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Network and communications fundamentals (Net222) أساسيات اتصالات وشبكات

Lecture 1: Ch1 -Classifications of Signals-• What does a Signal mean? a function representing a physical quantity or

variable, typically contains information about the behavior or nature of the phenomenon.

• What does a System mean? viewed as transformation (mapping) of x into y, to process input signals to produce output signals.

• Types of Signals: - Electrical. e.g: voltage & current.- Electromagnetic. e.g: radio & light.- Sound. e.g: humans sounds & music.

• Data: entities that convey info• Signals: electric or electromagnetic representation of data• Signaling: physically propagates along a medium• Transmission: communication of data by propagation and processing

of signals

• Types of Signals:- Continuous-Time: Signal x(t) is continuous-

time signal if the independent variable t is continuous. it can be defined at every instant of time. it contains values for all real numbers along the X-axis. for e.g: Analog Signals. (intervals : ∞) data: Audio and video.

- Discrete-Time: A signal x(t) is defined at discrete times, it’s often identified as a sequence of numbers, denoted by x[n].

for e.g: Digital Singles. (only finite number of values) data: text, numbers and IRA(ASCII). Advantages: cheaper and less susceptible to noise interference. Disadvantages: suffer more from attenuation.

- Even Signals: any signal ‘x’ such that x(t) = x(-t), symmetric around y axis.- Odd signals: any signal ‘x’ such that x(-t) = -

x(t), symmetric around origin.

- Periodic Signals: pattern repeated over time.- Aperiodic signals: no pattern repeated.

Lecture 2: Ch1 -Classifications of Signals cont.-

- Sinusoidal Signals: Sine wave is the fundamental periodic signal.

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1/11/1439 AH+ it’s the simplest periodic signal.represented by: 1. Peak Amplitude (A) < Highest value in y axis. measured in: volts.2. Frequency (f) < # of cycle/s. measured in: Hz or cycle/s. period (T)= 1/f.3. Phase (φ) < relative position in time within a single period of signal.s(t) = A sin(2π f t +Φ) > see e.g in slide 19.

• Time domain: shows changes in signal amplitude with respect to time.

- wave length: distance occupied by a single cycle.

- light speed in free space = 3*10^8 m/s

• Frequency domain: show the relationship between amplitude and frequency. -more useful-

- measured in Hz (cycle/s) - Spectrum: Range of frequencies that a signal spans form minimum to

maximum.- Bandwidth: Absolute value of the difference between the lowest and highest

frequencies of a signal.> check e.g slide 26.

Lecture 2: Ch1 -Transformations of the independent variable (time)-

Lecture 2: Ch1 -The Unit impulse and the Unit steps function-

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Time Reversal Time Shifting Time Scaling

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Unit step function Unit impulse function

- Used in signal processing to represent a signal that switches on at a specified time and stays switched on indefinitely.

- It called also Heaviside function.

- Used in signal processing as an infinitesimally narrow pulse of unit area centers around 0.

- It called also Dirac delta function.

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Lecture 3: Ch1 -Continuous-Time and Discrete-Time Systems-

for more clear pictures of systems > check slide 3-5.

• Linear System (L): when an input to given system is scaled by a value, the output of the system is scaled by the

same amount. - it obeys the principle of superposition. (that means if two

inputs are added together and passed through a linear system, the output will be the sum of the individual inputs’ outputs.

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Cont.-time and Dust.-time Systems

Cascade(series) Interconnection

Parallel Interconnection

Series-parallel Interconnetion

Feedback Interconnection

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Invertible System The Inverse System

A system S is invertible if the input signal can always be uniquely recovered from the output signal. Y(t) = 2x(t)

foramally written as, S^-1, such that the cascade interconnection in the figure below is equivalent to the identity system, which leaves the input unchanged. y(t) = 1/2 x(t)

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System with memory Without memory (memory-less)

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∑−∞=

=n

k

kxny ][][

if the output at anytime depends on only the input at that same time. Otherwise, the system is said to have memory.

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• Time-Invariant System (TI): has the property that a certain input will always give the same output, without regard to when the input was applied to the system.

- Because the system is (TI), the input x(t) and x(t-to) produce the same output. The only difference is that the output due to x(t-to) is shifted by a time to.

