Transcript PPT Slide - Gurukul Galaxy

COMMUNICATION TECHNOLOGY
by
Shashi Bhushan
School of Computer and Information Sciences
Q. Discuss the relationship between data
rate and bandwidth.
Answer:
Direct relationship
• Higher data rate of signal, the greater the
bandwidth.
• The greater the bandwidth of a signal the
higher the data rate that can be
transmitted using the signal.
Q.
What is the difference between
bandwidth, baudrate and channel capacity?
Answer:
Bandwidth
Bandwidth is an electrical characteristic of
transmission line or circuit. It indicates the
range of frequencies measured in Hz. (cycles
per second) which can be successfully
transmitted over the line.
Baudrate
Baudrate is the number of signal elements.
This defines the signalling rate on the
transmission line. A signal element is a
discrete voltage, phase or frequency value.
Channel Capacity
Channel Capacity is the rate at which data
can be transmitted over a given
Communication Path or channel under given
condition.
In case of noise free channel, the limitation of
data rate is simply the bandwidth of the
channel.
Measuring the Channel Capacity
• Nyquist Law
• Shannon’s Law
Nyquist Law
Given a bandwidth of w, the highest
signal rate that can be carried is 2w.
Nyquist Law
If the signal to be transmitted are binary
(two voltage levels) then the data rate
that can be supported by whz is 2 wbps
Nyquist Law
Q. Suppose a bandwidth is 3100 hz then
what is the channel capacity if
(a)
Signal to be transmitted are two
voltage levels?
(b)
If the signal to be transmitted are
voltage levels?
8
Nyquist Law
Answer:
• For two voltage levels
C =2 * 3100 = 6200 bps
(b) For 8 voltage levels with multilevel
signalling the Nyquist formulation
becomes
C = 2 W log2M
M = digital levels = 8
W = bandwidth
Nyquist Law
For multivoltage level
C = 2 * 3100 * log28
= 2 * 3100 * 3 = 18,600 bps
For a given bandwidth the data rate can be
increased by increasing the number of
different level of signals.
Shannon’s Law
• Used for noisy channel
C = W log2(1 + S/N)
S/N = Signal Power/Noise Power
Shannon’s Law
Q. Calculate a channel capacity of W =
3100 Hz, S/N =1000:1 or 30db
Answer:
C = 3100 log2 (1 +1000) = 30894 bps
Shannon’s Law
Observation:
Data rate could be increased by either
increasing signal strength or bandwidth.
However, as the signal strength increases, so
do the nonlinearities in the system; leading
to an increase intermodulation noise.
Shannon’s Law
• More
amplifiers
increasing S/N ratio.
needed
for
• Since noise is constant throughout
the line the location of the amplifiers
must be closely inserted.
Shannon’s Law
• Frequent spacing
increases costs.
of
amplifiers
• Amplifiers
must
be carefully
designed to minimise the amount of
noise that is amplified along with the
signal.
Transmission Impairements
• Attenuation and attenuation distortions
• Delay distortion
• Noise
Network Devices
Methods for connecting to a network or an
independently connected computer to
another computer are:
•
•
•
•
Through a modem connection
Through an ISDN connection
Through a gateway
Through a bridge or router
Modem
Modem converts digital data into an analog
form that can be transmitted over a standard
telephone line
Modem
The term modem is a composite word that
refers to the two functional entities that
make up the device; a modulator and a
demodulator.
Modem
Normally, telephone type connections are
unsuitable for digital data as they have a
limited bandwidth of 300 to 400 hz. A
modem must then be used to convert the
digital information into an analog form that
is transmittable over the telephone line.
Modem
A modulator treats a digital signal as a series
of 1s and 0s and so can transform it into a
completely analog signal by using the digital
to analog mechanisms of ASK, FSK, PSX &
QAM.
Modem
•
•
•
•
ASK manipulates amplitude
FSK manipulates frequency
PSK manipulates phase
QAM manipulates both phase and
amplitude
Theoretical Bit Rates for Modems
ENCODING
ASK, FSK, 2-PSK
4-PSK, 4-QAM
8-PSK, 8-QAM
16-QAM
32-QAM
64-QAM
128-QAM
HALF
DUPLEX
2400
4800
7200
9600
12,000
14,400
16,800
FULL
DUPLEX
1200
2400
3600
4800
6000
7200
8400
Modem Standards
Today, many of the most popular modems
available are based on standards published
by ITU-T.
ITU-T
Bell
V.21
V.22
V.23
V.26
V.27
V.29
103
212
202
201
208
209
Baudrate
300
600
1200
1200
1600
2400
Bit
rate
300
1200
1200
2400
4800
9600
Modulation
PSK
4-PSK
FSK
4-PSK
8-PSK
16-QAM
Routers
• Examine the network address field to
determine the best route for the packets
• Supports several different
network layer protocols
types
of
Routers
• Routers communicate with other routers
to exchange routing information.
• Most network O.S. have associated
routing protocols to support the transfer
of routing information.
Routing Protocols
• BGP (Border Gateway Protocol)
• EGP (Exterior Gateway Protocol)
• OSPF (Open Shortest Path First)
• RIP (Routing Information Protocol)
Routing Protocols
• RIP – Most popular protocols in the past.
• Distributed with UNIX.
Routing Protocols
• RIP is based on distance vector algorithm
which measures the number of hops to
the destination router.
• Disadvantage – The smallest hops may
not be the best route.
Routing Protocols
• With distance vector routing each router
maintains a table by communicating with
neighbouring routers.
• Updating the table takes time when
changes occur.
Routing Protocols
Link state algorithm is based, not only on
hops, but on other parameters such as delay
capacity, reliability and throughput.