Transcript Part I - Austin Community College

Transmission Basics
ITNW 1325, Chapter III
OSI Physical Layer
Physical Layer
Overview:
 Facilitates transmission of signals over network media –
copper cable, fiber optics cable, or a wireless medium
 Signals travel as electrical current in a copper cable, as
light pulses, and as EM waves in these media
 Defines and implements physical communications
principles – signaling, multiplexing, duplex modes, etc.
 Communications problems that occur have affect all
other layers and thus security of communications
 Better understanding of its principles and technologies
enables fast recovery from network failures
Physical Layer
Network Media:
Physical Layer
Network Media (continued):
Physical Layer
Network Media (continued):
Signaling Types
Signaling Types
Analog:
 Implies continuously changing voltage or intensity –
signal appears as a wavy line when graphed over time
 Possesses four common characteristics – amplitude,
frequency, wavelength, and phase
 Amplitude – the measure of the wave’s strength at any
given point in time (maximum deviation from center)
 Frequency – the number of full cycles of the amplitude
in a second (measured in Hz, KHz, MHz, GHz, etc.)
 Wavelength – the distance between consequent similar
points on a wave (measured in length units)
Signaling Types
Analog (continued):
 Phase – a measure of the progress of a wave over time
in relation to a fixed initial point
 Quite variable – can convey greater subtleties with less
energy (human vs. computer voice)
 Continuous in nature – carry imprecise signal levels that
are further affected by interference and environment
Signaling Types
Analog (continued):
Signaling Types
Digital:
 Implies encoding logical bits – binary zeroes and ones –
into precise levels of voltages or medium intensities
 Fit perfectly the binary nature of computer data – both
wired and wireless LANs use digital signaling only
 Transmission of discrete pulses is more resistant to
interference – brings lower compensation overhead
 Requires more complex communication equipment
Signaling Types
Compared:
Analog Modulation
Analog Modulation
Overview:
 Enables modification of analog signals to carry useful
data – not all media can carry digital signals
 Employs two devices – transmitter and receiver – and
two waves – a carrier wave and a data wave
 A carrier wave has well-known wavelength, frequency,
amplitude, and phase – conveys information
 A data wave carries data to be transmitted – used for
alteration of one of the carrier wave’s parameters
 A transmitter combines the two waves for data – by
modifying one of the the carrier wave’s parameters
Analog Modulation
Overview (continued):
 Alterations of the carrier wave’s amplitude, frequency,
or phase produce AM, FM, or PM analog modulations
 The resultant analog wave carries useful information –
transmitted over the medium to the receiver
 The receiver is aware of the carrier wave’s original
parameters – reads information from it by comparing
the actual wave received to the original one
Analog Modulation
Amplitude (AM):
 Implies modifying the maximum amplitude at each
peak of the carrier wave – with higher peaks standing
for logical 1s and lower peaks representing logical 0s
 Susceptible to interference
Frequency (FM):
 Implies modifying the duration of consequent carrier
wave’s cycles – with shorter cycles representing logical
1s and longer cycles representing logical 0s
 Less susceptible to interference than AM
Analog Modulation
Amplitude, Illustration:
Analog Modulation
Frequency, Illustration:
Analog Modulation
Phase (PM):
 Implies modifying the carrier wave’s phase according to
bit changes between 1 and 0 in the data signal
 Requires most complex equipment types of all
Analog Modulation
Use Examples:
 Radio broadcast stations use AM or FM
 Television broadcast stations use AM for video, FM for
sound, and PM for color
Digital Modulation
Digital Modulation
Overview:
 Employs three techniques that are similar to AM, FM,
and PM – abbreviated ASK, FSK, and PSK
 Relies on discrete signal levels – not affected by
interference as much as analog signals
 Digitally modulated signals enable effective errorcorrecting techniques and require less power
 Used broadly by modern communication systems
Digital Modulation
Amplitude Shift Keying (ASK):
