Intro to Telecom

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Intro to Telecom
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Fig 6.2
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Analog and Digital Signals
Digital signal
Analog signal
Fig. 6.4

• Analog
• Continuous fluctuations over time between high
  and low voltage
• Digital
• A discrete voltage state

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Fig 6.3
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Source/Signal Combinations
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Basic Modulation Techniques

• Amplitude modulation (AM)
• Converts digital data to analog signals using a
  single frequency carrier signal
• High-amplitude wave denotes a binary 1
• Low-amplitude wave denotes a binary 0
• Frequency modulation (FM)
• Uses a constant amplitude carrier signal and two
  frequencies to distinguish between 1 and 0
• Phase modulation
• Uses a phase shift at transition points in the
  carrier frequency to represent 1 or 0

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Examples Analog shifts
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Data Transmission Speeds

• Measured in bits per second (bps)
• Kilobits per second (kbps)
• Megabits per second (Mbps)
• Gigabits per second (Gbps)

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Types of Communications Media

• Guided Media
• Twisted wire cable
• Coaxial cable
• Fiber-optic cable
• Unguided Media
• Microwave transmission - satellite
• Microwave transmission - terrestrial
• Cellular transmission
• Infrared transmission

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Cable/Wire Types

• Twisted Pair Wire
• A cable consisting of pairs of twisted wires
• The twist helps the signal from bleeding into
  the next pair
• Cheapest
• Limited bandwidth
• Coaxial Cable
• Inner conductor wire surrounded by insulation,
  called the dielectric
• Dielectric is surrounded by a conductive shield,
  which is in turn covered by a layer of
  nonconductive insulation, called the jacket
• More expensive than twisted pair, but higher
  bandwidth

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Twisted Pair

• Fig 6.4

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Coaxial Cable

• Fig 6.5

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Cable/Wire Types, Continued

• Fiber Optic Cable
• Consists of many extremely thin strands of solid
  glass or plastic bound together in a sheathing
• Transmits signals with light beams
• No risk of sparks, safe for explosive
  environments
• More expensive than coaxial, but more bandwidth
• Different colors of light are used to
  simultaneously send
• Multiple signals

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Fiber Optic Cable

• Fig 6.6

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Microwave Transmission

• Fig 6.7

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Satellite

• Fig 6.8

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Cellular

• Fig 6.9

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Table 6.1
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Communications Efficiency

• A large part of telecommunication expense is cost
  of the medium
• Several approaches are used to efficiently use
  the medium
• Multiplexing
• Switching
• Compressing

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Multiplexing Time Division and Frequency
Division
Figure 6.14
Time division multiplexing (TDM) is where
multiple incoming signals are sliced into small
time intervals
Frequency division multiplexing (FDM) is where
incoming signals are placed on different
frequency ranges
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Multiplexing Freeway Analogy

• Frequency division multiplexing is analogous to
  having a 3-lane freeway. Each car has its own
  lane, three cars drive simultaneously in the same
  direction.
• Time division multiplexing is analogous to a
  freeway-onramp cars enter the on-ramp one at a
  time, and drive in single file.

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Frequency Division of Cable
Base video width is 4.2 MHz with guard bands 6 MHz
.....
Ch n
Ch 1
Ch 2
Ch 3
Ch 4
Ch 5
Ch 6
The default is 6 Mega Hertz slices of bandwidth
per channel
Cable modem
--Cable Bandwidth--
Cable phone gets 4 KHz slices
Q What limits the bandwidth on coaxial cable?
A The bandwidth of the amplifier.
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Switching

• Switching further advances the objective of
  efficiently utilizing the circuit
• Two types
• Circuit switching (e.g., public telephone
  network) requires end-to-end physical connection
• Packet switching (e.g. Internet) breaks up
  messages into small packets and routes them
  individually. No end-to-end physical connection
  required. Can be virtual circuit (all packets
  travel through same route) or datagram (packets
  may travel through any route)

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Circuit Switching
medium
You
Switch
To communicate a physical connection must be made
and maintained
Your Mom
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Packet Switching
Packets thrown into the internet cloud either
independently find the path from point to point
(datagram) of follow the same path (virtual
circuit)
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The message
Header
Message contents
Trailer
Direction of transmission
Block Check Character
Start of Header
Start of Text
End of Text
SOH
(STX)
(ETX)
BCC
If variable length header used
If variable length message used
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A Simple Protocol Stack
Application Protocol
Application
Application
Transport Protocol
Transport
Transport
Network Access
Network Protocol
Network Access
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A Simple Protocol Stack, Continued

• The application uses the protocol for its
  layer/level to determine how it should format its
  message for an application at a different
  computer
• However, it does not worry about getting the
  message to the application
• The transport layer is responsible for making
  sure that the message arrives at the correct
  application at the correct computer
• However, it does not concern itself with how it
  gets there. That is the responsibility of the
  network layer. The transport layer is only
  concerned with reliability of the communication
• The network layer determines how the message
  should be presented to the network

