Chapter 25

TDM Customer-Premise
Switching Systems

PBXs have evolved in a path parallel to central offices. The earliest versions
were manual switchboards, followed by electromechanical dial, and then stored
program systems that matured into the current TDM architecture. As with central
offices, PBXs are on the cusp of a transition from TDM to VoIP and for much the
same reasons. The legacy TDM technology is still adequate for most offices and in
the absence of compelling reasons for replacing existing systems, the transition is
likely to be gradual. Most manufacturers have modified their TDM systems to
support VoIP, which further postpones the transition to an all-IP system because
upgrades to most existing systems can provide equivalent services. Moreover,
many of the justifications for replacing existing systems are difficult to quantify in
tangible terms. We will discuss these considerations in the “Applications” section
at the end of the next chapter.
All CPE equipment manufacturers are touting their IP systems to the point
that the TDM alternatives are obscured in the hoopla, but TDM switches serve the
majority of CPE lines and will continue to do so for the next several years. Feature
development has reached its zenith with TDM systems, however, as manufacturers
concentrate their development efforts on their IP products. The primary interest
most companies have in TDM systems is preserving the existing investment, the
life span of which has historically been in the 7-to-10-year range.
This chapter discusses the architectures and configuration of the three TDM
product lines on the market: key systems, hybrids, and PBXs. Although the distinction
between the product lines is somewhat blurred, if an organization requires more
than about 100 central office line and station ports, a PBX is usually most effective
because of its greater line, trunk, and intercom capacity. Hybrids can grow to about
250 ports, and are satisfactory for midsized offices at a lower cost than PBXs. Key systems
are used in smaller line sizes. When the system supports more than a dozen
central office lines it is set up with pooled trunk capabilities and defined as a hybrid.
437
TDM KEY AND HYBRID ARCHITECTURES
Key and hybrid systems are typically wall mounted in cabinets similar to the one
in Figure 25-1. Many small systems have a fixed size and cannot be expanded.
These are designated by their line and station size such as 3×8 or 6×16. When the
system reaches capacity, it must be replaced if further growth is needed, often preserving
only the telephone instruments. Larger hybrid systems may also be designated
by line and station size, but the basic unit is expandable. These systems
are also housed in wall-mounted cabinets, but they have expansion slots to accept
additional line and station cards.
All key and hybrid systems support loop-start central office lines. Some
high-end systems support ground start, T1/E1, BRI, and PRI trunks. Some also
support tie lines between other switches, and may support networking either
between themselves or with the manufacturer’s PBX product line. All hybrids and
key systems are processor controlled with either a proprietary or a commercial
microprocessor. The generic program is usually stored in a chip, which is replaced
to upgrade to a later software release. This is in contrast to PBXs, in which the program
is upgraded in the field from tape or CD-ROM. Key and hybrid line and
station translations are typically stored in flash memory, which enables them to
restart quickly in case of power failure.
438 PART 4 Customer Premise Systems
F I G U R E 25-1
Nortel MICS Key System (Photo by Author)
KTSs usually include one or more intercom lines. These are used for stationto-
station communication—in smaller systems the intercom is primarily for conversations
between the attendant and the called party. In larger systems multiple
intercom paths are provided so several internal conversations can be held simultaneously.
Most systems provide a built-in speaker so the intercom line can be
answered without using the telephone handset. Optionally, the user can lift the
handset for privacy. The number of intercom lines provided is a feature that distinguishes
a PBX from a hybrid. Many hybrid systems support a limited number
of intercom paths, which may preclude their use in offices that require a large
amount of internal calling. Most PBXs have enough time slots that virtually all
stations can be talking simultaneously.
While the attendant can answer and transfer calls from an ordinary telephone,
a special telephone is often provided. The attendant has all the features of
regular stations and may also have a busy lamp field (BLF) to show which stations
are occupied and a DSS field, which allows the attendant to transfer calls to stations
by pushing a button instead of dialing the station number. To support the
attendant, many systems include paging either to an overhead system or to telephone
speakers. The paging system is accessed by pushing a button or dialing a
code and can be divided into zones if the building is large enough to warrant it.
