Tài liệu Lecture 25: The Telephone System doc

Bell foreseen the problem of jumbling wires all over the city and formed the Bell Telephone Company, which opens its first switching office in Connecticut in 1878.. Houses and offices ar

The Telephone System: (Ref: Ch-4, Page-113 of your Text book)

The telephone system remains as the backbone of low-cost telecommunications services in North America An understanding of the telephone system is necessary when considering its use for data communications

Telephone: invented in 1874 by Alexander Graham Bell and patented in 1876

Single wire: In the beginning, the market was for telephones only It is the customer who used to

wire them between telephones using single wires and the earth as the ground or the other conductor to complete the circuit

Bell foreseen the problem of jumbling wires all over the city and formed the Bell Telephone

Company, which opens its first switching office in Connecticut in 1878

The original problem soon returned: to connect every switching office became unmanageable, so

second level switching offices were invented Eventually, the hierarchy grew to five levels

Copper: At one time, 80% 0f AT&T's capital was the Copper in the local loops, the largest

Copper mine

Now a days a variety of transmission media are used for telecommunication Local loops consist of

twisted pairs, although in the early days of telephony, uninsulated wires spaced 25cm apart on telephone poles were common Between switching offices, coaxial cables, microwaves, and especially fiber optics are widely used

Local loops still carry analog signals but all interexchange lines are carrying digital signal

In summary, the telephone system consists of three major components:

1 Local loops (twisted pair, analog signaling)

2 Trunks (fiber optics or microwaves, mostly digital)

3 Switching offices

We will next look at three different topics related to these three major telephone components:

4 How to send digital data over the local loops, quick answer is modem

5 How to collect multiple calls together and send them together, the answer is multiplexing

6 How to do switching, mechanical and digital switching

The telephone system (commonly called POTS for plain old telephone service) enables voice

communication between any two telephones within its network

Houses and offices are connected to a telephone company end office (central office class 5) by a set

of two twisted pair wires (called the local loop)

Local Loop: refers to 2-wire copper circuits brought to the end-user premises In the local

loop the 2 wires are referred to as

TIP Positive (Green wire)

On-Hook: When the phone is "on-hook" it is not connected to the circuit The

open-circuit voltage presented to the phone is a 48 volts DC

Off-Hook: When the phone is lifted or connected to the local loop, about 5 volts DC is

present at the phone and current flows in the local loop This current is detected in the "end office" or local switch by a relay resulting in the presentation of a dial tone on the circuit

Ring: When the phone is ringing, a 20 Hz 85-volt RMS voltage is present on the

line

Standard Rotary Telephone Handset (Type 500 Telephone)

Dialing is done by interrupting the line current at a 10 Hz rate 1 to 10 pulses are sent to dial digits 1 to 0

(Hayes modem command: ATDP)

Touch-Tone Telephone: (DTMF or Dual Tone Multiple Frequency)

Requires DTMF circuitry on the local telephone office switch It uses pairs

of frequencies (or "tones") to identify the digits of the number being dialed Two tones are used instead of one to avoid dialling by unwanted single tones generated acidentally in front of the mouth piece (Hayes modem command

to tone dial is ATDT)

1209 1336 1477

4

1

5

6

7

8

9

0

Telephone Switching Facilities:

• Switching machines (or "switches") are used to interconnect any pair of subscribers in the public telephone system Within a radius of about 10 km from the "end office" (local switching office)

up to 10,000 subscribers can be interconnected directly They share a common "exchange" or

switch This method of making connections is called Circuit Switching

759-XXXX

949-XXXX <====10,000 (= 9999) different phone numbers in each exchange 254-XXXX

Switching Center Hierarchy: (Fig 4.5, p.113)

In North America there is a hierarchy of switching centers based on the number of subscribers they handle:

Local Office

Can connect up to 10,000 subscribers in each exchange

(759-, 254, 949-, 942-, etc.) CLASS 4 Toll Center Sault Ste Marie Handles multiplexed calls

between exchanges

CLASS 3 Primary Center Sudbury (increased capacity to handle inter-toll

trunk lines)

In 705-949-8384 for example, the "705"

identifies the Primary center

CLASS 2 Sectional Centers Next level

CLASS 1 Regional Centers Major switches between regions There are

only 2 in Canada: one in Regina and one in

Montreal

The end office is connected to a central office class 4 by a trunk line

The central offices are arranged in a hierarchy; a class 4 is connected to a class 3 office which is connected

to a class 2 etc

The telephone system was originally designed as an analog system, but today, most trunk lines are digital

Types of Switches:

There have been 3 major types of telephone switches developed over the past 100 years

1 STEP-BY-STEP or Direct Progressive (used to be called Strowger Switch)

The oldest style of electromechanical switches They require a lot of space and maintenance

There are only a few of these switches left in service

• Generate line noise while switching calls, which causes errors in data communications

2 CROSS-BAR or Common-Control Switches

• They too are electromechanical in nature but have certain advantages over step-by-step

switches They consist of electrical contacts arranged in a grid

• They are faster, take up less space and are less expensive to maintain than step-by-step

switches

• They are rapidly being replaced by digital switches

3 DIGITAL SWITCHES (ESS or Electronic Switching Systems)

• Digital switches are basically computers using stored-program control to handle the

switching of calls

• They can handle 100,000s of subscribers at a time

• They are much faster, more reliable and flexible

• They don't generate electrical noise when switching making them more desirable for data

communications

• Because they handle so many subscribers it may be disastrous if they fail

• Virtually all common carriers now have digital switching

Leased Lines vs Dial-up Lines:

