Transcript Data Transmission

Networking H15
Analog and Digital Data
Data, Signal, Transmission
Encoding and Decoding ( Amplitude Shift,
Frequency Shift, PCM etc.)
Transmission
Simplex, Half - Duplex, Duplex
Serial and Parallel Transmission
Synchronous + Asynchronous
Transmission impairments
Attenuation and Attenuation Distortion
Networking H15
Media (twisted pair etc.)
Switching Techniques
Why do we need to switch?
Switching Concepts(Crossbar, Multi-Stage)
Message, Packet, Circuit Switching
Multiplexing
Frequency Division, Time Division,
Statistical Time Division
Networking H15
LANS and WANS
Network Topologies
Bus, Star, Ring
Media Access Control Techniques
802.3, 802.4, 802.5
Protocols
Why would you want Protocols in the first place?
OSI Model
Interconnection
Repeaters, Bridges, Routers, Hubs, Gateways
Networking H15
TCP/IP Protocol and Addressing +WWW
Frame Relay, Cell Relay, FDDI etc
X.25
13 Step Approach to Network Design
Wireless Networks
Distributed Computing
Client Server
Technologies in Client Server

Middleware

Groupware
Communication considerations
1) AHHH!! What do those electrical signals mean?
2) How can I send a bit, what signal do I use for 0
and which for 1?
3) How do devices make use of the wire?
4) How do I derive meaningful information from all
of these bits
5) How are transmission errors discovered and dealt
with?
6) How do packets get from one system to another?
Communication Considerations
1) How do I send large amounts of data and how do I
ensure that I receive all of my data?
2) How do machines keep track of who there are
talking to?
3) What language is this, how can I the computer
understand different formats?
4) How does a user gain access to the network?
5) How do programmers write programs to use the
network?
Our solution the OSI model
Application
Presentation
Session
Transport
Network
Data link
Physical
What is a Protocol?
Allows entities (i.e. application programs)
from different systems to communicate
Shared conventions for communicating
information are called protocols
Includes syntax, semantics, and timing
Why Use Protocol Architecture?
Data communications requires complex procedures
Sender identifies data path/receiver
Systems negotiate preparedness
Applications negotiate preparedness
Translation of file formats
For all tasks to occur, high level of cooperation is
required
Modular Approach
Breaks complex tasks into subtasks
Each module handles specific subset of tasks
Communication occurs
between different modules on the same system
between similar modules on different systems
OSI Lower Layers
Physical
Data Link
Network
OSI Physical Layer
Concerned with transmission of unstructured
bit stream over physical medium
Deals with accessing the physical medium
Mechanical characteristics
Electrical characteristics
Functional characteristics
Procedural characteristics
OSI Data Link Layer
Responsible for error-free, reliable
transmission of data
Flow control, error correction
OSI Network Layer
Responsible for routing of messages through
network
Concerned with type of switching used
(circuit v. packet)
Handles routing between networks, as well as
through packet-switching networks
OSI Upper Layers
Transport
Session
Presentation
Application
OSI Transport Layer
Isolates messages from lower and upper
layers
Breaks down message size
Monitors quality of communications channel
Selects most efficient communication service
necessary for a given transmission
OSI Session Layer
Establishes logical connections between
systems
Manages log-ons, password exchange, logoffs
Terminates connection at end of session
OSI Presentation Layer
Provides format and code conversion services
Examples
File conversion from ASCII to EBDIC
Invoking character sequences to generate bold,
italics, etc on a printer
OSI Application Layer
Provides access to network for end-user
User’s capabilities are determined by what
items are available on this layer
OSI in Action: Outgoing File
Transfer
 Program issues command to
Application Layer
 Application passes it to
Presentation, which may
reformat, passes to Session
 Session requests a connection,
passes to Transport
 Transport breaks file into
chunks, passes to Network
 Network selects the data’s
route, passes to Data Link
 Data Link adds errorchecking info, passes to
Physical
 Physical transmits data,
which includes information
added by each layer
OSI in Action: Incoming File
Transfer
 Physical receives bits, passes to
 Presentation may reformat,
Data Link
 Data Link checks for errors,
passes to Network
 Network verifies routing,
passes to Transport
 Transport reassembles data,
passes to Session
 Session determines if transfer is
complete, may end session,
passes to Presentation
perform conversions, pass
to Application layer
 Application presents results
to user (e.g. updates FTP
program display)
Data Communication Terms
Data - entities that convey meaning, or
information
Signals - electric or electromagnetic
representations of data
Transmission - communication of data by the
propagation and processing of signals
Examples of Analog and Digital
Data
Analog
Video
Audio
Digital
Text
Integers
Analog Signals