Many systems provide for parking a call so a paged user can go to any telephone,
dial a park number, and pick up an incoming call.
Several manufacturers produce multiline systems that do not require a KSU.
Most KSU-less systems require one pair of wires per line, which limits the size of
the system to four or fewer lines. The primary advantages of KSU-less systems are
low cost and ease of installation. Anyone who knows how to install a single-line
telephone can install KSU-less telephones because they do not require setup,
which makes them ideal for small offices and residences. The primary drawbacks
of KSU-less systems are limited expandability and lack of features. Since the systems
have no KSU, the only features available are those contained in the telephone
set itself. Some KSU-less systems also lack an intercom path, which means calls
cannot be announced over an intercom as they are with most key systems.
Voice mail is provided almost as a matter of course in practically all key and
hybrid installations. Other add-on peripherals such as ACD and call accounting
are available for many product lines.
PBX ARCHITECTURES
PBX technology has progressed through three generations and is now starting the
fourth generation of IP PBXs. First-generation systems were manual switchboards,
which were common in the first half of the twentieth century. Secondgeneration
systems used wired logic and analog step by step or crossbar switching
fabric. Second-generation telephones were nonproprietary rotary dial or DTMF
analog sets. If key features were needed with a second-generation PBX, a separate
CHAPTER 25 TDM Customer-Premise Switching Systems 439
key telephone system was required. The third generation introduced stored program
control processors. Some used analog switching fabric and others were
TDM. The processor-controlled logic enabled PBXs to support proprietary telephones,
which controlled a limited number of key telephone features. All systems
supported POTS phones, which required users to dial feature access codes.
Proprietary telephones either used an ISDN-like interface with a separate signaling
channel, or they signaled over wires separate from the talking path.
All of today’s third-generation switches employ TDM switching technology
and support both analog and proprietary digital telephones. TDM PBXs are the de
facto standard of the industry, having been operation for nearly three decades.
Products have continually improved with feature and hardware enhancements
and have reached the stage where the risk is slight in buying a PBX from any
reputable manufacture and distributor. Proponents of IP PBXs contend that
TDM PBXs are obsolete, and eventually they will be, but TDM systems meet the
communications needs of most users and will continue to do so for several years.
All TDM PBXs are processor controlled. Some use proprietary processors
and some use industry-standard microprocessors. PBXs are rated by busy hour
call attempts (BHCA) and busy hour call completions (BHCC). A call attempt is
any event that accesses the processor. This includes functions such as switch hook
flashes and feature accesses to hold, transfer, conference, park, or any such event
that occurs during the normal course of a call. The number of call attempts
an office requires is difficult to estimate, but the BHCA and BHCC factors are
convenient metrics for comparing products.
The switching matrix is evaluated by busy hour hundreds of calls seconds
(BHCCS). Traffic engineers use CCS in evaluating trunking requirements. A discussion
of traffic engineering is beyond the scope of this book, but it is covered in
the companion volume, The Irwin Handbook of Telecommunications Management.
A circuit occupied for an hour is busy for 3600 s or 36 CCS. If a switching matrix
is nonblocking, it is capable of supporting all stations in an off-hook condition
simultaneously on station-to-station calls. Trunk groups are never engineered to
permit all stations to connect to trunk calls simultaneously, so the switching network
would never be overloaded by trunk calls. Since each call involves two
stations, a nonblocking switch must support half the quantity of stations times
36 CCS per station. To illustrate, assume a PBX has 100 stations. If it is nonblocking,
the switching network should support 50×36 or 1800 BHCCS.
The architecture of a TDM PBX is similar to that of a TDM local switching
system. As Figure 25-2 shows, PBXs have a line side and a trunk side. The interfaces
are contained in hardware modules that fit into slots in a card cage. The line
and trunk ports connect to the TDM switching matrix, which makes the connections
under processor control. The features and configuration information are
contained in memory. Smaller systems use flash memory for the program and
configuration information. Larger systems usually employ volatile memory for the
program and for station and trunk translations. Generic programs are upgraded
440 PART 4 Customer Premise Systems
from tape or CD. Larger systems usually provide tape drives for backing up the
program and configuration, which must reload if the power fails.
