Transcript Slide 1

Intro to Telecom
Fig 6.2
Analog and Digital Signals
Digital
signal
Analog
Fig. 6.4
signal

Analog


Continuous fluctuations over time between high and low voltage
Digital

A discrete voltage state
Fig 6.3
Source/Signal Combinations
Analog
Signal
Analog Voice, Telephone,
Source Television....
Digital
Signal
Voice over
digital
media,
Audio files,
CODEC
Digital Fax, Any Computer Computer
Source over POTS, Digital over digital
T-V
lines (T-1,
ATM,
Frame
relay...)
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
Examples: Analog shifts
Data Transmission Speeds
Measured in bits per second (bps)
Kilobits per second (kbps)
Megabits per second (Mbps)
Gigabits per second (Gbps)
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
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
Twisted Pair
Fig 6.4
Coaxial Cable
Fig 6.5
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
Fiber Optic Cable
Fig 6.6
Microwave Transmission
Fig 6.7
Satellite
Fig 6.8
Cellular
Fig 6.9
Table 6.1
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
Multiplexing:
Time Division and Frequency Division
Time division multiplexing (TDM) is where multiple
incoming signals[Figure
are sliced6.14]
into small time intervals
Frequency division multiplexing (FDM) is where incoming signals are
placed on different frequency ranges
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.
Frequency Division of Cable
Base video width is 4.2 MHz with guard bands 6 MHz
Ch 1Ch 2Ch 3Ch 4Ch 5Ch 6
….……………....
Ch n
The default is 6 Mega Hertz slices of bandwidth per channel
Cable
modem
--Cable Bandwidth-Q: What limits the bandwidth on coaxial cable?
A: The bandwidth of the amplifier.
Cable phone
gets 4 KHz
slices
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)
Circuit Switching
You
Switch
To communicate a physical
connection must be made and
maintained
Your Mom
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)
The message
Header
Message contents
Direction of transmission
Start
of
Header
SOH
Start
of
Text
(STX)
If variable length header used
Trailer
Block
Check
End Character
of
Text
(ETX)
BCC
If variable length message used
A Simple Protocol Stack
Application Protocol
Application
Transport Protocol
Transport
Network
Access
Network Protocol
Application
Transport
Network
Access
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
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
Communications Protocols
Fig 6.22
Relationship of TCP/IP to OSI
OSI
TCP/IP
7
Application
6
Presentation
5
Session control
4
Transport control
Host to Host
3
Network control
Internet
2
Data link control
1
Physical link control
Process /
Application
Network
Access
Controls the user’s 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 protoc’l)
• NNTP (Net news transport protocol)
TCP: Virtual circuit maintained, ack
UPD: No acknowledgment
IP: routing, fragmentation, assembly
ICMP: Above IP, error handling
ARP: Address resolution sw to hw addr
RARP: hardware to sw address convert
Physical layer, such as Ethernet or
Token Ring
Fig 6.23
Ethernet Evolution
New, taking over
Predominant
Legacy
10 Mbps
Ethernet
Old installations
100 Mbps
Ethernet
Most new installations
1000 Mbps
Gigabit
Ethernet
Battling ATM
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
Token Ring
T
data
T
data
40008065402
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)
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!!!
Body
Header
ATM Pros and Cons
•Very fast
•Reliable – mission critical applications
•Efficient bandwidth >75% 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
Body
Header
real time need ATM
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+
Synchronous Optical Network (SONET)
Define Optical Carrier Levels (OC)
Basic transmission rate STS-1 51.84
Mbps
OC-3 = 3*51.84 Mbps = 155.52 Mbps
OC-12 = 12* 51.84 Mbps = 622.08 Mbps
OC-48 = 2.488 Gbps
OC-768 = ?????
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.
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
Brief History of Telecom
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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 - AT&T introduces much higher quality insulated wire
1934 - Federal Communications Commission (FCC) founded
1945 - AT&T 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 AT&T 1984 - Cellular phones enter service
1996 - Telecommunications Act of 1996 deregulates U.S. telephone system