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Formatting and Decoding a Message
Protocols strip header information from the
message
Protocols add header information to the message
Application
Application
Data
Transport
Transport
Transport Header
Network Access
Network Access
Network Header
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Communications Protocols

• Fig 6.22

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Relationship of TCP/IP to OSI
TCP/IP

• Controls the users interface and applications
  between two hosts, e.g.
• File transfer protocol (ftp)
• HTTP (Hypertext trans. protocol)
• Telnet
• SMTP (Simple mail transfer protocol)
• SNMP (Simple Network Mgt protocl)
• NNTP (Net news transport protocol)

Process / Application
Host to Host
TCP Virtual circuit maintained, ack UPD No
acknowledgment
Internet
IP routing, fragmentation, assembly ICMP Above
IP, error handling ARP Address resolution sw to
hw addr RARP hardware to sw address convert
Network Access
Physical layer, such as Ethernet or Token Ring
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Fig 6.23
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Ethernet Evolution
New, taking over
1000 Mbps Gigabit Ethernet
Predominant
100 Mbps Ethernet
Battling ATM
Legacy
Most new installations
10 Mbps Ethernet
Old installations
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Ethernet Pros and Cons

• Operates by contention packets collide
• Inefficient many aborted transmissions
• Rates of only 37 of raw wire speed
• 10 Gbit Ethernet on the way
• Inexpensive
• Simple circuitry
• Cheapest bandwidth ratios

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Token Ring
T
data
40008065402
T
data
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Token Ring Pros and Cons

• Very efficient 75 of raw bandwidth
• A better technology
• Expensive
• Used for mission critical applications like
  banking
• Lost battle to fast-Ethernet (like beta vs. VHS)

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ATM

• Sends 53-byte cells not variable length packets
  like Token Ring and Ethernet
• Hardware knows where header ends and data begins
• Speeds up to 622 Mbps
• Predictable throughput rates very reliable,
  guaranteed service
• Military, Safety valve in nuclear power reactor.
  No Delay or Jitter!!!

Header
Body
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ATM Pros and Cons

• Very fast
• Reliable mission critical applications
• Efficient bandwidth gt75 of raw capacity
• No delays or sequence re-configuring
• Very expensive and complex
• Not compatible with 10/100 Mbps Ethernet
  installations
• Most applications do need this efficient
  management of data cells only messages used in
  real time need ATM

Header
Body
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Connectivity
Type Bandwidth Users Rel.Cost Modem 28.kbps 1
-5 1 DSL 256 Kbps 1-50 2 ISDN 128
Kbps 5-50 3 T1 (DS1) 1.54 Mbps 50-500 10 T3
(DS3) 45 Mbps 4000 100 ATM 155-622
Mbps 10,000 200
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Synchronous Optical Network (SONET)
Define Optical Carrier Levels (OC) Basic
transmission rate STS-1 51.84 Mbps OC-3 351.84
Mbps 155.52 Mbps OC-12 12 51.84 Mbps
622.08 Mbps OC-48 2.488 Gbps OC-768 ?????
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Bringing in the fiber

• 48 strands - OC 48
• 96 strands - OC 96
• Dense Wave Division Multiplexing 48 strands can
  yield OC 192
• Optical Switches do not convert from light to
  electricity and back to light. 100 light.

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Current Status Fiber

• Massive investments by telecoms in 1990s.
• Current fiber utilization at 2.5!!!!
• Mostly between major corporate infrastructures in
  major cities. CO to CO
• Limitations on last mile to smaller
  infrastructures
• Abundance trickled to equipment manufacturers as
  well predicted to last through 2002

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Brief History of Telecom

• 1837 - Invention of the telegraph
• 1876 - Alexander Graham Bell invents the
  telephone
• 1876 - Edison invents the electric bulb and the
  phonograph
• 1880 - American Bell founded
• 1892 - Telephone system regulation begins in
  Canada
• 1893 - Broadcasting was started in Budapest.
• 1906 - Lee de Forest invents the vacuum tube.
• 1910 - Interstate Commerce Commission starts to
  regulate telcos
• 1914 - Underground cables link Boston, NYC and
  Washington
• 1925 - Bell Telephone Laboratories founded
• 1930 - ATT introduces much higher quality
  insulated wire
• 1934 - Federal Communications Commission (FCC)
  founded
• 1945 - ATT lays 2000 miles of coax cable
• 1952 - The first database was implemented on
  RCA's Bizmac computer
• 1954 - Gene Amdahl developed the first computer
  operating system for the IBM 704.
• 1968 - Carterfone court decision permits non-Bell
  telephone equipment to be used
• 1970 - Court permits MCI to provide long-distance
  services
• 1984 - Breakup of ATT 1984 - Cellular phones
  enter service
• 1996 - Telecommunications Act of 1996 deregulates
  U.S. telephone system