Basics of Telephony

Overview


Thou Art Welcome To The Dungeons Of Telephony!

The Dark Labirynths Of Twisted Pairs And Local Exchange Castles Thou Shall Pass To Proceed To The Next Level! Beware of Angry Trolls!

Godspeed!

Phone
A telephone remains one of the simplest devices ever



Making A Phone Call
Step 1: Picking up the handset completes the circuit between the phone and the local exchange office

Step 2: Call leaves through the pair of copper wires connected from your house to the "entrance bridge"

Step 3: Entrance bridge then proceeds the call to the digital concentrator, whicn digitizes the the signal at a sample rate of 8,000 samples per second and 8-bit resolution. Step 4: Concentrator sends the digitized signal to the local exchange, which then routes the call to the main exchange.

(NOTE: If the phone you are calling is connected to the same local exchange switch then it just creates the loop between your phone and the phone you are calling skipping the main exchange.)

Step 5: Main exchange then may reroute the call to another main exchage or to an international exchange depending on where the call is going. International calls are sent through the satellites or undersea cables.

Visualization

Long Distance Call
Long distance calls travel through the following route: Local Office Switch (access database, look up PIC) -> Long Distance Switch -> Local Carrier.

PIC Code - Primary Interchange Carrier Code, indicates the long distance carrier

Public Switched Telephone Network
PSTN, also refered to as POTS (Plain Old Telephone System), is the network of the world's public circuit-switched telephone networks.

Functions of a network

 * access


 * transport


 * switching


 * service delivery

Components of a PSTN

 * signaling
 * access and switching
 * transport
 * customer-premises equipment (CPE)

Regional Bell Operating Companies
RBOC's are the local exchange service operating companies originally derived from AT&T Corp. in 1984. Originally there were 7 "Baby Bells", but many of the merged leaving only 3 RBOC's today: AT&T, Verizon, and Qwest.



Local Access & Transport Area (LATA) - area within which RBOC is permitted to offer exchange telecom and access services.

Exchange Carriers
Local Echange Carrier (LEC) - local telephone company Interexchange Carrier (IXS or IEC) - long distance telephone company Local Loop - Line from local telephone company to the end user's premis (access line)
 * Incumbent LEC - existed before RBOC's split
 * Competitive LEC- competes with other already estblished companies
 * Point Of Presence (POP) - area where IXC could terminate services and provide connections into LATA

Carrier Identification Code (CIC) - a code used for billing and call routing

Four basic function of signaling

 * 1) Alerting
 * 2) Transmitting information signals
 * 3) Transmitting address info
 * 4) Supervising circuit status, network management

Alerting (someone's calling): Ringer - 20 Hz sine wave, 2 seconds on, 4 seconds off

Info Signals: Dial tone, Ring Back tone, Line Busy tone, Trunk Busy tone

Addressing: Dual Tone Multy Frequency (DTMF) - each number on the keypad is a combination of two different frequencies. Numbering Plan:  Ex: 1 517 580 5555
 * International Dialing Code - Country Code
 * Area code (first digit restricted to 2-9) - 3 digits
 * Exchange Code (first digit restricted to 2-9) - 3 digits
 * Subscriver Code - 4 digits

Modern Signaling Systems: SS7
Signaling System #7 (SS7) is a set of telephony signaling protocols which are used to set up most of the world's public switched telephone network telephone calls.

In-band signaling vs. Out-of-band signaling
In-band - voice and signaling are transferred through the same channel (Ex: DTMF)

Out-of-band - voice and signaling data travel on separate trunks (Ex: SS7)

Switching

 * Public vs. PrivatePSTN_Switching.png
 * Manual vs. Electronic
 * Analog vs. Digital
 * Circuit Switched vs. Packet Switched

Transport
Carriage of conversations over trunks multiplexed T1 combines 24 separate voice channels onto a single link. The T1 data stream is broken into frames consisting of a single framing bit plus 24 channels of 8-bit bytes.
 * digital
 * T1 - most common trunk

PBX
Private Branch Exchange - telephone exchange that serves a particular business or office; moves a portion of a Central Office to a customer premise

KTS
Allowed the station user to see and control the calls directly (lighted line buttons)

CENTREX

 * PBX-like
 * CO operated
 * PROS: Investment and operating cost, Networking
 * CONS:Limited features, Lack of control

The Grand Finale
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