The expansion cards plug into the PBX’s backplane, which ties the lines,
trunks, and central control circuits to the switching fabric and busses over which
the circuit elements communicate. Although the structure of PBXs is similar, there
are significant differences in products on the market and the way features are
packaged and sold. PBXs grow incrementally up to the total capacity of the system.
The main module contains common equipment cards such as processor, memory,
TDM switch, power supply, and tape and CD-ROM drives. These fit in dedicated
slots that cannot be used for any other purpose. System recovery following power
failure is much slower than key and hybrid systems, typically requiring 3 to
10 min. Figure 25-3 shows a cabinet stack from a Nortel SL-100 PBX.
Added to the main module are modules that support line and trunk cards.
These plug into a backplane that connects to the TDM switch modules. Figure 25-2
shows separate line and trunk modules for clarity, but most systems have universal
card slots—that is, either line or trunk cards can plug into any slot. Up to a given
line size, any port can connect through the TDM switch to any other port. At some
point, depending on the manufacturer’s design, direct port-to-port connectivity
becomes impractical and a center-stage switch is required to connect modules.
Station Connections
Stations connect to the line ports through a 64 Kbps circuit that runs on twisted-pair
wire. The typical digital station range is in the order of 1500 to 2500 ft (460 to 760 m).
CHAPTER 25 TDM Customer-Premise Switching Systems 441
Voice Mail
ACD Positions
Line Modules Trunk Modules
Remote Switch Unit
Local Trunks
IXC Trunks
Tie Trunks
Local Trunks
Fax
Maintenance
Terminal
Switch Modules
Processor
Bus
CTI Server
Database
F I G U R E 25-2
TDM PBX Architecture
Analog telephones connect to analog ports, or in some systems to digital ports
through an analog adapter. PBXs have at least two different types of line interface
cards, analog and digital. Most systems also support BRI cards, which are typically
used for videoconferencing. Digital line cards support proprietary telephones that
work only with that manufacturer’s system. Analog and ISDN cards support
telephone sets that are independent of the PBX manufacturer. Although ISDN
telephones should work with any manufacturer’s PBX, ISDN standards do not
define all of the features that the system may be capable of. Therefore, ISDN sets
from the PBX manufacturer will usually provide features that other manufacturers’
telephones cannot support.
442 PART 4 Customer Premise Systems
F I G U R E 25-3
Nortel SL-100 PBX (Photo by Author)
Line card density, which ranges from eight to 32 ports, is a distinguishing
feature among products. High-density cards allow for smaller cabinet size, which
is usually a plus. In smaller PBXs, however, high-density cards may result in more
spare ports than the owner would normally purchase. For example, if the system
has 32-port cards and 33 ports are required, 64 ports must be purchased, leaving
nearly half of them unused.
Trunk Connections
PBXs, like central offices, interface the outside world through trunk circuits that
exchange signals with other switching systems through a variety of signaling
interfaces. TDM systems support both analog and digital trunks, but as the network
evolves toward all digital, analog trunk interfaces for the PBX are gradually
disappearing. Transmission quality is better on digital trunks and they take fewer
card slots. Analog trunks are usually provided over three separate trunk groups:
incoming to the main listed number, outgoing, and DID. A separate type of analog
trunk card is required for DID trunks in most PBXs, although some manufacturers
offer universal trunk cards that will support either DID or two-way CO.
Incoming and outgoing analog trunks can be combined into a two-way trunk
group. Calls to the main listed number route to the console, and any trunk can be
seized for outgoing calls. Analog trunk cards contain from 4 to 16 trunks per card.
Analog trunk cards support two-way central office trunks and foreign exchange
lines. Trunks to the IXC are normally digital.
