1. Unit-4 Telephone system

2. What we cover
Telephony
Telecommunications

3. Introduction either data communications or voice communications.
Students used to focus on
either data communications
or voice communications.
Today, the two fields are merging.
Most voice systems are computer controlled and data networks support voice.
Anyone studying the field of data communications and networks must learn some basic telecommunications too.
Telecommunications used to be just “voice.”

4. Basic Telephone Systems
POTS is the plain old telephone system that connects most homes and small businesses.
POTS lines were designed to transmit the human voice, which has a bandwidth less than 4000 Hz.
A telephone conversation requires two channels, each occupying 4000 Hz.

5. Various telelphone channels and their
assignment of frequencies

6. Basic Telephone Systems Limitations
A 4000 Hz analog signal can only carry about 33,600 bits per second of information while a 4000 Hz digital signal can carry about 56,000 bits per second.
If you want to send information faster, you need a signal with a higher frequency.
POTS cannot deliver faster signals.

7. Basic Telephone Systems - Loops
The local loop is the telephone line that runs from the telephone company’s central office to your home or business.
The central office is the building that houses the telephone company’s switching equipment and provides a local dial tone on your telephone.
If you place a long distance call, the central office passes your telephone call off to a long distance provider.

8. Local Loops

9. Local Access Transport Areas
The USA is divided into a few hundred local access transport areas (LATAs).
If your call goes from one LATA to another, it is a long distance call and is handled by a long distance telephone company.
If your call stays within a LATA, it is a local distance call and is handled by a local telephone company.

10. Trunks
A trunk is a special telephone line that runs between central offices and other telephone company switching centers.
A trunk is usually digital, high speed, and carries multiple telephone circuits.
A trunk is typically a 4-wire circuit, while a telephone line is a 2-wire circuit.
Not associated with a single telephone number like a line is.
A telephone number consists of an area code, an exchange, and a subscriber extension.
The area code and exchange must start with the digits 2-9 to separate them from long distance and operator services.

11. Composition of a Telephone Number

12. Telephone Connections
When the telephone company installs a line, it must not proceed any further than 12 inches into the building. This point is the demarcation point, or demarc.
Modular connectors, such as the RJ-11, are commonly used to interconnect telephone lines and the telephone handset to the base.
When the handset is lifted off the base (off-hook), an off-hook signal is sent to the central office.

13. Basic Telephone Systems
Data Communications and Computer Networks
Chapter 12
Basic Telephone Systems
When the off-hook signal arrives at the central office, a dial tone is generated and returned to the telephone.
When the user hears the dial tone, they dial (or press) the number.
The central office equipment collects the dialed digits, and proceeds to place the appropriate call.

14. Basic Telephone Systems Services
Data Communications and Computer Networks
Chapter 12
Basic Telephone Systems Services
Foreign exchange service (FX) - customer calls a local number which is then connected to a leased line to a remote site.
Wide area telecommunications services (WATS) - discount volume calling to local and long distance sites.
Off premises extensions (OPX) - dial tone at location B comes from the PBX at location A.

15. The Telephone Before and After 1984
Data Communications and Computer Networks
Chapter 12
The Telephone Before and After 1984
In 1984, the U.S. government broke up AT&T.
Before then, AT&T owned a large majority of all local telephone circuits and all the long distance service.
With the Modified Final Judgment of 1984, AT&T had to split off the local telephone companies from the long distance company.
The local telephone companies formed seven Regional Bell Operating Companies. Today, there are only 4 left: Bell South, SBC, Qwest (US West), and Verizon (Bell Atlantic).

16. Data Communications and Computer Networks
Chapter 12

17. ISDN Architecure Devices
NT1
±TE1 -
ISDN
devices

19. Terminal Equipment type 1 (TE1)*
ISDN compatible device (Router with ISDN Interface)
* TE1s connect to the ISDN network through a four-wire,
twisted-pair digital link
Terminal Equipment type 2 (TE2)
ISDN Non-compatible devices.
*Will require a terminal adapter.
Terminal Adapter (TA)
*Converts standard electrical signals into the form used by ISDN
Needed for connection with TE2 devices