Post on 25-Feb-2016
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Transmission BasicsITNW 1325, Chapter III
OSI Physical Layer
Physical LayerOverview: 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 LayerNetwork Media:
Physical LayerNetwork Media (continued):
Physical LayerNetwork Media (continued):
Signaling Types
Signaling TypesAnalog: 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 TypesAnalog (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 TypesAnalog (continued):
Signaling TypesDigital: 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 TypesCompared:
Analog Modulation
Analog ModulationOverview: 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 ModulationOverview (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 ModulationAmplitude (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 interferenceFrequency (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 ModulationAmplitude, Illustration:
Analog ModulationFrequency, Illustration:
Analog ModulationPhase (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
Use Examples: Radio broadcast stations use AM or FM Television broadcast stations use AM for video, FM for
sound, and PM for color
Analog Modulation
Digital Modulation
Digital ModulationOverview: 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 error-
correcting techniques and require less power Used broadly by modern communication systems
Digital ModulationAmplitude 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 ModulationFrequency 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 ModulationPhase 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 ModesOverview: 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 ModesOverview (continued):
Duplex ModesFull 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 ModesFull Duplex (continued):
Relationships
RelationshipsOverview: 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)
RelationshipsOverview (continued): Multicast – a point-to-multipoint relationship that
implies one specific sender and multiple defined receivers (example – audio and video conferences)
RelationshipsOverview (continued):
RelationshipsOverview (continued):
RelationshipsOverview (continued):
Throughput and Bandwidth
Throughput and BandwidthOverview: 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 BandwidthExamples: 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 BandwidthExamples (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 BandwidthExamples (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
MultiplexingOverview: 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
MultiplexingOverview (continued):