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?

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, 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

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 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

1 0 0 1

ASK Illustration

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

1 1 0 1

FSK Illustration

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)

0 0 1 1

PSK Illustration

0 1 1

Differential Phase Shift Keying (DPSK)

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