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A Probe into Basic Telephony and
Jon F. Johnson
Extension Area Specialist
Virginia Cooperative Extension
• The making and operation of
telephones – Webster’s New
World Dictionary
• More advanced products, services, and
applications of traditional telephone
services that not only carry voice traffic but
data as well
Talking Beyond the Teleset…
Cable media – copper, fiber optics, microwave
The POTS line - Plain Old Telephone Service
Analogue and Digital Telephone Signals
Circuit Switched Operations
The C.O. – The Central Office
PSTN – The Public Switched Telephone
• Traditional and enhanced TELCO (Telephone
Company) services
Analog Telephone Signals…
• Analog transmissions began with invention of
the telephone in 1876
• Analog signals move along a telephone line as
an electromagnetic wave
• Analog signal strength is measured by
frequency and requires amplification as it
• Analog signals can easily pick-up electrical
interference or noise
Digital Telephone Signals…
• Digital signals are transmitted as binary
bits (binary means two values for bits: on
and off)
• Digital “on” bits carried by copper medium
represent positive voltage while “off” bits
represent no voltage
• Bits make Bytes and the amount of
transferred Bytes determines Bandwidth
• The measure of the capacity of a
communications channel (path).
• Digital channels are measured in bits per
• Analog telephone lines are measured by
hertz (cycle of a wave from crest to trough)
Digital versus Analog…
• Digital signals are easily recreated as they
travel along a telephone line by digital
regenerators or repeaters
• Unlike analog amplification, noise (static
from electrical interference) is discarded
• Digital service is more reliable and
requires less equipment to boost signals
Switching defined…
• The establishment on demand, of an
individual connection from a caller to a
desired receiver … as long as is
required for the transfer of information.
- International Telecommunications Union (ITU)
The Public Switched Telephone
Network (PSTN)
The Central Office (CO)
The Tandem Office
The Toll Office
Interexchange Carrier’s (IEXs) Point of
Presence (POP)
The CO defined…
• A telephone company facility that houses
the local switching equipment and serves
as the place of termination for the wires
from all telephones (the local loop) in a
specified geographical area served by that
telephone company.
• The Exchange!
Who’s in the CO?
• The Local Exchange Carrier or LEC
• CLEC or Competitive Local Exchange
Carriers - Maybe?
• Interexchange Carriers or IEX (Long
distance services providers)
• Local ISPs - Internet Service Providers
• Physical location for Interexchange
carriers (MCI, Sprint, AT&T, etc.) or IEXs
to have their Point of Presence or POP
• The POP is the connection between the
incumbent network and the carrier’s
An ISP is…
• Internet Service Provider connects endusers to the Internet via telephone lines,
cable media, or other media.
ISP Services
• ISPs often supply services such as voice
mail, web hosting, and domain name
registration for websites
• ISDN – Integrated Services Digital
Network (protocol) via twisted pair
Some Telco Services
• Value added services (voice mail, call waiting,
conference calling, etc.)
• DSL (Digital Subscriber Line)
• ISDN (BRI and PRI)
• ATM (Asynchronous Transmission Mode)
• Frame Relay
• T1, T3, DS1, DS3
• Intrastate and interstate connections
• Long-distance service
Transmission Media
• T1 uses 2 copper pairs
• DS1 uses fiber optics with the same bandwidth
as a 2 copper pairs
• Fiber-optics cable is made from glass that
transfers light and not electricity and is used for
higher-speed transmissions
• Coaxial cable is capable of delivering up to
10Mbps or more (voice, data, & video)
• Ethernet cabling typically uses 2 of 4 pairs of
Category 5 or 6 cable to deliver 10 or 100Mbps
of bandwidth
Important Protocols for Most
• DSL – Digital Subscriber Line (a glorified
twisted pair with broadband capabilities
128kps to 6 Mbps with distance
Residential or Business?
• POTS basic service - $20/mth
• Dial-up internet access - $15 to $30/mth with
56kps max. capability
• DSL basic service - $50/mth with 6Mbps to
128kps (download and upload) with distance
limitations Note: DSL carries voice and data
• Broadband cable internet service - $70/mth with
10Mbps, add $18/mth for basic voice and more
for TV signal Note: broadband is a shared
Government, Business,
Educational Uses
• T1 lines are most commonly used by larger
business and government agencies with greater
bandwidth needs
• T1 and DS1 lines can range from $700 to
$1000+/mth, depending on competition, and
offer up to 1.54Mbps (64kps/channel)
• T3 and DS3 offer 44.7 Mps or the equivalent of
28 T1 (that’s is a lot of money)
Special Notes and Recommended
• Pricing is relevant to September 2002 and
is based on the Richmond Metro Area
• The Essential Guide to Telecom by
Annabel Dodd
• Verizon Communications at