Transcript Part II Data Transmission

Data Transmission
The basics of media, signals, bits,
carries, and modems
(Part I)
Physical Layer
• The lowest layer in any network architecture model
• It is concerned with the transparent transmission of “raw”
bits across a communications medium
• It deals with the physical characteristics (mechanical,
electrical, procedural) of data transmission and
communication.
• It is responsible for:
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–
–
–
–
–
providing basic signaling (control, data)
signal modulation
encoding/decoding
activate/deactivate physical medium (PM)
bit-timing (clocking)
mapping between different formats
Physical Layer
Received Signal
(Attenuated &
Distorted)
Because of
Propagation Effects
Transmitted Signal
Propagation
Transmission Medium
Sender
Receiver
Data Communication
• Source initiates the communication.
• Destination address (identifier) is required for the
network to establish a communication path between the
source and the destination
• Destination must be prepared to receive data
• Source and destination hosts may belong to different
types of networks.
• Line speed and/or packet formats mismatches should be
taken care of (i.e., via fragmentation, conversion, etc.)
• Example: consider a file transfer (e.g., FTP)
Analog and Binary Data
Analog Data
Binary Data
1101011000011100101
Smoothly changing
among an infinite
number of states
(loudness levels, etc.)
Two states:
One state represents 1
The other state represents 0
Binary Data and Binary Signal
15 Volts
(0)
0
There are two states (in this case,
voltage levels).
One, (high) represents a 0.
The other (low) represents a 1.
0
0 Volts
1
-15 Volts
(1)
Transmitted
Signal
Binary Data and Binary Signal
15 Volts
(0)
Clock Cycle
0
0
0 Volts
1
-15 Volts
(1)
Time is divided into clock cycles
The State is held constant
within each clock cycle.
It can jump abruptly at the end of
each cycle.
One bit is sent per clock cycle.
Transmitted
Signal
Binary Data and Digital Signal
11
11
10
10
01
01
Client PC
00
01
00
Server
In binary transmission, there are two states.
In digital transmission, there are few states (in this case, four).
With four states, two information bits can be sent per clock cycle.
00, 01, 10, and 11
Binary transmission is a special case of digital transmission.
Baud Rates for Digital Signals
11
11
10
01
Client PC
00
Baud Rate =
# of Clock Cycles/Second
10
01
01
00
Server
Suppose that the clock cycle is 1/10,000 second.
Then the baud rate is 10,000 baud (10 kbaud).
The bit rate will be 20 kbps (two bits/clock cycle
times 10,000 clock cycles per second).
(The bit rate gives the number of information bits per second.)
In Summary- Bit Rate and Baud Rate
• Two terms frequently used in data communication
• Bit rate: the number of bits sent in one second, usually
expressed in bits per second (bps)
• More important to know how long it takes to process
each piece of information
• Duration of a bit (bit interval): the time required to
send one single bit. bit interval = 1/bit rate
• Example: bit rate = 55.6 kbps,
duration of a bit = 1/55600 = 1.8 10 5 second
= 18 microseconds
Baud Rate
• Refers to the number of signal units per second that are
required to represent those bits.
• Related to the bandwidth -- the fewer signal units
required, the more efficient the system and the less
bandwidth required to transmit more bits
• More important to know how efficiently we can move
those data from place to place
• Bit rate = Baud rate * the number of bits represented by
each signal unit
• Example: An analog signal carries 4 bits in each signal
element. If 1000 signal elements are sent per second,
then baud rate = 1000 bauds per second,
bit rate = 1000 * 4 = 4000 bps
Perspective
• Analog Data
– Smooth changes among an infinite number of states—
like hands going around an analog clock
• Digital Data
– Few states
– In a digital clock, each position can be in one of ten
states (the digits 0 through 9)
• Binary Data
– Two states (a special case of digital)
Basic Idea For Transmission Media
• Encode data as energy and transmit energy
• Decode energy at destination back into data
• Energy can be electrical, light, radio, sound,
...
• Each form of energy has different properties
and requirements for transmission
Transmission Media
• Transmitted energy is carried through some sort of
medium Transmitter encodes data as energy and transmits
energy through medium
– Requires special hardware for data encoding
– Requires hardware connection to transmission medium
– Media can be copper, glass, air, ...
Copper Wires
• Twisted pair: a pair of insulated copper
wires. Can run for a few kms (oldest and
most common). Several standards: STP and
UTP (Unshielded Twisted Pair).
4-Pair Unshielded Twisted Pair
Cable with RJ-45 Connector
Single Twisted Pair
Four pairs (each pair is twisted)
Jacket
There is insulation around each wire.
4-Pair Unshielded Twisted Pair
Cable with RJ-45 Connector
A length of UTP is called a cord.
There is no metal shielding around
The individual pairs or around the entire
Cord. Hence the name unshielded UTP
UTP Cord
Copper Wires (continued)
• Coaxial cable: copper wire surrounded by
an insulator encased by another copper
conductor (mesh) covered by a protective
plastic sheath
Glass Fibers
• Thin glass fiber carries light with encoded data
• Plastic jacket allows fiber to bend (some!) without breaking
• Fiber is very clear and designed to reflect light internally for
efficient transmission
• Light emitting diode (LED) or laser injects light into fiber
• Light sensitive receiver at other end translates light back
into data
Glass Fibers (continued)
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•
•
•
Very reliable and high capacity
Can run up to tens of kms
Not affected by electromagnetic inference
Easy to install but a costly technology
Multimode & Single-Mode Fiber
Cladding
Light
Source
Modes
Core
Multimode Fiber
Light only travels in one of several allowed modes
Multimode fiber must keep its distance short or limit modal distortion
Multimode fiber goes a few hundred meters and is inexpensive to lay
It is dominant in LANs
Multimode & Single-Mode Fiber
Single Mode
Cladding
Core
Light
Source
Single Mode Fiber
Core is so thin that only one mode can propagate.