T1/E1 trunk cards support 24 or 32 circuits. Some PBXs use a single type of
card for T1/E1 or PRI; others have separate card types. It is important to understand
the difference between a line-side and trunk-side T1/E1. Line-side connections,
which are always the case with analog trunks, allow the user to flash the
central office and get second dial tone to activate features such as conferencing
and transfer. Trunk-side connections do not offer this capability. Most key systems
and hybrids allow the user to flash the central office, but PBXs do not. Therefore,
even though the central office may provide line-side features toward a PBX, they
cannot be accessed. For example, users may want incoming calls transferred to
cell phones. With a line-side connection, an attendant can flash the line, receive
second dial tone, and transfer the call. The connection is made in the central office
and the line is released. A PBX with T1/E1 trunk-side connections can also transfer
calls, but only if the switch has been configured for trunk-to-trunk transfer.
Many PBXs have this feature deactivated for reasons that will be explained later,
but if it is permitted, two trunks are tied up for the duration of the call. If the PBX
has PRI trunks, and if both the PBX and central office are equipped for antitromboning,
one of the two trunks will be released. Line-side T1 cards are used to connect
the PBX to peripherals such as interactive voice response and voice mail.
Most LECs offer both PRI and digital trunks. The latter may be connected to
the line side or the trunk side of the central office, but PRI is always a trunk-side
CHAPTER 25 TDM Customer-Premise Switching Systems 443
connection. Depending on the LEC, non-PRI T1 trunks may be set up to provide
two-way DID, offering a service similar to PRI, but with in-band signaling. The
main impediment to the all-digital PBX trunking network is the premium prices
that many LECs charge for digital or ISDN trunks, but even this is disappearing
as competition lowers trunk prices. Most LECs now offer digital trunks, either as
separate two-way and DID trunks or combination trunks that can be used for
either DID or outgoing service. Digital trunks have two drawbacks. One is their
lack of scalability. When additional trunks are required, they are added a full
T1/E1 at a time. The other is their greater vulnerability to failure since a single
failure can kill all the trunks to a PBX.
Signaling compatibility is an important issue in connecting a PBX to a central
office. If analog trunks are used, PBXs use ground start trunks to prevent glare
as discussed in Chapter 12. DID trunks are used for incoming service only, so they
may be loop start from the central office, with the DID digits passed with DTMF
signaling. Two-way DID trunks are normally connected to the central office as tie
lines using E&M signaling. PRI trunks use the D channel for signaling.
As discussed in Chapter 15, PRI in North America provides 23 64-Kbps
B channels plus one D channel. The rest of the world uses E-1, with 32 channels,
of which two 64 Kbps channels are reserved for signaling and controlling. PRI is
preferable to T1 because, among other advantages, it supports caller ID and callby-
call service selection, which permits the PBX and the central office to determine
for each call what type of service is needed. For example, if a PBX supports video
conferencing, multiple channels on the PRI will be needed to provide the desired
degree of picture quality. The PBX and the central office set up the appropriate
bandwidth by exchanging messages on the D channel. Many PBXs use BRI on the
line side to support video conferencing.
PBXs require an access digit, usually 9, to connect station lines to central
office trunks. When the user dials 9, the PBX seizes an idle central office trunk and
connects the talking path through to the station if the station is permitted off-net
dialing. The station hears central office dial tone as a signal to proceed with
dialing.
Trunk-to-trunk transfer enables a user to link an incoming trunk to an outgoing
trunk and hang up. The feature is available in most PBXs, but it is often disabled
to prevent misuse. For example, unauthorized callers may request
confederates on the inside to connect them to long distance trunks. The feature has
many authorized uses, however, such as transferring a call to a cell phone, so
many companies activate trunk-to-trunk transfer, but restrict it to local calls and
tie trunks or to a particular class of service.
Tie Trunks and Networking
Organizations operating multiple PBXs have two alternatives for interconnecting
them, tie trunks and networking. Tie trunks can be analog or digital, terminating on
444 PART 4 Customer Premise Systems
the trunk side of the PBX. Analog tie trunks are rare today because most organizations
have both voice and data networks running over digital circuits. If VoIP is not
used in the PBX, voice and data can share a T1/E1 by splitting the line through an
add-drop multiplexer as shown in Figure 25-4. This configuration avoids the QoS
issues of VoIP, but it does not allocate bandwidth dynamically. If the bandwidth balance
is not optimum, the multiplexer must be reconfigured. Nevertheless, it is an
inexpensive way of sharing a digital line with legacy equipment.
