abstract

The main purpose of both TDM and IP customer-premise switching systems is identical. They connect voice users in the office to one another and to external users over a pool of shared trunks. The differences lie in the way the two types of switch accomplish their objectives. This chapter discusses the principal features of CPE switching systems.
Most manufacturers offer separate key system and PBX product lines, although the functions may be nearly identical, at least in larger line sizes. Most TDM manufacturers also offer a cross between a PBX and a key system known as a hybrid.
sThese buttons and lamps define the following features, which are common to all key systems:
• Call pickup- Any station can access a line by pressing a line button.
• Call hold- A hold button (usually red) can be pressed to hold the line
in the central unit. By contrast, the hold button on a POTS phone holds
the line in the telephone so the line cannot be used for another call.
• Intercom- Acommon path shared by all telephones is used to announce calls.
• Supervisory signals- Lamps show when a line is ringing, in use, or on hold.
• Common bell- A bell common to all lines signals an incoming call. A slow
lamp flash shows which line is ringing.
• Automatic line selection: When the user picks up the phone, an outgoing
line is selected automatically.
• Bridged call appearance: The same extension number can be terminated
on multiple phone sets.
• Call drop: A call can be terminated without hanging up the receiver.
• Call forwarding: Users can forward their calls to another station
in the system.
• Call park: This feature places a call in a parking orbit so it can be
retrieved from any telephone in the system.
• Call transfer: An incoming or outgoing call can be transferred
to another user.
• Callback: If someone transfers a call to an extension that does not answer
after a set number of rings, the call returns to the original station.
• Camp-on: Users or the attendant can send an external call to another
telephone even if it is busy. The callee hears a faint camp-on tone.
When the user hangs up, the camped-on call rings at the station.
• Conferencing: Stations can bridge two or more lines together for
a multiparty conversation.
• Distinctive ringing: Different ringing tones enable users to distinguish
between internal and PSTN calls.
• Do not disturb: Users can press a button that silences the bell and
prevents intercom calls from reaching the station.
• Forward all calls: Users can redirect all calls to another station or
destination.
• Forward on busy or no answer: Users can redirect calls to another station
or destination if the line is busy or does not answer.
• Held-line reminder: After a call has been left on hold for a specified
period, the telephone emits a warning tone.
• Missed-call indicator: A list of unanswered calls is displayed
on the telephone.
• Music on hold: While a call is on hold, music or a promotional
announcement is played.
• Mute: A mute button on the telephone disables the microphone.
• Paging: Stations can page over the telephone speaker.
• Privacy: Prevents other stations from picking up a line that is in use.
In some systems privacy is automatic unless the user presses a privacy
release key.
• Station restriction: Stations can be assigned to different classes of service
for restricting long distance calls.
• Voice call: A user can place a call directly to the speaker of another user’s
telephone.
• Volume control: The volume of the handset, speaker, and ringer
can be adjusted.
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.
Direct Inward Dialing (DID)- DID offers station users the ability to receive calls from outside the system without going through the attendant.
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.
Networking Options- Most PBXs offer networking options, which allow multiple PBXs to operate as a
single system.
Direct Inward System Access (DISA)- The DISA feature enables external callers to dial a telephone number and password to gain access to PBX features.
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.
PBX Voice Features
As all PBXs are designed for voice switching service, they have features intended
for the convenience and productivity of the users. Not all the features listed below
are universally available, and many systems provide features not listed.
• Automatic call trace: Harassing or nuisance call can be traced to the origin
by dialing an access code.
• Call blocking: Users can selectively block calls such as specific extensions,
numbers, or calls from particular trunk groups.
• Call coverage: Users can have one or more coverage paths to direct
how calls route when the called station is busy, does not answer, or is
in do-not-disturb status. External calls can take a different path than
internal calls.
• Executive override: This feature allows a station to interrupt a busy line
or preempt a long distance trunk.
• Forced account code: On long distance calls, this feature prompts callers to
enter an identification code, which is registered on the CDR.
• Hoteling: A station user can temporarily move to another location, log in,
and have station features including the extension number follow to the
new location. Intervention from the administrator is not required.
• Paging access: The PBX can be equipped with paging trunks that
connect to an external paging system
• Personal call routing: Users can define routing of incoming calls based
on variables such as time of day, calling number, etc.
• Portable directory number: Allows a user on a networked PBX to move
from one switch to another without changing the telephone number.
• Priority ringing: A distinctive ring is used for calls from specified
numbers.
• Recorded announcements: This feature provides announcements for vacant
and disconnected numbers.
• Trunk answer any station: This feature allows stations to answer incoming
trunks when the attendant station is busy.
• Whisper page: A user can bridge into a call and speak to the local user
without the other end hearing.
Attendant Features
Most PBXs have attendant consoles for incoming call answer and supervision. The latter is increasingly popular because it can be easily integrated with a directory. The following features are important for most consoles and represent only a fraction of the features available.
• Attendant controlled conferencing: Attendant can set up multiport
conference calls.
• Automatic timed reminders: Alerts the attendant when a called line has not
answered within a prescribed time.
• Busy lamp field: When the station is busy or in do-not-disturb mode,
an LED associated with the station is lighted.
• Direct station selection (DSS): Allows the attendant to call stations by
pressing an illuminated button associated with the line.
• Directory features: Attendants with PC-based consoles may be able
to search by first and last name, department, and extension.
• Night service: Calls are automatically transferred to an alternate destination
when the console is closed.
System Administration Features
System administration is a costly element of every PBX, so features that ease
the administrator’s job are valuable. The following are some of the more popular
features.
• Automatic set relocation: Allows users to move their telephones from one
location to another without the need to retranslate. The administrator
gives users a code and instructions to carry the set to the new location,
plug it in, and dial the code.
• LDAP synchronization: Enables the system to update its PBX and voice
mail database from customer’s LDAP directory. Eliminates or reduces
redundant database entries
• Network move: Similar to automatic set relocation, this feature works
across a network, where automatic set relocation works only in the
same PBX.
TELECOMMUNICATIONS CONVERGENCE
Convergence: Merging real-time applications such as voice, video, and instant messaging together with data onto a single broadband infrastructure that is based on IP.
THE BUSINESS CASE CONVERGENCE
• Competitive Advantage- Businesses can tie customers to their internal processes through e-commerce.
• Lower Cost of Ownership- The cost equation is difficult to factor into the decision to migrate to a converged network.
• Flexibilty – The TDM PBX is a proprietary and inflexible device, closed in every respect except
for its CTI interface, which provides for limited call control.
• Create Value-Added Service- Convergence opens an enormous variety of opportunities to provide new services that are infeasible with circuit switching.
BARRIERS TO CONVERGENCE
New telecommunications developments have always been over-hyped, and with notable exceptions such as fiber optics, many of these have flared briefly and fizzled out.
• Network infrastructure- Carriers are converting portions of their network to VoIP, but islands of VoIP cannot support the performance and security that commercial-grade voice
• Flow control- Another key issue is congestion control, which is a vital feature of any voice or