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

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

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

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

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 error- correcting 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

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

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

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

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):