Tie trunks are generic, and can be set up between PBXs of different manufacture.
Signaling is usually E&M and it supports no feature transparency beyond call
origination and termination. A call into one PBX can terminate to a station in the
other PBX, and users can transfer calls across the tie lines, but sharing a voice mail
requires QSIG networking. If tie trunks terminate in a single location they are often
accessed by dialing a digit, such as 8, which connects them to the distant PBX. Many
multi-PBX organizations have a separate dialing plan for each system plus a single
organization-wide dialing plan. The PBX then is then programmed to provide the
translations necessary to reach the distant number over the tie trunk network. This
feature is called uniform dial plan. To avoid the need for users to understand the dialing
plan, many organizations use the PBX’s ARS to dial the necessary codes. Users
dial the number, and the PBX selects the route and dials any additional digits.
Most companies network their PBXs to obtain feature transparency.
Proprietary networking protocols may be used between PBXs of the same manufacture,
or, as discussed in Chapter 24, QSIG may be used between otherwise
incompatible products. Networked PBXs provide service equivalent to a central
CHAPTER 25 TDM Customer-Premise Switching Systems 445
T1/E1 line
Ehernet
Add-drop multiplexer
Router
PBX
T1/E1
F I G U R E 25-4
Voice-Data Line Sharing with an Add-Drop Multiplexer
PBX and RSU except that each PBX contains a separate database and is inherently
survivable. If the link to the main PBX is lost, features are lost, but local switching
is unaffected.
Distributed Switching
Many PBXs offer remote switching units. An RSU extends line and sometimes trunk
interfaces over T1/E1 lines to secondary locations. The main advantage of an RSU
as compared to networked PBXs is that the remote stations are in every respect
equivalent to those in the host. All configuration and management are centrally controlled,
in contrast to networked PBXs, each of which is separately administered. All
processing is in the central unit and connects to the remote over an umbilical. If the
remote is in a separate calling area it may be equipped with local trunks.
On the down side, if the umbilical fails, the remote is dead. Some remotes
have survivability features that permit limited stand-alone operation. A survivable
remote typically has a copy of the line and trunk database so the system
operates with reduced capabilities. If incoming calls terminate on the central unit
during normal operation, they will be lost during emergency conditions.
Remote units have several advantages compared to networked PBXs:
_ Only one processor and software set are needed. This is usually less
expensive than maintaining separate systems.
_ Administration is from the host. All database changes are made
on the host switch.
_ Wiring costs are reduced on a campus. It is often less costly to install
a remote than to cable from the central site.
_ Total feature transparency is achieved. Users in the remote location
share the same voice mail, numbering plan, and trunks as the central
site, and have access to exactly the same features.
Administration
All PBXs are administered from a maintenance and administration terminal
(MAT). In some products a dumb terminal connects directly to the PBX through a
serial port, but many current PBXs provide Ethernet connections to a PC. The
MAT terminal is used to enter station and trunk translations in a command language
that is unique to the product. Most products offer an optional PC-based
program that allows the administrator to make changes on a graphic screen and
upload them to the switch. The same terminal and associated printer are used to
display and diagnose maintenance messages that indicate hardware or trunking
faults. The same terminal is used to collect statistics such as processor activity, and
line, trunk, and feature usage. Most PBXs support remote administration either
over Ethernet or dialup to a remote maintenance port.
446 PART 4 Customer Premise Systems
The maintenance and administration process is designed to be closed and
proprietary. It is kept out of the hands of users as one of the ways the configuration
is controlled to support management’s objectives. The process is complex
enough that training in factory-authorized schools is required to become certified
in administering the system.
Redundancy
Modern TDM PBXs are inherently reliable. Total failures are rare, but not unheard
of. Reliability can be increased through redundancy. Some products offer redundant
processors while others offer redundant switching matrices, power supplies,
and common equipment items. With full redundancy and uninterruptible power
supplies, TDM PBXs can achieve reliability in the order of central offices.
