Principles of Electronic Communication Systems

© 2008 The McGraw-Hill Companies

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Principles of ElectronicPrinciples of ElectronicCommunication SystemsCommunication Systems

Third Edition

Louis E. Frenzel, Jr.

Modified by Sunantha Sodsee


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TelephonesTelephones

The telephone system The largest and most complex electronic

communication system in the world. The primary purpose

Provide voice communication. Widely used for

Facsimile transmission Computer data transmission.


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The telephone system Full-duplex analog communication of voice signals. Telephone can connect with any other telephone in the

world. Identification code

Telephone number Country code + Subscriber numbers : +66 XXXX

XXXX Trunk prefix + Subscriber numbers: 02 XXX XXXX Subscriber numbers: area code, local number


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The Local Loop Single central office

10,000 telephone lines can be connected The two-wire, twisted-pair connection

Telephone and central office local loop or subscriber loop.


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Telephone Set Analog baseband transceiver

Handset: a microphone and a speaker, transmitter and receiver.

Ringer and a dialing mechanism ringer: bell or an electronic oscillator

connected to a speaker. A switch hook

a double-pole mechanical switch Dialing circuits : dual-tone multifrequency

(DTMF) system. Hybrid circuit

special transformer used to convert signals from the four wires from the transmitter and receiver into a signal suitable for a single two-line pair to the local loop.

http://electronics.howstuffworks.com/telephone1.htm


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Basic telephone set.


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DTMF


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Standard Telephone and Local Loop Telephone wires:

color coded: tip wire is green and usually connected to ground, and the ring wire is red.


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Subscriber interface.


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Telephone SystemTelephone System

Telephone Hierarchy a telephone call,

your voice is connected through your local exchange to the telephone system.

Several other facilities may provide switching, multiplexing, and other services required to transmit your voice.

The telephone system is referred to as the public switched telephone network (PSTN).


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Organization of the telephone system in the United States.

Trunk: A communications path between two switching systems


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Private Telephone System Telephone service among the telephones in a company

or organization The two basic types :

Key systems Private branch exchanges


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Private Telephone System: Key Systems serve 2–50 user telephones within an organization. individual telephone units called stations,

all of which are connected to a central answering station.

The central answering station is connected to one or more local loop lines, or trunks, back to the local exchange.

The telephone sets in a key system typically have a group of pushbuttons that allow each phone to select two or more outgoing trunking lines.


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Private Telephone System: Private Branch Exchange For larger organizations: thousands of individual

telephones within an organization. private automatic branch exchanges (PABXs) computer branch exchanges (CPXs).

Advantages of efficiency and cost reduction when many telephones are required.

An alternative to PBX is Centrex. This service performs the function of a PBX but uses

special equipment and special trunk lines.


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A PBX.


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Circuit SwitchCircuit Switch


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Circuit-SwitchingCircuit-Switching

PSTN is a circuit-switched network Circuit establishment Transfer of information

point-to-point from endpoints to node internal switching/multiplexing among nodes

Circuit disconnect Circuit switching is well suited for analog voice

communications as in the telephone network. in-efficient for data networks due to its resource

allocation nature data traffic is BAD


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Setting up a Path

Before any data can be sent, the path between the caller and callee must be established.

It can easily take 10 seconds to set up the path (more if its an international call).

During this time interval, the switching equipment is searching for a ‘copper’ path through the network.


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Advantages of Circuit Switching

The advantages are: For the duration of the call, the communicating

computers have exclusive use of a connection. The full bandwidth of the connection can be used. Data can be sent at a constant rate (there are not

unexpected delays and data arrives in the order it was sent).

Circuit switching is also easier to administer, charge for and maintain.


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Disadvantages of Circuit Switching

The disadvantages are: There is along delay while the circuit is set up and

acknowledgement sent. The connection can be tapped (thus a potential security

problem). No error checking or flow control is done by network,

the computers must to it themselves. Traffic often consists of short bursts of data followed by

long periods of inactivity (thus line utilization is low).


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Examples of Circuit SwitchingExamples of Circuit Switching

Public Switched Telephone Network – PSTN Telephone service carried by the PSTN is often called

plain old telephone service (POTS).

Private Automatic Branch Exchange – PABX

Integrated Services Digital Network - ISDN


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POTSPOTS

POTS standard telephone service that most homes use.

