QSIG
TDM PBXs use a proprietary protocol similar to ISDN for networking between
their own products, but networking with switches of another manufacturer is
impractical. The QSIG protocol, named after Q.931 ISDN signaling, is designed to
support feature transparency and sharing of common resources such as voice mail
between disparate products in a private integrated services network (PISN). The
official ISO name for the protocol is private signaling system No. 1 (PSS1). QSIG
separates the bearer channels from the signaling, which uses a separate packetbased
signaling channel. QSIG can also be used with VoIP, where it offers the
advantage of potentially reducing the number of hops needed for a call.
Networking Comparison between TDM and VolP
The first layer of the protocol is called QSIG basic call, which supports transparency
between multivendor nodes. All products that claim QSIG compliance
must support basic services. The BC feature set is intended for call control, but a
higher layer known as QSIG generic function (GF) or QSIG supplementary services
supports additional services such as calling line identification. QSIG capability
is important for users with PBXs from different manufacturers to network
them together. QSIG support is rarely found in hybrids.
Station Restrictions
An important feature of every PBX is its ability to limit the calling privileges of
selected stations. Even companies that leave employees’ extensions unrestricted
normally require toll-restricted telephones in public locations such as waiting
areas and lunchrooms. The type of restriction varies with manufacturer, but it is
possible with most systems to restrict incoming, outgoing, and any type of long
distance. One class of employee could be given international access, for example,
while others are restricted to domestic calls. Some systems can restrict down to a
specific telephone number. All restriction systems should be able to restrict
selected area codes and prefixes. Area code restriction is necessary to prevent
users from calling certain chargeable numbers, such as 900 numbers and to certain
area codes that are known destinations for toll thieves.
Many systems provide an override feature that enables a user to dial an
access code and identification number. This removes the restriction from a phone
for the current session, and restores it when the call terminates. Another common
feature is time-of-day restriction, which leaves phones open during working
hours, but restricts them after hours.
Follow-Me Forwarding
With the increasing importance of telecommuting, several manufacturers are
offering this feature, which allows the user to receive telephone calls at home, on
a cell phone, or in a remote location such as a conference center. The user keeps
the PBX informed of his or her location, and the PBX forwards calls accordingly.
With caller ID and the appropriate programming, the system can screen calls as
well, and forward calls from only selected users. Forwarding can be selective
depending on time of day and day of week. At the user’s option, the system can
be programmed to ring to different destinations simultaneously or sequentially in
patterns that can be changed for different days or times. This feature is available
from both TDM and IP systems, usually as an extra-cost option. It requires a central
server that typically is accessed with a browser over the Web or a private IP
network. The protocol is usually proprietary on TDM systems and either proprietary
or SIP in IP systems. The most effective products link the application to an
electronic calendar.
When a user is available via e-mail, some applications can download voice
mail as e-mail file attachments that the user can play back on a laptop computer.
This is usually a feature of unified messaging (UM), which is discussed briefly
later in this chapter and in more detail in Chapter 29.
Call Detail Recording (CDR)
This feature, sometimes known as station message detail recording (SMDR), in
combination with a call accounting system provides the equivalent of a detailed
toll statement for PBX users. Many businesses require call detail to control
long distance usage and to spread costs among the user departments. The CDR
port is a serial connection that outputs the raw call details in ASCII using a
proprietary field format. A call accounting system, which is discussed later in
this chapter, connected to the serial port parses the detail, rates the calls, and
formats various management reports such as budgetary detail and individual
toll statements.
Voice Mail
Voice mail (see Chapter 28) is available as an optional feature of all PBXs and
hybrids and is one that is almost invariably applied. When a station is busy or
unattended, the caller can leave a message, which is stored digitally on a hard
disk. The station user can dial an access and identification code to retrieve the
message. Most voice-mail systems include automated attendant, an option that
enables callers with a DTMF dial to route their own calls within the system.
Incoming calls are greeted with an announcement that invites them to dial the
extension number if they know it or to stay on the line for an attendant. Most
voice-mail systems also support dial by name for callers who reach the automated
attendant and do not know the extension number.
Dialed Number Identification System (DNIS)
Offered by IXCs along with T1/E1-based toll-free services, DNIS provides the
equivalent of DID for toll-free calls. If multiple toll-free numbers are terminated
on the same switch, DNIS digits are sent with the call to identify the number
dialed to the PBX so it can route the call to the appropriate station or group. DNIS
enables an organization to have several toll-free numbers and to route each call to
a different station, UCD or ACD hunt group, voice mail, or any other destination
within the PBX. For example, if a company has different ACD groups for sales,
service, and order inquiry, it can assign each of these groups a different toll-free
number and use DNIS to route the calls appropriately. This alternative is often
more effective than using an auto attendant to answer the call and offer a menu of
choices.
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