Telephony

Principles of Telephony

The Telephone is:
–	The basic instrument of all communications technology
–	There are four primary types of data communications:
–	Voice, data, message, and image
–	Voice communication is Telephony
–	But the other kinds of communications make heavy use of phone lines

The Telephone has five parts:
–	Transmitter: is really a microphone
–	Receiver: is really a speaker
–	Ringer: bells, whistles, horns, buzzers, and musical interludes
–	Control unit: push buttons or rotary dial
–	Switchhook: signals the phone company and caller that the phone is either idle or in use

Transmission of Sound:
–	Sound waves are converted to electrical energy
–	Transmitted over wires or another medium
–	Converted back to sound
–	Transmission is an analog signal
–	Frequency range is from 300 to 3,400 Hz

Analog vs. Digital Transmission:
–	Analog is a continuous and continuously varying flow of electrical energy that varies with the frequency and strength of the sound
–	Digital is a stream of discrete on and off pulses called bits
–	Uses a technique called Pulse Code Modulation
–	Conversion from one to the other is done with a modem
–	It performs the function of modulation and demodulation
–	Digital is better quality because
–	No noise and distortion
–	More efficient and faster
–	More economical in the long run

Telephone Lines:
–	Every telephone line is a twisted pair of wires (2X)
–	Electricity for the phone is delivered via the 2X wire
–	2X wires connect phones to a central office
–	This local loop consist of three parts
–	Drop wire -- from house to pole to a:
–	Distribution cable -- runs down your street on poles to a:
–	Feeder cable -- connects to the central office
–	These are often underground nowadays
–	Trunk lines connect one central office to another

Central Offices:
–	Originally, every phone went from one phone directly to another
–	As the number of phones increased, this became unworkable, plus:
–	Telephones were more valuable if they connected to multiple phones
–	Switching is connecting one circuit to another
–	The central office began as a switching center
–	Each central office serves a specific geographic area
–	The size of area depends on the number of lines
–	The central office provides links to other central offices and to long distance providers
–	During times of unusually high use, central offices may be overwhelmed -- this results in blocking

Automatic Switching:
–	In the early days, switching required operators.
–	Automatic exchanges began to replace operators in the early 20s
–	The Strowger switch was fundamental to switching for decades
–	Banks of relays and switches occupying large buildings reaching from floor to ceiling
–	Technology improved from step-by-step system (10,000 users) to panel system (30,000 users), but both were mechanical and required mucho maintenance
–	Then came electronic switching systems
–	Minimum of 100,000 lines, low maintenance
–	Stored program control switching allows a variety of revenue-enhancing programmable services

Telephone Numbers

Nationwide Numbering Plan:
–	Nation is divided into numbering plan areas, each with an area code
–	Seven digits to dial within an area, ten outside the area
–	"1" signals long distance and "0" calls the operator, so 2-9 are left to begin prefixes
–	Mathematically no more than 8 million numbers per area code
–	Private phone systems may have other single-digit codes, such as "9" to get an outside line

World Numbering Plan:
–	International Telecommunications Union
–	Is a United Nations Agency
–	The ITU-T (as the CCITT) established world-wide numbering plan
–	There are nine international zones with prefixes beginning 1-9 which may be followed by another one or two numbers
–	Highest use areas receive single-digit codes
–	North America has "1"

Telephone Systems

Single-Line Systems:
–	Consist of an individual line and one or more telephones
–	each line can support several extensions
–	may be residential or small business, but business charges are higher
–	largest market for phone business
–	capable of numerous service features, as well as service inherent in the phone

Key Telephone Systems:
–	For the small business user
–	Permits multiple lines to terminate on one telephone with the punch of a button
–	Many features are inherent in the system, such as hold and call pickup

Private Branch Exchanges (PBX)
–	On-premises computer-based switching system for large organizations
–	Calls between stations are dialed directly, usually with a single-digit prefix
–	Can be purchased, leased, or rented from AT&T, RBOCs and many other vendors
–	Many highly-specialized services can be offered
–	Centrex is a service offered by telcos where the switch remains at central office but is dedicated to one business

Telephone Sets

Telephone Sets and Dialing:
–	Only basic black phones were available until the late 50s
–	Now there are a wide variety, with easy modular hook-ups
–	Rotary phones used dial pulsing signaling -- one digit per second
–	Push-button phones use Dual Tone Multifrequency (DTMF) -- sounds as a chord -- 10 digits per second
–	Much faster dialing speeds
–	Better dialing accuracy for mechanical and mnemonic reasons
–	Necessary for "talking" to computers

Cordless Phones:
–	Base station connected to phone line and cordless handset
–	Uses low-power radio transmission
–	Absolutely no expectations for privacy

Cellular and PCS
–	Cellular may be analog or digital, but is primarily digital since 1997-98
–	Must be within range of a compatible cell tower
–	Personal Communications Systems (PCS) are digital services
–	May be a satellite-based service
–	Incorporates phone service with paging, e-mail, Internet access