The main distinctions between POTS and non-POTS services speed and bandwidth.

POTS : about 33.6 kbps (33,600 bits per second) (modem manufacturers : rates of 56.6 kbps).


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Public Switched Telephone Network Public Switched Telephone Network (PSTN) Elements(PSTN) Elements

Subscribers Local loop

Connects subscriber to local telco exchange

Exchanges Telco switching centers Also known as end

office >19,000 in US

Trunks Connections between

exchanges Carry multiple voice

circuits using FDM or synchronous TDM

Managed by IXCs (inter-exchange carriers)


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Telephone Network StructureTelephone Network Structure


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Circuit Switching ConnectionCircuit Switching Connection


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SignalingSignalingOriginating

CPE

Terminating Switching

OfficeTerminating

CPE

Originating Switching

Office

Off-hook

Dial Tone

Dialed Digits

Off-hook

Off-hook (wink)

On-hook (wink)

Dialed Digits

Audible Ring Ringing

Answer Off-hook

Disconnect

Idle1

2

3


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SignalingSignalingTerminating Switching

Office

Originating CPE


OfficeTerminating

CPE


Office

Off-hook

Dial Tone

Dialed Digits

Off-hook

Off-hook (wink)

On-hook (wink)

Dialed Digits


Answer Off-hook

Disconnect

Idle1

2

4

3

5

6

6


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SignalingSignalingTerminating Switching

Office

Originating CPE


OfficeTerminating

CPE


Office

Off-hook

Dial Tone

Dialed Digits

Off-hook

Off-hook (wink)

On-hook (wink)

Dialed Digits


Answer Off-hook

Disconnect

Idle1

2

4

3

5

7

6

6

8


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SignalingSignalingOriginating

CPE


OfficeTerminating

CPE


Office

Off-hook

Dial Tone

Dialed Digits

Off-hook

Off-hook (wink)

On-hook (wink)

Dialed Digits


Answer Off-hook

Disconnect

Idle1

2

4

3

5

7

6

6

8

9

10 10


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PABXPABX

PBX = Private Branch Exchange connect customer telephones (and related equipment)

to LEC central office lines (trunks), and to switch internal calls within the customer's telephone system.

Modern PBX numerous software-controlled features such as call

forwarding and call pickup.

A PBX uses technology similar to that used by a central office switch (on a smaller scale). (The acronym PBX originally stood for "Plug Board

Exchange".)


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ISDNISDN

Integrated services digital network sending voice, video, and data over digital telephone

lines. requires special metal wires and supports data

transfer rates of 64 Kbps (64,000 bits per second). Most ISDN lines offered by telephone companies

give you two lines at once, called B channels. one line for voice and the other for data, or both lines for data: data rates of 128 Kbps


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B-ISDNB-ISDN

B-ISDN, broadband transmission support transmission rates of 1.5 million bits per

second and higher. requires fiber optic cables

It is not widely available.


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Issues in Circuit Switched NetworksIssues in Circuit Switched Networks Alternate RoutingAlternate Routing

Switch selects the best route for each call Routes listed in preference order Different sets of routes may be used at different times Routing paths can be fixed (1 route) or dynamic

(multiple routes, selected based on current and historical traffic)

Need to use algorithms to determine paths dynamically, based on load/congestion vectors


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Alternate RoutingAlternate Routing


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Message Switching


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Message Switching

message switching all the connections are permanently set up.

Message header containing

address of the source destination computer. routing information.

Each message is sent to the local switching office that stores the message (checking it for errors) and then forwards it on to the next appropriate switching office (this technique is called store-and-forward).


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Advantages of Message Switching

The advantages are: no waiting for setting up connections. Flow control and error checking Messages can be sent even when the receiving

computer is not ready they can be stored until it is ready.


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Disadvantages of Message Switching

The disadvantages are: no limit to the length of a message

single message may block a link for a long time. If messages are too long,

intermediate switching offices may not have sufficient memory to store them they cannot be passed on.


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Packet Switching


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Packet Switching

Packet switching, like message switching, uses permanent connections.

messages are broken up into smaller messages called packets (typically 512 bytes long).

header containing Address routing information position in the original message.


