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Principles of Electronic Communication Systems. Third Edition Louis E. Frenzel, Jr. Modified by Sunantha Sodsee. Telephones. The telephone system The largest and most complex electronic communication system in the world. The primary purpose Provide voice communication. Widely used for - PowerPoint PPT Presentation
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© 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
© 2008 The McGraw-Hill Companies
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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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TelephonesTelephones
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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TelephonesTelephones
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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TelephonesTelephones
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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TelephonesTelephones
Basic telephone set.
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TelephonesTelephones
DTMF
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TelephonesTelephones
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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TelephonesTelephones
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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Telephone SystemTelephone System
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Telephone SystemTelephone System
Organization of the telephone system in the United States.
Trunk: A communications path between two switching systems
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Telephone SystemTelephone System
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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Telephone SystemTelephone System
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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Telephone SystemTelephone System
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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Telephone SystemTelephone System
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
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
4
3
5
6
6
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SignalingSignalingTerminating Switching
Office
Originating 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
4
3
5
7
6
6
8
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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
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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