Emergency Communications
Because PBX stations are tied to a location, it is easy to convey location information
to an emergency center. Fixed station locations are changed only through
order activity, which gives the administrator an opportunity to update the emergency
database. This is in contrast to IP stations, which may change location without
management’s involvement. Systems employing remotes that do not have
separate trunks, must take precautions to ensure that the true location of the
reporting station is transmitted to the PSAP.
Computer–Telephony Interface
Most PBXs provide an open interface for limited call control from an external
processor. The physical interface and the command language are proprietary to
the manufacturer, and allow application programmers to connect the PBX to a
standard application program interface (API). The most common are telephony
API (TAPI), which is for connecting a PC running MS Windows to telephone services
and telephony server API (TSAPI), which is a server-based interface. CTI is
discussed in Chapter 27.
Wireless Capability
Many organizations have classes of users who must roam the building. Wireless
systems allow use of the telephone anywhere in a building or within a restricted
range on a campus. Two types of wireless systems are available. One type plugs
into analog ports on the PBX, and gives the user capabilities of analog telephones.
Proprietary wireless systems provide the features of digital telephones including
multi-line capability and button access to features. Wireless phones are discussed
in Chapter 21.
CHAPTER 25 TDM Customer-Premise Switching Systems 447
TDM CPE Application Issues
Nearly every business that has more than a handful of stations is in the market for a
key system, hybrid, PBX, or its central office counterpart Centrex. PBXs are economical
for some small businesses that need features such as restriction, networking, and
ARS that are beyond the capabilities that most key systems provide. Very large businesses
may use central office switching systems of a size that rivals many metropolitan
public networks. Between these two extremes lie hundreds of thousands of PBXs.
PBX Standards
PBXs, almost by definition, are proprietary in their internal switching and features,
but trunk interfaces are standard T1/E1 or analog trunks that are covered in
ANSI/TIA/EIA-464-C. This document is a detailed set of standards for transmission,
signaling, framing, and private network synchronization. It describes digital
and analog interfaces to a local central office in detail. The interfaces are so well
documented that incompatibility is practically non-existent.
QSIG, as discussed in Chapter 24, is an ISO standard for networking PBXs of
disparate manufacture. SMDI, also discussed in Chapter 24, is a Telcordia
Technical Reference TSR-TSY-000283, Interface Between Customer Premises
Equipment; Simplified Message Desk and Switching System. Both of these are open
interfaces for connecting peripherals to proprietary PBXs. The TAPI and TSAPI
standards for connecting CTI peripherals are open standards, but the protocols are
unique to each switch, which publishes its own APIs.
Evaluation Considerations
In choosing a key system or PBX it is important that you understand exactly what
you want it to do. The variety of key and hybrid systems on the market is so vast
that managers must carefully evaluate their requirements before selecting a system.
The differences between systems are often subtle, and differences in function
and support are not apparent until you have lived with the system for several
months. This makes it important to check references carefully. Considerations that
are important in some applications will have no importance in others and the
buyer should weigh them accordingly.
The first consideration in selecting any office phone system is to determine
how many station and line or trunk ports are needed initially and to accommodate
growth. The following are some general rules, but be aware that exceptions are
many, and product lines are changing constantly, which may invalidate some of
these distinctions:
_ If more than 24 central office trunks are required, favor a PBX or a hybrid.
_ If fewer than eight central office trunks are required, favor a key system
unless the system will grow significantly.
448 PART 4 Customer Premise Systems
_ If the system will never grow beyond three or four lines and about
eight stations and if PBX features are not needed, consider a KSU-less
system.
_ If ACD or voice processing is required, favor a PBX or a hybrid, with
a PBX providing superior features.
_ If half the total system traffic is intercom, favor a PBX or, depending
on size, a hybrid.
Line and Trunk Interfaces
Every system must conform to the standard EIA-464 interface to a local telephone
central office and must be registered with the FCC for network connection. In
addition, interfaces such as these should be considered:
_ PRI and BRI interfaces
_ Computer–telephony integration interface
_ QSIG interface
_ T1/E1 interface to external trunk groups or to internal devices such
as remote access servers
_ IP line and trunk interfaces (Chapter 26)
A key consideration in evaluating any system is the type of terminals it supports.