 Carrier signal (positive voltage or intensity) encodes a
binary 1 and no carrier signal encodes a binary 0
 Resembles analog amplitude modulation
Digital Modulation
Frequency Shift Keying (FSK):
 Higher frequency (tighter wave) encodes a binary 1 and
lower frequency (wider wave) encodes a binary 0
 Resembles analog frequency modulation
Digital Modulation
Phase Shift Keying (PSK):
 One change in phase encodes transition to a binary 1
while other change encodes transition to a binary 0
 Resembles analog phase modulation
Duplex Modes
Duplex Modes
Overview:
 Reflect possible directions of a data flow – as well as
possible utilization of both directions at a time
 Simplex – signals can travel in only one direction
(example – a broadcast radio station)
 Half-duplex – signals can travel in both directions but in
only one direction at a time (example – a walkie-talkie)
 Full-duplex – signals can travel in both directions
simultaneously (example – a telephone conversation)
 The duplex mode can be specified by humans or
negotiated between computer devices
Duplex Modes
Overview (continued):
Duplex Modes
Full Duplex:
 Maximizes data rates in both directions – beneficial for
modern computer networks that use it widely
 One physical channel would commonly be used for
transmitting data while another one – for receiving it
 Example – multiple wires used for sending and
receiving data combined into single network cable
 Must be supported by both communication peers in
order for them to communicate – may be negotiated too
Duplex Modes
Full Duplex (continued):
Relationships
Relationships
Overview:
 Reflect possible numbers and types of hosts sending
and receiving data over a network
 Point-to-Point (PtP, Unicast) – implies one specific
sender and one specific intended receiver (example – a
WAN connection between business locations)
 Point-to-Multipoint (PtM) – implies one specific sender
and multiple defined or undefined receivers
 Broadcast – a point-to-multipoint relationship that
implies one specific sender and multiple undefined
receivers (example – TV and radio stations)
Relationships
Overview (continued):
 Multicast – a point-to-multipoint relationship that
implies one specific sender and multiple defined
receivers (example – audio and video conferences)
Relationships
Overview (continued):
Relationships
Overview (continued):
Relationships
Overview (continued):
Throughput and Bandwidth
Throughput and Bandwidth
Overview:
 Bandwidth – a difference between the highest and
lowest frequencies that the medium can transmit (Hz)
 Throughput – a number of bits transmitted per second
(reflects a real communication data rate)
 Bandwidth correlates with maximum achievable data
rate while throughput measures the actual data rate
 The two are not the same thing but get mixed up often
Throughput and Bandwidth
Examples:
 Bit per second – equivalent to 1 bit per second,
abbreviated bps
 Kilobit per second – equivalent to 1000 bits per second,
abbreviated Kbps
 Megabit per second – equivalent to 1,000,000 bits per
second, abbreviated Mbps
 Gigabit per second – equivalent to 1,000,000,000 bits
per second, abbreviated Gbps
Throughput and Bandwidth
Examples (continued):
 Hertz – equivalent to 1 oscillation per second,
abbreviated Hz
 Kilohertz – equivalent to 1000 oscillations per second,
abbreviated KHz
 Megahertz – equivalent to 1,000,000 oscillations per
second, abbreviated MHz
 Gigahertz – equivalent to 1,000,000,000 oscillations per
second, abbreviated GHz
Throughput and Bandwidth
Examples (continued):
 Residential cable and DSL connections provide
throughput of up to 30 and 3 Mbps, respectively
 Modern wired and wireless local area networks provide
up to 10 Gbps and up to 1.3 Gbps, respectively
Multiplexing
Multiplexing
Overview:
 Enables splitting the network medium into multiple data
channels in order for multiple signals to travel at once
 Effectively increases the amount of data transmitted
over the medium available during a time frame
 A multiplexer combines signals at the sending end –
with a demultiplexer separating them at the receiving
end to obtain the original separate data streams back
 Type of multiplexing used depends on what the media,
transmission, and reception equipment can handle, with
several types used most commonly
Multiplexing
Overview (continued):