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):