• Linear Time-Invariant System (LTI): It obeys the principle

of superposition (if two inputs are added together and passed through a linear system, the output will be the sum of the individual inputs’ outputs).

Lecture 4: Ch3 -Data Transmission-Physical Layer: Foundation on which other layers build.Properties of wires, fiber, wireless limit what the network can do.Modulation: Key problem is to send digital bits using only analog signals.• Transmission medium and physical layer:

• Transmission media: anything that carry info between a source to a destination.

• Located below the sender physical layer and is connected to the other receiver physical layer.

• The successful transmission of data depends on:

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LTI Systems in Series LTI Systems in Parallel

if two or more LTI systems are in series with each other, their order can be interchanged without affecting the overall output of the system. also called cascaded systems.

if two or more LTI systems are in parallel with one another, an equivalent system is one that is defined as the sum of these individual systems.

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1/11/1439 AH1. Quality of the signal being transmitted.2. Characteristics of the transmission medium.

• Types of connections: 1. Point-to-point (direct link between 2 devices) 2. Multipoint (more then 2 devices share the same medium) • Direction of data flow: - Simplex: signals transmitted in 1 direction < eg: television - Half duplex: both stations transmit, but only 1 at a time. < e.g.: police radio - Full duplex: simultaneous transmissions. < eg: telephone • Time domain and frequency domain: - How to find frequency? Number of cycles per sec Data rate: amount of data that is moved from one place to another in a given time. Bandwidth: -see lecture 2- There is a direct relationship between data rate & bandwidth. Lecture 5: Ch3 -Data Transmission-• Transmission Impairments: - Signal received may differ from signal transmitted causing: 1. Analog - degradation of signal quality.2. Digital - bit errors- Most significant impairments are: 1. Attenuation2. Delay distortion3. Noise• Attenuation: - Signal strength falls off with distance over transmission medium. - we can use amplifiers/repeaters to increase strength. - Signal strength varies with frequency(Attenuation is greater at higher

frequencies and this causes distortion. - we can equalize attenuation by using loading coils/amplifiers.• Received signals must be: - Strong enough to be detected- Higher than noise to be received without error.- Attenuation: Db = 10 log10 (Pd / Ps) answer = - > loss of data , answer = + > gain of data

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Thermal noise Intermodulation noise Crosstalk Impulse noise

- agitation of electrons - referred to as white

noise - can’t be eliminated- significant for satellite

communication.

- produced by nonlinearities in the transmitter and receiver.

- signal from one line picked up by another.

- caused by external electromagnetic interference (lighting, faults, flaws).

- consisting of irregular pulses or spikes of short duration and high amplitude.

1/11/1439 AH• Delay Distortion occurs because propagation velocity of a signal through a guided

medium varies with frequency.• Noise: unwanted signals inserted between transmitter and receiver. < major

limiting factor. • Noise categories: - SNR(Signal-to-Noise-Ratio): SNR = Signal / Noise - SNR(db) = 10 log10 (SNR) • Channel Capacity: maximum (largest) rate at which data can be transmitted

over a given communications channel under given conditions. • Nyquist Bandwidth: C = 2B log2 M - where M = number of discrete signal/voltage levels. • Capacity: C = B log2 (1+SNR) Lecture 6.1: Ch3 -Transmission Media-• guided media - wire: electromagnetic waves are guided along a solid

medium. • unguided media - wireless: transmission occurs through

the atmosphere. • Transmission characteristics of guided media: - just to compare between them numbers are not important>• Twisted pair: - decrease cross talk - cheaper - easier to work with

- Application: - Telephone network- between house and local exchange- within building- local area network (LAN)

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unshielded (UTP) shielded (STP)

- telephone wire - cheapest - easier to install - suffers from external EM

interference

- metal braid or seating that reduces interference

- more expensive - harder to handle because it’s thick

and heavy.

