Data Communication and Computer Networks Lecture-1 Prepared by: Abdul Hasib Lecture, IICT BUET.

Data Communication and Computer Networks

Lecture-1

Prepared by:

Abdul HasibLecture, IICT

BUET


Time domain (examining the signal over time):

• Continuous signal - signal with no breaks or discontinuities

• Discrete signal - signal with a finite number of values

Amplitude - the instantaneous value of a signal

•Frequency - inverse of the period in cycles per second, Hertz

•Phase - measure of relative position in time within a single period

Frequency, Spectrum and Bandwidth


Frequency Domain (signal viewed as a function of frequency):

• any signal is made up of components at various frequencies, each sinusoid

• Spectrum- the range of frequencies in a signal

• Absolute bandwidth - is the width of the spectrum (fn - f1) where, fn is largest frequency in signal and f1 is the smallest

Frequency, Spectrum and Bandwidth

Audio signal Digital signal


Signal Strength

• Effective bandwidth (bandwidth) - width of spectrum containing most of the energy in the signal

• Signal strength gain, losses and relative levels are expressed in decibels:

• Decibel measure difference in two power levels:


Communication mode

• Point-to-point - if direct link is shared between only twodevices

• Multipoint - if direct link shared between multiple devices

• Simplex- one way transmission (commercial radio/TV)

• Half-duplex - one way transmission at a time, endpoints taketurns

• Full-Duplex - simultaneous two way transmission

» NOTE: These are US (ANSI) definitions, in Europe (CCITT)simplex refers to half-duplex and duplex to full-duplex


Data versus signal

• Data: Convey meaning within a computer (stored in files)

Need to be converted into a signal before transfer

• Signals: Electric or electromagnetic encoding of dataNetworks and communication systems transmit signals

• Data and signal can be analog or digital– Analog information can be audio or video– Digital information is binary

• Issues related to the transmission of signals– Impairments (also transmission flaws)– Capacity of the media


Analog to Digital Conversion : Why?

• Digital Technology : a. VLSI, LSI b. lower cost

• Data Integrity : Use repeaters instead of amplifiers=> Noise not accumulated, i.e., transmit data for longer distance

• Capacity utilization : More easily to multiplex digital data than analog

• Security and privacy

• Integration : integrate voice , data ( when digitize analog data)


Modulation and Encoding schemes

Modulation:

Encoding:


Conversion

Encoding: •Digital data to digital signal : less complex and less expensive equipment than analog modulation equipment

• Analog data to digital signal : To use the modern digital transmission and switching equipment

Modulation:• Digital data to analog signal : Some transmission media can propagate analog signals only. Example: fibre , wire

• Analog data to analog signal : 1. Transmit baseband signal over wire transmission (Microwave) => Low frequency baseband means few kilometer antenna !!!

2. shift baseband signals of several voice channel (FDM)


• Factors to improve receiving data: Decrease data rate => decrease error rate Increase S/N ratio => decrease bit error rate Increase bandwidth => Increase data rate

Digital data , Digital signals

• Digital signal encoding formats: Nonreturn-to-zero-level (NRZ-L) Nonreturn-to-zero Interted (NRZI) Bipolar-AMI Manchester Differential Manchester etc.


Digital signal encoding formats


Analog Data, Digital Signals

• Codec : Coder – decoder Device to convert analog to digital and digital to analog at transmitting and receiving side

• Sampling Theorem : “If a signal f(t) is sampled at a regular intervals of time and at a rate higher than twice the highest significant signal frequency, then the samples contains all the information of the original signal.”

example: voice data < 4000Hz, then 8000 samples/sec (Nyquist formula)

• Data Rate:

C = capacity or data transfer rate in bps

B = bandwidth (in hertz)

M = number of possible signaling levels


Analog to Digital Conversion (PCM)


Modulation

• The process of encoding source data onto a carrier signal with frequency f.• Three basic modulation technique:

-> Amplitude-> Frequency-> Phase

• Baseband signal :Input signal (digital or analog) to modulator.


