chapter 1 • Old World Technologies
PBXs via T-1s. Historically these were centrally located and copper was run from
each building in the town to the central office.Today, a wide variety of devices
are deployed to convert copper local loops into fiber and the central office terminates
a small number of fiber pairs that service hundreds of lines.The central
office would also provide a Direct Inward Dial (DID), although such connections
are typically bi-directional today. In order to directly connect from the public
phone system to a PBX, the caller must either be manually routed to the extension
or a relationship between the extension and a public number must be established.
DID provides the latter service—a block of numbers can be assigned to a
trunk line from the telephone provider to the PBX, and the PBX administrator
can route those numbers to related extensions. Figure 1.2 illustrates the logical
configuration of number 415-555-1234 to extension 51234. Please note that it is
quite common to create five-digit extensions in North America that relate to the
assigned DID numbers.