A continuously varying electromagnetic wave that
may be propagated over a variety of media,
depending on frequency
Examples of media:
Copper wire media (twisted pair and coaxial cable)
Fiber optic cable
Atmosphere or space propagation
Analog signals can propagate analog and digital data
Digital Signals
A sequence of voltage pulses that may be
transmitted over a copper wire medium
Generally cheaper than analog signaling
Less susceptible to noise interference
Suffer more from attenuation
Digital signals can propagate analog and digital data
Analog Signaling
Digital Signaling
Reasons for Choosing Data and
Signal Combinations
Digital data, digital signal
Equipment for encoding is less expensive than digitalto-analog equipment
Analog data, digital signal
Conversion permits use of modern digital transmission
and switching equipment
Digital data, analog signal
Some transmission media will only propagate analog
signals
Examples include optical fiber and satellite
Analog data, analog signal
Analog data easily converted to analog signal
Analog Transmission
Transmit analog signals without regard to
content
Attenuation limits length of transmission link
Cascaded amplifiers boost signal’s energy for
longer distances but cause distortion
Analog data can tolerate distortion
Introduces errors in digital data
Digital Transmission
Concerned with the content of the signal
Attenuation endangers integrity of data
Digital Signal
Repeaters achieve greater distance
Repeaters recover the signal and retransmit
Analog signal carrying digital data
Retransmission device recovers the digital data from
analog signal
Generates new, clean analog signal
About Channel Capacity
Impairments, such as noise, limit data rate
that can be achieved
Channel Capacity – the maximum rate at
which data can be transmitted over a given
communication path, or channel, under given
conditions
Impairments and Capacity
Impairments exist in all forms of data
transmission
Analog signal impairments result in random
modifications that impair signal quality
Digital signal impairments result in bit errors
(1s and 0s transposed)
Transmission Impairments:
Guided Media
Attenuation
loss of signal strength over distance
Attenuation Distortion
different losses at different frequencies
Delay Distortion
different speeds for different frequencies
Noise
distortions of signal caused by interference
Transmission Impairments:
Unguided (Wireless) Media
Free-Space Loss
Signals disperse with distance
Atmospheric Absorption
Water vapor and oxygen contribute to signal loss
Multipath
Obstacles reflect signal creating multiple copies
Refraction
Noise
Types of Noise
Thermal (aka “white noise”)
Uniformly distributed, cannot be eliminated
Intermodulation
When different frequencies collide (creating “harmonics”)
Crosstalk
Overlap of signals
Impulse noise
Irregular spikes, less predictable
Why Use Analog Transmission?
Already in place
Significantly less expensive
Lower attentuation rates
Fully sufficient for transmission of voice
signals
Analog Encoding of Digital Data
Data encoding and decoding technique to
represent data using the properties of analog
waves
Modulation: the conversion of digital signals
to analog form
Demodulation: the conversion of analog data
signals back to digital form
Modem
An acronym for modulator-demodulator
Uses a constant-frequency signal known as a carrier
signal
Converts a series of binary voltage pulses into an
analog signal by modulating the carrier signal
The receiving modem translates the analog signal
back into digital data
Methods of Modulation
Amplitude modulation (AM) or amplitude
shift keying (ASK)
Frequency modulation (FM) or frequency
shift keying (FSK)
Phase modulation or phase shift keying
(PSK)
Amplitude Shift Keying (ASK)
In radio transmission, known as amplitude
modulation (AM)
The amplitude (or height) of the sine wave
varies to transmit the ones and zeros
Major disadvantage is that telephone lines are
very susceptible to variations in transmission
quality that can affect amplitude
ASK Illustration
1
0
0
1
Frequency Shift Keying (FSK)
In radio transmission, known as frequency
modulation (FM)
Frequency of the carrier wave varies in accordance
with the signal to be sent
Signal transmitted at constant amplitude
More resistant to noise than ASK
Less attractive because it requires more analog
bandwidth than ASK
FSK Illustration
1
1
0
1
Phase Shift Keying (PSK)
Also known as phase modulation (PM)
Frequency and amplitude of the carrier signal
are kept constant
The carrier signal is shifted in phase according
to the input data stream
Each phase can have a constant value, or
value can be based on whether or not phase
changes (differential keying)
PSK Illustration
0
0
1
1
Differential Phase Shift Keying
(DPSK)
0
1
1
0
Analog Channel Capacity: BPS vs. Baud
Baud=# of signal changes per second
BPS=bits per second
In early modems only, baud=BPS
Each signal change can represent more than one bit,
through complex modulation of amplitude,
frequency, and/or phase
Increases information-carrying capacity of a channel
without increasing bandwidth
Increased combinations also leads to increased
likelihood of errors