• Transmitting digital data through public telephone network (0.3 - 3.4 KHz)• Example : Modem (modulator, demodulator)

• Digital Modulation techniques: 1. Amplitude-Shift Keying (ASK):

amplitude of carrier freqency vary betwwen two level susceptible to sudden gain changes and is rather inefficient technique up to 1200 bps on voice-grade lines

Digital data, Analog signals


Digital Modulation techniques:

2. Frequency-Shift Key (FSK)

A cos(2 f1 t + Qc) binary 1s(t) =

A cos(2 f2 t + qc) binary 0

f1 and f2 are carrier frequency• Used in early low bit modem


Digital Modulation techniques

3. Phase Shift Key (PSK)

•The phase of the carrier signal is shifted to represent data

Fig: Phase coherent PSKZero : represented by a signal with the same phase of the preceding oneOne: represent by signal of opposite phase (180° shift)to the preceding one

• Disadvantage: - Reference carrier signal phase is required at the recever - Bit rate= Signalling rate

M=2EE


Analog Data, Analog Signals

• Techniques:AM, FM, and PM

•Amplitude Modulation

Carrier

f tccos( )2

Baseband

signal sm

Bandpass

s t tm

signal

cos 2 fc( )

Figure 7.2: amplitude modulation of a sinusoidal carrier by the baseband PAM signal


Amplitude-modulation.

0 r

G fr ( )2

WW-

1

f

U fm ( )1

2

0- fc + W

- fc - W

- fc

fc + W

fc

fc - W

Figure: Spectra of (a) baseband and (b) amplitude-modulated signal.


Analog Modulation Techniques


Simple switching network

end node

Network node-- provide routing

Purpose:- provide interconnection between all the nodes on a network without the need for single connections between each pair of nodes


Digital Telephone Networks, Mobile Networks

• Circuit switched of fixed bit rate (n x 64 kbps / 13 kbps) • Connection oriented


Public switched telephone network (PSTN)


Circuit Switching (CS)

• Communication in which a dedicated communications path is established between two devices through one or more intermediate switching nodes

• Dominant in both voice and data communications today (PSTN is a circuit-switched network)

• Relatively inefficient (100% dedication even without 100% utilization)

•Three stages:

–Circuit establishment

–Transfer of information

–Circuit disconnect


Circuit-Switching Stages

• Circuit establishment– Based on routing information

• Transfer of information– Point-to-point from endpoints to node– Internal switching among nodes– Usually a full-duplex connection throughout

• Circuit disconnect– Signal initiated by one of the stations and propagated

to used nodes to de-allocate the dedicated resources


Example: space division switch

• The interconnection of network consists of a rectangular matrix of cross-points


• Multiplexing provides a mechanism to share the use of a common channel or circuit by two or more devices.

• Multiplexing minimizes number of transmission lines.

Multiplexing


• FDM (Frequency Division Multiplex)

• TDM (Time Division Multiplex)

• WDM (Wave Length division Multiplex)

• CDM (Code division Multiplex)

Type of multiplexer


Frequency Division Multiplex

• FDM is a broadband analog transmission technique.

• Each data signal is modulated onto a carrier with a different frequency

• All signal travel simultaneously over a channel.


• TDM is a base-band technique.

• Individuals circuit are identified by their position in a stream.

• Analog inputs are digitized using PCM

• Digitized information are insert into the pre-allocated, fixed timing called timed slot.

Time Division Multiplex


Wave Length division Multiplex

• Used for photonic communication.

• Realised by laser modulation of different wave length.

• Superposition of optical signals of different channels on one fibre


Code division Multiplex

• Each channel uses a different code sequence for modulation

• Codes are mutually orthogonal

• Spread spectrum technique

• Multiplexing is acheived by superposition of the products of the signal with their code.


Digital Carrier Systems

• T-carrier

North America,Japan

• E-carrier

Europe,South America

• SONET/SDH

world-wide new standard


E1-frame

E1 bit rate :(32x8 bit)/125 microsec =2.048Mbps

30 voice channels+2 control channels


• The type of switching depends on connection nature.

• Transfer modes:

-Circuit Switching (CS)

-Packet switching

Transfer Modes

• Problems of Circuit Switch:

- Collisions: when more than one inputs are destined for the same output.

- Blocking: when the progress of one message through the network is stopped by a message that is not destined for the same output.