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1/11/1439 AH- Types: • Near end Crosstalk: • occurs when transmit signal entering the

link couples back to receiving pair. • Optical Fiber: uses reflection to guide light through a channel. • a glass or plastic core is surrounded by a cladding of less dense glass or plastic.- used in: 1. Cable TV network2. Local area networks3. Backbone networks.• Propagation modes using fiber optics: 1. Multimode Fiber: any light ray incident on thou boundary above the critical

angle will be reflected internally, many 2. Single-mode Fiber: light can propagate only in a straight line, without bouncing.• Benefits: - greater capacity - smaller size & weight - lower attenuation - electromagnetic isolation - greater repeater spacing. -- just to compare between them numbers are not

important >

Lecture 6.2: Ch3 -Transmission Media-

• Electromagnetic waves are formed when an electric field (shown in blue) couples with a magnetic field (red arrows).

- travels at light speed- don’t require a medium.• Antenna: electrical conductor used to radiate or

collect electromagnetic energy. • Types: 1. Horn antenna2. Dish antenna(parabolic reflective antenna)

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Radio wave Microwave Infrared wave

- omnidirectional - point-to-point communication

- for satellite communication

- local point-to-point and multipoint application

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• Types of radiation patterns:

Antenna Gain: measure of directionality of an antenna. G = 4(Pi) Um / PT

Um : maximum radiation intensity. PT : total input power isotropically.

Radiation Efficiency: ratio of total power radiated by an antenna to the net power accepted by the antenna from the connected transmitter. er = Pr/Pin Pin : portion of input power.

Antenna Effective Aperture: Pt = P Ae

• Impairments specific to wireless line-of-sight transmission: - Free space loss - Atmospheric absorption - Multipath - Refraction.

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Isotropic antenna Omnidirectional Directional antenna

- a point in space that radiate power in all directions equally.

- Endfire

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- Broadside

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- doughnut shape

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Broadcast Radio Microwave Infrared

- omnidirectional - Communication need not line

of sight- can penetrate walls- less sensitive to rainfall- suffers form multipath

interference.

- Terrestrial Microwave- Satellite Microwave- Satellite communication

configuration:- Point-to-point link- Broadcast link

- Infrared communications is achieved using transmitters/receivers that modulate non-coherent infrared light.

- Transceivers must be within the line of sight

- blocked by walls- no licenses required- typical uses: TV remote

control, IRD port.

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Lecture 7.1: Ch5 -Signal Encoding Techniques-- for digital signaling, a data source g(t), which may be either digital

or analog, is encoded into a digital signal x(t).- the basis for analog signaling is a continuous constant-frequency

signal AKA the carrier signal. - Digital data, digital signals: simplest form of digital encoding

of digital data. < equipment less complex and less expensive than digital-to-analong modulation equipment.

• Digital signal is a sequence of discrete voltage pulses. • Line coding schemes: - unipolar > 1 voltage level - polar > 2 voltage levels (+,-): NRZ-I,NRZ-L, Manchester,

Differential Manchester - bipolar > 3 voltage levels (+,0,-) Encoding scheme: mapping from data bits to signal elements. include:

• Multilevel Binary issues: - synchronization with long runs of 0s or 1s- scramble data- not as efficient as NRZusage of scrambling: to replace sequences that would produce constant voltage.• Biphase: Pros+ synchronization on mid bit transition (self clocking)+ has no dc component+ has error detection Cons - at least one transition per bit time and possibly two- maximum modulation rate is twice NRZ- requires more bandwidth• Modulation rate: rate at which signal elements are generated.

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NRZ-L NRZ-I Bipolar AMI Pseudoternary Manchester Manchester Differential

0 : +

1 : -

0 : no transition 1 : high to low/low

to high

0 : no line signal

1 : -/+

0 : -/+ 1 : absence of line

signal

0 : high to low 1 : low to high

0 : transition at start

1 : no transition at start

lack of synchronizaition when data contain long streams of 0/1

loss of synchronizaition when data contain long streams of 0

no loss of sync if long string of 1s

Pros + easy to engineer + make good use

of bandwidth Cons - dc component - lack of

synchronization capability

< same

- long runs of zeroes still a problem

- no net dc component

- lower bandwidth

- easy error detection

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Lecture 7.2: Ch5 -Signal Encoding Techniques-Encoding is the conversion of streams of bits into signal (digital/analog). Categories of encoding techniques: Digital Transmission: - Digital data > Digital signal - Analog data > Digital signal Analog Transmission: - Digital data > analog signal- Analog data > analog signal

