chapter24

Direct Inward System Access (DISA)
The DISA feature enables external callers to dial a telephone number and password
to gain access to PBX features. The DISA port can be restricted to limit calls
to internal extensions, tie lines, local calls, or any other restriction level used in the
PBX. If the DISA port is unrestricted, callers can gain access to long distance services.
DISA helps reduce credit card calls by enabling users outside the PBX to
access low-cost long distance services.
Security is an obvious problem with DISA. It is one of the most prevalent targets
for toll thieves, who use it to place calls at the company’s expense. The best
practice is to disable DISA. If it must be used, managers should change the password
frequently and check the call accounting system for evidence of misuse.
N × 64 Capability
With the growth of video conferencing, it is often desirable to dial more bandwidth
than an ordinary BRI connection provides. Conference-quality video usually
requires at least 384 Kbps, which is six 64 Kbps channels. A PBX with N × 64
capability enables the user to dial as many channels of contiguous bandwidth as
required.
Centralized Attendant Service (CAS)
CAS enables attendants at one location to handle attendant functions for remote
PBXs over a network. Although each PBX has its own group of trunks, all calls
routed to the attendant flow to the centralized location over the network. The
attendant can terminate the call to any station or hunt group. A related feature is
release-link trunk, which enables the PBX to release the attendant trunk after setting
up the call. Without this feature the trunk is tied up for the duration of the call.
Power-Fail Transfer
Unless a PBX is configured to run from batteries or from an uninterruptible power
supply, a commercial power failure will cause the system to fail. The power-failtransfer
feature connects central office trunks to standard DTMF telephones. Since
most PBXs require ground-start trunks, provisions must be made to operate from
loop-start telephones. This can be accomplished by two methods: use a separate
loop-start-to-ground-start converter or equip the telephones with a ground start
button. The former method is prevalent.
Power-fail transfer is an inexpensive and effective way to obtain minimum
service during power failure conditions. Even users of systems with battery
backup or UPS should consider power-fail transfer to retain some service if the
PBX itself fails. Some manufacturers offer power-fail transfer for digital or ISDN
trunks, which can enable the owner to avoid analog trunks.
Automatic Call Distribution (ACD)
ACD enables PBXs to route incoming calls to a group of service positions. Typical
applications are sales and customer service positions. Incoming calls route to an
agent position based on logic programmed into the switch. Calls can be routed
based on the toll-free number that was dialed using DNIS. The caller’s telephone
number may be delivered by the network and used to route calls, or an automated
attendant or call-prompting software in the switch can prompt the caller to select
from a menu of routing options.
When agent positions are idle, the call routes to an agent immediately. If all
positions are occupied, the ACD holds calls in queue and notifies the caller by
recorded announcement that the call is being delayed. Calls can be overflowed to
other agent groups, routed to voice mail so the caller can request a callback, or
handled in a variety of different ways, which Chapter 27 discusses in more detail.
ACD is one of the most important features in a PBX, and is included in more than
three-fourths of the systems shipped.
Uniform Call Distribution (UCD)
UCD distributes calls evenly among a group of stations. When one or more active
stations are idle, incoming calls are directed to the station that is next in line to
receive a call. When all stations in the UCD group are busy, incoming calls are
answered with a recording and held in queue. When a UCD station becomes idle,
the call that has been in queue the longest is directed to the station. In many UCD
systems, a station user can toggle between active and inactive status by dialing a
code or pressing a feature button. Compared to ACD, UCD is unsophisticated,
lacking the supervisory, management, and reporting features that an ACD offers.
Chapter 27 discusses UCD further