PBX FEATURES
This section discusses the main features that most PBXs and many hybrids, both
TDM and IP, support. These features are in addition to the key system features
discussed in the previous section. Two features on the key system list, flash and
paging, are generally available on hybrids but unavailable on PBXs. Although the
features in this section are common to most PBXs, users will find operational differences
among products. There are also differences in whether the features are
standard or an extra-cost option.
Direct Inward Dialing (DID)
DID offers station users the ability to receive calls from outside the system without
going through the attendant. The LEC’s central office contains a software table
with the location of the DID trunk group. When a call for a DID number arrives,
the central office seizes an idle trunk and outpulses the extension number, usually
with DTMF tones or over a channel. DID is effective in reducing the load on PBX
attendants. It also enables users to receive calls when the switchboard is closed.
DID is provided on both analog and digital trunks. On analog trunks a separate
trunk group is required. Digital trunks may be provisioned as tie lines between
the PBX and the central office to provide two-way DID. PRI trunks offer call-bycall
service selection, enabling any trunk to be used for any purpose.
Automatic Route Selection (ARS)
Most PBXs terminate a combination of public switched and private trunks on the
system. For example, in addition to local trunks, the PBX may terminate T1/E1
lines to the IXC, FEX lines, and tie trunks to another PBX. Educating users about
which service to use is impractical, particularly as rates vary with time of day and
terminating location, and the dialing plan varies with the type of service. It is a
reasonably simple matter, however, to program route selection into the central
processor of the PBX. With ARS, sometimes called least-cost routing (LCR), the
user dials the number and the system determines the preferred route and dials the
digits to complete the call over the appropriate trunk group.
The most sophisticated ARS systems can screen calls on the entire dialed
number, but some simple systems, typical of hybrids, can screen on only the NPA
and prefix. The ability to screen on the entire number is important for many companies.
With it, for example, it is possible to allow users to dial some 900 numbers,
but deny others. If a company has an IP gateway that enables it to call other company
numbers in an overseas location, the ARS can route those calls to the gateway,
and domestic calls to the PSTN. ARS can also select the trunk group based
on class of service. One class could call internationally only over IP trunks, while
international calls for another class always use the PSTN.
A related issue is digit insertion and deletion. Some services, such as FEX,
may require the PBX to insert or delete an area code for correct routing. Telephone
service is easiest for users if they always dial the same way regardless of the route
the call takes. For example, if the PBX has FEX trunks to another area code, the
user would dial the area code, but the PBX would strip it off before passing the
digits forward to the FEX trunks if the central office does not require all 10 digits.
Users cannot be expected to understand the logic of this arrangement.
Networking Options
Most PBXs offer networking options, which allow multiple PBXs to operate as a
single system. Networking is available on most PBXs, but it is rare in hybrids.
Call-processing messages pass between PBXs over a separate data channel using
IP messages or some form of common channel signaling. With the networking
option, call-processing information such as a station’s identification and class of
service travel across the network to permit features to operate in a distant PBX as
they do in the local system. This feature is known as traveling class mark.
The objective of networking is to provide complete feature transparency,
which is the ability of users to have the same calling features across the network
as they have at the main PBX. For example, users want to be able to camp on a
busy station, regardless of whether it is in their PBX or in a distant system, and
they want to share a voice-mail system across the network. Some features do not
work across a network in some products. Call pickup, for example, enables a user
who hears a ringing telephone to press a button and bring the call to his or her
telephone. The lack of this feature across a network is usually unimportant since
users are normally in separate locations and cannot hear the bell. Some companies,
however, start with separate systems in separate locations and later merge
them. The PBXs are collocated in the same equipment room and remain networked
together. If features such as call pickup do not work across the network,
users in one work group must be assigned to the same switch, which often
requires moving people from one PBX to another and possibly changing numbers.
TDM and IP PBXs have significant differences in the way they implement
networking. Figure 24-1 illustrates some of the differences. In the top half of
the figure three TDM switches are used. Each switch has a unique number range
from a different central office and a local trunk group to that office. The database
in each switch contains the details on each number range in its domain. Its
ARS knows which trunk group to use to reach an extension in either of the other
switches, but it does not contain the translations for the stations in the other
switches.
The IP configuration, by contrast, has three servers, each with an identical
database. If one of the servers fails, the other servers can support its stations,
which are attached directly to the LAN behind the routers. Each server has direct
access into the IP network, but the connectionless nature of IP enables any switch
to set up a path to either of the other switches. This configuration provides survivability
that the TDM model lacks. It also enables users to move between
switches while retaining their telephone numbers
No comments:
Post a Comment