Leased Lines

• Leased phone lines (also called private circuits or dedicated circuits) are phone lines that are

available permanently (without having to dial) from one site to another

• They are leased on a monthly basis and you pay for them whether you use them or not Can be expensive unless usage is high

• You never get a busy signal, but you are always connected to the same site

• Ideal for permanent computer links between two sites that have significant traffic

like air-line reservation terminals or Internet connections

• Lines can be conditioned for high quality data transmission by having noise and distortion

minimized

Dial-up or Switched Circuits

• Normal dial-up phone lines are inexpensive but have some disadvantages:

• Calls may be blocked if there are no lines available or the line may be busy

• There is a dial-up delay

• Long-distance charges are based on usage so heavy use will be expensive

• May get poor quality connections since each time you dial a long-distance line you may get a different circuit

Store-and-Forward Switching:

If a called device is busy, the central switching office stores the incoming message and retransmits it to the destination when it becomes available Commonly used for data communications because it provides the highest level of throughput on circuits

Transmission Impairments:

Transmission lines suffer from three major problems:

Attenuation - is the loss of energy as the signal propagates outward The amount of energy lost depends

on the frequency Each Fourier component is attenuated by a different amount

Delay distortion - It is caused by the fact that different Fourier components travel at different speeds For

digital data, fast components from one bit may catch up and overtake slow components from the bit ahead, mixing the two bits and increasing the probability of incorrect reception

Noise - unwanted energy from the sources other than the transmitter Thermal noise, cross talk, impulsive

noise

• These transmission impairments and the bandwidth limitation makes digital signaling (have a wide spectrum) unsuitable for transmission over the local loop

• To get around the problems associated with the dc signaling or digital signaling, especially on

telephone lines, ac signaling is used A continuous tone in the range 1000Hz to 2000Hz, called a sine wave carrier is used Its amplitude, frequency, or phase can be modulated (changed with the

information) to transmit information

Brief Summary of PBX’s, Cellular Technology and Fax Machines

PBX: (Ref: Appendix-B, Page-426 of your Text book)

PBX (Private Branch Exchanges) is a switch also called a switchboard You can think it as a miniature version of the large scale switching exists in a Telephone company

PBXs (Private Branch Exchanges) are used by organizations to manage their voice (and sometimes their data) communications They reduce costs by reducing the number of connections to the telephone

company’s trunk lines Essentially a PBX is like a “smart” switch placed on your premises instead of the phone company’s

There are digital PBXs and wireless PBXs (operates just above 900MHZ)

Cellular Technology: (Ref: Appendix-C, Page-430 of your Text book)

Commonly used to provide wireless mobile (high frequency radio) telephone service but is also used for data communications, pagers and personal digital assistants (PDAs) Regions are divided into cells, each with its own antenna and mobile phones transmit and receive using the nearest antenna They are

automatically switched from cell to cell as they travel Using frequencies in the 870 to 890 MHz range

they are accessible by scanners and should not be considered private Cellular telephone fraud is

becoming common as user’s security codes are discovered and used illegally

FAX (Facsimile Machines):

A method or device for transmitting images (documents, drawings, photographs, or the like) by means of radio or telephone for exact reproduction elsewhere

The Basic operation of a FAX requires converting the image into binary bits for transmission This is achieved by scanning the image: a photoelectric scanning device built into the facsimile system shines light onto the page, and a special sensor reads the reflection of the light on a line by line basis

The smallest individual position on a line is known as a picture element A white area reflects light and sets the binary value of the picture element (PEL) to '1', while a black area absorbs light and sets the binary value of the PEL to '0' or vice-versa

A FAX has a horizontal resolution of 200 dots per inch (dpi) and the vertical resolution can be set to either

100 (standard) or 200 (fine) dpi

• Horizontal resolution - dot density in a line

• Vertical resolution - line density in a page

Evolved through 4 generations (ITU-T classifications):

Group 1 - analog, carrier frequency is 1700Hz, uses FM plus and minus of 400Hz For a letter size page it

takes about 6 minutes to transmit Data rate of 2400 bps

Group 2 - uses Vestigial Sideband Amplitude Modulation with suppressed carrier Takes 3 minutes for

transmitting a letter size paper The carrier frequency is 2100Hz Data rate of 9600 bps

Group 3 - uses 9600bps with Quadrature Amplitude Modulation Takes 1 minute for transmitting a letter

size paper

Group 4 - uses 14400bps with Trellis-coded Modulation Takes 30 seconds for transmitting a letter size

paper

FAX Class Designation:

To standardize the Computer-to-FAX interface, the EIA developed a series of recommendations Those recommendations are designed to standardize the commands used to control the Fax operation

• Class 1

• Class 2

Fax commands start with the prefix AT+F

e.g., AT+FCLASS? Means List the current mode setting

• A FAX machine scans an image and then transmits over the telephone line

• The call can be established by dialing manually or automatically

• A basic FAX machine is simply a modem with an added scanner and printer

There are 5 phases to a fax transmission:

A Call establishment

B Negotiation of data rates and capabilities

C Image transmission

D Confirm page image received by the other party

E Hang up