No modal dispersion, so can span long distances without distortion.
Expensive, so not widely used in LANs. Popular in WANs
Multimode and Single-Mode Fiber
• Multimode
– Limited distance (a few hundred meters)
– Inexpensive to install
– Dominates fiber use in LANs
• Single-Mode Fiber
– Longer distances: tens of kilometers
– Expensive to install
– Commonly used by WANs and telecoms carriers
Wireless
• Air is the medium
• Radio
– Data transmitted using radio waves
– Energy travels through the air rather than copper or
glass
– Conceptually similar to radio, TV, cellular phones
– Can travel through walls and through an entire
building
– omnidirectional (broadcast)
Radio Wave
Wavelength
Amplitude
Frequency
Measured in Hertz (Cycles per Second)
2 Cycles in one Second, so 2 Hz
Wavelength * Frequency = Speed of Propagation
Wireless
• Satellite: unidirectional, costly, propagation
is a consideration
Wireless
• Microwave
– High frequency radio waves
– Unidirectional, for point-to-point communication
– Antennas mounted on towers relay transmitted data
• Infrared
– Infrared light transmits data through the air
– Similar to technology used in TV remote control
– Can propagate throughout a room (bouncing off
surfaces), but will not penetrate walls
Wireless
• Laser
– Unidirectional, like microwave
– Higher speed than microwave
– Uses laser transmitter and photo-sensitive
receiver at each end
– Point-to-point, typically between buildings
– Can be adversely affected by weather
Wireless Propagation Problems
Inverse Square
Law Attenuation
Laptop
Comm. Tower
Very Rapid Attenuation with Distance
Compared to Wires and Fiber
Wireless Propagation Problems
Multipath
Interference
Laptop
Shadow
Zone:
No Signal
Comm. Tower
Signals Arriving at Slightly
Different Times Can Interfere
Choosing A Medium
• Copper wire is mature technology, rugged and
inexpensive; maximum transmission speed is limited
• Glass fiber:
– Higher speed
– More resistant to electromagnetic interference
– Spans longer distances
– Requires only single fiber
– More expensive; less rugged
Choosing A Medium
• Radio and microwave don't require physical
connection
• Radio and infrared can be used for mobile
connections
• Laser also does not need physical
connection and supports higher speeds
Data Transmission
• Data transmission requires:
 Encoding bits as energy
 Transmitting energy through medium
 Decoding energy back into bits
• Energy can be electric current, radio,
infrared, light
• Transmitter and receiver must agree on
encoding scheme and transmission timing
Encoding--Using Electric Current To
Send Bits
 Simple idea - use varying voltages to represent 1s and 0s
 One common encoding use negative voltage for 1 and positive
voltage for 0
 In following figure, transmitter puts positive voltage on line for
0 and negative voltage on line for 1
Encoding Details
• All details specified by a standard
• Several organizations produce networking
standards
– IEEE Institute for Electrical and Electronics
Engineers
– ITU (International Telecommunications Union)
– EIA (Electronic Industries Association)
• Hardware adheres to standard interoperable
Transmission Modes
• Asynchronous and synchronous communications
• Asynchronous communication: data are
transmitted one character at a time
– transmitter and receiver do not explicitly coordinate
each data transmission
– transmitter can wait arbitrarily long between
transmissions
– receiver does not know when a character will arrive.
May wait forever
Asynchronous Communication
• To ensure meaningful exchange
– start bit before character
– one or more stop bits after character
– 1s when idle
• Used, for example, when transmitter such as a
keyboard, may not always have data ready to send
Synchronous Communication
• No start/stop bits. Sends bytes contiguously
• Periodically transmit clocking information
• Uses flags (special bit sequences) as delimiters
for frames
• More efficient transmission
The RS-232C Standard
• Example use
– Connection to keyboard/mouse
– Serial port on PC (as opposed to parallel
transmission)
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•
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Specified by EIA
Voltage is +15 or –15
Cable limited to ~50 feet
Use asynchronous communication
Illustration Of RS-232
• Start bit
– Same as 0
– Not part of data
• Stop bit
– Same as 1
– Follows data
Duration Of A Bit In RS-232
• Determined by baud rate
– Typical baud rate: 9.6 kbaud, 14.4 kbaud, 28.8
kbaud
– bit_rate = baud_rate
– Duration of a bit is 1/baud_rate
• Sender and receiver must agree a priori
• Receiver samples signal
• Disagreement results in framing error
Two-Way Communication
• Desirable in practice
• Requires each side to have transmitter and
receiver
• Called full duplex
Illustration Of Full-Duplex Communication
• Transmitter on one side connected to receiver on
other
• Separate wires needed to carry current in each
direction
• Common ground wire
Electric Transmission
• In real world
– Electric energy dissipates as it travels along
– Wires have resistance, capacitance, and
inductance which distort signals
– Magnetic or electrical interference distorts
signals
– Distortion can result in loss or misinterpretation
Illustration Of Distorted Signal
For A Single Bit
• In practice
– Distortion can be much worse than illustrated
Consequences
• RS-232 hardware must handle minor
distortions
– Take multiple samples per bit
– Tolerate less than full voltage
• Can not use electrical current for longdistance transmission
Reading Materials
• Chapter 4
• Chapter 5: Sections 5.1-5.7