All systems have, at a minimum, a two-wire station interface to a standard
analog DTMF telephone, either directly or through an analog adapter. Ordinary
telephones are the least expensive terminals and because of the quantities of stations
involved in a large PBX, inability to use standard telephones can add significantly
to the cost. The standard analog telephone falls short as a user device in
most offices, but it is usually preferable in residential facilities such as hospitals,
hotels, and dormitories.
Proprietary telephones are the most practical way of accessing integrated
key telephone features. A proprietary terminal makes some features, such as call
pickup and transfer, easier to use by assigning features to buttons to avoid the
switch hook flashes and special codes required with analog telephones.
These features, among others discussed in Chapter 24, should be considered
in evaluating a PBX or key system terminal interface:
_ Proprietary or nonproprietary telephone interface
_ Number of lines and characters on the telephone set display
_ Station conductor loop range (in a campus environment)
_ Integrated key telephone system features
_ Message waiting or nonmessage waiting analog line card
_ Availability of BRI interface
CHAPTER 25 TDM Customer-Premise Switching Systems 449
Voice Mail
Voice mail evaluation is discussed in Chapter 28. Once available only in PBXs and
some hybrids, voice mail is now one of the most desired features among users
purchasing new key systems.
Wireless Capability
Many organizations need wireless to enable some users to roam all or part of a
building. Wireless systems that support analog telephones can be used with any
PBX or key system, but the ability to use proprietary telephones may be important
if button access to features is required.
ISDN Compatibility
PRI capability is an essential feature for all PBXs and many hybrids. Key systems
and hybrids may support BRI toward the central office. Determine whether ISDN
compatibility is likely to be required within the life of the system, and if so,
whether the system can be purchased or retrofitted to interface either BRI or PRI
lines. If ISDN is not used, ADSI may furnish equivalent service on key systems and
hybrids. BRI line interfaces are needed in many PBXs to support video conferencing.
Administrative Interface
All PBXs have some form of administrative interface through a terminal or
attached PC. The ease of use of this interface differs significantly among products.
The most difficult products to use have a command-driven terminal interface. At
the other end of the spectrum are systems with graphical user interfaces that allow
users to make point-and-click changes. Key systems and hybrids are administered
by plugging a laptop computer into a serial interface or in some systems a telephone
can be used for system setup.
The ease of changing classes of service and telephone numbers is an important
evaluation consideration. If an easy-to-use maintenance terminal can control
these, it is possible to add, remove, and move stations and change features such
as restrictions without using a trained technician.
The degree to which a PBX can diagnose its own trouble and direct a
technician to the source of trouble is important in controlling maintenance
expense. It is also important that a system has remote diagnostic capability so the
manufacturer’s technical assistance center can access the system over a dial-up port.
Cost
The initial purchase price of a PBX or key system is only part of the total lifetime
cost of the system. All systems require maintenance and administration, and the
method of accomplishing them can be significantly different among products. As
with all types of telecommunications apparatus, the failure rate and the cost of
restoring failed equipment are critical and difficult to evaluate. The most effective
way to evaluate them is by reviewing the experience of other users.
450 PART 4 Customer Premise Systems
Installation cost is another important factor. One factor is the method of programming
the station options in the processor. Some systems provide such
options as toll call restriction, system speed calling, and other such features in an
external database that can be uploaded to the switch.
Maintenance costs may be significant over the life of the system. The best
way to evaluate maintenance cost is to request a quotation on a maintenance contract,
which most vendors offer. Cost savings are possible with systems that provide
internal diagnostic capability. Virtually all PBXs provide remote diagnostic
capability so the vendor can diagnose the system over an ordinary telephone line.
These features can offer cost savings in hybrids, but are less important in key
systems.
Power Failure Protection
During power outages, a PBX or key system is inoperative unless battery backup
or a UPS is provided. Some systems include emergency battery supplies, while
others are inoperative until power is restored. Lacking battery backup, the system
should at least maintain its system memory during power failures.