Local Area Networks (LAN)

• Connectionless


Internet


Alternate Routing


• Datagram– Connectionless service – Individual packets can follow different routes– Packets can arrive out of sequence and are reassembled on the destination

host.

Packet-Switching

• Data is broken into packets, each of which can be routed separately


•Virtual Circuit

–Establishes an end-to-end circuit between the sender and receiver

–All packets for that transmission take the same route over the virtual circuit

–Similar to circuit switching, but the circuit is not dedicated

Packet Switching


• Advantages: – Better line efficiency, – Signals can always be routed– Prioritization option

• Disadvantages: – Transmission delay in nodes,– Variable delays can cause jitter– Extra overhead for packet addresses

Packet-Switching Networks:Pros and Cons


Example: Virtual Connection


Development of Internet Hosts and Web


Internet History

• Arpanet 1960s : studies of packet switching

1980-1983: Introduction of TCP/IP

1989: first proposal for Web (Tim Berners, Robert Cailliau)

1994: Internet known to public

Everything over IP, IP over everything


Arpanet History


Growth

• Number of users• Traffic demand per application

Web item sizes (imazes, java applets, audio, video) • New applications• Access line bit rate• Number of servers• Penetration into leisure / entertainment sector


Worldwide Ranks


Challenges

• Connectivety: connecting various systems to support communication among disparate technologies

• reliability

• network management: must provide centralized support and troubleshooting capabilities configuration, security, performance

• flexibility: to change with new demands.


Standardization Body

• Internet Activities Board (IAB):

discuss issues pertinent to the Internet and set Internet policies through decisions and task forces. The IAB designates some Request For Comments (RFC) documents as Internet standards, including Transmission Control Protocol/Internet Protocol (TCP/IP) and the Simple Network Management Protocol (SNMP).

• create formal standards by:

organizing ideas discussing the approach developing draft standards voting on draft formally releasing the completed standard to the public


Standardization Body


Towards a global information infrastructure


Applications for Broadband Infrastructure


Transmission impairment

• Damage caused to signal during transmission

• Impairments exist in all forms of data transmission– Analog signal impairments degrade the signal quality

– Digital signal impairments result in bit errors (1s and 0s transposed)

• Type of impairment depends on type of media used


Categories of transmission media

• Conducted or guided media– Use a conductor to move the signal from sender

to receiver– TP, Coax and fiber optic cable

• Wireless or unguided media– Use radio waves at different set of frequencies to

move the signal from sender to receiver– Microwaves, radio frequencies, infrared light


Transmission Impairments:Guided media

• Attenuation: Loss (in dB) of signal strength over distance

Prevention: Repeater or amplifier

• Noise: Distortion caused by interference from external sources

Prevention: Depends on type of noise


Transmission Impairments: Wireless media

• Free-Space Loss : Attenuation due to distance

• Atmospheric Absorption: Attenuation due to atmospheric conditions (humidity)

• Multi-path: Due to lack of direct line of sight path. Obstacles reflect signal (creating multiple copies) or block it (no signal)

• Thermal noise: Due to thermal activity of devices used for transmission


Channel capacity

• Determine the rate at which data can be transmitted over a given path, under given conditions

• Network designers must deal with….– Data rate: in bps

– Bandwidth: in Hz

– Noise: Average level of noise

– Error rate: rate at which errors occur

– Packet loss

– Packet delay

• … and manage to get the highest data rate possible


Frequency spectrum for transmission media


Implications for networking• Media selection:

– Type of network: LAN v/s WAN– Cost: Varies depending on media and is evolving – Transmission distance: Varies depending on media and is evolving – Security: Wireless less secure (signal can be intercepted)– Error rates: Highest with wireless (more susceptible to interference)– Transmission speeds:Varies depending on media and is evolving

• Design factors for transmission media– Bandwidth: All other factors remaining constant, the greater the

bandwidth of a signal, the higher the data rate– Number of receivers: Each attachment introduces some attenuation

and distortion, limiting distance and/or data rate– Transmission impairments: Limit the distance a signal can travel


Thank you

Data Communication and Computer Networks Lecture-1 Prepared by: Abdul Hasib Lecture, IICT BUET.

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