Networking H15 Analog and Digital Data Data, Signal, Transmission Encoding and Decoding ( Amplitude...
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Transcript of Networking H15 Analog and Digital Data Data, Signal, Transmission Encoding and Decoding ( Amplitude...
Networking H15Analog 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 H15Media (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 considerations1) 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 bits5) How are transmission errors discovered and dealt with?6) How do packets get from one system to another?
Communication Considerations1) 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?
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 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 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, log-offs
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 error-checking 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
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
Presentation may reformat, 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
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
Reasons for Choosing Data and Signal Combinations
Digital data, digital signal Equipment for encoding is less expensive than digital-to-
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
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
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)
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