The system should include a power-failure transfer system that connects
incoming lines to ordinary telephone sets so calls can be handled during power
outages. The method of restarting the system after a power failure is also important
because of the time required to get the system restarted. Some systems use
nonvolatile memory that does not lose data when power fails. Other systems
reload the database from a backup tape or disk, which results in a delay before the
system can be used following restoral.
Key System Considerations
Capacity
Key telephone systems should be purchased with a view toward long-term
growth in central office lines and stations. This specified size figure is the capacity
of the cabinet, and further expansion may be expensive or impossible. Some
systems can grow by adding another cabinet, but it also may be necessary to
replace the power supply and main control module. With some systems it is possible
to move major components, such as line and station cards, to a larger cabinet
to increase capacity. Most key systems use plug-in circuit cards. These are less
costly than wired systems, which must be purchased at their ultimate size. The
number of internal or intercom call paths also should be considered.
Station Equipment Interfaces
Many key telephone systems support only a proprietary station interface so
analog telephones cannot be used. The lack of a single-line interface is not
a disadvantage in many applications, but some companies need to connect
modems or facsimile machines to key system ports. An important feature for
CHAPTER 25 TDM Customer-Premise Switching Systems 451
many users is upward compatibility of telephone sets and line and trunk cards
across the manufacturer’s entire product line. This capability reduces the cost of
converting from a key system to a PBX and enables users to keep their instruments,
which not only reduces cost, but also minimizes retraining.
Key Service Unit versus KSU-Less Systems
Some systems support from two to four lines without a KSU. For small systems
these can be effective, providing many of the capabilities of a key telephone system
without the need for a central unit. KSU-less systems have disadvantages,
however, which make them inappropriate for many installations. First, they have
limited capacity, so they are usable only for small locations and cannot grow.
Second, they usually lack intercom paths, on-hook voice announcing, and other
features that are essential in a multiroom office.
Centrex Compatibility
Key systems are often used behind Centrex. Many Centrex features cannot be activated
unless the key system is Centrex compatible. For example, call transfer
requires a switch hook flash to get second dial tone. To make a key system Centrex
compatible, it must have a special button to flash the central office line. Many key
systems are provided with buttons to make them directly compatible with
Centrex features.
Number of Intercom Paths
A nonblocking switching network is one that provides as many links through the
network as there are input and output ports. For example, one popular key system
has capacity for 24 central office trunks, 61 stations, and eight intercom lines.
The system provides 32 transmission paths, which support calls to and from all 24
central office trunks. The eight intercom paths limit intrasystem conversations to
eight pairs of stations. A nonblocking network provides enough paths for all line
and trunk ports to be connected simultaneously. In this system, if all central office
trunks are connected, of the remaining 37 stations, only eight pairs can be in
conversation over the intercom paths. Although this system is not nonblocking, it
meets an important test of having sufficient paths to handle all central office
trunks and intercom lines.
PBX Considerations
Universal-Shelf Architecture
Universal-shelf architecture permits various types of line and trunk cards to be
installed in any slot. Lacking this feature, slots are dedicated to a particular type
of card. It is, therefore, possible to have spare slot capacity in the PBX but have no
room for cards of the desired type. Check to determine if all port cards are universal
or whether some specialized types require dedicated slots.
452 PART 4 Customer Premise Systems
Switch Network
A key evaluation consideration is whether the switch network is blocking or nonblocking.
Blocking networks are acceptable, but may require additional administrative
effort to keep them in balance. Also, consider the number of BHCAs the
PBX is capable of supporting to determine if the processor limits the capacity of
the system. Some manufacturers use the term “virtually nonblocking” to indicate
that there are nearly as many time slots as stations.
Redundancy
Organizations that cannot tolerate PBX outages can improve reliability by purchasing
redundant systems. Several levels of redundancy are available. The lowest
level provides redundant processors. Higher reliability can be achieved with
redundant power supplies and switching networks. Even with redundancy failures
will still occur, but reliability should be much higher than with a nonredundant
system.
Application Programming Interface
An open architecture interface is important for future computer–telephony applications
that will be appearing in the next few years. Determine if the PBX has such
an interface, and if the standards are readily available to developers. Consider that
outside developers will apply the greatest amount of development effort to the
most popular PBXs.