Content Overview Government and
commercial organizations have used telephone services for over
100 years. During that time, technologies have advanced to
provide more services, more reliability, and lower cost to the
public and clients. In the past decade, with huge advances in
computing power and networking technology, the capability for
enterprises to leverage new opportunities demands that voice,
video, and data services be converged over a single network.
This module describes the concept of converged networks that
carry voice, video, and data. The module begins with a brief
description of telephone services that companies have come to
rely on prior to the movement toward converged networks. The
first lesson describes traditional telephone company services
and defines many of the legacy terms and concepts that affect
the network convergence technologies presented in this course.
The second lesson illustrates the concept of converged networks
through examples of architectures and network models that
accommodate the integrated services within converged networks.
The lesson explains the vision Cisco has of the Intelligent
Information Network (IIN), Cisco Service-Oriented Network
Architecture (SONA), and Cisco Enterprise Architectures, with a
focus on the traffic conditions and requirements imposed by
converged applications. Web Links At-a-Glance: What
is Cisco SONA?
http://www.cisco.com/application/pdf/en/us/guest
/
netsol/ns477/c643/cdccont_0900aecd8039b324.pdf
INTELLIGENT INFORMATION NETWORK; BUILDING BUSINESS
TRANSPARENCY AND AGILITY
http://www.cisco.com/warp/public/779/largeent/
landingPage/iin/pdf/cisco_refreshbrochure_05132_r2.pdf
Content 1.1 The Evolution of Telephony in the
Enterprise 1.1.1 A Basic Telephone System
Businesses have used the telephone for over 100 years. Over
this period, there have been many significant changes to the
equipment that is used and the companies who provide the
services. Today VoIP is rapidly becoming the dominant
technology. However, it is important to review past
technologies since they may have significant impact on network
deployment within the enterprise. A telephone system consists
of four elements as shown in Figure : - Each subscriber
uses a telephone set that converts sound into electrical
signals and then back again to sound. The base has a dial to
initiate a call and a bell or other signaling device to notify
the user that a call from another user is being received.
- One or more central switching facilities or telephone
exchanges interconnect groups of local subscribers. Modern
telephone exchanges use time-division multiplexing (TDM) to
digitize analog signals from subscribers. Exchanges assign
these digitized signals to the appropriate time slots.
Exchanges interconnect both digital data and voice
circuits.
- Inter-exchange transport facilities connect
multiple exchanges across telephone networks.
- Wiring
and cables connect subscribers to the exchange.
Figures and show examples of telephones over the years. There
are three ways for a subscriber to connect to the telephone
exchange: - By using dedicated physical wire connections
run in overhead or underground cables
- By radio
(cellular, satellite or radiotelephone)
- By using
VoIP
Figure shows the components of a modern
telephone system that connects across the world through a
variety of media.
Content 1.1 The Evolution of
Telephony in the Enterprise 1.1.2 Traditional
Telephone Company Services The Public Switched Telephone
Network (PSTN) in North America and the Postal, Telephone and
Telegraph (PTT) services, in much of the rest of the world,
carry analog voice over copper wires. This basic telephone
service is often called POTS. In some places, POTS means the
Post Office Telephone Service or Post Office Telephone System.
However, as legislation removed telephone services from the
control of national post offices, the term became more widely
known as meaning “plain old telephone service.” POTS remains
the basic form of residential and small business telephone
service nearly everywhere in the world. Until the introduction
of mobile telephones, POTS was the only telephone service that
most people knew. POTS has been available almost since the
introduction of the telephone system in the late 19th century.
Telephone service remained mostly unchanged to the average user
since the middle of the 20th century, despite the introduction
of electronic telephone exchanges into the PSTN and PTT. Here
are some of the POTS services that are available: -
Bi-directional or full duplex signals that carry the sound of
the human voice both ways at once
- Dial tone and
ringing signals
- Subscriber dialing
- Operator
services, such as directory assistance, long distance, and
conference calling assistance
- Power to the
telephone
Content 1.1 The Evolution
of Telephony in the Enterprise 1.1.3 Digital
Telephone Technologies The Public Switched Telephone
Network (PSTN) has evolved towards digital telephony and the
capacity and quality of the network has improved. In the 1970s,
telephone companies (telcos) began modifying their analog
telephone networks by upgrading the long-haul transmission
sections of their networks with fiber optic technology. Digital
transmission made it possible to carry multiple digitized
switched circuits on a single transmission medium (known as
multiplexing). Although the user equipment remains largely
analog-based, the switches at the telephone exchange (called
the analog voice signal switches) convert the analog signals to
digital signals. Recently, telcos began moving their digital
networks closer to the customer premises. This change has all
but relegated the analog local loop to legacy status.
Generally, telcos offer digital transport services in one of
two forms. Integrated Services Digital Network (ISDN) provides
a digital loop from the exchange (or central office) to the
customer premises. In North America and Japan, transmission 1
(T1) carrier technology provides trunk lines between exchanges.
Europe 1 (E1), used in Europe, is a similar carrier, though the
two services differ in these ways: - T1 and E1 lines
are dedicated telephone connections supporting high data rates.
The T1 carrier is the most commonly used digital transmission
service in the United States, Canada, and Japan. The E1
standard is used in Europe and other parts of the world.
- A T1 line has 24 individual channels, each of which is
capable of supporting one analog telephone call. The total
capacity of a T1 line is 1.544 Mbps. Large Internet service
providers (ISPs) commonly use transmission 3 (T3) lines that
provide 44.736 Mbps. T3 circuits are now commonly leased by
small and medium businesses.
- An E1 line has
thirty-two 64-kbps channels and supports an aggregate of 2.048
Mbps.
- T1 and E1 use pulse code modulation (PCM)
signals with time-division multiplexing (TDM). Each 64 kbps
channel can be configured to carry voice or data traffic. Many
telephone companies offer customers individual channels or
fractional T1 access. T1 lines originally used copper wire but
now also include optical and wireless media.
- T1 lines
are a popular leased line option for businesses connecting to
the Internet and for ISPs connecting to the Internet backbone.
In the early days of the Internet, it was common for ISPs to