connect to the Internet as a point-of-presence
(POP) on a T1 line owned by a major telephone network. Many
businesses also use T1 (and T3) lines to connect to an Internet
access provider. ISDN provides digital
transmission over ordinary telephone copper wire as well as
over other media. - Conceptually, ISDN integrates both
analog or voice traffic with digital data over the same
network. Home and business users receive data at rates up to
twice the rate of a modem connection.
- ISDN requires
adapters at both ends of the transmission (customer premises
and service provider end). Most telephone companies in urban
areas in North American and Europe can provide ISDN. In many
areas where service providers offer digital subscriber line
(DSL) and cable modem service, ISDN is no longer a popular
option.
- There are two levels of ISDN service. Both
rates include a number of B-channels and a D-channel. Each
B-channel carries data, voice, and other services at a rate of
64 kbps. Each D-channel carries control and signaling
information:
- Basic Rate Interface (BRI) is for home
and small business use. BRI contains two B-channels and one
D-channel.
- Primary Rate Interface (PRI) is for larger
users. PRI includes 23 B-channels in North America and Japan
(30 in Europe) and one D-channel.
- Service
providers usually transmit PRI service through a T1 line in
North America and an E1 line in Europe.
- From a
consumer point of view, and as a data connection service, DSL
replaces ISDN.
- ISDN is also used as a smart network
technology intended to add new services to the PSTN by giving
users direct access to end-to-end circuit-switched digital
services.
- ISDN PRI is commonly used in recording
studios when a voice-over actor is in one studio but the
director and producer are in a studio at another location.
ISDN is used because of its "guaranteed" real-time
service that does not rely on the Internet and audio fidelity
that is superior to POTS service. A few companies make video
conference call equipment that combines 3 BRI lines (six 64K
channels), to create a good quality picture.
Content 1.1 The Evolution of Telephony in
the Enterprise 1.1.4 Digital Telephone
Services When the computerization of electronic telephone
exchanges began in the 1970s and 1980s, many new network
services became available. Here are some of the digital
telephone services that were developed: - Voice
mail
- Caller ID
- Call waiting
- Reminder
calls
- (Three-way) conference calling
- Enhanced
911 (U.S.)
- Central office exchange service
(Centrex)
Content 1.1 The Evolution
of Telephony in the Enterprise 1.1.5 PBXs and
Centrex Services Many enterprises use a private branch
exchange (PBX). Medium-sized and larger companies use a PBX
because it is much less expensive than connecting an external
telephone line to every telephone in the organization. In
addition, it is easier to make internal calls within a PBX
using only three or four digits. Many telephones can be
attached to a PBX, but users can only share a certain number of
outside lines for external calling. Usually to secure an
outside line, the caller dials the digit ”9” before the full
outside telephone number. For companies that are not willing or
able to invest in PBX technology, many telephone companies
offer central office exchange service (Centrex). Centrex is a
virtual PBX with all the switching occurring at the telephone
office instead of at the company's premises. Typically, the
telephone company owns and manages all the communications
equipment that is needed to implement the PBX and then sells
various services to the company. Here is a list of some typical
Centrex services: - Call Transfer
- Call
Divert—on no reply or on busy calls
- Call
Waiting
- Three-Party Conference
- Call Pick Up
(Group)
- Ring Back
- Reminder or Alarm
Call
- Last Number Redial
- Centrex Hotline
(non-dialed connection)
- Centrex Warm Line (delayed
Hotline)
- Centrex Hunt Groups, with optional bypass
numbers
Centrex customers can choose from a variety
of special services and features: - Packaged
Centrex: Packaged Centrex customers are offered a fixed set
of features in different packages. These packages are offered
at a relatively low cost, as little or no customization is
permitted. The lack of customization minimizes the operational
costs of programming and maintaining the services.
-
Centrex Data: Centrex Data services can provide
relatively low speed (56 and 64 kbps) data services using the
circuit-switched telephone network. Although they are now
overshadowed by Internet and other data networks, Centrex Data
services can offer very flexible and wide-reaching network
configurations since connections can be made almost anywhere
within the reach of the telephone network.
-
Customized Centrex: Customized Centrex offers a highly
customizable set of options that require specialized
programming and troubleshooting skills to maintain. A typical
Customized Centrex setup allows four digit dialing between
people in the same network, called "locals"
(sometimes even if the locals are located in different parts of
a city), customized routing through the telephone network (such
as Least Cost Routing or Time of Day Routing), and customized
codes for invoking features. Customized Centrex is the most
flexible, but usually the most expensive service
offering.
Content 1.1 The Evolution
of Telephony in the Enterprise 1.1.6
Long-Distance Services Enterprises increasingly rely on
their ability to communicate directly with customers,
suppliers, partners, branches, and teleworkers. Long-distance
services represent a significant expense. When long-distance
costs were relatively high, companies kept control over
long-distance access. Figure shows a historical example of how
operator intervention was required for a caller inside the PBX
to access long-distance service. Long-distance trunk lines
connecting exchanges usually use TDM technologies and T1 or E1
transport. The most common long-distance service that is
offered in North America is Wide Area Telephone Service (WATS).
Other parts of the world have similar services, see Figure .
WATS-type plans provide access to long-distance telephone lines
for commercial use at reduced rates. WATS is outward
(OUT-WATS), inward (IN-WATS), or both: - With outward
WATS, the calling party can make an unlimited number of
long-distance calls (toll calls) for a fixed price within
pre-determined time and distance constraints. Most telephone
companies now market OUT-WATS as something called a flat-rate
plan. Initially, operator assistance was necessary to control
access to OUT-WATS from within a PBX. Following company policy,
callers asked the company operator at their PBX to connect them
to the “tie-line,” or the OUT-WATS line that tied into the long
distance system. However, as long distance calling rates fell,
operator intervention has almost disappeared. Callers making a
call from inside a PBX usually only need to add a prefix to
gain access to outside long-distance numbers.
- With
IN-WATS, subscribers have a toll-free telephone number. IN-WATS
service reduces the time that operators spend processing
toll-collect calls for businesses. The IN-WATS area codes in
North America are 800, 888, 877, and 866. More 800 numbers are
reserved for the future (855, 844, 833 and 822). Telephone
users within a designated area can call the IN-WATS telephone
number of an organization without having to pay a toll charge.
Many businesses have toll-free 800 numbers that can ring at
multiple call-centers.
Content 1.1
The Evolution of Telephony in the Enterprise
1.1.7 The Concept of Convergence Before advanced
networking technologies allowed convergence, enterprises