What Voice Designers Do
The art of voice system design is very different from data installations,
although there are similarities. A voice designer is typically confronted
with two challenges—the tariff, or cost-per-minute-per-mile, and the
redundancy within the network itself. These designs are based on the
number of channels needed, and are greatly simplified by the lack of
routing protocols and intelligent end-stations.
For example, in a data network installation, the designer will typically
draw upon elements of the three-tier model. This model defines a
network core, which interconnects different distribution layer devices,
and these, in turn, connect to the access layer, which services users. This
design is based upon the concept that data packets will take alternate
paths between devices based on load, in addition to the premise that
the network devices themselves are prone to failure.
Voice designs are different in regards to both hierarchy and redundancy.
First, the modern PBX is internally self-redundant, which means
the physical box itself attempts to provide its own redundancy. Data networking
systems have only recently reached this level of redundancy,
and, typically, they still experience a short outage as the system changes
from the primary to standby engine. In addition, the illusion of redundancy
within the box in data networking often requires alterations to
the connected devices—Cisco’s Hot Standby Router Protocol (HSRP) is a
good example of how workstations are tricked into thinking that two
physically redundant routers are actually one device. The trick is a shared
IP address and virtual Media Access Control (MAC) address to make two
routers appear as a single router. This, coupled with redundant
Supervisor and routing engines, can create the appearance of a redundancy
intradevice—however, because the end station has intelligence
(unlike the phone), these installations are more complex.
As noted, the end stations in voice networks do not have intelligence,
which greatly simplifies the redundancy model. The internally
self-redundant PBX, therefore, is not concerned with protocols and
other user-side functions to provide redundancy. Within the chassis, a
PBX only needs to provide redundant power, redundant processing, and
alternate egress paths. Advanced systems may also provide an ability to
redirect the physical port to another interface (engine) so the user’s
phone is also serviced in the event of a hardware failure. This is an
uncommon installation, however. Note that all of these redundancies
occur intrachassis, and, because of the static nature of the switching
paths, no convergence (compared to IP routing) occurs.
By now, you have likely guessed that the hierarchical design considerations
in many PBX systems are also very different from routers and
switches. For example, it is rare to have a PBX system with three tiers.
Most large installations are serviced with two tiers sufficiently. These
designs parallel hub-and-spoke data models much more than the threetier
requirements of large data networks. Part of this variance in design
is availed by the constant bit-rate of voice and the use of time division
multiplexing (TDM). Thus, a designer in a PBX environment need only
concern himself or herself with the number of concurrent calls between
points. All traffic consumes the same amount of bandwidth (a DS-0 in
most cases).
Let’s look at that another way. A data designer reviewing the
capacity of a link needs two variables—the number of flows and the size
of each flow. This is analogous to a freeway where semi trailers use the
same road as cars and motorcycles. Clearly the roadway can service
more motorcycles than trucks. In contrast, the voice designer needs only
one variable in addition to time—the number of flows. All flows are
exactly the same—in the highway example, they would all be
Volkswagens. Thus, a designer need only consider the number of flows
that will occur at the same moment. This may result in a peak of 12 calls
at 2:00 P.M.—a figure easily within the capacity of a T-1 circuit including
growth and bursts in call volume. The voice designer then adds resiliency
and redundancy to the design, in addition to tariffs, or pricing, to
develop a network.