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Analog data > analog signal Digital data > analog signal Analog data > digital signal

Modulation: process of encoding source data onto a carrier signal with frequency. it involves:- Amplitude (AM) -simplest- - Frequency (FM) - Phase (PM) -FM and PM requires greater bandwidth--Amplitude modulation requires 2B < twice bandwidth-

Modulation involves:- Amplitude Shift Keying (ASK) - susceptible to sudden gain changes- inefficient - Frequency Shift Keying (FSK) • Binary Frequency Shift Keying (BFSK) - most common- less susceptible than ASK• Multiple FSK - more than 2 f used- more bandwidth efficient- more prone to error

- Phase Shift Keying (PSK) 1. Binary phase shift keying (BPSK)2. Differential phase shift keying (DPSK)0 : same phase as previous 1 : opposite to preceding one

• Multiple PSK - more efficient use of bandwidth- each signal represent more than 1 bit• Quadrature PSK (QPSK)

Digitization: conversion of analog data into digital data. it’s done using a codec(coder-decoder)

• Pulse code modulation: Sampling theorem: *rate higher than twice the highest signal frequency eg: 4000Hz > 8000 samples/second• Non-Linear Coding: PCM scheme is refined using a technique AKA nonlinear encoding, it means that quantization levels are not equally spaced.- reduces overall distortion- can significantly improve the PCM

SNR ratio.• Companding: a process that

compresses the intensity of a signal by setting more gain to weak signals than to strong signals on input.

• Delta Modulation:analog input is approximated by a staircase function.

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Lecture 8: Ch6 -Digital communication technique-

Transmission mode is the manner in which data is sent over the underlying medium. Transmission modes can be divided into two fundamental categories:- Serial -one bit is sent at a time-- Parallel -multiple bits are sent at the same time-

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Half Duplex(HDX) Full Duplex(FDX)

- provides communication in both directions

- but only one direction (not simultaneously)

- once a party begins receiving a signal, it must wait for the transmitter to stop transmitting before replying.

- examples: walkie talkies

- or it called double-duplex- allows communication in

both directions- unlike half-duplex, allows

this to happen simultaneously.

- examples: Land-line telephone networks.

Benefits to using full-duplex over half-duplex

1. time is not wasted, since no frames need to be retransmitted, as there are no collisions.2. the full data capacity is available in both directions because the send and receive functions are separated.3. stations(nodes) don’t have to wait until others complete their transmission, since therein only one

transmitter for each twisted pair.

Parallel transmission Serial transmission

- allows transfers of multiple data bits at the same time over separate media.

- it’s used with a wired medium- the signals on all wires are synchronized so that a bit travels

across each of the wires at precisely the same time- n wires are used to send n bits at one time- advantages: speed- disadvantages: cost; limited to short distances.

- it sends one bit at a time- most communication systems use serial mode, because:• serial networks can be extended over long distances at less cost• using only one physical wire means that there is never a timing

problem caused by one wire being slightly longer than another- advantages: reduced cost- disadvantages: requires conversation devices- in serial mode, when sending bits, which bit should be sent across

the medium first?- consider an integer: should a sender transmit the Most Significant

Bit or Least Significant Bit? Either form can be used, but the sender and receiver must agree.

- Serial transmission mechanisms can be divided into two board categories(depending on how transmission are spaced in time):

- Asynchronous > transmission can occur at any time- Synchronous > transmission occurs continuously

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• Asynchronous Transmission:- Asynchronous transmission allows the physical medium to be idle for an

arbitrary amount of time between two transmissions.- it’s well-suited to applications that generate data at random time intervals. for example: - a user typing on a keyboard- a user that clicks on a hyperlink - transfer of data with start and stop bits and a variable time interval between data

units.- timing is not important - Start bit > alerts receiver that new group of data is arriving.- Stop bit > alert receiver that byte is finished.- Synchronization achieved through start/stop bits with each byte received.- The beginning of a character is signaled by a start bit with a value of binary 0. - followed by the 5 to 8 bits that actually make up the character.- Then the data bits are usually followed by a parity bit, set by

the transmitter, the receiver uses this bit for error detection.- The final element is a stop element, which is a binary 1. - Characteristics: - Cheap and effective - ideal for low-speed communication when gaps may occur

