Transcript Lecture - Bangladesh University of Engineering and Technology

Lecture-1
Prepared by:
Abdul Hasib
Lecture, IICT
BUET
Data Communication and Computer Networks
Frequency, Spectrum and Bandwidth
 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
Data Communication and Computer Networks
Frequency, Spectrum and Bandwidth
Audio signal
Digital signal
 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
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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:
Data Communication and Computer Networks
Communication mode
• Point-to-point - if direct link is shared between only two
devices
• Multipoint - if direct link shared between multiple devices
• Simplex- one way transmission (commercial radio/TV)
• Half-duplex - one way transmission at a time, endpoints take
turns
• 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 Communication and Computer Networks
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 data
Networks 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
Data Communication and Computer Networks
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)
Data Communication and Computer Networks
Modulation and Encoding schemes
Modulation:
Encoding:
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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)
Data Communication and Computer Networks
Digital data , Digital signals
• 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 signal encoding formats:
 Nonreturn-to-zero-level (NRZ-L)
 Nonreturn-to-zero Interted (NRZI)
 Bipolar-AMI
 Manchester
 Differential Manchester etc.
Data Communication and Computer Networks
Digital signal encoding formats
Data Communication and Computer Networks
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
Data Communication and Computer Networks
Analog to Digital Conversion (PCM)
Data Communication and Computer Networks
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.
Data Communication and Computer Networks
Digital data, Analog signals
• 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
Data Communication and Computer Networks
Digital Modulation techniques:
2. Frequency-Shift Key (FSK)
A cos(2 f1 t + Qc) binary 1
s(t) =
A cos(2  f2 t + qc)
binary 0
f1 and f2 are carrier frequency
• Used in early low bit modem
Data Communication and Computer Networks
Digital Modulation techniques
3. Phase Shift Key (PSK)
•The phase of the carrier signal is shifted to represent data
E
E
M=2
Fig: Phase coherent PSK
Zero : represented by a signal with the same phase of the preceding one
One: 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
Data Communication and Computer Networks
Analog Data, Analog Signals
• Techniques:
AM, FM, and PM
•Amplitude Modulation
Baseband
signal sm

Bandpass signal
sm (t ) cos 2fc t
Carrier
cos( 2f c t )
Figure 7.2: amplitude modulation of a sinusoidal carrier by the baseband PAM signal
Data Communication and Computer Networks
Amplitude-modulation.
Gr ( f )
1
W 0
- fc
-W
fc
2
W r
Um ( f )
1
2
f
fc +
fc f
W
W
+W
c
Figure: Spectra of (a) baseband and (b) amplitude-modulated signal.
- fc 0
Data Communication and Computer Networks
Analog Modulation Techniques
Data Communication and Computer Networks
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
Data Communication and Computer Networks
Digital Telephone Networks, Mobile Networks
• Circuit switched of fixed bit rate (n x 64 kbps / 13 kbps)
• Connection oriented
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Public switched telephone network (PSTN)
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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
Data Communication and Computer Networks
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
Data Communication and Computer Networks
Example: space division switch
• The interconnection of network
consists of a rectangular matrix
of cross-points
Data Communication and Computer Networks
Multiplexing
• 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.
Data Communication and Computer Networks
Type of multiplexer
• FDM (Frequency Division Multiplex)
• TDM (Time Division Multiplex)
• WDM (Wave Length division Multiplex)
• CDM (Code division Multiplex)
Data Communication and Computer Networks
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.
Data Communication and Computer Networks
Time Division Multiplex
• 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.
Data Communication and Computer Networks
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
Data Communication and Computer Networks
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.
Data Communication and Computer Networks
Digital Carrier Systems
• T-carrier
North America,Japan
• E-carrier
Europe,South America
• SONET/SDH
world-wide new standard
Data Communication and Computer Networks
E1-frame
30 voice channels+2 control channels
E1 bit rate :(32x8 bit)/125 microsec =2.048Mbps
Data Communication and Computer Networks
Transfer Modes
• The type of switching depends on connection nature.
• Transfer modes:
-Circuit Switching (CS)
-Packet switching
• 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.
Data Communication and Computer Networks
Local Area Networks (LAN)
• Connectionless
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Internet
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Alternate Routing
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Packet-Switching
• Data is broken into packets, each of which can be routed separately
• Datagram
– Connectionless service
– Individual packets can follow different routes
– Packets can arrive out of sequence and are reassembled on the destination
host.
Data Communication and Computer Networks
Packet Switching
•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
Data Communication and Computer Networks
Packet-Switching Networks:Pros and Cons
• 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
Data Communication and Computer Networks
Example: Virtual Connection
Data Communication and Computer Networks
Development of Internet Hosts and Web
Data Communication and Computer Networks
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
Data Communication and Computer Networks
Arpanet History
Data Communication and Computer Networks
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
Data Communication and Computer Networks
Worldwide Ranks
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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.
Data Communication and Computer Networks
Standardization Body
• create formal standards by:
 organizing ideas
 discussing the approach
 developing draft standards
 voting on draft
 formally releasing the completed standard to the public
• 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).
Data Communication and Computer Networks
Standardization Body
Data Communication and Computer Networks
Towards a global information infrastructure
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Applications for Broadband Infrastructure
Data Communication and Computer Networks
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
Data Communication and Computer Networks
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
Data Communication and Computer Networks
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
Data Communication and Computer Networks
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
Data Communication and Computer Networks
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
Data Communication and Computer Networks
Frequency spectrum for transmission media
Data Communication and Computer Networks
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
Data Communication and Computer Networks
Thank you
Data Communication and Computer Networks