Intro to Telecom. Fig 6.2 Analog and Digital Signals Fig. 6.4 Analog signal Digital signal Analog...
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Transcript of Intro to Telecom. Fig 6.2 Analog and Digital Signals Fig. 6.4 Analog signal Digital signal Analog...
Intro to Telecom
Fig 6.2
Analog and Digital Signals
Fig. 6.4Analog signal
Digital signal
Analog Continuous fluctuations over time between high and low voltage
Digital A discrete voltage state
Fig 6.3
Source/Signal Combinations
AnalogSignal
DigitalSignal
AnalogSource
Voice, Telephone,Television....
Voice overdigitalmedia,Audio files,CODEC
DigitalSource
Fax, Any Computerover POTS, DigitalT-V
Computerover digitallines (T-1,ATM,Framerelay...)
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
[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
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
--Cable Bandwidth--
Ch 1Ch 2Ch 3Ch 4Ch 5Ch 6 Ch n….……………....
Base video width is 4.2 MHz with guard bands 6 MHz
The default is 6 Mega Hertz slices of bandwidth per channelCable modem
Cable phone gets 4 KHz slicesQ: What limits the bandwidth on coaxial cable?
A: The bandwidth of the amplifier.
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
Your Mom
To communicate a physical connection must be made and maintained
Switch
medium
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 Trailer
Startof Header Start
of Text
Endof
Text
BlockCheck
Character
SOH (STX) (ETX) BCC
If variable length header used If variable length message used
Direction of transmission
A Simple Protocol Stack
Application
Transport
NetworkAccess
Application
Transport
NetworkAccess
Application Protocol
Transport Protocol
Network Protocol
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
Application
Transport
NetworkAccess
Application
Transport
NetworkAccess
Transport Header
Data
Network Header
Protocols add header information to the message
Protocols strip header
information from the message
Communications Protocols
Fig 6.22
Relationship of TCP/IP to OSI
6
7
4
5
2
3
1
Presentation
Application
Transport control
Session control
Data link control
Network control
Physical link control
OSI
Process /Application
Internet
Host to Host
NetworkAccess
TCP/IP 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)IP: routing, fragmentation, assemblyICMP: Above IP, error handlingARP: Address resolution sw to hw addrRARP: hardware to sw address convert
TCP: Virtual circuit maintained, ackUPD: No acknowledgment
Physical layer, such as Ethernet or Token Ring
Fig 6.23
Ethernet Evolution
10 MbpsEthernet
100 MbpsEthernet
1000 MbpsGigabitEthernet
Legacy
Predominant
New, taking over
Old installations
Most new installations
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
data
T
data
T40008065402
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!!!
HeaderBody
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 real time need ATM
HeaderBody
Connectivity
Type Bandwidth # Users Rel.CostModem 28.kbps 1-5 1DSL 256+ Kbps 1-50 2ISDN 128 Kbps 5-50 3T1 (DS1) 1.54 Mbps 50-500 10T3 (DS3) 45 Mbps 4000+ 100ATM 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 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