during transmission (ex: keyboard)- Asynchronous disadvantages: - Asynchronous technologies usually require the sender to transmit a few extra bits

before each data item:- to inform the receiver that a data transfer is starting- extra bits (preamble or start bits) allow the receiver to synchronize with the

incoming signal- Slower • Synchronous Transmission:- Requires constant timing relationship - Bit stream is combined into longer frames, possibly contain multiple bytes- any gaps between bursts are filled in with a special sequence of 0s and 1s

indicating idle- advantages: - speed, no gaps or extra bits - more efficient than asynchronous - block of data transmitted, sent as frames in a steady stream without start and

stop codes.- clocks must be synchronized - can use separate clock line, or embed clock signal in data- need to indicate start and end of block- use preamble and post-amble (flags)

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• Transmissions Types: In serial transmission a character is converted from parallel to serial form when transmitting and form serial to parallel form when receiving.

Lecture 9: Ch8 -Multiplexing-- Multiplexing process allows several transmission sources to share a larger

transmission capacity.- multiple links on 1 physical line - most common use of multiplexing is in long-haul communication using

coaxial cable, microwave and optical fiber. - the multiplexer combines (multiplexes) data from the n input lines and

transmits over a single data link(medium). - The demultiplexer separates (demultiplexes) the data according to channel,

and delivers data to the appropriate output lines. - Multiplexing: the set of techniques that allows

the simultaneous transmission of multiple signals across a single data link. types:

1. Frequency-Division Multiplexing (FDM)2. Wavelength-Division Multiplexing (WDM)3. Time-Division Multiplexing (TDM)4. Code-Division Multiplexing (CDM)

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FDM

- can be used with analog signals- put different signals on different frequency bands using modulation- each signal is modulated onto a different carrier- all the modulated signals are combined to form a composite signal for

transmission- signals are carried simultaneously on the same medium - to prevent interference, the channels are separated by guard bands

which are unused portions of the spectrum.- television and radio uses FDM to broadcast many channels over the

same media.

FDM system overview

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• Synchronous Time Division Multiplexing - can be used with digital signals or analog signals carrying digital data. - in this form of multiplexing, data from various sources are carried in

repetitive frames.- Each frame consists of a set of time slots, and each source is

assigned one or more time slots per frame.- Synchronous TDM is called synchronous not because

synchronous transmission is used, but because the time slots are preassigned to sources and fixed.

- the multiplexer allocates exactly the same time slot to each device at all times, wether or not a device has anything to transmit.

- a frame consists of one complete cycle of time slots - thus the number of slots in frame is equal to the number of

inputs. - How Synchronous TDM Works? >• Asynchronous TDM(Statistical Time Division Multiplexing) - each slot in a frame is not dedicated to the fix device - the number of slots in a frame is not necessary to be equal to

the number of devices - more than one slots in a frame can be allocated for an input device - allows maximum utilization of the link - it allows a number of lower speed input lines to be multiplexed to a

single higher speed line• Statistical TDM - in synch TDM many slots are wasted - Statistical TDM allocates time slots dynamically based on demand- multiplexer scans input lines and collects data until frame full - may have problems during peak periods (must buffer inputs). - How Asynchronous TDM Works? >

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WDM

- theoretically identical to Frequency Division Multiplexing- used in optical systems while FDM is used in electrical systems- Requires more spacing between channels- also have Dense Wavelength Division Multiplexing (DWDM)

TDM

- multiple transmission can occupy a single link by subdividing them and interleaving the portions

- TDM can be implemented in two ways:1. Synchronous TDM 2. Asynchronous TDM

1/11/1439 AHSynchrnous vs. Statistical TDM >

• Multiplexing Real Examples Asymmetrical Digital Subscriber Line(ADSL) - Digital Subscriber Line is the link between subscriber and network. - it uses currently installed twisted pair cable - is Asymmetric -bigger downstream than up- - uses frequency division multiplexing - has a range up to 5.5km • Code Division Multiplexing - sends many signals or “chips” per bit - each sender uses a unique pattern of chips - may use multiple frequencies for spread spectrum communication - common with wireless systems.

الحمدلله الذي بنعمته تتم الصالحات.

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