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Packet Switching

Packets are reassembled by the receiving computer to form the original message.

Packet switching widespread in many computer networks and the

internet.


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Advantages of Packet Switching

The advantages are: take less time to transmit across links. less memory to store and forward. More secure because line taps will reveal only

fragments of messages.


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Disadvantages of Packet Switching

The disadvantages are: Packets may arrive at their destination out-of-order

long delay while a small number of slow packets find their way through the network.

It is not certain how long it will take a packet to pass through the network or how long to wait before deciding to request its

retransmission). Packet switching is not ideal for supplying streams of

data (as required for radio or T.V).


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Virtual Circuits

Virtual circuit is a fixed path through a network establish when a call starts.

Data is transmitted as packets. The packets follow the fixed path through the network. packets from other sources can share common links. The packets are guaranteed to arrive in the correct order. It is usually left to the receiving computer to ask for

damaged or missing packets to be retransmitted this reduces the workload of the network and allows higher transfer

rates in general.


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Virtual Circuits

transmitting video and speech data occasional missing or damaged packets are ignored.

file transfers.

When a packet is lost, it’s absence is detected immediately because of the guaranteed order of packets.


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Crossbar Switches

Several kinds of switches are (or were) common within the telephone system.

The simplest kind is the crossbar switch (sometimes called a crosspoint switch).

The switch has N inputs and N outputs for N full duplex lines.

There are N2 intersections, called crosspoints.


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The connection is a direct electrical connection jumper

Every line can be connected to every other line.


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Space Division Switches

smaller connected crossbar switches. Theses are called space division switches.

For example, if we had 16 lines, we could have four crossbar switches each taking 4 lines.

The output of the crossbar switches can themselves be fed into crossbar switches.


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Space Division Switches

Each stage of the space division switch is fully connected to the next stage. This means that an electrical connection can be made

from any input to any output.


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Pros and Cons of Space Division Switches

Because the space division switches use many smaller crossbar switches, if one fails it can be easily replaced without

disrupting all the calls. it is possible for a Space Division Switch to be

jammed i.e. a lot of calls had to go through one crossbar

switch, all its input or output lines may be used up. Setting k=2n-1 will ensure this will not happen


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Time Division Switches

the n inputs are scanned in sequence to build a frame with n slots.

For T1 switches, the slots are 8 bits including 1 control bit.

8,000 frames are processed every second.


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Time Division Switches

Each input is mapped (using an n word mapping table) to one of the n output lines.

The slots are reordered so that they are sent to the correct output lines.


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Finally….Advantages of Time Division Switches

Time Division Switches use digital technology.

The number of switches involved (be they electronic gates) grows linearly with the number of inputs.

The Time Division switch must, however, store and forward the n inputs within 1/8000 of a second(125 sec).


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FacsimileFacsimile

Facsimile, or fax, an electronic system for transmitting graphic

information by wire or radio. Facsimile

send printed material by scanning it converting it into electronic signals that modulate a

carrier to be transmitted over the telephone lines. Since modulation is involved, fax transmission can

also take place by radio.


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FacsimileFacsimile

Components of a facsimile system.


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FacsimileFacsimile

How Facsimile Works? High-tech electro-optical machine. Scanning is done electronically

the scanned signal is converted into a binary signal. Digital transmission with standard modem techniques is used.

image scanner that converts the document into hundreds of horizontal scan lines.

all incorporate a photo- (light-) sensitive device to convert light variations along one scanned line into an electric voltage.

The resulting signal is then processed in various ways to make the data smaller and faster to transmit.


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FacsimileFacsimile

How Facsimile Works? The signal is sent to a modem

it modulates a carrier set to the middle of the telephone voice spectrum bandwidth.

The signal is then transmitted over the public-switched telephone network.

The receiving machine’s modem demodulates the signal processed to recover the original data.

The data is decompressed and printed out.


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FacsimileFacsimile

Block diagram of modem fax machine.


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FacsimileFacsimile

Most fax machines use charge-coupled devices (CCDs) for scanning. A CCD is a light-sensitive semiconductor device that

converts varying light amplitudes into an electrical signal.

Data compression is a digital data processing technique that looks for redundancy in the transmitted signal.

Every fax machine contains a built-in modem that is similar to a conventional data modem for computers.

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